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Remediation of highly fuel oil-contaminated soil by food waste composting and its volatile organic compound (VOC) emission
Accepted Manuscript Remediation of highly fuel oil-contaminated soil by food waste composting and its volatile organic compound (VOC) emission
Huu-Tuan Tran, Chi-Thanh Vu, Chitsan Lin, Xuan-Thanh Bui, Wen-Yen Huang, Thi-Dieu-Hien Vo, Hong-Giang Hoang, Wen-Yi Liu PII: DOI: Reference:
S2589-014X(18)30108-7 doi:10.1016/j.biteb.2018.10.010 BITEB 107
To appear in:
Bioresource Technology Reports
Received date: Revised date: Accepted date:
24 August 2018 21 October 2018 23 October 2018
Please cite this article as: Huu-Tuan Tran, Chi-Thanh Vu, Chitsan Lin, Xuan-Thanh Bui, Wen-Yen Huang, Thi-Dieu-Hien Vo, Hong-Giang Hoang, Wen-Yi Liu , Remediation of highly fuel oil-contaminated soil by food waste composting and its volatile organic compound (VOC) emission. Biteb (2018), doi:10.1016/j.biteb.2018.10.010
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ACCEPTED MANUSCRIPT Remediation of highly fuel oil-contaminated soil by food waste composting and its volatile organic compound (VOC) emission Huu-Tuan Tran1#, Chi-Thanh Vu2#, Chitsan Lin1*, Xuan-Thanh Bui3, Wen-Yen Huang1, Thi-
Institute of Marine Science and Technology, National Kaohsiung University of Science and
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Dieu-Hien Vo1, Hong-Giang Hoang1, Wen-Yi Liu1
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Technology, Kaohsiung 81157, Taiwan.
Civil and Environmental Engineering Department, University of Alabama in Huntsville,
Faculty of Environment and Natural Resources, University of Technology, Vietnam National
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Huntsville, AL 35899, USA.
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University - Ho Chi Minh City, Vietnam.
These authors contributed equally to this work
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Corresponding author: Tel: +886-7-3651472; Fax: +886-7-3651472; E-mail:
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#
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[email protected] (C. Lin).
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ACCEPTED MANUSCRIPT Abstract Food waste composting was used to treat highly fuel oil-contaminated soil (two particle size fractions - fine < 0.3 cm and coarse > 1.3 cm). The initial fuel oil concentrations in the fine and coarse piles were 12,000 and 11,850 mg/kg, respectively. After 45-day incubation, the
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removal efficiencies of the fine and coarse piles were 82% and 93%, respectively. Residual fuel
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oil concentration (847 mg/kg) in the coarse pile met the Taiwan EPA standard limit (1,000 mg/kg). The results indicated that the soil particle size fraction affected the efficiency of the
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composting treatment. Besides, emissions of oxygenated volatile organic compounds (OVOCs) and BTEXs from the food waste composting treatment were investigated. Ketones and toluene
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were the most abundant species of OVOCs and BTEXs, respectively. This study demonstrated
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that food waste composting would be a promising treatment technology for highly fuel oil-
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contaminated soil.
Keywords: Food waste composting; Highly fuel oil-contaminated soil; Oxygenated volatile
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organic compound (OVOCs); BTEXs.
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ACCEPTED MANUSCRIPT 1. Introduction Petroleum contaminants are widespread in the soil and the remediation of petroleumcontaminated soil has attracted wide attention from scientific community. Petroleum products include gasoline, fuel oil, diesel and/or lubricants, which are released to the environment through
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leaking storage tank, industrial runoff, spills or commercial activities, construction activities and
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agriculture (Adedokun et al., 2007; Zappi et al., 2017). In Taiwan, approximately 400 gas stations have been found contaminated with petroleum hydrocarbons (Tsai et al., 2009).
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Petroleum hydrocarbons are highly toxic and carcinogenic substances, posing negative effects to
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the ecosystem and human health (Asgari et al., 2017).
Fuel oil is a type of petroleum hydrocarbons, which the main characteristics are low
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volatility, high viscosity, and low mobility, making fuel oil difficult to treat compared to other
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petroleum hydrocarbons such as gasoline and diesel fuel (Tsai et al., 2009). Many treatment methods have been developed to address petroleum hydrocarbon concentrations in soil such as
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composting (Lin et al., 2012), electrochemical processes (Huguenot et al., 2015), thermal
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desorption (Yi et al., 2016), etc. Among these treatment technologies, composting is not only easy to operate and cost-effective, but also environmentally friendly (Tradler et al., 2018).
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Composting uses microbial metabolism in the presence of optimum environmental conditions and sufficient nutrients to break down organic contaminants. In composting process, physical and chemical parameters, i.e. temperature, dissolved oxygen nutrients, C/N ratio, organic matter content and moisture need to be monitored and adjusted for improving the growth of microbial communities as well as the degradation of contaminants. The most common materials used for composting are biodegradable organic waste such as food waste (Joo et al., 2008), yard waste (Tan et al., 2017) and manures (Jiang et al., 2015). Among them, food waste has the most 3
ACCEPTED MANUSCRIPT potential to be widely applied for bioremediation purposes owing to its potentially unlimited supply from restaurants, hotels, cafeterias, household kitchens, etc. The major components of food waste include rice, meat, noodles, fruit and vegetable waste, which can be generated in the processes of harvesting, transportation, storage, processing and merchandising (Shen et al., 2013;
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Zhang et al., 2018). In Taiwan, approximately 2.3 million tons of food waste are discharged
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every year (Thi et al., 2015). This is an abundant food waste source that can be used for
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composting bioremediation. In food waste composting, different materials, e.g. sawdust (used to adjust the moisture content) and/or mature compost (used enhance the density and diversity of
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microorganisms), are mixed together in order to optimize the composting conditions. Studies on
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employing composting process for the treatment of highly petroleum-contaminated soil with up to 85% - 96% degradation efficiencies are available in the literature (Lin et al., 2012; Franco et
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al., 2014; Zappi et al., 2017). However, there is very little research on composting treatment of
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fuel oil-contaminated soil.
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The limitation of the composting process is the release of large quantities of volatile organic compounds (VOCs), which are toxic to humans and negatively affect the environment;
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therefore, they can be considered a source of secondary pollution (Franco et al., 2015). Among
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VOCs, oxygenated volatile organic compounds (OVOCs) have received special attention because they are the main component (46-83%) of the total VOCs emitted from the composting process (Gray et al., 2010; Kumar et al., 2011). OVOCs play important roles in tropospheric chemistry, influence the ground level ozone formation and significantly contribute to the formation of secondary organic aerosols (Kumar et al., 2011). Ethanol, acetone, butanone, acetaldehyde are the most abundant species in the total OVOCs released from the composting of organic waste (Wu and Wang, 2015). OVOCs emitted from orange fruit waste (Wu and Wang, 4
ACCEPTED MANUSCRIPT 2015), garden waste (Kumar et al., 2011) and municipal solid waste (Tan et al., 2017) have been investigated during aerobic composting process. Apart from OVOCs, recent studies have also shown that BTEX compounds (benzene, toluene, ethylbenzene, xylenes) are the second main component (7-10%) of the total VOCs emitted from the composting process of petroleum
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hydrocarbon-contaminated soil (Chang et al., 2010). BTEX are highly toxic and exposure to
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them can cause different harmful effects to human health. BTEX are also considered as
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carcinogenic substances.
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Therefore, the main objectives of this study were to investigate the removal efficiency of fuel oil from the contaminated soil using aerobic food waste composting, evaluate the influence
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of composting parameters such as temperature, pH and oxygen content on the removal efficiency
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and analyze fuel oil degradation using kinetic modelling. Additionally, OVOC and BTEX
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emissions from different stages of the composting process was characterized and discussed. 2. Materials and methods
Fuel-oil contaminated soil
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2.1.
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In this study, fuel oil-contaminated soil was achieved from a former gas station in Kaohsiung city, southern Taiwan. The soil was excavated from a previous fuel storage site,
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which was contaminated by the transport and storage of fuel oil. The soil was slightly alkaline and characterized to be a sandy loam texture according to the classification of the United States Department of Agriculture (USDA, 1993). Soil screening was performed according to the method issued by the American Society for Testing and Materials (ASTM, 2013). Because the bulk of fuel oil content in the contaminated soil was mainly found in the fine (size < 0.3 cm) and coarse (size > 1.3 cm) fractions (data not shown), these two particle sizes of the soil were
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ACCEPTED MANUSCRIPT selected for the composting experiments. These piles were mixed homogenously, oven-dried at 40oC, stored in polyethylene bottles at room temperature until the experiment. The initial average fuel oil concentrations in both the fine and coarse piles were approximately 12,000 mg/kg and 11,850 mg/kg respectively, which are about twelve times higher than the regulatory limit issued
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by Taiwan EPA (Environmental Protection Administration, 2005) (1,000 mg/kg) . The moisture
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content and pH of the fine and coarse piles were 65% and 45%, and 7.9 and 8.7 respectively,
Compost pile composition
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2.2.
