Bio-reduction Of Hexavalent Chromium Using Methane As A Sole Electron Donor

The heavy metal pollution in the water environment in China is becoming more and more serious,among which chromium pollution is more prominent.The sources of chromium pollution in aquatic environments can be roughly divided into three categories:industrial production,daily life,and chemical products.Chromium is mainly present in the water as trivalent（Cr（Ⅲ））and hexavalent（Cr（Ⅵ））forms.Cr（Ⅵ）has high solubility and high toxicity in water,and generally exists in the form of anion.In contrast,Cr（Ⅲ）generally forms precipitated hydroxides under neutral or alkaline conditions and can be removed by centrifugation or filtration.The Cr（Ⅵ）bioreduction method is a simple and highly efficient method for the remediation of chromium contamination in water,that is,microorganisms use organic or inorganic substances as electron donors and Cr（Ⅵ）as an electron acceptor to reduce Cr（Ⅵ）to Cr（Ⅲ）.Methane（CH₄）is a kind of strong greenhouse gas,which is commonly seen in natural and human activities.It mainly comes from the anaerobic fermentation process in urban sewage treatment and landfill.In recent years,it has been used more and more widely in wastewater bioremediation.In this biological process,methane can act as an electron donor and a carbon source,providing enough energy for the growth of microorganisms to drive the biological reduction of oxidized pollutants.Membrane biofilm reactor（MBfR）can transfer CH₄ safely and efficiently to microorganisms on the membrane to drive microbes to carry out material and energy metabolism.It is an ideal process for removing Cr（Ⅵ）from wastewater.In this dissertation,the microbiological process of Cr（Ⅵ）bioreduction driven by CH₄ as the sole electron donor was studied.The reduction kinetics and reduction processes of Cr（Ⅵ）,NO₃-,SO₄^2-and Se（Ⅵ）under different loads were investigated.The interactions among them illustrate the mechanism of Cr（Ⅵ）bioreduction and the mechanism of NO₃-,SO₄^2-and Se（Ⅵ）on Cr（Ⅵ）reduction.The main conclusions are as follows:1)Cr（Ⅵ）bioreduction process in CH₄-MBfRIt was confirmed that microorganisms can use CH₄ as the sole electron donor to drive Cr（Ⅵ）bioreduction.When Cr（Ⅵ）is the sole electron acceptor in CH₄-MBfR,the ability of microbes to reduce Cr（Ⅵ）is continuously enhanced as the microbial community attached to the biofilm continuously adapts to Cr（Ⅵ）.The experiment lasted 90 days and was divided into five phases.Finally,when the influent Cr（Ⅵ）concentration is 3mg-Cr/L and the surface loading is 370 mg-Cr m^-2d^-1,the removal efficiency reaches 95%.Analysis by XPS,TEM and EDS showed that Cr（Ⅲ）precipitate was the final product of Cr（Ⅵ）reduction and was distributed intracellularly and extracellularly in bacteria.2)The interaction between reduction of Cr（Ⅵ）and NO₃-,SO₄^2-and Se（Ⅵ）in CH₄-MBfRWhen Cr（Ⅵ）is the sole electron acceptor in CH₄-MBfR,Cr（Ⅵ）with a surface loading of 500mg-Cr/m²-d in the influent is all reduced to Cr（Ⅲ）;When the NO₃-of 280mg-N/m²-d,Cr（Ⅵ）reduction ability significantly reduced（<25%）;when the influent NO₃-load is zero,Cr（Ⅵ）reduction rate only restored to 70%.Therefore,NO₃-has an irreversible inhibitory effect on Cr（Ⅵ）reduction.In CH₄-MBfR,when the SO₄^2-load increases from zero to 4.7 mg/m²-d,the Cr（Ⅵ）removal rate increases from 60%（stage 1）to 70%;after the SO₄^2-load decreases to zero,Cr（Ⅵ）The removal rate was further increased to 90%,indicating that sulfate-reducing bacteria（SRB）are enriched on biofilms and drive the reduction of Cr（Ⅵ）.However,high loading of SO₄^2-（26.6 mg/m²-d）significantly inhibited the reduction of Cr（Ⅵ）（reduction rate was 40%）.Similarly,when CH₄-MBfR contains 0.5 mg Se/L of Se（Ⅵ）in the feed water,the removal rate of Cr（Ⅵ）slightly decreases to 60%;when the feed water does not contain Se（Ⅵ）,Cr（Ⅵ）removal rate rose back to 80%.3)Changes in microbial ecology on CH₄-MBfR membranes.In CH₄-MBfR,as microorganisms use CH₄ as an electron donor to drive the bioreduction of Cr（Ⅵ）,Meiothermus（Deinococci）and Methylosinus（typeⅡmethanotrophs）have gradually become dominant bacteria on the biofilm.Community structure analysis showed that the synergy between methanotrophs and chromate reducing bacteria is the main biological mechanism:Methanotrophs activate CH₄ to generate and release some metabolic intermediates.Chromate-reducing bacteria use these intermediates as electron donors drive the bioreduction of Cr（Ⅵ）.With the introduction of NO₃-,the relative abundance of Meiothermus（Deinococci）on the membrane was greatly reduced,and the abundance of Chitinophagaceae（a.denitrifying bacteria）was significantly increased.This shows that NO₃-has a significant effect on the microbial community on the biofilm.Afterwards,when the influent water does not contain NO₃-,the relative abundance of Pelomonas increases significantly,indicating that it may have Cr（Ⅵ）reduction capability.In addition,through the prediction of functional genes by PICRUSt,it was found that after the introduction of NO₃-,the abundance of chromate reduction-related genes on biofilms was significantly reduced,whereas the abundance of genes associated with denitrification and methane oxidation processes increase.The SO₄^2-coexisting with Cr（Ⅵ）led to the evolution of three dominant genera on the CH₄-MBfR biofilm.Based on the correlation between the relative abundances of these three genera and the loads of Cr（Ⅵ）and SO₄^2-,we conclude that Methylocystis（typeⅡ methanotrophs）can simultaneously reduce Cr（Ⅵ）and SO₄^2-,While Meiothermus only acts on Cr（Ⅵ）reduction,Ferruginibacter only acts on SO₄^2-reduction.Se（Ⅵ）coexisting with Cr（Ⅵ）led to the evolution of two dominant genera on the CH₄-MBfR biofilm:Meiothermus（which can simultaneously reduce Cr（Ⅵ）and Se（Ⅵ））and Methylophilus.Thus,we conclude that Methylophilus activates CH₄,which releases the released electrons and energy to Meiothermus to drive the reduction of Cr（Ⅵ）and Se（Ⅵ）.