RESEARCH ARTICLE

Enhanced reductive dechlorination of polychlorinatedbiphenyl-contaminated soil by in-vessel anaerobic compostingwith zero-valent iron

Yu-Yang Long & Chi Zhang & Yao Du & Xiao-Qing Tao &

Dong-Sheng Shen

Received: 6 October 2013 /Accepted: 29 November 2013# Springer-Verlag Berlin Heidelberg 2013

Abstract Anaerobic dechlorination is an effective degrada-tion pathway for higher chlorinated polychlorinated biphenyls(PCBs). The enhanced reductive dechlorination of PCB-contaminated soil by anaerobic composting with zero-valentiron (ZVI) was studied, and preliminary reasons for the en-hanced reductive dechlorination with ZVI were investigated.The results show that the addition of nanoscale ZVI canenhance dechlorination during in-vessel anaerobiccomposting. After 140 days, the average number of removedCl per biphenyl with 10 mg g−1 of added nanoscale ZVI was0.63, enhancing the dechlorination by 34% and improving theinitial dechlorination speed. The ZVI enhances dechlorinationby providing a suitable acid base environment, reducing vol-atile fatty acid inhibition and stimulating the microorganisms.The C/N ratios for treatments with the highest rate of ZVIaddition were smaller than for the control, indicating that ZVIaddition can promote compost maturity.

Keywords Polychlorinated biphenyl . Dechlorination .

Zero-valent iron . Anaerobic composting

Introduction

Polychlorinated biphenyls (PCBs) are typically persistent or-ganic pollutants that have been globally dispersed through the

global distillation effect (Dalla Valle et al. 2005). Consideringthe historical mass production of PCBs (Lee 1995), largeamounts of PCBs have been released into the environment(Holoubek 2001). The low biodegradability of PCBs meansthat they remain in the environment for long periods of timeand gradually accumulate in the soil (Seija and Jaakko 2000).Previous studies have shown that at least 21,000 tons of PCBsare present in surface soil worldwide (Meijer et al. 2003).PCBs tend to accumulate in biota and are magnified throughthe food chain because of their low aqueous solubility andhigh affinity for organic phases (Jacob et al. 1994).

Although PCBs are refractory, microbial degradation canstill occur if the surrounding environment is suitable. Lowerchlorinated PCB congeners can be aerobically co-metabolizedby the biphenyl catabolic pathway (Sylvestre 2004), and alarge number of aerobic microorganisms have been isolatedand identified (Borja et al. 2005). However, the biodegrad-ability of PCBs in aerobic conditions decreases with increas-ing chlorination. Only rarely are aerobic microorganisms ca-pable of degrading PCB congeners with five ormore chlorines(Bedard et al. 1986). Therefore, higher chlorinated PCBs aremore persistent than lower chlorinated PCBs. Furthermore,higher chlorinated PCBs usually have higher biological tox-icity and bioaccumulation rates. Twelve PCB congeners areconsidered to have “dioxin-like” properties. These PCBs arecalled dioxin-like PCBs and are mainly higher chlorinatedPCBs (Ross 2004).

The higher chlorinated PCBs are very stable and cannotreadily be aerobically biodegraded. Reductive dechlorinationhas been shown to be the required biodegradation mechanismfor higher chlorinated PCBs. However, naturally occurringanaerobic microorganisms suitable for PCB dechlorinationare difficult to isolate because they are scarce and slow grow-ing (Jacobus et al. 1995). Only a small number of PCB-dechlorinating microorganisms have been reported (Wattset al. 2005). These microorganisms use the higher chlorinated

Responsible editor: Leif Kronberg

Y.<Y. Long :Y. Du :X.<Q. Tao :D.<S. Shen (*)Zhejiang Provincial Key Laboratory of Solid Waste Treatment andRecycling, School of Environmental Science and Engineering,Zhejiang Gongshang University, Hangzhou 310018, Chinae-mail: shends@zju.edu.cn

C. ZhangZhejiang Environmental Science and Design Institute,Hangzhou 310007, China

Environ Sci Pollut ResDOI 10.1007/s11356-013-2420-4

PCBs as electron acceptors provided that electron donors areavailable (Morris et al. 1992). Therefore, the presence ofadequate and suitable electron-donating substrates is a keyfactor in the success of reductive dechlorination of PCBs.Organic carbon is commonly used as the electron-donatingsubstrate. Previous studies have shown that many organiccarbon sources can provide electrons for the reductive bio-dechlorination (Liu et al. 2010). Other research also indicatesthat the different carbon/electron sources can affect dechlori-nation rates but not the reductive dechlorination pathway(Nollet et al. 2005).

