Biodesulfurization of diesel fuels – Past, present and future perspectives

International Biodeterioration & Biodegradation 110 (2016) 163e180

Contents lists available at ScienceDirect

International Biodeterioration & Biodegradation journal homepage: www.elsevier.com/locate/ibiod

Review

Biodesulfurization of diesel fuels e Past, present and future perspectives Ghasemali Mohebali a, *, Andrew S. Ball b a b

Microbiology and Biotechnology Research Group, Research Institute of Petroleum Industry, Tehran, Iran Centre for Environmental Sustainability and Remediation School of Science, RMIT University, Australia

a r t i c l e i n f o

a b s t r a c t

Article history: Received 10 April 2015 Received in revised form 19 January 2016 Accepted 11 March 2016

The world focus on environmentally friendly fuels requires refiners to convert the increasingly poorquality crude oil into high-quality finished products. Refineries are facing many challenges including heavier crude oils and increased fuel quality standards. Global society is moving towards zero-sulfur fuel and hydrodesulfurization (HDS) is the most common technology used by refineries to remove sulfur from intermediate streams. However, HDS has several disadvantages and therefore recent research has focused on improving HDS catalysts and processes and also on the development of alternative technologies. Among the alternative technologies one possible approach is biodesulfurization (BDS). BDS is a process that is based around bacterial potential. In this process, bacteria remove organosulfur from oil fractions without degrading the carbon skeleton of the compounds. BDS operates at ambient temperature and pressure with high selectivity, resulting in decreased energy costs, low emission and no generation of undesirable side-products. For assessing the potential of BDS as a biorefining process, pilot plants have been operated. The results obtained for BDS may be generally applicable to other areas of biorefining. In this review the history, current status and future challenges of BDS will be discussed. The integration use of BDS systems with existing HDS technology is discussed as a future approach by the oil industry, providing an efficient and environmentally friendly approach to desulphurization. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Biodesulfurization Diesel fuels Dibenzothiophene Bacteria

Contents 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

13. 14. 15. 16.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164 Sulfur in petroleum and its fractions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164 Air pollution as a result of fuel combustion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164 Legislative regulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165 Hydrodesulfurization (HDS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165 Deep desulfurization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165 BDS as a complementary technology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165 Biodesulfurization in environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165 Model sulfur compounds in BDS studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166 Susceptibility of DBT to microbial attack . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166 Ring-destructive pathways . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166 Sulfur-specific pathways . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166 12.1. Anaerobic sulfur-specific pathway . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166 12.2. Aerobic sulfur-specific pathways . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166 Desulfurizing microorganisms at a glance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167 Non-cellular desulfurizing biocatalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167 Dsz enzymes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 168 Sulfur substrate specificity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169

* Corresponding author. E-mail address: [email protected] (G. Mohebali). http://dx.doi.org/10.1016/j.ibiod.2016.03.011 0964-8305/© 2016 Elsevier Ltd. All rights reserved.

164

17. 18. 19. 20. 21. 22. 23. 24. 25.

26. 27.

G. Mohebali, A.S. Ball / International Biodeterioration & Biodegradation 110 (2016) 163e180

Sulfur substrate and end product inhibition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169 Genetics of DBT desulfurization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169 Dsz genes regulation and repression . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170 Desulfurization of alkylated DBTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171 Biodesulfurization of diesel oil fraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171 BDS process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171 22.1. Biocatalyst production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172 Bioreactors for BDS studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 Separation of biocatalyst from reaction mixture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174 Mass transfer studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174 25.1. Oxygen mass transfer and uptake rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174 25.2. Sulfur substrate mass transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174 25.3. The coupling of BDS and other desulfurization techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175 Improvement of BDS performance using nanotechnology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175 Future perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175 27.1. Refinery challenges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175 27.2. Research needs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176

1. Introduction Sulfur is the third most abundant heteroatom in crude oil and can vary from 0.05% to 10% of the composition. The types of sulfur compounds vary greatly within a crude supply (Blumberg et al., 2003). In addition to elemental sulfur, sulfate, sulfite, thiosulfate and sulfide, together with more than 200 sulfur-containing organic compounds have been identified in crude oils (Ma, 2010). Sulfurcontaining heterocyclic compounds are among the most potent environmental pollutants. Reducing sulfur levels in fuels can decrease harmful emissions in three ways: (i) directly reducing sulfur dioxide (SO2) and sulfate particulate matter (PM), (ii) achieving better performance from the emissions control systems, especially catalysts, and (iii) enabling the use of new emission control technologies such as diesel PM filters, NOx absorbers and selective catalyst reduction systems (Stanislaus et al., 2010). Conventional HDS processes have been employed by refineries to remove organic sulfur from liquid fuels for several decades. This technology is economic in terms of removal of a number of classes of compounds containing sulfur, other than refractory organic sulfur compounds (Breysse et al., 2003). Deep desulfurization of diesel fuel has become an important research subject due to the upcoming legislative regulations to reduce sulfur content. However, to meet the challenges of producing ultraclean diesel fuels, especially with sulfur content lower than 15 ppm, both capital investment and operational costs would be high due to more stringent operating conditions. Consequently, several alternative approaches have been used, including selective adsorption, extraction by ionic liquid, oxidative desulfurization and BDS. Microbial desulfurization of organosulfur pollutants is attracting more attention because of cost effectiveness and environmental friendliness. However, this technology is not yet available for largescale applications, so future research must investigate modifications of this process for industrial applications (Xu et al., 2006). Several previous reviews outline progress in microbial desulfurization from the basic and practical point of view (McFarland et al., 1998; Monticello, 1998; McFarland, 1999; Ohshiro and Izumi, 1999; Tong et al., 2001; Acero et al., 2003; Gray et al., 2003; Gupta et al., 2005; Kilbane, 2006; Soleimani et al., 2007; Mohebali and Ball, 2008; Xu et al., 2009; Debabov, 2010; Nuhu, 2013; Boniek et al., 2015). A recent mini-review on the role of biotechnology in the petroleum industry (Bachmann et al., 2014) highlighted the potential significance of BDS although few details were presented. In

this current review, attention is focused solely on the biodesulfurization of diesel fuels as an alternative technology, which has become an important research subject. 2. Sulfur in petroleum and its fractions In crude oil, sulfur is present in soluble organic form, the principal generic groups being: (i) aliphatic and aromatic thiols and their oxidation products (disulfides); (ii) aliphatic, aromatic and mixed thioethers, and (iii) heterocyclics based on the thiophene ring: thiophene itself, benzothiophene (BT), dibenzothiophene (DBT), and their alkyl substituted derivatives (Oldfield et al., 1998). The most abundant form of sulfur in petroleum is usually the thiophenic form. Thiophenic sulfur often comprises 50%e95% of the sulfur in crude oil and derived fractions, and alkylated DBTs are the most common organosulfur compounds typically found in crude oil and fractions used to produce diesel (Kilbane and Le Borgne, 2004). In other words the organosulfur compounds found in crude oil are generally classified into two types: non-heterocyclics and heterocyclics. The former comprise thiols, sulfides and disulfides. Cyclic or condensed multicyclic organosulfur compounds are referred to as sulfur heterocyclics (Mohebali and Ball, 2008). 3. Air pollution as a result of fuel combustion A typical flue gas from the combustion of fossil fuels will contain quantities of NOX, SO2 and particulate matter (PM); these gases react in the atmosphere with water, oxygen and other chemicals to form a mild solution of sulfuric and nitric acids. The acid rain dissolves buildings, kills forests and poisons lakes as well as damaging agricultural areas located downwind of combustion facilities (Mohebali and Ball, 2008). Acid rain also damages the environment by upsetting the natural balance of chemicals and can decrease biological diversity of the ecosystems. Traces of sulfur present in the diesel fuels poison the oxidation catalysts in the emission control system and reduce their effectiveness for the oxidation of harmful carbon monoxide, hydrocarbons and volatile organic matter (Stanislaus et al., 2010). Sulfur is one of the key causes of PM and total PM emissions are proportional to the amount of sulfur in the diesel fuel. According to the U.S. Environmental Protection Agency (USEPA), approximately 2% of the sulfur in the diesel fuel is converted to direct PM emissions. PM has been found to be a human carcinogen (Stanislaus

G. Mohebali, A.S. Ball / International Biodeterioration & Biodegradation 110 (2016) 163e180

et al., 2010); fine particulate and sulfur oxide-related pollution have been associated with lung cancer and cardiopulmonary mortality. Sulfur dioxide gas at elevated levels can also cause bronchial irritation and trigger asthma attacks in susceptible individuals (Pope et al., 2002).

4. Legislative regulations The quality of crude oil is changing; the American Petroleum Institute (API) gravity of oil is decreasing and sulfur content is increasing (Swaty, 2005). The sulfur is preferentially associated with the higher molecular weight components of crude oils and consequently, heavy crude oils typically have more sulfur than light crude oils (Kilbane and Le Borgne, 2004). Increasing sulfur concentrations in crude oil supplies results in an increase in the sulfur content of finished petroleum products. New environmental legislation has put the oil industry under increasing pressure to limit the level of sulfur in gasoline and diesel fuels. The regulations in the developed world have also forced car manufacturers to produce vehicles that meet these legislative goals. Sulfur in gasoline and diesel fuels has been targeted, since the sulfur dioxides, besides being responsible for pollution, also poisons the catalytic converters in automobile exhaust systems (Gupta et al., 2005). In response to the US Clean Air Amendment (1990), the sulfur content of transportation fuels has been reduced. The USEPA reduced non-road diesel fuel sulfur from an average of 3400 ppm down to 500 ppm (Song, 2003). Further, the USEPA has required that the concentration of sulfur present in highway and non-road diesel fuels not exceed 15 ppm through 2010e2014 (Highway, non-road, locomotive and marine diesel fuels sulfur standards, www.epa.gov/otaq/standards/fuels). The European Union also legislated that the sulphur concentration of diesel fuel be reduced to <10 ppm by 2009 (European Directive (2003/17/CE)). Overall new approaches are needed for producing affordable ultra-lowsulfur transportation and non-road fuels.

5. Hydrodesulfurization (HDS) In a refinery, hydrotreating refers to a variety of hydrogenation processes which saturate unsaturated hydrocarbons and remove S, O, N and metals from different petroleum streams. The main aim of hydrotreating is to diminish air pollution emissions, to avoid poisoning of catalysts and to improve fuel quality. HDS also improves the characteristics of the material making it easier to crack (Swaty, 2005). HDS is a catalytic process converting organic sulfur to hydrogen sulfide gas by reacting crude oil fractions with hydrogen in the presence of an efficient catalyst. Hydrogen sulfide is then readily separated from the fuel. Diesel desulfurization is more challenging due to higher initial sulfur content and the difficulty in removing sulfur compounds from the feedstock. The HDS process operates at a temperature of 200e450
C and employs a pressure of 150e200 psig in the presence of an inorganic catalyst, depending upon the level of desulfurization required (Gupta et al., 2005). HDS removes relatively simple sulfur compounds such as thiols, sulfides and disulfides effectively. Some complex aromatic sulfurcontaining compounds such as DBTs, BTs and polyaromatic sulfur heterocyclics are resistant to HDS and form the most abundant organic sulfur compounds after HDS (Ma, 2010). Therefore, research has been focused on the hydrodesulfurization of thiophenic compounds.

165

6. Deep desulfurization Deep (ultra-deep) desulfurization refers to processes to remove sulfur to below 15 ppm for diesel fuels (Song, 2003). Deep reduction of diesel sulfur is dictated largely by the least reactive sulfur compounds, refractory sulfur compounds (Pawelec et al., 2011). The reactivity of sulfur compounds in HDS follows the order: thiophene (TH)> alkylated TH > BT > alkylated BT > DBT and alkylated DBT without substituents at the 4 and 6 positions > alkylated DBT with alkyl substituents at the 4 and 6 positions (Babich and Moulijn, 2003). The shift from normal to ultra-deep desulfurization represents a complicated technical problem. The changes in diesel properties as a consequence of ultra-deep desulfurization by the hydrotreating process have been reported by Stanislaus et al. (2010). In summary, deep HDS (i) is energy intensive, (ii) results in large greenhouse gas emissions, and (iii) is costly to install and operate. In view of these, efforts have been devoted to develop alternative processes for desulfurization under mild, less severe operating conditions (Ito and van Veen, 2006). A variety of alternative routes, such as BDS, oxidative desulfurization (Wu and Ondruschka, 2010; CamposMartin et al., 2011), adsorptive desulfurization (Zhang et al., 2008a) and extractive desulfurization of sulfur compounds using solvents and ionic liquids (Kulkarni and Afonso, 2010; Kowsari, 2013) have been investigated. 7. BDS as a complementary technology While other alternatives to HDS are under development to desulfurize various refinery products, BDS would be a complete breakthrough in process development. BDS can potentially offer a low-cost alternative to HDS, reducing capital and operating costs (Worrell and Galitsky, 2004). With respect to the Kyoto Accord the interest in reducing greenhouse gas emissions led to calculations showing that CO2 emissions and energy requirements are reduced if BDS is used instead of HDS (Linguist and Pacheco, 1999). Therefore, BDS has attracted attention as a promising alternative to conventional HDS used in petroleum refineries. BDS could complement the hydrotreating process, as sterically hindered alkyl DBTs are least reactive in HDS, but are the preferred substrates for BDS. Therefore, BDS should be viewed as a complementary technology to remove recalcitrant molecules present in HDS-treated oils, not as a replacement technology. In order to achieve very low sulfur levels in diesel range fuel, it has been suggested that it is useful to perform the BDS process in conjunction with conventional HDS technology (Monticello, 1996; Grossman et al., 2001). 8. Biodesulfurization in environment In natural systems bacteria assimilate sulfur in very small amounts for their maintenance and growth. The sulfur present in both agricultural and uncultivated soils is largely in the form of organic-bound sulfur either as sulfonates and sulfate esters and not as free as bioavailable inorganic sulfate (Singh and Schwan, 2011); bacteria which are able to transform sulfur-containing compounds for utilization of either the sulfur or the carbon skeleton are widespread in nature (Le Borgne and Ayala, 2010). The occurrence of desulfurizing bacteria in diverse environments and geographic locations suggests an important and fairly common survival strategy for some bacterial species (Duarte et al., 2001; Kilbane, 2006). The development of BDS processes is dependent on provision of a microbial system with the potential to desulfurize a broad range of organosulfur compounds present in crude oil fractions.

166

G. Mohebali, A.S. Ball / International Biodeterioration & Biodegradation 110 (2016) 163e180

9. Model sulfur compounds in BDS studies Although there is no common model compound that can be used for all the various crude oil fractions, DBT and its derivatives have been reported to account for as much as 70% (w/w) of total sulfur content of West Texas crude oil and up to 40% (w/w) of the total sulfur content of some Middle East crude oils (Monticello and Finnerty, 1985). In brief, DBT as a model compound represents a reasonable choice on the basis that: (i) DBT and its derivatives represent a major proportion of thiophenic sulfur in crude oil and its distillates; (ii) alkyl-substituted DBTs seem to be the most difficult of all organosulfur compounds to remove, certain isomers surviving even deep HDS treatment (Oldfield et al., 1998). Therefore, most basic scientific work on the biodesulfurization of diesel fuels has been performed using DBT. 10. Susceptibility of DBT to microbial attack Many bacteria can degrade DBT aerobically following three major pathways (Fig. 1) (Díaz and García, 2010): in the first type, the carbon skeleton of DBT is partially oxidised, with the CeS bond remaining intact (Kodama pathway). In the second type, DBT is utilised as the sole source of carbon, sulfur, and energy. In the third type, DBT is desulfurized and the carbon skeleton remains intact (the 4S pathway). In another classification, two main types of pathways have been reported: ring-destructive and sulfur-specific pathways. 10.1. Ring-destructive pathways Until now two ring-destructive pathways for metabolism of DBT have been recognised. The most common pathway of DBT degradation, known as the “Kodama pathway” is analogous to that of naphthalene (Kodama et al., 1973). There are several reports showing that DBT can be utilised via this pathway by several bacterial genera including Pseudomonas (Kodama, 1977; Hou et al., 2005), Beijerinkia (Labord and Gibson, 1977) and Rhizobium (Frassinetti et al., 1998). In this pathway initial dioxygenation is carried out at the peripheric aromatic ring (benzyl ring) of DBT, followed by cleavage of the ring. This process leads to the accumulation of 3-hydroxy-2-formyl-benzothiophene as a watersoluble end product. In this pathway no desulfurization of the organosulfur substrate occurs. Another ring-destructive pathway that results in mineralization of DBT is one described by Van Afferden et al. (1990). They isolated Brevibacterium sp. strain DO capable of using DBT for growth as the sole source of carbon, sulfur and energy. During DBT mineralization three metabolites were identified: DBT sulfoxide (DBTO), DBT sulfone (DBTO2) and benzoate. This pathway results in the complete mineralization of DBT with the release of the sulfur atom as sulphite stoichiometrically, which is then oxidised to sulphate abiotically (Van Afferden et al., 1990; Van Afferden et al., 1993). There are no detailed studies of the enzymology or molecular biology of the attack of DBT by this strain (Bressler and Fedorak, 2000). This carbon destructive pathway may be valuable in the biodegradation of DBT in the environment. In another work, Xanthobacter polyaromaticivorans 127W has been reported as a strain capable of degrading a wide range of cyclic aromatic compounds including DBT (Hirano et al., 2004). 10.2. Sulfur-specific pathways 10.2.1. Anaerobic sulfur-specific pathway Sulphate-reducing bacteria (SRB) have been reported to

desulfurize model compounds and fossil fuels (Kim et al., 1995; Lizama et al., 1995). This assimilatory desulfurization route produces H2S. Desulfovibrio desulfuricans M6 can anaerobically reduce DBT to biphenyl and H2S (Kim et al., 1995). Desulfomicrobium escambium and Desulfovibrio longreachii have been reported to desulfurize DBT following a pathway in which biphenyl was not the end-product (Díaz and García, 2010). In anaerobic conditions, reduction of DBT by a thermopilic mixed culture has been reported; production of H2S as well as an end product other than biphenyl has been detected (Bahrami et al., 2001). However, under wellcontrolled sulphate-reducing anaerobic conditions, no significant reduction in sulfur content of DBT or in total sulfur content of vacuum gas oil, deasphalted oil or bitumen has been observed (Armstrong et al., 1995, 1997). The conserved nature of the desulfurization (dsz) genotypes among aerobic desulfurising strains from different geographic locations has been documented (Denis-Larose et al., 1997); sulphate-reducers showed no cross-reactivity, suggesting the anaerobic sulfur-specific removal sometimes reported (Kim et al., 1995; Lizama et al., 1995) occurs by a different pathway. The desulfurization of oil under anaerobic conditions avoids costs associated with aeration and has the advantage of liberating sulfur as a gas. However, an anaerobic BDS process has not currently been developed due to low reaction rate, safety and cost concerns (Gupta et al., 2005).

