Membrane-assisted crystallization: A molecular view of NaCl nucleation and growth

Membrane-assisted crystallization, aiming to induce supersaturation in a solution, has been successfully tested in the crystallization of ionic salts, low molecular organic acids, and proteins. Membrane crystallization is an emerging membrane process with the capability to simultaneously extract fresh water and valuable components from various streams. Successful application of crystallization for produced water treatment, seawater desalination, and salt recovery has been demonstrated. Recently, membrane crystallization has been developed to recover valuable minerals from highly concentrated solutions, since the recovery of high-quality minerals is expected to impact agriculture, pharmaceuticals, and household activities. In this work, molecular dynamics simulations were used to study the crystal nucleation and growth of sodium chloride in bulk and with hydrophobic polymer surfaces of polyvinylidene fluoride (PVDF) and polypropylene (PP) at a supersaturated concentration of salt. In parallel, membrane crystallization experiments were performed utilizing the same polymeric membranes in order to compare the experimental results with the computational ones. Moreover, the comparison in terms of nucleation time between the crystallization of sodium chloride (NaCl) using the traditional evaporation process and the membrane-assisted crystallization process was performed. Here, with an integrated experimental- computational approach, we demonstrate that the PVDF and PP membranes assist the crystal growth for NaCl, speeding up crystal nucleation in comparison to the bulk solution and leading to smaller and regularly structured face-centered cubic lattice NaCl crystals. This results in a mutual validation between theoretical data and experimental findings and provides the stimuli to investigate other mono and bivalent crystals with a new class of materials in advanced membrane separations.