Solute displacement in the aqueous phase of water–NaCl–organic ternary mixtures relevant to solvent-driven water treatment

Abstract

Twelve water miscible organic solvents (MOS): acetone, tetrahydrofuran, isopropanol, acetonitrile, dimethylsulfoxide, 1,4-dioxane, dimethylacetamide,N-methyl-2-pyrrolidone, trifluoroethanol, isopropylamine,dimethylformamide, and dimethyl ether (DME) were used to produce ternary mixtures of water–NaCl–MOS relevant to MOS-driven fractional precipitation. The aqueous-phase composition of the ternarymixture at liquid–liquid equilibrium and liquid–solid endpoint was established through quantitativenuclear magnetic resonance and mass balance. The results highlight the importance of considering thehydrated concentrations of salts and suggest that at high salt concentrations and low MOSconcentration, the salt concentration is governed by competition between the salt ions and MOSmolecules. Under these conditions a LS phase boundary is established, over which one mole of salt isreplaced by one mole of MOS (solute displacement). At higher MOS concentrations, MOS with higherwater affinity deviate from the one-to-one solute exchange but maintain a LS boundary witha homogenous liquid phase, while MOS with lower water affinity form a liquid–liquid phase boundary.DME is found to function effectively as an MOS for fractional precipitation, precipitating 97.7% of theCaSO4from a saturated solution, a challenging scalant. DME-driven water softening recycles the DMEwithin the system improving the atom-efficiency over existing seawater desalination pretreatments byavoiding chemical consumption.