Molecular modeling of ice crystallization and salt rejection mechanisms in freeze desalination

Abstract

This work investigates the complex dynamics of ice crystal growth and salt entrapment in saltwater freeze desalination (FD). Classical molecular dynamics (MD) simulations have been developed to analyze the behavior of salt ions at the evolving ice-liquid interface during saline water freezing process. Valuable atomic-level insights have been gained into the ion rejection mechanisms by systematically exploring various salinity levels, ranging from 0 g/L (pure water) to 70 g/L (brine) and supercooling degrees of ΔT = -5 K to −25 K. Results reveal a significant limitation to ice formation in the presence of salt ions, with crystallization rates reducing by 40 % to 90 % for 35–70 g/L solutions compared to pure water at various supercooling temperatures. The Péclet number illustrates the interplay between ice growth rate and ion diffusion at the ice-liquid interface. At lower salinity levels (35 g/L) and higher supercooling (ΔT = -25 °C), the Péclet number (Pe = 1.1) indicates intensified salt ion entrapment within the growing ice lattice. Conversely, at higher salinity levels (70 g/L), the average effective diffusion coefficient of salt ions remains relatively stable, indicating a consistent diffusion process despite temperature variations. This stability in diffusion results in uniformly higher rejection rates for the 70 g/L solution compared to the 35 g/L solution, leading to a Péclet number of < 1 at various freezing temperatures. Results also revealed ion-specific entrapment behavior, with a higher likelihood of chloride entrapment in the ice phase. The evolution of salt concentration in the growing ice crystal was validated experimentally, showing a strong correlation and matching findings to MD simulations. These findings shed light on the interrelated mechanisms in saltwater crystallization. Understanding these details could lead to advancements in the FD process, thereby increasing its potential for industrialization in various fields.