The effect of water exchange on the leaching of alum shale

In recent years, there has been an increasing focus on the adverse environmental effects from naturally occurring radioactive materials (NORM). Acid rock drainage (ARD) produced by debris from mining and construction work is a major environmental issue, leading to release of highly acidic water as well as both NORM and stable trace elements. Batch leaching experiments with alum shale demonstrated that exchange of water will increase leaching of elements whose mobility is limited by concentration effects, such as Ba and the extremely radiotoxic, naturally occurring uranium daughter Ra-226. Periods of drying the alum shale in air increased leaching of Li, V, Mo and Ra-226, increasing their mobility in the environment. Acid production from sulphide oxidation did not cause pH values below 6.4 in the 28 weeks experiment. However, exchange of water did lead to reduction of inherent buffer capacity of the alum shale, which increases the risk of ARD as well as likely reducing the time before onset of ARD.