Some inorganic reducing agents (such as zero-valentmetals (Kirschling et al. 2010; Xiu et al. 2010) and molecularhydrogen (Son et al. 2006; Yu et al. 2006)) can also provideelectrons to dechlorinating bacteria. Zero-valent iron (ZVI)has attracted increasing attention, given its ease of productionand low ecotoxicity. ZVI has been shown to significantlyimprove the efficiency of PCB bio-dechlorination in sedimentbecause of its hydrogen producing (Winchell and Novak2008) and reduction of competition from other hydrogen-utilizing microorganisms (Wu et al. 2000). Rysavy et al.’s(2005) research showed that the addition of ZVI in sedimentscan reduce the lag time for PCBs dechlorination. SrinivasaVaradhan et al. (2011) also demonstrated that a slight alkalineenvironment and sufficient supply of hydrogen were condu-cive to sustaining the population ofDehalococcoides species,a genus of bacteria that obtain energy via the oxidation ofhydrogen gas and subsequent reductive dehalogenation, in thesediments amended with ZVI. Composting is a traditionalwaste disposal method that is commonly used on a widevariety of organic wastes including kitchen waste, livestockmanure, agricultural waste, and municipal sewage sludge(Bernal et al. 2008; Paradelo et al. 2012; Rodríguez et al.2012), despite its greenhouse gas emission. Remediation ofsoil contaminated by organic pollutants via composting is anew bioremediation technology (Antizar-Ladislao et al. 2006;Purnomo et al. 2010; Semple et al. 2001; Wang et al. 2011).Anaerobic composting were also effective in PCBs dechlori-nation in a previous study (Zhang et al. 2013). The benefits ofsoil composting to promote organic pollutant removal include:(1) the composting materials provide a rich co-metabolismmatrix for the microbes, so the growth of most microorgan-isms is enriched during the composting (Narihiro et al. 2010);(2) the presence of different microbial communities at differ-ent stages of the composting, and a somewhat increasedabundance of microbial communities, providing the possibil-ity of pollutant degradation (Albrecht et al. 2010); and (3) thesoluble organic matter formed during the composting processaids the separation of organic pollutants from soil colloids andenhances bioavailability fat soluble organism (Oleszczuk2009).

This study aims to investigate the potential for enhancedreductive dechlorination of PCB-contaminated soil by in-

vessel anaerobic composting with added ZVI. The effects ofdifferent ZVI particle size (micron- and nanoscale ZVI) onPCBs dechlorination and compost maturity were also studiedand preliminary reasons for the enhanced reductive dechlori-nation with ZVI are discussed.

Materials and methods

Soil and organic waste

PCB-contaminated soil was simulated using soil and Aroclor1260 (a commercial mixture of PCBs containing 12 carbonatoms and 60 % chlorine from AccuStandard Inc.). The soilwas collected from the surface layer (0 to 20 cm) of cultivatedyellow soils in Hangzhou, Zhejiang, China. The soil wasclassified as a yellow-brown loam and analyzed to ensure itcontained no Aroclor 1260. The soil was air-dried at roomtemperature. The dried soil was sieved through a 1-mm meshsieve to remove large stones and plant roots prior to use. TheAroclor 1260 was dissolved in acetone and then thoroughlymixed with the soil. The acetone was evaporated, leavingbehind spiked soil. The total concentration of the 35 PCBcongeners in soil was 1.09±0.08mg kg−1. The spiked soil wasequilibrated in a greenhouse for 15 days at field capacitymoisture level before use.

