Speciation and Structural Properties of Hydrothermal Solutions of Sodium and Potassium Sulfate Studied by Molecular Dynamics Simulations

Abstract

Aqueous solutions of salts at elevated pressures and temperatures play a key role in geochemical processes and in applications of supercritical water in waste and biomass treatment, for which salt management is crucial for performance. A major question in predicting salt behavior in such processes is how different salts affect the phase equilibria. Herein, molecular dynamics (MD) simulations are used to investigate molecular-scale structures of solutions of sodium and/or potassium sulfate, which show contrasting macroscopic behavior. Solutions of Na-SO4 exhibit a tendency towards forming large ionic clusters with increasing temperature, whereas solutions of K-SO4 show significantly less clustering under equivalent conditions. In mixed systems (Nax K2-x SO4 ), cluster formation is dramatically reduced with decreasing Na/(K+Na) ratio; this indicates a structure-breaking role of K. MD results allow these phenomena to be related to the characteristics of electrostatic interactions between K(+) and SO4 (2-) , compared with the analogous Na(+) -SO4 (2-) interactions. The results suggest a mechanism underlying the experimentally observed increasing solubility in ternary mixtures of solutions of Na-K-SO4 . Specifically, the propensity of sodium to associate with sulfate, versus that of potassium to break up the sodium-sulfate clusters, may affect the contrasting behavior of these salts. Thus, mutual salting-in in ternary hydrothermal solutions of Na-K-SO4 reflects the opposing, but complementary, natures of Na-SO4 versus K-SO4 interactions. The results also provide clues towards the reported liquid immiscibility in this ternary system.

Molecular Dynamics Study on the Nucleation Characteristics of Mixed Na2SO4/K2SO4 Solution in Supercritical Water

“…More SO 4 2– accumulated around Na + , which led to an increase in the coordination number. Reimer et al conducted molecular dynamics simulations of mixed Na 2 SO 4 /K 2 SO 4 solutions near the critical point at temperatures ranging from 25 to 320 °C. The results showed that as the temperature increased, Na 2 SO 4 solutions tended to form large ionic clusters, while K 2 SO 4 solutions formed significantly fewer clusters under the same conditions.…”

“…. Reimer et al 37 used the RDF of cation-S as the basis for analyzing the interaction between cations and sulfate ions. In order to ensure a better fit between the simulation and actual situation, cations and tetrahedral SO 4 2− were selected for calculation in this work.…”

Salts in Hot Water: Developing a Scientific Basis for Supercritical Desalination and Strategic Metal Recovery

“…For instance, the presence of other salts may either enhance or suppress cluster formation via "salting-out" or "salting-in" effects. 25 This work will build on our understanding of clustering in aqueous NaCl solutions 23 (see Figure 1). Finally, we will explore more challenging counter-ions, e.g., common naturally-occurring carbonate and sulfate ligands.…”