Soil attenuation of the leaching potential of mine-related metallic elements (Zn, As, and Cd) under different leachate solute compositions

Abstract

Leachate from abandoned mine is frequently enriched with toxic elements but their off-site movement is not well addressed. In this study, the attenuation potential of mine-related metallic elements (Zn, As, and Cd) through downward soil was investigated using batch equilibrium sorption and seepage column studies under simulated leachate composition (single, binary, and ternary solutes in 5-mM CaSO4). In the batch result, the retention of Cd was suppressed by 40-45% in the presence of Zn while the Zn was less affected by Cd. The retention of As increased by 14-25% in the presence of both cations, with a greater effect from Zn. The phenomena were explained by the combined effects of sorption selectivity, the relative element abundance, and the operating sorption mechanism (nonspecific vs. specific). These effects also influenced the effluent element concentrations in the seepage study, as numerically indicated by a two-site model fit and moment analysis (e.g., the peak arrival time and peak concentration). For 500 PV seepage, element retention by the column (M-retention) was strongly correlated (r(2) = 0.907) with the sorption constant (K-d*) during the sorption-dominant stage, but the same correlation was poor (r(2) = 0.346) during the depletion-dominant stage, due to the desorption resistance of As compared to Zn and Cd. Therefore, the attenuation of the leaching potential by surrounding soils and the effect of cosolutes dissolved in the leachate phase must be concurrently understood when assessing the off-site leaching of metallic elements from abandoned mine sites.