The Transformation Mechanism Of Selenite By Shewanella Oneidensis MR-1

Selenite(Se(IV))is highly water-soluble and toxic,and widely exists in mining,smelting and other wastewater.Once the wastewater containing selenium enters the water body,it will accumulate in the environment and enter the human body through the biological chain to cause selenium poisoning.Biological treatment is a common water treatment method,but the transformation mechanism of Se(IV)by microorganisms is still unclear and needs further investigation.In this paper,Shewanella oneidensis MR-1,a model strain of metal-dissimilating bacteria,was used as the main research object to reveal the extracellular and intracellular transformation mechanism of Se(IV)induced by S.oneidensis MR-1.Selenium nanoparticles(Se NPs)synthesized by S.oneidensis MR-1 were separated and recovered.The results provide theoretical support for exploring the biological function and potential application of S.oneidensis MR-1,which has academic significance and application prospect.The main research conclusions are as follows:(1)The extracellular transformation mechanism of Se(IV)induced by S.oneidensis MR-1 was investigated through reduction kinetics experiments and modern test characterization methods.The reduction effect of tightly bound extracellular polymer(TB-EPS)on Se(IV)was stronger than that of loosely bound extracellular polymer(LB-EPS).The contents of protein,polysaccharide and humic acid of TB-EPS were higher than those of LB-EPS.The C-O-C of uronic acid of LB-EPS and TB-EPS under aerobiotic conditions,as well as the amide bonds in proteins and the aromatic rings in humic acid of LB-EPS and TB-EPS under anaerobic conditions,were found to play major roles in the Se(IV)reduction.The ~1H-NMR results showed that some fatty amine hydrogen and aromatic amine hydrogen in LB-EPS and TB-EPS were converted to acetamide group after reacting with Se(IV).The methylene content of TB-EPS before and after reaction with Se(IV)was higher than that of LB-EPS.In addition,LB-EPS and TB-EPS were found to have clearly visible selenium nanoparticles after reacting with Se(IV);TB-EPS had a higher selenium particle content than LB-EPS.(2)The intracellular transformation mechanism of Se(IV)induced by S.oneidensis MR-1 was investigated by means of molecular biological tests.The reduction of Se(IV)in S.oneidensis MR-1 cells occurred mainly in the cell membrane and cytoplasm under aerobic conditions,while in the cytoplasm under anaerobic conditions.Glutathione and cytochrome c in cells were involved in the reduction of Se(IV).The results of q PCR showed that the reduction of Se(IV)under anaerobic conditions depended on sulfur metabolism.Under aerobic conditions,only thioredoxin reductase expression increased after the addition of Se(IV).TEM results showed that Se NPs generated by S.oneidensis MR-1 under both aerobic and anaerobic conditions were mainly distributed outside the cell.The particle size of Se NPs produced under anaerobic conditions was larger both extracellularly and intracellularly than that under aerobic conditions,and the extracellular particle size of Se NPs was larger than that in the intracellular.(3)The separation and recovery performance of Se NPs produced by S.oneidensis MR-1 by ultrasonic crushing and lysozyme methods were revealed.The Se NPs generated by S.oneidensis MR-1 were found to be coated with proteins and other substances;Lysozyme method could not effectively separate and extract Se NPs,while ultrasonic crushing method could effectively separate and extract Se NPs when the ultrasonic power was set at 450 W.The results of SEM-EDS and dynamic light scattering revealed that the Se NPs generated under aerobic and anaerobic conditions were spherical nanoparticles.The particle size of Se NPs produced under anaerobic condition(210 nm)was larger than that under aerobic condition(165 nm).The results of XRD and Raman spectra also showed that Se NPs produced by S.oneidensis MR-1 under aerobic conditions were amorphous selenium,whereas under anaerobic conditions are triangular selenium.