The removal of heavy metals from refinery effluents by sulfide precipitation

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%.