Sulfide precipitation is superior to hydroxide precipitation for removal of heavy metals from wastewaters as it results in lower effluent concentrations and less interference from chelating agents. However, sulfide precipitation is not widely applied in practice because the dosing of sulfide cannot adequately be controlled, and excess sulfide in the effluent is toxic and corrosive. A new process was developed where a sulfide-selective electrode (pS-electrode) controls the sulfide addition. Experiments were performed on lab-scale in batch and continuous systems for synthetic wastewater containing Cd, Cu, Ni, Ph, and Zn. The heavy metals were removed to ppb levels (<0.05 mg l(-1)) at pH 6.0 by sulfide precipitation while maintaining the total sulfide concentration <0.02 mg l(-1) The response of the pS-electrode during precipitation was unique, for each. heavy metal and was directly related to the solubility product of the corresponding metal sulfide. By control of the pS at different levels, the metals in mixtures of Cu-Zn and Pb-Zn were selectively precipitated from solution. This resulted in the production of pure metal sulfide sludges that possibly can be reused. Formation of colloidal sulfide precipitates was prevented by applying a membrane to keep the solids in the reactor to offer high surface area for growth and by controlling pS at optimal levels. The precipitate was separated from the water phase by a microfiltration membrane at a flux of 10001 m(-2) h(-1) bar(-1) at a total solids content of 30 g l(-1). Control of sulfide precipitation by the pS-electrode results in very low metal effluent concentrations, and the selectivity of the precipitation results in pure metal sulfide sludges, which can be reused.
|Journal||Separation Science and Technology|
|Publication status||Published - 2003|
- crystal precipitation