Long-term leachability and stabilization of lead and cadmium co-contaminated soils using hydroxyapatite-modified...

Hydroxyapatite (HAP) modified drinking water treated sludge (DWTS) was developed for the remediation of lead (Pb) and cadmium (Cd) co-contaminated soils, thereby simultaneously achieving the dual objectives of solid waste management and contaminated site remediation. However, long-term leachability and stabilization of Pb and Cd co-contaminated soils under environmental exposure were not very clear. In this work, batch stabilization tests, freeze-thaw cycles, soil column leaching test, and wet-dry cycles were conducted. The results showed that HAP and DWTS mixed at a mass ratio of 2:1 (2H‑DWTS) and applied at 5 % (w/w) with 14 days of incubation stabilized the leaching concentrations of Pb and Cd at 0.05 mg/L and 0.10 mg/L, respectively. 2H‑DWTS stabilized Pb and Cd through dissolution-precipitation, cation exchange, and adsorption/complexation, resulting in the formation of insoluble phosphates, carbonates, and hydroxides. The 2H‑DWTS treated soil showed strong resistance to freeze-thaw‑induced deterioration, with Pb and Cd leaching concentrations increasing by only 0.09 mg/L and 0.03 mg/L, respectively, after 12 cycles. In the soil column leaching tests, 2H‑DWTS reduced the cumulative leaching of Pb and Cd by 28.7 % and 26.5 %, respectively, under CaCl₂ solution, and by 24.6 % and 47.3 %, respectively, under simulated acid rain. Wet-dry cycles combined with CaCl₂/simulated acid rain aging decreased the leaching concentrations in each cycle but led to an increase in the cumulative leaching concentrations. This study not only provides new insights into the management of DWTS but also lays a theoretical foundation for the application of H-DWTS composites in the remediation of Pb and Cd co-contaminated soils.