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Journal of Hazardous Materials 261 (2013) 826– 832

Contents lists available at SciVerse ScienceDirect

Journal of Hazardous Materials

j o ur nal homep age: www.elsev ier .com/ locate / jhazmat

echlorination of polychlorinated biphenyl-contaminated soil vianaerobic composting with pig manure

hi Zhanga,b, Yao Dua, Xiao-Qing Taoa, Kun Zhanga, Dong-Sheng Shena, Yu-Yang Longa,∗

Zhejiang Provincial Key Laboratory of Solid Waste Treatment and Recycling, School of Environmental Science and Engineering, Zhejiang Gongshangniversity, Hangzhou, 310012, ChinaZhejiang Environmental Science & Design Institute, Hangzhou, 310007, China

i g h l i g h t s

Increase of PCB concentration not disrupt the anaerobic composting process but inhibit dechlorination.Hormesis effect increases nitrogen consumption and methane production when the PCB was 10 mg kg−1.Dechlorination of PCB-contaminated soil via anaerobic composting with pig manure is feasible.

r t i c l e i n f o

rticle history:eceived 29 August 2012eceived in revised form 30 April 2013ccepted 7 May 2013vailable online 9 July 2013

a b s t r a c t

Anaerobic dechlorination is an effective degradation pathway of higher chlorinated polychlorinatedbiphenyls (PCBs). The efficiency of anaerobic composting remediation of PCB-contaminated soil usingpig manure was determined. The results show that the dechlorination of PCB-contaminated soil viaanaerobic composting with pig manure is feasible. PCB concentration is the most critical factor. Ele-vated PCB concentrations can inhibit dechlorination but does not disrupt the anaerobic fermentation

eywords:olychlorinated biphenylechlorinationnaerobicompostingig manure

process. At 1 mg kg−1 PCBs, the degradation rate of five or more chlorinated biphenyls is 43.8%. The high-est dechlorination performance in this experiment was obtained when the soil-to-organic waste ratio,carbon-to-nitrogen ratio, moisture content, and PCB concentration were 2:3, 20, 60%, and 1 mg kg−1,respectively.

© 2013 Elsevier B.V. All rights reserved.

. Introduction

Polychlorinated biphenyls (PCBs) belong to the first batch of 12ompounds listed as persistent organic pollutants (POPs) by thetockholm Convention on POPs in 2001. PCBs are typical POPs char-cterized by their persistence in the environment, bioaccumulationn human and animal tissues, biomagnification in food chains, andignificant biological toxicity. The mass production in the 1930so the 1970s resulted in the discarding of at least 300,000 t ofCBs into the environment [1]. Moreover, the moderate volatil-ty and long-range atmospheric transport of PCBs facilitated their

ide distribution throughout the global environment. Surface soils

ave become an important reservoir for PCBs because of their highydrophobicity [2,3]. In China, soil PCB contamination has becomeore prominent with the development of related contamination

∗ Corresponding author. Tel.: +86 571 88832369; fax: +86 571 88832369.E-mail address: longyy@zjgsu.edu.cn (Y.-Y. Long).

304-3894/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.jhazmat.2013.05.060

surveys [4]. Pollution concentration can reach tens or hundredsof mg kg−1, especially in the surrounding soils of some capacitor-dismantling and stockpiling facilities [5,6].

Although PCBs are refractory and chemically stable, a largenumber of aerobic microbes can transform these compounds intochlorobenzoic acids under a suitable aerobic environment [7]. TheBurkholderia xenovorans strain LB400, the best microbe in PCBdegradation yet discovered, can degrade more than 20 PCB con-geners [8]. However, the biodegradability of PCBs decreases withincreasing chlorine content. PCBs with five or more chlorines areextremely difficult to degrade. PCB dechlorination and the accumu-lation of lower chlorinated congeners were observed in anaerobicsediments from different sites [9–11]. A number of studies also con-firmed that the reductive dechlorination of PCBs occurs in naturalor artificial anaerobic environments [12,13]. The carbon source is

a critical factor that enhances microbial activity and PCB bioavail-ability. It also provides electrons for the reductive dechlorinationof PCBs [14]. Variations in the carbon/electron source affect thedechlorination rate but not the dechlorination pathway [15]. In

C. Zhang et al. / Journal of Hazardous Materials 261 (2013) 826– 832 827

Table 1Characteristics of raw materials.

