Effect of chemical oxidants on the Pb-Cd-As co-contaminated soil solidified by waste-derived geopolymer: performance, mechanisms and environmental benefits.

Waste-derived geopolymers have garnered widespread attention for heavy metal immobilization in contaminated soils, but the simultaneous solidification of cationic heavy metals and As(III) remains challenging. As(III) exists as arsenic acid (H3AsO3) and/or anionic species (e.g., H2AsO3-), which exhibit low affinity for the negatively charged geopolymer matrix and are difficult to immobilize during hydration. Potassium permanganate (KMnO4), as a strong oxidant and arsenic-affinity reagent, is expected to significantly improve As(III) immobilization, yet its effects on the speciation, occurrence, and fate of As in geopolymer systems remain unclear. This study develops a novel, sustainable strategy by combining a waste-derived steel slag-based geopolymer (SSG) with KMnO4 and ferrous sulfate (FeSO4) for the effective treatment of Pb-Cd-As co-contaminated soil. After 28 days of curing, the composite system (20% SSG + 0.1% KMnO4 + 1% FeSO4) yields exceptionally low leaching concentrations of 12.15 μg/L for Pb, 14.96 μg/L for Cd, and 21.63 μg/L for As, all complying with stringent Chinese regulatory limits. Mechanistic investigations reveals that the geopolymer gel effectively immobilizes Pb and Cd through physical encapsulation, ion exchange, and chemical bonding within its aluminosilicate network. Critically, the chemical additives plays a vital synergistic role: KMnO4 oxidizes highly mobile and toxic As(III) to As(V), which is subsequently sequestered via adsorption and co-precipitation with iron (oxy)hydroxides formed from FeSO4 hydrolysis. Furthermore, this approach reduces the treatment cost by 31.19% and decreases carbon emissions compared to ordinary Portland cement. This work provides a cost-effective and environmentally benign "waste-treats-waste" paradigm for the remediation of complexly contaminated soils.