Separation of Toxic Heavy Metals by Sulfide Precipitation

Sulfide precipitation with Na₂S is found to be highly effective to obtain a high degree of separation of heavy metal cations (Cd, Zn, Cu, and Pb) and of the oxyanions of arsenic and selenium from complex wastewaters. The metal separation characteristics are evaluated with a dilute synthetic mixture and with an actual copper smelting plant wastewater. The overall separation of arsenic and other heavy metals and precipitate settling rates are optimum at sulfide dosages about 60% of the theoretical values and at a final pH greater than 8.0. The removals of Cd, Zn, and Cu from the actual wastewaters are greater than 99%, and As and Se removals are 98 and >92%, respectively. Cd, Cu, and Zn concentrations in the range of 0.05 to 0.1 mg/1 can be achieved with sulfide precipitation. The metal separations and settling rates obtained with conventional hydroxide precipitation (lime) are considerably lower than those obtained with sulfide precipitation.