TOC (g kg−1) TKN (g kg−1) Moisture content (%) C/N

Pig manure 340 20.9 64.5 16.27

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Table 2Detailed design of orthogonal experiments.

Test S/W C/N Moisture content (%) PCBs (mg kg−1)

A 3:2 25 50 10B 3:2 30 60 1C 3:2 20 40 50D 1:1 30 40 10E 1:1 20 50 1F 1:1 25 60 50G 2:3 20 60 10

50 mL min−1, a column temperature of 180 ◦C, an injector temper-

Sawdust 494 7.4 8.5 66.76Soil 3 0.4 – 7.5

ddition, the introduction of organic fertilizers or urea in soil canignificantly promote dechlorination [16].

At present, the treatment of solid waste such as kitchen waste,ivestock manure, agricultural waste, and municipal sewage sludgeia composting is widely applied. The composting of soil contami-ated with organic pollutants is a new bioremediation technologyhat has attracted increasing attention [17]. Aerobic compostingas been used in soil contaminated by organics such as polycyclicromatic hydrocarbons [18], chlorophenol [19], petroleum hydro-arbons [20], pesticides [21], and trinitrotoluene [22]. Although thenaerobic composting of contaminated soil is energy saving, it haseen less studied than aerobic composting to date mainly becausef odor emissions as well as the time-consuming process. Moremportantly, anaerobic dechlorination is an irreplaceable metabolicathway for persistent chlorinated organic pollutants. The envi-onmental risks can be reduced in three aspects when the higherhlorinated PCBs are converted to the lower chlorinated ones.irst, the lower chlorinated PCBs are more biodegradable than theigher ones. Hence, the dechlorination process can enhance theiriodegradation and removal efficiency. Second, the bioaccumula-ion effects of the higher chlorinated PCBs are greater than those ofhe lower ones. Thus, the anaerobic transformation of PCBs can helpeduce the accumulation risk in the food chain. Finally, the transfor-ation of higher chlorinated PCBs to less toxic lower ones can also

educe the carcinogenicity and toxicity of dioxin analogs. Para- andeta-dechlorination have higher probabilities of occurring than

rtho-dechlorination. Thus, coplanar PCBs (non-ortho-substitutedCB congeners, which have similar structure and toxicity to dioxin)re hardly generated and can be easily degraded [23].

Pig manure is a common agricultural organic waste that can besed as highly effective carbon and nitrogen sources for compost-

ng. This study aims to investigate the possibility of co-processingCB-contaminated soil with pig manure via anaerobic composting.he corresponding influencing factors on PCB dechlorination andhe effects of PCBs on the anaerobic digestion system were alsonvestigated.

. Materials and methods

.1. Soils and organic wastes

PCB-contaminated soil was simulated using soil and Aroclor260 (a commercial mixture of PCBs containing 12 carbon atomsnd 60% chlorine). The soil was obtained from the surface layer0–20 cm) of cultivated yellow soils in Hangzhou, Zhejiang, China.he soil was classified as a yellow-brown loam and was determinedo be free of Aroclor 1260. Dried soils were sieved through a 1 mm

esh sieve to remove large stones and plant roots prior to use. Aro-lor 1260 was dissolved in acetone and then thoroughly mixed withhe soils. Finally, the spiked soils were equilibrated in a greenhouse5 days before use.

Pig manure was collected from a livestock farm in Hangzhou.ried pig manure was crushed to an approximately 1 mm grain

ize. Sawdust was used as a composting amendment to adjust the

arbon-to-nitrogen ratio (C/N). The characteristics of the organicastes and soil are shown in Table 1. Anaerobic sludge was col-

ected from a papermaking factory (Zhejiang) for the treatment of

H 2:3 25 40 1I 2:3 30 50 50

papermaking wastewaters in an internal circulation reactor. Theseed wet sludge had a concentration of 14.90% TS and 11.07% VS.

