The Effect Of Exogenous Electron Acceptors On The Reductive Dechlorination Of PCP

Pentachlorophenol(PCP)is widely used in China as herbicide,insecticide,fungicide and wood antiseptic.Due to its relative persistence,high toxicity and long half-life in the natural environment,it still has a profound impact on the environment.In the long-term flooding conditions,microbial anaerobic dechlorination process is the most important way to degrade PCP.Iron,sulfate and nitrate are important competitive electron acceptors in the natural soils,they could occur simultaneously with the reductive dechlorination of PCP,which made the process of PCP degradation more complexed.PCP was selected as the target compound in this paper.By adding the exogenous electron acceptors,the anaerobic batch experiment was conducted to explore the anaerobic PCP degradation in two different soils(paddy soil and sulfate soil)and the corresponding reponse in microbial structure.We also studied the reductive dechlorination of PCP in the enrichment by enriching the sulfate-reducing bacteria(SRB)culture.The main findings are as follows:(1)The completed reductive dechlorination of all treatments in the paddy soil was up to 40 days incubation.Both sulfate and nitrate showed inhibitory effect on the reductive dechlorination of PCP,compared with the CK without any sulfate or nitrate addition.The increased sulfate and nitrate concentrations all resulted in the decreased removal rates of PCP successively.And once the electron donors were sufficient for completing dechlorination of PCP,the increased sulfate and nitrate reduction only inhibited the early process of dechlorination.The pathway of PCP dechlorination was started from ortho position,and then dechlorinated at ortho,para,meta position to form 3-MCP as final product.The dominant genera were detected including the genus Desulfobacca,Syntrophomonas,Pelotomaculum,Geobacter,Pesudomonas,Desulfitobacterium,Desulfovibrio,Clostridium,Sedimentibacter and Caloramator based on 16S rRNA data.The known dechlorinator,the genera Desulfitobacterium that was detected in all treatments was likely accounted for the reductive dechlorination of PCP.Although soil bacterial community structure shared similar dominate species following addition of nitrate and sulfate respectively,some specialized functional species were also responded contrastingly to the addition of sulfate and nitrate,with the genera of Desulfovibrio and Caldicoprobacte enriched in the treatments with sulfate and nitrate individually.(2)Reductive dechlorination of PCP in the sulfate soil started on day 60.Among all treatments,the maximal PCP transformation rate was about 22.5%in the treatment with 20 mM Na2MoO4 and the smallest PCP transformation rate was 7.5%in the treatment with 20 mM NaNO3 at the end of incubation(on the day 90).The residual amount of PCP in different treatments with nitrate was in the order of 20 nM>10 mM>1 mM>5 mM.Also,the intermediate product of PCP was only 2,3,4,5-TeCP dechlorinated by ortho-position dechlorination.The effect of nitrate addition on PCP degradation showed two different results compared to CK:low concentrations of nitrate(1 mM and 5 mM nitrate)promoted the dechlorination of PCP;while high concentrations of nitrate(10 mM and 20 mM nitrate)inhibited the dechlorination of PCP.The microbes of sulfate soil are similar to other mangrove soils,mainly composed of Clostridia,Alphaproteobacteria,Gammaproteobacteria and Deltaproteobacteria;the genus of Dethiobacter and Alkaliphilus was rich in the sulfate soil due to its high salinity.In addition,the genera such as Anaeromyxobacter,Geobacter,Dehalobacter that may have dechlorination function were also dectected in the tested mangrove soil.(3)By enriching the sulfate-reducing bacteria(SRB)culture,we found that SRB culture was tolerant to PCP but could not degrade PCP.The high concentration of PCP resulted in the decrease of the sulfate reduction rate.Therefore,the high concentration of PCP concentration was toxic to the sulfate-reducing culture and had negative effect on the reduction of PCP.By studying the reductive dechlorination of PCP in two completely different soils(paddy soil and sulfate soil),this study has demonstrated that:by adding sufficient electron donors,the inhibiton of reductive dechloriantion of PCP resulted from the electron donors competition between the reduction of SO42-and NO3-and the dechlorination of PCP would be reduced;In addition,by inhibiting the sulfate reduction and adding appropriate amount of nitrate,we could control the reductive dechlorination of PCP in the rich-sulfate but deficient-nitrogen soil,to achieve more efficiently degradation.This study would provide a theoretical basis for the remediation of different types of PCP polluted soil.