A review of electrokinetically enhanced bioremediation technologies for PHs

J O U RN A L OF E N V I RO N ME N TA L SC IE N CE S 8 8 (2 0 2 0) 3 1–4 5

Available online at www.sciencedirect.com

ScienceDirect www.elsevier.com/locate/jes

Review

A review of electrokinetically enhanced bioremediation technologies for PHs Anish Saini1,2 , Dawit Nega Bekele1,2 , Sreenivasulu Chadalavada1,2 , Cheng Fang1,2 , Ravi Naidu1,2,⁎ 1. Global Centre for Environmental Remediation, University of Newcastle, Callaghan, Newcastle 2308, NSW, Australia 2. Cooperative Research Centre for Contamination Assessment and Remediation of the Environment (CRC CARE), University of Newcastle, Callaghan, Newcastle 2308, NSW, Australia

AR TIC LE I N FO

ABS TR ACT

Article history:

Since the early 1980’s there have been several different strategies designed and applied to

Received 10 May 2019

the remediation of subsurface environment including physical, chemical and biological

Revised 13 August 2019

approaches. They have had varying degrees of success in remediating contaminants from

Accepted 13 August 2019

subsurface soils and groundwater. The objective of this review is to examine the range of

Available online 21 August 2019

technologies for the remediation of contaminants, particularly petroleum hydrocarbons, in subsurfaces with a specific focus on bioremediation and electrokinetic remediation.

Keywords:

Further, this review examines the efficiency of remediation carried out by combining

Subsurface contamination

bioremediation and electrokinetic remediation. Surfactants, which are slowly becoming the

Remediation

selected chemicals for mobilizing contaminants, are also considered in this review. The

Petroleum hydrocarbons (PH)

current knowledge gaps of these technologies and techniques identified which could lead to

Electrokinetics

development of more efficient ways of utilizing these technologies or development of a

Bioremediation

completely new technology. © 2019 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . 1. Petroleum hydrocarbons (PH) contamination . . . 1.1. Fate of PH contaminants in the subsurface . 2. Bioremediation of PH . . . . . . . . . . . . . . . . 2.1. Bioremediation techniques . . . . . . . . . 3. Electrokinetics . . . . . . . . . . . . . . . . . . . . 3.1. Electrokinetic remediation process . . . . . 3.2. Electrokinetic remediation technologies . . 3.2.1. Conventional anode–cathode . . . .

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⁎ Corresponding author. E-mail: [email protected] (Ravi Naidu).

https://doi.org/10.1016/j.jes.2019.08.010 1001-0742 © 2019 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.

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3.2.2. Alternative anode and cathode approach . . 3.2.3. Two-anode technique (TAT) . . . . . . . . . 3.2.4. Approaching anode electrokinesis (AA-EK). . 3.2.5. Approaching cathode electrokinesis (AC-EK). 4. Coupled electrokinetic and bioremediation approach. . . . 4.1. Electrokinetic (EK) bioattenuation . . . . . . . . . . 4.2. Electrokinetic (EK) biostimulation . . . . . . . . . . 4.3. Electrokinetic (EK) bioaugmentation . . . . . . . . . 4.4. Electrokinetic soil flushing (EKSF) . . . . . . . . . . 5. Surfactants. . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Surfactants enhanced in situ remediation . . . . . . 6. Concluding remarks . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . References. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Introduction The industrial revolution brought about massive expansion of industries in developed as well as developing countries. Over the past century global population has quadrupled and the gross domestic product (GDP) grew more than 20-fold (Krausmann et al., 2009; Maddison, 2007). Development and industrialization often come at the expense of the environment. In Europe there exists an estimated 2.5 million contaminated sites (Panagos et al., 2013), up to 530,000 in the USA (EPA, 2013) and approximately 160,000 in Australia (Smith et al., 2015). In the recent past, the spectacular economic growth of developing countries has spawned a growing number of contaminated sites and waste disposal problems. Although contamination has been recognized as an issue for over 70 years, hardly a tenth of such sites have been remediated (Naidu, 2013). This may be attributed to the complex and challenging nature of contamination that varies from site to site and to high costs of clean-up (Gavrilescu et al., 2009; Mulligan et al., 2001; Naidu, 2013). Most contaminated sites have associated groundwater contamination problems that prevent their effective and reliable remediation and pose health risks to the communities. During the late 20th century, efforts to contain and remediate contaminated groundwater increased (Gavrilescu, 2005; Harrison, 2001; Kovalock Jr, 2000). Increasing demand for remediation of persistent contaminants has brought-about development of numerous remediation technologies, some of which are promising for remediating contaminated water and soils (Farhadian et al., 2008; Pavel and Gavrilescu, 2008; Varjani, 2017). Remediation of contaminants can be challenging from both technical and financial perspectives. Estimates of the annual global cost of remediation ranges from $65 billion in 2016 to in excess of $82 billion by 2022 (Boots, 2017). Several reports suggest that despite many years of focusing on remediation it has proven challenging, problematic and expensive, to meet regulatory prescribed end-points (Scherer et al., 2000) which are often extremely conservative. The selection of suitable technology is often a difficult but crucial step for the successful remediation of a contaminated site (da Silva and Maranho, 2019; Khan et al., 2004). The objective of this paper is to critically review existing

