| Reveal the mechanism on biological and chemical changes between the reaction of microbial organisms and toxic metals from microcosmic perspective, is conducive to form deeply understanding of the nature of biological treatment for toxic metals pollution, and then develop more mature and efficient technologies on the removal and recycling of toxic metals. In this work, we chose the non-pathogenic soil bacteria (Bacillus thuringiensis, Bacillus subtilis, Bacillus cereus and other bacteria) as target strains, investigated the microscopic changes and transformation mechanism in the reaction process of toxic metals and bacteria, and utilized the bean curd wastewater for bacterial culture and applied it into the removal of toxic metals.The main results were as follows:(1) Two B. thuringiensis strains, which were isolated from uranium-contaminated soil samples in Xinjiang, had highly resistant and possessed super accumulation ability to U(VI), and the maximum accumulation capacity was around400mg U/g biomass (dry weight). X-ray powder diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy (FT-IR) analyses indicated that U(VI) was adsorbed on the bacterial surface firstly through coordinating with phosphate,-CH2and amide groups, then needle-like amorphous uranium compounds were formed, and the uranium immobilized on bacteria was still at the U(VI) state. With the extension of time, the adsorbed uranium nanoparticles entered into the intracellular region, and then the amorphous uranium compound was transformed into crystalline tetragonal-uramphite. Besides, the cytoplasm of B. thuringiensis had better uranium-immobilization ability than its cell debris, suggesting that the formation of uramphite was attributed to the rich phosphate or other organics in cytoplasm. (2) Previous studies by our group member indicated that the planktonic cells and biofilms of B. subtilis had different effects on Cr(Ⅵ) reducution and Cr(Ⅲ) immobilization. Furthermore, electron microscope observation and valence analysis were applied in the reaction process betweeen chromium and bacteria, in order to investigate the mechanism and explore the reasons for the different ability of reduction and immobilization. TEM analysis indicating that the planktonic cells of B. subtilis became lean and rough after treated with100mg/L Cr(Ⅵ) for120h, and the chromium signal could be detected inside of bacteria by EDS analysis. However, after the Cr(Ⅵ) treatment, neither morphologic change nor chromium was detected inside of biofilm cells. Moreover, the result of X-ray photoelectron spectroscopy (XPS) indicating that the chromium immobilized on biofilms and planktonic cells was at Cr(Ⅲ) state. Additionally, a strategy combining the advantages of planktonic cells and biofilms was proposed, and the chromium removal from electroplating wastewater was successfully achieved in a10-L pilot-scale experiment, the remaining non-immobilized Cr was lower than the wastewater release standard of China (total Cr(?)1.5mg/L). Simultaneously, the immobilization Cr(Ⅲ) on bacteria can be further recycled in the form of Cr2O3.(3) The biosorption mechanism of Ni(Ⅱ) by B. cereus was investigated in this study. This bacterium was isolated from a toxic metals contamination site in Pingnan, Fujian Province. It was found that the adsorption equilibrium reached rapidly in2h and the maximum nickel adsorption capability of B. cereus was17.7mg/g (dry weight). Additionally, electron microscopic (AFM, SEM and TEM), ultrasonic disrupted experiment, FT-IR and XRD analysis confirmed that the extracellular immobilization was the main biosorption process, the bacterial amido and carboxyl function groups had involved in nickel immobilized, and the Ni(Ⅱ) collected by the bacteria was amorphous.(4) In order to reduce the cost and increase the feasibility of real application in removal of heavy metals by bacteria, the discharged soybean wastewater was used as cheap culture for bacterial cultivation, and the cultured bacteria were applied to remove toxic metals from electroplating wastewater. It was found that B. cereus and Ochrobactrum anthropi CTS-325could be well grown in dilute soybean wastewater, means that the soybean wastewater could ensure the necessary nutrition for bacterial growth. In addition, soybean wastewater with adding sucrose residue leach solution (pH7.5) was the optimum condition for bacterial growth. Further studies revealed that when the initial concentrations of chromium and nickel in electroplating wastewater were214mg/L and367mg/L respectively, the cultured bacteria in optimum medium had a strong capacity of Cr(VI) and Ni(II) removal, Meanwhile, the biomass of B. cereus and O. anthropi CTS-325could be greatly affected the removal efficiency, the more bacterial biomass, the better removal ability, and the removal efficiency could be higher than80%. |
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