Regulation Of Biochar On The Reductive Transformation Of Typical Organochlorine Pesticides In Flooded Soil And The Involoved Mechanism

The dechlorination rate of organochlorine pesticides?OCPs?under anaerobic conditions would be comprehensively regulated by factors such as degrading microorganisms,electron donors/mediators,and redox conditions.Research on its dechlorination in anaerobic soils has attracted much attention in recent decades.Biochar,as a new comprehensive remediation material for soil pollution and degradation,has shown great potential in environmental remediation.However,most of the previous studies were mainly focused on the aerobic conditions of the dry farming system,its remediation mechanism under the anaerobic conditions of the wet system is not clear yet.Therefore,we took pentachlorphenol?PCP?as an example at first,and investigated the effects of biochar on the reductive transformation of PCP and the typical soil redox processes?such as the dissimilatory iron and sulfate reduction and methanogenesis?in natural flooded soil.Then,in the presence of biochar,the regulation effect of sulfate reduction on the reductive transformation of PCP was investigated.Furthermore,taking lindane as an example,the effects and involved microbial mechanisms of biochar mediated electron donors/acceptors regulation on the reductive transformation of typical OCPs and the methanogenesis process were studied.Finally,the possibility and its influencing factors of biochar as an electron shuttle affecting the reductive transformation of OCPs in the anaerobic environment were analyzed in a pure-phase system.Based on the compound-specific isotope analysis?CSIA?,the mechanism of biochar on anaerobic reduction of the typical OCPs was revealed.The main findings of this study are as follows:?1?The effects of biochar on reductive dechlorination of PCP under iron/sulfate reduction and methanogenesis co-occurring environment were systematically investigated.Biochar amendment suppressed the dechlorination of PCP,but promoted iron/sulfate reduction and accelerated methanogenesis.By analyzing the co-occurrence relationship between microorganisms and reduction processes,this was likely mediated by the core functional microbial groups that responded sensitively to the addition of biochar and/or PCP,including the genus Dethiobacter,Clostridium,Geosporobacter,Desulfuromonas,Desulfatitalea and Methanosarcina.Biochar was also possibly involved as electron mediator through regulating electron distribution during microbial anaerobic respiration.?2?By adding molybdate as a microbial sulfate reduction inhibitor to regulate the soil sulfate reduction process,and setting up sodium 2,6-anthraquinone disulfonate?AQDS?treatment as an electron shuttle to regulate the electron transfer process,the effect and mechanism of sulfate reduction process on the reductive transformation of PCP in the presence of biochar were further explored.The main mechanism of biochar addition on soil sulfate reduction process was different from the iron reduction process,which was mainly achieved by changing the abundance and activity of functional microorganisms,rather than affecting the process as an electron transfer medium.Combined with the analysis of 16S r RNA high-throughput sequencing,it showed that biochar addition significantly changed the soil microbial community structure,increased the relative abundance of sulfate reducing bacteria?Desulfobulbaceae,Desulfobacteraceae?,and methanogens?Methanosarcina,Methanolobus?,thereby relatively inhibited PCP dechlorination.Noticeably,when molybdate addition inhibited the sulfate reduction process completely,biochar might improve the competitive relationship among microorganisms involved in the reduction processes.It might increase the relative abundance of dechlorinating bacteria?such as Dehalobacteriaceae?,and thereby promote the anaerobic dechlorination process of PCP.Therefore,there might be uncertainties about the remediation effect of biochar on reductive organic pollutants in flooded soil.Its impact on other indigenous soil reduction processes and the involved mechanisms should be comprehensively considered.?3?Through exogenous amendment of sodium acetate/pyruvate and Fe Cl₃,the initial content of the electron donor/acceptor in the experimental system were regulated.The influence and the underpinning chemical-microbiological coupling mechanisms of biochar under different redox condiction for lindane dechlorination and the downstream reduction process?methanogenesis?were also disclosed.Biochar could promote the biodegradation of lindane in flooded soil,which was more significant with the replenishment of carbon sources.Although biochar had no significant effect on the methanogenesis process in different soils during the reaction,the addition of carbon sources resulted in more than doubled methane emissions,especially in the presence of both carbon sources and biochar.The amount of methane produced in red soil and blue clayey paddy soil were as high as 25.8?mol/g and 20.1?mol/g soil,respectively.Additionally,the degradation of lindane and methanogenesis processes were completely suppressed in the treatment with Fe Cl₃ addition,regardless of the presence or absence of biochar.By extracting soil RNA and reverse transcription into c DNA for high-throughput sequencing,it was found that biochar didn't change the community structure of bacteria and archaea significantly with sufficient electron donor.It increased the modularity of the microbial co-occurrence network and the relative abundance of some rare microorganisms,which might indirectly affect the reductive transformation of lindane.When pyruvate and acetate were added exogenously,their effects on the degradation of lindane and the methanogenesis process were similar,but the responded soil microorganisms were different.During these processes,Methanosarcina?Methanosarcinales?and Clostridiales?Firmicutes?might be the key microorganisms.?4?The effects and reaction mechanisms of initial concentrations of lindane,biochar addition dosages,and different biochar redox characteristics on the transformation of lindane in anaerobic condition were further explored in the phosphate buffer.The results showed that when the amount of added biochar was constant,the lower the initial concentration of lindane,the higher the removal rate.Meanwhile,the removal rate of lindane increased with the increase of biochar dosages,especially after two times equivalent addition.The removal rate of lindane were 59.1%,34.6%,22.4%,and 7.3%in the biochar additions of 5%,1%,0.2%,and control,respectively.The role of biochar in the anaerobic reductive transformation of lindane was further revealed by CSIA.The extent of carbon isotope fractionation of lindane increased with the increase of biochar addition dosages,and the?¹³C value was larger with lower initial concentration of lindane?10 mg/L?.The carbon isotope enrichment factor??_C?of lindane in the presence of biochar was-3.4±0.4‰by Rayleigh equation fitting,which indicated that the anaerobic degradation of lindane by biochar might be proceed via dichloro-elimination,whereby two C-Cl bonds are reductively cleaved.Further analysis of the functional groups on the biochar surface before and after the reaction by X-ray photoelectron spectroscopy?XPS?showed that the oxygen-containing functional groups changed significantly in the redox process.Hence,biochar could play the role of promoting the adsorption and reduction of lindane simultaneously in the anaerobic environment.This study is important for the improved understanding of biochar's environmental function in soil pollution control and remediation,and may also provide important theoretical basis for improved understanding of the anaerobic transformation of typical pollutants in the complex anaerobic soil conditions with different co-existing electron donors/acceptors and meiators.All in all,we proposed that the comprehensive control measures based on regulating the microbial metabolism could be used to to amplify the self-purification function and ecological environmental effect of typical anaerobic environment such as the paddy soil,and provide theoretical basis and technical support for optimizing in-situ remediation of OCPs polluted farmland soils.