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remediation technologies with a focus on in situ treatments, including bioremediation (Part 3) and electrokinetic methods (Part 4) and an approach based on coupling these technologies (Part 5). Surfactant-enhanced remediation is discussed (Part 6) because it has received increasing attention recently to enhance remediation efficiency by accelerating the mobilization of adsorbed petroleum hydrocarbons from soil matrices. The review will also provide environmental practitioners and scientists with crucial information for the selection of the most applicable technology for remediating priority recalcitrant contaminants in subsurface media. The review concludes with recommendations, and highlights research gaps, which, if plugged, could lead to improvements in situ remediation technologies.

1. Petroleum hydrocarbons (PH) contamination Petroleum hydrocarbons (PH) products are some of the most extensively used chemicals both in industrial processes and in day to day use as fuel (Huang et al., 2019; Sarkar et al., 2005). About 1.7–8.8 million metric tons of oil are discharged to the global water resources each year (Megharaj et al., 2011). Incidents due to human failures and other activities such as deliberate disposal of PH wastes, account for more than 90% of oil pollution of water (Varjani and Upasani, 2017; Zhu et al., 2001). Petroleum hydrocarbon contaminated surface and subsurface soils are currently treated using chemical, physical or biological processes (Moliterni et al., 2012). Most commonly contaminated soils are, disposed to a landfill or incinerated. Both these processes are expensive, not very efficient and are generally unsustainable (Moliterni et al., 2012; Varjani, 2017). Biological treatments, including natural bioremediation and/ or enhanced bioremediation involve decomposing contaminants to nontoxic products by microbiological processes (Varjani and Upasani, 2017; Zhao et al., 2011). High molecular weight fractions of PHs from oil refinery sludge are particularly challenging to remediate (EPA, 2000; Varjani and Upasani, 2017). Remediating higher molecular weight fractions of PH from soils is slow, costly and is even more difficult for the most recalcitrant fractions of PH

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(Huang et al., 2005; Varjani and Upasani, 2017). Combining multiple remediation techniques, both in parallel or in serial combinations (ex, remediation train), can help overcome the limitations of individual techniques (Burd et al., 2000; CarrilloCastañeda et al., 2002; Huang et al., 2005). Furthermore, in severely contaminated soils the population of microorganisms is small so that remediation and plant growth are also suppressed (Baldwin, 1922; Megharaj et al., 2011; Siciliano and Germida, 1997). Pre-treating PH contaminated soils with bacteria that utilize PH enhances remediation (Megharaj et al., 2011; Ron and Rosenberg, 2014).

1.1. Fate of PH contaminants in the subsurface The soil acts like a chromatographic column for PH, separating individual constituents according to their physicochemical properties (IPCS, 1996). Contaminants have a tendency to migrate through soil until the pressure, gravity and capillary forces are balanced (Dror et al., 2001). Two hydrocarbon fractions can be defined in soil (Fuentes et al., 2014): One fraction remains irreversibly adsorbed to particles and is considered non-toxic and non-biodegradable because it is not bioavailable. The other fraction is bioavailable and is reversibly bound, and able to desorb and diffuse into the aqueous phase. However, “bioavailability” is inadequately defined, used ambiguously in the literature, and the concepts on which it is based are unclear (Ehlers and Luthy, 2003; Naidu et al., 2008; Semple et al., 2004). There are numerous definitions for bioavailability: (1) The degree to which a chemical in a potential source is free for uptake (into an organism) (Hamelink et al., 1994). (2) The extent to which a contaminant is available for biological conversion which is a function of the biological system and physicochemical properties of the contaminant and environmental factors (Juhasz and Naidu, 2000). (3) The portion of a chemical in a soil that can be utilized or transformed by living organisms (Semple et al., 2003) Bioavailability is closely related to the sorption and desorption kinetics between the soil and PH, and is dependent on soil properties. Hydrocarbons can associate with the soil matrix either through absorption or through adsorption (together termed “sorption”). The sorption of PH, as hydrophobic compounds, is mostly determined by the organic carbon in the soil and the octanol–water partition coefficient of the contaminant (Rodgers and Bunce, 2001). Recalcitrant hydrocarbons generally have large octanol–water coefficients and high soil organic-carbon water partition coefficients. These properties result in preferential partitioning of contaminants from the water phase to the organic matrix. Humic acids, the main organic chemical constituent of unpolluted soil, contain aromatic rings with carboxylic and phenolic surface groups, sugars and peptides (Cho et al., 2002) providing binding sites for hydrophobic regions for PH, hence reducing their bioavailability. Petroleum hydrocarbon contaminants retained in a soil for an extended period of time are exposed to weathering, photolytic loss, evaporation, hydrolysis, and biotransformation, and the residual contaminants tend to be resilient to further degradation (Pollard et al., 1994). Contaminants that are physically entrapped or sorbed form the residual phase in

