Study On The Reduction Of Cr(Ⅵ)and V(Ⅴ)in A Hydrogen Fermenter By Thermophilic Bacteria

As industrialization progressed,a large amount of heavy metals are discharged into surface water and soils from various industries,which not only causes pollution to the ecological environment,but also causes different degrees of harm to microorganisms,plants and animals.What’s more,heavy metals can enter the human body with the enrichment of the food chain,inducing health risks.Chromium and vanadium,as the typical representatives of anionic heavy metals,sometimes appear in the form of symbiosis in nature due to their similar properties.For example,vanadium containing titanium magnetite,in the smelting process of ore,chromium-vanadium containing wastewater flows into the environment.The toxicity and mobility of heavy metals are greatly affected by the valence state.Hexavalent chromium[Cr(Ⅵ)]and pentavalent vanadium[V(Ⅴ)]are the most toxic,while trivalent chromium[Cr(Ⅲ)]and tetravalent vanadium[V(Ⅳ)]are less toxic and easy to precipitate.Therefore,more and more technology and processes for heavy metal reduction have been developed to lessen the toxicity of heavy metals,wherein the biological process,in particular anaerobic digestion,was favored by many researchers because of its low cost and no secondary pollution.Previous studies have focused on the reduction of heavy metals in normal temperature anaerobic methane fermentation.However,the high temperature of wastewater from battery,metallurgy and refractory industries poses new challenges to the operation of traditional anaerobic digestion.The thermophilic hydrogen-producing bacteria can show good performance in high-temperature environment,and the anaerobic hydrogen fermentation is more resistant to external environment.Therefore,in this paper,Cr(Ⅵ)and V(Ⅴ)were reduced by high temperature anaerobic hydrogen fermenter.At the initial pH value of 5.8 and temperature of 55℃,the acid anaerobic hydrogen-producing thermophilic bacteria showed an efficient reduction performance.The experiment is divided into three partsPart Ⅰ:Reduction of Cr(Ⅵ)by a high temperature anaerobic hydrogen production fermentation.Results show that Cr(Ⅵ)with a cumulative concentration of 300 mg/L can be completely reduced(99.5%)to Cr(Ⅲ)through chemical and biological reactions.Bio-reduction dominates Cr(Ⅵ)removal in a firstorder exponential decay mode with both glucose and its metabolites(volatile fatty acids)as electron donors.Bacilli,Clostridia and Thermotogae in the fermenter dominated the reduction of Cr(Ⅵ)by regulating the production and composition of extracellular polymers(EPSs),in which carboxyl and hydroxyl groups play an important role for Cr(Ⅵ)reduction by coordinationPart Ⅱ:Reduction of V(Ⅴ)using high temperature anaerobic hydrogen fermentation.The results show that 100 mg/L of V(Ⅴ)can be completely reduced(97.75%)to tetravalent vanadium through biochemical reduction.Bio-reduction dominates the reduction of V(Ⅴ)in the first-order exponential decay mode.Both glucose and its metabolites(volatile fatty acids,hydrogen)serve as electron donors.In addition,clostridium dominates the reduction of V(Ⅴ)by regulating their own metabolism and secretion of extracellular polymers(EPSs),in which the tryptophan and tyrosine residues might play important roles in microbial cell protection and V(Ⅴ)reduction.Part Ⅲ:Simultaneous reduction experiments of heavy metals chromium and vanadium were carried out by using agricultural waste corn stalk as the substrate of anaerobic hydrogen fermentation.The results show that simultaneously reduction of heavy metals Cr(Ⅵ)and V(Ⅴ)can be achived in high-temperature anaerobic hydrogen fermentater within 24 hours.The reduction efficiency of Cr(Ⅵ)and V(Ⅴ)is 99.97±0.1%and 95.37±1.9%,respectively,among which the physicochemical effect can account for 17～23%.The presence of Cr(Ⅵ)and V(Ⅴ)promoted the digestion and decomposition of straw,which produced more glucose and VFAs.The increase of electron donors in the system accelerated the reduction of heavy metalsIn summary,the high-temperature anaerobic hydrogen fermentation is expected to be applied to the treatment of high-temperature wastewater containing chromium and vanadium,which also provides a new direction for the utilization of straw resources.