The Study Of Microbial Reduction Of Cr(?)-schwertmannite By Shewanella Oneidensis MR-1

Schwertmannite is a kind of very common high iron sulfate minerals in the environments polluted by acid mine drainage?AMD?,and it usually forms by oxidation of Fe?II?by thiobacillus ferrooxidans in low-pH,high-sulfate and high-iron environments.Meanwhile,schwertmannite obtained from nature samples was always loaded and coprecipitated with many ion metals,because the high specific surface area of schwertmannite and the replace of SO₄^2-and Fe?III?by ions in the surroundings.However,the contaminants adsoped or incorporated on the schwertmannite are likely to release to make an impact on the surroundings when the schwertmannite transforms to other minerals because of its unstable structure.The structural Fe?III?can be reduced to Fe?II?by the dissimilatory iron reducing bacteria?DIRB?,and this process will destroy the mineral structure and accelerate the dissolution of schwertmannite and the formation of secondary minerals.Cr???,a common pollutant in AMD environment,is easier to replace SO₄^2-in the schwertmannite for their similar ion radius,leading to high concentration of Cr???loaded on the mineral.Therefore,Cr???-loaded schwertmannites were synthesized in the study by slow chemical method,and the minerals obtained and a kind of typical DIRB S.oneidensis MR-1were added to a anaerobic system to study the reduction of the mineral by S.oneidensis MR-1.The study was carried for a better understanding of mobility of Cr???and the formation of secondary mineral during the reduction process,to provide a theoretical foundation for contamination remediation of mines.The main results of the study shows as follows:?1?The reduction of shcwertmannite by S.oneidensis MR-1 when Cr exist:in the system with Cr???and pure schwetmannite,from the study of the reduction of Cr???by S.oneidensis MR-1,it can be indicated that the bacteria showed strong ability of reduction of Cr???,and the reduction rates were about 0.625 mg/?L·h??0.073 mg/?L·h??0.080 mg/?L·h?with the initial concentrations of Cr???were 5?20?50 mg/L respectively.Cr in the solution was adsorbed immediately in the system with Cr???and schwertmannite existing both.Fe?II?could be detected in the solution after 5 days when the initial concentration of Cr???was 5mg/L,and there was no Fe?II?detected when the initial concentration of Cr???reached 50mg/L.It shows that the Cr???was reduced firstly in the system.The main secondary mineral was goethite probably with a little chrome oxide and ferrihydrite during the reduction process when the initial concentration of Cr???was 5 mg/L.While the solid phase did not changed with higher initial concentration of Cr????50 mg/L?.?2?The reduction of Cr???-schwertmannite by S.oneidensis MR-1:experiments showed that the Cr???loaded to the schwertmannite did not change the XRD data of minerals,but the SEM showed that the rough cloud surface changed to a hedgehog surface and the particles united to become larger with more Cr???loaded to the shcwertmannite.This phenomenon occurred probably due to the replace of SO₄^2-by CrO₄^2-,to make the particles units and the micro morphology of schwertmannite changes.From the study of reduction of Cr???-schwertmannite by S.oneidensis MR-1,Fe?II?released to the solution immediately and no Fe?III?was detected in study of the reduction of Cr???-schwertmannite by S.oneidensis MR-1.The Cr in the solution released from the surface of schwertmannite decreased over time,and no Cr released to the solution during the reduction process.It demonstrated that on other Cr released from the mineral tructure during the whole reduction process.According to the result of XRD,SEM,FT-IR,the main secondary minerals during the reduction process was goethite with a little siderotil.The formation of goethite retarded with more Cr???loaded in the schwertmannite,which indicated the more stable of the mineral.The goethite formed after 1,3 and 7 days,when the content of Cr???in the schwertmannite was 0,0.22,0.37%.There was no secondary mineral detected with a higher content of Cr????0.60%?.It can be concluded that the incorporation of Cr???to the shcwertmannite can strength the microbial stability of this mineral.