Abstract
Supercritical water desalination (SCWD) shows great potential in the treatment of high-salt wastewater with zero liquid discharge. To investigate the salt precipitation behavior and mechanism in supercritical water, experiments and molecular dynamics simulations (MDs) were used to study the salting-out process of different salts in supercritical water. The equilibrium concentrations of NaCl, KCl, CaCl2, Na2SO4, and Na2CO3 in supercritical water were experimentally measured. When the temperature exceeded 693 K, the salt equilibrium concentration measured in the experiment was less than 130 mg/L. The solubility decreased in the order of KCl > NaCl > CaCl2 > Na2SO4 > Na2CO3. To elucidate the effects of different cations and anions in supercritical water on salt dissolution and precipitation behavior, the potential energy, radial distribution function (RDF) and coordination number in the system were obtained via molecular dynamics simulation. Experimental and MD results showed that salt solubility has significant positive correlation with systemic potential energy and hydration number. MD results indicated that a small ionic radius, large ionic charge, and low hydration coordination number are favorable for inorganic salts to precipitate and crystallize since these factors can strengthen the interaction between free ions and salt clusters. Moreover, due to the formation of multilayer coordination structure, polyatomic ions can achieve a lower equilibrium concentration than that of the corresponding monatomic ions.
Highlights
Due to the significant decrease in the number of hydrogen bonds and dielectric constant, the dissolving property of supercritical water (T > 647 K, P > 22.1 MPa) approaches a nonpolar solvent [1,2], which becomes immiscible with inorganic salts and leads to the precipitation of inorganic salts from water [3,4,5]
Through the combination of supercritical water desalination (SCWD) and supercritical water oxidation (SCWO) [2,3,4], it is expected that high-salt organic wastewater can be treated economically and efficiently
The aqueous solution of sodium chloride under ambient temperature and supercritical conditions was simulated by Koneshan and Rasaiah [13] through molecular dynamics simulations (MDs)
Summary
Due to the significant decrease in the number of hydrogen bonds and dielectric constant, the dissolving property of supercritical water (T > 647 K, P > 22.1 MPa) approaches a nonpolar solvent [1,2], which becomes immiscible with inorganic salts and leads to the precipitation of inorganic salts from water [3,4,5]. Zhang et al [10] summarized and analyzed the precipitation behavior and salting-out mechanism of inorganic salts in supercritical water through the phase equilibrium, dissolution, crystallization, and deposition of salt. Since current studies in the open literature are mainly focused on a single type of salt, a systematic comparison of different salts on their different solubilities in SCW with the related mechanism analysis in salt precipitation is not available To fill this gap, in this paper, NaCl, KCl, CaCl2, Na2SO4, and Na2CO3 were selected as model salts, and through a combination of experiment and molecular dynamic simulations, the influence of different anions and cations on the dissolution and precipitation behavior of salt in supercritical water is explored. Where R refers to the universal gas constant, T is the system temperature, Gsolv the Gibbs free energy, Hsolv the enthalpy of the solvent, and Ssolv the entropy of the solvent
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