| Chromium and its compounds are widely used in many industries and a large quantity of Cr(Ⅵ)-containing wastes are released into environment. Chromium, especially hexavalent chromium has posed great threat on environmental safety and human health. The reduction of quite toxic hexavalent chromium into less toxic trivalent chromium is the key for the detoxification of chromium. Many bacterial species have been found to be able to reduce Cr(Ⅵ) and microbial remediation has been regarded as a promising approach for the treatment of chromium pollution. In this paper, the ability and mechanisms of Cr(Ⅵ) reduction by several different strains were researched in detail.A novel Cr(Ⅵ)-reducing strain Cr-4 was isolated from Cr(Ⅵ)-contaminated environment and identified as Bacillus anthracis by sequence analysis of 16SrRNA. The isolate Cr-4 could grow in the presence of 125mg/L Cr(Ⅵ) and its cellular morphology keep quite intact in liquid medium containing 50mg/L Cr(Ⅵ).The parameters affecting Cr(Ⅵ) reduction by isolated strain, Cr-4, were thoroughly investigated, such as carbon source, initial pH value of liquid medium, initial Cr(Ⅵ) concentration, cells inoculum amount, temperature, the anions of SO42- and NO)3-, the heavy metals of Zn2+ and Cu2+ and the exogenous strains. The results demonstrated that addition of carbon source enhanced Cr(Ⅵ) reduction obviously. Among three carbon sources applied in the experiments, malate was the best, followed by glucose, succinate. With the initial Cr(Ⅵ) concentration of 50 mg/L and initial pH value of 7.2, more than 98%, 89% and 83% of Cr(Ⅵ) was removed after 48h by addition of malate, glucose and succinate respectively, but only 60% of Cr(Ⅵ) was removed after 72h if none of carbon source was added. The initial pH value of liquid medium also had important effect on Cr(Ⅵ) reduction. With the initial Cr(Ⅵ) concentration of 50 mg/L and glucose as carbon source, the isolate Cr-4 reduced more than 85%-99% of Cr(Ⅵ) after 72h when initial pH value was in the range of 7.0-11.0, but only 35%-65% of Cr(Ⅵ) was reduced when initial pH value was in the range of 4.0-6.0. The isolated strain, Cr-4 was able to reduce Cr(Ⅵ) at initial concentration from 10 to 80 mg/L, but the Cr(Ⅵ) removal rate decreased with the increase of initial Cr(Ⅵ) concentration. With the cells inoculum amount from 0.5%-5%, the increase of cells inoculum amount leaded to the increase of Cr(Ⅵ) removal rate. The isolate Cr-4 reduced Cr(Ⅵ) more efficiently at 37℃, compared with that at 20℃and 47℃. The anions of SO42- and NO3-, exogenous strains (Pseudomonas aeruginosa and Cellulomonas flavigena) and 25mg/L Zn2+ did not affect the bacterial Cr(Ⅵ) reduction, but 100mg/L Zn2+ inhibited Cr(Ⅵ) reduction, and Cu2+, however, had significant enhancing effects on Cr(Ⅵ) reduction.The relation between cells growth and Cr(Ⅵ) reduction was detected. It was observed that the cells growth of Cr-4 and Cr(Ⅵ) reduction were well correlated. The growth of cells stimulated Cr(Ⅵ) reduction and efficient Cr(Ⅵ) reduction conversely promoted the cells growth. The cell-free extracts and filtrates of cultures were used to reduce Cr(Ⅵ) in order to research the mechanisms of Cr(Ⅵ) reduction by the isolate Cr-4. The results demonstrated that the cell-free extracts was able to reduce Cr(Ⅵ), indicating that the enzyme-catalyzed mechanism was applied in Cr(Ⅵ) reduction by the isolate Cr-4. Additionally, it was found that the mechanism was different when different carbon source was applied as the electron donors. When glucose was used as carbon source, a part of Cr(Ⅵ) was reduced by the metabolites, but