Pig manure, the main organic waste used in this compoststudy, was collected from a livestock farm in Hangzhou,Zhejiang, China. The dried pig manure was ground to approx-imately 1-mm grain size. Sawdust was used as a compostingamendment to adjust the carbon-to-nitrogen ratio to 20, acommonly used ratio in traditional composting. The charac-teristics of the organic wastes and soil are shown in Table 1.The soil and organic waste were mixed with a proportion of2:3 for composting, with an initial moisture content of 60 %.

Anaerobic sludge, 16.2 % total solids and 10.5 % volatilesolids, was collected from the internal circulation reactor of awastewater treatment plant in a papermaking factory inZhejiang.

Experimental design

The detailed experimental design is described in Table 2. Twotypes of ZVI (commercial 98 % micron-scale ZVI and 99 %nanoscale ZVI) and three additive concentrations (10, 2, and0.4mg g−1) were used. The highest additive concentration wasreferred to previous relevant studies (Long et al. 2009;Winchell and Novak 2008) and accord with the possibleengineering. Four control treatments were undertaken: MI-ST and NA-ST were sterilized controls of treatments A andD, respectively; CK was a control treatment without ZVI, andCS was a sterilized control without ZVI. Neutral formalinsolution (containing formaldehyde, disodium hydrogen

Environ Sci Pollut Res

phosphate, and sodium dihydrogen phosphate) was added toinhibit the microbial activity in the sterilized control treat-ments at the start of composting. Concentrations of formalde-hyde, disodium hydrogen phosphate, and sodium dihydrogenphosphate in compost were 2.4 %, 3.9 g kg−1, and 2.4 g kg−1,respectively. Anaerobic compostings were test at a greenhousewith 25±2 °C. The sterilized effect was verified when few gasof anaerobic fermentation was produced.

Experimental setup

The anaerobic composting was conducted in 50 mL serumbottles sealed with halogenated butyl rubber. The total dryweight of the composting material including soil and organicwaste was 20.0 g in each serum bottle. At the start ofcomposting, no extra microorganism was inoculated. Theanaerobic sludge (2.0 g, wet weight) was used to inoculatetreatments MI-10, MI-2, MI-0.4, NA-2, NA-0.4, and CK at15 days to prevent acidification. Each serum bottle was con-nected to a Smith fermentation tube containing 3.0 M NaOHsolution. The acidic gases (CO2, H2S, and SO2) producedduring composting were absorbed, and the gas collected viathe vacuum-dewatering system was approximated as the vol-ume of methane. During composting, the composts weresampled at 0, 7, 14, 28, 42, 70, 105, and 140 days using adestructive sampling method. Each sample collection wasperformed in triplicate.

Analyses

After sampling, a portion of the compost sample was imme-diately extracted using deionized water with a solid-to-liquidratio of 1:10 at high-speed shaking (150 rpm) for 1 h. Allextracts were filtered using a 0.22-μm microfiltration mem-brane. The pH, soluble organic carbon (SOC), volatile fattyacids (VFAs), and soluble total nitrogen (STN) were thendetermined. The pH and SOC were measured using a pHmeter (Mettler Toledo SevenMulti S40, Switzerland) and totalorganic carbon analyzer (Shimadzu TOC-L, Japan), respec-tively. The STN was determined via the alkaline potassiumpersulfate digestion–UV spectrophotometric method (Lu1999). The VFAs, including acetic acid, propionic acid, n -butyric acid, isobutyric acid, n -pentanoic acid, andisopentanoic acid, were determined using a gas chromato-graph (GC; Techcomp GC7890II, China) with a flame ioni-zation detector (Ren et al. 2008). The GC parameters were asfollows: a carrier gas (N2) flow of 50 mL min−1, columntemperature of 180 °C, injector temperature of 230 °C, anddetector temperature of 250 °C.

