Removal Of Vanadium From Groundwater By Ethane Based Membrane Biofilm Reactor

Vanadium（V）contamination in groundwater is a serious threat to human health.V mainly exists in the form of pentavalent V（V（Ⅴ））with high solubility and strong toxicity in groundwater.Biological method is an effective method to recover V contamination in groundwater by reducing V（Ⅴ）to precipitate tetravalent V（V（Ⅳ））,which is difficult to dissolve in water and has low toxicity.Nitrate（NO₃^-）is a common concomitant contamination in water.In some V（Ⅴ）polluted groundwater,the concentration of NO₃^-also exceeds the prescribed limit value,and the influence of NO₃^-on the biological reduction of V（Ⅴ）cannot be ignored.In the selection of microbial electron donor,the traditional soluble organic electron donor has a good reduction effect on V（Ⅴ）,but it is easy to lead to secondary pollution,large sludge and other problems.Solid organic electron donors can not only avoid excessive reproduction of microorganisms,but also provide support for their growth.However,as a slow-release carbon source,their electron contribution is difficult to calculate,and the absence of carbon and the presence of other electron acceptors tend to lead to the oxidation of V（IV）,which increases the concentration of V（V）in water.The amount of sludge with inorganic material as electron donor is relatively small,but the price of chemical agents is high,and additional carbon source is needed,which increases the storage and transportation cost.Gaseous organic electron donors can provide both electron and carbon sources for microorganisms and are not easy to remain in water,which is an environmentally friendly and cost-effective option.The feasibility of reducing V（Ⅴ）with methane（CH₄）as the sole electron donor and carbon source has been verified.However,the price of pure CH₄ is relatively high compared to natural gas.In natural gas and shale gas,ethane（C₂H₆）has the highest content except CH₄.C₂H₆ is able to provide more electrons to microorganisms and theoretically has a higher pollutant reduction efficiency.In addition,C₂H₆ is also a kind of greenhouse gas,and the biodegradation of C₂H₆ while removing contamination from water is of positive significance for"carbon reduction".Previous studies have shown that microorganisms using C₂H₆ as electron donor and carbon source can achieve NO₃^-,perchlorate,chromate and selenate reduction.In this paper,the biological reduction of V（V）was studied for the first time using C₂H₆ as the sole electron donor and carbon source in membrane biofilm reactor（MBf R）.The reduction performance of ethane based membrane biofilm reactor（C₂H₆-MBf R）on V（V）and the effect of NO₃^-on V（V）reduction were investigated,and the microbial mechanism of C₂H₆ oxidation coupled with V（V）reduction was clarified.The main conclusions are as follows:（1）Microbial reduction of V（V）can be achieved by using C₂H₆ as the sole electron donor and carbon source.Oxygen（O₂）played an important role in the oxidation of C₂H₆.Under the same conditions,the V（V）removal efficiency of aerobic C₂H₆-MBf R was 10%higher than that of anaerobic.Under long-term operation,with the decrease of hydraulic residence time（HRT）（78 h,54 h,22 h）and the increase of V（V）concentration(2 mg L^-1,4 mg L^-1,8 mg L^-1),the removal ability of V（V）and the resistance to V（V）load impact of C₂H₆-MBf R were significantly enhanced.Finally,When HRT was 22 h,the removal efficiency of 8 mg L^-1 V（V）reached 93.5±1.0%.（2）The effect of NO₃^-on V（V）biological reduction in C₂H₆-MBf R varied with the difference of V（V）and NO₃^-concentration ratio（V/N）.When V/N was 1:1,the reduction of V（V）would be slightly inhibited by NO₃^-,while the reduction of NO₃^-was almost unaffected by V（V）.When the system was stable,the removal rates of V（V）and NO₃^-were 86.4±0.6%and 95.1±0.1%,respectively.When V/N was 2:1,the reduction of V（V）was not affected by NO₃^-.As V/N increased,V（V）became more competitive with the electron donor.When V/N was 4:1,the removal efficiency of C₂H₆-MBf R on 16 mg L^-1 V（V）and 4 mg L^-1 NO₃^-could reach 90.6±0.2%and 92.1±0.5%,respectively.（3）The synergistic action between ethane oxidizing bacteria and vanadium reducing bacteria promoted the efficient reduction of V（V）.The results of scanning electron microscope coupled to energy dispersive X-ray analysis（SEM-EDS）and X-ray photoelectron spectroscopy（XPS）showed that the reduction product of V（V）was V（Ⅳ）precipitation.Combined with 16S r RNA sequencing results and metagenomic prediction analysis,the possible microbial mechanism of V（V）reduction by C₂H₆-MBf R was inferred:In the presence of O₂,ethane oxidizing bacteria Mycobacterium and Methylocystis had used alkane 1-monooxygenase,alcohol dehydrogenase and aldehyde dehydrogenase to produce metabolic intermediates（such as ethanol,acetaldehyde and acetic acid）.These metabolites were utilized by vanadium reducing bacteria Geothrix,Rudaea and UTBCD1 to achieve V（V）reduction in both intracellular and extracellular cells.