Divergent Aging Mechanisms of Calcium Arsenic Residue under Dry-Wet and Freeze-Thaw Cycles: Toxic Metal Mobility, Multiscale Physicochemical Characterization, and Escalated Ecological Risks

This study investigates the long-term mobility and ecological risks of As, Zn, and Cd in calcium arsenic residue (CAR) under simulated dry-wet (DW) and freeze-thaw (FT) cycles. Accelerated aging experiments, combined with multiscale characterization (XRD, XPS, SEM, FTIR), revealed distinct transformation mechanisms. DW cycles promoted carbonate-driven dissolution, As(III) oxidation to As(V) (resulting in an 18.4% increase in As(V) as shown by XPS), and sulfide oxidation (with reductions of 47.7% in ZnS and 15.08% in CdS). These processes increased the acid-soluble metal fractions (F1: As by 11.3%, Zn by 6.0%, and Cd by 8.7%) and metal release rates (52.39% for As, 42.63% for Zn, and 68.55% for Cd under DW conditions). In contrast, FT cycles induced mechanical fracturing and ice-mediated stabilization, which limited ion migration, partially amorphized ZnO, and promoted the precipitation of Cd(OH)₂. Ecological risk assessments indicated rising risks, with integrated potential ecological risk indices (IPER) reaching 11,187.85 under DW conditions and 10,668.29 under FT conditions, with arsenic contributing over 80%. The Risk Assessment Code (RAC) reclassified all metals into moderate-risk categories (As: 11.9–19.7%, Zn: 9.4–15.2%, Cd: 12.1–18.6%). Weibull modeling (α = 6.98–10.98, R² > 0.96) described the nonlinear kinetics, showing that cadmium aged the fastest (λ: Cd > As > Zn), with delayed but persistent risks under FT conditions. These results underscore the importance of developing climate-resilient stabilization strategies. The integrated framework combining mineral evolution, kinetics, and risk forecasting offers significant insights for managing legacy CAR pollution under changing climate conditions.