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which were within suitable ranges for biological treatment (Lin et al., 2012; Zappi et al., 2017).
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The materials used for composting treatment of the contaminated soil were food waste, sawdust and mature compost. Food waste used for the composting in this study contained
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discarded dairy products, grains, bread, fruits, vegetables, meat scraps, seafood and kitchen
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waste, which were collected from restaurants and schools in Kaohsiung City. The food waste ingredient composition was selected to be 50% of meat scraps and seafood, 35% of vegetables,
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and 15% of others, which was reported suitable for bacterial growth during the composting
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process in our previous study (Lin et al., 2012). Fresh food waste was shredded into fractions of less than 2 cm diameter before the experiment. Sawdust was used to adjust the moisture content.
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Mature compost was added to enrich the microbial population considered beneficial for soil bioremediation. Contaminated fine and coarse soil fractions were separated into two compost piles (from now addressed as fine pile and coarse pile) and mixed with the compost materials at a dry weight ratio shown in Table 1. All those materials were manually mixed to create homogenous fine and coarse compost piles. Water was added to maintain 35-40% moisture, which is considered beneficial for microbial fermentation (Lin, 2008; Lin et al., 2012). All the
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ACCEPTED MANUSCRIPT composting experiments were conducted in the compost facility of National Kaohsiung University of Science and Technology, Taiwan. Table 1. Characteristics of two compost piles (fine pile and coarse pile) of highly fuel oil-
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contaminated soil Moisture
pH
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Dry weight (kg)
Saw
Mature
Contaminated
waste
dust
compost
soil
Fine pile
80
50
20
150
Coarse pile
80
50
20
Total
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Food
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content (%)
65
7.9
300
45
8.7
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150
300
Experimental operation and sampling
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2.3.
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The compost piles were incubated under aerobic condition for 45 days. During the
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incubation, the moisture content in two piles was controlled at 45% by adding water. Daily operation included temperature and oxygen content measurements, and homogenously mixing.
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Temperature was measured at 3 sampling points (top, center and bottom) of each pile for the representative value using a TES 1310 K-type digital thermometer. Oxygen content was collected using a gas collector, which was plugged down 20 cm deep into each pile. Then, a handheld multi-function gas-meter (ISC M40 Multi-Gas Monitor) was connected to the gas collector to measure oxygen content. After that, compost samples were taken to examine pH, humidity and periodically fuel oil concentrations. Samples were taken at five locations (at four corners and the center) at 15 to 20 cm deep in each pile, mixed well for a representative sample 7
ACCEPTED MANUSCRIPT and then kept in the zip-lock PE bags at 4oC until the analysis. The pH and humidity were evaluated daily. Based on the temperature profiles, the fuel oil concentration in the soil was measured continuously every day for the first two weeks. After the first two weeks, the fuel oil concentration in the soil was measured once a week until finishing the composting process. Also,
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monitored natural attenuation (MNA) approach was employed so that after the composting
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finished (on day 46), a sample from each pile was taken on day 100 for fuel oil analysis. Gaseous
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VOCs samples were collected on the 2th, 4th, 6th, 8th, 10th, 12th, 14th, 16th, 18th, 20th, 30th and 45th days of the experiment. Gaseous VOCs were collected using the 1L Teflon-lined sampling bags
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(SKC Inc., USA), which were connected with the gas collector (used for the oxygen content
Analytical methods
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2.4.
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measurement described above).
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During 45 days of composting incubation, variations of temperature, moisture, oxygen content, and pH were monitored. A GasTech portable multigas (ISC M40 gas detector) was used
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to analyze oxygen content. The temperature was measured by a TES 1310 K-type digital
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thermometer equipped with a 1.2 m probe (± 0.1oC sensitivity). The moisture content was measured using the standard method NIEA R203.02C issues by the Taiwan Environmental
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Protection Administration (TEPA,2009); pH was determined according to the USEPA method 9045C (USEPA, 1995). Since there is currently no specific method for the analysis of fuel oil, modified analytical method of total petroleum hydrocarbon (TPH) was employed for the quantification of the heavy fuel oil. Concentrations of fuel oil were measured using a gas chromatography/flame ionization detector (GC/FID). The Soxhlet extraction method using dichloromethane (DCM) was employed
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ACCEPTED MANUSCRIPT using the Taiwan NIEA method M165.00C. The details of extraction and analysis methods were described in our previous study (Lin et al., 2012). Briefly, 10 g of compost sample was mixed with 4 g sodium sulfate for dehydration, which was then added with 10 mL of DCM and shaken by a vibration machine for 1h. Then, the supernatant was collected and purified by filtering with
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acid silica gel. For GC/FID analysis, the injector inlet temperature and the detector temperature
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were both 340oC. The oven temperature was programmed to increase from 50oC to 340oC at
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10oC/ min and stayed there for 34 min. Fuel oil was quantified a range from C6 to C40.
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The analytical method employed for the identification and quantification of emitted OVOCs and BTEX (benzene, toluene, ethylbenzene, xylenes) in this study was TO-15 (USEPA,
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1999). The OVOCs were divided into 3 groups, ketones (acetone, 2-butanone, 4-methyl 2-
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pentanone and 2-hexanone), esters (2-methoxy-2-methyl-propane, vinyl acetate, ethyl acetate), aldehydes (acrolein). Details of the method are given in the Supporting Information. Briefly, a
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DB-VRX capillary column (60 m x 250 mm x 1.4 mm, Agilent Technologies, USA) was used
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with helium as carrier gas to separate the target compounds. The emitted OVOC and BTEX samples were analyzed using a gas chromatography-mass spectrometry (GC/MS) system
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(6890/5973, Agilent Technologies, Santa Clara, USA) coupled with an Entech 7100A pre-
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concentrator (Entech Instruments Inc., Simi Vally, USA). Kinetic analysis of fuel oil degradation
Kinetic modelling was used to assess the biodegradation of fuel oil-contaminated soil. In the previous studies, first-order kinetics was used to describe the biodegradation rate of motor oil (Abdulyekeen et al., 2016) and lubricating oil (Lee et al., 2007) in contaminated soil. In this study, kinetic analysis (first and second-order kinetic models) was employed to investigate the
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ACCEPTED MANUSCRIPT degradation of fuel oil. The mathematical models of first and second-order kinetics were described by the following equations: 𝐿𝑛𝐶𝑡 = 𝐿𝑛𝐶0 − 𝑘1 𝑡 (first-order kinetics) 1
= 𝑘2 𝑡 +
1 𝐶0
(second-order kinetics)
Eq. (2.2)
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𝐶𝑡
Eq. (2.1)
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where C0 is the initial concentration of fuel oil, Ct is the concentration of fuel oil at time t, t is time (days), k1 and k2 are first and second order rate constants, respectively. Statistical analysis
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2.6.
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Pearson’s correlation test was performed to analyze the correlation between temperature, moisture content, and pH profiles. The differences were considered significantly based on 95%
analyses.
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3. Results and discussion
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confidence interval. SPSS statistics for Windows (version 20) was used for all statistical
Variation profiles of temperature, oxygen content and pH of the composting of
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highly fuel oil-contaminated soil
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The composting process was divided three stage, thermophilic stage (day 1 to day 14), mesophilic stage (day 15 to day 30), maturation stage (day 15 to day 45). Fig. 1a shows the temperature profiles of the composting piles of the fine and coarse piles (from now addressed as the fine and coarse piles, respectively). The temperature of the two piles increased rapidly to the thermophilic stage (> 60oC) on the 5th day, indicating that bioactivity was not hindered and limited by the high fuel oil content (approximately 12,000 mg/kg). The temperature reached its highest value at 77oC on the 7th day. Then, it remained at 70oC until the 10th day (thermophilic 10
ACCEPTED MANUSCRIPT stage). After that, the temperature declined slowly during the mesophilic stage to ambient condition on the 30th day (maturation stage), indicating that the composting process had finished. The variation trend of temperature profiles in this study was similar to previous studies on food waste composting (Lin et al., 2012; Pereira et al., 2018).
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During the thermophilic stage (from the 8th day to the 14th day), due to the higher
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moisture content of the fine pile, the temperature of the fine pile was higher than that of the coarse pile. Similar observation was also reported in the municipal solid waste composting study
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of Zhang et al. (2013). In the thermophilic stage, the temperature stayed at 45-70oC, which was proven beneficial for the development of microorganisms (Lin et al., 2012). This development
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further facilitated the degradation of the high fuel oil concentration. High temperature was
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reported not only to increase the solubility but also to decrease the viscosity of hydrocarbon pollutants and transfer long-chain n-alkanes from solid phase to liquid phase, improving
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degradation rates of fuel oil (Aislabie et al., 2006). Therefore, the temperature of 45-70oC
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occurring the thermophilic stage of this study should favor microbial degradation of fuel oil.
