The Promotion Mechanism Of Sulfate-reducing Bacteria And Their Metabolites On The Removal Of Hexavalent Chromium From Water By Zero-valent Iron

With the rapid development of steel,smelting,and pesticide manufacturing industries,hexavalent chromium（Cr（Ⅵ））pollution has brought enormous environmental pressure.The widely used environmental remediation method of sulfate-reducing bacteria（SRB）combined with zero-valent iron（ZVI）can economically,environmentally friendly,and effectively remove Cr（Ⅵ）as well as other pollutants from water.However,current studies on the removal of pollutants by SRB in combination withZVI have mainly focused on the promotion and enhancement of SRB growth and metabolism byZVI,while ignoring the fact that SRB can enhance the reaction activity ofZVI.In this study,we elucidated that SRB enhanced the reactivity ofZVI through various facilitation mechanisms,and we performed a hydrogen-based membrane biofilm reactor for the co-reduction of SO₄^2-and CrO₄^2-byZVI and SRB,thus elucidating the synergistic effect of SRB andZVI in this process.Carboxymethylcellulose stabilized ferrous sulfide nanoparticles（CMC-Fe S）and sulfidated nano-zerovalent iron（S-nZVI）were synthesized in SRB-secreted extracellular polymeric substances（EPS）solution,and carboxymethylcellulose stabilized ferrous sulfide nanoparticles@extracellular polymeric substances（CMC-Fe S@EPS）and sulfidated nano-zero-valent iron@extracellular polymeric substances（S-nZVI@EPS）were successfully synthesized,respectively.Compared with CMC-Fe S and S-nZVI,the Cr（Ⅵ）removal capacity of CMC-Fe S@EPS and S-nZVI@EPS were enhanced from 218.57 and 41.83 mg/g to 285.71 and 64.64 mg/g,respectively.EPS in CMC-Fe S@EPS and S-nZVI@EPS enhanced Cr（Ⅵ）adsorption through its functional group ability to bind to metals,and the Cr（Ⅵ）adsorption ability of EPS was found to enhance the solid-liquid reaction between S-nZVI@EPS and Cr（Ⅵ）.In addition,EPS enhanced the Cr（Ⅵ）reduction by improving the electron transfer efficiency of CMC-Fe S@EPS and S-nZVI@EPS,and promoted the co-precipitation of Cr（III）and Fe（III）by providing additional binding sites.Thus,EPS played an important role in improving the Cr（Ⅵ）removal capacity of CMC-Fe S@EPS and S-nZVI@EPS.After adding micron-scaleZVI（mZVI）and nanoscaleZVI（nZVI）to SRB for incubation,Fe_xS_y precipitation and covered EPS were formed on the surface of both mZVI and nZVI,resulting in the synthesis of bio-generated sulfidated mZVI（BS-mZVI）and bio-generated sulfidated nZVI（BS-nZVI）,respectively.Compared with mZVI and nZVI,the Cr（Ⅵ）removal capacity of BS-mZVI and BS-nZVI were enhanced from 0.059 and 0.095 mg/g to 8.801 and 18.91 mg/g,respectively.The Cr（Ⅵ）removal capacity of BS-nZVI was higher than that of chemical sulfidated nZVI（CS-nZVI）（18.80 mg/g）.Fe_xS_y formed on the surface of BS-ZVI had an efficient electron transfer capacity,which enhanced the Cr（Ⅵ）reduction capacity of BS-ZVI.In addition,the EPS covered on the surface of BS-ZVI accelerated the electron transfer between S-ZVI and Cr（Ⅵ）,enabling faster Cr（Ⅵ）reduction by BS-nZVI compared to CS-nZVI.During Cr（Ⅵ）removal by BS-ZVI,the involvement of covered EPS on the coprecipitation of Cr（III）and Fe（III）led to the formation of Cr（III）-Fe（III）-EPS coprecipitates,which impaired the formation of dense passivation layer that would impede electron transfer between the inner coreZVI and Cr（Ⅵ）,and thus the Cr（Ⅵ）removal capacity of BS-nZVI superior to that of CS-nZVI.More importantly,the Cr（Ⅵ）removal capacity of BS-nZVI can be partially recovered by SRB through continuous biological sulfidation,which failed through chemical sulfidation.Furthermore,a hydrogen-based membrane biofilm reactor with non-ZVI（Reactor-NZ）and a reactor combined withZVI（Reactor-Z）were constructed to study the co-reduction process of SO₄^2-and CrO₄^2-by SRB withZVI.Under the stress of CrO₄^2-,the reactor-Z achieved efficient co-reduction of SO₄^2-and CrO₄^2-with removal fluxes of 618.52 and 66.29 g/（m²·d）,respectively,due to the succession of microbial communities and the synergistic effect betweenZVI and SRB.ZVI facilitated the growth and metabolism of SRB,thus reducing the adverse effect of CrO₄^2-on the Reactor-Z.Furthermore,ZVI help to maintain species diversity,thus evolving many strains with effective co-reduction capacity of both SO₄^2-and CrO₄^2-.Therefore,the SRB of Reactor-Z possessed a strong adaptive ability towards CrO₄^2-to achieve CrO₄^2-reduction inside and outside the SRB cell.In contrast,the microbial community of Reactor-NZ evolved toward chromate-reducing bacteria-dominated microorganisms under the stress of CrO₄^2-,Reactor-NZ basically lost the reduction capacity of SO₄^2-and achieved extracellular and intracellular CrO₄^2-reduction mainly through chromium reductase,with SO₄^2-and CrO₄^2-removal fluxes of 1.41 and 24.57 g/（m²·d）,respectively.TheZVI in Reactor-Z covered with Fe_xS_y and SRB biofilm was continuously biological sulfidated by SRB to form BS-ZVI.Under the stress of CrO₄^2-,the EPS secreted by SRB contained more proteins,especially tryptophan protein-like substances with strong complexing ability of CrO₄^2-.The Cr（III）and Fe（III）released during CrO₄^2-reduction by BS-ZVI can be co-precipitated with EPS to form a loose Cr（III）-Fe（III）-EPS co-precipitate,which was discharged from the reactor under the shear stress of the circulating water flow,helping the Reactor-Z to maintain a thin biofilm with high mass transfer efficiency between H₂ and the external biofilm.In summary,SRB changed the physicochemical properties of Fe S,S-ZVI andZVI through the metabolic release of S^2-and/or secreted EPS,which enhancing the reactivity ofZVI through this facilitation mechanism and strengthening the Cr（Ⅵ）reduction from water.In the co-reduction process of SO₄^2-and CrO₄^2-by SRB in combination withZVI,ZVI maintained species diversity by stimulating the metabolic activity of SRB,and the continuous biological sulfidation ofZVI triggered by SRB enhanced the reactivity ofZVI.In this synergistic effect,the efficient co-reduction of SO₄^2-and CrO₄^2-was achieved.The results of this study contribute to the practical application ofZVI in combination with SRB in chromium-containing wastewater treatment process.