10.2.2. Aerobic sulfur-specific pathways The 4S pathway is a sulphur-specific pathway in which DBT is desulfurized and the carbon skeleton of the substrate released intact (Fig. 1). There is almost no reduction in calorific value of the petroleum products. The 4S pathway is an energetically expensive pathway because the carbon skeleton is not mineralized in order to get back the energy invested. The pathway can be divided into three stages: (i) activation of the thiophene ring for cleavage by oxidation of the sulfur moiety; (ii) cleavage of the thiophene ring to give an aromatic sulphinate, and (iii) removal of the sulfinate group (Oldfield et al., 1998). The use of this pathway has been proposed for the desulfurization of petroleum in production fields and also refineries. Several bacterial genera have been reported that were capable of converting DBT to intermediate compounds of the 4S pathway or partially performed this pathway (Marzona et al., 1997). An extended 4S pathway has also been reported in some thermophilic Mycobacterium strains. In this pathway, the final product of the 4S pathway, 2HBP becomes methoxylated to 2-methoxybiphenyl (2MBP) (Li et al., 2003; Xu et al., 2006; Chen et al., 2009). Li et al. (2005b) reported that with Microbacterium sp. strain ZD-M2 the metabolites produced by DBT desulfurization were identified as 2MBP and biphenyl as well as 2-HBP. Compared with 2-HBP, 2-MBP has a reduced inhibition effect on cell growth and desulfurization activity. The pathway, with 2-MBP as the end product may be an alternative for the further desulfuration of the fossil fuels. A new BDS pathway has been found that is different from the 4S pathway. Rhodococcus sp. strain WU-K2R was reported to grow on naphtho[2,1-b]thiophene (NTH) as the sole sulfur source but not as the sole carbon source (Kirimura et al., 2002). NTH is an asymmetric structural isomer of DBT and in addition to DBT derivatives, NTH derivatives can also be detected in HDS-treated diesel oil. Strain WU-K2R could not utilize DBT, DBTO2 or 4,6-DMDBT. However, desulfurization metabolites of NTH were identified as NTH sulfone and 2-hydroxynaphthylethene and naphtho[2,1-b]furan, indicating that strain WU-K2R could preferentially desulfurize asymmetric heterocyclic sulfur compounds such as NTH through sulfur-specific pathways.

G. Mohebali, A.S. Ball / International Biodeterioration & Biodegradation 110 (2016) 163e180

167

Fig. 1. The aerobic pathways for DBT degradation: (1) the 4S pathway (black thick arrows) and the extended 4S pathway (white arrow), (2) the CeC cleavage pathway that releases sulfur (dotted thin arrows), (3) the sulfur-oxidation pathway (black thin arrow) and (4) the Kodama pathway (dotted thick arrows) (Díaz and García, 2010).

11. Desulfurizing microorganisms at a glance Maliyantz (1935) reported bacterial desulfurization of petroleum oil with the accumulation of hydrogen sulfide (Yamada et al., 1968). Strawinski (1950, 1951) and Zobell (1953) issued patents concerned with the procedures for microbial desulfurization (Yamada et al., 1968). Isbister and Koblynski (1985) described a Pseudomonas sp. strain CB-1 that could accomplish sulfur specific metabolism of DBT. The intermediates were DBTO, DBTO2 and the end product was dihydroxybiphenyl. Unfortunately this strain was lost before the metabolic pathway could be fully characterized (Gallager et al., 1993). Rhodococcus erythropolis strain IGTS8 has been isolated on its ability to use coal as the sole sulfur source for growth (Kilbane and Jackowski, 1992). Several bacterial genera have been reported that can selectively desulfurize DBT and its derivatives or can desulfurize a range of

sulfur compounds in several crude oil fractions via the 4S pathway (Table 1). By screening the available genomic databases, Bhatia and Sharma (2010b) identified 13 novel potential DBT desulfurizing microorganisms belonging to 12 genera. 12. Non-cellular desulfurizing biocatalysts Besides cells of wild type and genetically modified bacteria, BDS studies using other isolated biocatalysts such as monooxygenases, oxidases and peroxidases have been carried out. The use of monooxygenases was determined not useful as they required, besides O2, the high-cost flavin mononucleotide co-factor (Stachyra et al., 1996). Better results have been obtained with oxidases and peroxidases (Madeira et al., 2008). Eibes et al. (2006) evaluated fungal peroxidases, including manganese peroxidases to degrade DBT and observed the formation of 4-methoxybenzoic acid as the ring

168

G. Mohebali, A.S. Ball / International Biodeterioration & Biodegradation 110 (2016) 163e180

Table 1 The bacterial genera that can use the 4S pathway to desulfurize DBT and its derivatives. Genera

References

Agrobacterium Alcaligenes Arthrobacter Bacillus Beijerinkia Brevibacillus Corynebacterium Desulfobacterium Desulfovibrio Gordonia Lysinibacillus Microbacterium Mycobacterium Nocardia Paenibacillus Pantoea Pseudomonas Rhodococcus

Constanti et al., 1994 Ranson and Rivas, 2008 Lee et al., 1995; Labana et al., 2005 Kirimura et al., 2001; Jiang et al., 2002; Ma et al., 2006b; Xiaojuan et al., 2008 Labord and Gibson, 1977 Abo-State et al., 2014 Maghsoudi et al., 2000; Wang et al., 2006b Aribike et al., 2009 Zobell, 1953 (Yamada et al., 1968) Rhee et al., 1998; Acero et al., 2003; Alves et al., 2005; Li et al., 2006a; Santos et al., 2006a; Wang et al., 2006a; Mohebali et al., 2007; Bahuguna et al., 2011 Li et al., 2005b; Chen et al., 2008a Wang and Krawiec, 1994; Nekodzuka et al., 1997; Kayser et al., 2002; Li et al., 2003; Srinivasaraghavan et al., 2006 Olson, 2000; Jiang et al., 2002; Chen et al., 2008b; Konishi et al., 1997 Bhatia and Sharma, 2010a Yamada et al., 1968; Jiang et al., 2002; Shan et al., 2003; Martin et al., 2004; Hou et al., 2005; Tao et al., 2011; Kilbane and Jackowski, 1992; Purdy et al., 1993; Izumi et al., 1994; Lee et al., 1995; Ohshiro et al., 1995; Omori et al., 1995; Denis-Larose et al., 1997; Honda et al., 1998; Folsom et al., 1999; Kobayashi et al., 2000; Maghsoudi et al., 2001; Castorena et al., 2002; Gou et al., 2002; Ma et al., 2002; Tanaka et al., 2002a; Ting et al., 2002; Xu et al., 2002; Akbarzadeh et al., 2003; Zhongxuan et al., 2003; Guchhait et al., 2005a; Labana et al., 2005; Wei et al., 2005; Yang and Marison, 2005; Ma et al., 2006a; Yu et al., 2006; Li et al., 2007b; Santos et al., 2007; Xiong et al., 2007; Zhang et al., 2007b; Davoodi-Dehaghani et al., 2010; Etemadifar et al., 2014; De Araújo et al., 2012 Ansari et al., 2007 Darzins and Mrachko, 1998; Gunam et al., 2013 Constanti et al., 1994 Abbad-Andaloussi et al., 2003b

Serratia Shewanella Sphingomonas Xanthomonas Several unidentified bacteria

cleavage final product with DBTO2 as a reaction intermediate compound. As DBT oxidation generated DBTO and DBTO2 that could be compatible to a subsequent microbial BDS, the combination of an enzymatic reaction with a further bacterial metabolism step to remove sulfur has been proposed (Klyachko and Klibanov, 1992). Ayala et al. (1998) described a method that includes biocatalytic oxidation of organosulfides and thiophenes with hemoproteins to form sulfoxides and sulfones, followed by a distillation step in which these oxidized compounds are removed from the fuel; straight-run diesel fuel (1.6% sulfur) was oxidized with chloroperoxidase from Caldariomyces fumago in the presence of 0.25 mM hydrogen peroxide. The organosulfur compounds were effectively transformed to their respective sulfoxides and sulfones which were then removed by distillation. The resulting fraction contained only 0.27% sulfur. The above mentioned results may lead to improvements in the BDS process, as the low solubility of sulfur compounds in aqueous media and their transference from the oil phase into cells has been considered to be a rate-limiting step in the metabolism of DBTs. 13. Dsz enzymes As mentioned earlier, the 4S pathway involves sequential oxidation of the sulfur moiety and cleaving of the C-S bonds (Fig. 1). In the sequential oxidation, 4 key enzymes are involved, two monooxygenases, one desulphinase and one NADH:FMN oxidoreductase. The latter supplies the two monooxygenases with reduced flavin. In strain IGTS8 each of the key Dsz pathway enzymes has been purified and characterised. In strain IGTS8 the pathway proceeds via two monooxygenases (DszC and DszA) supported by a flavin reductase (DszD), and a desulphinase (DszB).

DBT/DBTO/DBTO2 /HBPS/2  HBP þ sulphite DszC catalyses the sequential conversion of DBT / DBTO / DBTO2. The two catabolic steps require oxygen and FMNH2 for activity (Gray et al., 1996; Oldfield et al., 1997; Xi et al.,

1997). The properties of and the reactions catalyzed by the purified DszC enzyme from strain IGTS8 have been characterized (Lei and Tu, 1996). On the basis of isotope labeling patterns, DBT sulfoxide and sulfone obtained their oxygen atom(s) from molecular oxygen rather than water in their formation from DBT. This monooxygenase is unique among microbial flavomonooxygenases in its ability to catalyze two consecutive monooxygenation reactions. The DszC of a desulfurizing bacterium Mycobacterium goodii X7B has also been purified (Li et al., 2009a). The monooxygenase catalyzing sulfur-atom specific oxidation of both DBT and BT has been purified and characterized from Paenibacillus sp. strain A11-2 (Konishi et al., 2002). DszA catalyses the transformation of the sulphone to a sulphinate (HPBS), with a reaction rate 5e10 fold higher than DszC (Gray et al., 1996). This reaction requires molecular oxygen and is NADHdependent (Oldfield et al., 1997). DszB is an aromatic sulphinic acid hydrolase and uses a nucleophilic attack of a base-activated water molecule on the sulphinate sulfur to form 2-HBP in a rate-limiting reaction for the 4S pathway (McFarland et al., 1998; McFarland, 1999; Ohshiro and Izumi, 1999). To enhance the DBT metabolic flux rate, the intrinsic catalytic properties or the specific production of DszB in the cell must be improved. HPBS desulfinase has been purified and characterized from Paenibacillus sp. strain A11-2 (Konishi and Maruhashi, 2003a). The Dsz pathway is an energy intensive process with ~4 mol of NADH required per mol DBT desulfurized (Oldfield et al., 1997). DszC and DszA do not use NADH directly, but use FMNH2 from a FMN:NADH oxidoreductase (DszD). DszD couples the oxidation of NADH with substrate oxidation by DszA and DszC:

DBT þ 3O2 þ 4NADH þ 2H þ /2  HBP þ SO3 2þ þ 3H2 O þ 4NADþ Flavin reductase (Frd) A has been found to be essential for expression of DBT desulfurizing activity by strain IGTS8. When the frd gene is destroyed by insertional inactivation DBT

G. Mohebali, A.S. Ball / International Biodeterioration & Biodegradation 110 (2016) 163e180

desulfurization activity is lost completely. Interestingly, inactivation of frdA seems to have no effect on the viability and growth of strain IGTS8. Twenty-fold over-expression of the enzyme in strain IGTS8 results only in a modest two-fold increase in DBTdesulfurization activity indicating that provision of FMNH2 is not rate-limiting for DBT desulfurization in vivo (Oldfield et al., 1998). 14. Sulfur substrate specificity The broad substrate range of the Dsz system is one of the driving forces for the development of BDS as a commercial process. Despite the obvious chemical similarity of DBT and BT, the two desulfurization pathways are mutually exclusive. Thus BT cannot be desulfurized via the DBT-specific pathway and vice versa (Gilbert et al., 1998). Therefore these pathways are complementary in terms of their potential roles in development of a fuel BDS technology. Substituted BTs and DBTs remain in diesel oil following conventional HDS (Li et al., 2008a) and a bacterial mixture is more efficient for the practical BDS of diesel oil. A mixture of washed cells of R. erythropolis DS-3, capable of desulfurizing DBT and its derivatives (Ma et al., 2002) and Gordonia sp. C-6, capable of desulfurizing BT and its derivatives (Li et al., 2006a) have been employed to desulfurize HDS-treated diesel oil (Li et al., 2008a); approx 86% (w/w) of the total sulfur (from 1260 to 180 ppm) was removed from diesel oil after three cycles of BDS. The results demonstrated that application of mixed bacteria to desulfurization increased the number of targeted substrates and enhanced the desulfurization efficiency toward a variety of sulfur compounds; however several bacteria have been reported as desulfuring both BT and DBT: Rhodococcus sp. KT462 (Tanaka et al., 2002a), Mycobacterium goodie X7B (Li et al., 2003), Mycobacterium phlei WU-0103 (Ishii et al., 2005), R. erythropolis (Zhang et al., 2007b), Paenibacillus sp. A11-2 (Konishi et al., 2000), Sphingomonas subarctica T7b (Gunam et al., 2006), Mycobacterium sp. ZD-19 (Chen et al., 2008b), Lysinibacillus sphaericus DMT-7 (Bahuguna et al., 2011) and Rhodococcus sp. ECRD-1 (Grossman et al., 2001). However, the efficiency of desulfurization of BT and DBT by the strains is not equal. Improvement of the desulfurization enzymes to broaden substrate specificity is needed to apply biodesulfurization to the refining process. It has been reported that strain IGTS8 could not grow on BT or 5-methyl BT (Kayser et al., 1993; Gilbert et al., 1998; Matsui et al., 2000) as a sole sulfur source. Arensdorf et al. (2002) used a two-phase sulfur-limited chemostat to select for gain-offunction mutants with mutations in the Dsz system of strain IGTS8. Mutations arose that allowed growth with octyl sulfide and 5-methyl BT as the sole sulfur sources. An isolate was genetically characterized and found to contain mutations in two genes, dszA and dszC. The nature of the DszA mutations has not been characterized. A transversion (G to T) in dszC codon 261 resulted in a V261F mutation that was determined to be responsible for the 5methyl BT gain-of-function phenotype. The V261F DszC mutant represents an enzyme that can transform both DBT and BT. It has been found that strain IGTS8 containing DszA mutated at residue 345 (Q345A) can degrade octyl sulfide on which the wild strain cannot grow (Konishi and Maruhashi, 2003b). Only DszA, changed at residue 345 gave an altered CeS bond cleavage pattern of 3-methyl DBT sulfone. This residue is therefore involved in CeS bond cleavage specifically for alkylated DBT sulfone. The substrate specificity of Mycobacterium sp. G3 with Dsz activity against DBT has also been investigated (Okada et al., 2002a, b). 4,6-Dipropyl DBT, 4,6-dibutyl DBT and 4,6-dipentyl DBT were metabolized to the hydroxybiphenyl form in the water reaction system. The results indicated that strain G3 has superior substrate specificity for high molecular weight alkyl DBTs. Enzyme MdsC from strain G3 oxidized derivatives of DBT to each sulfone form,

169

suggesting that MdsC cover a broad specificity for alkyl DBTs (Nomura et al., 2005). 15. Sulfur substrate and end product inhibition The growth kinetics of Rhodococcus sp. strain JUBT1 has been examined using DBT, alkylated DBT and diesel as limiting substrates (Guchhait et al., 2005a,b). The substrate inhibited growth followed Haldane type kinetics. Kinetic parameters were highly affected by the increase in the extent of alkylation of DBT and linear correlations have been observed between the functionality of the parameters with the number of alkylation. Zhang et al. (2013) investigated interactions among three typical Cx-DBTs, DBT, 4-MDBT and 4,6-DMDBT, using Mycobacterium sp. ZD-19 in an airlift reactor. The results indicated that the Dsz rates would decrease in the multiple Cx-DBTs system compared to the single Cx-DBT system. The extent of inhibition depended upon the substrate numbers, concentrations and affinities of the co-existing substrates. This phenomenon was caused by an apparent competitive inhibition of substrates, which was well predicted by a MichaeliseMenten competitive inhibition model. Several authors have reported that cell growth and Dsz activity were inhibited by the presence of 2-HBP (Xu et al., 2002; Kim et al., 2004; Yang and Marison, 2005; Chen et al., 2008a). Because the 2-HBP oil-water partition coefficient is very high, this hydrophobic compound migrates to the oil phase when biphasic media are tested, and so it has a lower inhibitory effect in biphasic environments than that in the sole aqueous system (Yang and Marison, 2005; Caro et al., 2007; Chen et al., 2008a). Chen et al. (2008a) reported that 2-HBP inhibits cell growth and Dsz activity of Microbacterium sp. when added in the reaction media. A mathematical model explaining the product formation kinetics with DBT as the sole sulfur source elucidated that along with 2-HBP accumulation, the inhibitory effect of 2-HBP on DBT desulfurization and cell growth was enhanced. Abin-Fuentes et al. (2013) indicated that the inhibition of the Dsz enzymes by HBP is responsible for the observed reduction in IGTS8 resting cells activity concomitant with HBP generation. Xu et al. (2002) reported that 2-HBP inhibited the growth of Rhodococcus sp. 1awq, the synthesis of the Dsz enzymes and the activity of the enzymes. The effect of 2-HBP on cell growth of Gordonia alkanivorans and R. erythropolis has been determined using chronic bioassays on batch and continuous cultures (Alves ~o, 2011); the half maximal inhibitory concentration and Paixa (IC50) values obtained showed the high toxicity of 2-HBP. Caro et al. (2008b) investigated the inhibition process in aqueous and oil-water systems using R. erythropolis IGTS8 and Pseudomonas putida CECT5279 resting cells; with strain CECT5279 no decrease in DBT biodegradation was observed when 2-HBP was added both in aqueous and biphasic media; in contrast, strain IGTS8 was strongly affected from very low 2-HBP concentrations in aqueous medium. 16. Genetics of DBT desulfurization The genes involved in DBT metabolism have been called bds, dsz, tds, mds, and sox. Because several other unrelated genes have been labelled sox, the sox designation has been generally rejected. Bds, Dsz, Tds, and Mds have all been accepted as gene products (Mohebali and Ball, 2008). R. erythropolis IGTS8 (Piddington et al., 1995; Gray et al., 1996; Li et al., 1996; Oldfield et al., 1997) and Rhodococcus sp. strain X309 (Denis-Larose et al., 1997, 1998) were among the first strains to be characterised at the molecular level. The conversion of DBT into 2HBP is carried out by strain IGTS8 via the 4S pathway, consisting of two monooxygenases (DszC and DszA) and one desulfinase (DszB)

