Study On The Characteristics And Mechanism Of Bioreduction And Accumulation Of Hexavalent Chromium By Rhodobacter Sphaeroides H

The pollution of heavy metals in water environment is becoming more and more serious in China,especially the pollution of hexavalent chromium.The toxicity of different forms of chromium is different.The toxicity of hexavalent chromium[Cr???]is much greater than that of trivalent chromium[Cr???].The reduction process of Cr???can reduce the toxicity of Cr???.In recent years,the microbial reduction of Cr???has become a research hotspot at home and abroad.In this paper,Rhodobacter sphaeroides Strain H,which was preserved by our research group,was used as experimental strain.The factors affecting the reduction and accumulation of Cr???and the mechanism by H strain were investigated.The major conclusions are as follows:?1?The minimum inhibition concentration of strain H to Cr???was 300 mg/L;the living cells were the main factor of reducing Cr???;under conditions of anaerobic light,aerobic light,anaerobic dark and aerobic dark,the strain H could reduce Cr???;The reduction rate of100 mg/L Cr???by strain H in 96 hours was reached 98.4%under anaerobic light,the initial concentration of Cr???was 100 mg/L,the initial pH was 7.0,the inoculation amount was 5%,and the incubation temperature was 30?.Co-existence of metal ions and oxygen anions had different effects on the reduction of Cr???by strain H,of which the most promotive effect was Co²⁺,while the most inhibitory metal ion was Cu²⁺.Among the four oxygen-containing anions,SO₄^2-and PO₄^3-had greater effect on the reduction of Cr???by strain H,but the inhibition of other ions was not obvious.Different electron donors could also produce effects on the reduction of Cr???by strain H.Lactose,glucose,sucrose and sodium acetate had the most significant effect,while maltose and citric acid had significant inhibitory effects.In addition,the reduction of Cr???by strain H was in accordance with the first order reaction at lower concentration.With the concentration increasing,the reaction order of strain H was between the first order and the second order.?2?The time-dependent relationship between Cr???and Cr???in the extra-cellular and sub-cellular fractions of strain H showed that 98%of the Cr???in the supernatant could be removed within 96 hours,of which 55.3%could be converted to Cr???on the cell surface,19.9%to Cr???in the cell surface,and 24.8%to Cr???accumulated in the cell.Further study on the reduction of Cr???by extra-cellular and sub-cellular components showed that the reduction of Cr???by extra-cellular components was the best,followed by cytoplasm and cell wall components.The determination of the activity of reducing substances of extracellular components proved that this component was the main substance for the reduction of Cr???.According to the reduction properties of extra-cellular enzymes,the optimum pH,temperature and substrate concentration were 7.0,30?and 0.2 mmol/L.Different electron donors have great influence on enzyme activity,among which NADPH improves the reduction efficiency by 50%.Metal ions like Cu²⁺,Fe²⁺and Ca²⁺had strong inhibitory effects on the activity of reductase,while Co²⁺and K⁺could significantly promote the activity of reductase.?3?FT-IR results showed that the hydroxyl,amino and amide functional groups of strain H appeared strong absorption peaks at the corresponding positions after the accumulation of Cr???in cell wall.The results of scanning electron microscopy?SEM-EDX?further proved that the toxicity of Cr???on the cell wall had a certain effect on the surface morphology and cell structure of strain H,making the cells rough and irregular.The results of transmission electron microscopy?TEM?showed that there were many black dense particles in the cells,indicating that the reduction products of Cr???or Cr???entered the cells.The results of XPS and XRD characterization showed that Cr???was reduced to Cr???,and the reduction product was Cr₂O₃.