soil. The residual phase can act as a continuing source of contaminant in soil or ground water. Hence, during the planning of any remediation programs it is important to consider contaminants in the residual phase. When aquifers are polluted with PH, pore-scale diffusion limitations increase the complexity of remediation (Li and Yu, 2015). They may either dissolve in the pore water or the water film surrounding soil particles, get trapped in the pores or fractures of rock as non-aqueous phase liquids (NAPL), float on the water table (for those less dense less than water), or get trapped under the water table (most of those with density greater than water and some of those with density less than water) Once a contaminant enters fractured rocks, it continues diffusing into the rock fractures or pores. This stored fraction of contaminants could readily dissolve into the groundwater when changes in the water table occur (Lerner et al., 2003).

2. Bioremediation of PH Bioremediation is seen as a green and cost-effective option due to its efficacy and cost-effectiveness (Speight and Arjoon, 2012) for remediating PH-impacted soils and groundwater. Bioremediation can lead to complete mineralization of organic contaminants by microorganisms, either by metabolism by bacteria for their growth and energy for cell maintenance or by transformation to benign compounds (Speight and Arjoon, 2012; Varjani, 2017). However, their significant advantage comes with limitations, and essential conditions need to be maintained for effectual remediation (Chandra et al., 2013; Varjani and Upasani, 2017) (Table 1). Bioremediation of PH contaminated soils has been investigated since the late 1940s, but the interest peaked after the Exxon Valdez oil spill in 1989 (Jackson and Pardue, 1999; Margesin and Schinner, 1997). Subsequently, there have been several studies where bioremediation was found to be effective for treating PH-contaminated sites (Liebeg and Cutright, 1999; Lima et al., 2009; Moliterni et al., 2012). A number of in situ or ex situ processes can be applied to bioremediation. The differences between implementing bioremediation by in situ or ex situ processes are compared in Table 2. In their review, Leahy and Colwell (1990) mentioned that the portion of the total heterotrophic community

Table 1 – Essential factors for microbial bioremediation (Speight and Arjoon, 2012). Factor Microbial population Oxygen Water Nutrients Temperature pH

Optimal conditions Appropriate species of organisms that can biodegrade the contaminants Sufficient to sustain aerobic biodegradation (~ 2% oxygen in gas phase or 0.4 mg/L in soil/water) Soil moisture should be between 50% and 70% of the water holding capacity of the soil Nitrogen, phosphorus, sulphur, etc., to support adequate microbial growth Suitable temperatures for microbial growth (0–40°C) Optimum range is between 6.5 and 7.5

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Table 2 – Differences between in situ and ex situ bioremediation (Speight and Arjoon, 2012) In situ bioremediation

Ex situ bioremediation

Do not require excavation of soil Causes little disturbance on site Dispersal of contaminants on site is minimized

Soils are excavated and transferred to a treatment plant Major disturbance is caused on site

Due to excavation procedures dispersal of contaminants is a concern Large volumes of soil can be Smaller volumes of soil are treated treated at once at a time Effective on sites with Can be applied to all soil types permeable soils Difficult to manage Can be easier to manage Slow remediation process Faster as conditions can be modulated in the treatment plant Can be used as both primary Cannot be used as primary and secondary treatments treatments as it is not preferred when physical and chemical remediation methods are employed as secondary treatments Low equipment and High equipment and operating operating costs costs

represented by hydrocarbon-utilizing bacteria in soil is inconsistent, with frequencies ranging from 0.13% (Jones et al., 1970) to 50% (Pinholt et al., 1979). This indicates that indigenous microorganisms at a contaminated site may have a wide-range of metabolic capability for bioremediation. The bacterial genus Pseudomonads (Gram negative and generally aerobes) are hydrocarbon degraders that are ubiquitous in soils. Some other important bacterial genera that degrade hydrocarbons are Pseudomonas, Burkholderia, Zymomonas, and Xanthomonas (Appendix A Table S1).

