Geochemistry and Mineralogy of Precipitates from Passive Treatment of Acid Mine Drainage: Implications for Future Management Strategies

Traditional mining activities in Zaruma-Portovelo (SE Ecuador) have led to high concentrations of pollutants in the Puyango River due to acid mine drainage (AMD) from abandoned waste. Dispersed alkaline substrate (DAS) passive treatment systems have shown efficacy in neutralizing acidity and retaining metals and sulfates in acidic waters, achieving near a 100% retention for Fe, Al, and Cu, over 70% for trace elements, and 25% for SO42−. However, significant solid residues are generated, requiring proper geochemical and mineralogical understanding for management. This study investigates the fractionation of elements in AMD precipitates. Results indicate that Fe3+ and Al3+ predominantly precipitate as low-crystallinity oxyhydroxysulfate minerals such as schwertmannite [Fe3+16(OHSO4)12–13O16·10–12H2O] and jarosite [KFe3+3(SO4)2(OH)6], which retain elements like As, Cr, Cu, Pb, and Zn through adsorption and co-precipitation processes. Sulfate removal occurs via salts like coquimbite [AlFe3(SO4)6(H2O)12·6H2O] and gypsum [CaSO4·2H2O]. Divalent metals are primarily removed through carbonate and bicarbonate phases, with minerals such as azurite [Cu(OH)2·2CuCO3], malachite [Cu2(CO3)(OH)2], rhodochrosite [MnCO3], and calcite [CaCO3]. Despite the effectiveness of DAS, leachates from the precipitates exceed regulatory thresholds for aquatic life protection, classifying them as hazardous and posing environmental risks. However, these residues offer opportunities for the recovery of valuable metals.