| With the continuous progress of industrial technology,a large amount of hexavalent chromium,has been released into the environment.Hexavalent chromium can migrate in the environment and accumulate with the food chain.If too much hexavalent chromium is absorbed in living organisms,it can cause cancer,mutation and malformation,which puts the life and health of humans,animals and plants at risk.Therefore,finding an effective method to reduce hexavalent chromium in the environment,is one of the hot issues of current research.The reduction of hexavalent chromium to trivalent chromium using microbial reduction,is considered as an economical and green method.However,microorganisms can reduce Cr(Ⅵ)at low concentrations,while high concentrations inhibit microorganisms,while Cr(Ⅲ)still has low toxicity and risk of being reoxidized to hexavalent chromium.Therefore,solving the problems of microbial susceptibility to interference by high Cr(Ⅵ)concentration,long remediation cycle,and removal of Cr(Ⅲ)are the key problems to be solved in this study.In this study,a strain of Cr(Ⅵ)reducing bacteria was screened from a soil contaminated with heavy metals,and the reduction characteristics of the bacteria for Cr(Ⅵ)were investigated.On this basis,corn stover was modified and polycationic straw was prepared,with which the strain was immobilized,and its effect and mechanism on Cr(Ⅵ)reduction and Cr(Ⅲ)immobilization were investigated,mainly as follows:(1)Screening and identification of Cr(Ⅵ)-reducing bacteria.Six strains with hexavalent chromium tolerance were isolated from the molybdenum mine contaminated soil around Jiaohe City,Jilin Province.The six strains were found to have the function of hexavalent chromium reduction through re-screening,among which strain MZ-1 had better reduction ability than others.The reduction efficiency for hexavalent chromium of MZ-1 at 50 mg/L and 100 mg/L were 96.74%and 68.45%,respectively.The strain was used as a follow-up study and was identified by physiological and biochemical tests and 16s rRNA,etc.The strain was identified and named Bacillus licheniformis MZ-1 who had the highest homology with Bacillus licheniformis.(2)Study of the reduction performance of strain MZ-1.The conditions for the reduction of hexavalent chromium by strain MZ-1 were optimized using single-factor experiments.It was found that the optimal reduction conditions for strain MZ-1 were 5%inoculum,pH 7,and temperature 42℃.Under the optimal conditions,strain MZ-1 could completely reduce 50 mg/L Cr(Ⅵ)in 28 h.In addition,MZ-1 can still reach 100%reduction rate when coexisting with various heavy metal ions(Fe3+,Mn2+,Pb2+).It was also found that strain MZ-1 had good heat and salt tolerance,and could effectively reduce hexavalent chromium even at 52℃ or 11%salt concentration,It has the potential to be applied to the biological treatment of complex chromium-containing wastewater.(3)The corn stover pellets were modified by cross-linking polyethyleneimine(PEI)and glutaraldehyde to prepare poly cationic straw(PEI-CS).After that,MZ-1 cells were immobilized on the polycationic straw using electrostatic forces to produce a polycationic straw immobilization system(PEI-CS immobilized MZ-1).It was found that at 50-250 mg/L Cr(Ⅵ)concentrations,the free bacteria were only able to completely reduce Cr(Ⅵ)at 50 mg/L,and were unable to reduce Cr(Ⅵ)at 250 mg/L.In contrast,immobilization conferred the ability of MZ-1 to completely reduce 100 mg/L Cr(Ⅵ),with a reduction rate of only 24 h required to reach 100%,and a high reduction rate for 250 mg/L Cr(Ⅵ).Therefore,the immobilization of MZ-1 by PEI-CS not only increased the reduction rate of hexavalent chromium,but also increased the upper limit of the concentration that could be reduced.In addition,free bacteria can reduce hexavalent chromium in the range of pH 6-9,but none of them can reach 100%reduction rate.In contrast,the PEI-CS immobilized MZ-1 has strong adaptability to pH changes and can achieve complete reduction of Cr(Ⅵ)in the pH range of 5,6 and 7 at a Cr(Ⅵ)concentration of 100 mg/L.This is more effective and ecologically meaningful for the removal of Cr(Ⅵ)from acid and alkaline wastewater using immobilized microorganisms.It is noteworthy that the immobilization method improved the stability and efficiency of MZ-1 under high salt shock,and the reduction rate remained up to 100%at 1%,3%and 5%salt concentrations,while the free bacteria were unable to completely reduce hexavalent chromium at any salt concentration.Even at salt concentrations of 7%,9%and 11%,the immobilized MZ-1 still had high Cr(Ⅵ)reduction capacity and good and stable Cr(Ⅲ)fixation rate,indicating that this technology has the potential to be applied to the bioremediation of Cr(Ⅵ)under extreme operating conditions.Moreover,compared with free MZ-1,PEI-CS immobilized MZ-1 enhanced the adsorption of trivalent chromium after hexavalent chromium reduction in the range of 50-250 mg/L hexavalent chromium,pH 5-9 and salt concentration 1-11%.In addition,the reduction rate of PEI-CS immobilized MZ-1 for the reduction of hexavalent chromium remained 89.8%after up to 10 cycles,indicating its good reduction ability and stability.Finally,the mechanism of Cr(Ⅵ)reduction and Cr(Ⅲ)immobilization by PEI-CS immobilized MZ-1 was initially investigated,and it was found that the adsorption of hexavalent chromium by PEI-CS alleviated the pressure of MZ-1 during the biological reduction of hexavalent chromium by MZ-1,and at the same time,the functional groups on the surface of PEI-CS also participated in the reduction process of hexavalent chromium,and the combined effect of both improved the 1 and the reduction rate and capacity of hexavalent chromium reduction.It was also found that after the reduction of hexavalent chromium by immobilized MZ-1,a large number of functional groups on PEI-CS could effectively immobilize trivalent chromium.In summary,hexavalent chromium reducing bacteria MZ-1 belongs to Bacillus licheniformis,and it has a good ability to reduce hexavalent chromium up to 100%of 50 mg/L at 28 h.And the strain MZ-1 has a wide temperature(28～52℃)and salt concentration(0～11%)tolerance ability.Immobilization of MZ-1 with poly cationic straw could effectively improve the reduction rate and reduction ability of hexavalent chromium,and improve the tolerance ability of the strain to pH and salt concentration.The results of this study can provide a theoretical basis for the efficient remediation of hexavalent chromium-contaminated wastewater by immobilized microorganisms. |
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