2.1. Bioremediation techniques Ranges of techniques for bioremediation are available (Appendix A Table S2). Bioremediation can have limited effectiveness due to several factors including: (1) Sites are generally contaminated with a mixture of contaminants, but no organism has the capability to degrade or tolerate them all (Huesemann et al., 2003). (2) High concentrations of contaminants may be inhibitory or lethal to bacteria (Admon et al., 2001). (3) Contaminants may bind to soil, which decreases their bioavailability (Guo et al., 2014; Kuppusamy et al., 2017) aggravated by poor contaminant solubility and pollutant “aging.” (4) Limited availability of nutrient and electron acceptors. (5) The microbial community is not homogeneously dispersed across the contaminated site (Guo et al., 2014). (6) Mass transfer of electron acceptors and nutrients to microorganisms responsible for biodegradation (Simoni et al., 2001). (7) Adaptation of the indigenous microorganisms for biodegradation of a particular contaminant (Mrozik and Piotrowska-Seget, 2010). (8) Build-up of toxic transformation products (Lorton et al., 2001; Pollard et al., 1994; Wackett and Hershberger, 2001). Although few synthetic compounds are entirely resistant to microbial digestion, environmental conditions greatly affect remediation (Thompson, 2002). It was identified in a

CLARINET (Contaminated LAnd Rehabilitation Network for Environmental Technologies) publication (Vik and Bardos, 2003) that available in situ and ex situ remediation technologies are ineffective when there is complex infrastructure blocking access to the contamination. Hence, there is a demand for a consistent in situ technology capable of remediating PH by overcoming problems of bioavailability, nutrient delivery and also the ability for optimization without direct access to the pollutant plume (McCarty, 1991). Electroremediation has the potential to create optimum conditions for the biodegradation of PH by indigenous microorganisms and to mobilize PH from under buildings within an acceptable period.

3. Electrokinetics Electrokinetics (EK) uses low electrical current between an anode and a cathode placed in soil. It was first performed by F.F. Reuss in 1809 (Elektorowicz and Boeva, 1996; Reuss, 1809). Since then, it has been used for dewatering clays, consolidation and stabilization of fine-grained soils through dewatering (Alshawabkeh et al., 2004; Station and Casagrande, 1947), insertion of metal grouts (Gray and Schlocker, 1969; Ozkan et al., 1999), geochemical exploration (Acar, 1992; Shmakin, 1985), metal extraction (Will, 1995), and controlling salt levels in agricultural land (Eid et al., 2000; Gibbs, 1966). Electrokinetics is a green technology that can be applied in situ and hence is inexpensive and non-disruptive. Another practical and important reason for the use of this technology is its efficiency in low-permeability soils. The applied electrical field can readily reach contaminants embedded deep in the subsurface, which other technologies are incapable of reaching (Reddy and Saichek, 2003; Schnarr et al., 1998). However, electrokinetics has several limitations and drawbacks. The factors governing the effectiveness of this technology are the soil type, the nature and extent of contamination, the presence of obstructions at the site (e.g., small stones, buried objects, fractured rocks, etc.) (Lageman, 1993), the strength of electrical field applied to the site, and the corrosion/erosion of the electrodes (Van Cauwenberghe, 1997) when iron electrodes are used. Another common phenomenon is accumulation of calcium in the form of bicarbonates or hydroxides near the electrode that can significantly decrease remediation efficiency (Acar et al., 1995; Maini et al., 2000).

3.1. Electrokinetic remediation process Contaminants occur as charged states that can move towards electrodes when an external electrical field is applied. Heavy metals were the first contaminants to be remediated with electrokinetics (Hamnett, 1980; Segall et al., 1980). This technology has also been developed for the remediation of other inorganic and organic contaminants, such as PH. The remediation of contaminants by electrokinetic processes may take place and be involved with the following phenomena (Fig. 1 and Appendix A Table S3).

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Fig. 1 – Schematic representation of the different Electrokinetic phenomenon

3.2. Electrokinetic remediation technologies A range of technologies has been developed to remediate contaminants by electrokinetics. Most have been used for the remediation of metals from subsurface soil by electrolysis; however, some have been modified for the effective remediation of PH.

jump is unfavorable for microbial growth and impedes the remediation efficiency. Diverse methods have been developed to prevent the pH jump, such as injecting solutions (Gonzini et al., 2010; Kim and Lee, 1999), fixing the cathode and having approaching anodes (Li et al., 2012; Shen et al., 2007), electrolyte circulation (Chang and Liao, 2006; Lee and Yang, 2000), and nonuniform electrokinetics (Cang et al., 2009; Luo et al., 2005b).

