Oxidative stabilization of arsenic sulfide sludge via As-Ca-S solid solutions: From mechanisms to field demonstration

Arsenic sulfide sludge (ASS), a hazardous by-product of non-ferrous smelting, poses serious environmental risks due to its high arsenic leaching toxicity. Herein, an oxidative stabilization method employing H₂O₂ as the oxidant and CaO as the stabilizer was developed for the efficient disposal of ASS. The raw sludge, primarily composed of amorphous As₂S₃ and As₂O₃, contained arsenic predominantly as highly toxic and mobile As(III), with an initial arsenic leaching concentration of 1311.5 mg/L at pH 1.07. Under optimized conditions (H₂O₂ liquid-to-solid ratio of 1:1, Ca/As molar ratio of 3.67:1, stirring speed of 800 rpm, and temperature of 40 ℃), the treated product achieved an arsenic leaching concentration as low as 0.47 mg/L, corresponding to a 99.96% reduction and well below the regulatory limit of 1.2 mg/L (GB18598-2019). Mechanistic analysis revealed that H₂O₂ efficiently oxidized As(III) to As(V), with arsenate accounting for 97.46% of total arsenic in the stabilized product. The dominant solid phases were identified as low-solubility calcium arsenates, including Ca₅(AsO₄)₃OH and Ca₄(AsO₄)₂(OH)₂·4 H₂O. Meanwhile, sulfur species were oxidized and incorporated into As-Ca-S solid solutions via coprecipitation, forming CaSO₄·2 H₂O. An industrial-scale facility with a treatment capacity of 2750 t/a was successfully implemented, with 33 out of 43 consecutive batches meeting the regulatory arsenic leaching limit of 1.2 mg/L. Overall, this study provides an effective and scalable strategy for arsenic immobilization, offering strong potential for the safe disposal of hazardous arsenic-bearing sulfide wastes.