2.2. Experimental design

The orthogonal test L9(34) was used to investigate the effects offour factors: soil-to-organic waste ratio (S/W), C/N, moisture con-tent, and PCB concentration. First, pig manure and sawdust weremixed at different ratios to obtain 20, 25, and 30 C/Ns. S/Ws of 2:3,1:1, and 3:2 were obtained by adjusting the soil and organic wasteproportions. Tap water was used to adjust the matrix moisture con-tent. PCB concentrations of 1 mg kg−1 (potential public health risklevel of China), 10 mg kg−1, and 50 mg kg−1(hazardous waste level)were prepared using stock PCB-contaminated soil. The experimen-tal details are shown in Table 2.

2.3. Experimental setup

The anaerobic composting was conducted in 50 mL serum bot-tles sealed with halogenated butyl rubber. The total dry weight ofthe compost substance, including soils and organic wastes, in eachserum bottle was 20.0 g. Anaerobic sludge (2.0 g, wet weight) wasthen inoculated. Each serum bottle was connected to a Smith fer-mentation tube containing 3.0 M NaOH solution. The acidic gases(CO2, H2S, and SO2) produced during composting were absorbed,and the gas collected via the vacuum-dewatering method wasapproximated as the volume of methane. During composting, thecomposts were sampled at 0, 7, 21, 35, 63, and 98 days using thedestructive sampling method. Each sampling was performed intriplicate.

2.4. Analyses

After sampling, the compost samples were immediatelyextracted using deionized water with a solid-to-liquid ratio of 1:10at 150 rpm for 1 h. All extracts were filtered using a 0.22 �m micro-filtration membrane. The pH, soluble organic carbon (SOC), volatilefatty acid (VFA), total nitrogen (TN), and ammonia nitrogen (NH4

+-N) were then determined.

The values of pH and SOC were measured using a pH meter(Mettler Toledo SevenMulti S40, Switzerland) and a total organiccarbon analyzer (Shimadzu TOC-L, Japan), respectively. TN andNH4

+-N were determined via the alkaline potassium persulfatedigestion-UV spectrophotometric method and the Nessler colori-metric method [24]. VFAs, including acetic acid, propionic acid,n-butyric acid, isobutyric acid, n-pentanoic acid, and isopentanoicacid, were determined using a gas chromatograph (GC, Tech-comp GC7890II, China) with a flame ionization detector [25]. Thedetection parameters are as follows: a carrier gas (N2) flow of

ature of 230 ◦C, and a detector temperature of 250 ◦C.A portion of the compost sample was used for the determina-

tion of PCB congeners via a modified EPA 8082 method. Each 2.0 g of

828 C. Zhang et al. / Journal of Hazardous Materials 261 (2013) 826– 832

Table 3Changes of pH and C/N of composting.

Time/d A B C D E F G H I

pH 0 6.37 6.41 6.53 6.46 6.98 6.30 6.43 7.37 7.6635 8.61 8.19 8.74 8.72 8.55 8.30 8.38 8.58 8.4898 8.29 8.02 8.62 8.57 8.44 8.12 8.25 8.66 8.58

SOC (mg kg−1) 0 6128 5326 5095 6002 5606 6821 6813 8391 788335 27,365 26,723 26,935 24,490 23,010 33,123 33,675 26,685 27,90598 9751 6773 10,178 9978 10,375 10,273 11,060 11,633 9141

TN (mg kg−1) 0 549 202 351 371 321 395 744 1117 70335 1307 928 1810 1254 1820 1776 2384 1988 135498 1486 1106 1527 1036 1230 1802 2357 2031 1045

