Membrane-Assisted Crystallization: A Molecular View of NaCl Nucleation and Growth

Abstract

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.

Characteristics of soil salt crust formed by mixing calcium chloride with sodium sulfate and the possibility of inhibiting wind-sand flow

“…During water evaporation, ions migrate from the pores of sand to the evaporation front, and the salt concentration on the evaporation surface increases continuously. When the salt concentration reaches supersaturation, the salt crystals precipitate 23 , and the precipitated salt crystals bond with aeolian soil to form a salt crust.…”

“…The compressive strength of the salt crust can indicate the wind erosion resistance of soil to a certain extent 23 . The higher the compressive strength is, the stronger the wind erosion resistance 24 .…”

Influence of Polyvinylidene Fluoride Polymorphs in Membrane-Assisted Crystallization: NaCl Crystals Nucleation and Growth

“…In particular, the aim of the work was to compare the behaviour of crystallization process of NaCl solution approaching the α and β PVDF and the amorphous phase. We have compared the results obtained in this work with that of crystallization of NaCl simulated in bulk solution and with amorphous PVDF [12,14] and validated the theoretical data with the experimental ones available. In this paper, we show the prepared new amorphous models, assuming that the membranes were without any polymorphism.…”

“…The effects can be due to both structural and chemical properties of membrane surface that might operate as a physical substrate for heterogeneous nucleation, by inducing a reduction in the free energy barrier. [11] For example, PVDF membranes reduce induction time of crystals compared to conventional crystallization without the presence of a membrane as a physical barrier supporting the vapor-liquid interface; [12] moreover, the addition of nanoparticles in membranes has an amplifying effect that reduces the nucleation rate of crystals more and more. For instance, the addition of graphene platelets in PVDF membranes induced faster supersaturation and faster nucleation when compared to the pristine PVDF.…”

“…[13,14] To our knowledge, only the influence of a hydrophobic polymeric membrane of polypropylene (PP) and PVDF was observed computationally to reduce nucleation times in comparison with NaCl crystallization in simple bulk solution. [12] No study is available if polymer matrices have polymorphs.…”