Environmental behavior of metal-rich residues from the passive treatment of acid mine drainage.

In closed or abandoned mine sites, passive systems are often used for acid mine drainage (AMD) treatment. They generate metal-rich residues with variable chemical stability, which is rarely reported. The objective of the present study was to evaluate the potential mobility of contaminants (metals and sulfates) from AMD post-treatment residues to better anticipate their fate and enable their proper management. Sampling of a field tri-step passive system, consisting of two passive biochemical reactors (PBR1 and PBR2), separated by a wood ash reactor (WA), implemented in the reclaimed Lorraine mine site, QC, Canada, was carried out. Samples were collected from the inlet (In) and the outlet (Out) of each treatment unit. Physicochemical and mineralogical characterization was performed. The potential mobility of the metals was then assessed via static and kinetic leaching tests. Results showed that all residues had high metal contents (e.g. Fe content >29 g/kg in PBR1-In, > 76 g/kg in WA-In and > 80 g/kg in PBR2-Out). A high residual neutralizing potential was also found in the WA residues (inorganic carbon 6.5%). Native and organic sulfur were found in the PBR2 residues, while Fe-oxyhydroxide (hematite, goethite and magnetite), carbonate and sulfate minerals were present in all residues. According to USEPA regulations, all residues were considered non-hazardous, but Quebec's provincial regulation relative on mining effluents classifies these residues as leachable for some metals, such as Fe, Al, Ni, Zn and Mn. A potential generation of contaminated neutral mine drainage (Al, Ni, Mn and Zn concentrations exceeding criteria) could occur from PBR1 (In & Out) and WA (In & Out) residues. Moreover, the PBR2 residues (In & Out) regenerated AMD rich in Fe and sulfates, especially for PBR2-Out (1 g/L Fe and 6 g/L sulfates). Therefore, all residues were proven to require stabilization prior to their landfill (co-)disposal with municipal waste.