170

G. Mohebali, A.S. Ball / International Biodeterioration & Biodegradation 110 (2016) 163e180

which are encoded by the dsz operon. Sequence and subcloning analyses revealed that the three genes, dszA, dszB, and dszC, were transcribed and expressed in the same orientation. The termination codon for dszA and the initiation codon for dszB overlap, and there is a 13-bp gap between dszB and dszC (Piddington et al., 1995). DBT desulfurization activity was improved by removing the gene overlap in the dsz operon (Li et al., 2007a). It has been reported that the three genes are clustered on a 120-kb linear plasmid of strain IGTS8 (Denome et al., 1994). The desulfurization ability of Rhodococcus sp. strain ECRD-1, strain IGTS8 and other desulfurization strains appeared to be an exclusive property of a 4-kb gene locus on a large plasmid, the loss of which results in the loss of the desulfurization phenotype (Prince and Grossman, 2003). In Rhodococcus the dsz genes are found on conjugative plasmids and located in the proximity of insertion sequences (Denome and Young, 1995; DenisLarose et al., 1997; Kilbane and Le Borgne, 2004). Therefore, the Dsz system is organised as one operon with three genes (dszA, dszB, dszC) transcribed in the same direction, coding for three proteins DszA, DszB, and DszC under the control of a single promoter (Oldfield et al., 1997; Gray et al., 1998). Li et al. (1996) detected that there was only one dsz promoter before the start of dszA. They cloned 385 bp of the DNA immediately 50 to dszA from strain IGTS8 in Escherichia coli and showed that this region contains a Rhodococcus promoter and at least three dsz regulatory regions. Immediately upstream of the promoter was a protein-binding domain. Deletion of this region did not affect sulfate repression, but promoter activity appeared to be reduced by three-fold. Thus, it could be an activator binding site or an enhancer region. The DBT biodesulfurization pathway in R. erythropolis has also been optimized by genetic rearrangement (Li et al., 2008b). Three genes (mdsA, mdsB, and mdsC) for desulfurization which form a cluster have been cloned from Mycobacterium sp. G3 (Nomura et al., 2005). The expression of each gene in E. coli JM109 showed that MdsC oxidized DBT to DBTO2, MdsA transformed DBTO2 into HPBS and MdsB desulfurized HPBS into 2-HBP. The distribution of dsz genes in various bacteria strongly supports the hypothesis that these genes are commonly subjected to horizontal transfer in nature. Indeed the DNA sequences of dsz genes from numerous bacterial cultures isolated in geographically distinct locations have been found to be nearly identical. PCR amplification of dsz genes from soil samples revealed relatively few variations in dsz gene sequences, with the majority of variations found in dszA, and even then homology to the R. erythropolis IGTS8 dszA sequence was 95% or more (Duarte et al., 2001). The conserved nature of the dsz genotypes among desulfurizing strains from different geographic locations has been documented (Denis-Larose et al., 1997). Various species belonging to Arthrobacter, Bacillus, Corynebacterium, Entrobacter, Gordonia, Klebsiella, Mycobacterium, Nocardia, Paenibacillus, Rhodococcus, Sphingomonas and Stenotrophomonas have been isolated that possess dsz gene sequences that are identical or highly homologous to the DNA sequence of the dsz genes of R. erythropolis IGTS8 (Darzins and Mrachko, 2000; Ishii et al., 2000; Kobayashi et al., 2000; Zhongxuan et al., 2003; Kilbane and Le Borgne, 2004; Kilbane, 2006; Ma et al., 2006c; Alves et al., 2007; Kilbane and Robbins, 2007; Santos et al., 2007; Shavandi et al., 2009). An interesting finding is that bacterial cultures that possess identical dsz gene sequences can have vastly different desulfurization phenotypes. This was clearly illustrated by examining Dsz activity of M. phlei GTIS10 having mdsABC gene sequences identical to R. erythropolis IGTS8; the temperature at which maximum desulfurization activity was detected in the cultures was around 50
C and 30
C, respectively and the concentrations of metabolites produced by the two cultures varied (Kayser et al., 2002; Kilbane, 2006). Characterization of five bacterial cultures capable of

utilizing DBT as the sole source of sulfur revealed that four of the cultures had identical dsz genes, but the cultures differed significantly with regards to their substrate range, Dsz activity and yield of metabolites (Abbad-Andaloussi et al., 2003a). At the intraspecies level, a comparative study of the bacterial strains M. phlei SM120-1 and M. phlei GTIS10 showed that there are considerable differences in the phenotypic and genotypic characteristics of these two strains (Srinivasaraghavan et al., 2006). The range of desulfurization phenotypes observed in different cultures may reflect the ability of each bacterial species to provide cofactors and reaction substrates under the conditions tested. Transport of substrates and products might also contribute to Dsz activity, as demonstrated by the fact that cell-free lysates of desulfurization cultures can exhibit a broader substrate range than the intact cell culture (Kilbane, 2006). Ishii et al. (2000) introduced molecular evidence for the presence of the dsz-like genes and Dsz-like enzymes in Paenibacillus sp. strain A11-2. The nucleotide sequences of the gene cluster as well as the structures and functions of the gene products showed that the tds (thermophilic desulfurization) sequence seemed to be a thermophilic equivalent of the mesophilic dsz operon. Organization of the tds genes in the cluster was also quite similar to that of the dsz genes. This pathway for DBT conversion into 2-HBP was practically the same as the 4S pathway. Distinct variants of the dsz genes have also been found among thermophilic bacteria. The thermophilic bacteria Bacillus subtilis WU-S2B and M. phlei WU-F1 desulfurize DBT and alkylated DBTs through specific cleavage of the C-S bonds over a temperature range up to 52
C (Furuya et al., 2001; Kirimura et al., 2004). The gene cluster containing bdsA, bdsB, and bdsC was cloned from B. subtilis WU-S2B (Kirimura et al., 2004); a nucleotide sequence homology of 61.0% to dszABC of strain IGTS8 and 57.8% to tdsABC of the thermophilic bacterium Paenibacillus sp. A11-2 was found. The nucleotide sequences of B. subtilis WU-S2B bdsABC and the corresponding genes from M. phlei WU-F1 were found to be identical to each other although the strains are genetically different. It has been confirmed that the operon structure of the bds genes is similar to those of DBT-desulfurization genes, such as the dsz genes and the tds genes. Shavandi et al. (2010) analysed the dsz promoter from G. alkanivorans RIPI90A, a DBT desulfurizing strain. Despite the high homology between dsz genes of strain RIPI90A and strain IGTS8, promoter sequences of the strains were not similar (52.5% homology). Unlike the dsz operon of strain IGTS8, the operon of strain RIPI90A was located on the chromosome. 17. Dsz genes regulation and repression Several investigators reported that the Dsz activity in various bacteria was completely repressed by sulphate or other readily bioavailable sulfur sources including methionine, cysteine, taurine, methanesulfonic acid and casamino acids. In rhodococci, these sulfur compounds repress the promoter of the dsz gene sequence (Li et al., 1996) or enzyme synthesis at the transcription level (Monticello, 1998), but are not inhibitors of the Dsz enzymes. It has been reported that Dsz activity of strain IGTS8 was repressed by methionine, cysteine, casamino acids and sulfate but not by DBT (Li et al., 1996). It was found that transcription from the dsz promoter was strongly repressed by SO2 4 , cysteine and casamino acids indicating that there could be a Dsz repressor. Analysis showed that each deletion that retained promoter activity was fully repressible indicating that if there is a Dsz repressor protein it bound to a site that overlaps the promoter. Darzins and Mrachko (2000) revealed that the sequences (400 bp) directly upstream of the dszA start site contained regulatory elements (i.e. promoter elements) that control transcription of the strain AD109 dsz gene

G. Mohebali, A.S. Ball / International Biodeterioration & Biodegradation 110 (2016) 163e180

cluster. A comparison of this potential promoter region with the strain IGTS8 dsz promoter region failed to reveal any significant homology. It has been reported that the dsz cluster from strain IGTS8 could be engineered as a DNA cassette under the control of heterologous regulatory signals to expand the ability of Pseudomonas strains to efficiently desulfurize DBT (Gallardo et al., 1997). The alleviation of the native sulfur repression of the dsz cluster by heterologous regulation, and the chromosomal location of dsz, providing stability and containment, were relevant features of the recombinant strains. The dszAB genes of G. alkanivorans strain 1B have been expressed in E. coli (Alves et al., 2007); the recombinant strain could desulfurize DBTs in LB medium containing other sulfur compounds such as sulfates, showing no sulfate repression of the dszAB genes expression. Several researchers were interested in promoter replacement as a possible alternative to expression of the dsz genes in a heterologous host as a means of decreasing the sulfur repression problem; recombinant Rhodococcus sp. strain T09 has been constructed that could desulfurize DBT even in the presence of methionine, cysteine, or inorganic sulfate as the sulfur source (Matsui et al., 2002). The putative rrn promoter was replaced upstream from the Dsz enzyme-coding region to construct a Dsz enzyme expression vector for strain T09. The obtained recombinant strain could degrade DBT, irrespective of the sulfur source used. The dszABC genes from a DBT-desulfurizing bacterium, G. alkanivorans RIPI90A have been cloned and sequenced (Shavandi et al., 2009). The E. coli-Rhodococcus shuttle vector pRSG43 was applied to cloning and efficient expression of the dsz genes under the control of the lac promoter. The recombinant strain was able to desulfurize DBT in the presence of inorganic sulfate and sulfur-containing amino acids. The maximum Dsz activity by recombinant resting cells was increased 2.67-fold in comparison to the highest Dsz activity of native resting cells. The expression of dsz genes in R. erythropolis strain KA2-5-1 was repressed by sulfate (Noda et al., 2002). A 340 bp putative promoter element, designated kap1 was isolated from a strain KA2-5-1 recombinant. The activity of kap1 was not affected by 1 mM sulfate. Two mutants of strain KA2-5-1, strains MS51 and MS316 also express a high level of Dsz activity in the presence of sulfate (Tanaka et al., 2002b). The level of DBT desulfurization by cell-free extracts prepared from the mutants grown on sulfate was about 5-fold higher than that by cell-free extracts of wild type. 18. Desulfurization of alkylated DBTs Much of the residual post-HDS organic sulfur in intermediates and combustible products is thiophenic sulfur. The majority of this residual sulfur is present in DBT and derivatives thereof having one or more alkyl or aryl groups attached to one or more carbon atoms present in one or both flanking benzylic rings. The alkyl side chains have been shown to significantly affect the relative reactivity of the thiophenic molecules with chemical or biological catalysts. The most refractory Cx-DBTs have substitutions at the 4 and 6 positions, which are adjacent to the sulfur moiety and are believed to strictly hinder access of the sulfur atom to the catalyst surface (Rhee et al., 1998; Grossman et al., 1999, 2001). As regulation on sulfur levels in fuels become stricter, more of the HDS-refractory compounds must be removed. When the sulfur content of the diesel fuel has to be reduced to ultra-low levels by catalytic hydrotreating, even the very refractory sulfur compounds must be removed (Stanislaus et al., 2010). Alkylated DBTs and alkylated BTs as the major sulfur components in the HDS-treated oil fractions are more resistant to HDS treatment than mercaptans and sulphides (Folsom et al., 1999). Because of the ubiquity of these compounds in practically all crude oils, these compounds represent

171

the bulk of sulfur (Marcelis, 2002). Heavy crude oils contain higher concentrations of refractory sulfur in the form of stable alkylated DBTs (Olson, 2000). The results of desulfurization studies revealed that all DBT desulfurizing bacteria could desulfurize alkylated DBTs via the 4S pathway. The manner of the attack on DBT is affected not only by the position but the number and length of the alkyl substituents (Onaka et al., 2001a, b; Tao et al., 2006); in general, increasing alkylation decreases the reactivity. Several bacterial isolates have been shown to desulfurise sterically hindered DBTs (Table 2). In medium containing mixtures of DBT and its alkylated derivatives, a clear preference for DBT over 4,6-diethyl DBT (4,6DEDBT) was observed (Grossman et al., 2001; Chen et al., 2008b). However, desulfurization of a mixture containing DBT and 4,6dimthyl DBT (4,6-DMDBT) dissolved in dodecane by Nocardia globerula R-9 proceeded simultaneously without preference for either of them (Mingfang et al., 2003); the desulfurization rate for each compound was decreased when they were mixed together. The Dsz rate of DBT or 4,6-DMDBT in the mixture was lower than when they were desulfurized separately by Mycobacterium sp. ZD19, indicating that substrate competitive inhibition occurs when DBT and 4,6-DMDBT are mixed (Chen et al., 2008b). Konishi et al. (1997) found that the thermophilic bacterium, Paenibacillus sp. A11-2, in the growing state could decrease sulfur in the following four methylated DBTs when used as the sole sulfur sources in the presence of n-tetradecane at 50
C: 4-methyl DBT (4MDBT), 4,6-DMDBT, 2,8-dimethyl DBT (2,8-DMDBT), and 3,4,6trimethyl DBT (3,4,6-TMDBT). Konishi et al. (1999) found that the manner of the degrees of growth at the expense of the sulfur substrates was as follows: control < 4,6-DMDBT < 3,4,6TMDBT < 4-MDBT < 2,8-DMDBT. Rhodococcus sp. ECRD-1 has been reported to be effective at completely removing 4,6-DEDBT from the oils tested, light catalytic-cycle oil and a middle distillate fraction (Prince and Grossman, 2003). The experiments described the substrate preference of the initial enzymes of the desulfurization pathway. Light gas oil desulfurization by resting cells of Pseudomonas aeruginosa PAR41 carrying the dsz gene cluster from R. erythropolis KA2-5-1, grown in a medium with 4,6-DMDBT in n-tetradecane (50% (v/v)) showed that 4-MDBT, 4,6-DMDBT and 3,4,6-TMDBT in light gas oil were desulfurized by 68.5, 34.2 and 12.3%, respectively during a 4 h reaction, with no further Dsz activity observed (Watanabe et al., 2002). Mycobacterium sp. MR65 carrying dszABCD genes has been used for desulfurization of 10-methylbenzo[b]naphtho[2,1-d]thiophene (10-methyl BNT) in the hexadecane phase (Watanabe et al., 2003). The specific Dsz activity of 10-methyl BNT in n-hexadecane by resting cells of the strain was approximately 25% of that of DBT but approximately 4 and 20 times more than those of 4,6-diethyl DBT and 4,6-dipropyl DBT, respectively. 19. Biodesulfurization of diesel oil fraction Research work is currently being undertaken around the world to bring about deeper desulfurization of middle distillate oil fractions. Biodesulfurization of the fractions has been reported (Table 3). 20. BDS process In order to make a BDS process competitive with deep HDS a five-step process is needed: (i) production of active resting cells (biocatalysts) with a high specific activity; (ii) preparation of a biphasic system containing oil fraction, aqueous phase and biocatalyst; (iii) biodesulfurization of a wide range of organic sulfur

172

G. Mohebali, A.S. Ball / International Biodeterioration & Biodegradation 110 (2016) 163e180

Table 2 Bacterial isolates capable of desulfurizing DBTs bearing alkyl substitutions adjacent to the sulfur atom. Bacterium

Growing/ resting state

Sulfur substrate

End product

Organic phase

Temp. (
C)

Strain RIPI-S81

Growing cells

DBT 4-MDBT 4,6-DMDBT

e

30

96

Bacillus subtilis WU-S2B

Growing cells

e

50

120

Kirimura et al., 2001

Paenibacillus sp. A11-2

Growing cells

Corresponding monohydroxy biphenyls

ntetradecane

50

22

Konishi et al., 1997 Onaka et al., 2001b

Rhodococcus erythropolis H-2

Resting cells

a-hydroxy-b-phenylnaphthalene monohydroxy dimethyl biphenyls

e

30

12

Ohshiro et al., 1996

Rhodococcus sp. ECRD-1

Growing cells

25

12

Lee et al., 1995

Growing cells

HBP Corresponding hydroxydiethyl biphenyl 2-HBP 2-hydoxy-30 -methyl-biphenyl 2-hydroxy-3-methyl-biphenyl 2-hydroxy-3,30 -dimethyl-biphenyl a-hydroxy-b-phenyl-naphthalene 1-methoxy-2-(3-methylphenyl) naphthalene 1-hydroxy-2-(3-methylphenyl)naphthalene 2-(2-methoxy-3-methylphenyl) naphthalene 2-(2-hydroxy-3-methylphenyl)naphthalene 2-ethylNTH sulfoxide 1-(20 -hydroxynaphthyl)-1-butene 1-naphthyl-2-hydroxy-1-butene

e

Rhodococcus erythropolis XP

DBT 2,8-DMDBT 4,6-DMDBT 3,4-benzo DBT 4-MDBT 2,8-DMDBT 3,4,6-TMDBT Ethyl DBT Propyl DBT 3,4-benzo DBT 2,8-DMDBT 4,6-DMDBT DBT 4,6-DEDBT DBT 4-MDBT

