Research On Remediation Of Soil Polluted By Cadmium And Lead Through The Sulfate Reduction Based On Bioelectrochemistry

Lead(Pb)and cadmium(Cd)are common metal pollutants in farmland soils in China,which threaten human health through the food chain.In situ immobilization of Pb and Cd in agricultural soil with S2-is an effective remediation strategy.However,the activity of sulfate-reducing bacteria(SRB)in the soil is always limited by organic carbon and pH of soil,which makes it difficult to quickly and effectively reduce SO42-to S2-.In addition,the use of S2-in soil remediation has trouble of high cost,difficulty in storage and safety.Therefore,this study established a bioelectrochemical system-sulfate reducing reactor(BES-SRR),which used hydrogen produced by electrolysis of water as the electron donor for autotrophic SRB,and used SO42-containing wastewater as the sulfur source.The S2+ containing solution produced by BES-SRR was used as an immobilization agent to immobilize the heavy metals in farmland soil.The work of the research was as follows:First,a sulfate reduction system based on bioelectrochemistry was constructed.The optimization results showed that under the conditions of pH=7.5 and 20 mA current,the maximum reduction efficiency of the BES-SRR system to 60 mg/L SO42-was 51.8%.The H2 utilization efficiency was 25.1%.When the current was 15-30 mA,the current was proportional to the SO42-reduction efficiency.As the concentration of SO42-increased,the SO42-reduction efficiency decreased due to the inhibitory of S2-on SRB.Secondly,the use of BES-SRR to remediate Cd contaminated farmland soil was studied.When the ratio of Cd immobilization to soil was 160g:40g and the S2-concentration was 30 mg/L,the bioavailable Cd immobilization efficiency was maximum(51.0%).The results showed that the ratio of immobilization agent to soil and the concentration of S2-were important factors that influenced the Cd immobilization efficiency.In addition,the remediation on soils taken from different regions showed that the content of Fe,Cu and other metals in the soil had an impact on the immobilization efficiency of Cd.Furthermore,the use of the BES-SRR to remediate Pb and Cd co-contaminated farmland soil was studied.Under the conditions of a current of 20 mA and a remediation time of 21.5 hours,77.4%of bioavailable Pb was converted to organic matter bound and residual bound.While,49.0%of bioavailable Cd was converted to carbonates bound and iron and manganese bound due to the increase of soil pH.In addition to immobilization,BES-SRR also had washing effect on the bioavailable Cd in the soil,and the heavy metal ions and NO3-were removed in the BES-SRR.Remediation experiments on soils taken from 4 different regions showed that the BES-SRR was efficient on Pb and Cd contaminated soils in most areas.Finally,the biological toxicity of the remediation soil,the change of heavy metal concentration and the microbial community were analyzed.Compared with the original soil,the phytotoxicity of the treated soil was significantly reduced.After remediation,the germination rate of Brassica chinensis L.seeds in the soil increased from 50.0%in the original soil to 68.2%.After 35 days of continuous cultivation,the bioavailable Pb and Cd concentrations in the waterlogged soil further decreased by 46.3%and 23.1%,confirming that BES-SRR has a certain sustained remediation effect for soils.Through 16S rRNA sequencing,it was identified that SRB activity in the soil significantly increased after remediation,and Pseudomonas,Bacillus,Stenotrophomonas and Pullulanibacillus with heavy metal resistance increased significantly.The results of this study provides a promising technology to remediate heavy metal polluted soil.