Fundamentals of freeze desalination: Critical review of ion inclusion and rejection studies from molecular dynamics perspective

As water needs and shortages increase, so has focus on freeze desalination as a potential solution to lower cost and more efficient desalination systems. At the molecular level, freeze desalination is caused by the higher solubility of ions in water than in ice - a phenomenon known as the ion rejection. This work reviews the published works and findings since 2004 on ion rejection and ion inclusion via Molecular Dynamics (MD) simulations. It begins with a brief historical background on the ion rejection phenomenon and MD simulations surrounding water and ionic solutions. The fundamentals of MD theory are reviewed and MD simulation practices related to ion rejection are explained. Finally, the current findings on ion inclusion and ion rejection - both the mechanisms and kinetics - are compared and contrasted among various studies. It is hypothesized that the ion rejection phenomenon is caused by dramatic structural rearrangements in the molecular structure around the ions and localized dislocations in the ice lattice that cause energetic differences between phases. To maximize ion rejection for freeze desalination systems, the current data supports temperatures closer to the melting point of the saltwater solution. However, there is not yet an optimized temperature available. Furthermore, further research is required to determine the best water-ion molecular model combination to simulate ion rejection. When compared with the experimental results of large-scale tests, MD simulations overestimate the rate of ion rejection at a given temperature. This is most likely due the inability of MD simulations to account for brine pockets that form due to dendritic ice growth. The aim of this critical review is to serve as an introduction to MD simulations and their use in understanding the fundamental phenomena governing freeze desalination and will facilitate increased interest in use of MD simulations in future freeze desalination research.