Selenium Leaching Kinetics and In situ Control

Selenium leached from coal tailings and spoil is a challenge for mining operations in southern West Virginia. Selenium discharges are not supposed to exceed 5 mu g/L, and yet are commonly in the range of 10-25 mu g/L. Once in the selenate form, selenium removal can be extremely difficult and expensive, particularly in the narrow valleys and highly variable flow regimes of southern West Virginia. This study reports on the first 96 weeks of a leaching study. Selenium leached at the rate of 0.06% of the extant selenium pool per day. After 96 weeks, about 35% of the original, potentially mobile selenium had leached. While sulfur was far more abundant, its leach rate was about 10% of the selenium rate. Iron oxyhydroxide was found to reduce the concentration of dissolved selenium by about 70%, which indicates that selenite is the dominant, mobile selenium species during initial weathering, and that selenium could be controlled at its source, through special handling and treatment of selenium-rich rock units. Iron oxyhydroxide kept selenium near the regulatory limit of 5 mu g/L throughout the experiment.