Another portion of the compost sample was used to deter-mine the PCB congener concentrations using a modified EPA8082 method (Payne et al. 2011). Two grams (2.0 g) of eachsample was accurately weighed into a soxhlet thimble. Thesamples were then extracted with 50 mL hexane-acetone (1:1,v /v ) using a Soxhlet extraction system (FOSS Soxtec 2043,Denmark) by boiling for 2 h and then rinsing for 1 h at 90 °C.The extracts were concentrated to approximately 1 mL using arotary evaporator and then transferred into test tubes. Toremove any fat, wax, and other interferences, the extracts werediluted to 4.0 mL with hexane and then sulfonated twice with1.0 mL concentrated sulfuric acid. 2.0 mL of each extract wasthen purified through a Florisil column using 10mL hexane asthe eluent. The eluents were dried using a gentle stream ofhigh-purity N2 to evaporate the hexane, before re-dissolvingin 1.0 mL isooctane for analysis using a GC (Agilent 7890A,USA) equipped with a DB-1 capillary column (30 m×320 μm×0.25 μm) and an electron-capture detector. The GCparameters were: N2 as the carrier gas; an injection volume of2.0 μL in the splitless mode; a temperature program starting at100 °C for 1 min, increasing to 150 °C at 5 °C min−1, holdingfor 2 min, then increasing to 250 °C at 2 °Cmin−1 and holdingfor 5 min, and an injector temperature of 250 °C. A total of 35PCB congeners were detected: PCB4, PCB9, PCB6, PCB8,PCB19, PCB18, PCB16, PCB25, PCB28, PCB22, PCB52,

Table 1 Characteristics of rawmaterials TOC (g kg−1) TKN (g kg−1) Moisture content (%) C/N

Pig manure 324 20.0 75.4 16.18

Sawdust 494 7.4 8.5 66.76

Soil 3 0.4 – 7.5

Table 2 Detailed experimental design

Test ZVI Additive amount Sterilization

MI-10 Micron scale 10 mg g−1 No

MI-2 Micron scale 2 mg g−1 No

MI-0.4 Micron scale 0.4 mg g−1 No

MI-ST Micron scale 10 mg g−1 Yes

NA-10 Nanoscale 10 mg g−1 No

NA-2 Nanoscale 2 mg g−1 No

NA-0.4 Nanoscale 0.4 mg g−1 No

NA-ST Nanoscale 10 mg g−1 Yes

CK None added – No

CS None added – Yes

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PCB44, PCB67, PCB74, PCB66, PCB56, PCB99, PCB87,PCB110, PCB82, PCB147, PCB148, PCB153, PCB179,PCB138, PCB187, PCB174, PCB177, PCB173, PCB182,PCB199, PCB203, PCB195, PCB194, and PCB206. AllPCB standards were purchased from AccuStandard.

Results and discussion

Compost development

The SOC-to-STN ratio (C/N) is an effective indicator ofcompost maturity. Usually, lower C/N ratios indicate ahigher degree of maturity in the compost material. Thecompost can be considered mature when the C/N ratiois less than 7 (Wang et al. 2011). Based on the C/Nratios, all composting treatments reached maturity by140 days. The C/N ratios of treatments MI-10 andNA-10 were less than for CK, indicating that the addi-tion of ZVI can promote compost maturity.

The composting process is coupled with the consumptionof nutrients and stabilization of materials. The changes in theSTN and SOC represent such processes, and these trends weregenerally consistent for each treatment. As shown in Fig. 1,the SOC concentrations increased continuously from the startof composting, peaking at day 28 for all treatments exceptNA-10, after which the SOC sharply decreased and thenstabilized. The SOC concentrations for each treatment showthat the addition of ZVI can significantly reduce the amount ofSOC accumulation at day 28, except F. The differences be-tween the amounts of added ZVI were more pronounced forthe nanoscale ZVI than the micron-scale ZVI. Effect of addednanoscale ZVI was concentration dependent. Although thelower dose of nanoscale ZVI (E and F) did not significantlyaffect the composting at day 7, the addition of 10 mg g−1 hadthe greatest impact, accelerating the SOC consumption andforming an earlier SOC peak. Compare the SOC between MI-10 and NA-10, nanoscale ZVI stimulated SOC consumptionmore effective than micron-scale ZVI. It may because

nanoscale ZVI has higher chemical activity and faster rate ofelectron donating. At the beginning of composting, enoughelectron donors can contribute to the activity of methanogensand promote the consumption of VFA and SOC.

