Sulphide precipitation of heavy metal containing wastewaters results in low effluent concentrations. However, sulphide precipitation is not widely applied in practice because the dosing of sulphide cannot adequately be controlled. A new process was developed where the combination of a sulphide-selective electrode (pS-electrode) and pH electrode controls the sulphide addition. Precipitation experiments were performed on a laboratory-scale in batch and continuous reactor systems with synthetic wastewaters containing Cd, Cu, Ni, Pb or Zn. 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 sulphide. The metals were removed to levels <0.05 mg.l-1 at pH 6.0 by sulphide precipitation while maintaining a total sulphide concentration <0.02 mg.l-1. By the control of pS at different levels and keeping the pH constant at 6.0, the metals in solutions of both Cu and Zn could be precipitated selectively from solution in a batch and continuous reactors. The use of a membrane reactor in combination with control of pH and pS offers a simple, cheap, efficient and sustainable technique with respect to obtaining very low effluent concentrations and selective removal of metals as pure metal sulphides which can be reused.

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