The Cr(Ⅵ) Removal Mechanism Of The Interaction Between Pyrite And Shewanella Under Light Irradiation

Pyrite,as a widely occurring semiconductor mineral in nature,plays a various significant role in driving various biogeochemical processes via the conversion of Fe and S elements.Under light irradiation,the photocatalytic performance of pyrite is crucial in the interaction the interaction between minerals and microorganisms as well as the transformation of heavy metals in the environment.However,the influence of light on the growth metabolism of microorganisms and the electron transfer process between iron sulfide minerals and microorganisms still not clear.Therefore,in this study,pyrite a semiconductor mineral,and S.oneidensis MR-1,a dissimilatory metal reducing bacterium S.oneidensis MR-1 were selected as experimental materials to investigate the effect of light irradiation on the growth-metabolism of Shewanella,the electron transfer pathways in the pyrite-S.oneidensis MR-1-Cr（Ⅵ）system under visible light irradiation,and the mechanism of Cr（Ⅵ）removal.The following main conclusions were obtained:（1）Under dark condition,the removal rate of Cr（Ⅵ）by pyrite-S.oneidensis MR-1composite was 50%,while under light irradiation,the corresponding removal rate could reach 64%.Both of light and dark conditions exhibited a synergistic promotion effect on the removal of Cr（Ⅵ）by pyrite and S.oneidensis MR-1,with the synergistic factors of2.24 and 1.65,respectively.However,the synergistic promotion effect between mineral and microorganism was stronger under light irradiation.The mutant experiment revealed that the removal rate of Cr（Ⅵ）by outer membrane protein mutantΔomc A-Δmtr C and the inner membrane protein mutantΔcym A were lower than that of the wild type of S.oneidensis MR-1,meanwhile,the interaction of inner and outer membrane mutant with pyrite also exhibited a synergistic effect on Cr（Ⅵ）removal under light/dark conditions.（2）Light irradiation had no significant effect on the survival rate of S.oneidensis MR-1,while in the pyrite-S.oneidensis MR-1 composite,the survival rate of S.oneidensis MR-1 under light irradiation was higher than under dark condition,this is ascribed to the photoelectrons generated by light excitation of pyrite in which were acquired and utilized by S.oneidensis MR-1,thus maintaining the microbial activity.Under both light and dark conditions,the removal of Cr（Ⅵ）by pyrite,S.oneidensis MR-1 and pyrite-S.oneidensis MR-1 composite systems was mainly dominated by reduction,with a smaller contribution of adsorption was small.（3）Under light irradiation,the sequential interaction of functional groups in the pyrite-S.oneidensis MR-1 composite with Cr（Ⅵ）was as follows:1236 cm^-1（P=O）→1397 cm^-1（COOH）→1540 cm^-1（amide II）→1651 cm^-1（amide I）→1078 cm^-1（P-O）.Under dark condition,the order was as follows:1078 cm^-1（P-O）→1651 cm^-1（amide I）→1540 cm^-1（amide II）→1397 cm^-1（COOH）→1236 cm^-1（P=O）.（4）The products of Cr（Ⅵ）removal by pyrite-S.oneidensis MR-1 composite mainly exist as soluble Cr（III）in the supernatant,with a small fraction present as precipitates such as Cr（OH）₃on the surface of mineral solid phase.The amount of Cr（III）generated in both of the solution and on the surface of solid phase under light irradiation was higher than under dark condition,indicating light irradiation affected the morphology and distribution of Cr（Ⅵ）products.（5）Under both light and dark conditions,Bipyridine inhibited the removal of Cr（Ⅵ）by pyrite-S.oneidensis MR-1 composite.The free S₂（-II）in the system would be converted into S₀,S_n^2-and other intermediate substances with the same reducibility,and further oxidized into sulfite(SO₃^2-)and other sulfur oxides,ultimately,transforming into sulfate(SO₄^2-)and stored in the supernatant.The dissolved Fe（II）generated was oxidized to Fe（III）,which was then reduced back to Fe（II）by S.oneidensis MR-1 and other polysulfides such as S₂（-II）and S₀,and the Fe（II）,participating in the reduction of Cr（Ⅵ）and making the generated free Fe（II）difficult to exist for a long time,forming a Fe（II）/Fe（III）cycling mechanism.In the supernatant,Fe mainly existed in the form of free Fe（III）.At the same time,some iron hydroxide and chromium-containing iron composite precipitations would be formed on the surface of mineral solid phase.