2-HBP 2-hydroxydimethylbiphenyl 2-hydroxy-3'-methyl-biphenyl 2-hydroxy-3-methyl-biphenyl Corresponding monohydroxy biphenyls

e

30

42

Yu et al., 2006

nhexadecane

30

120

n-tridecane

50

8

Mycobacterium sp. MR65

Growing cells

4,6-DMDBT BNT BNT 10-methyl BNT

Mycobacterium phlei WU-F1

Resting cells

2-ethylNTH

Reaction time (h)

Reference

Rashidi et al., 2006

Watanabe et al., 2003

Furuya et al., 2002

2-hydroxydimethylbiphenyl (2-HDMBP), 2-hydroxy-3'-methyl-biphenyl (2-HMBP), 10-methylbenzo[b]naphtho[2,1-d]thiophene (10-methyl BNT).

compounds at a suitable rate; (iv) separation of desulfurized oil fraction, recovery of the biocatalyst and its return to the bioreactor; and (v) efficient wastewater treatment (Mohebali and Ball, 2008). Each step is affected by a number of factors. Reactor configuration, processing and engineering studies have important roles to play in the development and commercialization of BDS technology. 20.1. Biocatalyst production The 4S pathway is a complex enzyme system, and its cofactors requirement prohibit the use of purified enzyme systems rather than whole cells for a practical BDS process (Kilbane and Le Borgne, 2004). Therefore, resting cells are regarded as the best biocatalysts. Preparation of desulfurizing resting cells consists of the following stages: growing the selected strain in a suitable medium in such a way as to obtain cells that exhibit the highest possible level of Dsz activity, harvesting these active cells and using them as the biocatalysts (Monot et al., 2002). To make the BDS process economically competitive with the deep HDS process, it is necessary to improve several factors, including the production cost of the biocatalyst and its biocatalytic activity. As mentioned earlier, the Dsz phenotype is repressed by readily bioavailable sulfur compounds, including sulfate because Dsz enzymes are sulfate-starvation-induced proteins (Tanaka et al., 2002b). In this context, sulfate contamination of the growth medium is one of the main barriers to the mass production of desulfurizing resting cells (Tanaka et al., 2002b). Repression can be avoided by replacing sulfate with DBT as the sulfur compound for

growth (Yoshikawa et al., 2002; Kim et al., 2004). Mass production of biocatalyst using DBT has been considered to be commercially impractical because of its high price, low water solubility and growth inhibition by 2-HBP (Yoshikawa et al., 2002; Kilbane and Le Borgne, 2004; Wang et al., 2004; Guobin et al., 2006). Therefore, some researchers have tried to find a suitable sulfur source alternative to DBT. Yoshikawa et al. (2002) investigated the specific desulfurization activity of R. erythropolis KA2-5-1 in a fed-batch culture system containing 2-aminoethanesulfonic acid as the sole source of sulfur. They achieved a specific Dsz activity of 111 mmol 2HBP kg-cell1 h1 and a cell concentration of 20 g l1 after 89 h of cultivation. In another exponential fed-batch culture technique, the same productivity level was achieved using inexpensive ammonium sulfate as the sole source of sulfur (Hirasawa et al., 2001; Konishi et al., 2005). Dsz activity of Mycobacterium sp. G3 has been expressed in medium containing MgSO4 at 0.1 mM, when 217 mM ethanol was used as the sole carbon source (Okada et al., 2001); the cultivation time was shortened compared to that using DBT, and the Dsz activity was higher than those of DBT. Dimethyl sulfoxide (DMSO) has been reported as a sulfur substrate that does not repress desulfurization activity in R. erythropolis IGTS8, and it is not a substrate for the DBT desulfurizing enzymes (Li et al., 1996). Expression of DBT desulfurizing activity in the presence of DMSO is due to the de-repression of the dsz operon in the absence of more readily bioavailable sulfur sources (e.g. sulfide, sulfate, cysteine and methionine) (Li et al., 1996; Oldfield et al., 1997). DMSO has been employed as the sulfur source for growth by several researchers (Matsui et al., 2002; Abbad-Andaloussi et al., 2003a, b; Luo et al., 2003; Bouchez-Naïtali et al., 2004; Del Olmo et al., 2005; Ma et al.,

G. Mohebali, A.S. Ball / International Biodeterioration & Biodegradation 110 (2016) 163e180

173

Table 3 Biodesulfurization of oil middle distillate fractions. Bacterium

Sulfur substrate

Total sulfur (ppm)

Desulfurization Temp. (%) (
C)

Desulfobacterium anilini Growing cells

Diesel

e

82

30

72

Rhodococcus erythropolis FSD-2 Strain CYKS1

Resting cells

HDS-treated diesel

198

~94

30

12

Resting cells

1500 3000 280

70 50 59

30

12

Growing cells

Middle distillate unit feed Light gas oil HDS-treated light gas oil

27

36

Growing cells Resting cells Growing cells

HDS-treated light gas oil (three types with 34e390 different sulfur contents) Crude straight-run light gas oil (12-fold-diluted) 1000

60e70 56e74 52.5

45 45

e e 72

Resting cells

Diesel oil

250

76

30

20

303 1000 535 591

48.5 23.7 86 90.5

30

24

45 30

24 20

Sphingomonas subarctica T7b Mycobacterium phlei WU-F1 Mycobacterium phlei WU-0103 Gordonia nitida CYKS1

Growing/resting state

Rhodococcus sp. P32C1 Resting cells

Reaction time (h)

Mycobacterium sp. X7B Pseudomonas delafieldii R-8 Pseudomonas delfieldii R-8 Pseudomonas delafieldii R-8 Rhodococcus erythropolis XP Nocardia globerula R-9

Resting cells Resting cells

HDS-treated light diesel oil Diesel oil HDS-treated diesel oil HDS-treated diesel oil

Culture suspension

HDS-treated diesel oil

591

47

30

16

Resting cells

Straight-run diesel oil

1807

55.3

30

24

Resting cells

HDS-treated diesel oil

259

94.5

30

24

Resting cells

Straight-run diesel oil

1807

59

Rhodococcus sp. IMPS02 Rhodococcus erythropolis LSSE8-1 Rhodococcus erythropolis I-19 Rhodococcus sp. ECRD-1

Growing cells

HDS-treated diesel oil

500

Up to 60

Resting cells/Culture suspension Resting cells

HDS-treated diesel

248

79.5

30

24

HDS-treated-middle distillate petroleum (149 e428
C) Middle-distillate fraction of Oregon basin crude oil (232e343
C)

1850

66.8

30

24

21,700

8.1

25

168

Growing cells

2006a; Mohebali et al., 2007). The Dsz phenotype in G. alkanivorans RIPI90A was expressed via the 4S pathway in the presence of DMSO as the sulfur source (Mohebali et al., 2008). Resting cells grown on DMSO were more active than resting cells grown on DBT (Mohebali et al., 2007). However, with resting cells of Rhodococcus sp. 1awq grown on DMSO as the sole sulfur source, Dsz activity was shown to be similar to that obtained with DBT (Ma et al., 2006a). In an attempt to induce the Dsz phenotype in cells grown on sulfate as the sole sulfur source, several researchers have reported the use of a two-stage process. In their method, growth and Dsz-phenotypeinduction stages were performed separately: after acquiring a high-cell-density culture at the expense of sulfate/DMSO, the Dsz phenotype was induced in the second stage using DBT (Honda et al., 1998; Chang et al., 2001; Ma et al., 2006a; Mohebali et al., 2008). Alves et al. (2008b) studied the effect of Zn2þ on growth and DBT desulfurization by G. alkanivorans 1B. Resting cells grown in the presence of Zn2þ exhibited a 2-HBP specific productivity 7.6-fold higher than the specific productivity obtained by resting cells grown in the absence of Zn2þ. In this context, Alves et al. (2008a) evaluated the performance of the hydrolyzates obtained by enzymatic saccharification of recycled paper sludge, as nutrient source for low-cost DBT desulfurization by G. alkanivorans 1B. Alves and ~o (2014) found that DBT desulfurization by strain B1 was Piaxa enhanced using sugar beet molasses as carbon source. Martinez et al. (2015) reported that the addition of co-substrates improves the yield and the rate of biodesulfurization of DBT following the 4S pathway by resting cells of Pseudomonas putida CECT5279; acetic acid as the co-substrate increased the intracelullar concentration of NADH and ATP.

168

Reference Aribike et al., 2009 Zhang et al., 2007b Rhee et al., 1998 Gunam et al., 2006 Furuya et al., 2003 Ishii et al., 2005 Chang et al., 2001 Maghsoudi et al., 2001 Li et al., 2003 Guobin et al., 2005b Guobin et al., 2006 Mingfang et al., 2003 Yu et al., 2006 Mingfang et al., 2003 Castorena et al., 2002 Li et al., 2007b Folsom et al., 1999 Grossman et al., 1999

20.2. Bioreactors for BDS studies Two liquideliquid bioreactors, a stirred-tank and a novel electrostatic-dispersion system have been used to investigate biodesulfurization of oil by sulfate reducing bacteria (SRB) (Tsouris et al., 1996). Biodesulfurization of diesel has been studied in airlift bioreactors (Nandi, 2010; Irani et al., 2011). Amin (2011) used an integrated two-stage process for biodesulfurization of a model oil using a vertical rotating immobilized cell reactor with the bacterium R. erythropolis. Biodesulfurization of DBT and gas oil in a bioreactor packed with a catalytic bed of silica containing immobilized Rhodococcus rhodochrous was also studied (Alejandro Dinamarca et al., 2014a). An aqueous-organic two-layer partitioning continuous process has been designed to make efficient use of growing cells of Rhodococcus globerulus DAQ3 for the biodesulfurization of diesel oils (Yang et al., 2007). In this process the organic phase and the cellcontaining aqueous phase were kept as two layers and the cells were grown under steady-state conditions in the aqueous phase. The organosulfur compounds diffused from the organic layer into the aqueous phase and sulfur was removed by the cells, while water-soluble inhibitory compounds from cell metabolism or cell lysis in the aqueous phase were washed out of the bioreactor (Yang and Marison, 2005). This system was shown to have a significant advantage over batch and fed-batch processes in the maintenance of the BDS activity for an extended period of time. Microchannel reactor systems generate a higher yield of products more quickly than bulk reaction systems. A microchannel reactor system has been used in a BDS process in which the Dsz rate

174

G. Mohebali, A.S. Ball / International Biodeterioration & Biodegradation 110 (2016) 163e180

in the oil/water phase was more than nine-fold that in a batch (control) reaction (Noda et al., 2008); in addition, this system using a bacterial cell suspension degraded an alkylated DBT that was not degraded by the control system. Biodesulfurization of model oil (DBT-containing n-dodecane) and HDS-treated diesel has been carried out using Pseudomonas delafieldii R-8 in a ceramic microsparging aeration system (Lin et al., 2012). Microbubble aeration gave an activity of 1.3 mg DBT removed g1 h1 and 277 mg sulfur g1 h1 for model oil and hydrodesulfurized diesel, respectively. These values were 1.9- and 1.6-times higher than using a traditional bubble aeration process. 20.3. Separation of biocatalyst from reaction mixture Post-reaction separation of cells is another critical step in the BDS process. With hydrophobic biocatalysts, the separation of cells from the organic phase is a problem; with increasing catalyst concentrations, particle-stabilized emulsions are formed (Pacheco, 1999; Mohebali et al., 2007). It has been suggested that the formation of a stable water/oil emulsion could be avoided in order to facilitate oil recovery (Ayala and Vazquez-Duhalt, 2004). A BDS process using immobilized cells has been also described (Naito et al., 2001; Lee et al., 2005); using this method it is possible to devise a two-phase system (immobilized cells and oil) to desulfurize oil without any particle-stabilized emulsion formation. Although the free dispersal of microorganisms in a fluid reaction volume optimizes mass transport, it is sometimes difficult to carry out the necessary separation afterwards, and usually the separation procedure compromises viability. Immobilization of enzymes and whole cells as biocatalysts provide advantages such as enhanced stability, repeated or continuous use, easy separation from the reaction mixture and possible modulation of catalytic properties. For BDS, several separation schemes can be evaluated, including settling tanks, hydrocyclones and centrifuges. However, these procedures are time-consuming and costly. In order to overcome the problems immobilized whole cells can be used in the BDS process (Guobin et al., 2005b; Hou et al., 2005; Lee et al., 2005; Li et al., 2008d; Derikvand and Etemadifar, 2014). Hou et al. (2005) reported that the stability and life-time of immobilized cells were much better than those of the non-immobilized cells. Chang et al. (2000) immobilized DBT-desulfurizing strains, Gordonia sp. CYKS1 and Norcardia sp. CYKS2 in Celite for the desulfurization of light oil. Naito et al. (2001) entrapped R. erythropolis KA2-5-1 cells in a photocrosslinkable pre-polymer (ENT4000); the immobilized cells could catalyze biodesulfurization repeatedly in the model oil system for more than 900 h with reactivation; recovery of both the biocatalyst and the desulfurized model oil was simple. Li et al. (2008d) studied the immobilization of P. delafieldii R-8 in calcium alginate beads in order to improve its Dsz activity in oil/water biphasic systems. Alejandro Dinamarca et al. (2014b) immobilized IGTS8 cells on Silica, Alumina and Sepiolite by adsoption. The immobilization of Pseudomonas stutzeri TCE3 using adsorption on different inorganic supports has been studied in relation to the number of adsorbed cells, metabolic activity and biodesulfurization (Alejandro Dinamarca et al., 2010). Efforts have been also made to immobilize bacteria, typically in the form of biofilms coating reaction vessels with high surface to volume ratios and thus no separation is subsequently required (Ansari et al., 2009). In order to combine the advantages of immobilization, ease of separation and microbial longevity it is possible to decorate the bacterial cells with magnetic nanoparticles (Shan et al., 2005; Xu et al., 2011). Li et al. (2009b) have developed a simple and effective technique by integrating the advantages of magnetic

separation and cell immobilization for BDS process with R. erythropolis LSSE8-1, with hydrophobic nature, using superparamagnetic Fe3O4 nanoparticles. These nanoparticles were modified with ammonium oleate, to produce hydrophilic magnetic fluids. The nanoparticles were strongly absorbed to the surface of and coated the cells. Compared to free cells, the coated cells not only had the same Dsz activity but could also be easily separated from fermentation broth by magnetic force. Bardania et al. (2013) confirmed that magnetic nanoparticles (Fe3O4) do not affect the Dsz activity of nanoparticle-coated bacterial cells. Biodesulfurization of DBT has been carried out by strain IGST8 cells decorated with magnetic Fe3O4 nanoparticles in order to facilitate the post-reaction separation of the cells (Ansari et al., 2009); the decorated cells had a 56% higher DBT desulfurization activity compared to the nondecorated cells. Model experiments with black lipid membranes demonstrated that the nanoparticles indeed enhance membrane permeability facilitating the entry and exit of reactant and product, respectively. P. delafieldii has been immobilized in magnetic polyvinyl alcohol beads (Shan et al., 2003). The main advantage was that the magnetic immobilized cells maintain a high Dsz activity. 20.4. Mass transfer studies 20.4.1. Oxygen mass transfer and uptake rates All the microorganisms employed for BDS are strictly aerobic, as oxygen is essential for cell growth and Dsz activity. In many cultures, dissolved oxygen concentration becomes the limiting factor and therefore, oxygen mass transfer from the gas to the liquid phase is important not only for growth but also for the BDS capability of the cells. Oxygen uptake rate during growth of R. erythropolis IGTS8 has been measured using two experimental methods (dynamic and process methods) for the same set of experiments performed in a commercial bioreactor (Santos et al., 2006b). The experimental values obtained by both methods are modeled to describe oxygen consumption. Oxygen-uptake and oxygen mass-transfer rates in cultures of P. putida CECT5279 capable of desulfurizing DBT and its derivatives have been determined in a stirred tank bioreactor under different transport conditions (Gomez et al., 2006); influences of mass-transfer conditions have been observed in growth and Dsz capability of the cells. 20.4.2. Sulfur substrate mass transfer Mass transfer of substrate is one of the key factors which impact BDS rates (Kobayashi et al., 2001). Setti et al. (2003) showed that the transfer rate of DBT from oil phase to water and then to Pseudomonas cells, does impact BDS rates. In oil-water polyphasic systems access to organic sulfur by biocatalysts requires the costly dispersal of the oil fraction in the aqueous phase. It has been reported that in BDS bioreactors at higher cell densities, the created emulsion is composed of smaller droplets (Pacheco, 1999). In some bioreactors, such as mechanically mixed reactors and electro-spray reactors, the emulsions are created by physical means. Under these conditions, emulsion stabilization can help to prolong the longevity of the created emulsion. In polyphasic systems, certain bacterial cells are also able to stabilize oil/water emulsions without changing the interfacial tension, by inhibition of droplet coalescence (Dorobantu et al., 2004; Mohebali et al., 2007); these bacterial cells are often hydrophobic by nature. Mohebali et al. (2007) reported that G. alkanivorans RIPI90A could stabilize water/gas oil emulsions efficiently by adhering to the oilewater interface and by inhibition of droplet coalescence, without