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Fig. 1b shows the oxygen content profile of fine and coarse piles. The oxygen concentrations of fine and coarse piles decreased rapidly in the first week and reached their
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lowest at 1.8% and 2.2% on the 7th and the 6th days, respectively. In this stage, the fact that the temperature increased sharply to 77oC indicates that the development of microorganisms in the soil was consuming a significant amount of oxygen and the high temperature was a respiratory outcome of such high microbial activity (Liang et al., 2003; Lin et al., 2012). After the 8th day, the oxygen content of the two piles increased rapidly to 12.9% and 15.9% up to the 12th day; but from that to the 20th day, while the coarse pile continued to increase to 17.4%, the fine pile fluctuated between 9% and 18%. Also, the oxygen content in the fine pile was lower than that in 11
ACCEPTED MANUSCRIPT the coarse pile. Those results could be due to lower permeability of the fine pile. Such low permeability could prevent oxygen transfer between soil particles, resulting in lower oxygen air diffusion in the fine pile (Haghollahi et al., 2016). From the 20th day to the end of the experiment, the oxygen concentrations of all treatments remained between 18% and 20%
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(approximately ambient condition). The fact that temperature and oxygen content decreased
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gradually after the thermophilic stage indicates that the microbial activity was diminishing,
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which would lead to reduced degradation rates (Zappi et al., 2017). The trend of variations in oxygen content in this study has also been observed in other similar composting studies using
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either food waste or municipal solid waste (Joo et al., 2008; Zhang et al., 2013).
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The pH variations of the fine and coarse piles were shown in Fig. 1c. After thorough
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mixing, the pH values of fine and coarse pile increased slightly from 7.6 and 8.0 to 7.9 and 8.7, respectively. The pH of the fine and coarse piles dropped to 6.5 and 7.4 on the 1st day but rose to
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7.3 and 7.6 on the 3rd day, respectively. Thereafter, it increased to 8.3 and 8.4 on the 6th day and
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remained around 8.5 on the 17th day for the fine and coarse piles, respectively. On the 22th day, the pH of both piles decreased again to 8.2 and 7.6 and remained steady until the end of the
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experiment. In this stage, the pH of fine pile was higher than that of the coarse pile. Short-chain
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organic acids and alcoholic compounds were generated from the degradation of organic waste might contribute to variations in pH during the composting process (Beck Friis et al., 2001). For example, produced humic acid could have decreased the pH of the composting process on the first and the 22th days, while the alcoholic compounds, e.g. those alcoholic VOCs reported below, could have increased the pH at various stages of the composting. Our observations of pH variations were similar to those of other similar composting and bioremediation studies (Lin, 2008; Paladino et al., 2016). 12
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Fig. 1. Variation profiles of (a) temperature, (b) oxygen content and (c) pH during 45 days of composting treatment of highly fuel oil-contaminated fine and coarse piles
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ACCEPTED MANUSCRIPT 3.2. Degradation of fuel oil-contaminated soil Fig. 2 shows the degradation of fuel oil in two experimental composting piles. The initial fuel oil concentrations in the fine and coarse piles were 12,200 and 11,850 mg/kg, respectively. Fuel oil concentrations in the fine and coarse piles decreased rapidly to 3215 and 1342 mg/kg on
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the 14th day with degradation rates of 599 and 700 mg/kg dry weight/day, respectively. From the
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beginning to the 14th day was the thermophilic stage, where microbial activity reached its highest degradation efficiency. The biodegradation efficiency was positively correlated with
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temperature, and negatively correlated with pH and oxygen content (all p > 0.05) due to the fact that microbial activity was exothermic, released acidic compounds and consumed available
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oxygen.
Fig. 2. Degradation of fuel oil during composting treatment and MNA of highly fuel oilcontaminated fine and coarse piles.
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The mesophilic stage (from the 15th day to the 30th day) followed the thermophilic stage. In the mesophilic stage, fuel oil concentrations and degradation rates of both the fine and coarse piles decreased slightly. Previous studies showed that since microbial activity was lessening,
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decreasing degradation of petroleum hydrocarbons occurred in this stage (Lin et al., 2012; Asgari et al., 2017). During the maturation stage (from the 30th day to the 45th day), the temperature
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went down to ambient value (25oC). After 45 days of the experiments, fuel oil concentrations
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were 2,115 and 847 mg/kg for fine and coarse piles, respectively. The removal efficiency of the coarse pile was higher than that of the fine pile, indicating that the particle size plays an
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important role in determining the success of the composting treatment. The porosity of different
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particle sizes can significantly impact air permeability, regulate the diffusion of moisture and the gas/water exchange in the composting process (Zhang and Sun, 2016). The fine particle might
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have smaller pores, therefore limiting transport of oxygen and nutrients and negatively hindering
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microbial activity (Haghollahi et al., 2016). The final fuel oil concentration of the coarse pile was lower than the standard limit issued by Taiwan EPA (1,000 mg/kg), indicating that this kind of
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soil particle is more suitable for composting bioremediation than the finer ones. Similar
2010).
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observation of higher performance of coarse particle was also reported previously (Chang et al.,
Monitored natural attenuation (MNA) is an often-selected option for near-standard-limit concentrations. MNA helps reduce the cost and efforts for environmental remediation projects. Bioremediation is considered time-consuming and sometimes costly. Therefore, MNA approach can help achieve the lower-than-standard-limit concentration while pursuing environmentally friendly bioremediation. MNA was also reported to be a promising approach for the degradation 15
ACCEPTED MANUSCRIPT of petroleum hydrocarbons in contaminated soil (Guarino et al., 2017). In this study, although fuel oil concentration of the coarse pile was lower than the regulatory limit after 45 days of composting, the concentration of the fine pile (2,115 mg/kg) was still far higher than the limit. However, we hypothesized that the enhanced microbial activity in the contaminated soil would
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remain after the composting and help lower fuel oil concentration. Therefore, samples were
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taken after 45 days of composting to monitor fuel oil removal. On the 100th day, fuel oil of both
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fine and coarse piles decreased to 665 mg/kg and 360 mg/kg, respectively, which are lower than the regulatory limit of Taiwan EPA (1,000 mg/kg). The removal of fuel oil during MNA occurs
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through various mechanisms, including sorption, dilution, volatilization and biodegradation
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(Sarkar et al., 2005). In this study, the composting piles were left in the composting facility of National Kaohsiung University of Science and Technology (NKUST). Hence, the sorption and
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dilution mechanisms, which are more suitable to describe MNA processes in the open
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environment, are less likely. Volatilization might happen in this study. However, since Henry law’s constants of fuel oil compounds were low (around 10-7 to 10-3 at 25oC), volatilization was
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also less likely to contribute significantly to fuel oil removal. Thus, biodegradation should be the
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main mechanism of fuel oil removal during MNA process in our study. The biodegradation was attributed to microbial reduction and hydroxylation, which could occur in both aerobic and
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anaerobic environments (Lin, 2008; Shen et al., 2013). In our previous study of food waste composting of diesel oil contaminated soil, various degradation-facilitating microorganisms were detected, such as Bacillus sp, Acinetobacter sp, Pseudoxanthomonas sp (Lin et al., 2012). Thence, biodegradation is the most important mechanism for fuel oil removal from the contaminated soil during the composting treatment as well as the MNA.
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ACCEPTED MANUSCRIPT Kinetic modelling was used to assess the biodegradation of fuel oil-contaminated soil. In the previous studies, first-order kinetics was used to describe the biodegradation rate of motor oil (Abdulyekeen et al., 2016) and lubricating oil (Lee et al., 2007) in contaminated soil. In this study, first and second order kinetic models were both studied for the degradation of fuel oil in
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contaminated soil. The degradation rate constants of first and second orders kinetics were
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calculated according to Asgari et al. (2017). The kinetic constants (k) and correlation coefficients
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(R2) of the fine and coarse piles were shown in Fig. S1. The first order rate constants of the fine pile (k1 = 0.0266 day-1) was smaller than that of the coarse pile (k1 = 0.0325 day-1). Similarly,
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the second order rate constant of fuel oil biodegradation in the fine pile (k2 = 10-5 kg mg-1 day-1)
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was three times lower than that of the coarse pile (k2 = 3 10-5 kg mg-1 day-1), indicating that the soil particle size has significant effects on the biodegradation rate. The fine piles could in fact
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prevent some of the oil phase from approaching microbes due to their low permeability, which
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explained why the rate constant of fine pile was lower than that of the coarse pile (Chang et al.,
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2010). The R2 values for the second order kinetics of the two experiments (0.95 and 0.91) were higher than those for the first order kinetics (0.90 and 0.68). Therefore, the degradation of fuel
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oil in fine and coarse piles followed the second order kinetics. The results of kinetic modelling showed that biodegradation was the main mechanism for the degradation of fuel oil, which is in
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agreement with previous composting studies of fuel oil-contaminated soil (Lin et al., 2012; Asgari et al., 2017).
