Studies on the Characteristics and Behaviors of the Ion Association Structures of SO42- in Na+, Mg2+//SO42-, Cl-, H2O System by Raman Spectroscopy

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 SO42- in Na+, Mg2+ // SO42- 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 SO42- as non-associated SO42- and SO42- groups in the Na2SO4-H2O system, while in the MgSO4-H2O, MgSO4-MgCl2-H2O, and Na+, Mg2+//SO42-, Cl-, H2O systems, there were also Mg2+-H2O-SO42- and Mg2+-OSO32- structures. Non-associated SO42- was the main structure in the v(1)-SO42- band of binary (MgSO4 Na2SO4) and ternary (MgSO4-MgCl2-H2O) subsystems, and with varying SO42- concentration, these four types of SO42- ion association structure varied regularly. Likewise, the ion association structures of SO42- in the Na+, Mg2+//SO42-, 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 SO42- decreased, the chance of Mg2+-H2O-SO42- and Mg2+-OSO32- structure formation increased, and the SO42- 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 Janecke index (J) value of SO42- were positively related to the intensity and peak area of the v(1)-SO42- band, and the concentration of Mg2+ affected the contents of four ion association structures in the v(1)-SO42- band mainly.