Remediation Of Hexavalent Chromium Contaminated Soil By BC@?-Fe₂O₃@loihica Complex With Electron Shuttle

There have been extensive researches and some engineering practices in the restoration of hexavalent chromium-contaminated soil by using microbial dissimilation iron reduction.However,under natural restoration conditions,the restoration effect is unstable due to the lack of conditions to protect and stimulate microbial activity.In this study,biochar was used as a carrier,loaded with hematite and Shewanella loihica to synthesize BC@a-Fe2O3@loihica complex to enhance the protective effect of biochar on bacteria.Electron shuttles was added to promote electron transfer and shorten the repair time.The restoration effect of BC@?-Fe2O3@loihica on Cr(?)in the soil was explored,the changes of the complex before and after restoration were analyzed.The soil heavy metal form,biological toxicity and microbial community structure after restoration were analyzed,and the restoration mechanism of BC@?Fe2O3@loihica was discussed.First,researched the growth curve of Shewanella loihica in the medium,as well as the tolerance and reduction in Cr(?)solution,and explore the bacteria reduction effect on three different electron shuttles,sodium anthraquinone-2sulfonate(AQS),riboflavin(RF)and humic acid(HA)in different dosage.When the Cr(?)concentration is greater than 2 mM,it will have a serious impact on the propagation of bacteria.Adding humic acid as an electron shuttle can increase the reduction rate of bacteria to Cr(?)in the soil by 3.02 times in the same time.Secondly,the BC@?-Fe2O3@lohica complex was synthesized and analyzed by SEM,XRD,XPS,and FTIR.It can be seen that hematite was successfully loaded on the surface of the biochar,and the bacteria in the complex were intact and grew on the surface of the biochar.The process parameters of the composite preparation were optimized,and the best composite composition ratio was:the mass ratio of BC and ?-Fe2O3 was 10:1,and the dosage of BC@?-Fe2O3 was 5.0g.Then the effects of different factors on the remediation effect of 1000 mg/kg Cr(?)contaminated soil were explored,and the response surface method was used for optimization experiments.The best experimental parameters for each factor are:complex,humic acid,sodium lactate and watersoil ratio are 1.0 mL/g,32 mg/g,12%and 3:1 respectively;after Box-Behnken response surface optimization,the predicted value And the measured value are 99.2961%and 98.26%,respectively,the predicted value is not much different from the measured value,and the result of model fitting is valid.The repaired complex was characterized by SEM,XRD,XPS,and FTIR.The oxygencontaining functional groups such as the carboxyl group of the biochar and aromatic carbon can be adsorbed and reduced to remove Cr(?)in the solution.The final trivalent chromium has many forms,mainly Cr(OH)3,Cr2O3,and a small amount of Cr(?)-Fe(?)combined state.Then,analyze the heavy metal form,biological toxicity and microbial community of the reacted soil to evaluate the effect of soil remediation.The heavy metal chromium in the soil changes greatly after the remediation,and the chromium changes from an exchangeable state to an oxidized state;the restored soil can greatly reduce the biological toxicity of the soil,and the germination rate of wheat reaches 92.11%of that of the non-polluted soil,which improves the soil utilization.And it can significantly improve the abundance and diversity of soil microbial species,increase the abundance of Bacillus,Desulfitobacterium,Acinetobacter and other chromium-resistant bacteria,chromium-reducing bacteria and iron-reducing bacteria in the soil,and improve soil community structure.Summary and analysis of the mechanism of complex restoration of Cr(?)contaminated soil:Biochar can quickly adsorb and reduce Cr(?)in the soil due to its strong adsorption capacity,surface oxygen-containing functional groups and the reduction of aromatic structure,and its layered shape And porous structure can provide protection for microorganisms;Shewanella loihica directly or indirectly reduces Cr(?)in the soil.Under the mediation of the electron shuttle humic acid,the reduction rate of Cr(?)can be increased,and the reduction rate of Cr(?)can be increased.The remediation time is shortened to 7 days.For 1000 mg/kg of chromium-contaminated soil,the reduction efficiency can reach 98.26%,and the Cr(?)content is reduced to 17.4 mg/kg,realizing high-efficiency reduction and remediation of hexavalent chromiumcontaminated soil.