The STN formed during composting is mainly from thehydrolysis of organic macromolecules. As these compostswere formed under strict anaerobic conditions, the main typeof STN was ammoniacal nitrogen. As there was a lowerpossibility of denitrification, the reduction in the amount ofSTN is mainly from the consumption of the microorganismsduring composting. As shown in Fig. 1, the amount of STNaccumulated rapidly after the start of composting, thendropped slightly before stabilizing within a certain range. Atday 7, the addition of ZVI did not have a significant impact onSTN accumulation, but by day 70, the results indicated thataddition of ZVI may somewhat promote the consumption ofSTN, possibly because of the stimulating effect of ZVI on thegrowth of anaerobic microorganisms.

Dechlorination of PCBs during composting

The average number of Cl per biphenyl is a common param-eter that used to indicate the degree of chlorination in chlori-nated organic pollutants. Many researchers use a related index(the average number of Cl removed per biphenyl, ANDCPB)to evaluate the dechlorination of PCBs (Master et al. 2002;Payne et al. 2011). In this study, dechlorination was observedduring anaerobic composting with each treatment except thesterilized treatments (Fig. 2). Treatment NA-10, with10 mg g−1 nanoscale ZVI added at the beginning ofcomposting, showed the best dechlorination performance,with an ANDCPB of 0.63 after 140 days’ composting.Significant differences were observed in the final dechlorina-tion performances between treatment NA-10 and all othertreatments except MI-10. The other non-sterilized treatmentswere not significantly different to each other in this study. Theinitial dechlorination speed of treatment NA-10 was alsofaster than the other groups with significant dechlorinationoccurring by day 28.

Fig. 1 Changes in SOCconcentrations during anaerobiccomposting

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These results suggest that the addition of nanoscale ZVIcan enhance PCB dechlorination during anaerobiccomposting. The enhancement dechlorination due to nano-scale ZVI was concentration dependent. The addition of10 mg g−1 nanoscale ZVI sped up and enhanced the dechlo-rination by 34 % (F =2.32, P <0.05) after 140 days comparedwith the control group with no ZVI. The difference betweennanoscale and micron-scale ZVI could be because the nano-scale ZVI is more evenly distributed in compost and hasgreater reactivity than micron-scale ZVI.

Adding nanoscale ZVI can significantly improve the effi-ciency of the anaerobic dechlorination of chlorinated organics.Previous studies have identified two reasons for these effects.First, the chlorinated organic molecule reacts with ZVI on thesurface of the metal particle, removing chlorine atoms(Marshall et al. 2002). The other reason is that the corrosionof ZVI generates H2, which can stimulate microbial reductivedechlorination in anaerobic environments (Winchell andNovak 2008). No significant dechlorination was identifiedfor treatments MI-ST and NA-ST, the sterilized controls oftreatments MI-10 and NA-10, respectively, during thecomposting process, which indicates that the addition of ZVIalone cannot dechlorinate PCBs through chemical reactions ina compost system. Therefore, the enhanced dechlorinationmay be related to the stimulation of biochemical reactionsand reductive bio-dechlorination during anaerobiccomposting.

Possible enhanced dechlorination mechanisms

pH

During anaerobic composting, the pH generally decreases atfirst, then increases. The decrease in pH is caused by thehydrolytic acidification of organic material. When the pH isless than 6.5, the anaerobic system is inhibited by acid. If thehydrolytic acidification can be overcome, then the pHwill riseto 8–9, and a good anaerobic reducing environment is provid-ed by this slightly alkaline condition (Paredes et al. 2000).

As shown in Fig. 3, each treatment rapidly became acidic atthe start of composting, with the pH of most treatments below6.5 at day 7. All of the treatments except NA-10 presentedsevere acidification at day 14, and this acidification phenom-enon is not automatically reversed. Therefore, treatments MI-10, MI-2, MI-0.4, NA-2, NA-0.4, and CK were seeded withfresh anaerobic sludge and sodium hydrogen carbonate toreverse the acidification inhibition at day 15. The pH quicklyincreased and returned to normal by day 35, then continued toincrease to more than 8.

