The Mechanism Of Fe(?) Produced By Iron/Sulfatereducing Bacteria On The Synthesis Of Mackinawite And Its Removal Of Cr(?)

With the development of industry in recent years,heavy metal chromium pollution has become more and more serious A large amount of chromium-containing wastewater and residues are discharged into the environment without treatment,which seriously threatens water,soil and human health.Chromium mainly exists in the form of Cr(?)and Cr(?I)in the environment.Cr(?I)is less toxic and easy to be precipitated and fixed,while Cr(?)mainly exists in the form of oxygen anions and is highly toxic and easy to migrate.Therefore,reduction and reprecipitation is the main way to deal with Cr(?)pollution.As a precursor of many iron-sulfur minerals,mackinawite(FeS)widely exists in nature.It contains both Fe(?)and S(-?).It has high reducing activity and high specific surface area.It can quickly adsorb Cr(?)and reduced to Cr(?I)to significantly reduce the toxicity and migration ability of Cr(?).The FeS obtained through biosynthesis is not only environmentally friendly,low cost,but also has sustainable in-situ remediation potential.However,there are many influencing factors in the biosynthesis process that will affect the removal effect of FeS on Cr(?)removal.The mineralization mechanism in the biosynthesis process and the subsequent removal effect is still unclear.Therefore,this study selected representative iron-sulfur-reducing microorganisms:Ferric reducing bacteria(Fe RB)and Sulfate-reducing bacteria(SRB),and explored the biosynthetic process under different microbial systems.The main research conclusions include:1.Biosynthesis of mackinawite by iron/sulfate-reducing bacteria:a mixed group of iron-reducing bacteria and sulfate-reducing bacteria(HM group)and a single-group of sulfate-reducing bacteria(DM group)were used to synthesize mackinawite.The source and production rate of Fe(?)in the HM group and DM group are different.The HM group uses MR-1 for rapid direct biological reduction of Fe(?I),and the DM group uses SRB to produce a large amount of S^2-and then use S^2-to reduce Fe(?I).Different Fe(?)sources and production rates determine different biomineralization mechanisms:Mackinawite(FeS)in the HM group mainly develops and grows on the surface of the bacterial cell,with faster speed and higher crystal crystallinity;in the DM group,production rate of Fe(?)is slow,the mackinawite(FeS)mainly uses Fe(?I)precipitation as the carrier,and it grows while reducing led to a poor crystallinity.The content of Fe(?)and S(-?)finally entering the materials HM-nFeS and DM-nFeS is approximately 6.7m M and 3 m M.2.The mechanism of Cr(?)removal by biosynthetic mackinawite:removal thermodynamic studies show that both HM-nFeS and DM-nFeS are more corresponding to the Freundlich isotherm model,indicating that heterogeneous adsorption occurred during the removal of Cr(?).The maximum removal capacities of HM–nFeS and DM–nFeS are 213.95mg/g and 262.66 mg/g,respectively.The kinetic study showed that the removal kinetics of Cr(?)by HM-nFeS and DM-nFeS were in line with the pseudo-second-order kinetic model,indicating that the rate limiting step of the entire reaction was chemical adsorption.The different distribution of Fe(?)and the influence of organic matter on the surface of the material are the main reasons that the removal effect of HM-nFeS worse than that of DM-nFeS.The mechanism of removing Cr(?)can be described as that Cr(?)is first adsorbed on the surface of the material,and then Cr(?I)is reduced by Fe(?)on the surface,while S(-?)reduces Fe(?I)to Fe(?),which is oxidized to SO₄^2-and SO₃^2-.Cr(?I)is finally precipitated into Cr(OH)₃ and Cr_xFe_1-x(OH)₃.The contribution ratio of Fe(?)and S(-?)in removing Cr(?)is about 7:3.3.The influence of environmental factors on the removal of Cr(?)by biosynthetic mackinawite:the study of the effect of common coexisting anions Cl^-,NO^3-,HCO₃^-,SO₄^2-on the removal of Cr(?)found that Cl^-increases the background ion intensity and enhances the removal effect,NO^3-has a weak effect on the removal effect.HCO₃^-can converted into CO₃^2-which is easy to form precipitates.SO₄^2-also shows the inhibition because of the competitive adsorption with HCr O₄^-.When HA concentration of 0-10 mg/L,the removal ability is enhanced through the organic functional groups in complexation and reduction.The increase of organic matter enhances the surface electronegativity of the material,and the electrostatic repulsion between HCr O₄^-decreases the removal effect when HA concentration of 10-20 mg/L.Exploring the oxidation resistance of the material found that the oxidation resistance of DM-nFeS is stronger than that of HM-nFeS,because the Fe(?)content on the surface of HM-nFeS is higher and it is easier to be oxidized and deactivated by oxygen.