3.3. Emissions of OVOCs and BTEXs from fuel oil composting process Total OVOC concentrations were 4153 ppbv and 9279 ppbv in the fine and coarse piles, respectively, indicating that the composting process using food waste was successful as the decomposition of food waste by microbes should release high amount of VOCs containing 17
ACCEPTED MANUSCRIPT oxygenated functional groups (Kim et al., 2009). According to Kumar et al. (2011), OVOCs accounted for 83.8% of the total VOCs emitted from green waste composting. Also, over 23% of the total VOCs were occupied by OVOCs from the composting of fruit and garden wastes (Defoer et al., 2002).
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Fig. 3a shows variation in OVOC emissions from the fine and coarse piles. During the
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thermophilic stage, emissions of OVOCs from the fine and coarse piles peaked at 590 and 3256 ppbv but decreased during the maturation stage and then dropping rapidly to only 8 and 0 ppbv
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on day 20, respectively. After that, OVOCs dropped rapidly to 20ppbv. In the maturation stage, the degradation rates of both the fine and coarse piles decreased, however, degradation rate of
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the fine pile (37 mg/kg dry weight/day) was higher than that of the coarse pile (17 mg/kg dry
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weight/day). These results suggest that the biodegradation rates influenced the emissions of OVOCs (p > 0.05). Similar observation was also previously reported from a bioremediation
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study of diesel-contaminated soil (Franco et al., 2015). In addition, these results indicate that the
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variations in temperature highly influenced the emissions of OVOCs (p > 0.05), especially in the initial stage of the composting (Tan et al., 2017). The main components of the OVOCs are
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alcohols, ketones, esters and aldehydes, which were previously reported to be generated from the
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process of food waste composting (Tsai et al., 2008; Zhang et al., 2012). Ketones, which main components were acetone, 2-butanone and 4-methyl 2- pentanone, were found to be the most abundant species among the OVOCs released from both fine and coarse piles, followed by esters and aldehydes. Ketones accounted for 70% and 84% of OVOCs emitted from the fine and coarse piles, respectively. During the first stage of the composting, ketones concentrations occupied 98% and 97% of OVOCs emitted from the fine and coarse piles, respectively and stayed at 100% during both mesophilic and maturation stages of both piles. This observation agrees with the 18
ACCEPTED MANUSCRIPT results reported in a decomposition study of orange waste conducted by Wu and Wang (2015). Ketones were produced via microbial metabolism, in which the large bio-organic compounds were turned into humus as the composting process aged (Eitzer, 1995). Our result was also similar to the VOC monitoring study of composting process reported by Romain et al. (2005),
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which claimed that the amount of ketones released increased as the compost aged. Esters and
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aldehydes, however, decreased during the composting process, which was also previously
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observed by Zhang et al. (2012) in their bioremediation study of municipal solid waste. Esters and aldehydes could be emitted as intermediates of biological metabolism, which occurred
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mainly in the early stage of the composting process (Shao et al., 2014).
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Total BTEX concentrations were 242 ppbv and 927 ppbv in the fine and coarse piles,
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respectively. Previous studies showed that the composting of food waste (Tsai et al., 2008) and municipal solid waste (Tan et al., 2017) could release 10.63-59.73 ppbv of BTEXs. Also, high
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concentrations of BTEXs (955 ppbv) were found to be emitted from the composting
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bioremediation of petroleum-contaminated soil (Mihial et al., 2006). BTEX emissions largely depend on the compost materials and the operational conditions (i.e. the shredding, screening,
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moisture adjustment and aeration) of the composting (Cerda et al., 2017). Higher BTEX
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emissions from the coarse pile could be attributed to this pile’s higher microbial activity, which was proven by its higher fuel oil degradation rate discussed above. The thermophilic stage saw emissions of BTEXs from the fine and coarse piles reach a peak at 146 and 879 ppbv, but drop during the maturation stage and further decrease to 76 and 28 ppbv on day 20, respectively (Fig. 3b). Then, BTEX concentrations fell to 20 ppbv.
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Fig. 3. Emission profiles of (a) OVOCs and (b) BTEXs at different stages of the composting process of highly fuel oil-contaminated fine and coarse piles
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ACCEPTED MANUSCRIPT Among BTEX compounds, toluene shared the largest proportion, followed by xylene and benzene. Toluene was identified as the most easily biodegradable among BTEX (El Naas et al., 2014). Fig. S2 shows the composition of BTEX compounds emitted from the fine and coarse piles at different stages of the composting. BTEX were produced mainly from the degradation of
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organic matter (Shao et al., 2014). Emissions of BTEX depend largely on the types of organic
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matter existing in the composting pile and the conditions (e.g. temperature, moisture, pH, etc.) of
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the composting, and were reported not correlated with compost age (Shao et al., 2014). In this study, there was no obvious increasing or decreasing trend in BTEX composition during the
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composting.
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3.4. Cost analysis of fuel oil composting bioremediation
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In the recent years, economic feasibility of bioremediation projects has attracted wide
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attention from the scientific community. Though bioremediation is generally considered environmentally-friendly, the selection of bioremediation technologies for the remediation
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projects is still largely dependent on the degradation efficiencies and the consideration of the
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relevant cost. In this study, we provided a simple economic estimation for the cost incurred from the composting of highly fuel oil-contaminated soil.
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Since electricity is needed for mechanical operation during the composting process, the cost of the whole process depends largely on the electricity bills. The unit price of electricity in Kaohsiung, Taiwan, is 0.16 USD/kWh. In this study, a mechanical mixer (2 kW) was employed to overturn the compost piles every day (1 hour/day) during 45 days of the composting. The total electricity cost for overturning each pile was 14.12 USD. The raw materials used in this study were donated without charge by the industries (i.e. mature composting and sawdust were
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ACCEPTED MANUSCRIPT provided by Yes-Sun environmental biotech Co., Ltd and Chuen Fa green energy Co., Ltd., respectively). For actual application, the prices of mature compost and saw dust can be around 60.13 and 45.75 USD/tonne, respectively. In this study, each composting pile needed 0.02 tonne of mature compost and 0.05 tonne of saw dust. Therefore, the total cost of mature compost and
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saw dust for each composting pile can be 1.20 and 2.29 USD, respectively. Food waste in this
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study was donated by restaurants and schools’ kitchens in Kaohsiung City. For actual
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application, food waste can also be achieved gratis from local restaurants, households/schools’ kitchens, department stores, etc., which is also a salient advantage of food waste composting.
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The average cost for the composting treatment of one tonne of contaminated soil in this study is
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estimated to be 17.61 USD, which is lower than other bioremediation technologies employed for soils contaminated with petroleum hydrocarbons, e.g. windrow turning, land farming,
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bioventing, bioslurry and biopiles (Houghton, 1996) (Fig. 4), demonstrating the promising
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future of using composting technology for the remediation of highly fuel oil-contaminated soil.
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Fig. 4. Cost comparison between different bioremediation technologies for soil contaminated
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4. Conclusions
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with organic contaminants
The food waste composting treatment of highly fuel oil-contaminated soil was influenced
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by the soil fraction, and profiles of temperature, oxygen content and pH. After 100 days of the composting treatment and MNA, fuel oil concentrations in compost fine and coarse piles dropped to 665 and 360 mg/kg respectively (lower than the regulatory limit of Taiwan EPA 1000 mg/kg). OVOCs and BTEXs were mostly emitted during the thermophilic stage and highly influenced by temperature. Ketones were the most abundant species among the OVOCs, followed by esters and aldehydes. Toluene shared the largest proportion of the total BTEX concentration. 23
ACCEPTED MANUSCRIPT Acknowledgments This research was financially supported by AECOM Co., Ltd., Taiwan. The authors are grateful to Mrs. Wen Ming Mao of the Center of Environmental Analysis Services (CEAS),
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National Kaohsiung University of Science and Technology, for helping with fuel oil analysis.
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Graphical abstract
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ACCEPTED MANUSCRIPT Highlights Composting showed high efficiency in treating highly fuel oil-contaminated soil.
Volatile organic compounds were mainly emitted during the thermophilic stage.
Particle size greatly affected the degradation efficiency during composting.
The degradation of fuel oil–contaminated soil followed the second-order kinetics.
