Long-term effectiveness of different amendments on Pb/Cd co-contaminated soil under accelerated ageing

Long-term effectiveness is the primary challenge for the immobilization of heavy metals in contaminated soils. This study developed an accelerated aging method based on proton flux conservation to simulate long-term acid rain exposure. This method was used to assess the longevity of biochar (BC), lime (CaO), and magnesia (MgO) in remediating a soil severely co-contaminated by Pb and Cd. Biochar proved ineffective for immobilizing heavy metals in both short and long term. While both CaO and MgO were effective in the short term, MgO demonstrated superior performance, reducing the initial Toxicity Characteristic Leaching Procedure (TCLP) leaching concentrations of Pb from 84.62 to 0.02 mg/L and Cd from 9.91 to 0.01 mg/L. After 100 years of simulated aging, CaO showed poor durability, as leaching risk increased by 577 % (Pb) and 483 % (Cd). Conversely, MgO maintained its high efficacy with these concentrations remaining low at 0.07 mg/L (Pb) and 0.03 mg/L (Cd). The sustained efficacy of MgO is attributed to its hydration into brucite, which provides high acid resistance due to its low solubility, and the formation of a passivation shell that prevents further carbonation, thereby maintaining a stable, high-pH environment favorable for heavy metal precipitation. This study thus identifies MgO as a highly effective and durable amendment for long-term remediation.