Study On The Stabilization Mechanism Of Microbial Detoxification And Encapsulation Of Chromium Slag

Chromate and chromium are widely used in metallurgical,chemical,electroplating,tanning,pharmaceutical and aviation industry.However,chromium slag derived from the production of chromate and chromium is in large quantity,hazardous and hard to control,which has become a serious environmental problem.The toxicity of chromium slag is mainly from hexavalent chromium,which has strong oxidability and carcinogenicity.In this paper,a strain with composite function of chromium reduction and mineralization was applied to comprehensive stabilization of chromium slag,which provides an ecological view of biological detoxification and solidification for chromium slag.In this study,Microbacterium laevaniformans GM-1 with composite function of both urease activity and Cr(VI)reduction was selected from existing 5 strains in the laboratory.Microbacterium laevaniformans GM-1 has good environmental adaptability and it can grow well under the environment with pH 5~10,salinity 0.5%~1%,15~40 ?.It was shown in the orthogonal experiment that the order of the influence to the deposition rate of Ca2+(mineralization of Microbacterium laevaniformans GM-1)was urea concentration > time > pH > Ni2+ concentration.The optimal culture condition for Microbacterium laevaniformans GM-1 mineralization was 50?M Ni2+,60g/L of urea,pH 10 and 96 h of culture time.By Cr(VI)reduction shake flask experiment,the tolerance of Microbacterium laevaniformans GM-1 to Cr(VI)concentration was as high as 160 mg/L.Under the initial conditions of pH 9,10g/L of urea and concentration of 100?M Ni2+,the reduction rate of 90mg/L Cr(VI)was the fastest,which respectively were 1.88,1.91,and 1.87mg/L/h.Cr(VI)concentration in the same system For the same Cr(VI)concentration,the initial inoculation amount had no obvious regularity of the influence to bacterial growth.But in general,the larger the initial inoculation amount is,the more beneficial it is to the growth of bacteria in the early time.To some extent,the increase of the inoculation amount can enhance the reduction of Cr(VI).A biological detoxification experiment on the chromium slag from a chromate factory in Chongqing was carried out by using Microbacterium laevaniformans GM-1.It was known that when the amount of chromium slag was 10%(W/V)and the the inoculation amount was 5%,chromium slag could be effectively detoxified by Microbacterium laevaniformans GM-1 within 120h(5d),hexavalent chromium removal rate could reach 51.08%,and commutative hexavalent chromium in the chromium slag was reduced from 223 mg/kg to 14 mg/kg.Besides,in the whole process of detoxification,Microbacterium laevaniformans GM-1 continuously reduced hexavalent chromium in the detoxification medium and promoted the leaching and dissolution of the hexavalent chromium in the chromium slag at the same time.By XRD analysis,Cr(VI)morphological analysis and SEM-EDS analysis of the chromium slag before and after detoxification,it was known that the detoxification process of chromium slag included adsorption and dissolution,and it could also be related to the mineral phase and crystal lattice.Stabilization of chromium slag was studied by preparing solidified forms with different proportions of chromium slag,river sand and kaolin using Microbacterium laevaniformans GM-1 which had good adaptability to the environment.By testing the uniaxial compressive strength,Cr(VI)leaching toxicity and long-term toxicity(7d)of the solidified forms,it was shown that the strength of solidified forms with Microbacterium laevaniformans GM-1 was significantly higher than that of the control group,and the leaching concentration of Cr(VI)and 7d leaching amount were much lower than that of the control group.When the proportion was 8:1:1,the maximum of solidified strength was 0.6851 MPa,the lowest leaching toxicity Cr(VI)could reach 24.86 mg/L,and 7d leaching amount remained below 1.5mg.Through thermogravimetric analysis,NH3-N analysis,X-ray diffraction analysis,Cr(VI)morphological analysis and SEM-EDS analysis of solidified forms,it was known that the stabilization mechanism of chromium slag included simultaneous biological detoxification and encapsulation of biomineralization CaCO3 deposition.The reductase localization experiment was carried out and it was shown that the major way to reduce Cr(VI)by using Microbacterium laevaniformans GM-1 was direct reduction.The reductase mainly exists in cell-free extracts and also on membrane.The Cr(VI)reductase activity in the cell-free extracts was 48.19 ± 1.30 ?M/h/mg protein and Cr(VI)reductase activity on membrane was 12.59 ± 3.65 ?M/h/mg protein.Culturing with chromium can improve the Cr(VI)reductase activity of bacteria.It was shown in experiment of the Cr(VI)reductase activity that the optimum pH of Cr(VI)reductase in Microbacterium laevaniformans GM-1 was 7.4,and the optimal temperature is 30 ?.Cu2 +,Ni2+,Co2+,Pb2+,Zn2+,Ca2+ and Cr3+ stimulated the Cr(VI)reductase activity in the cell-free extracts and 1mM of respective ions could stimulate the reductase activity by 9.77%,13.94%,3.90%,3.27%,4%,5.33% and 6.03%.Ag +,Hg2+,Mg2+ and Ba2+ significantly inhibited the Cr(VI)reductase activity in the cell-free extracts;1mM NADH,fructose and glucose stimulated the reductase activity in Microbacterium laevaniformans GM-1 by 40.21%,11.15% and 40.21% respectively;1mM 2-mercaptoethanol and depressant sodium azide strongly inhibited the reductase activity in Microbacterium laevaniformans GM-1 by 81.64% and 81.64% respectively;EDTA had no effect on the composition of chromium reductase and its activity.