G. Mohebali, A.S. Ball / International Biodeterioration & Biodegradation 110 (2016) 163e180

decreasing the surface tension of their environment. In these conditions a stable and long-lasting creamy emulsion can be formed; the strain can sequester organic sulfur substrates directly from the hydrophobic phase with minimum mass transfer limitation. Owing to the high hydrophobicity of strain IGTS8, it has been suggested that the uptake of the DBT takes place on the interface surface (Monticello, 2000; Le Borgne and Quintero, 2003). Other reports suggest that high mass transfer limitations occur when this surface is saturated by adhered biocatalyst cells (Maghsoudi et al., 2001). Using desulfurizing resting cells of strain IGTS8 and P. putida CECT5279 with hexadecane as model oil, it has been proven that in aqueous and biphasic media strain CECT5279 was more sensitive for DBT mass transfer limitation (Caro et al., 2007). Caro et al. (2008b) found that mass transfer limitations decreased the yields obtained with strain CECT5279 in biphasic media. Moreover, mass transfer limitations were less important with IGTS8 than with CECT5279, as IGTS8 takes up compounds dissolved in oil. The work confirms that Rhodococci hydrophobicity in the presence of organic solvents (Abbad-Andaloussi et al., 2003a; Caro et al., 2007), enhances its capacity to uptake compounds dissolved in this phase. It has also been proven that when DBT was added as a sulfur source, in the presence of the oil phase (hexadecane), negative effects on both growth of strain CECT5279 and BDS yields were observed (Caro et al., 2008a). The Haldane model has been used to simulate the BDS experimental results obtained by growing cells under biphasic conditions. 21. The coupling of BDS and other desulfurization techniques Desulfurization by adsorption is an alternative method to remove sulfur compounds with modified metal oxides, molecular sieves and activated carbon as adsorbents under ambient conditions (Li et al., 2008c). Solvent extraction and oxidation in the air ndez-Maldonado are used in the regeneration of adsorbents (Herna et al., 2004). However, these two methods have some disadvantages. The coupling of adsorption desulfurization and biodesulfurization is a new approach to produce clean fuels. Sulfur compounds are firstly adsorbed on adsorbents which are bioregenerated. Bioregeneration of the used adsobents have been studied using DBT as the model compound with P. delafieldii R-8 (Li et al., 2005a). Li et al. (2006b) studied the use of a combined physical-biological procedure for desulfurization of fuels. During this process, sulfur compounds were removed through adsorption and then biodegraded in a subsequent stage by strain R-8, with the regeneration of the adsorbent. The particle size of cells was similar to that of desulfurization adsorbents. Therefore, in order to separate regenerated adsorbents and cells, Fe3O4 nanoparticles-modified strain R-8 cells was used (Li et al., 2008c). Aiming for the deep desulfurization of hydrotreated diesel, a novel adsorptionbioregeneration system has been constructed by combining adsorption and BDS processes (Li et al., 2009c). The improved regeneration process of several adsorbents was carried out by adding strain R-8 cells. Agarwal and Sharma (2010) investigated the use of BDS, reactive adsorption, oxy-desulfurization, photo-desulfurization and solvent extraction for the removal of sulfur from heavy crude oil (HCO) and light crude oil (LCO). Biodesulfurization under anaerobic conditions followed by oxy-desulfurization and reactive adsorption integration resulted in maximum removal, 95.21% of sulfur from HCO and 94.30% from LCO. 22. Improvement of BDS performance using nanotechnology Nanotechnology has attracted great interest in recent years due

175

to its impact on many scientific areas. Looking at the nanoscale has stimulated the development and use of novel and cost-effective technologies for catalytic degradation (Zhao et al., 2011). The present progress of nanobiocatalysis suggests that nanobiocatalytic approaches offer significant potential for the future (Kim et al., 2008). Guobin et al. (2005a) assembled g-Al2O3 nanoparticles onto biodesulfurizing cells of Pseudomonas delafieldii R-8 to increase the transfer rate of sulfur compounds from model diesel oil to the microbial cell, and thus to improve BDS rates. These nanosorbents had the ability to adsorb DBT from oil phase and the rate of adsorption was far higher than that of biodesulfurization. Thus, the nanosorbents increased DBT transfer rate from the organic phase to the biocatalyst surface resulting in an increase in BDS rates, approximately two-fold higher than that of original cells. Zhang et al. (2008b) applied widely used adsorbents including g-Al2O3, which had the ability to selectively adsorb DBT from the organic phase. The results showed that g-Al2O3 can adsorb DBT from the oil phase quickly and transfer it to microbial cells and thus increase the Dsz rate. The application of g-Al2O3 nanoparticles is however limited due to their aggregation during and after synthesis. Thus, the preparation of well-dispersed and biocompatible g-Al2O3 nanoparticles is a key for their biological applications. These nanoparticles have been synthesized and modified using gum arabic to avoid agglomeration in aqueous solutions (Zhang et al., 2007a). The adsorption desulfurization capacity of modified g-A12O3 used in the experiment was 1.12 times that of unmodified g-A12O3. P. delafieldii strain R-8 cells have been coated with surfacemodified Fe3O4 nanoparticles (Shan et al., 2005). The nanoparticles were synthesized and then modified with ammonium oleate. The surface-modified nanoparticles were monodispersed in an aqueous solution and did not precipitate in over 18 months. The coated superparamagnetic cells not only had the same Dsz activity as free cells but could also be reused more than five times. 23. Future perspectives 23.1. Refinery challenges With the new USEPA Tier II regulations, refineries are facing major challenges to meet the fuel sulfur specification along with the required reduction of aromatics (Song and Ma, 2007). Although the new environmental regulations limiting the sulfur levels of diesel and other transportation fuels are beneficial from an environmental point of view, meeting the required stringent specifications represent a major operational and economic challenge for the petroleum refining industry. The problem faced by the refiners is the difficulty in meeting the increasing market demand for ultra-low sulfur diesel. At the same time, the quality of crude and diesel feed streams available to the refiners is declining. The refiners are, thus, required to produce a high quality diesel product such as ultra-low sulfur diesel from lower quality feedstocks, which is a tough challenge (Stanislaus et al., 2010). 23.2. Research needs Significant research has been made during the last two decades in the application of the BDS process for diesel desulfurization. It is evident from the results reported that current diesel biodesulfurization is not a very effective process to meet the ultra-low sulfur levels (15 ppm) as required by new diesel fuel specifications. The development of bioprocesses for upgrading large volumes of fossil fuels is one of the greatest challenges facing biotechnology. However all the studies outlined in various reviews

176

G. Mohebali, A.S. Ball / International Biodeterioration & Biodegradation 110 (2016) 163e180

represent significant steps to realise the biotechnological potential for developing an efficient BDS process. An industrial-scale process for petroleum biodesulfurization using aerobic bacteria has not yet been demonstrated. We still need a much better understanding of more aspects of the Dsz process in order to develop a commercial process. However, the commercialization of the BDS process does not seem to be realistic in the near future because of the low Dsz rate of known microorganisms. The currently available biocatalysts require an activity increase in Dsz rate of about 500-fold. Hence, future development will depend on either genetically modifying the currently available bacteria or identifying novel biodesulfurizers (Bhatia and Sharma, 2010b). Some key research needs for improving biocatalysts for an efficient and commercial BDS process for petroleum and its fractions are the design of engineered cells with (1) higher specific Dsz activity, (2) broader substrate range, (3) higher substrate affinity in biphasic reaction systems containing toxic solvents, (4) activity for a long period of time and (5) higher thermal tolerance (Díaz and García, 2010; Abin-Fuentes et al., 2013). The availability of genome sequences of desulfurizing biocatalysts will make it possible to regulate the metabolism through engineering (Ma, 2010). Issues regarding gene regulation are still almost unknown. Host cell contributions play also a pivotal role in achieving the higher activities. The water/oil volume ratio is among the most important technical bottlenecks in the development of petroleum biotechnological processes, and therefore in order to reduce the operational costs associated with water handling, separation and disposal, ideally the water/oil volume ratio should be minimized (Foght, 2004). Another limiting factor in the BDS process is the transport rate of the sulfur compounds from the oil phase to the bacterial cell membrane. Efficient separation of desulfurized oil fraction, recovery of the biocatalyst and its return to the bioreactor are also important issues which need attention. It has been suggested that more work is needed in the above areas to make a BDS process competitive with catalytic deep HDS processes currently used in the refineries (Kilbane, 2006; Mohebali and Ball, 2008). The most attractive option at present for the industrial application of the BDS process in deep desulfurization of diesel is to integrate it with existing HDS units in the refineries. The operations costs will be reduced and the process economics will be improved when BDS and catalytic HDS are integrated. References Abbad-Andaloussi, S., Lagnel, C., Warzywoda, M., Monot, F., 2003a. Multicriteria comparison of resting cell activities of bacterial strains selected for biodesulfurization of petroleum compounds. Enz Microbiol. Technol. 32 (3e4), 446e454. Abbad-Andaloussi, S., Warzywoda, M., Monot, F., 2003b. Microbial desulfurization of diesel oils by selected bacterial strains. Oil Gas. Sci. Technol. 58, 505e513. Abin-Fuentes, A., El-Said Mohamed, M.E., Wang, D.I.C., Prather, K.L.J., 2013. Exploring the mechanism of biocatalyst inhibition in microbial desulfurization. Appl. Environ. Microbiol. 79 (24), 7807e7817. Abo-State, M.A., El-Gendy, N.S., El-Temtamy, S.A., Mahdy, H.M., Nassar, H.N., 2014. Modification of basal salts medium for enhancing dibenzothiophene biodesulfurization by Brevibacillus invocatus C19 and Rhodococcus erythropolis IGTS8. World Appl. Sci. J. 30 (2), 133e140. Acero, J., Berdugo, C., Mogollon, L., 2003. Biodesulfurization process evaluation with a Gordona rubropertinctus strain. Cieneia Tecnol. YFuturo 2 (4), 43e54. Agarwal, P., Sharma, D.K., 2010. Comparative studies on the biodesulfurization of crude oil with other desulfurization techniques and deep desulfurization through integrated processes. Energy Fuels 24 (1), 518e524. Akbarzadeh, S., Raheb, J., Aghaei, A., Karkhane, A.A., 2003. Study of desulfurization rate in Rhodococcus FMF native bacterium. Iran. J. Biotechnol. 1, 36e40. Alejandro Dinamarca, M., Ibacache-Quiroga, C., Baeza, P., Galvez, S., Villarroel, M., Olivero, P., Ojeda, J., 2010. Biodesulfurization of gas oil using inorganic supports biomodified with metabolically active cells immobilized by adsorption. Bioresour. Technol. 101, 2375e2378. Alejandro Dinamarca, M., Orellana, L., Aguirre, J., Baeza, P., Espinoza, G., Canales, C., Ojeda, J., 2014a. Biodesulfurization of dibenzothiophene and gas oil using a

bioreactor containing a catalytic bed with Rhodococcus rhodochrous immobilized on silica. Biotechnol. Lett. 36, 1649e1652. Alejandro Dinamarca, M., Rojas, A., Baeza, P., Espinoza, G., Ibacache-Quiroga, C., Ojeda, J., 2014b. Optimizing the biodesulfurization of gas oil by adding surfactants to immobilized cell systems. Fuel 116, 237e241. Alves, L., Matos, J., Tenreiro, R., Gírio, F.M., 2008b. Evidence for the role of zinc on the performance of dibenzothiophene desulfurization by Gordonia alkanivorans strain 1B. J. Ind. Microbiol. Biotechnol. 35, 69e73. ~es, F., Matos, J., Tenreiro, R., Gírio, F.M., 2007. Alves, L., Melo, M., Mendonça, D., Simo Sequencing, cloning and expression of the dsz genes required for dibenzothiophene sulfone desulfurization from Gordonia alkanivorans 1B. Enz Microb. Technol. 40 (6), 1598e1603. Alves, L., Marques, S., Matos, J., Tenreiro, R., Gírio, F.M., 2008a. Dibenzothiophene desulfurization by Gordonia alkanivorans strain 1B using recycled paper sludge hydrolyzate. Chemosphere 70, 967e973. ~o, S.M., 2011. .Toxicity evaluation of 2-hydroxybiphenyl and other Alves, L., Paixa compounds involved in studies of fossil fuels biodesulphurisation. Bioresour. Technol. 102 (19), 9162e9166. Alves, L., Paix~ ao, S.M., 2014. Enhancement of dibenzothiophene desulfurization by Gordonia alkanivorans strain 1B using sugar beet molasses as alternative carbon source. Appl. Biochem. Biotechnol. 172, 3297e3305. Alves, L., Salgueiro, R., Rodrigues, C., Mesquita, E., Matos, J., Gírio, F.M., 2005. Desulfurization of dibenzothiophene, benzothiophene, and other thiophene analogs by a newly isolated bacterium, Godonia alkanivorans strain 1B. Appl. Biochem. Biotechnol. 120, 199e208. Amin, G.A., 2011. Integrated two-stage process for biodesulfurization of model oil by vertical rotating immobilized cell reactor with the bacterium rhodococcus erythropolis. J. Pet. Environ. Biotechnol. 2 (1), 100e107. Ansari, F., Grigoriev, P., Libor, S., Tothill, I.E., Ramsden, J.J., 2009. DBT degradation enhancement by decorating Rhodococcus erythropolis IGST8 with magnetic Fe3O4 nanoparticles. Biotechnol. Bioeng. 102 (5), 1505e1512. Ansari, F., Prayuenyong, P., Tothill, I., 2007. Biodesulfurization of dibenzothiophene by Shewanella putrefaciens NCIMB 8768. J Biol. Phys. Chem. 7 (2), 75e78. Arensdorf, J.J., Loomis, A.K., DiGrazia, P.M., Monticello, D.J., Pienkos, P.T., 2002. .Chemostst approach for the directed evolution of biodesulfurization gain-offunction mutants. Appl. Environ. Microbiol. 68, 691e698. Aribike, D.S., Susu, A.A., Nwachukwu, S.C.U., Kareem, S.A., 2009. Microbial desulfurization of diesel by Desulfobacterium aniline. Acad. Arena 1 (4), 11e14. Armstrong, S.M., Sankey, B.M., Voordouw, G., 1995. Conversion of dibenzothiophene to biphenyl by sulfate-reducing bacteria isolated from oil field production facilities. Biotechnol. Lett. 17, 1133e1137. Armstrong, S.M., Sankey, B.M., Voordouw, G., 1997. .Evaluation of sulfate-reducing bacteria for desulfurizing bitumen or its fractions. Fuel 76, 223e227. Ayala, M., Tinoco, R., Hernandez, V., Bremauntz, P., Vazquez-Duhalt, R., 1998. Biocatalytic oxidation of fuel as an alternative to biodesulfurization. Fuel Process Technol. 57, 101e111. Ayala, M., Vazquez-Duhalt, R., 2004. Enzymztic catalysis on petroleum products. In: Vazquez-Duhalt, R., Quintero-Ramirez, R. (Eds.), Petroleum Biotechnology, Developments and Prospectives. Elsevier, The Netherland, pp. 67e112. Babich, I.V., Moulijn, J.A., 2003. Science and technology of novel processes for deep desulfurization of oil refinery streams: a review. Fuel 82, 607e631. Bachmann, R.T., Johnson, A.C., Edyvean, R.G.J., 2014. Biotechnology in the petroleum industry: an overview. Inter. Biodeter. Biodeg. 86, 225e237. Bahrami, A., Shojaosadati, S.A., Mohebali, G., 2001. Biodegradation of dibenzothiophene by thermophilic bacteria. Biotechnol. Lett. 23, 899e901. Bahuguna, A., Lily, M.K., Munjal, A., Singh, R.N., Dangwal, K., 2011. Desulfurization of dibenzothiophene (DBT) by a novel strain Lysinibacillus sphaericus DMT-7 isolated from diesel contaminated soil. J. Environ. Sci. 23 (6), 975e982. Bardania, H., Raheb, J., Mohammad-Beigi, H., Rasekh, B., Arpanaei, A., 2013. Desulfurization activity and reusability of magnetite nanoparticle-coated Rhodococcus erythropolis FMF and R. erythropolis IGTS8 bacterial cells. Biotechnol. Appl. Biochem. 60 (3), 323e329. Bhatia, S., Sharma, D.K., 2010a. Biodesulfurization of dibenzothiophene, its alkylated derivatives and crude oil by a newly isolated strain Pantoea agglomerans D23W3. Biochem. Eng. J. 50 (3), 104e109. Bhatia, S., Sharma, D.K., 2010b. Mining of genomic databases to identify novel biodesulfurizing microorganisms. J. Ind. Microbiol. Biotechnol. 37, 425e429. Blumberg, K.O., Walsh, M.P., Pera, C., 2003. Low-sulfur gasoline and diesel: the key to lower vehicle emission. In: Prepared for the May 2003 Meeting in Napa, California, of the International Council on Clean Transportation (ICCT). www. cleantransportcouncil.org/docs/. Boniek, D., Figueiredo, D., dos Santos, A.F.B., de Resende Stoianoff, M.A., 2015. Biodesulfurization: a mini review about the immediate search for the future technology. Clean. Techn. Environ. Policy 17 (1), 29e37. Bouchez-Naïtali, M., Abbad-Andaloussi, S., Warzywoda, M., Monot, F., 2004. Relation between bacterial strain resistance to solvents and biodesulfurization activity in organic media. Appl. Microbiol. Biotechnol. 65, 440e445. Bressler, D.C., Fedorak, P.M., 2000. Bacterial metabolism of fluorene, dibenzofuran, dibenzothiophene, and carbazole. Can. J. Microbiol. 46, 397e409. rot, G., Breysse, M., Djega-Mariadassou, G., Pessayre, S., Geantet, C., Vrinat, M., Pe Lemaire, M., 2003. Deep desulfurization: reactions, catalysts and technological challenges. Catal. Today 84 (3e4), 129e138. Campos-Martin, J.M., Capel-Sanchez, M.C., Perez-Presas, P., Fierro, J.L.G., 2011. Oxidative processes of desulfurization of liquid fuels. J. Chem. Technol. Biotechnol. 85 (7), 879e890.

