Thermodynamics equilibrium of M(Ⅱ)-S-H₂O system

Diagrams for the distribution of S(Ⅱ) species αn-pH in S(Ⅱ)-H₂O system, the concentration offour kinds of heavy metal ions(Cu²⁺、Pb²⁺、Zn²⁺、Cd²⁺) pM-pH in M(Ⅱ)-S-H₂O system are drawn, respectively.The various existing forms of S(Ⅱ) in S(Ⅱ)-H₂O system at different pH are illustrated in α_n-pH diagram.The concentration of S^2- occurs at pH>12.5. The dominant S(Ⅱ) species is S^2- at pH=16 with the percentage98.61 %. The pM-pH diagram indicates the relationship between the equilibrium concentration of total heavymetal ions and pH in M(Ⅱ)-S-H₂O system, when dissolution of metal sulfide is in equilibrium. The solubilityof CuS, PbS, ZnS, CdS is at the minimal level at pH 7.75, 8.87, 8.36, 9.83, respectively. Meanwhile, thethermodynamic results were verified by a series of experiments. The results show that the total Cu²⁺, Pb²⁺, Zn²⁺, Cd²⁺concentrations reach the minimum value when pH is 8.2, 8.7, 8.3, 10.3, respectively under a certainconcentration, which was largely in line with thermodynamic calculation. The thermodynamics study is ofgreater practical value and can offer theoretical foundation for treatment of heavy metal wastewater by sulfideprecipitation.