Passive multi-unit field-pilot for acid mine drainage remediation: Performance and environmental assessment of post-treatment solid waste

This study evaluated the performance of a passive multi-unit field-pilot operating for 16 months to treat acid mine drainage (AMD) from a coal mine in Colombia Andean Paramo. The multi-unit field-pilot involved a combination of a pre-treatment unit (550 L) filled with dispersed alkaline substrate (DAS), and six passive biochemical reactors (PBRs; 220 L) under two configurations: open (PBRs-A) and closed (PBRs-B) to the atmosphere. The AMD quality was 1200 ± 91 mg L⁻¹ Fe, 38.0 ± 1.3 mg L⁻¹ Mn, 8.5 ± 1.6 mg L⁻¹ Zn, and 3200 ± 183.8 mg L⁻¹ SO₄²⁻, at pH 2.8. The input and output effluents were monitored to establish AMD remediation. Physicochemical stability of the post-treatment solids, including metals (Fe²⁺, Zn²⁺, and Mn²⁺) and sulfates for environmental contamination from reactive mixture post-treatment, was also assessed. The passive multi-unit field-pilot achieved a total removal of 74% SO₄²⁻, 63% Fe²⁺, and 48% Mn²⁺ with the line of PBRs-A, and 91% SO₄²⁻, 80% Fe²⁺, and 66% Mn²⁺ with the line of PBRs-B, as well as 99% removal for Zn²⁺ without significant differences (p < 0.05) between the two lines. The study of the physicochemical stability of the post-treatment solids showed they can produce acidic leachates that could release large quantities of Fe and Mn, if they are disposed in oxidizing conditions; contact with water or any other leaching solutions must be avoided. Therefore, these post-treatment solids cannot be disposed of in a municipal landfill. The differences in configuration between PBRs, open or closed to the atmosphere, induced changes in the performance of the passive multi-unit field-pilot during AMD remediation.