no Cr(Ⅵ) was reduced by the metabolites when malate or succinate was used as carbon source. The energy-dispersive X-ray microanalyzer was employed to test the adsorption of chromium by cells after Cr(Ⅵ) reduction. The results of energy spectrums showed that Cr(Ⅲ) was adsorbed by cation exchange during Cr(Ⅵ) reduction at initial pH value of 7.2, but no chromium was adsorbed when Cr-4 reduced Cr(Ⅵ) at initial pH value of 9.0.Another strain, Pseudomonas aeruginosa, was also used to reduce Cr(Ⅵ). Its resistance to Cr(Ⅵ), the factors influencing Cr(Ⅵ) reduction and the mechanisms of Cr(Ⅵ) reduction were researched. It was found that the resistance of Pseudomonas aeruginosa to Cr(Ⅵ) was not as good as that of the isolate Cr-4. The cellular morphology was affected by 40mg/L Cr(Ⅵ), and the cells was not grown in 24h in the presence of 100mg/L Cr(Ⅵ). Three carbon sources, glucose, malate and succinate applied in the experiments, were found to promote Cr(Ⅵ) reduction, and malate had the best effects. With the initial Cr(Ⅵ) concentration of 40mg/L and initial pH value of 7.0, only 34% of Cr(Ⅵ) was removed by Pseudomonas aeruginosa after 72h if there was no carbon source was applied, and the Cr(Ⅵ) removal rate increased to 60% when malate was used as carbon source. Pseudomonas aeruginosa reduced Cr(Ⅵ) more efficiently at initial pH value of 8.0-11.0 than that at 4.0-7.0. With the initial Cr(Ⅵ) concentration of 40mg/L and glucose as carbon source, more than 95% of Cr(Ⅵ) was removed after 72h at initial pH value of 8.0-11.0, but only 10%-50% of Cr(Ⅵ) was removed at initial pH value of 4.0-7.0. The Cr(Ⅵ) removal rate decreased with the increase of initial Cr(Ⅵ) concentration ranging from 10 to 80 mg/L, and the increase of cells inoculum amount (0.5%-5%) leaded to the increase of Cr(Ⅵ) removal rate. The anions of SO42- and NO3- had no effects on Cr(Ⅵ) reduction. Zn2+ inhibited Cr(Ⅵ) reduction significantly even the concentration of Zn2+ was only 25mg/L, but Cu2+ was found to promote Cr(Ⅵ) reduction significantly.The mechanism of Cr(Ⅵ) reduction by Pseudomonas aeruginosa is like that of the isolate Cr-4. Pseudomonas aeruginosa reduced Cr(Ⅵ) by enzyme-catalyzed mechanism, and a part of Cr(Ⅵ) was reduced by the metabolites when glucose was applied as carbon source, but no Cr(Ⅵ) was reduced by the metabolites when malate or succinate was used as carbon source. The energy spectrum showed that Pseudomonas aeruginosa adsorbed chromium by cation exchange during Cr(Ⅵ) reduction (initial pH=7.0).The effect of Fe(Ⅲ) on Cr(Ⅵ) reduction by dissimilatory iron-reducing strain, Cellulomonas flavigena, was also investigated. The results demonstrated that addition of FeCl3 or lepidocrocite promoted Cr(Ⅵ) reduction and increased Cr(Ⅵ) removal rate, but addition of hematite did not lead to the increase of Cr(Ⅵ) removal rate, which indicates that the effect of Fe(Ⅲ) on Cr(Ⅵ) reduction appears different due to the diversity of iron-oxides. In addition, the results demonstrated that the Cr(Ⅵ) removal rate increased with the increase of initial concentration ratio of Fe(Ⅲ)/ Cr(Ⅵ), and the change of pH value reflected Cr(Ⅵ) reduction to some extent.The isolate Cr-4 and Pseudomonas aeruginosa were respectively applied to treat the Cr(Ⅵ)-containing industrial wastewater. The isolate Cr-4 was found to have a higher Cr(Ⅵ) removal efficiency than Pseudomonas aeruginosa. The isolate Cr-4 was also inoculated to Cr(Ⅵ)-contaminated soil. It was observed that the cells of Cr-4 promoted Cr(Ⅵ) reduction, but increased the activity of chromium in soil.
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