Remediation Of Hexavalent Chromium Contaminated Water And Soil By Shewanella

The situation of heavy metal soil pollution in China' is severe.And Cr(?)contaminated soil is attracting more and more attention because of its carcinogenic,teratogenic and mutagenic properties.The main remediation methods of Cr(?)contaminated soil are physical remediation and chemical remediation,but these technologies have the problems of high cost and easy to cause secondary pollution.As an environmentally friendly and low-cost technology,microbial remediation is increasingly favored by researchers.The iron-reducing bacterium are widely distributed in the nature.Except the Fe(?)produced by reducing iron oxides is a good reducing agent for Cr(?),iron-reducing bacterium also have a certain reducing ability to Cr(?).Therefore,this study selected three strains of iron-reducing bacterium,which are Shewanella alga,Shewanella putrefaciens and Shewanella oneidensis,to explore their ability to remediate Cr(?)in water and soil,and analyzed the bacterial community structure in the soil before and after remediation.At first,the reaction conditions were optimized by Box-Benhnken response surface method.The results showed that the reduction rate,which was predicted to be 102.51%,was the highest at the temperature of 22.16?,the pH value of 6.86,and the number of bacteria of about 108 per mL,and there were weak interactions between the three factors.Then,we studied the tolerance and direct reduction rate of Shewanella to Cr(?)as well as the effects of iron oxide,sodium lactate and humic acid on reduction of Shewanella to Cr(?)in water.The results showed that Shewanella had high tolerance to a Cr(VI)concentration of 8 mM and could remove about 25%Cr(?).The addition of iron oxide and sodium lactate could improve the reduction rate of Shewanella on Cr(?)to 96.40%-97.20%.And adding humic acid could shorten the reaction time by 4 hours.Secondly,the Cr(?)removal rate of Shewanella to Cr(?)contaminated soil under the above optimal conditions was explored.It was found that the removal rate of total Cr(?)in the less polluted soil was between 75.92%and 92.63%,while the removal rate was 22.59%-27.48%in heavily polluted soil.Among the three strains of bacterium,the removal rate from high to low were Shewanella.alga,Shewanella.putrefaciens,Shewanella.oneidensis.By further optimizing the dosage of iron oxide,sodium lactate,bacterial number and moisture content,it was found that the rise of them could improve the removal rates to over 96.45%.But as soon as they reached the threshold,iron oxide dosage would not work,and the other would have the opposite effects.Finally,by invsetigating the succession regulations of bacterial community structure in the Cr(?)contaminated soil with adding Shewanella,it was found that the adding Shewanella had a great effect on Actinobacteria,Deinococcus-Thermus,Cyanobacteria,Bacteroidetes and Patescibacteria,and could also promote the growth of some iron-reducing bacteria and chromium-reducing bacteria in the soil.The addition of Shewanella alga had the least effect on indigenous microorganisms,and it had a strong adaptablility to the Cr(?)-contaminated environment.