Brine rejection and hydrate formation upon freezing of NaCl aqueous solutions

TL;DR

This study combines experimental and simulation methods to analyze the molecular processes of saltwater freezing, brine rejection, and hydrate formation, providing insights relevant for freeze desalination.

Contribution

It introduces a combined x-ray diffraction and molecular dynamics approach to quantify ice and brine fractions and hydrate formation in NaCl solutions during freezing.

Findings

Brine rejection quantified by a superposition model.

NaCl hydrates form alongside ice at 233 K.

Simulations estimate low salinity in formed ice.

Abstract

Studying the freezing of saltwater on a molecular level is of fundamental importance for improving freeze desalination techniques. Here, we investigate the freezing process of NaCl solutions using a combination of x-ray diffraction and molecular dynamics simulations (MD) for different salt-water concentrations, ranging from seawater conditions to saturation. A linear superposition model reproduces well the brine rejection due to hexagonal ice Ih formation and allows us to quantify the fraction of ice and brine. Furthermore, upon cooling at T = 233 K we observe the formation of NaCl$\cdot$2H$_2$O hydrates (hydrohalites), which coexist with ice Ih. MD simulations are utilized to model the formation of NaCl crystallites. From the simulations we estimate that the salinity of the newly produced ice is 0.5% mass percent (m/m) due to ion inclusions, which is within the salinity limits of fresh water. In addition, we show the effect of ions on the local ice structure using the tetrahedrality parameter and follow the crystalite formation by using the ion coordination parameter and cluster analysis.