| The problem of heavy metal pollution has become a major challenge for human beings today.Different from the biodegradable organic matter,heavy metals can not be degraded,which brings more difficulties to the removal of pollution.The general strategy to deal with heavy metal pollution is to reduce toxicity by enriching,removing or changing chemical forms.Highly toxic Cr(?)is widely used in electroplating,leather tanning,textile printing and dyeing,metal processing and other industries.At present,the remediation of chromium pollution through various physical,chemical and biological methods has been widely used.Microbial remediation technology has been widely recognized as an economical,effective and ecologically friendly Cr(?)detoxification method because of its advantages of low cost and no secondary pollution.At present,screening strains with high Cr(?)tolerance and reduction ability is still the focus of research.In this study,a strain of Cr(?)resistant reducing bacteria was isolated and screened from the sewage of an electroplating plant in Xiamen,Fujian Province.By physiological and biochemical analysis and 16S rRNA gene sequence analysis,it was identified as Microbacterium sp.,named BD6.Strain BD6 is suitable to grow in medium temperature and alkaline environment.Meanwhile,it can tolerate high salt environment up to 50.0 g/L NaCl.The test showed that growth of the strain was strongly inhibited by Mn2+,slightly inhibited by Ni2+,Zn2+,and Cd2+and promoted by Cu2+.The minimum inhibitory concentration of Cr(?)on BD6 was 1700 mg/L.The reduction effect has been significantly improved by electronic donors including glycerol,fructose,lactose,glucose and sodium pyruvate.Glycerol was selected as the best electron donor for the reduction of Cr(?)by the strain.The reduction rate of Cr(?)with concentration of 100 mg/L had been increased from 69.63%within 96h to 100%within 36h in presence of 2 g/L glycerol.Reduction test showed that the optimum reduction conditions and growth conditions of the strain were consistent.In the high salt environment with NaCl of 50.0 g/L and toxic environment with Cd2+of 50 mg/L,the reduction rate of the strain to 100 mg/L Cr(?)were 96.79%within 72 h and 99.86%within 54 h respectively in presence of 2 g/L glycerol.The reduction of the strain to higher initial concentration of Cr(?)was increased by adding the amount of glycerol,but it would be limited by the toxicity of Cr(VI).By measuring the total chromium in cells and the reduction products of Cr(?)and Cr(?),and the reduction of Cr(?)by cell components,it was speculated that the Cr(?)reductase of strain BD6 was mainly located in the cytoplasm.The reduced product of,Cr(?)was not precipitated in the form of Cr(OH)3,but mainly combined with the extracellular secretion of bacteria to form a soluble complex.The main physical and chemical indexes of sweet potato starch wastewater simulated in laboratory are basically the same as those of actual sweet potato starch wastewater,and its rich nutrients can provide basic nutritional requirements for microbial fermentation.The fermentation concentration of strain BD6 was the best in the wastewater containing 80%sweet potato starch,and the growth and reproduction of the strain were not conducive to the growth and reproduction of the strain if the strain was not diluted or diluted too high.When the temperature was 25?-35?,the strain was suitable for fermentation,and the optimum growth was obtained at 30?,and the growth of the strain was not conducive to the growth of the strain when the temperature was lower than 20? or higher than 40?.The strain grew best in the fermentation environment of sweet potato starch wastewater with pH of 7.0,and the growth of the strain was obviously inhibited under the condition of pH greater than 9.0.When the amount of broth was more than 250 mL,the growth of the strain was inhibited because of the decrease of dissolved oxygen in the fermentation broth,so the amount of broth at 200 mL was selected as the optimUm amount of broth for the fermentation strain.The reducibility of the strain to Cr(?)in sweet potato starch wastewater can be improved by adding nitrogen sources,in which the growth of the strain can be promoted by adding peptone and yeast extract,and the reduction of the strain can also be promoted obviously.The germination rate of soybean could be improved by adding fermentative bacteria in the simulated Cr(?)contaminated soil,27 soybeans germinated in the contaminated soil without bacteria,37 germinated in the contaminated soil with bacteria,and 34 germinated in the control group.The germination rates were 67.5%,92.5%and 85.0%,respectively.The bud length of contaminated soil remediation group was significantly better than that of uncontaminated soil group.The germination rate of contaminated soil without remediation was the lowest and the bud length was short,which indicated that Cr(?)significantly inhibited the germination growth of soybean,and the germination rate of contaminated soil remediation group was the highest,and the bud growth condition remained good.In a word,sweet potato starch wastewater fermentation strain BD6 was used to repair Cr(?)contaminated soil,and good results were obtained. |
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