Major Processes Dominating the Release of Aluminum from Smectite Clays When Leached with Acid Mine Drainage

Abstract

Elevated concentrations of Al in acid rock drainage (ARD) from a highway embankment in Centre County, PA, are reported to have ruined a former trout fishery stream. Long-term data (1999–2008) on the release of Al in this ARD site suggest a first stage (1999–2003) of Al release from the subsurface clays to surface seeps that follows first-order kinetics, followed by a slower second stage (2003–2008) that follows zero-order kinetics. Batch laboratory leaching of smectite clays with synthetic acid mine drainage (AMD) was consistent with the observed evolution of Al from this ARD field site. Initially, rapid release of Al from clays appeared to result from exchange reactions. This was followed by a slower release of Al that appeared to be dominated by longer term dissolution processes. PHREEQC simulations were consistent with these experimental results, suggesting that Fe2+ (in addition to H+), which is common in AMD/ARD, can enhance the release of Al from smectite clays at low pH. The outcome of this study assists in a better understanding of the occurrence of high concentrations of Al in AMD/ARD by pointing out the significance of ferrous iron in AMD/ARD on the release of Al from smectite clays.