Reaction of CO2 with alkaline solid wastes to reduce contaminant mobility

The effects of carbon dioxide treatment on the pH and solubility of inorganic contaminants in alkaline fly ash and spent shale solid wastes were investigated at 2 levels of moisture, time and carbon dioxide pressure. Minerals were identified by X-ray diffraction, elements by inductively coupled plasma emission spectroscopy or atomic absorption spectrophotometry after acid digestion. Carbon dioxide treatment at 40 psi and 20 per cent moisture for 120 h precipitated calcite and lowered the pH from 12.5 to 8.8-9.7. Leachable concentrations of cadmium, lead, zinc, manganese, arsenic and selenium were greatly reduced without the need for high carbon dioxide pressures. Such treatment would prevent substantial leaching of pollutants from alkaline wastes into groundwater.