Long-term Stability Evaluation Of Iron-based Materials For Remediation Of Chromium-contaminated Soil

A large amount of waste gas,water and residues are produced in industrial production processes such as electroplating,printing and dyeing,leather tanning,and chromium salt production,resulting in serious soil chromium pollution.Chemical reduction stabilization method is currently the most widely used chromium-contaminated soil remediation technology,but the site after reduction and remediation often appears"return to yellow",and the reduced Cr（Ⅲ）is oxidized to Cr（Ⅵ）again,resulting in pollution hazards.However,the mechanism of Cr（Ⅲ）reoxidation after soil remediation is still unknown,and the dominant factor affecting the long-term stability of Cr-contaminated soil remediation is unclear.Therefore,in this study,the widely used typical iron-based materials:ferrous sulfate（Fe SO₄）,nano-zero-valent iron（n ZVI）and pyrite-loaded zero-valent iron composite（Fe S₂/Fe⁰）were selected to repair high-concentration chromium-contaminated soils.optimization of experimental conditions,and long-term stability evaluation of the restored soil,to explore the long-term stability of the remediation effects of different iron-based materials in extreme environments such as freeze-thaw cycles,dry-wet alternation,acid rain leaching,and oxidation conditions The re-oxidation behavior of different reduction products under oxidative conditions was analyzed and the stability mechanism was studied,which provided the basis and guidance for the management and subsequent reuse of soil after remediation.The main findings are as follows:（1）The optimal process conditions of typical iron-based materials Fe SO₄,n ZVI and Fe S₂/Fe⁰for the remediation of chromium-contaminated soils were explored,and the transformation of soil chromium forms and changes in microbial communities after remediation with different materials were compared.The results showed that the removal rate of total Cr（Ⅵ）in the soil treated with Fe S₂/Fe⁰,Fe SO₄and n ZVI were 99%,90%,and 26%,respectively,which reached the maximum when the material dosage was 5%,the water-soil ratio was2:1,and the restoration time was 15 d.The effective state of Cr in the restored soil decreased to 3.12%,11.76%,and 9.59%,respectively,and the combined state of iron and manganese oxides increased to 85.3%,72.79%,and 74.59%,respectively.The bioavailability of soil after iron-based material remediation decreased and was mainly converted into stable iron-manganese oxide bound state.In addition,the soil community distribution abundance and diversity after stable restoration of the three iron-based materials were higher than those before stabilization,and the effect of remediation is Fe S₂/Fe⁰>n ZVI>Fe SO₄.（2）The release,migration and transformation of chromium in soils stabilized by Fe S₂/Fe⁰,Fe SO₄and n ZVI under extreme conditions were investigated,and the key factors affecting the long-term stability of iron-based materials were identified.The concentrations of total Cr（Ⅵ）,water-soluble Cr（Ⅵ）,and toxic leaching total Cr（Ⅵ）and total Cr（Ⅵ）in the soil after remediation of the three iron-based materials remained basically unchanged under the conditions of freeze-thaw cycles,dry-wet alternation and acid rain leaching.The available state of Cr in soil decreased,and it was mainly converted into the bound state of iron and manganese oxides.Under extreme oxidizing conditions,the total Cr（Ⅵ）in soil increased with the increase of manganese dioxide content and reaction time.When the Mn O₂content was 5%and the reaction time was30 d,the total Cr（Ⅵ）concentration in the soil of the control,Fe SO₄,Fe S₂/Fe⁰increased by 297.68,118.68 and 117.07 mg/kg,respectively,which indicated that the oxidation condition was the key factor affecting the stability of Cr（Ⅲ）in the soil after remediation.（3）The kinetic properties and reoxidation mechanism of the reduction products under oxidizing conditions after the reaction of different iron-based materials with Cr（Ⅵ）were revealed.The results showed that the reduction products of Fe SO₄are amorphous Cr（OH）₃,the reduction products of n ZVI are a mixture of Fe Cr O₃,Fe₂O₃and Fe⁰,and the reduction products of Fe S₂/Fe⁰are Fe S₂and Cr_xFe_1-x（OH）₃.After reacting with Mn O₂,the Cr（Ⅲ）in the reduction products of Fe SO₄,n ZVI and Fe S₂/Fe⁰was reoxidized,and the Cr（Ⅵ）phase increased by 2.19%,21.71%,and 13.85%,respectively.The apparent oxidation rates were4.72×10^-2h^-1,5.37×10^-4h^-1and 4.14×10^-3h^-1.The synthesized Cr（OH）₃reacted with Mn O₂to generate 9.56%Cr（Ⅵ）,and the product contained Mn O₂,Mn₃O₄and Mn OOH.XPS showed that Mn（IV）decreased by33.84%after the reaction,and was reduced to Mn（Ⅲ）)and Mn（Ⅱ）.The apparent oxidation rate of Cr（OH）₃is 7.59×10^-3h^-1,and Cr（OH）₃has a great risk of reoxidation.37figures,8tables and 147 references...