Role of electrolytes on the transfer of organic matter through Ion-Exchange Membrane: computational and experimental approaches

Membrane processes are largely used for sea water desalination in order to avoid the problem of drinkable water scarcity due to the growing of the world population. Experimental studies dealing with electrodialysis (ED) or nanofiltration membrane systems have shown that the transfer of the organic matter can change according to the ionic composition. Such effects have an influence on the quality of the treated water. Handling with ion exchange membranes, it was pointed out that the conditioning with various electrolytes has a strong influence on the transport properties of the membrane. It was further shown to be related to the hydration properties of the membrane counter ions. For instance, it was observed that the fluxes of sugars, like xylose or glucose for instance, obtained in diffusion condition through a cation exchange membrane soaked in NaCl are more than twice those obtained in the same condition with the membrane soaked in MgCl2 [1]. It was then considered that the non-covalent interactions could be responsible for these experimental observations. A computational and experimental investigation was thus carried out to know how these interactions could be involved in the observed phenomenon. Fundamental knowledge-based ab-initio approaches devoid of fitting or adjustable parameters are important for an accurate description of the non-covalent interactions and then to understand the molecular scale mechanisms underlying the correlation between the ions properties and the organic matter flux [2][3]. Quantum Mechanics (QM-DFT), Molecular Mechanics (MM-CHARMM) and hybrid approaches (QM/MM) were thus used to investigate the non-covalent interactions involved in model systems, composed of different cations (Na+, Ca2+ and Mg2+), glucose as organic matter and a few polymer fragments Two phenomena were investigated concerning the influence of the electrolyte (cations) on the organic solute transfer. On one hand, it was considered that the electrolyte can affect the sugar-polymer interactions, and thus the sugar solubility inside the membrane. On the other hand, it was considered that the electrolyte can change the polymer-polymer interactions and then the organic solute transfer following the modifications of the properties of the hydrated polymer network. From this computational approach it was concluded that, in the conditions investigated, the change of the sugar flux is mainly driven by a modification of the interaction energy between the hydrated polymer chains depending on the electrolyte used to soak the membrane. Experimental characterization of the membrane samples, using Infra Red spectroscopy, was also carried out for comparison and a good agreement was found with the computational results.