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C/N 0 11.2 26.4 14.5

35 20.9 28.8 14.9

98 6.6 6.1 6.7

ir-dried sample was accurately weighed in a filtration paper cylin-er. The samples were then extracted with 50 mL hexane–acetone1:1, v/v) using a Soxhlet extraction system (FOSS Soxtec 2043,enmark) by boiling for 2 h and then rinsing for 1 h at 90 ◦C. Thextracts were concentrated to approximately 1 mL using a rotaryvaporator and then transferred into test tubes. To remove theat, waxy, and other interfering substances, the extracts were fur-her diluted to 4.0 mL with hexane and then sulfonated twiceith 1.0 mL concentrated sulfuric acid. Approximately 2.0 mL of

ach extract was then purified through a Florisil column using0 mL hexane as the eluent. The effluents were dried using a gen-le stream of high-purity N2 to evaporate the hexane and thenissolved in 1.0 mL isooctane for GC analysis using a gas chromatog-aphy (Agilent 7890A, USA) equipped with a DB-1 capillary column30 m × 320 �m × 0.25 �m) and an electron-capture detector. N2as used as the carrier gas. The injection volume was 2.0 �L, and

he analysis was performed in the splitless mode. The tempera-ure program was initiated at 100 ◦C for 1 min, increased to 150 ◦Cor 2 min at 5 ◦C min−1, and then to the final temperature 250 ◦Cor 5 min at 2 ◦C min−1. The injector temperature for all analysesas 250 ◦C. A total of 35 PCB congeners were detected: PCB4, PCB9,

CB6, PCB8, PCB19, PCB18, PCB16· ·PCB25, PCB28, PCB22, PCB52,CB44, PCB67, PCB74, PCB66, PCB56, PCB99, PCB87, PCB110, PCB82,CB147, PCB148, PCB153, PCB179, PCB138, PCB187, PCB174,CB177, PCB173, PCB182, PCB199, PCB203, PCB195, PCB194, andCB206. All standard PCBs were purchased from AccuStandard.

. Results and discussion

.1. Characteristics of the composting process

The variations in the matrix pH, SOC, and TN during anaer-

bic composting are shown in Table 3. The pH of all testedroups increased from the initial neutral (6.30–7.66) to alkaline8.19–8.76) on day 35 and then remained stable. These results sug-est that none of the tested groups underwent an acidification

able 4NDCPB of different treatments.

Test 7 d 21 d

A −0.029 ± 0.054 0.02 ± 0.095

B −0.029 ± 0.069 0.256 ± 0.014

C −0.021 ± 0.014 0.113 ± 0.023

D 0.073 ± 0.020 0.198 ± 0.044

E 0.271 ± 0.032 0.400 ± 0.054

F 0.032 ± 0.015 0.058 ± 0.034

G 0.004 ± 0.043 0.168 ± 0.126

H 0.084 ± 0.055 0.053 ± 0.050

I −0.016 ± 0.012 −0.034 ± 0.006

16.2 17.5 17.3 9.2 7.5 11.219.5 12.6 18.7 14.1 13.4 20.6

9.6 8.4 5.7 4.7 5.7 8.7

phase, which is a routine stage of anaerobic digestion. Under suchpH circumstances, the compost has a higher reducing capacity [26].

The matrix SOC and TN continuously increased until day 35.Afterward, SOC sharply decreased to approximately 1000 mg kg−1

on day 98. However, TN remained at a relatively stable level. TheSOC-to-TN ratio (C/N) in the water extract is an effective indicatorof compost maturity [20]. The composting process can be consid-ered complete when the C/N is below 7. Table 3 shows that the C/Nrapidly decreased after day 35, reaching a value <7 at day 98, exceptfor those of groups D, E, and I. However, the range and varianceanalysis results of C/N based on the PCB concentration (Table 6)indicates that the PCB concentration did not significantly affectcompost maturity. Therefore, the co-disposal of PCBs via com-posting is feasible, but the dechlorination effect requires furtherinvestigation.

3.2. Dechlorination of PCBs in composting

3.2.1. Effects of factorsGenerally, the average number of Cl per mole is regarded as

an ideal and visualized parameter for the assessment of the chlo-rination extent of chlorinated organic compounds. Therefore, theaverage number of dechlorinated Cl per biphenyl (ANDCPB) is usedto evaluate the dechlorination performance. In this study, the PCBsunderwent a certain degree of dechlorination after 98 days ofmature composting. Table 4 shows that the ANDCPBs of groups B, E,and H, which all contain low PCB concentrations, were 0.455, 0.457,and 0.392, respectively. These groups exhibited higher dechlori-nation effects than others groups with higher PCB concentrations.

