Relationship between volumetric properties and mass transfer through NF membrane for saccharide/electrolyte systems

Recent studies have shown that unexpected performances can be obtained with NF membranes, when applied to the treatment of solutions containing significant amount of electrolyte. This study deals with the investigation of the mechanisms governing the mass transfer of neutral species through NF membranes with a focus on the role of ions. More precisely, it consists to determine the apparent molar volume of saccharides, which characterizes the hydration state of solutes in presence of electrolyte, and to assess the relationship between these parameters and those characterizing the mass transfer. The transfer of saccharides of increasing molecular weights (xylose, glucose and sucrose) through a NF membrane is studied in a diffusion regime. Different electrolytes (NaCl, Na2SO4, CaCl2, MgCl2) are chosen with respect to their hydration level. A specific procedure was developed to dissociate the flux variation due to the modifications of the solute properties and of the membrane material induced by the electrolyte. The results show that the mass transfer modification is mainly due to the influence of the electrolyte on the solute properties. Then, the mass transfer parameters are put in parallel with the apparent molar volume of saccharides measured in solutions of different ionic compositions.For a given electrolyte, Na2SO4, a good quantitative relationship is obtained regardless of saccharide nature, concerning the influence of the electrolyte concentration. This result confirms that the saccharide transfer increase can be due to its dehydration in presence of electrolyte. However, from these results, it is not possible to establish a clear relationship regardless of the electrolyte nature.

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► Influence of the ion composition on the mass transfer of neutral solute through NF.
► Determination of the apparent molar volume of saccharides in electrolyte.
► Characterization of the hydration state of saccharides in presence of electrolytes.
► Relationship between mass transfer increase and dehydration of neutral solutes.