Effect of drying treatment on the leachability of metallic elements (Zn, As, Cd, and Pb) from amended mine soils during batch leaching experiments

Chemical amendments are frequently applied to immobilize toxic elements in contaminated soils. However, remobilization of elements in amended soils is poorly understood. The elution of metallic elements (Zn, As, Cd, and Pb) from mine soil amended with mine sludge (MS), steel slag (SS), and limestone (LS) was evaluated using batch leaching tests under continuous wetting and intermittent drying. The elements were effectively immobilized by the three amendments, as evidenced by the reductions in the labile fraction (10.6–78.8 %) of SEP (sequential extraction procedure) and cumulative mass (42.0–98.5 %) during 14 leaching runs. Drying events increased the leaching potential by 2.9-fold and 4.4-fold for the eluted mass (μg) and depletion rate (k, h⁻¹), respectively. The depletion rate of cationic elements (e.g., Zn, Cd, and Pb) from the amended samples (MSS, SSS, and LSS) correlated with the leachate pH (r² > 0.583), while the leaching of anionic element (e.g., As) from MSS and SSS correlated well with the leachate concentration of Fe (r² = 0.898). The findings indicate that drying events can substantially increase the remobilization of metallic elements from amended soils. Moreover, change in the leachate phase, such as the pH drop (up to 1.5 unit) and/or the Fe concentration rise (up to 62 %), can be an early sign of the increased remobilization potential of metallic elements in amendment-treated remediation sites.