| Heavy metal removals and particle size distributions were presented for both hydroxide precipitation and sulfide precipitation of zinc, cadmium, copper, and nickel in the presence of various complexing agents. Both batch and continuous experiments were performed for synthetic and actual metal-containing wastewaters. Metal concentrations complexing agent concentrations, sulfide dosages, pH levels of the solutions, and reaction or residence times were varied to determine the reaction kinetics.;Low kinetic orders were obtained for continuous precipitation of both metal sulfide and hydroxide precipitations. As a consequence, little effect on the particle size distributions were observed by varying detention time.;Sulfide precipitation always achieved lower residual metal concentrations than the corresponding hydroxide precipitation. The presence of strong complexing agents, such as EDTA, severely inhibited the removal of heavy metal by the precipitation process. Weak complexing agents, such as citrate and ammonia, exhibited little effect on the chemical equilibria for both hydroxide and sulfide precipitation systems due to their weak complexing ability with metal ions. Weak complexing agents did affect the particle size distribution through a nucleation/growth inhibition, generally forming smaller particles. |
