Prediction of the environmental behavior of residues from the passive treatment of acid mine drainage

Passive treatment systems are frequently used for acid mine drainage (AMD) remediation in closed and abandoned mine sites, but they have a limited lifespan and often need renewing. The generated metal-rich residues from these types of treatment show an inconsistent stability. To ensure their proper management, residues require a complete investigation of their potentially mobile and leachable contaminants, including metals and sulfates. To do so, three different post-treatment solids-WA50, PBR and C50-were collected from a laboratory Fe-AMD passive treatment system. Chemical and mineralogical characterization, followed by toxicity characteristic leaching procedure (TCLP) and kinetic tests, in weathering cells under wetting/drying cycles, were undertaken to assess the environmental behavior of the collected residues. Results showed that metals and sulfates occurred in the form of hydroxides, oxy-hydroxides, sulfate and native sulfur. They also showed that a possible generation of contaminated neutral drainage by Ni, Cu, Mn and Zn could occur from WA50 residues. Several discrepancies were found in the classification of the PBR and C50, since they were classified as "low risk" wastes but showed high release of other contaminants (Ba, Cu, Mn and Zn) that exceeded the regulatory limits during kinetic testing. To ensure the stability of the contaminants in the residues, any contact with water and oxygen must be avoided. Storage in a dry environment could be the appropriate option for their responsible management and prevention of a subsequent metal release.