The pH values of groups MI-10, MI-2, and MI-0.4 werevaried at day 7, with the pH of treatments A and B remainingover 6.5, which indicated that larger amounts of micron-scaleZVI, added at the beginning, increased the capacity for acid-ification resistance. Treatment NA-10 was not acidified al-though its pH still decreased at day 7. Both treatments MI-10

Fig. 2 Changes in STNconcentrations during anaerobiccomposting

Fig. 3 Change in ANDCPB withdifferent treatments

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and NA-10 had the same amount of added ZVI, but themeasured pH values indicate that the acidification resistancecapacity of nanoscale ZVI was greater than micron-scale ZVI.No significant changes were seen in the pH values of treat-ments NA-2, NA-0.4, and CK at days 7 and 14, and similarly,no differences were seen in the pH of the treatments withadded nanoscale ZVI in the early stages of composting.

ZVI is a neutral, easily available electron donor, but it tendsto corrode in aqueous solutions. The corrosive action gener-ates free hydrogen that is used to stimulate microbes that liveoff hydrogen, and also produces hydroxyl groups that increasethe alkalinity (Fe0+2H2O→Fe2++2OH−+H2, ΔEθ=0.44 V).This corrosive action happens spontaneously.

An analysis of the influence of pH on PCB dechlorinationfound that microbial dechlorination in neutral and alkalineconditions were more efficient than in acid condition.Previous studies had revealed that dechlorination microbesthrive in a neutral or slightly alkaline pH range (SrinivasaVaradhan et al. 2011; Yu et al. 2006), and the activities of thesemicrobes are inhibited during the acidification phase.Therefore, the addition of ZVI helps to stabilize the acid baseenvironment in compost and thus may promote bio-dechlorination.

The corrosion of nanoscale ZVI is significantly greater thanmicro-scale ZVI with 10 mg g−1 added to compost. Nanoscale

ZVI has a smaller particle size, larger surface area and higherreactivity then micron-scale ZVI. However, nanoscale ZVI iseasily adsorbed to and surrounded by soil particles (Sirk et al.2009), which will lead to its reactivity decreasing. Therefore,the corrosion of nanoscale ZVI will be weaker than the sameamount of micron-scale ZVI at lower addition rates (2 and0.4 mg g−1).

Volatile fatty acids

At the beginning of composting, different degrees of acidifi-cation were observed with each treatment. Because of thehydrolytic acidification effect of organic materials, VFAsaccumulated quickly, and the highest concentration in thecompost reached 100,000 mg kg−1. As shown in Fig. 3, theaccumulation of VFAs is somewhat suppressed by the addi-tion of micron-scale ZVI (Zhang et al. 2012), and the VFAconcentrations in treatments MI-10 and MI-2 are significantlysmaller than in CK in day 7. The addition of 10 mg g−1

nanoscale ZVI had a greater impact on the VFA concentra-tions than micron-scale ZVI, and the highest concentration intreatment NA-10 reached 60,000 mg kg−1 at day 14, beforedecreasing to about 2,500 mg kg−1 at day 42.

Acid-forming bacteria are strongly resistant to VFAs andproduce large amounts of VFAs in the initial composting

Fig. 4 Change in pH duringanaerobic composting

Table 3 Parameters and goodness fit obtained with the logistic model

Test A (mL) Km (mL/day) λ (day) R2

MI-10 1,671±94 a 31.80±2.49 ab 38.28±0.15 c 0.9961

MI-2 1,723±82 a 32.42±0.82 a 39.70±0.76 bc 0.9955

MI-0.4 1,720±63 a 33.68±0.69 a 41.20±0.39 abc 0.9958

NA-10 1,629±151 a 25.46±5.00 b 3.45±1.53 d 0.9394

NA-2 1,799±18 a 35.19±0.10 a 43.49±1.42 ab 0.9967

NA-0.4 1,805±39 a 35.20±1.15 a 44.40±1.11 a 0.9957

CK 1,724±12 a 34.19±3.12 a 44.66±1.28 a 0.9964

The logistic model is given by: y ¼ A1þexp 4Km λ−tð Þ

Λ þ2ð Þ . Within the same columns, values with different lowercase letters (a–c) differ (P<0.05)

A biogas production potential (in milliliters), Km maximum biogas production rate (in milliliters per day), λ lag phase (in days)

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stages. The VFA accumulation can lead to the inhibition ofmethanogenesis and also dechlorination (Shrout et al. 2005).Methanogenesis is a major route for VFA consumption, and

the suitable pH range for methanogenesis is generally 6.5–7.5.The pHs of each treatment showed that MI-10, MI-2, and NA-10 were not inhibited by acid at day 7. Therefore, the lower

Fig. 5 Change in VFAconcentrations during anaerobiccomposting

Fig. 6 Daily (empty circles) and accumulated methane production (broken lines) during anaerobic composting

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VFA concentration in treatment NA-10 was not due to acidinhibition, but it may be because the nanoscale ZVI canstimulate methanogenesis by providing electron donors atthe start of composting.

