Mechanistic insights into fly ash-stabilized arsenic sulfide residues: Leaching suppression and stabilization pathways

Arsenic sulfide residue (ASR) is a solid waste that harms the environment, especially its inherent difficulty in handling sulfur (S). Here, a stabilization pathway based on fly ash (FA) is proposed by directional control and design of curing material proportion and conditions. The leaching behavior and the leaching suppression mechanism of ASR were studied systematically. The results demonstrate that FA-based cement materials effectively stabilize ASR. The leaching concentration of arsenic (As) decreased dramatically from 500 mg/L to 0.4 mg/L after 14 days of optimized curing, achieving a 99 % reduction in leaching toxicity. Response surface methodology (RSM) was employed to analyze the interactive effects of dual factors on solidification, and it was found that CaO, cement content, and liquid/solid (L/S) had strong interaction between the three factors, and L/S played a leading role. During the solidification process of FA-based materials, the initial network and scattered granular loose structure gradually changed to the network/fibrous and acicular dense structure. Moreover, the content of As (V) increased from 49.3 % (3 d) to 79.1 % (14 d), and the structure of metal As in ASR changed from S-As to S-Ca. The stabilization mechanism of arsenic involves four key path ways: calcium silicate hydrate (C-S-H) adsorption, calcium aluminosilicate hydrate (C-A-S-H) precipitation, ettringite incorporation, and physical encapsulation. Therefore, this study provides a stabilization pathway for the harmless treatment of hazardous material by treating it with waste.