G. Mohebali, A.S. Ball / International Biodeterioration & Biodegradation 110 (2016) 163e180 n, P., García-Calvo, E., 2008a. Biodesulfurization of dibenCaro, A., Boltes, K., Leto zothiophene by growing cells of Pseudomonas putida CECT 5279 in biphasic media. Chemosphere 73, 663e669. n, P., Garcia-Calvo, E., 2007. Dibenzothiophene bioCaro, A., Boltes, K., Leto desulfurization in resting cell conditions by aerobic bacteria. Biochem. Eng. J. 35 (2), 191e197.  n, P., García-Calvo, E., 2008b. Description of by-product inCaro, A., Boltes, K., Leto hibition effects on biodesulfurization of dibenzothiophene in biphasic media. Biodegradation 19 (4), 599e611. rez, C., Valdez, I., Amador, G., Ferna ndez, L., Le Borgne, S., 2002. Castorena, G., Sua Sulfur-selective desulfurization of dibenzothiophene and diesel oil by newly isolated Rhodococcus sp. strains. FEMS Microbiol. Lett. 215, 157e161. Chang, J.H., Chang, Y.K., Ryu, H.W., Chang, H.N., 2000. Desulfurization of light gas oil in immobilized-cell systems of Gordona sp. CYKS1 and Nocardia sp. CYKS2. FEMS Microbiol. Lett. 182, 309e312. Chang, J.H., Kim, Y.J., Lee, B.H., Cho, K.-S., Rye, H.W., Chang, Y.K., Chang, H.N., 2001. Production of a desulfurization biocatalyst by two-stage fermentation and its application for the treatment of model and diesel oils. Biotechnol. Progr. 17, 876e880. Chen, H., Cai, Y., Zhang, W., Li, W., 2009. Methoxylation pathway in biodesulfurization of model organosulfur compounds with Mycobacterium sp. Bioresour. Technol. 100, 2085e2087. Chen, H., Zhang, W., Chen, J., Cai, Y., Li, W., 2008b. Desulfurization of various organic sulfur compounds and the mixture of DBT þ 4,6-DMDBT by Mycobacterium sp. ZD-19. Bioresour. Technol. 99 (9), 3630e3634. Chen, H., Zhang, W., Cai, Y., Zhang, Y., Li, W., 2008a. Elucidation of 2hydroxybiphenyl effect on dibenzothiophene desulfurization by Microbacterium sp. strain ZD-M2. Bioresour. Technol. 99 (15), 6928e6933. Constanti, M., Giralt, J., Bordons, A., 1994. Desulfurization of dibenzothiophene by bacteria. World J. Microbiol. Biotechnol. 10, 510e516. Darzins, A., Mrachko, G.T. (1998). Sphingomonas biodesulfurization catalyst. Patent No. WO9845446A1. Darzins, A., Mrachko, G.T. (2000). Sphingomonas biodesulfurization catalyst. U.S. Patent No. 6,133,016. Davoodi-Dehaghani, F., Vosoughi, M., Ziaee, A.A., 2010. Biodesulfurization of dibenzothiophene by a newly isolated Rhodococcus erythropolis strain. Bioresour. Technol. 101, 1102e1105. De Araújo, H.W.C., de Freitas Siva, M.C., Lins, C.I.M., do Nascimento, A.E., da Silva, C.A.A., Campos-Takaki, G.M., 2012. Oxidation of dibenzothiophene (DBT) by Serratia marcescens UCP 1549 formed biphenyl as final product. Biotechnol. Biofuels 5, 33. Debabov, V.G., 2010. Microbial desulfurization of motor fuel. Appl. Biochem. Microbiol. 46 (8), 733e738. Del Olmo, C.H., Santos, V.E., Alcon, A., Garcia-Ochoa, F., 2005. Production of a Rhodococcus erythropolis IGTS8 biocatalyst for DBT biodesulfurization: influence of operational conditions. Biochem. Eng. J. 22, 229e237. Denis-Larose, C., Bergeron, H., Labbe, D., Greer, C.W., Hawari, J., Grossman, M.J., Sankey, B.M., Lau, P.C.K., 1998. Characterization of the basic replicon of Rhodococcus plasmid pSOX and development of a Rhodococcus e Escherichia coli shuttle vector. Appl. Environ. Microbiol. 64, 4363e4367. Denis-Larose, C., Labbe, D., Nergron, H., Jones, A.M., Greer, C.W., Al-Hawari, J., Grossman, M.J., Sankey, B.M., Lau, P.C.K., 1997. Conservation of plasmid-encoded dibenzothiophene desulfurization genes in several Rhodococci. Appl. Environ. Microbiol. 63, 2915e2919. Denome, S.A., Oldfield, C., Nash, L.J., Young, K.D., 1994. Characterization of the desulfurization genes from Rhodococcus sp. strain IGTS8. J. Bacteriol. 176, 6707e6716. Denome, S.A., Young, K.D., 1995. Identification and activity of two insertion sequences elements in Rhodococcus sp. strain IGTS8. Gene 161, 33e38. Derikvand, P., Etemadifar, Z., 2014. Improvement of biodesulfurization rate of alginate immobilized Rhodococcus erythropolis R1. Jundishapur J. Microbiol. 7 (3), e9123. Díaz, E., García, J.L., 2010. Genetics engineering for removal of sulfur and nitrogen from fuel heterocycles. In: Timmis, K.N. (Ed.), Handbook of Hydrocarbon and Lipid Microbiology. Springer-Verlag, Berlin Heidelberg, pp. 2788e2801. Dorobantu, L.S., Yeung, A.K.C., Foght, J.M., Gray, M.R., 2004. Stabilization of oil-water emulsion by hydrophobic bacteria. Appl. Environ. Microbiol. 70, 6333e6336. Duarte, G.F., Rosado, A.S., Seldin, L., de Araujo, W., van Elsas, J.D., 2001. Analysis of bacterial community structure in sulfurous-oil-containing soils and detection of species carrying dibenzothiophene desulfurization (dsz) genes. Appl. Environ. Microbiol. 67, 1052e1062. Eibes, G., Cajthaml, T., Moreira, M.T., Feijo, G., Lema, J., 2006. Enzymatic degradation of anthracene, dibenzothiophene and pyrene by manganese peroxidase in media containing acetone. Chemosphere 64, 408e414. Etemadifar, Z., Cappello, S., Zarkesh-Esfahani, S.H., 2014. Characteristics of dibenzothiophene desulfurization by Rhodococcus erythropolis R1 and its Dsz-negative mutant. Biol. J. Microorg. 2 (8), 1e14. Foght, J.M., 2004. Whole-cell bio-processing of aromatic compounds in crude oil and fuels. p. 145e176. In: Vazquez-Duhalt, R., Quinterao-Ramirez, R. (Eds.), Petroleum Biotechnology, Developments and Perspectives. Elsevier, The Netherlands. Folsom, B., Schieche, D., Di Gracia, P., Wener, J., Palmer, S., 1999. Microbial desulphurization of alkyl-dibenzothiophenes from a hydrodesulfurized middle distillate by Rhodococcus erythropolis I-19. Appl. Environ. Microbiol. 65, 4967e4972.

177

Frassinetti, S., Setti, L., Corti, A., Farrinelli, P., Montevecchi, P., Vallini, G., 1998. Biodegradation of dibenzothiophene by a noduling isolate of Rhizobiom meliloti. Can. J. Microbiol. 44, 289e297. Furuya, T., Ishii, Y., Noda, K., Kino, K., Kirimura, K., 2003. Thermophilic biodesulfurization of hydrodesulfurized light gas oils by Mycobacterium phlei WUF1. FEMS Microbiol. Lett. 221 (1), 137e142. Furuya, T., Kirimura, K., Kino, K., Usami, S., 2001. Thermophilic biodesulfurization of dibenzothiophene and its derivatives by Mycobacterium phlei WU-F1. FEMS Microbiol. Lett. 204, 129e133. Furuya, T., Kirimura, K., Kino, K., Usami, S., 2002. Thermophilic biodesulfurization of naphthothiophene and 2-ethylnaphthothiophene by a dibenzothiophenedesulfurizing bacterium, Mycobacterium phlei WU-F1. Appl. Microbiol. Biotechnol. 58, 237e240. Gallager, J.R., Olson, E.S., Stanly, D.C., 1993. Microbial desulfurization of dibenzothiophene: a sulfur-specific pathway. FEMS Microbiol. Lett. 107, 31e36. Gallardo, M.E., Ferr andez, A., De Lorenzo, V., García, J.L., Díaz, E., 1997. Designing recombinant Pseudomonas strains to enhance biodesulfurization. J. Bacteriol. 179, 7156e7160. Gilbert, S.C., Morton, J., Buchanan, S., Oldfield, C., McRoberts, A., 1998. Isolation of a unique benzothiophene desulphurising bacterium: Gordona sp. strain 213E (NCIMB 40816) and characterization of the desulphurisation pathway. Microbiology 144, 2545e2553. Gomez, E., Santos, V.E., Alcon, A., Martín, A.B., García-Ochoa, F., 2006. Oxygen-uptake and mass-transfer rates on the growth of Pseudomonas putida CECT5279: influence on biodesulfurization (BDS) capability. Energy Fuels 20 (4), 1565e1571. Gou, Z.X., Liu, H.Z., Luo, M.F., Li, S., Xing, J.M., Chen, J.Y., 2002. Isolation and identification of nondestructive desulfurization bacterium. Sci. China Ser. B 45, 521e531. Gray, K.A., Mrachko, G.T., Squires, C.H., 2003. Biodesulfurization of fossil fuels. Curr. Opin. Microbiol. 6 (3), 229e235. Gray, K.A., Pogrebinsky, O.S., Mrachko, G.T., Xi, L., Monticello, D.J., Squires, C.H., 1996. Molecular mechanisms of biocatalytic desulfurization of fossil fuels. Nat. Biotech. 16, 1705e1709. Gray, K.A., Squires, C.H., Monticello, D.J. (1998). DszD utilization in desulfurization of DBT by Rhodococcus sp. IGTS8. U.S. Patent No. 5,846,813. Grossman, M.J., Lee, M.K., Prince, R.C., Garrett, K.K., George, G.N., Pickering, I.J., 1999. Microbial desulfurization of a crude oil middle-distillate fraction: analysis of the extent of sulfur removal and the effect of removal on remaining sulfur. Appl. Environ. Microbiol. 65, 181e188. Grossman, M.J., Lee, M.K., Prince, R.C., Minak-Bernero, V., George, G.N., Pickering, I.J., 2001. Deep desulfurization of extensively hydrodesulfurized middle distillate oil by Rhodococcus sp. strain ECRD-1. Appl. Environ. Microbiol. 67 (4), 1949e1952. Guchhait, S., Biswas, D., Bhattacharya, P., Chowdhury, R., 2005a. Biodesulfurization of model organo-sulfur compounds and hydrotreated diesel-experiments and modeling. Chem. Eng. J. 112, 145e151. Guchhait, S., Biswas, D., Bhattacharya, P., Chowdhury, R., 2005b. Biphasic bioconversion of sulfur present in model organo-sulfur compounds and hydro-treated diesel. Catal. Today 106 (1e4), 233e237. Gunam, I.B.W., Yaku, Y., Hirano, M., Yamamura, K., Tomita, F., Sone, T., Asano, K., 2006. Biodesulfurization of alkylated forms of dibenzothiophene and benzothiophene by Sphingomonas subarctica T7b. J. Biosci. Bioeng. 101 (4), 322e327. Gunam, I.B.W., Yamamura, K., Sujaya, N., Antara, N.S., Aryanta, W.R., Tanaka, M., Tomita, F., Sone, T., Asano, K., 2013. Biodesulfurization of dibenzothiophene and its derivatives using resting and immobilized cells of Sphingomonas subarctica T7b. J. Microbiol. Biotechnol. 23 (4), 473e482. Guobin, S., Huaiying, Z., Jianmin, X., Guo, C., Wangliang, L., Huizhou, L., 2006. Biodesulfurization of hydrodesulfurized diesel oil with Pseudomonas delafieldii R-8 from high density culture. Biochem. Eng. J. 27, 305e309. Guobin, S., Huaiying, Z., Weiquan, C., Jianmin, X., Huizhou, L., 2005a. Improvement of biodesulfurization rate by assembling nanosorbents on the surfaces of microbial cells. Biophys. J. Biophys. Lett. 89, L58eL60. Guobin, S., Jianmin, X., Chen, G., Huizhou, L., Jiayong, C., 2005b. Biodesulfurization using Pseudomonas delafieldii in magnetic polyvinyl alcohol beads. Lett. Appl. Microbiol. 40, 30e36. Gupta, N., Roychoudhury, P.K., Deb, J.K., 2005. Biotechnology of desulfurization of diesel: prospects and challenges. Appl. Microbiol. Biotechnol. 66, 356e366. ndez-Maldonado, A.J., Stamatis, S.D., Yang, R.T., He, A.Z., Cannella, W., 2004. Herna New sorbents for desulfurization of diesel fuels via p Complexation: layered beds and regeneration. Ind. Eng. Chem. Res. 43, 769e776. Hirano, S., Kitauchi, F., Haruki, M., Imanaka, T., Morikawa, M., Kanaya, S., 2004. Isolation and characterization of Xanthobacter polyaromaticivorans sp. nov. 127W that degrades polycyclic and heterocyclic aromatic compounds under extremely low oxygen conditions. Biosci. Biotechnol. Biochem. 68, 557e564. Hirasawa, K., Ishii, Y., Kobayashi, M., Koizumi, K., Maruhashi, K., 2001. Improvement of desulfurization activity in Rhodococcus erythropolis KA2-5-1 by genetic engineering. Biosci. Biotechnol. Biochem. 65, 239e246. Honda, H., Sugiyama, H., Saito, I., Kobayashi, T., 1998. High cell density culture of Rhodococcus rhodochrous by pH-state feeding and dibenzothiophene degradation. J. Fermen Bioeng. 85, 334e338. Hou, Y., Kong, K., Yang, J., Zhang, J., Shi, D., Xin, W., 2005. Biodesulfurization of dibenzothiophene by immobilized cells of Pseudomonas stutzeri UP-1. Fuel 84 (14e15), 1975e1979. Irani, Z.A., Mehrnia, M.R., Yazdian, F., Soheily, M., Mohebali, G., Rasekh, B., 2011.

178

G. Mohebali, A.S. Ball / International Biodeterioration & Biodegradation 110 (2016) 163e180

Analysis of petroleum biodesulfurization in an airlift bioreactor using response surface methodology. Bioresour. Technol. 102, 10585e10591. Ishii, Y., Konishi, J., Okada, H., Hirasawa, K., Onaka, T., Suzuki, M., 2000. Operon structure and functional analysis of the genes encoding thermophilic desulfurizing enzymes of Paenibacillus sp. A11-2. Biochem. Biophys. Res. Commun. 270, 81e88. Ishii, Y., Kozaki, S., Furuya, T., Kino, K., Kirimura, K., 2005. .Thermophilic biodesulfurization of various heterocyclic sulfur compounds and crude straightrun light gas oil fraction by a newly isolated strain Mycobacterium phlei WU0103. Curr. Microbiol. 50 (2), 63e70. Ito, E., van Veen, J.A.R., 2006. On novel processes for removing sulphur from refinery streams. Catal. Today 116 (4), 446e460. Izumi, Y., Ohshiro, T., Ogino, H., Hine, Y., Shimao, M., 1994. Selective desulfurization of dibenzothiophene by Rhodococcus erythropolis D-1. Appl. Environ. Microbiol. 60, 223e226. Jiang, C., Liu, H., Xie, Y., Chen, J., 2002. Isolation of soil bacteria species for degrading dibenzothiophene. Chin. J. Chem. Eng. 10, 420e426 (abstract). Kayser, K.J., Bielaga-Jones, B.A., Jackowski, K., Odusan, O., Kilbane, J.J., 1993. Utilization of organosulphur compounds by axenic and mixed cultures of Rhodococcus rhodochrous IGTS8. J. Gen. Microbiol. 139, 3123e3129. Kayser, K.J., Cleveland, L., Park, H., Kwak, J., Kolhatkar, A., Kilbane, J.J., 2002. Isolation and characterization of a moderate thermophile, Mycobacterium phlei GTIS10, capable of dibenzothiophene desulfurization. Appl. Microbiol. Biotechnol. 59, 737e746. Kilbane, J.J., 2006. Microbial biocatalyst developments to upgrade fossil fuels. Curr. Opin. Biotechnol. 17, 305e314. Kilbane, J.J., Jackowski, K., 1992. Biodesulfurization of water-soluble coal-derived material by Rhodococcus rodochrous strain IGTS8. Biotechnol. Bioeng. 40, 1107e1114. Kilbane, J.J., Le Borgne, S., 2004. Petroleum biorefining: the selective removal of sulfur, nitrogen, and metals. In: Vazquez-Duhalt, R., Quintero-Ramirez, R. (Eds.), Petroleum Biotechnology, Developments and Perspectives. Elsevier, The Netherlands, pp. 29e65. Kilbane, J.J., Robbins, J., 2007. Characterization of the dszABC genes of Gordonia amicalis F.5.25.8 and identification of conserved protein and DNA sequences. Appl. Microbiol. Biotechnol. 75, 843e851. Kim, Y.J., Chang, J.H., Cho, K., Ryu, H.W., Chang, Y.K., 2004. A physiological study on growth and dibenzothiophene (DBT) desulfurization characteristics of Gordonia sp. CYKS1. Korean J. Chem. Eng. 21 (2), 436e441. Kim, J., Grate, J.W., Wang, P., 2008. Nanobiocatalysis and its potential applications. Trends Biotechnol. 26 (11), 639e646. Kim, B.H., Kim, H.Y., Kim, T.S., Park, D.H., 1995. Selectivity of desulfurization activity of Desulfovibrio desulfuricans M6 on different petroleum products. Fuel Process Technol. 43, 87e94. Kirimura, K., Furuya, T., Nishii, Y., Ishii, Y., Kino, K., Usami, S., 2001. Biodesulfurization of dibenzothiophene and its derivatives through the selective cleavage of carbon-sulfur bonds by a moderately thermophilic bacterium Bacillus subtilis WU-S2B. J. Biosci. Bioeng. 91, 262e266. Kirimura, K., Furuya, T., Sato, R., Ishii, Y., Kino, K., Usami, S., 2002. Biodesulfurization of naphthothiophene and benzothiophene through selective cleavage of carbon-sulfur bonds by Rhodococcus sp. strain WU-K2R. Appl. Environ. Microbiol. 68, 3867e3872. Kirimura, K., Harada, K., Iwasawa, H., Tanaka, T., Iwasaki, Y., Furuya, T., Ishii, Y., Kino, K., 2004. Identification and functional analysis of the genes encoding dibenzothiophene-desulfurizing enzymes from thermophilic bacteria. Appl. Microbiol. Biotechnol. 65, 703e713. Klyachko, N.L., Klibanov, A.M., 1992. Oxidation of dibenzothiophene catalyzed by hemoglobin and other hemoproteins in various aqueous-organic media. Appl. Biochem. Biotechnol. 37, 53e68. Kobayashi, M., Onaka, T., Ishii, Y., Konishi, J., Takaki, M., Okada, H., Ohta, Y., Koizumi, K., Suzuki, M., 2000. Desulfurization of alkylated forms of both dibenzothiophene and benzothiophene by a single bacterial strain. FEMS Microbiol. Lett. 187 (2), 123e126. Kobayashi, M., Horiuchi, K., Yoshikawa, O., Hirasawa, K., Ishii, Y., Fujino, K., Sugiyama, H., Maruhashi, K., 2001. Kinetic analysis of microbial desulfurization of model and light gas oils containing multiple alkyl dibenzothiophenes. Biosci. Biotechnol. Biochem. 65 (2), 298e304. Kodama, K., 1977. Co-metabolism of dibenzothiophene by Pseudomonas jianii. Agric. Biol. Chem. 41, 1305e1306. Kodama, K., Umehara, K., Shimizu, K., Nakatani, S., Minoda, Y., Yamada, K., 1973. Identification of microbial products from dibenzothiophene and its proposed oxidation pathway. Agric. Biol. Chem. 37, 45e50. Konishi, M., Kishimoto, M., Omasa, T., Katakura, Y., Shioya, S., Ohtake, H., 2005. Effect of sulfur sources on specific desulfurization activity of Rhodococcus erythropolis KA2-5-1 in exponential fed-batch culture. J. Biosci. Bioeng. 99 (3), 259e263. Konishi, J., Onaka, T., Ishii, Y., Suzuki, M., 2000. Demonstration of the carbon sulfur bond targeted desulfurization of benzothiophene by thermophilic Paenibacillus sp. Strain A11-2 capable of desulfurizing dibenzothiophene. FEMS Microbiol. Lett. 187, 151e154. Konishi, J., Maruhashi, K., 2003b. Residue 345 of dibenzothiophene (DBT) sulfone monooxygenase is involved in C-S bond cleavage specificity of alkylated DBT sulfones. Biotechnol. Lett. 25, 1199e1202. Konishi, J., Ishii, Y., Onaka, T., Maruhashi, K., 2002. Purification and characterization of the monooxygenase catalyzing sulfur-atom specific oxidation of