3.2.1. Conventional anode–cathode

3.2.2. Alternative anode and cathode approach

Fluid, usually water, is applied to soils for the effective application of electrokinetics. The fluid enables movement of the contaminants towards the electrodes by electroosmosis. However, the emission of hydrogen at the cathode leads to liberation of OH−, which increases pH and the precipitation of cations nearby. In order to prevent alkaline precipitation around cathodes several improvements have been proposed and developed: (1) addition of strong complexing agents such as ammonia, citrate, oxalate, acetic acid, and EDTA into soil, which compete with soil for cationic heavy metals and prevent precipitation near cathodes (Ottosen et al., 2005; Reddy and Chinthamreddy, 2003; Zhou et al., 2004); (2) conditioning the anode and/or the cathode reservoirs, or use of an ion exchange membrane (IEM) to control the pH and zeta potential, and also enhance contaminant mobility (Desharnais and Lewis, 2002; Li et al., 1998). IEM-enhanced EK operates similar to electrodialysis (Ottosen et al., 1997). Even with these improvements, the conventional anode– cathode method has apparent drawbacks. It may take a number of days to even a few years to remediate soil contamination and also involves significant amounts of energy (Virkutyte et al., 2002), significantly limiting its application. In addition, altered pH zones are created at both electrodes, which over time greatly affects treatment efficiency. Electrolysis generates an acidic front zone and a base zone near the respective electrodes. When these zones approach each other to meet mid-way they cause a pH jump. This pH

In the equipment shown in Fig. 2, a constant electrical potential is applied using a DC power supply and the polarity is reversed after a set period. The results from such set-ups indicated that polarity reversal maintained suitable soil conditions for bacterial growth including higher electrical intensity and higher dissolved oxygen concentrations near the anode. It also enhanced the distribution of nutrients (Li et al., 2016). Polarity reversal helps to neutralize soil pH and enhances bioremediation of organic-contaminated soils (Huang et al., 2013; Li et al., 2015). Polarity reversal could also increase the temperature, bioavailability and transformation of organic contaminants (Luo et al., 2006; Luo et al., 2005a).

3.2.3. Two-anode technique (TAT) Two-anode technique employs a secondary anode that generates hydrogen ions reducing the distance advanced by the basic front. It is less labor intensive modification. Both the primary and secondary anodes are coupled to one cathode. The secondary anode is positioned between the primary anode and cathode (Fig. 3). Hydrogen ions generated by the secondary anode move towards the cathode by electroosmotic and electromigration. During this movement the hydrogen ions lower the pH near the secondary anode and react with the hydroxyl ions generated by the cathode to form water. Therefore, the distance between the secondary anode and the primary cathode influences the outcome of TAT. Positioning the secondary anode closer to the cathode

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Fig. 2 – Schematic diagram of Electrokinetic set-up with polarity reversal (Li et al., 2016).

produced better results. The electricity required for electrokinetic remediation can be generated by solar panels, making the process even more environmentally friendly.

3.2.4. Approaching anode electrokinesis (AA-EK) As the name suggests, in AA-EK the anode in the system is not fixed but sequentially approaches a cathode that is fixed

(Fig. 4). The number of anodes required depends on the contaminant concentration, site geology, and duration of the treatment. Each anode is kept equidistant. The switchover time from one anode to the other is adjustable but most of time kept constant. As the anode approaches the cathode, an acidic pH front moves towards the cathode. The movement of the front is much faster than the conventional acidic pH

Fig. 3 – Schematic of electrokinetic remediation cell (Hassan et al., 2015).

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Fig. 4 – Schematic representation of EK-AA (Zhang et al., 2014 )

approach in soil when a single anode – cathode system is applied. Additionally, the amount of energy consumed by the set-up is directly proportional to the distance between anode and cathode (Shen et al., 2007).

3.2.5. Approaching cathode electrokinesis (AC-EK) Similar to the approaching anode modification of the conventional EK set-up, the approaching cathode method was developed for anionic pollutants. Hydrogen emission generates an alkaline environment around the cathode. Under alkaline condition some contaminants are efficiently desorbed from soil (Arnesen and Krogstad, 1998). Rather than an array of anodes, approaching cathodes were placed equidistant from the cathode compartment (Fig. 5). They were switched on sequentially after a pre-defined time. Since desorption was driven by the alkaline environment around the approaching cathode, remediation of anionic contaminants was enhanced when compared with a traditional setup. In addition, the energy consumption was lower than the conventional set-up.

