Research On Simultaneous Remediation Of Arsenic,cadmium,lead Co-contaminated Soil By Amorphous Iron-manganese Binary Oxides

Industrial activities such as metal mining and smelting often lead to the coexistence of heavy metals such as As,Cd,and Pb,causing serious multi-metal pollution to the water and soil.Due to the different chemical properties of divalent cations Cd and Pb and oxygenated anions As,the remediation of As,Cd and Pb co-contaminated soils poses a challenge.The widespread Fe/Mn oxides in soil have a strong enrichment and oxidation on heavy metals.When Fe/Mn oxides are amorphous structures,more surface defects will greatly improve their immobilization performance;while amorphous iron/manganese oxides will spontaneously transform into more stable crystalline oxides in the environment,which can directly achieve long-term stability of immobilized heavy metals.In this study,Amorphous iron-manganese binary oxides was successfully prepared by microwave hydrothermal method,and the immobilization performance and long-term stability of amorphous iron-manganese binary oxides were investigated;the simultaneous immobilization mechanism of As,Cd and Pb was also discussed,which provided an important theoretical basis for the application of amorphous iron-manganese binary oxides to As,Cd and Pb co-contaminated soil.The main results are as follows:（1）Amorphous iron-manganese binary oxides was successfully prepared by microwave hydrothermal method,and the Fe/Mn mass ratio and the synthesis p H were the main factors affecting the immobilization performance of amorphous iron-manganese binary oxides,which is a mesoporous material composed of round spherical nanoscale particles with a specific surface area of 182.76 m²·g^-1 and abundant surface hydroxyl active sites.The Fe and Mn elements in amorphous iron-manganese binary oxides are mainly present in Fe（III）and Mn（IV）.The above properties give the amorphous iron-manganese binary oxides the ability to immobilize As,Cd and Pb efficiently.The immobilization rates were up to 85%,100%and 100%for As O₃^3-,Cd²⁺and Pb²⁺co-contamination system(pollutant concentration of 100 mg·L^-1).98.45%oxidation of As（III）at 10 mg·L^-1 was achieved by amorphous iron-manganese binary oxides within 120 min.（2）Amorphous iron-manganese binary oxides has excellent remediation efficiency on As,Cd,Pb co-contaminated soil and has a high stability of heavy metals after remediation.After 5%（w/w,soil mass ratio）amorphous iron-manganese binary oxides reaction with co-contaminated soil for 7 days,the contents of available As,Cd and Pb in the soil decreased by 87.35%,36.70%and 48.67%,respectively,and the contents of water-soluble As,Cd and Pb were reduced by above 99%.Compared with crystalline iron-manganese binary oxides,the immobilization efficiency was improved by 5.68%,8.5%and 9.55%,respectively.As,Cd,and Pb in the soil showed a significant tendency to change from unstable bound state to stable bound state under amorphous iron-manganese binary oxides remediation.100%,35%,and 85.45%reduction in As,Cd,and Pb toxic leaching concentrations,respectively,with 21.62%and 52.5%higher reduction in As and Cd relative to crystalline iron-manganese binary oxides.Leaching experiments showed that amorphous iron-manganese binary oxides maintained a low heavy metal leaching rate even under acidic conditions.In the solutions with p H=7 and p H=2,the leaching of As was reduced by 82.5%and 81.5%,respectively,compared to crystalline iron-manganese binary oxides.The leaching of Cd was reduced by 98.4%and 26.23%,respectively.（3）The results of the characterization techniques demonstrate that amorphous iron-manganese binary oxides immobilizes As,Cd,and Pb by oxidation-adsorption,surface complexation,surface precipitation,and crystalline phase transition.As,Cd,and Pb are first enriched by electrostatic adsorption in large amounts;Mn（IV）oxidizes As（III）to As（V）;meanwhile As,Cd,and Pb replace H⁺or combine with surface hydroxyl groups to form inner sphere complexes;and is accompanied by the formation of Cd（OH）₂,Pb（OH）₂,Pb As₂O₆ precipitates were formed.The crystalline phase transition process is crucial for the immobilization of As,Cd,and Pb.During the transformation of amorphous iron-manganese binary oxides into well-crystallized magnetite hematite,a part of the unimmobilized free states As,Cd,and Pb enter into the magnetic hematite crystals through homocrystalline substitution;a part of the heavy metals immobilized on the surface of amorphous iron-manganese binary oxides are wrapped into the internal structure;a part of the immobilized As,Cd,and Pb desorb,and the re-generated oxygenated anions As（V）and hydrated cations Cd（OH）⁺and Pb（OH）⁺are combined with amorphous iron-manganese binary oxides surface hydroxyl groups to undergo surface precipitation or surface complexation reactions,providing reaction conditions for the formation of ternary complexes.37 Figureures,11 tables and 144 references...