The Study Of Perchlorate Reduction In A Methane-based Membrane Biofilm Reactor

Perchlorate is widely used in the defense industry,fireworks manufacturing,leather processing,rubber manufacturing and other industries.A lot of perchlorate-containing wastewater is discharged into the environment without treatment.Perchlorate interfere the normal function of the human thyroid and make the nerve damage,especially for infants and children.The methods of removing perchlorate pollutants from water include physical,chemical and biological methods,in which biological methods are attracting people's attention because of their low cost and environmental friendliness.Perchlorate bioreduction is limited by many environmental conditions.Enrichment of more efficient microorganism and optimization of reduction conditions are keys to improve the efficiency of bioremediation.Microorganisms need electrons to reduce contaminants,and methane anaerobic oxidation can provide the electrons for microbial reduction.The way which combine perchlorate reduction and methan anaerobic oxidation not only to achieve the perchlorate pollution remediation but also reduce global carbon emissions.In this study,methane-based hollow fiber membrane bioreactor was used for the research.We used nitrite?NO₂^-?as the electron acceptor to enrich the biofilm at the beginning.Then the perchlorate?CIO4-?was used as the sole electron acceptor to enrich of high-efficient perchlorate-reducing bacteria,and to study the metabolic kinetics of perchlorate reduction using methane as the sole electron donor and carbon source for the microorganism.After 40 days culture the experiment was carried out.When the electron donor supply was sufficient?10 psi CH4,electronically equivalent 462.808 mmol e-/m²-d?,1 mg/L ClO₄^-and 1.1 mg N/L NO₃^-can be completely reduced by microbe.The reduction of ClO₄^-was inhibited and the removal rate declined to 30%when 2 mg N/L NO₂^-presence.When the electron acceptor loading in the reactor increased to 1 mg/L ClO₄^-+ 11.3 mg N/L NO₃^-,electron donor was insufficient and the microorganism gave priority to reducing NO₃^-.High-throughput sequencing results showed that Methylocystis,Methylomomas,Methylophilus,Pelomonas and uncultured Chloroflexi were dominant microorganisms in this reactor.Methylocystis and Methylomonas were methane-oxidizing bacteria.Methylomonas relative abundance was not detected in the first stage,and as the experiment continued the relative abundance of Methylomonas increased.At the beginning of the experiment Methylocystis was the main methane-oxidizing bacteria in the reactor,but the relative abundance of Methylocystis decreased gradually with the increase of Methylomonas relative abundance.Methylophilus,a methanol-oxidizing bacteria,appeared in the reactor after a period of incubation,while Methylophilus may have the ability to reduce perchlorate.Pelomonas was the major functional reducing bacteria,Pelomonas relative abundance in nitrate-containing stage continued to rise.Pelomonas was similar to the known Dechloromonas which was perchlorate-degrading bacterium in the phylogenetic tree and was presumably involved in the perchlorate reduction process.We continued to study the saline effect of methane anaerobic oxidation coupling with perchlorate reduction and found the salinity has significant effect of this bioreduction process.Under the condition of enough electron donor,the 1%salinity?10 g/L NaCl?led the ClO₄^-?2 mg/L?and NO₃^-?10 mg/L?reduction rate to decrease from 57.2%and 91.9%to 5,9%and 55.9%,respectively.After removing the salinity,the reduction rate of the pollutants could not be restored to the saline-free stage.Perchlorate?nitrate?as an electron acceptor under 0.2%salinity?2 g/L NaCl?,the reduction rate of 2mg/L CIO4-?5mg/L NO₃^-?was only 53.5%?59.1%?.Scanning electron microscopy revealed that salinity caused irreversible damage to microbial morphology.Microorganisms became ruptured,wrinkled,filamentous and the cell volume declined in salt environment,while the ruptured cells produced a large number of extracellular polymers and soluble microbial products.Real-time PCR result showed that the total amount of microorganisms in the reactor was decreased under salinity conditions.High-throughput sequencing revealed that salt stress changed the microbial morphology in the reactor.Methylomonas was sensitive to salinity,the Methylomonas relative abundance was 31.3%under saline-free environment,and decreased rapidly to 5.9%under 1%salinity.The relative abundance of Methylocystis increased with the decreasing of Methylomonas relative abundance.The relative abundance of Methylophilus was 9%at 1%salinity environment,but the abundance of Methylophilus was very low at other stages.It was speculated that Methylophilus is tolerant to salinity.Methylophilus was similar to Dechloromonas agitate in the phylogenetic tree,it was presumed that Methylophilus has achieved perchlorate reduction and denitrification under salt conditions.The relative abundance of unclassified Xanthomonadaceae increased to 11.5%under salt environment.Unclassified Xanthomonadaceae was similar to Thermomonas haemolytica which has denitrifying function in the phylogenetic tree,suggesting that unclassified Xanthomonadaceae was involved in the denitrification process.Environmental pH and concentration of copper ions observably impact methane anaerobic oxidation coupled with perchlorate reduction.Appropriate pH and concentration of copper ions can promote the growth of microorganisms and improve the reduction efficiency.The results showed that when the pH was 7-8,the reduction rate of perchlorate was the highest.And the concentration of copper ion?100?g/L could promote the perchlorate reduction.