Caption title. At head of title: Project summary. "Mar. 1984." "EPA/600-S2-84-023."

"Sulfide precipitation behavior of heavy metal ions in the presence of chelating agents was evaluated in terms of type of metal ion, chelating agent type and concentration, pH, sulfide dosage and reaction time. Theoretical solubilities of metal sulfides were calculated for the various metal-sulfide-chelating agent systems and compared with the experimental values. Experiments with Zn and Cd showed that Ksp's of ZnS and CdS of fresh precipitates were one or two log cycles higher than the aged precipitate Ksp values. With metal sulfides of higher solubilities (such as, zinc and nickel), high concentrations (greater than 10 mg/l) of strong chelating agent (such as EDTA) substantially reduced the extent of sulfide precipitation. For sparingly soluble metal sulfides (such as, copper and cadmium), the effect of the chelating agent was insignificant; residual concentration of less 1mg/l was obtained even with the strong chelating agent at a pH 4. The presence of a high concentration of citrate caused partial dissolution of copper sulfide. NiS precipitation was effective only for short reaction times (less than 5 minutes) or in the absence of oxygen. CdS precipitation was effective even in the presence of a strong chelating agent, but the removals of cadmium were interfered with in the Ni-containing-systems. Continuous experiments were also conducted to evaluate the effectiveness of the settling operation and to establish selective metal sulfide separation."