Effect Of Soil Typical Redox Processes On The Reductive Dechlorination Of Pentachlorophenol Under Anaerobic Condition

Pentachlorophenol(PCP)is a highly toxic,mutagenic and carcinogenic polychlorinated organic compound which is widely used as a wood preservative and insecticide versatile in our daily life.Due to the difficult degradation of PCP in the environment,it is included in the priority pollutants.The reduction of PCP in soil is mainly the biochemical process of microbial participation.In general,there is iron reduction,sulfur reduction,methane production in the soil.These processes are more active under anaerobic conditions.Therefore,the reduction of PCP in soil,especially in flooded soil,must be related to the typical redox process in soil.In this paper,we studied the relationship between the reductive dechlorination of PCP and the typical redox processes(iron reduction,sulfur reduction,methane production)in soil under anaerobic conditions.Specifically,the effects of different electron donors on the reductive dechlorination of PCP under methanogenic condition,the effect of different electron donors on the reductive dechlorination of PCP under sulfate-reducing condition,and the effect of methane production on the reductive dechlorination of PCP were studied.The main findings are as follows:(1)We tested four electron donors(formate,acetate,pyruvate,and lactate)for the degradation of PCP under methanogenic condition.In general,supplemented electron donors stimulated the growth and abundance of the functional microbial groups fordechlorination,iron/sulfate reduction,and methaogenesis.Among the tested electron donors,pyruvate showed the best effect for facilitating PCP dechlorination.However,it accelerated mostly,meanwhile,the emission of greenhouse gases,as well as aggravated the toxic effects of reducing substances.For a comprehensive consideration,the acetate should be the best carbon source for regulating the interaction of PCP anaerobic degradation and soil natural redox processes.High-throughput sequencing results revealed dominant genera were Pseudomonas,Sedimentibacter,Clostridium,Desulfosporosinus,Desulfovibrio,Pelobacteraceae,Geosporobacter Thermotalea and Methanosaeta.The main dechlorinator in acetate,pyruvate and lactate treatments was Pseudomonas,Sedimentibacter and Clostridium,respectively.Other non-dechlorinating bacteria,especially the iron-reducing bacteria Pelobacteraceae and Geosporobacter Thermotalea,might also be able to assist the known dechlorinator in reductive dechlorination of PCP under anaerobic conditions.And once the electron donors were sufficient for the typical soil redox processes,the archaea Methanosaeta was likely to simulate dechlorination via transport H2 to dechlorinating bacteria(2)In the same manner as in part(1),the effect of sulfate was further considered,and the sulfate-reducing conditions were set by addition of exogenous sulfate.It was found that the reduction rate of PCP decreased in most of the treatments,but the reduction of Fe(Ⅲ)was promoted under the sulfate-reducing condition.Among the tested electron donors,sodium pyruvate and sodium lactate exhibited the best results in promoting PCP degradation.At the same time,RDA analysis showed consistent results with methanogenic conditions,that is,the process of methanogenesis and iron reduction showed synergistic relationship with the reductive dechlorination of PCP.But the RDA results also found that the sulfate reduction process and PCP reductive dechlorination has a mutual inhibitory relationship.High-throughput sequencing results revealed the dominant genera were Pseudomonas,Sedimentibacter,Clostridium,Desulfosporosinus,Desulfovibrio,Pelobacteraceae,Geosporobacter Thermotalea and Methanosaeta.The main dechlorinator in acetate,pyruvate and lactate treatments was Pseudomonas,Sedimentibacter and Clostridium,respectively.Other non-dechlorinating bacteria,especially the iron-reducing bacteria Pelobacteraceae and Geosporobacter Thermotalea,might also be able to assist the known dechlorinator in the reductive dechlorination of PCP.Once the electron donors were sufficient for the typical soil redox processes,the archaea Methanosaeta was likely to simulate dechlorination via transport H2 to dechlorinating bacteria.The addition of sulphate inhibited the growth of Dehalobacterium,Pelobacteraceae,Sedimentibacter and Pseudomonas,promoted the growth of Desulfosporosinus,Desulfovibri,Clostridium and Geosporobacter Thermotalea.The addition of sulfate has no effect on the growth of methanogens,but only delays the process of producing methane.(3)The reductive dechlorination of PCP under methanogeneic condition was studied by exogenous addition of coenzyme M(CoM)or sodium 2-ethanesulfonate(BES).The addition of CoM inhibited the growth of iron-reducing bacteria in soil,promoted the growth of methanogens and the reductive dechlorination of PCP.The addition of BES promoted the growth of iron-reducing bacteria in the soil.It also completely inhibited the growth of methanogens and inhibited the reductive dechlorination of PCP in the soil.High-throughput sequencing results revealed that the dominant microorganisms were Pseudomonas,Sedimentibacter,Desulfosporosinus,Desulfovibrio,Pelobacteraceae,Geosporobacter Thermotalea and Methanosaeta.In this study,the key functional dechlorination microorganisms were Sedimentibacter and Pseudomonas.CoM promoted the growth of Sedimentibacter and inhibited the growth of Pseudomonas;and the effect of BES was just the opposite.As such,theBES and CoM showed no effect on the key functional dechlorination microbial groups(Sedimentibacter + Pseudomonas).Methane production in the soil can promote the reductive dechlorination of PCP.Therefore,the result that BES inhibited the reductive dechlorination of PCP might be caused by their inhibition effect on the process of methanogenesis.Additionally,Methanosaeta is a key microorganism in the process of PCP reductive dechlorination in flooded soil.