Study On Reduction And Stabilization For Remediation Of Chromium-contaminated Grey Calcium Soil In The Semi-arid Region Of Northwestern China

Heavy metal soil contamination in the semi-arid region of northwestern China has become more troublesome with the rapid growth of industry and agriculture,adding to the ongoing soil pollution crisis in China.And chromium,a type of heavy metal element in soil pollution that is hazardous and heavy,has created a significant danger to the soil environment and human living environment.Reduction and stabilization technology is commonly used in the remediation of chromium-contaminated soil,but there are few reports on the application to the remediation of chromium-contaminated gray calcium soil in the semi-arid region of northwestern China.In this study,chromium-contaminated grey calcium soil in the semi-arid region of northwestern China were taken as the research object.Remediation of chromium-contaminated soil with combined chemical reduction and stabilization techniques was conducted to study the effects of reducing agent types and dosage,water（m L）-soil（g）ratio,and maintenance time（remediation time）on Cr leaching toxicity in soils,and screened out the best reducing agent and its corresponding reducing conditions.In order to prevent the reoxidation of Cr（VI）after reduction,the chromium-contaminated soil were stabilized by combining three stabilizers,namely,quicklime,calcium-magnesium phosphate fertilizer,and biochar.By analyzing the leaching concentration of total Cr and Cr（VI）,Cr（VI）content,the bioeffective speciation Cr（VI）content extracted by DTPA and EDTA,and the morphological distribution of Cr（VI）in the contaminated soil before and after remediation,the remediation mechanism of reduction and stabilization technology on chromium-contaminated soil was clarified.The research of this paper can provide feasible technical solutions for the remediation and control of chromium-contaminated sites in the semi-arid region of northwestern China.The main research conclusions of this paper are as follows:（1）Four reducing agents,FeSO₄,Fe（NH₄）₂（SO₄）₂,Na₂S and Na₂SO₃,were used to remediate the chromium-contaminated soil.The results showed that the stabilization efficiency of the four reducing agents for total Cr and Cr（VI）in soils was in descending order of FeSO₄>Fe（NH₄）₂（SO₄）₂>Na₂S>Na₂SO₃.FeSO₄ was the best reducing agent,and the best reducing conditions were as follows:FeSO₄ dosage was 2.5 times the theoretical reaction amount（2.5x）,water and soil ratio was 50%,and maintenance time was 7 days.Under these conditions,the leaching concentration of total Cr and Cr（VI）in the soil were reduced from 65.65 mg L^-1 and 61.98 mg L^-1 in the contaminated soil before remediation to 0.674 mg L^-1 and 0.471 mg L^-1 in the contaminated soil after remediation,respectively,which met the requirements of the wastewater discharge standards.（2）Remediation of chromium-contaminated soil using two stabilizers,quicklime and calcium-magnesium phosphate fertilizer,in combination with the above-mentioned optimal reducing agents.The results showed that the effects of different factors such as stabilizer types,dosage,combination sequence,and maintenance time on the remediation effect of chromium contamination in soil were significantly different.The best experimental conditions for the combined remediation method were:FeSO₄ as the reducing agent with dosage of 2.5x and maintenance time of 3 d;quicklime as the stabilizer with dosage of 5%of the soil mass and maintenance time of 30 d,and the sequence of the combined reducing and stabilizing agent was a two-step method.Under this experimental condition,the leaching concentration of total Cr and Cr（VI）in soil were 0.150 mg L^-1 and 0.141 mg L^-1,respectively.They were according with comprehensive wastewater discharge standards.The Cr（VI）content in soil was 21.164mg kg^-1,which met the requirements of the soil control standard.The bioeffective speciation Cr（VI）content extracted by DTPA and EDTA in soil decreased from 760.727mg kg^-1 and 641.455 mg kg^-1 in the contaminated soil before remediation to 0.455 mg kg^-1 and 1.996 mg kg^-1 in the contaminated soil after remediation,respectively.The speciation of Cr（VI）in soil was mainly dominated by the residual（RS）,followed by the bound to Fe and Mn oxides（OX）and bound to organic matter（OM）,and the speciation of exchangeable（EX）and bound to carbonates（CB）accounted for less than1%,and the degree of soil stabilization was high.（3）The corn straw biochar was prepared by oxygen-limited pyrolysis method,and it was modified by KOH to prepare alkali-modified biochar（KBC）.The prepared KBC was used for the remediation of chromium-contaminated soil.The results showed that when FeSO₄ combined with KBC remediated chromium-contaminated soil,the FeSO₄combined with KBC remediation had good reduction and stabilization effect on Cr（VI）in soil within the range of 0.5%-8%of the dosage,and the reduction efficiency and stabilization efficiency of Cr（VI）were above 95%.The percentage of EX,CB,OX,and OM speciation of Cr（VI）in soil after remediation decreased from 55.73%,1.91%,14.51%and 11.32%in the contaminated soil before remediation to 0.22%,0.06%,2.17%,and7.15%,respectively,while the percentage of RS speciation increased from 16.53%in the contaminated soil before remediation to 90.39%.Therefore,the speciation of Cr（VI）was mainly transferred from the unstable effective speciation to the stable residual speciation.During the maintenance period,the leaching concentration of total Cr and Cr（VI）,Cr（VI）content,and bioeffective speciation Cr（VI）content extracted by DTPA and EDTA in soil showed a gradual decrease with the extension of maintenance time.The percentage of EX,CB,OX,and OM speciation of Cr（VI）decreased to different degrees,while the percentage of RS speciation gradually increased.Therefore,the FeSO₄ combined with KBC remediation could rapidly reduce Cr（VI）to Cr（III）,significantly improve the stability of chromium,reduce the environmental risk of chromium,and have better sustained stability.In summary,from the consideration of economic benefits and technical feasibility,the best experimental scheme for using reduction and stabilization technology was:the dosage of FeSO₄ was 2.5x,and the dosage of KBC was 8%of the soil mass.This program had a significant treatment effect on the remediation of chromium-contaminated soil.It reduced the environmental hazards of the contaminant chromium.