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 regulated at 5 μg/L and discharges are commonly in the range of 10 to 25 μg/L.  Once oxidized to selenate ion, removal is extremely difficult and expensive, particularly in the narrow valleys and highly variable flow regimes of southern West Virginia.  This study attempts to understand the potential to control selenium at source, through special handling and treatment of selenium rich rock units.  Earlier study has indicated that the bulk of selenium in a given overburden sequence is associated with organic shales, primarily located adjacent to the coal seams.  Rock units with greater than 1 mg/kg of total selenium are generally considered a strong risk for producing selenium leachate

Fresh samples were taken from a wash plant in southern West Virginia where chloride also leached from the tailings.  As a conservative ion, chloride concentrations from the tailings pile and the laboratory leachate were used to estimate a leaching factor from the field.  When applied to leachate concentrations, selenium concentrations from the control leaching cells fell within the range observed in the field.  Laboratory leaching results were compared to projected mobile selenium fractions derived from sequential extraction i.e. leachable, exchangeable and sulfide associated fractions.  

After an initial rapid leaching of the exchangeable fraction, selenium leaching rate stabilized at about 0.07%/day for 32 weeks after which it declined gradually.  After 60 weeks about 25% of the total selenium had leached.  Selenite is known to bind to ferrihydrite and a series of leaching cells included ferrihydrite.  It has kept [Se] at or below the regulatory limit of 5 μg/L throughout the experiment confirming selenite as the initial, mobile selenium species during weathering and ferrihydrite’s potential as a sorbent.