Fractions and colloidal distribution of arsenic associated with iron oxide minerals in lead-zinc mine-contaminated soils: Comparison of tailings and smelter pollution.

The mining and smelting of lead-zinc (Pb-Zn) ores cause widespread As contamination. The fractions and colloidal distribution of As associated with Fe oxide minerals in Pb-Zn mine-contaminated soils have not been well understood. In this study, As fractions associated with Fe oxide minerals in Pb-Zn tailings- and smelter-contaminated soils were compared using sequential extraction techniques. Kinetic experiments were conducted to characterize the reactivity of Fe oxide minerals. The distribution of As and Fe oxide minerals in soil colloids were analyzed. The results show that in mining-contaminated soils (both tailings and smelter) the relatively active fraction (amorphous hydrous oxide-bound As, AsF3) has a strong relationship with easily reducible Fe (Feox1). In smelter-contaminated soils, relatively stable fractions (crystalline hydrous oxide-bound As, AsF4) were closely associated with reducible Fe (Feox2). Although the average proportions of specifically-bound As (AsF2) and AsF3 in contaminated soils were similar, high As release in tailings-contaminated soils was observed because of the high reactivity of Fe oxide minerals in those soils compared with that in smelter-contaminated soils. Some slightly polluted soils with high pH and TOC concentrations formed As-bearing colloidal suspensions. Especially in smelter-contaminated soils, many small-sized soil colloids could facilitate As migration with surface runoff or vertical transport.