Performance And Mechanism Of Cathode Biodechlorination Promoted By Sulfur Autotrophic Denitrification Process

Chlorinated aliphatic hydrocarbon(CAH)and nitrate are typical organic and inorganic pollutants in groundwater,which can coexist stably and permanently in groundwater.Especially,the serious coexistence of CAHs and nitrate in the groundwater of chemical sites has become an urgent environmental problem in the world.The lack of electron donors in groundwater and the competition between denitrification and reductive dechlorination process are the main problems of biological denitrification and dechlorination of groundwater.In this study,the electrode and elemental sulfur were used as electron donors to reduce pollutants.The biocathode was used to enhance the reductive dechlorination of typical CAHs,and the response mechanism of microbial structure and functional genes of cathode biofilm to operating condition change was discussed.To reduce the inhibitory effect of nitrate on biocathode dechlorination,a coupling system(SAD-BES)of sulfur autotrophic denitrification(SAD)and biocathode(BES)was constructed to accelerate nitrate removal and achieve the self-balance of acidity-alkalinity of the reaction system.Under the coexistence of CAH and nitrate,the promotion of SAD on biocathode dechlorination was clarified,and the abundance of potential functional bacteria was analyzed.The mechanism of sulfur autotrophic denitrification coupled with biocathode dechlorination to promote the removal of nitrate and CAH was revealed.The effects of potential regulation and carbon source conditions on biocathode dechlorination of CAH were investigated.The reductive dechlorination of tetrachloroethene(PCE),trichloroethene(TCE)and 1,2-dichloroethane(1,2-DCA)were well fitted with the first-order kinetics.PCE and TCE showed a metabolic pathway with cis-1,2-dichloroethene as the main product,whereas 1,2-DCA was dechlorinated entirely to the ethene.Higher potential(-0.06 V)resulted in the insufficiently available electrons for reductive dechlorination.However,lower potential(-0.46 V and-0.66 V)resulted in the generation of H2 and reduced the efficiency of electron utilization for dechlorination.The highest CAH removal rate of greater than 99% within 24 h was observed at-0.26 V.The decrease of cathode potential to-0.26 V stimulated the enrichment of electroactive bacteria and dechlorination bacteria and also promoted the expression of reductive dehalogenase genes and extracellular electron transport gene.During the reductive dechlorination of PCE and TCE,Bacillus,Pseudomonas and Desulfovibrio were the main functional bacteria,and the reductive dehalogenase gene pce A and tce A were highly expressed.However,during the reductive dechlorination of 1,2-DCA,Geobacter and Desulfobulbus were the main functional bacteria,and the reductive dehalogenase gene rhd A1 and tce A were highly expressed.The dominant functional bacteria under glucose,sodium acetate and sodium bicarbonate conditions were Lactococcus,Geobacter and Bacillus,respectively.Compared with sodium bicarbonate,organic carbon sources were more conducive to enrich putative reductive dehalogenase and electroactive genes,which promoted the dechlorination respiration of cathode biofilms.However,the enriched cathodophilic biofilm with sodium bicarbonate could utilize electrode as an electron donor to achieve more than 50% dechlorination efficiency within 24 h.With cathode as the sole electron donor,denitrification severely inhibited the dechlorination process,resulting in a PCE removal rate at 12 h and k value reduced by more than 70% and 82%,respectively.Therefore,the introduction of SAD into the biocathode to accelerate nitrate removal was proposed.The self-balance of acidity-alkalinity(p H value 7.05-7.49)of the reaction system and the complete removal of nitrate within 12 h were achieved in SAD-BES system at an optimized voltage of 0.5 V.The nitrate removal rate constant k of SAD-BES system was 4.7 and 2.4 times that of the BES and SAD systems,respectively.Sulfur autotrophic denitrification process was the main pathway for nitrate removal(75.3%-83.1%)and the sulfate yield of the SAD-BES system was 5.67-6.26 mg/mg NO3--N.Pseudomonas,Ralstonia and Brevundimonas were the main denitrifying bacteria in sulfur particle biofilm,and Pseudomonas,Chryseobacterium,Pantoea and Comamonas were the main denitrifying bacteria in the cathode biofilm.The efficient nitrate removal by the introduction of elemental sulfur was the prerequisite for simultaneous denitrification and dechlorination.Under the coexistence of nitrate and PCE,95.9% removal of nitrate within 12 h and 87.2% removal of PCE were achieved in SAD-BES system at an optimized voltage of 0.5 V.With the cathode as the sole electron donor,the denitrifying genus including Thauera,Azoarcus and Comamonas were dominated in the cathode biofilm.The accelerated nitrate removal in the cathode by the introduction of elemental sulfur weakened the competition intensity and period between reductive dechlorination and denitrification for electrode electrons,which promoted the biocathode dechlorination.At the same time,the abundance of bacteria capable of dechlorination and denitrification(Pseudomonas,Chryseobacterium,Desulfovibrio,Mesorhizobium and Geobacter)in the cathode biofilm was significantly improved,and reductive dechlorination was the dominant metabolic process in the middle and late stages of the biocathode reaction.The biofilm of sulfur particles was enriched with denitrifying bacteria including Arcobacter,Pseudomonas and Azospira,which could use elemental sulfur as an electron donor.The microbial ecological network showed that bacteria without definite electroactivity were positively correlated with the predominant functional bacteria in the cathode biofilm.These bacteria might cooperate to complete denitrification and dechlorination.The bacteria that could not use elemental sulfur as the electron donor in sulfur particle biofilm were positively correlated with the dominant genus Arcobacter,which might survive using metabolites produced by sulfur autotrophic denitrifying bacteria.This study proposes a method for the efficient removal of nitrate and CAH by sulfur autotrophic denitrification coupled with biocathode dechlorination,which provides a new idea for the bio-enhanced remediation of groundwater contaminated by the coexistence of nitrate and CAHs.