Methane production rate

Methane production is a key parameter in anaerobiccomposting, and is the most direct indicator of pH and VFAinhibition. As shown in Fig. 4, all treatments underwent a longperiod of acidification except treatment NA-10. Acidificationdid occur for treatment NA-10 and its lag phase was veryshort. A logistic model was used to estimate the methaneproduction performance parameters (Li et al. 2012). The λvalues (representing the lag phase) for each treatment, showedthat the addition of ZVI can shorten the lag phase formethanogenesis (Table 3), and there was very little delay inmethane production with the addition of 10 mg g−1 nanoscaleZVI. The λ values of MI-0.4, NA-2, and NA-0.4 have nosignificant differences with CK. This may because the lowconcentration of ZVI cannot provide adequate electron andadsorption of nanoscale ZVI largely reduces its reactivity(Figs. 5 and 6).

No significant differences were found in the A values(representing the biogas production potential) between thetreatments. This result indicates that although the addition ofZVI can provide electrons to reduce the methanogenesis lagphase, it does not affect the biogas production potential duringcomposting. It also proves that the biogas production potentialwas mainly related to the amount of organic material in thisanaerobic composting system. Compared with organic mate-rial, ZVI provided few electron donors for methaneproduction.

The lag times of the methanogenesis and dechlorina-tion processes were closely related. This may becausethe methanogenic bacteria and dechlorination bacteriahave relatively similar growing conditions, and bothuse hydrogen and VFAs as energy sources. The largenumber of electrons donated by ZVI can stimulate theactivity of methanogenic bacteria (Xiu et al. 2010) andmay also be used by dechlorination bacteria (SrinivasaVaradhan et al. 2011).

Methane production mainly occurs during a short timeperiod in anaerobic composting. In this study, large amountsof methane were produced about 40 days after the lag phase,and large amounts of hydrogen and VFAs were consumedduring this time. When the VFA concentrations reduced, themethane production rates decreased rapidly, but the dechlori-nation rates did not decrease much. This suggests that thedechlorination reaction does not require much VFA or hydro-gen as electron donors. In this study, competitive inhibitionbetween the methanogenic and dechlorination microorgan-isms was not found. This indicates that composting can

provide adequate substrate to methanogenesis and dechlori-nation. In addition, previous studies also observed that reduc-tive dechlorination occurred under methanogenic conditions(Holoman et al. 1998; Nies and Vogel 1990).

Overall, the addition of ZVI can enhance the dechlorinationeffect of anaerobic composting. Hydroxyl groups formed byZVI corrosion can buffer the pH changes, maintaining the pHin an optimal range and creating a suitable environment formicrobial dechlorination. Free hydrogen generated by ZVIcan stimulate the activity of methanogenic bacteria and theconsumption of VFAs, thus reducing the amount of VFAs ableto inhibit dechlorination.

Conclusions

The addition of nanoscale ZVI can enhance dechlorinationduring anaerobic composting. After 140 days of composting,the average number of removed Cl per biphenyl with10 mg g−1 of added nanoscale ZVI was 0.63. This equatesto a 34 % improvement in dechlorination, and an improvedinitial dechlorination speed. The possible explanations for theenhanced dechlorination by ZVI include providing a suitableacid base environment, reducing VFA inhibition, and a degreeof microbial stimulation effect. Finally, the C/N ratios fortreatments MI-10 and NA-10 were lower than the control,indicating that added ZVI can also promote compost maturity.

Acknowledgments This work was financially supported by FundedProject for Youth Researcher of Zhejiang Gongshang University (QY11-22) and Innovative Research Team in Higher Educational Institutions ofZhejiang Province (T200912).

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