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.