dibenzothiophene and benzothiophene from the thermophilic bacterium Paenibacillus sp. strain A11-2. Appl. Microbiol. Biotechnol. 60, 128e133. Konishi, J., Maruhashi, K., 2003a. 2-(2'-Hydroxyphenyl) benzene sulfinate desulfinase from the thermophilic desulfurizing bacterium Paenibacillus sp. strain A112: purification and characterization. Appl. Microbiol. Biotechnol. 62, 356e361. Konishi, J., Ishi, Y., Okumura, K., Suzuki, M. (1999). High-temperature desulfurization by microorganisms. U.S. Patent No. 5,925,560. Konishi, J., Ishii, Y., Onaka, T., Okumura, K., Suzuki, M., 1997. Thermophilic carbonsulfur-bond-targeted biodesulfurization. Appl. Environ. Microbiol. 63, 3164e3169. Kowsari, E., 2013. Recent advances in the science and technology of desulfurization of diesel fuel using ionic liquids. In: Kadokawa, J.-I. (Ed.), Ionic Liquids-new Aspects for the Future. InTech, Croatia, pp. 277e298. Kulkarni, P.S., Afonso, C.A.M., 2010. Deep desulfurization of diesel fuel using ionic liquids: current status and future challenges. Green Chem. 12, 1139e1149. Labana, S., Pandey, G., Jain, R.K., 2005. Desulphurization of dibenzothiophene and diesel oils by bacteria. Lett. Appl. Microbiol. 40, 159e163. Labord, A.L., Gibson, D.T., 1977. Metabolism of dibenzothiophene by a Beijerinkia species. Appl. Environ. Microbiol. 34, 783e790. Le Borgne, S., Quintero, R., 2003. Biotechnological processes for the refining of petroleum. Fuel Process Technol. 81, 155e169. Le Borgne, S., Ayala, M., 2010. Microorganisms utilizing sulfur-containing hydrocarbons. In: Timmis, K.N. (Ed.), Handbook of Hydrocarbon and Lipid Microbiology. Springer-Verlag, Berlin Heidelberg, Germany. Lee, I.S., Bae, H., Ryu, H.W., Cho, K., Chang, Y.K., 2005. Biocatalytic desulfurization of diesel oil in an air-lift reactor with immobilized Gordonia nitida CYKS1 Cells. Biotechnol. Prog. 21 (3), 781e785. Lee, M.K., Senius, J.D., Grossman, M.J., 1995. Sulfur-specific microbial desulfurization of sterically hindered analogos of dibenzothiophene. Appl. Environ. Microbiol. 61, 4362e4366. Lei, B., Tu, S., 1996. Gene over-expression, purification and identification of a desulfurization enzyme from Rhodococcus sp. strain IGTS8 as a sulfide/sulfoxide monooxygenase. J. Bacteriol. 178, 5699e5705. Li, M.Z., Squires, C.H., Monticello, D.J., Childs, J.D., 1996. Genetic analysis of the dsz promoter and associated regulatory regions of Rhodococcus erythropolis IGTS8. J. Bacteriol. 178, 6409e6418. Li, F.L., Xu, P., Ma, C.Q., Luo, L.L., Wang, X.S., 2003. Deep desulfurization of hydrodesulfurization-treated diesel oil by a facultative thermophilic bacterium Mycobacterium sp. X7B. FEMS Microbiol. Lett. 223, 301e307. Li, W., Xing, J., Xiong, X., Shan, G., Liu, H., 2005a. Bio-regeneration of p-complexation desulfurization adsorbents. Sci. China Ser. B Chem. 48 (6), 538e544. Li, W., Zhang, Y., Wang, M.D., Shi, Y., 2005b. Biodesulfurization of dibenzothiophene and other organic sulfur compounds by a newly isolated Microbacterium strain ZD-M2. FEMS Microbiol. Lett. 247 (1), 45e50. Li, G.Q., Lee, K., Ma, T., Li, S.S., Li, H., Liang, F.L., Liu, R.L., 2006a. Isolation and characterization of a benzothiophene desulfurizing bacterium. Huan Jing Ke Xue 27 (6), 1181e1185. Li, W., Xing, J., Xiong, X., Huang, J., Liu, H., 2006b. Feasibility study on the integration of adsorption/bioregeneration of p-complexation adsorbent for desulfurization. Ind. Eng. Chem. Res. 45 (8), 2845e2849. Li, G., Ma, T., Li, S., Li, H., Liang, F., Liu, R., 2007a. Improvement of dibenzothiophene desulfurization activity by removing the gene overlap in the dsz operon. Biosci. Biotechnol. Biochem. 71, 849e854. Li, Y., Xing, J., Li, W., Xiong, X., Li, X., Liu, H., 2007b. Medium optimization of Rhodococcus erythropolis LSSE8-1 by Taguchi methodology for petroleum biodesulfurization. Korean J. Chem. Eng. 24 (5), 781e786. Li, G., Li, S., Qu, S., Liu, Q., Ma, T., Zhu, L., Liang, F., Liu, R., 2008a. Improved biodesulfurization of hydrodesulfurized diesel oil using Rhodococcus erythropolis and Gordonia sp. Biotechnol. Lett. 30, 1759e1764. Li, G., Li, S., Zhang, M., Wang, J., Zhu, L., Liang, F., Liu, R., Ma, T., 2008b. Genetic rearrangement strategy for optimizing the dibenzothiophene biodesulfurization pathway in Rhodococcus erythropolis. Appl. Environ. Microbiol. 74 (4), 971e976. Li, W., Xing, J., Li, Y., Xiong, X., Li, X., Liu, H., 2008c. Desulfurization and bioregeneration of adsorbents with magnetic P. delafieldii R-8 cells. Catal. Commun. 9, 376e380. Li, Y.G., Xing, J.M., Xiong, X.C., Li, W.L., Gao, H.S., Liu, H.Z., 2008d. Improvement of biodesulfurization activity of alginate immobilized cells in biphasic systems. J. Ind. Microbiol. Biotechnol. 35 (3), 145e150. Li, J., Feng, J., Li, Q., Ma, C., Yu, B., Gao, C., Wu, G., Xu, P., 2009a. Both FMNH2 and FADH2 can be utilized by the dibenzothiophene monooxygenase from a desulfurizing bacterium Mycobacterium goodii X7B. Bioresour. Technol. 100, 2594e2599. Li, Y., Gao, H., Li, W., Xing, J., Liu, H., 2009b. In situ magnetic separation and immobilization of dibenzothiophene-desulfurizing bacteria. Bioresour. Technol. 100 (21), 5092e5096. Li, W., Tang, H., Liu, Q., Xing, J., Li, Q., Wang, D., Yang, M., Li, X., Liu, H., 2009c. Deep desulfurization of diesel by integrating adsorption and microbial method. Biochem. Eng. J. 44, 297e301. Lin, X., Liu, H., Zhu, F., Wei, X., Li, Q., Luo, M., 2012. Enhancement of biodesulfurization by Pseudomonas delafieldii in a ceramic microsparging aeration system. Biotechnol. Lett. 34, 1029e1032. Linguist, L., Pacheco, M., Feb. 1999. Enzyme-based diesel desulfurization process offers energy, CO2 advantages. Oil Gas J. 45e48. Lizama, H.M., Wilkins, L.A., Scott, T.C., 1995. Dibenzothiophene sulfur can serve as

G. Mohebali, A.S. Ball / International Biodeterioration & Biodegradation 110 (2016) 163e180 sole electron acceptor during growth by sulfate reducing bacteria. Biotechnol. Lett. 17, 113e116. Luo, M., Xing, J., Gou, Z., Li, S., Liu, H., Chen, J., 2003. Desulphurization of dibenzothiophene by lyophilized cells of Pseudomonas delafieldii R-8 in the presence of dodecane. Biochem. Eng. J. 13, 1e6. Ma, T., 2010. The desulfurization pathway in Rhodococcus. In: Alvarez, H.M. (Ed.), Biology of Rhodococcus, Microbiology Monographs 16. Springer-Verlag Berlin Heidelberg, pp. 207e230. Ma, C.Q., Feng, J.H., Zeng, Y.Y., Cai, X.F., Sun, B.P., Zhang, Z.B., Blankespoor, H.D., Xu, P., 2006a. Methods for the preparation of a biodesulfurization biocatalyst using Rhodococcus sp. Chemosphere 65, 165e169. Ma, T., Li, G., Li, J., Liang, F., Liu, R., 2006b. Desulfurization of dibenzothiophene by Bacillus subtilis recombinants carrying dszABC and dszD genes. Biotechnol. Lett. 28 (14), 1095e1100. Ma, T., Li, S., Li, G., Wang, R., Liang, F., Liu, R., 2006c. Cloning and expressing DBT (dibenzothiophene) monooxygenase gene (dszC) from Rhodococcus sp. DS-3 in Escherichia coli. Front. Biol. China 4, 375e380. Ma, T., Li, J., Tong, M.Y., Zhang, X.P., Liu, R.L., 2002. Desulfurization of dibenthiophene by Rhodococcus sp. DS-3. Acta Microbiol. Sin. 42, 126e131. Madeira, L.D., Ferreira-Leit~ ao, V.S., Bon, E.P.D., 2008. Dibenzothiophene oxidation by horseradish peroxidase in organic media: effect of the DBT: H2O2 molar ratio and H2O2 addition mode. Chemosphere 71, 189e194. Maghsoudi, S., Kheirolomooma, A., Vossoughib, M., Tanakac, E., Katohd, S., 2000. Selective desulfurization of dibenzothiophene by newly isolated Corynebacterium sp. strain P32C1. Biochem. Eng. J. 5, 11e16. Maghsoudi, S., Vossoughi, M., Kheirolomoom, A., Tanaka, E., Katoh, S., 2001. Biodesulfurization of hydrocarbons and diesel fuels by Rhodococcus sp. strain P32C1. Biochem. Eng. J. 8, 151e156. Marcelis, C., 2002. Anaerobic Biodesulfurization of Thiophenes. Thesis. Wageningen University, Wageningen, The Netherlands. Martin, A.B., Alcon, A., Santos, V.E., García-Ochoa, F., 2004. Production of a Biocatalyst of Pseudomonas putida CECT5279 for Dibenzothiophene (DBT) biodesulfurization for different media compositions. Energy Fuels 18, 851e857. Martinez, I., Santos, V.E., Alcon, A., Garcia-Ochoa, F., 2015. Enhancement of the biodesulfurization capacity of Pseudomonas putida CECT5279 by co-substrate addition. Process Biochem. 50 (1), 119e124. Marzona, M., Pessione, E., Di Martino, S., Giunta, C., 1997. Benzothiophene and dibenzothiophene as the sole sulfur source in Acinetobacter: growth kinetics and oxidation products. Fuel Process. Technol. 52 (1e3), 199e205. Matsui, T., Noda, K., Tanaka, Y., Maruhashi, K., Kurane, R., 2002. Recombinant Rhodococcus sp. strain T09 can desulfurize DBT in the presence of inorganic sulfate. Curr. Microbiol. 45, 240e244. Matsui, T., Onaka, T., Tanaka, Y., Tezuka, T., Suzuki, M., Kurane, R., 2000. .Alkylated benzothiophene desulfurization by Rhodococcus sp. Strain T09. Biosci. Biotechnol. Biochem. 64, 596e599. McFarland, B.L., 1999. Biodesulfurization. Curr. Opin. Microbiol. 2, 257e264. McFarland, B.L., Boron, D.J., Deever, W.R., Meyer, J.A., Johnson, A.R., Atlas, R.M., 1998. Biocatalytic sulfur removal from fuels: applicability for producing low sulfur gasoline. Crit. Rev. Microbiol. 24, 99e147. Mingfang, L., Jianmin, X., Zhongxuan, G., Huizhou, L., Jiayong, C., 2003. Microbial desulfurization of dibenzothiophene and 4,6-dimethyldibenzothiophene in dodecane and straight-run diesel oil. Korean J. Chem. Eng. 20 (4), 702e704. Mohebali, G., Ball, A.S., 2008. Biocatalytic desulfurization (BDS) of petrodiesel Fuels. Microbiology 154, 2169e2183. Mohebali, G., Ball, A.S., Kaytash, A., Rasekh, B., 2007. Stabilization of water/gas oil emulsions by desulfurizing cells of Gordonia alkanivorans RIPI90A. Microbiology 153, 1573e1581. Mohebali, G., Ball, A.S., Kaytash, A., Rasekh, B., 2008. Dimethyl sulfoxide (DMSO) as an alternative sulfur source to DBT for the production of desulfurizing resting cells of Gordonia alkanivorans RIPI90A. Microbiology 154, 878e885. Monot, F., Abbad-Andaloussi, S., Warzywoda, M. (2002). Biological culture containing Rhodococcus erythropolis and/or Rhodococcus rhodnii and process for desulfurization of petroleum fraction. U.S. Patent No. 6,337,204. Monticello, D.J. (1996). Multistage process for deep desulfurization of a fossil fuel.U.S. Patent 5,510,265. Monticello, D.J., 1998. Riding the fossil fuel biodesulfurization wave. ChemTech 28 (7), 38e45. Monticello, D.J., 2000. Biodesulfurization and the upgrading of petroleum distillates. Curr. Opin. Biotechnol. 11, 540e546. Monticello, D.J., Finnerty, W.R., 1985. Microbial desulfurization of fossil fuels. Annu. Rev. Microbiol. 39, 371e389. Naito, M., Kawamoto, T., Fujino, K., Kobayashi, M., Maruhashi, K., Tanaka, A., 2001. Long-term repeated biodesulfurization by immobilized Rhodococcus erythropolis KA2-5-1 cells. Appl. Microbiol. Biotechnol. 55, 374e378. Nandi, S., 2010. Biodesulfurization of hydro-desulfurized diesel in airlift reactor. J Sci. Ind. Res. 69, 543e547. Nekodzuka, S., Nakajima-Kambe, T., Nomura, N., Jie, L., Nakahara, T., 1997. Specific desulfurization of dibenzothiophene by Mycobacterium sp. strain G3. Biocatal. Biotransform. 15, 17e27. Noda, K., Watanabe, K., Maruhashi, K., 2002. Cloning of a rhodococcal promoter using a transposon for dibenzothiophene biodesulfurization. Biotechnol. Lett. 24, 1875e1882. Noda, K., Kogure, T., Irisa, S., Murakami, Y., Sakata, M., Kuroda, A., 2008. Enhanced dibenzothiophene biodesulfurization in a microchannel reactor. Biotechnol. Lett. 30, 451e454.

