The chemical attenuation of As by soils from abandoned mine sites was evaluated. Several soil samples, including As contaminated soil from the mine impacted areas, as well as As-free soils down-gradient from the mine sites, were collected across abandoned mine sites. Leaching and adsorption experiments were conducted under batch and 1-D water flow conditions. The cumulative As mass from 10 step sequential leaching experiments with six As contaminated soils, using 10mM CaCl2 solution, was less than 1% of the total As present in soils, indicating that As in contaminated soils is strongly adsorbed onto soil particles, which can serve as a long term potential As source. As adsorption by As-free soils was clearly nonlinear, with Freundlich N values (sorption nonlinearity) ranging from 0.56 to 0.87. Both the total As content in mine soils and the concentration-specific adsorption coefficient for arsine-free soils were best described by coupling the pH with various forms of Fe/Al oxides. In the breakthrough curves (BTCs) for As contaminated soils, an initial high concentration of As (called first-flush) was observed, and this flush export leveled off after the displacement of a few pore volumes. In the BTCs from layered soils, where clean down-gradient soils were overloaded above the mine soil, the appearance of measurable As was retarded, showing that the As attenuation by soils was effective in a flow water system. Also, the observed perturbation in the concentration of As during flow interruption supports that leaching/attenuation of As via flowing water occurs under nonequilibrium conditions. The results from both batch leaching/adsorption and column displacement experiments strongly suggested that the leaching of As from mine soils was rate limited and the risk of As leaching from soils can be mitigated by attenuation mechanisms, such as adsorption, provided by down-gradient clean soils.
ASJC Scopus subject areas
- Environmental Chemistry