The removal of heavy metals from refinery effluents by sulfide precipitation

This study investigated the selective precipitation of copper (Cu²⁺), nickel (Ni²⁺) and zinc (Zn²⁺) ions from industrial wastewater by using sodium sulfide (Na₂S). The effects of pH, time and free sulfide concentration on the selective precipitation were systematically investigated using synthetically prepared solutions and gold refinery wastewater. First, the optimum precipitation pH values of metals, the amount of Na₂S and time were determined by using synthetic solutions containing Cu, Ni and Zn single metal ions. The highest precipitation efficiencies were obtained at a pH of 4, 5.3 and 5.3 for Cu, Ni and Zn, respectively. Based on these results, the effects of Na₂S stoichiometry and time of precipitation were investigated with synthetically prepared solutions containing mixed metal ions (100 mg/L Cu²⁺, 90 mg/L Ni²⁺ and 100 mg/L Zn²⁺). Metal sulfide precipitation efficiencies increased with increasing amounts of Na₂S and reached a maximum at a ratio of 1:1.75 (Metal ion: Na₂S molar ratio). Lastly, the results obtained from the experiments performed with synthetic solutions were used for the metal sulfide precipitation from industrial wastewater. The precipitation from refining wastewater containing 10 g/L Cu²⁺, 0.3 g/L Ni²⁺ and 3 g/L Zn²⁺ ions was achieved at pH values from 2.3 to 5.3. At a pH of 2.3, Cu²⁺ precipitated as CuS (Cu₂S) at 100% efficiency. While the Ni²⁺ concentration in the solution decreased by only 44% at a pH of 2.3, the Zn²⁺ concentration decreased by 52%. At a pH of 5.3, Cu²⁺ and Zn²⁺ precipitated together at 100%.