Based on the range and variance analysis results of ANDCPBon day 98 (Table 5), the ranking of the factors affecting PCBdechlorination is as follows: PCB concentration > C/N > moisturecontent > S/W. The optimum dechlorination performance was

obtained when the S/W, C/N, moisture content, and PCB concentra-tion were 2:3, 20, 60%, and 1 mg kg−1, respectively. Of these factors,PCB concentration was the most sensitive. It also exhibited a sig-nificant effect on dechlorination. Increases in PCB concentration

35 d 63 d 98 d

0.009 ± 0.066 0.026 ± 0.033 0.096 ± 0.0250.329 ± 0.197 0.434 ± 0.150 0.455 ± 0.0540.121 ± 0.010 0.106 ± 0.019 0.101 ± 0.0390.212 ± 0.093 0.200 ± 0.003 0.120 ± 0.0090.375 ± 0.039 0.470 ± 0.044 0.457 ± 0.0090.053 ± 0.018 0.059 ± 0.010 0.098 ± 0.0050.213 ± 0.149 0.155 ± 0.051 0.234 ± 0.0250.320 ± 0.051 0.287 ± 0.041 0.392 ± 0.020

−0.004 ± 0.019 0.040 ± 0.011 0.101 ± 0.006

C. Zhang et al. / Journal of Hazardous

Table 5Parameters and goodness fit obtained with the logistic modela

Test A (mL) Km (mL day−1) � (days) R2

A 884 19.97 17.14 0.9985B 816 18.37 12.37 0.9945C 753 14.14 18.40 0.9991D 561 8.36 14.94 0.9983E 1076 22.65 18.23 0.9964F 1323 36.07 18.53 0.9980G 1770 41.19 19.81 0.9978H 535 7.45 8.95 0.9963I 825 13.19 16.85 0.9945

A, Km , � represented biogas production potential (in milliliters), maximum biogasp

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roduction rate (in milliliters per day), and lag phase (in days), respectively.a The logistic model is given by: y = A

1+exp((4Km(�−t)/A)+2) .

nhibit dechlorination. Therefore, the appropriate PCB concentra-ion is a key factor of dechlorination, which is consistent with theesults of Ho and Liu [11].

Meanwhile, C/N also had a significant effect on dechlorination.he effects of moisture content and S/W were not significant.igher moisture contents and lower S/Ws showed negligible effectsn reducing the inhibition.

.2.2. Dechlorination behaviorThe lower chlorinated PCBs were significantly degraded and

echlorinated by the reductive conditions in groups B, E, and Hfter 98 days of anaerobic composting. The amount of chlorinatedCB congeners containing five or more Cl decreased, whereas thatf PCB congeners containing four Cl and below increased (Fig. 1).his phenomenon indicates that higher chlorinated PCB congenersere converted to lower chlorinated PCB congeners during anaer-

bic composting. After 98 days, the average degradation rates ofve- to nine-chlorinated PCB congeners were 43.5%, 39.8%, 42.7%,2.6%, and 78.9%, respectively. The total degradation rate of five-nd above-chlorinated PCB congeners was 43.8%.

Meanwhile, the dechlorination efficiencies under different C/Natios were similar, except for that at a C/N ratio of 20, in which a sig-ificantly higher efficiency was observed. A lower C/N condition canrovide significantly more nitrogen for the growth of dechlorinat-

ng microbes. The moisture content and S/W also slightly affectedhe dechlorination efficiency. Therefore, all conventional compost-ng conditions in this experiment can provide a suitable growingnvironment and relatively sufficient electron donors for dechlori-ating microbes.

The results show that anaerobic composting using pig manures an effective PCB dechlorination and degradation method. Theigher the chlorination of PCB congeners, the more efficienthe dechlorination performance would be. During the anaerobiciodegradation of organics, hydrogen-producing acetogenic bacte-ia can produce metabolites such as acetic acid and hydrogen,hich decrease the redox potential in the environment. Hydrogen

r acetic acid can release electrons under the function of hydroge-ase and dehydrogenase. Electrons are then used by some specieshat can reductively dechlorinate PCBs by utilizing PCBs as termi-al electron acceptors [27]. In a low redox potential status, theigher the chlorination of PCB congeners, the easier the nucleo-hilic attack by the reductant. Therefore, higher chlorinated PCBongeners have higher anaerobic biodegradability [28]. Many stud-es showed that the introduction of organic substrates contributeso PCB dechlorination [15,16]. Thus, dechlorinating microbes mayave a broad spectrum for organic substrates. Overall, the dechlo-

ination of PCB-contaminated soil by anaerobic composting withig manure provides nutrition for microbial growth and offers a

ower redox potential and electrons for effective biological dechlo-ination.