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Figure 1
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Huu-Tuan Tran, Chi-Thanh Vu, Chitsan Lin, Xuan-Thanh Bui, Wen-Yen Huang, Thi-Dieu-Hien Vo, Hong-Giang Hoang, Wen-Yi Liu PII: DOI: Reference:
S2589-014X(18)30108-7 doi:10.1016/j.biteb.2018.10.010 BITEB 107
To appear in:
Bioresource Technology Reports
Received date: Revised date: Accepted date:
24 August 2018 21 October 2018 23 October 2018
Please cite this article as: Huu-Tuan Tran, Chi-Thanh Vu, Chitsan Lin, Xuan-Thanh Bui, Wen-Yen Huang, Thi-Dieu-Hien Vo, Hong-Giang Hoang, Wen-Yi Liu , Remediation of highly fuel oil-contaminated soil by food waste composting and its volatile organic compound (VOC) emission. Biteb (2018), doi:10.1016/j.biteb.2018.10.010
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ACCEPTED MANUSCRIPT Remediation of highly fuel oil-contaminated soil by food waste composting and its volatile organic compound (VOC) emission Huu-Tuan Tran1#, Chi-Thanh Vu2#, Chitsan Lin1*, Xuan-Thanh Bui3, Wen-Yen Huang1, Thi-
Institute of Marine Science and Technology, National Kaohsiung University of Science and
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Dieu-Hien Vo1, Hong-Giang Hoang1, Wen-Yi Liu1
2
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Technology, Kaohsiung 81157, Taiwan.
Civil and Environmental Engineering Department, University of Alabama in Huntsville,
Faculty of Environment and Natural Resources, University of Technology, Vietnam National
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3
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Huntsville, AL 35899, USA.
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University - Ho Chi Minh City, Vietnam.
These authors contributed equally to this work
*
Corresponding author: Tel: +886-7-3651472; Fax: +886-7-3651472; E-mail:
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#
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[email protected] (C. Lin).
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ACCEPTED MANUSCRIPT Abstract Food waste composting was used to treat highly fuel oil-contaminated soil (two particle size fractions - fine < 0.3 cm and coarse > 1.3 cm). The initial fuel oil concentrations in the fine and coarse piles were 12,000 and 11,850 mg/kg, respectively. After 45-day incubation, the
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removal efficiencies of the fine and coarse piles were 82% and 93%, respectively. Residual fuel
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oil concentration (847 mg/kg) in the coarse pile met the Taiwan EPA standard limit (1,000 mg/kg). The results indicated that the soil particle size fraction affected the efficiency of the
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composting treatment. Besides, emissions of oxygenated volatile organic compounds (OVOCs) and BTEXs from the food waste composting treatment were investigated. Ketones and toluene
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were the most abundant species of OVOCs and BTEXs, respectively. This study demonstrated
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that food waste composting would be a promising treatment technology for highly fuel oil-
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contaminated soil.
Keywords: Food waste composting; Highly fuel oil-contaminated soil; Oxygenated volatile
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organic compound (OVOCs); BTEXs.
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ACCEPTED MANUSCRIPT 1. Introduction Petroleum contaminants are widespread in the soil and the remediation of petroleumcontaminated soil has attracted wide attention from scientific community. Petroleum products include gasoline, fuel oil, diesel and/or lubricants, which are released to the environment through
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leaking storage tank, industrial runoff, spills or commercial activities, construction activities and
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agriculture (Adedokun et al., 2007; Zappi et al., 2017). In Taiwan, approximately 400 gas stations have been found contaminated with petroleum hydrocarbons (Tsai et al., 2009).
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Petroleum hydrocarbons are highly toxic and carcinogenic substances, posing negative effects to
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the ecosystem and human health (Asgari et al., 2017).
Fuel oil is a type of petroleum hydrocarbons, which the main characteristics are low
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volatility, high viscosity, and low mobility, making fuel oil difficult to treat compared to other
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petroleum hydrocarbons such as gasoline and diesel fuel (Tsai et al., 2009). Many treatment methods have been developed to address petroleum hydrocarbon concentrations in soil such as
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composting (Lin et al., 2012), electrochemical processes (Huguenot et al., 2015), thermal
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desorption (Yi et al., 2016), etc. Among these treatment technologies, composting is not only easy to operate and cost-effective, but also environmentally friendly (Tradler et al., 2018).
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Composting uses microbial metabolism in the presence of optimum environmental conditions and sufficient nutrients to break down organic contaminants. In composting process, physical and chemical parameters, i.e. temperature, dissolved oxygen nutrients, C/N ratio, organic matter content and moisture need to be monitored and adjusted for improving the growth of microbial communities as well as the degradation of contaminants. The most common materials used for composting are biodegradable organic waste such as food waste (Joo et al., 2008), yard waste (Tan et al., 2017) and manures (Jiang et al., 2015). Among them, food waste has the most 3
ACCEPTED MANUSCRIPT potential to be widely applied for bioremediation purposes owing to its potentially unlimited supply from restaurants, hotels, cafeterias, household kitchens, etc. The major components of food waste include rice, meat, noodles, fruit and vegetable waste, which can be generated in the processes of harvesting, transportation, storage, processing and merchandising (Shen et al., 2013;
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Zhang et al., 2018). In Taiwan, approximately 2.3 million tons of food waste are discharged
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every year (Thi et al., 2015). This is an abundant food waste source that can be used for
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composting bioremediation. In food waste composting, different materials, e.g. sawdust (used to adjust the moisture content) and/or mature compost (used enhance the density and diversity of
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microorganisms), are mixed together in order to optimize the composting conditions. Studies on
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employing composting process for the treatment of highly petroleum-contaminated soil with up to 85% - 96% degradation efficiencies are available in the literature (Lin et al., 2012; Franco et
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al., 2014; Zappi et al., 2017). However, there is very little research on composting treatment of
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fuel oil-contaminated soil.
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The limitation of the composting process is the release of large quantities of volatile organic compounds (VOCs), which are toxic to humans and negatively affect the environment;
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therefore, they can be considered a source of secondary pollution (Franco et al., 2015). Among
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VOCs, oxygenated volatile organic compounds (OVOCs) have received special attention because they are the main component (46-83%) of the total VOCs emitted from the composting process (Gray et al., 2010; Kumar et al., 2011). OVOCs play important roles in tropospheric chemistry, influence the ground level ozone formation and significantly contribute to the formation of secondary organic aerosols (Kumar et al., 2011). Ethanol, acetone, butanone, acetaldehyde are the most abundant species in the total OVOCs released from the composting of organic waste (Wu and Wang, 2015). OVOCs emitted from orange fruit waste (Wu and Wang, 4
ACCEPTED MANUSCRIPT 2015), garden waste (Kumar et al., 2011) and municipal solid waste (Tan et al., 2017) have been investigated during aerobic composting process. Apart from OVOCs, recent studies have also shown that BTEX compounds (benzene, toluene, ethylbenzene, xylenes) are the second main component (7-10%) of the total VOCs emitted from the composting process of petroleum
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hydrocarbon-contaminated soil (Chang et al., 2010). BTEX are highly toxic and exposure to
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them can cause different harmful effects to human health. BTEX are also considered as
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carcinogenic substances.
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Therefore, the main objectives of this study were to investigate the removal efficiency of fuel oil from the contaminated soil using aerobic food waste composting, evaluate the influence
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of composting parameters such as temperature, pH and oxygen content on the removal efficiency
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and analyze fuel oil degradation using kinetic modelling. Additionally, OVOC and BTEX
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emissions from different stages of the composting process was characterized and discussed. 2. Materials and methods
Fuel-oil contaminated soil
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2.1.
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In this study, fuel oil-contaminated soil was achieved from a former gas station in Kaohsiung city, southern Taiwan. The soil was excavated from a previous fuel storage site,
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which was contaminated by the transport and storage of fuel oil. The soil was slightly alkaline and characterized to be a sandy loam texture according to the classification of the United States Department of Agriculture (USDA, 1993). Soil screening was performed according to the method issued by the American Society for Testing and Materials (ASTM, 2013). Because the bulk of fuel oil content in the contaminated soil was mainly found in the fine (size < 0.3 cm) and coarse (size > 1.3 cm) fractions (data not shown), these two particle sizes of the soil were
5
ACCEPTED MANUSCRIPT selected for the composting experiments. These piles were mixed homogenously, oven-dried at 40oC, stored in polyethylene bottles at room temperature until the experiment. The initial average fuel oil concentrations in both the fine and coarse piles were approximately 12,000 mg/kg and 11,850 mg/kg respectively, which are about twelve times higher than the regulatory limit issued
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by Taiwan EPA (Environmental Protection Administration, 2005) (1,000 mg/kg) . The moisture
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content and pH of the fine and coarse piles were 65% and 45%, and 7.9 and 8.7 respectively,
Compost pile composition
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2.2.
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which were within suitable ranges for biological treatment (Lin et al., 2012; Zappi et al., 2017).
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The materials used for composting treatment of the contaminated soil were food waste, sawdust and mature compost. Food waste used for the composting in this study contained
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discarded dairy products, grains, bread, fruits, vegetables, meat scraps, seafood and kitchen
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waste, which were collected from restaurants and schools in Kaohsiung City. The food waste ingredient composition was selected to be 50% of meat scraps and seafood, 35% of vegetables,
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and 15% of others, which was reported suitable for bacterial growth during the composting
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process in our previous study (Lin et al., 2012). Fresh food waste was shredded into fractions of less than 2 cm diameter before the experiment. Sawdust was used to adjust the moisture content.
