Abstract:

Stable and metastable solid-liquid equilibria phenomena and complex salt-forming behaviors exist in complex salt-water systems. To realize the relationship between salt-forming behavior and liquid structure, the characteristics of ion association structures of SO₄^2- in Na⁺, Mg²⁺//SO₄^2-, Cl^-, H₂O system and its binary, ternary subsystems were studied by Raman spectroscopy, combined with a Gauss-Lorentz peak fitting program. The spectrum experimental results show that there were two ion association structures of SO₄^2- as non-associated SO₄^2- and SO₄^2- groups in the Na₂SO₄-H₂O system, while in the MgSO₄-H₂O, MgSO₄-MgCl₂-H₂O, and Na⁺, Mg²⁺//SO₄^2-, Cl^-, H₂O systems, there were also Mg²⁺-H₂O-SO₄^2- and Mg²⁺-OSO₃^2- structures. Non-associated SO₄^2- was the main structure in the ν₁-SO₄^2- band of binary (MgSO₄ Na₂SO₄) and ternary (MgSO₄-MgCl₂-H₂O) subsystems, and with varying SO₄^2- concentration, these four types of SO₄^2- ion association structure varied regularly. Likewise, the ion association structures of SO₄^2- in the Na⁺, Mg²⁺//SO₄^2-, Cl^-, H₂O system changed regularly during both the processes of NaCl crystallization and isothermal evaporation. This was evident when, during the process of decreasing NaCl concentration and increasing MgSO₄, the content of non-associated SO₄^2- decreased, the chance of Mg²⁺-H₂O-SO₄^2- and Mg²⁺-OSO₃^2- structure formation increased, and the SO₄^2- group structure appeared in the astrachanite region. More importantly, the adaptive changes in solution structure during the progress of evaporation can result in the appearance of metastable phenomena. Further linear analysis showed that the concentration and Jänecke index (J) value of SO₄^2- were positively related to the intensity and peak area of the ν₁-SO₄^2- band, and the concentration of Mg²⁺ affected the contents of four ion association structures in the ν₁-SO₄^2- band mainly.

Key words: SO₄^2-, Ion association structure, Raman spectroscopy, Solid-liquid phase equilibrium