Materials 261 (2013) 826– 832 829

3.3. Effect of PCB concentration on composting coupled withdechlorination

3.3.1. Methane productionMethane production is a key parameter of anaerobic com-

posting. As shown in Fig. 2, the methane production significantlydiffered among the tested groups. Group G had the highest accu-mulated methane production in 98 days, whereas group H had thelowest one. The average methane production rates of these twogroups were 18.6 and 5.4 mL day−1, respectively. More importantly,group G also had the highest daily methane production on day 36.Although the methane production of the groups showed signifi-cant differences, their changing trends were similar. Moreover, theamounts of accumulated methane production showed an S-shapedgrowth. The initial and final inhibition points occurred at days 7 and65, respectively.

A logistic model was used to estimate the methane produc-tion performance parameters [29]. The R2 of each group exceeded0.99, indicating that methane production was well fitted by thelogistic model (Table 5). The range and variance analysis results ofthe biogas production potential (A-value) (Table 6) suggest that allfactors had significant effects on methane production. The effectswere ranked as follows: moisture content > C/N > PCBs concen-tration > S/W. The methane production of the groups containing10 mg kg−1 PCB was higher than that of the groups contain-ing 1 mg kg−1 PCB. However, the methane production decreasedwhen the PCB concentrations reached 50 mg kg−1. In other words,the methane production initially increased and then substantiallydecreased with as PCB concentration increased. As an environ-mental hormone, PCBs could stimulate methane production [30].It showed a trend of “weak promotion and strong inhibition.”

3.3.2. VFAs and ammonia nitrogenIn anaerobic composting, the accumulation of VFAs and ammo-

nia nitrogen can lead to acid inhibition and ammonia inhibition,respectively. That is, the excess acid and ammonia concentrationscan disrupt the anaerobic composting system, as indicated by thedecrease in the methane production rate. Therefore, analyses ofacid and ammonia inhibitions during the composting process canprovide additional information on the effect of PCB concentrationon composting with dechlorination.

As shown in Fig. 3, the VFA concentrations increased until day 7and then continually dropped. As a result, the daily methane pro-duction of each group showed a significant decrease at around day 7(Fig. 2). These results indicate the occurrence of acid inhibition. Thecontinuous consumption of SOC and accumulation of water-solublenitrogen decreased the daily methane production at around day 65,indicating the occurrence of ammonia inhibition.

The range and variance analysis results for the VFA concen-tration at day 7 are shown in Table 6. All factors had significanteffects on the VFA concentration. The VFA concentrations in thegroups containing 10 mg kg−1 PCB were higher than those con-taining 1 mg kg−1 PCB. In addition, the 50 mg kg−1 PCB groupsgenerated less VFAs than the other groups. The range and varianceanalysis results for the ammonia concentration at day 65 indicatethat all factors exhibited significant effects. The ammonia concen-trations in the 10 mg kg−1 PCB groups were clearly lower than thosein the other two groups.

The range analysis results for the VFA and methane productionshowed a similar trend of “weak promotion and strong inhibi-tion.” VFAs are important intermediates of anaerobic digestion andare necessary raw materials for methane production. However,

high VFA concentrations can lead to acid inhibition, such as thatobserved on day 7. However, a rapid decline in VFAs indicates thenecessity for the rapid adjustment of the compost. Therefore, VFAshad no significant inhibition on the anaerobic digestion system.

830 C. Zhang et al. / Journal of Hazardous Materials 261 (2013) 826– 832

Fig. 1. The difference between day 0 and day 98 of PCB congeners mole percent in group B, E and H.

Fig. 2. Variation of daily (©) and accumulated methane production (...) of anaerobic composting.