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Mature compost was added to enrich the microbial population considered beneficial for soil bioremediation. Contaminated fine and coarse soil fractions were separated into two compost piles (from now addressed as fine pile and coarse pile) and mixed with the compost materials at a dry weight ratio shown in Table 1. All those materials were manually mixed to create homogenous fine and coarse compost piles. Water was added to maintain 35-40% moisture, which is considered beneficial for microbial fermentation (Lin, 2008; Lin et al., 2012). All the
6
ACCEPTED MANUSCRIPT composting experiments were conducted in the compost facility of National Kaohsiung University of Science and Technology, Taiwan. Table 1. Characteristics of two compost piles (fine pile and coarse pile) of highly fuel oil-
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contaminated soil Moisture
pH
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Dry weight (kg)
Saw
Mature
Contaminated
waste
dust
compost
soil
Fine pile
80
50
20
150
Coarse pile
80
50
20
Total
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Food
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content (%)
65
7.9
300
45
8.7
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150
300
Experimental operation and sampling
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2.3.
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The compost piles were incubated under aerobic condition for 45 days. During the
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incubation, the moisture content in two piles was controlled at 45% by adding water. Daily operation included temperature and oxygen content measurements, and homogenously mixing.
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Temperature was measured at 3 sampling points (top, center and bottom) of each pile for the representative value using a TES 1310 K-type digital thermometer. Oxygen content was collected using a gas collector, which was plugged down 20 cm deep into each pile. Then, a handheld multi-function gas-meter (ISC M40 Multi-Gas Monitor) was connected to the gas collector to measure oxygen content. After that, compost samples were taken to examine pH, humidity and periodically fuel oil concentrations. Samples were taken at five locations (at four corners and the center) at 15 to 20 cm deep in each pile, mixed well for a representative sample 7
ACCEPTED MANUSCRIPT and then kept in the zip-lock PE bags at 4oC until the analysis. The pH and humidity were evaluated daily. Based on the temperature profiles, the fuel oil concentration in the soil was measured continuously every day for the first two weeks. After the first two weeks, the fuel oil concentration in the soil was measured once a week until finishing the composting process. Also,
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monitored natural attenuation (MNA) approach was employed so that after the composting
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finished (on day 46), a sample from each pile was taken on day 100 for fuel oil analysis. Gaseous
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VOCs samples were collected on the 2th, 4th, 6th, 8th, 10th, 12th, 14th, 16th, 18th, 20th, 30th and 45th days of the experiment. Gaseous VOCs were collected using the 1L Teflon-lined sampling bags
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(SKC Inc., USA), which were connected with the gas collector (used for the oxygen content
Analytical methods
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2.4.
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measurement described above).
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During 45 days of composting incubation, variations of temperature, moisture, oxygen content, and pH were monitored. A GasTech portable multigas (ISC M40 gas detector) was used
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to analyze oxygen content. The temperature was measured by a TES 1310 K-type digital
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thermometer equipped with a 1.2 m probe (± 0.1oC sensitivity). The moisture content was measured using the standard method NIEA R203.02C issues by the Taiwan Environmental
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Protection Administration (TEPA,2009); pH was determined according to the USEPA method 9045C (USEPA, 1995). Since there is currently no specific method for the analysis of fuel oil, modified analytical method of total petroleum hydrocarbon (TPH) was employed for the quantification of the heavy fuel oil. Concentrations of fuel oil were measured using a gas chromatography/flame ionization detector (GC/FID). The Soxhlet extraction method using dichloromethane (DCM) was employed
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ACCEPTED MANUSCRIPT using the Taiwan NIEA method M165.00C. The details of extraction and analysis methods were described in our previous study (Lin et al., 2012). Briefly, 10 g of compost sample was mixed with 4 g sodium sulfate for dehydration, which was then added with 10 mL of DCM and shaken by a vibration machine for 1h. Then, the supernatant was collected and purified by filtering with
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acid silica gel. For GC/FID analysis, the injector inlet temperature and the detector temperature
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were both 340oC. The oven temperature was programmed to increase from 50oC to 340oC at
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10oC/ min and stayed there for 34 min. Fuel oil was quantified a range from C6 to C40.
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The analytical method employed for the identification and quantification of emitted OVOCs and BTEX (benzene, toluene, ethylbenzene, xylenes) in this study was TO-15 (USEPA,
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1999). The OVOCs were divided into 3 groups, ketones (acetone, 2-butanone, 4-methyl 2-
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pentanone and 2-hexanone), esters (2-methoxy-2-methyl-propane, vinyl acetate, ethyl acetate), aldehydes (acrolein). Details of the method are given in the Supporting Information. Briefly, a
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DB-VRX capillary column (60 m x 250 mm x 1.4 mm, Agilent Technologies, USA) was used
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with helium as carrier gas to separate the target compounds. The emitted OVOC and BTEX samples were analyzed using a gas chromatography-mass spectrometry (GC/MS) system
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(6890/5973, Agilent Technologies, Santa Clara, USA) coupled with an Entech 7100A pre-
2.5.
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concentrator (Entech Instruments Inc., Simi Vally, USA). Kinetic analysis of fuel oil degradation
Kinetic modelling was used to assess the biodegradation of fuel oil-contaminated soil. In the previous studies, first-order kinetics was used to describe the biodegradation rate of motor oil (Abdulyekeen et al., 2016) and lubricating oil (Lee et al., 2007) in contaminated soil. In this study, kinetic analysis (first and second-order kinetic models) was employed to investigate the
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ACCEPTED MANUSCRIPT degradation of fuel oil. The mathematical models of first and second-order kinetics were described by the following equations: 𝐿𝑛𝐶𝑡 = 𝐿𝑛𝐶0 − 𝑘1 𝑡 (first-order kinetics) 1
= 𝑘2 𝑡 +
1 𝐶0
(second-order kinetics)
Eq. (2.2)
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𝐶𝑡
Eq. (2.1)
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where C0 is the initial concentration of fuel oil, Ct is the concentration of fuel oil at time t, t is time (days), k1 and k2 are first and second order rate constants, respectively. Statistical analysis
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2.6.
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Pearson’s correlation test was performed to analyze the correlation between temperature, moisture content, and pH profiles. The differences were considered significantly based on 95%
analyses.
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3. Results and discussion
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confidence interval. SPSS statistics for Windows (version 20) was used for all statistical
Variation profiles of temperature, oxygen content and pH of the composting of
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highly fuel oil-contaminated soil
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The composting process was divided three stage, thermophilic stage (day 1 to day 14), mesophilic stage (day 15 to day 30), maturation stage (day 15 to day 45). Fig. 1a shows the temperature profiles of the composting piles of the fine and coarse piles (from now addressed as the fine and coarse piles, respectively). The temperature of the two piles increased rapidly to the thermophilic stage (> 60oC) on the 5th day, indicating that bioactivity was not hindered and limited by the high fuel oil content (approximately 12,000 mg/kg). The temperature reached its highest value at 77oC on the 7th day. Then, it remained at 70oC until the 10th day (thermophilic 10
ACCEPTED MANUSCRIPT stage). After that, the temperature declined slowly during the mesophilic stage to ambient condition on the 30th day (maturation stage), indicating that the composting process had finished. The variation trend of temperature profiles in this study was similar to previous studies on food waste composting (Lin et al., 2012; Pereira et al., 2018).
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During the thermophilic stage (from the 8th day to the 14th day), due to the higher
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moisture content of the fine pile, the temperature of the fine pile was higher than that of the coarse pile. Similar observation was also reported in the municipal solid waste composting study
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of Zhang et al. (2013). In the thermophilic stage, the temperature stayed at 45-70oC, which was proven beneficial for the development of microorganisms (Lin et al., 2012). This development
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further facilitated the degradation of the high fuel oil concentration. High temperature was
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reported not only to increase the solubility but also to decrease the viscosity of hydrocarbon pollutants and transfer long-chain n-alkanes from solid phase to liquid phase, improving
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degradation rates of fuel oil (Aislabie et al., 2006). Therefore, the temperature of 45-70oC
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occurring the thermophilic stage of this study should favor microbial degradation of fuel oil.
