Passive elimination of sulfate and metals from acid mine drainage using combined limestone and barium carbonate systems

Acid mine drainage contains high concentrations of sulfate and metals. Conventional neutralization systems available are capable to eliminating acidity and metals with varying success. However, they are not always able to consistently abate sulfate below the recommended value of 250 mg/L. We propose a system of two successive treatments using limestone and barium carbonate dispersed in a porous inert matrix of wood shavings. The system was tested at bench scale using two columns in series. With a residence time of 30 h, the first limestone column eliminated acidity, trivalent metals (aluminum and iron), arsenic, lead and copper as well as decreased sulfate from 7500 to 1800 mg/L. The metals were precipitated as oxyhydroxides and the sulfate as gypsum. The second column (barium carbonate) depleted the sulfate and divalent metals (zinc, manganese, nickel, cobalt, cadmium and thallium) concentrations below detection limits. Sulfate was precipitated as barite, and the metals were trapped inside calcite, both of which were stable under weathering conditions. Hardness was also depleted by 70%. The quality parameters of the outflowing water were within the required values for drinking water standards. The exception was the concentration of barium (up to 50 mg/L) measured at the initial stages of the treatment, which decreased over time to 3 mg/L. In a passive remediation system, the barium concentration was found to depend on the ratio of calcium and sulfate free ions of the inflowing water. This was difficult to estimate because it was based on the initial concentration of sulfate, calcite dissolution, gypsum precipitation and the aqueous chemical speciation.