4. Coupled approach

electrokinetic

and

bioremediation

Bioremediation of contaminated soils is limited by the bioavailabity of contaminants, especially in clayey soil (Guo

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et al., 2014; Kuppusamy et al., 2017). Electrokinetics (EK) is a technique that can be used to overcome these kinds of limitations to bioremediation. EK augments bioremediation by making nutrients, electron acceptors (EAs) and electron donors (EDs) more bioavailable to catabolically active microorganisms (Gomes et al., 2012; LUKAs, 2009) (Fig. 6). During initial stages of remediation redox reactions of dissolved ions in pore water produces electric current. The electric current then decreases over time under constant polarity due to depletion or diffusion limitations. As time elapses the concentration of ions in pore water are decreased as they migrate and precipitate (Wei et al., 2016). The electric field intensity has recently been reported to be an important factor for bacterial growth (Huang et al., 2013; Li et al., 2013, 2010). Electrolysis produces dissolved oxygen at the anode and promotes growth of aerobic bacteria (Alshawabkeh et al., 1997). Soil properties, including pH and moisture content, are critical for organic contaminant degradation by both electrolysis and microbial activity (Hamed and Bhadra, 1997). Electrokinetics with polarity reversal (EK-PR) has been combined with bioremediation (Huang et al., 2013; Kim et al., 2005a; Li et al., 2013). When employed together there is an increase in temperature, bioavailability and transformation of organic contaminants without creating extreme zones of pH around electrodes (Luo et al., 2006, 2005b). Additionally, polarity reversal facilitates mixing of bacteria and nutrients in soils (Shi et al., 2008). Consequently, it may also enhance aerobic microbial activity by oxygen production at anodes (Rabbi et al., 2000). In terms of EK-biotechnology, recent research shows that remediation of organic contaminants not only depends on biodegradation, but also on induced stimulation by applied electricity (Li et al., 2013). In total, the benefits of employing EK-biotechniques include: increasing diversity and connectivity of flow-paths for contact and mixing between microorganisms and contaminants (Luo et al., 2006); maintaining uniform pH and moisture conditions (Fan et al., 2007); maintaining a uniform microorganism distribution (Huang et al., 2013), and increasing the area over which the electric field and enhanced biodegradation are effective (Gill et al., 2014). Guo et al. (2014) used a set-up consisting of a perspex soil chamber (100 × 100 × 25 cm), 25 cylindrical graphite electrodes (20 × 1 cm) distributed into a matrix in a soil reactor

Fig. 5 – Schematic representation of AC-EK (Zhou et al., 2014)

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Fig. 6 – Conceptual model of electrokinetic processes that can enhance the biodegradation of contaminants. Expanded bubbles detail the abiotic and biotic effects of electrokinetic bioremediation (Gill et al., 2014).

(Fig. 7). The control apparatus was capable of reversing the polarity of the electric field rotationally by each row and column in turn at an optional interval, thus generating a 2-D electric field. They carried out four different experiments: BIO–EK, EK only, BIO only, and a control. The polarity reversal of the electrodes was changed at regular intervals. In the BIO– EK and BIO tests, the degrading-bacteria suspension was mixed into the petroleum-contaminated soil (Guo et al., 2014). At the end of the experimentation, they found out that the maximum PH remediation was achieved in the BIO–EK test.

4.1. Electrokinetic (EK) bioattenuation Bioattenuation is a low impact and inexpensive remediation technique, however it can only be successfully employed at sites where adequate mixing of contaminants, electron acceptors and microorganisms occurs (Rivett and Thornton,

2008). Electrokinetics can enhance natural mixing and can be optimized by the increasing number of electrodes, polarity reversal, and radial configuration of electrodes (Harbottle et al., 2009; Luo et al., 2006; Yuan et al., 2013). When EK is applied in conjunction with bioattenuation, the overall rate of remediation is increased.

4.2. Electrokinetic (EK) biostimulation Various studies have shown that EK can be employed for enhancing the delivery of nutrients (e.g., phosphate (Lee et al., 2007)), electron acceptors (e.g., nitrate and sulphate (Lohner et al., 2008) and electron donors (e.g., lactate (Wu et al., 2007)). EK can therefore benefit biostimulation and enhance in situ remediation. EK-biostimulation in soils has been successfully employed for PCE (Mao et al., 2012; Wu et al., 2012), toluene (Tiehm et al., 2010), diesel (Pazos et al., 2012) and PAHs (Xu et al., 2010) remediation.