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Fig. 1b shows the oxygen content profile of fine and coarse piles. The oxygen concentrations of fine and coarse piles decreased rapidly in the first week and reached their
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lowest at 1.8% and 2.2% on the 7th and the 6th days, respectively. In this stage, the fact that the temperature increased sharply to 77oC indicates that the development of microorganisms in the soil was consuming a significant amount of oxygen and the high temperature was a respiratory outcome of such high microbial activity (Liang et al., 2003; Lin et al., 2012). After the 8th day, the oxygen content of the two piles increased rapidly to 12.9% and 15.9% up to the 12th day; but from that to the 20th day, while the coarse pile continued to increase to 17.4%, the fine pile fluctuated between 9% and 18%. Also, the oxygen content in the fine pile was lower than that in 11
ACCEPTED MANUSCRIPT the coarse pile. Those results could be due to lower permeability of the fine pile. Such low permeability could prevent oxygen transfer between soil particles, resulting in lower oxygen air diffusion in the fine pile (Haghollahi et al., 2016). From the 20th day to the end of the experiment, the oxygen concentrations of all treatments remained between 18% and 20%
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(approximately ambient condition). The fact that temperature and oxygen content decreased
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gradually after the thermophilic stage indicates that the microbial activity was diminishing,
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which would lead to reduced degradation rates (Zappi et al., 2017). The trend of variations in oxygen content in this study has also been observed in other similar composting studies using
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either food waste or municipal solid waste (Joo et al., 2008; Zhang et al., 2013).
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The pH variations of the fine and coarse piles were shown in Fig. 1c. After thorough
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mixing, the pH values of fine and coarse pile increased slightly from 7.6 and 8.0 to 7.9 and 8.7, respectively. The pH of the fine and coarse piles dropped to 6.5 and 7.4 on the 1st day but rose to
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7.3 and 7.6 on the 3rd day, respectively. Thereafter, it increased to 8.3 and 8.4 on the 6th day and
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remained around 8.5 on the 17th day for the fine and coarse piles, respectively. On the 22th day, the pH of both piles decreased again to 8.2 and 7.6 and remained steady until the end of the
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experiment. In this stage, the pH of fine pile was higher than that of the coarse pile. Short-chain
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organic acids and alcoholic compounds were generated from the degradation of organic waste might contribute to variations in pH during the composting process (Beck Friis et al., 2001). For example, produced humic acid could have decreased the pH of the composting process on the first and the 22th days, while the alcoholic compounds, e.g. those alcoholic VOCs reported below, could have increased the pH at various stages of the composting. Our observations of pH variations were similar to those of other similar composting and bioremediation studies (Lin, 2008; Paladino et al., 2016). 12
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Fig. 1. Variation profiles of (a) temperature, (b) oxygen content and (c) pH during 45 days of composting treatment of highly fuel oil-contaminated fine and coarse piles
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ACCEPTED MANUSCRIPT 3.2. Degradation of fuel oil-contaminated soil Fig. 2 shows the degradation of fuel oil in two experimental composting piles. The initial fuel oil concentrations in the fine and coarse piles were 12,200 and 11,850 mg/kg, respectively. Fuel oil concentrations in the fine and coarse piles decreased rapidly to 3215 and 1342 mg/kg on
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the 14th day with degradation rates of 599 and 700 mg/kg dry weight/day, respectively. From the
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beginning to the 14th day was the thermophilic stage, where microbial activity reached its highest degradation efficiency. The biodegradation efficiency was positively correlated with
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temperature, and negatively correlated with pH and oxygen content (all p > 0.05) due to the fact that microbial activity was exothermic, released acidic compounds and consumed available
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oxygen.
Fig. 2. Degradation of fuel oil during composting treatment and MNA of highly fuel oilcontaminated fine and coarse piles.
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The mesophilic stage (from the 15th day to the 30th day) followed the thermophilic stage. In the mesophilic stage, fuel oil concentrations and degradation rates of both the fine and coarse piles decreased slightly. Previous studies showed that since microbial activity was lessening,
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decreasing degradation of petroleum hydrocarbons occurred in this stage (Lin et al., 2012; Asgari et al., 2017). During the maturation stage (from the 30th day to the 45th day), the temperature
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went down to ambient value (25oC). After 45 days of the experiments, fuel oil concentrations
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were 2,115 and 847 mg/kg for fine and coarse piles, respectively. The removal efficiency of the coarse pile was higher than that of the fine pile, indicating that the particle size plays an
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important role in determining the success of the composting treatment. The porosity of different
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particle sizes can significantly impact air permeability, regulate the diffusion of moisture and the gas/water exchange in the composting process (Zhang and Sun, 2016). The fine particle might
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have smaller pores, therefore limiting transport of oxygen and nutrients and negatively hindering
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microbial activity (Haghollahi et al., 2016). The final fuel oil concentration of the coarse pile was lower than the standard limit issued by Taiwan EPA (1,000 mg/kg), indicating that this kind of
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soil particle is more suitable for composting bioremediation than the finer ones. Similar
2010).
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observation of higher performance of coarse particle was also reported previously (Chang et al.,
Monitored natural attenuation (MNA) is an often-selected option for near-standard-limit concentrations. MNA helps reduce the cost and efforts for environmental remediation projects. Bioremediation is considered time-consuming and sometimes costly. Therefore, MNA approach can help achieve the lower-than-standard-limit concentration while pursuing environmentally friendly bioremediation. MNA was also reported to be a promising approach for the degradation 15
ACCEPTED MANUSCRIPT of petroleum hydrocarbons in contaminated soil (Guarino et al., 2017). In this study, although fuel oil concentration of the coarse pile was lower than the regulatory limit after 45 days of composting, the concentration of the fine pile (2,115 mg/kg) was still far higher than the limit. However, we hypothesized that the enhanced microbial activity in the contaminated soil would
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remain after the composting and help lower fuel oil concentration. Therefore, samples were
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taken after 45 days of composting to monitor fuel oil removal. On the 100th day, fuel oil of both
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fine and coarse piles decreased to 665 mg/kg and 360 mg/kg, respectively, which are lower than the regulatory limit of Taiwan EPA (1,000 mg/kg). The removal of fuel oil during MNA occurs
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through various mechanisms, including sorption, dilution, volatilization and biodegradation
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(Sarkar et al., 2005). In this study, the composting piles were left in the composting facility of National Kaohsiung University of Science and Technology (NKUST). Hence, the sorption and
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dilution mechanisms, which are more suitable to describe MNA processes in the open
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environment, are less likely. Volatilization might happen in this study. However, since Henry law’s constants of fuel oil compounds were low (around 10-7 to 10-3 at 25oC), volatilization was
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also less likely to contribute significantly to fuel oil removal. Thus, biodegradation should be the
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main mechanism of fuel oil removal during MNA process in our study. The biodegradation was attributed to microbial reduction and hydroxylation, which could occur in both aerobic and
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anaerobic environments (Lin, 2008; Shen et al., 2013). In our previous study of food waste composting of diesel oil contaminated soil, various degradation-facilitating microorganisms were detected, such as Bacillus sp, Acinetobacter sp, Pseudoxanthomonas sp (Lin et al., 2012). Thence, biodegradation is the most important mechanism for fuel oil removal from the contaminated soil during the composting treatment as well as the MNA.
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ACCEPTED MANUSCRIPT Kinetic modelling was used to assess the biodegradation of fuel oil-contaminated soil. In the previous studies, first-order kinetics was used to describe the biodegradation rate of motor oil (Abdulyekeen et al., 2016) and lubricating oil (Lee et al., 2007) in contaminated soil. In this study, first and second order kinetic models were both studied for the degradation of fuel oil in
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contaminated soil. The degradation rate constants of first and second orders kinetics were
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calculated according to Asgari et al. (2017). The kinetic constants (k) and correlation coefficients
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(R2) of the fine and coarse piles were shown in Fig. S1. The first order rate constants of the fine pile (k1 = 0.0266 day-1) was smaller than that of the coarse pile (k1 = 0.0325 day-1). Similarly,
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the second order rate constant of fuel oil biodegradation in the fine pile (k2 = 10-5 kg mg-1 day-1)
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was three times lower than that of the coarse pile (k2 = 3 10-5 kg mg-1 day-1), indicating that the soil particle size has significant effects on the biodegradation rate. The fine piles could in fact
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prevent some of the oil phase from approaching microbes due to their low permeability, which
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explained why the rate constant of fine pile was lower than that of the coarse pile (Chang et al.,
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2010). The R2 values for the second order kinetics of the two experiments (0.95 and 0.91) were higher than those for the first order kinetics (0.90 and 0.68). Therefore, the degradation of fuel
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oil in fine and coarse piles followed the second order kinetics. The results of kinetic modelling showed that biodegradation was the main mechanism for the degradation of fuel oil, which is in
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agreement with previous composting studies of fuel oil-contaminated soil (Lin et al., 2012; Asgari et al., 2017).
3.3. Emissions of OVOCs and BTEXs from fuel oil composting process Total OVOC concentrations were 4153 ppbv and 9279 ppbv in the fine and coarse piles, respectively, indicating that the composting process using food waste was successful as the decomposition of food waste by microbes should release high amount of VOCs containing 17
ACCEPTED MANUSCRIPT oxygenated functional groups (Kim et al., 2009). According to Kumar et al. (2011), OVOCs accounted for 83.8% of the total VOCs emitted from green waste composting. Also, over 23% of the total VOCs were occupied by OVOCs from the composting of fruit and garden wastes (Defoer et al., 2002).
