Experimental and molecular dynamics study of salt precipitation from oily wastewater under supercritical water conditions

This study investigates the crystallization behavior of inorganic salts during supercritical water oxidation of oily wastewater. Through integrated visual observation and continuous-flow experiments under subcritical and supercritical conditions, combined with molecular dynamics simulations of ion hydration structures and crystallization mechanisms, we quantitatively analyzed the solubility reduction of NaCl, CaCl₂, and MgCl₂. Under conditions of 400 °C and 25 MPa, the solubilities decreased to 455 mg/L, 198 mg/L, and 132 mg/L, respectively—more than two orders of magnitude lower than those under ambient conditions. In single-salt systems, compared to Ca²⁺ and Na⁺, Mg²⁺ formed a tighter hydration shell and experienced stronger electrostatic shielding from water molecules, making it more difficult for Cl⁻ to associate with Mg²⁺ and reducing the probability of direct collision and aggregation. In multi-ion systems, however, Na⁺ and Ca²⁺ significantly enhanced Mg²⁺–Cl⁻ association, shortening the Mg–Cl distance from 0.43 nm to 0.23 nm and weakening the hydration shell of Mg²⁺, thereby promoting MgCl₂ precipitation. These results elucidate the competitive effects between ion hydration and association in multi-salt systems and provide mechanistic insights for controlling salt deposition in supercritical water processes.