Fig. 7 – Schematic diagram of the set-up used by Guo et al. (2014)

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Contaminants can readily diffuse into soil or sediment matrices at low permeability areas and at particular sites due to the free-energy-favorite process. Once they diffuse into the matrices, they are trapped because the outward diffusion is slow due to the free-energy-unfavorable process. Contaminants trapped within the soil matrix become inaccessible and difficult to remediate. They also pose a recontamination risk over long periods as they can diffuse back into the accessible site (Reynolds and Kueper, 2002), which would extend the clean-up time. At low permeability sites, EK-biostimulation can stimulate bioremediation. When a current is applied to a bacterial cell hydrophilic pores are produced in the cell wall. The transfer rate of nutrients into bacterial cells could be increased via these pores (Weaver and Chizmadzhev, 1996). Luo et al. (2005a) studied the effects of direct current (DC) on bacteria in an aqueous system and found that when phenol-degrading microorganisms were exposed to 20 mA that the hydrophobicity increased. Whereas 40 mA of current increased the net negative charge of the cell surface. This was an important discovery since increasing the net negative charge on the bacterial cell surface could in turn lead to increased ability to mobilize bacteria towards an anode by electromigration, overcoming the sorption forces of the bacterial cells to the soil matrix.

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surfaces. DeFlaun and Condee (1997) showed that, by using electromigration it was possible to disperse bacteria, and to some extent induce electroosmosis as well. Wick et al. (2004) showed that electroosmosis played a vital role in the movement of non-motile bacteria. They were able to move up to 90% of non-motile bacteria by electroosmosis compared to only 0%–20% by electrophoresis. By maintaining a near neutral pH in the sub-surface electrokinetics can efficiently increase interaction between bacterial cells, the contaminant plume, and rate limiting nutrients, all of which benefit remediation (Kim et al., 2005b; Suni and Romantschuk, 2004).

4.4. Electrokinetic soil flushing (EKSF) Electrokinetic soil flushing (EKSF) is one of the earliest technologies developed for the remediation of contaminated soil. In contrast to the approaches described above it flushes contaminants from the soil matrix rather than degrade them. It is an in situ technique and not very cost effective. Initially, EKSF was employed as a technology on its own. Mena Ramirez et al. (2015) made several modifications to EKSF to improve its efficiency for soil remediation (Fig. 8, Appendix A Table S4).

4.3. Electrokinetic (EK) bioaugmentation

5. Surfactants During bioaugmentation, microorganisms added to contaminated sites may not be effective due to the low permeability of the subsurface strata. EK enhances the movement of microorganisms through such low permeability soils (Mao et al., 2012; Mena et al., 2012; Wick et al., 2004), despite cells being obstructed by small pore sizes in finegrained materials (DeFlaun and Condee, 1997). Such enhancement is possible due to electroosmotic flow which causes preferential movement of microorganisms along a flow path through macro-pores within the soil (Wick et al., 2004). The membrane integrity of microbes is sustained (Shi et al., 2008) and so is its functionality during movement by EK. In their study, Mao et al. (2012) first distributed lactate by EK and then for subsequent remediation moved a dechlorinating bacterial strain of Dehalococcoides through PCE-contaminated soil. EK-Bioaugmentation can also be performed as a pre-treatment for remediating heavy metals contaminated soils by efficiently redistributing bacteria (Lee and Kim, 2010). Microorganisms have a strong affinity to attach to sediment and organic matter, thus disrupting their movement (Mrozik and Piotrowska-Seget, 2010). This can be diminished by using surfactants and applying EK methods (Wick et al., 2004). Ensuring the survival of endogenous microbes introduced into a foreign environment is difficult (Megharaj et al., 2011), and substituting active bacteria with endospores could be researched. With high surface charges, endospores are robust and under EK travel faster than bacterial cells (Da Rocha et al., 2009). There has been a lot of interest on how electrokinetics can be efficiently used to bioaugment contaminated sub-

Surfactants are a group of amphiphilic chemicals that contain hydrophilic and hydrophobic parts in the same molecular structure (Mao et al., 2015). Surfactants have been widely used for industrial applications as adhesives, flocculating agents, de-emulsifiers, foaming agents and penetrating agents (Mulligan and Gibbs, 1993). They are also very widely employed in the petroleum industry since they are known to enhance the solubility of petroleum components (Falatko and Novak, 1992). This property led to their application in remediation to mobilize contaminants by increasing the solubility of contaminants (Abdul et al., 1992; Ellis et al., 1985; Johnson et al., 2004). Surfactants can be divided into two main types depending upon their source – synthetic or biological. They are also classified by the nature of the hydrophilic groups into ionic, non-ionic, and gemini surfactants. The ionic surfactants are classified further as anionic, cationic or zwitterionic surfactants (Paria, 2008; Rosen, 2004).

