Removal and co-transport of Zn, As(V), and Cd during leachate seepage through downgradient mine soils: A batch sorption and column study

Abstract

The removal of Zn, As(V), and Cd during the leachate seepage process was measured in single, binary, and ternary solute systems by batch sorption and 1-D column flow experiments, followed by a sequential extraction procedure (SEP). In single-solute systems, sorption (Kd⁎) occurred in the order of As(V)>Zn≫Cd, and this sequence did not change in the presence of other solutes. In multi-solute systems, the sorption of Zn (~20%) and Cd (~27%) was enhanced by As(V), while Zn and Cd suppressed the sorption of each other. In all cases, As(V) sorption was not affected by the cations, indicating that As(V) is prioritized by sorption sites to a much greater degree than Zn and Cd. Element retention by column soils was strongly correlated (r2=0.77) with Kd⁎. Across column segments, mass retention was in the order of inlet (36–54%)>middle (26–35%)>outlet (20–31%), except for Cd in the Zn–Cd binary system. The result of SEP revealed that most of the retained Cd (98–99%) and Zn (56–71%) was in the labile fraction (e.g., the sum of F1 and F2) while only 9–12% of As(V) was labile and most (>55%) was specifically adsorbed to Fe/Al oxides. Plots of the labile fraction (flabile) and the fast sorption fraction (ffast) suggested that the kinetics of specific As(V) sorption occur rapidly (ffast>flabile), whereas labile Zn and Cd sorption occurs slowly (flabile>ffast), indicating the occurrence of kinetically limited labile sorption sites, probably due to Zn–Cd competition. In conclusion, the element leaching potential of mine leachate can be greatly attenuated during downgradient soil seepage. However, when assessing the soil attenuation process, the impact of sorption competitors and the lability of adsorbed elements should first be considered.