Freezing/thawing effects on geochemical behavior of residues from acid mine drainage passive treatment systems

Abstract

Passive systems are promising for the treatment of acid mine drainage (AMD) but generate metal-rich solids or residues. In subarctic climate, freezing/thawing (F/T) cycles may influence the chemical stability of waste, including residues from AMD passive treatment. The objective of this study was to assess and better understand the effect of F/T cycles on contaminants (metal and sulfates) leaching from AMD post-treatment residues. To do so, samples of residues WA50 (50 % wood ash and 50 % wood chips), PBR (30 % inorganic materials and 70 % organic substrate) and C50 (50 % calcite and 50 % wood chips) were collected from a laboratory tri-unit passive treatment system for iron-rich AMD (Fe-AMD) operated for over 1-year. To better understand the effect of freezing upon contaminant leaching, kinetic tests in weathering cells under wetting/drying (W/D) and F/T cycles were performed. Results showed that F/T cycles are likely to enhance the potential of metal leaching from post-treatment residues. The Fe leaching was 1.6-(for WA50), 3.2-(for C50) and 5.2-(for PBR) times higher under F/T cycles relative to the W/D cycles. Similarly, Zn leaching was 2.4-(for WA50), 4.9-(for PBR) and 5-(for C50) times higher under F/T cycles compared to the W/D cycles. Finally, Mn leaching was 2.4-(for WA50) and 5.2-(for PBR) times higher under F/T cycles relative to the W/D cycles but no difference in Mn concentration was observed for C50 residues. These findings suggest that during snow melt or early spring significant metal concentrations could be released while disposing AMD passive treatment residues.