Nonequilibrium leaching behavior of metallic elements (Cu, Zn, As, Cd, and Pb) from soils collected from long-term abandoned mine sites

Leaching of metallic elements (Cu, Zn, As, Cd, and Pb) from two mine-impacted soils (DY and BS) was evaluated by batch decant-refill and seepage flow experiments. During eight consecutive leaching steps, aqueous As concentrations remained relatively constant (approx. 1.6 and 0.1mgL^-1 for DY and BS, respectively), while Cu (0.01-3.2mgL^-1), Zn (0.2-42mgL^-1), and Cd (0.004-0.3mgL^-1) were quickly reduced. The reduction of Pb concentration (0.007-0.02mgL^-1 and 0.2-0.9mgL^-1 for DY and BS, respectively) was much lesser. This pattern was well-explained by the biphasic leaching model by allocating a large fast leaching fraction (f_fast0.2) for Cu, Zn, and Cd while a negligible f_fast for As and Pb (<0.001). For all elements in column effluents, mass export through first-flush and steady-state concentration were elevated under slow seepage, with the greatest impact observed for As. Element export was enhanced after flow interruption, especially under fast seepage. A transient drop in As export in slow seepage was likely due to sorption back to soil phase during the quiescent period. The ratio of Fe²⁺/Fe³⁺ and SO42- concentration, related to the dissolution of sulfide minerals, were also seepage rate-dependent. The results of batch and column studies imply that the leachate concentration will be enhanced by initial seepage and will be perturbed after quiescent wetting period. The conversion from kinetically leachable pool to readily leachable pool is likely responsible for nonequilibrium metal leaching from the long-term abandoned mine soils.