Bio-reduction Of Vanadate In A Methane Based Membrane Biofilm Reactor And The Response Of Extracellular Polymeric Substances

Vanadium(?),a widely distributing transition metal on earth,is associated with a variety of natural resources and industries,e.g.metallurgical industry,battery manufacturing,chemical industry and biomedical fields.Although V is an essential element that benefits the organisms in trace amount,it becomes toxic when the concentration larger than 1 mg L-1.Due to the similar structure between vanadate and phosphate,excessive vanadate can interfere with the phosphate-metabolism-related enzymes,produce cytotoxicity and genetic toxicity,damage the respiratory,digestive and nervous system in human body.V(?)has high solubility and toxicity while V(?)is much less toxic and usually exists as precipitates in neutral or alkaline condition.So reduction of V(?)to V(?)is a feasible way to detoxify V in wastewaters.Bio-reduction of V(?)to V(?)attracted a lot of attentiondue to the simplicity and low cost.Some microorganisms are able to use methanol,lactate,hydrogen,and other materials as electron donor to reduce vanadate.If a certain microorganism can reduce vanadate using methane as electron donor and carbon source,not onlyvanadate pollution remediation,but also greenhouse gas methane mitigation,will be achieved.We demonstrated vanadate(V(?))bio-reduction in a methane-based hollow fiber membrane bioreactor(CH4-MBfR)We used a CH4-MBfR to conduct vanadate removal,with previously enriched "micro-aerobic methane oxidation coupled to nitrate reducing culture" as inoculum.CH4 was supplied sufficiently through experiment,while V(?)concentration were 2,5,10,and 5 mg L-1 in influent for 4 Stages.The V(?)flux reached highest(420 mg m-2 day-1)in stage 3 at a surface loading of 668 mg m-2 day-1,the V(?)removal percentage was 63%.When the influent V(?)loading decreased to 363 mg m-2 day-1 in stage 4,V(?)was completely removed.The amounts of proteins,polysaccharides and humic substances in EPS increased comparedto the inoculum,suggesting the secretion of EPS was stimulated by the reduction of V(?).The amounts of these constituents increased from Stage 1,2 to Stage 4,which were in accordance with the V(?)removal flux during these stages.However,when high surface loading of V(?)(668 mg m-2 day-1)was introduced,the amount of the constituents in EPS decreased sharply.In order to further understand the role of EPS in V(V)reduction,three-dimensional excitation-emission matrix(3D-EEM)spectra and Fourier transform infrared(FTIR)spectroscopy were employed to analyze the composition and functional groups of EPS.3D-EEM results showed that tryptophan and humic acid-like substances were the main componentsin EPS,might play important roles in protecting cells and binding V(V)under V(V)loading pressure.FTIR spectroscopy identified that hydroxyl(OH-),and carboxyl(COO-)groups were the main functional groupin EPS.The peak intensity of these two groups in the FTIR spectra increased significantly in accordance with the vanadate concentration,indicating these groups make a great contribution in maintaining the normal physiological function of the cells in adverse environmental conditions.Scanning electron microscope(SEM)observation of biofilm showed that a large amount of mineral precipitates accumulated,energy dispersive X-ray spectroscopy(EDS)and X-ray photoelectron spectroscopy(XPS)proved that these substances are likely to be the reduction of precipitation state V(IV).High-throughput sequencing results showed that the relative abundance of Methylomonas(Gammaproteobacteria)and Denitratisoma(Betaproteobacteria),a denitrifying methanotroph and an oestradiol-degradingdenitrifier,had strong correlation with V(V)removal flux.Considering the nitrate reductase from denitrifiers played an important role in V(V)reduction,these two genera might actively participate in V(V)reduction.Methylomonas and Methylococcus accumulated during the experiment,indicated that these two genera were mostly likely to take the function of methane oxidation.The relative abundance of a methanol-utilizing genera Methylophilus,continuously increased throughout the experiments,implying that intermediate metabolites methanol might be the electron carriers for synergism between methanotrophs and vanadate-reducing bacteria in the CH4 based biofilm.