5.1. Surfactants enhanced in situ remediation Bioremediation of PH has been reported as an efficient, cheap, and eco-friendly technology (Alexander, 1999; Madigan and Martinko, 2004; Rittmann and de Roda, 2001). However, bioavailability is a limiting factor (Guha and Jaffé, 1996; Mihelcic et al., 1993; Volkering et al., 1997). Adding surfactants can increase the dissolution and bioavailability of hydrophobic contaminants in the aqueous phase (Mulligan, 2005). To overcome low bioavailability bacteria can get access to hydrophobic substrates by reducing surface tension or by direct contact with hydrophobic droplet (Wentzel et al., 2007)

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Power supply Electrolite

Multimeter

10 cm

Level Control

Collector Compartment

Collector Compartment 10 cm

Polluted Soil

20 cm anodic Compartment

Graphite electrode Anode

Graphite electrode Cathode

Electroosmotic volume

Cathodic Compartment

Fig. 8 – Experimental set for EKSF (Ramírez et al., 2015)

(Fig. 9). Surface tension reduction can be achieved by the secretion of surfactants, molecules that disperse hydrocarbons into small droplets (Baboshin and Golovleva, 2012). In close contact to bacteria, hydrocarbon molecules enter the bacterial cell, where the catabolic machinery achieves their breakdown (Fuentes et al., 2014). EK enhances contact between surfactants and bound contaminants at the micro-scale and can deliver surfactants to polluted zones at the plume-scale (Saichek and Reddy, 2003). Variation in EK properties such as field strength and polarity reversals had limited effects on biodegradation the

Fig. 9 – Schematic representation of the surfactant enhanced remediation (Mao et al., 2015).

presence of a surfactant (Gonzini et al., 2010; Niqui-Arroyo and Ortega-Calvo, 2010). Such enhanced biodegradation has been demonstrated with synthetic surfactants (Saichek and Reddy, 2003), biosurfactants (Gonzini et al., 2010), cosolvents (Gómez et al., 2010) and cyclodextrin (Ko et al., 2000).

6. Concluding remarks It is evident from this review that despite the development of technology and nearly 75 years of remedial work, that remediation of subsurface contaminated media has been challenging and often difficult. As a consequence, < 5%–10% of contaminated sites have been remediated (Naidu, 2013). While natural attenuation is seen as a cost-effective remediation option, extension of this approach to subsurface remediation is still seen as a challenge. Common constraints include: bioavailability of contaminants; toxicity to microorganisms when the concentration of contaminant is high, especially, towards the centre of the plume; limited availability of growth limiting nutrients, unavailability of essential microbes for bioremediation, technical impractibility; and uncertainties relating to endpoints for remediation The difficulty of remediating subsurface contamination increases further in fractured rocks. The contaminant is able to lodge in cracks and fissures of rock, so that complete remediation at such sites is not possible. In addition, the sequestered contaminants can desorb into the soil and groundwater to cause contamination of the site in the future. Many researchers have proven successful remediation at their respective contaminated sites but few checked what became of the contaminants after they were remediated or

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bioremediated in situ. It is an important question to determine if the contaminant after remediation or bioremediation is transformed into some other harmful persistent contaminant. If the original contaminant is converted into a persistent contaminant it is not only a waste of resources for the remediation procedures applied but also adds another level of difficulty for remediation of the new persistent contaminant. Another provocative question arises when we encounter differing soil types. Every contaminated site has differing soil characteristics, and as most of the technologies are not onesize-fits-all remediation techniques, they cannot be successfully applied at any particular site. For effective remediation of PH the existing technologies needs be enhanced above currently available mechanical or microbial processes (Huang et al., 2005). Hence, there is an urgent need to develop an efficient and cheap in situ remediation technology, which could perform the subsurface remediation at any contaminated site with minimal disturbance or destruction to the natural environment at the site. Bioremediation is the most effective way forward to remediate a wide variety of contaminants. Because of its low cost and being an in situ technology, it ticks almost all the boxes a remediation technology should. However, as mentioned in the review, bioremediation becomes incapable of performing its job when the limitations over power at a given contaminated site. These limitations could be overcome by the use of electrokinetics. Hence, upon further research and development the combined use of EK-bioremediation technologies could provide the world with a relatively fast, cheap and non-intrusive remediation technology. In saying that both the technologies have their own drawbacks which need to be overcome to develop it into an extremely successful remediation technology. The first constraint is maintenance of optimal conditions for the microbial growth and proliferation. Since most contaminated sites are contaminated with contaminant mixtures, the efficiency of this technology to effectively remediate the mixtures should be further studied. There are very few field studies that have been carried out by this technology, and translating the bench studies into the field is an added complexity as variables in the lab can be controlled unlike in the field study. A big variable will be the positioning, type (material) and number of electrodes that are required for effective remediation in the field. To make it even more successful, there should be some focus of combining it with some of the successfully employed field technologies.

Acknowledgements This work was funded through Cooperative Research Center for Contamination Assessment and Remediation of the Environment (CRC CARE) and the University of Newcastle.

Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jes.2019.08.010.

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