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Both alkaline heavy metal (HM) contaminated red mud (RM) recycling and Cr­(VI) polluted soil remediation are severe hazardous environmental problems. Herein, we developed a novel integrated approach to neutralize and stabilize the hazardous RM via the introduction of Ca2+ and Cl– dual ions. When 3% CaCl2·2H2O is added, the pH of RM is reduced from 11.0 to 8.9, and the Pb, Cr, Zn, Cu, and As leaching concentrations from RM are suppressed lower than the environmental standards of China and the World Health Organization (Pb, Cr, Zn, Cu < 0.50 mg/L; As < 10 μg/L). Detailed mechanism studies reveal that the pH neutralization is attributed to the novel OH– flocculation effect ascribing to Ca2+ charge neutralization and Cl– promoted proton neutralization. Furthermore, as the pH decreases, the HM adsorption capacity of RM is enhanced, achieving stable HM immobilization. The treated RM can be further employed to remediate Cr­(VI) contaminated soil (Cr concentration 570 ppm; treated RM weight percentage in soil 5.0%), which can suppress 56.0% Cr mobility in soil and 68.4% bioaccumulated Cr in the harvested vegetables. Overall, this work demonstrates an ecofriendly “waste-to-soil remediation” strategy to solve both the solid waste and soil pollution problems.

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