Investigating the Effectiveness of Mineral Precipitate as a Tool in the Removal of Heavy Metals from Mine Waters

Abstract:

Mine water from a precious metal mine in Colorado drains into an underground tunnel and flows for about 8 km before being discharged into a series of sequentially connected settling ponds (5) aimed at removing suspended particulate. Our results suggest these ponds also remove heavy metals from solution through adsorption and mineral precipitation. Analyses of the precipitates and water in the settling ponds showed relatively higher metal concentration on the precipitates than in the corresponding aqueous solutions. Speciation modeling showed that the precipitates were mainly travertine, ferrihydrite, fe-oxyhdroxide and gypsum and these are expected to provide surfaces for metal adsorption. Overall, the average concentrations of trace metals were such that, Al concentration was 0.0 mg/L for the aqueous sample and 9.4 mg/L for the precipitate; Fe concentration was 0.04 mg/L for the aqueous sample and 20.1 mg/L for the precipitate; Mn concentration was 0.2 mg/L for the aqueous sample and 10.2 mg/L for the precipitate; Sr concentration was 3 mg/L for the aqueous sample and 8 mg/L for the precipitate; Zn concentration was 0.1 mg/L for the aqueous sample and 1.4 mg/L for the precipitate. Sulfate concentrations in solutions (1346 mg/L) were about seventeen times higher than on the precipitate (80 mg/L). As the water exits the tunnel, its carbon is expected to consistently decrease over space as it moves along the settling ponds while precipitating carbonates. The dissolved inorganic carbon (DIC) concentrations showed consistent drop from 109 mg/L at the tunnel exit to 96 mg/L at middle pond and 92 mg/L at the exit pond, which corresponds to decreasing pCO₂ and removal of carbon from solution through travertine precipitation and CO₂ outgassing. This data indicate a strong influence of mineral precipitate as an effective component in the attenuation of metals in mine