179

Nomura, N., Takada, M., Okada, H., Shinohara, Y., Nakajima-Kambe, T., Nakahara, T., Uchiyama, H., 2005. Identification and functional analysis of genes required for desulfurization of alkyl dibenzothiophenes of Mycobacterium sp. G3. J. Biosci. Bioeng. 100, 398e402. Nuhu, A.A., 2013. Bio-catalytic desulfurization of fossil fuels: a mini review. Rev. Environ. Sci. Bio Technol. 12 (1), 9e23. Ohshiro, T., Hirata, T., Izumi, Y., 1995. Microbial desulfurization of dibenzothiophene in the presence of hydrocarbon. Appl. Microbiol. Biotechnol. 44, 249e252. Ohshiro, T., Izumi, Y., 1999. Microbial desulphurization of organic sulphur compounds in petroleum. Biosci. Biotechnol. Biochem. 63, 1e9. Ohshiro, T., Hirata, T., Izumi, Y., 1996. Desulfurization of dibenzothiophene derivatives by whole cells of Rhodococcus erythropolis H-2. FEMS Microbiol. Lett. 142, 65e70. Okada, H., Nomura, N., Nakahara, T., Maruhashi, K., 2002a. Analysis of dibenzothiophene metabolic pathway in Mycobacterium strain G3. J. Biosci. Bioeng. 93, 491e497. Okada, H., Nomura, N., Nakahara, T., Maruhashi, K., 2002b. Analyses of substrate specificity of the desulfurizing bacterium Mycobacterium sp. G3. J. Biosci. Bioeng. 93 (2), 228e233. Okada, H., Nomura, N., Nakahara, T., Maruhashi, K., 2001. Cultivation of a desulfurizing bacterium, Mycobacterium strain G3. Biotechnol. Lett. 23, 2047e2050. Oldfield, C., Pogrebinsky, O., Simmonds, J., Olson, E.S., Kulpa, C.F., 1997. Elucidation of the metabolic pathway for dibenzothiophene desulfurization by Rhodococcus sp. strain IGTS8 (ATCC 53968). Microbiology 143, 2961e2973. Oldfield, C., Wood, N.T., Gilbert, S.C., Murray, F.D., Faure, F.R., 1998. Desulphurisation of benzothiophene and dibenzothiophene by actinomycete organisms belonging to the genus Rhodococcus, and related taxa. Antonie Leeuwenhoek 74, 119e132. Olson, G.J. (2000). Microbial catalyst for desulfurization of fossil fuels. U.S. Patent No. 6,124,130. Omori, T., Saiki, Y., Kasuga, K., Kodama, T., 1995. Desulfurization of alkyl and aromatic sulfides and sulfonates by dibenzothiophene desulfurizing Rhodococcus sp. strain SY-1. Biosci. Biotech. Biochem. 59, 1195e1198. Onaka, T., Kobayashi, M., Ishii, Y., Konishi, J., Maruhashi, K., 2001a. Selective cleavage of the two C-S bonds in asymmetrically alkylated dibenzothiophenes by Rhodococcus erythropolis KA2-5-1. J. Biosci. Bioeng. 92, 80e82. Onaka, T., Konishi, J., Ishii, Y., Maruhashi, K., 2001b. Desulfurization characteristics of thermophilic Paenibacillus sp.strain A11-2 against asymmetrically alkylated dibenzothiophenes. J. Biosci. Bioeng. 92, 193e196. Pacheco, M.A., 1999. Recent advances in biodesulfurization (BDS) of diesel fuel. In: Paper Presented at the NPRA Annual Meeting. San Antonio, TX, 21e23 March 1999. Pawelec, B., Navarro, R.M., Campos-Martin, J.M., Fierro, J.L.G., 2011. Towards near zero-sulfur liquid fuels: a perspective review. Catal. Sci. Technol. 1, 23e42. Piddington, C.S., Kovacevich, B.R., Rambosek, J., 1995. Sequencing and molecular characterization of a DNA region encoding the dibenzothiophenedesulfurization operon of Rhodococcus sp.IGTS8. Appl. Environ. Microbiol. 61 (2), 468e475. Pope III, C.A., Burnett, R.T., Thun, M.J., Calle, E.E., Krewski, D., Ito, K., Thurston, G.D., 2002. Lung cancer, cardiopulmonary mortality, and long-term exposure to fine particulate air pollution. J. Am. Med. Assoc. 287, 1132e1141. Prince, R.C., Grossman, M.J., 2003. Substrate preferences in biodesulfurization of diesel range fuels by Rhodococcus sp. strain ECRD-1. Appl. Environ. Microbiol. 69, 5833e5838. Purdy, R.F., Lepo, J.E., Ward, B., 1993. Bidesulfurization of organic-sulfur compounds. Curr. Microbiol. 27, 219e222. Ranson, I., Rivas, C.M. (2008). Biodesulfurization of hydrocarbons. U.S. Patent, 733, 879,5. Rashidi, L., Mohebali, G., Towfighi darian, J., Rasekh, B., 2006. Biodesulfurization of dibenzothiophene and its alkylated derivatives through the sulfur-specific pathway by the bacterium RIPI-S81. Afr. J. Biotechnol. 5, 351e356. Rhee, S.K., Chang, J.H., Chang, Y.K., Chang, H.N., 1998. Desulfurization of dibenzothiophene and diesel oils by a newly isolated Gordonia strain, CYKS1. Appl. Environ. Microbiol. 64, 2327e2331. Santos, S.C., Alviano, D.S., Alviano, C.S., Padula, M., Leitao, A.C., Martins, O.B., Ribeiro, C.M., Sassaki, M.Y., Matta, C.P., Bevilaqua, J., Sebastian, G.V., Seldin, L., 2006a. Characterization of Gordonia sp. strain F.5.25.8 capable of dibenzothiophene desulfurization and carbazole utilization. Appl. Microbiol. Biotechnol. 71, 355e362. dula, M., Leit~ Santos, S.C., Alviano, D.S., Alviano, C.S., Goulart, F.R.V., de Pa ao, A.C., Martins, O.B., Ribeiro, C.M.S., Sassaki, M.Y.M., Matta, C.P.S., Bevilaqua, J., Sebasti an, G.V., Seldin, L., 2007. Comparative studies of phenotypic and genetic characteristics between two desulfurizing isolates of Rhodococcus erythropolis and the well-characterized R. erythropolis strain IGTS8. J. Ind. Microbiol. Biotechnol. 34 (6), 423e431. Santos, V.E., Galdeano, C., Gomez, E., Alcon, A., Garcia-Ochoa, F., 2006b. Oxygen uptake rate measurements both by the dynamic method and during the process growth of Rhodococcus erythropolis IGTS8: Modelling and difference in results. Biochem. Eng. J. 32, 198e204. Setti, L., Bonoli, S., Badiali, E., Giuliani, S., 2003. Inverse phase transfer biocatalysis for a biodesulfurization process of middle distillates. BECTH MOCK YJ-TA CEP 2 XHMHR 44 (1), 80e83. Shan, G.B., Xing, J.M., Luo, M.F., Liu, H.Z., Chen, J.Y., 2003. Immobilization of Pseudomonas delafieldii with magnetic polyvinyl alcohol beads and its application in biodesulfurization. Biotechnol. Lett. 25, 1977e1981.

180

G. Mohebali, A.S. Ball / International Biodeterioration & Biodegradation 110 (2016) 163e180

Shan, G.B., Xing, J.M., Zhang, H.Y., Liu, H.Z., 2005. Biodesulfurization of dibenzothiophene by microbial cells coated with magnetite nanoparticles. Appl. Environ. Microbiol. 71 (8), 4497e4502. Shavandi, M., Sadeghizadeh, M., Khajeh, K., Mohebali, G., Zomorodipour, A., 2010. Genomic structure and promoter analysis of the dsz operon for dibenzothiophene biodesulfurization from Gordonia alkanivorans RIPI90A. Appl. Microbiol. Biotechnol. 87, 1455e1461. Shavandi, M., Sadeghizadeh, M., Zomorodipour, A., Khajeh, K., 2009. Biodesulfurization of dibenzothiophene by recombinant Gordonia alkanivorans RIPI90A. Bioresour. Technol. 100, 475e479. Singh, S.P., Schwan, A.L., 2011. Sulfur metabolism in plants and related biotechnologies. In: Moo-Young, M. (Ed.), Comprehensive Biotechnology, second ed., vol. 4. Elsevier, pp. 257e271. Soleimani, M., Bassi, A., Margaritis, A., 2007. Biodesulfurization of refractory organic sulfur compounds in fossil fuels. Biotechnol. Adv. 25 (6), 570e596. Song, C., 2003. An overview of new approaches to deep desulfurizaton for ultraclean gasoline, diesel fuel and jet fuel. Catal. Today 86, 211e263. Song, C.S., Ma, X., 2007. Ultra-clean diesel fuel by deep desulfurization and deep dearomatization of middle distillates. In: Hsu, C.S., Robinson, P.R. (Eds.), “Practical Advances in Petroleum Processing”, vol. 1, Chap. 11. Springer, New York, pp. 317e372. Srinivasaraghavan, K., Sarma, P.M., Lal, B., 2006. Comparative analysis of phenotypic and genotypic characteristics of two desulfurizing bacterial strains, Mycobacterium phlei SM120-1 and Mycobacterium phlei GTIS10. Lett. Appl. Microbiol. 42, 483e489. Stachyra, T., Guillochon, D., Pulvin, S., Thomas, D., 1996. Hemoglobin, horseradish peroxidase, and heme-bovina serum albumin as biocatalyst for the oxidation of dibenzothiophene. Appl. Biochem. Biotechnol. 59, 231e244. Stanislaus, A., Marafi, A., Rana, M.S., 2010. Recent advances in the science and technology of ultra low sulfur diesel (ULSD) production. Catal. Today 153 (1e2), 1e68. Swaty, T.E., Sep. 2005. Global refining industry trends: the present and future. Hydrocarb. Process. 35e46. Tanaka, Y., Matsui, T., Konishi, J., Maruhashi, K., Kurane, R., 2002a. Biodesulfurization of benzothiophene and dibenzothiophene by a newly isolated Rhodococcus strain. Appl. Microbiol. Biotechnol. 59, 325e328. Tanaka, Y., Yoshikawa, O., Maruhashi, K., 2002b. The cbs mutant strain of Rhodococcus erythropolis KA2-5-1 express high levels of Dsz enzymes in the presence of sulfate. Arch. Microbiol. 178, 351e357. Tao, F., Yu, B., Xu, P., Ma, C.Q., 2006. Biodesulfurization in biphasic systems containing organic solvents. Appl. Environ. Microbiol. 72, 4604e4609. Tao, F., Liu, Y., Luo, Q., Su, F., Xu, Y., Li, F., Yu, B., Ma, C., Xu, P., 2011. Novel organic solvent-responsive expression vectors for biocatalysis: application for development of an organic solvent-tolerant biodesulfurizing strain. Bioresour. Technol. 102 (20), 9380e9387. Ting, M., Jian, L., Mingyou, T., Xinping, Z., Rulin, L., 2002. Desulfurization of dibenthiophene by Rhodococcus sp. DS-3. Acta Microbiol. Sin. 42, 126e131. Tong, M.Y., Fang, X.C., Ma, T., Zhang, Q., 2001. The application of genetic engineering to the petroleum biodesulfurization. Sheng Wu Gong Cheng Xue Bao 17 (6), 617e620 (Abstract in English). Tsouris, C., Lizam, H.M., Spurrier, M.A., Takeuchi, T.L., Scott, T.C., 1996. Hydrodynamics of bioreactor systems for liquid-liquid contacting. Appl. Biochem. Biotechnol. 57/58, 581. Van Afferden, M., Schacht, S., Klein, J., Truperet, H.G., 1990. Degradation of dibenzothiophene by Brevibacterium sp. Do. Arch. Microbiol. 153, 324e328. Van Afferden, M., Tappe, D., Beyer, M., Truper, H.G., Klein, J., 1993. Biochemical mechanisms for the desulphurisation of coal-relevant organic sulfur compounds. Fuel 72, 635e643. Wang, M., Chen, H., Chen, J., Shi, Y., 2006a. Biodesulfurization of dibenzothiophene by growing cells of Gordonia sp. in batch cultures. Biotechnol. Lett. 28, 1175e1179. Wang, P., Krawiec, S., 1994. Desulfurization of dibenzothiophene to 2hydroxybiphenyl by some newly isolated bacteria strains. Arch. Microbiol. 161, 266e271. Wang, M., Li, W., Shi, Y., Wang, D., Feng, H., 2006b. Effects of surfactant on biodesulfurization by Corynebacterium sp. ZD-1 in the presence of organic phase. J Zhejiang Univ. Science A 7 (Suppl. II), 371e375. Wang, M., Li, W., Wang, D., Shi, Y., 2004. Desulfurization of dibenzothiophene by a newly isolated Corynebacterium sp. ZD-1 in aqueous phase. J. Environ. Sci. 16, 1011e1015.

Watanabe, K., Noda, K., Maruhashi, K., 2003. Selective cleavage of 10-methyl benzo [b]naphtho[2,1-d]thiophene by recombinant Mycobacterium sp. strain. Biotechnol. Lett. 25, 797e803. Watanabe, K., Noda, K., Ohta, Y., Maruhashi, K., 2002. Desulfurization of light gas oil by a novel recombinant strain from Pseudomonas aeruginosa. Biotechnol. Lett. 24, 897e903. Wei, Y.X., Yan, H., Xiao, B.Q., Li, Y.X., Lv, L., 2005. Biodesulfurization of 4,6-dimethyl dibenzothiophene by Rhodococcus erythropolis USTB03. Sci. Technol. Rev. 23, 10e13. Worrell, E., Galitsky, C., 2004. Profile of the petroleum refining industry in California. LBNL-55450. In: California industries of the Future Program. Environmental energy technologies division. Ernest Orlando Lawrence Berekley National Laboratory, CA, USA. http://ies.lbl.gov/iespubs/55450.pdf. Wu, Z., Ondruschka, B., 2010. Ultrasound-assisted oxidative desulfurization of liquid fuels and its industrial application. Ultrason. Sonochem. 17 (6), 1027e1032. Xi, L., Squires, C.H., Monticello, D.J., Childs, J.D., 1997. A flavin reductase stimulates DszA and DszC proteins of Rhodococcus erythropolis IGTS8 in vitro. Biochem. Biophys. Res. Commun. 230, 73e75. Xiaojuan, T., Lingtian, T., Li'e, P., Xinghong, L., 2008. Research on identification and screen of microbial desulfurization strains for petroleum. Earth Sci. Front. 15 (6), 192e198. Xiong, X., Xing, J., Li, X., Bai, X., Li, W., Li, Y., Liu, H., 2007. Enhancement of biodesulfurization in two-liquid systems by heterogeneous expression of Vitreoscilla hemoglobin. Appl. Environ. Microbiol. 73 (7), 2394e2397. Xu, P., Feng, J., Yu, B., Li, F., Ma, C., 2009. Recent developments in biodesulfurization of fossil fuels. Adv. Biochem. Eng. Biotechnol. 113, 255e274. Xu, P., Yu, B., Li, F.L., Cai, X.F., Ma, C.Q., 2006. Microbial degradation of sulfur, nitrogen and oxygen heterocycles. Trends Microbiol. 14 (9), 398e405. Xu, L., Yang, L., Luo, M., Liang, X., Wei, X., Zhao, J., Liu, H., 2011. Reduction of hexavalent chromium by Pannonibacter phragmitetus LSSE-09 coated with polyethylenimine-functionalized magnetic nanoparticles under alkaline conditions. J. Hazard. Mater. 189 (3), 787e793. Xu, P., Ma, C., Li, F., Tong, M., Zeng, Y., Wang, S., Blankespoor, H.D., 2002. Preparation of microbial desulfurization catalysts. Chin. Sci. Bull. 47 (13), 1077e1081. Yamada, K., Minoda, Y., Kodama, K., Nakatani, S., Akasaki, T., 1968. Microbial conversion of petro-sulfur compounds, Part I. Isolation and identification of dibenzothiophene-utilizing bacteria. Agr. Biol. Chem. 32, 840e845. Yang, J., Marison, I., 2005. Two-stage process design for the biodesulphurisation of a model diesel by a newly isolated Rhodococcus globerulus DAQ3. Biochem. Eng. J. 27, 77e82. Yang, J., Hu, Y., Zhao, D., Wang, S., Lau, P.C.K., Marison, I.W., 2007. Two-layer continuous-process design for the biodesulfurization of diesel oils under bacterial growth conditions. Biochem. Eng. J. 37, 212e218. Yoshikawa, O., Ishii, Y., Koizumi, K., Ohshiro, T., Izumi, Y., Maruhashi, K., 2002. Enhancement and stabilization of desulfurization activity of Rhodococcus erythropolis KA2-5-1 by feeding ethanol and sulfur components. J. Biosci. Bioeng. 94, 447e452. Yu, B., Xu, P., Shi, Q., Ma, C., 2006. Deep desulfurization of diesel oil and crude oils by a newly isolated Rhodococcus erythropolis strain. Appl. Environ. Microbiol. 72, 54e58. Zhang, S., Chen, H., Li, W., 2013. Kinetic analysis of biodesulfurization of model oil containing multiple alkyl dibenzothiophenes. Appl. Microbiol. Biotechnol. 97, 2193e2200. Zhang, J.C., Liu, Y.Q., An, G.J., Chai, Y.M., Fu, Q.T., Liu, C.G., 2008a. The adsorptive desulfurization technologies for ultra-clean oil production. Prog. Chem. 20 (11), 1834e1845. Zhang, H., Liu, Q., Li, Y., Li, W., Xiong, X., Xing, J., Liu, H., 2008b. Selection of adsorbents for in-situ coupling technology of adsorptive desulfurization and biodesulfurization. Sci. China Ser. B Chem. 51 (1), 69e77. Zhang, H.Y., Shan, G.B., Liu, H.Z., Xing, J.M., 2007a. Surface modification of g-Al2O3 nano-particles with gum arabic and its applications in adsorption and biodesulfurization. Surf. Coatings Technol. 201, 6917e6921. Zhang, Q., Tong, M.Y., Li, Y.S., Gao, H.J., Fang, X.C., 2007b. Extensive desulfurization of diesel by Rhodococcus erythropolis. Biotechnol. Lett. 29 (1), 123e127. Zhao, X., Lv, L., Pan, B., Zhang, W., Zhang, S., Zhang, Q., 2011. Polymer-supported nanocomposites for environmental application: a review. Chem. Eng. J. 170 (2e3), 381e394. Zhongxuan, G., Mingfang, L., Xin, L., Jianmin, X., Huizhou, L., 2003. Desulfurization metabolite of Rhodococcus erythropolis LSSE8-1 and its related desulfurizational gene fragments. Chin. Sci. Bull. 48 (24), 2703e2709.