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Fig. 3a shows variation in OVOC emissions from the fine and coarse piles. During the
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thermophilic stage, emissions of OVOCs from the fine and coarse piles peaked at 590 and 3256 ppbv but decreased during the maturation stage and then dropping rapidly to only 8 and 0 ppbv
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on day 20, respectively. After that, OVOCs dropped rapidly to 20ppbv. In the maturation stage, the degradation rates of both the fine and coarse piles decreased, however, degradation rate of
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the fine pile (37 mg/kg dry weight/day) was higher than that of the coarse pile (17 mg/kg dry
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weight/day). These results suggest that the biodegradation rates influenced the emissions of OVOCs (p > 0.05). Similar observation was also previously reported from a bioremediation
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study of diesel-contaminated soil (Franco et al., 2015). In addition, these results indicate that the
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variations in temperature highly influenced the emissions of OVOCs (p > 0.05), especially in the initial stage of the composting (Tan et al., 2017). The main components of the OVOCs are
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alcohols, ketones, esters and aldehydes, which were previously reported to be generated from the
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process of food waste composting (Tsai et al., 2008; Zhang et al., 2012). Ketones, which main components were acetone, 2-butanone and 4-methyl 2- pentanone, were found to be the most abundant species among the OVOCs released from both fine and coarse piles, followed by esters and aldehydes. Ketones accounted for 70% and 84% of OVOCs emitted from the fine and coarse piles, respectively. During the first stage of the composting, ketones concentrations occupied 98% and 97% of OVOCs emitted from the fine and coarse piles, respectively and stayed at 100% during both mesophilic and maturation stages of both piles. This observation agrees with the 18
ACCEPTED MANUSCRIPT results reported in a decomposition study of orange waste conducted by Wu and Wang (2015). Ketones were produced via microbial metabolism, in which the large bio-organic compounds were turned into humus as the composting process aged (Eitzer, 1995). Our result was also similar to the VOC monitoring study of composting process reported by Romain et al. (2005),
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which claimed that the amount of ketones released increased as the compost aged. Esters and
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aldehydes, however, decreased during the composting process, which was also previously
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observed by Zhang et al. (2012) in their bioremediation study of municipal solid waste. Esters and aldehydes could be emitted as intermediates of biological metabolism, which occurred
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mainly in the early stage of the composting process (Shao et al., 2014).
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Total BTEX concentrations were 242 ppbv and 927 ppbv in the fine and coarse piles,
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respectively. Previous studies showed that the composting of food waste (Tsai et al., 2008) and municipal solid waste (Tan et al., 2017) could release 10.63-59.73 ppbv of BTEXs. Also, high
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concentrations of BTEXs (955 ppbv) were found to be emitted from the composting
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bioremediation of petroleum-contaminated soil (Mihial et al., 2006). BTEX emissions largely depend on the compost materials and the operational conditions (i.e. the shredding, screening,
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moisture adjustment and aeration) of the composting (Cerda et al., 2017). Higher BTEX
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emissions from the coarse pile could be attributed to this pile’s higher microbial activity, which was proven by its higher fuel oil degradation rate discussed above. The thermophilic stage saw emissions of BTEXs from the fine and coarse piles reach a peak at 146 and 879 ppbv, but drop during the maturation stage and further decrease to 76 and 28 ppbv on day 20, respectively (Fig. 3b). Then, BTEX concentrations fell to 20 ppbv.
19
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3500
a Ketones
2500
Aldehydes
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2000
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1500 1000
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500 0 Fine
Coarse
Fine
b
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800 700
500
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100
Maturation stage
Benzene
Toluene
m & p - Xylene
o-Xylene
Ethylbenzene
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400
200
Coarse
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600
300
Mesophilic stage
Fine
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1000 900
Coarse
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Thermophilic stage
Concentration of BTEX (ppbv)
Esters
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Concentration of OVOCs (ppbv)
3000
0
Fine
Coarse
Thermophilic stage
Fine
Coarse
Mesophilic stage
Fine
Coarse
Maturation stage
Fig. 3. Emission profiles of (a) OVOCs and (b) BTEXs at different stages of the composting process of highly fuel oil-contaminated fine and coarse piles
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ACCEPTED MANUSCRIPT Among BTEX compounds, toluene shared the largest proportion, followed by xylene and benzene. Toluene was identified as the most easily biodegradable among BTEX (El Naas et al., 2014). Fig. S2 shows the composition of BTEX compounds emitted from the fine and coarse piles at different stages of the composting. BTEX were produced mainly from the degradation of
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organic matter (Shao et al., 2014). Emissions of BTEX depend largely on the types of organic
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matter existing in the composting pile and the conditions (e.g. temperature, moisture, pH, etc.) of
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the composting, and were reported not correlated with compost age (Shao et al., 2014). In this study, there was no obvious increasing or decreasing trend in BTEX composition during the
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composting.
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3.4. Cost analysis of fuel oil composting bioremediation
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In the recent years, economic feasibility of bioremediation projects has attracted wide
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attention from the scientific community. Though bioremediation is generally considered environmentally-friendly, the selection of bioremediation technologies for the remediation
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projects is still largely dependent on the degradation efficiencies and the consideration of the
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relevant cost. In this study, we provided a simple economic estimation for the cost incurred from the composting of highly fuel oil-contaminated soil.
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Since electricity is needed for mechanical operation during the composting process, the cost of the whole process depends largely on the electricity bills. The unit price of electricity in Kaohsiung, Taiwan, is 0.16 USD/kWh. In this study, a mechanical mixer (2 kW) was employed to overturn the compost piles every day (1 hour/day) during 45 days of the composting. The total electricity cost for overturning each pile was 14.12 USD. The raw materials used in this study were donated without charge by the industries (i.e. mature composting and sawdust were
21
ACCEPTED MANUSCRIPT provided by Yes-Sun environmental biotech Co., Ltd and Chuen Fa green energy Co., Ltd., respectively). For actual application, the prices of mature compost and saw dust can be around 60.13 and 45.75 USD/tonne, respectively. In this study, each composting pile needed 0.02 tonne of mature compost and 0.05 tonne of saw dust. Therefore, the total cost of mature compost and
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saw dust for each composting pile can be 1.20 and 2.29 USD, respectively. Food waste in this
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study was donated by restaurants and schools’ kitchens in Kaohsiung City. For actual
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application, food waste can also be achieved gratis from local restaurants, households/schools’ kitchens, department stores, etc., which is also a salient advantage of food waste composting.
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The average cost for the composting treatment of one tonne of contaminated soil in this study is
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estimated to be 17.61 USD, which is lower than other bioremediation technologies employed for soils contaminated with petroleum hydrocarbons, e.g. windrow turning, land farming,
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bioventing, bioslurry and biopiles (Houghton, 1996) (Fig. 4), demonstrating the promising
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future of using composting technology for the remediation of highly fuel oil-contaminated soil.
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Composting pile ( this study)
Biopiles
Bioslurry
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Bioventing
Windrow turning
50
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Land farming
100
150
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Cost (£/tonne)
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Fig. 4. Cost comparison between different bioremediation technologies for soil contaminated
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4. Conclusions
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with organic contaminants
The food waste composting treatment of highly fuel oil-contaminated soil was influenced
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by the soil fraction, and profiles of temperature, oxygen content and pH. After 100 days of the composting treatment and MNA, fuel oil concentrations in compost fine and coarse piles dropped to 665 and 360 mg/kg respectively (lower than the regulatory limit of Taiwan EPA 1000 mg/kg). OVOCs and BTEXs were mostly emitted during the thermophilic stage and highly influenced by temperature. Ketones were the most abundant species among the OVOCs, followed by esters and aldehydes. Toluene shared the largest proportion of the total BTEX concentration. 23
ACCEPTED MANUSCRIPT Acknowledgments This research was financially supported by AECOM Co., Ltd., Taiwan. The authors are grateful to Mrs. Wen Ming Mao of the Center of Environmental Analysis Services (CEAS),
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National Kaohsiung University of Science and Technology, for helping with fuel oil analysis.
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ACCEPTED MANUSCRIPT References
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Adedokun, O., and, Ataga, A., 2007. Degradation of crude oil, automotive gasoline oil and
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Asgari, A., Nabizadeh, R., Mahvi, A.H., Nasseri, S., Dehghani, M.H., Nazmara, S., Yaghmaeian, K., 2017. Biodegradation of total petroleum hydrocarbons from acidic
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Graphical abstract
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ACCEPTED MANUSCRIPT Highlights Composting showed high efficiency in treating highly fuel oil-contaminated soil.
Volatile organic compounds were mainly emitted during the thermophilic stage.
Particle size greatly affected the degradation efficiency during composting.
The degradation of fuel oil–contaminated soil followed the second-order kinetics.
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Figure 1
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Figure 4