C. Zhang et al. / Journal of Hazardous Materials 261 (2013) 826– 832 831

Table 6Range and variance analyses of orthogonal experiment.

Factors k1 k2 k3 Ranges P-value

C/N S/W 6.4986 7.9019 6.3168 1.5851 0.0001C/N 6.5988 6.0619 8.0565 1.9946 0.0001Moisture content 7.3013 7.8982 5.5178 2.3804 0.0001PCBs levels 6.7180 6.9750 6.9843 0.2664 0.6683

ANDCPB S/W 0.2112 0.2250 0.2419 0.0307 0.3997C/N 0.2637 0.1950 0.2194 0.0686 0.0317Moisture content 0.2039 0.2181 0.2560 0.0521 0.0937PCBs levels 0.4283 0.1499 0.0999 0.3285 0.0001

A-valuea S/W 819 988 1050 231 0.0001C/N 1202 916 935 464 0.0001Moisture content 1050 738 1302 682 0.0001PCBs levels 813 1072 971 258 0.0001

VFAs S/W 10,959 14,820 19,337 8378 0.0001C/N 17,912 16,681 10,523 7589 0.0001Moisture content 12,934 14,315 17,867 4932 0.0001PCBs levels 15,110 16,347 13,659 2420 0.0278

NH+4-N S/W 1054 1321 1350 265 0.0001

C/N 1355 1324 1046 277 0.0001Moisture content 1339 1195 1191 134 0.0001

ential

eAmadaPTa

PCBs levels 1405

a A-value is a parameter of logistic model that represented biogas production pot

In the composting process, C/N continuously decreases andventually leads to ammonia nitrogen inhibition in the late stage.mmonia nitrogen inhibition markedly differs with the environ-ental conditions and microbial ecosystems [31,32]. The range

nalyses results for the ammonia concentration and methane pro-uction showed trends that are opposite that of VFAs (Table 6). Themmonia nitrogen inhibition in the groups containing 10 mg kg−1

CBs was significantly lower than that in the other two groups.his result suggests that the addition of PCBs can promotemmonia consumption and alleviate the inhibition phenomenon.

Fig. 3. Variation of VFAs (�) and NH4+-N conc

1055 1265 316 0.0001

.

Stimulatory responses induced by low doses of toxic substanceshave been described in a number of studies [33] and are called“hormesis.” The stimulating effect of low concentrations of toxicsubstances can promote cell growth under stress [34]. Therefore,10 mg kg−1 PCBs increased nitrogen consumption and methaneproduction by promoting the growth of anaerobic microorganisms.

Overall, the increase in PCB concentration does not lead to dis-

ruptions in the anaerobic composting process. Methane productionand hydrogen production were also not inhibited. The condition inwhich the PCB concentration is constant is called “hormesis.” In this

entrations (�) of anaerobic composting.

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[33] E.J. Calabrese, L.A. Baldwin, Chemical hormesis: its historical foundations as a

32 C. Zhang et al. / Journal of Haza

ondition, the inhibitory effect of PCBs on microbial dechlorinationay be directly related to the dechlorinating microbes.

. Conclusions

The dechlorination of PCB-contaminated soil via anaerobic com-osting with pig manure is feasible. The highest dechlorinationerformance was obtained when the S/W, C/N, moisture content,nd PCB concentration were 2:3, 20, 60%, and 1 mg kg−1, respec-ively. PCB concentration is the most critical factor. The degradationate (of five or more chlorinated biphenyls) was 43.8%. This valueas obtained when the PCB concentration was 1 mg kg−1. The

ncrease in the PCB concentration did not disrupt the anaerobicomposting process but can inhibit dechlorination. It indicates thathe inhibitory effect of PCBs on the microbial dechlorination mayirectly relate to the dechlorination microbes.

cknowledgements

This work was financially supported by Funded project forouth researcher of Zhejiang Gongshang University (QY11-22),esearch Plan of Department of Education of Zhejiang ProvinceY201119953), Innovative Research Team in Higher Educationalnstitutions of Zhejiang Province (T200912), and New talent planf Zhejiang province (2011R408051).

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