How do ions enhance the transfer during nanofiltration of saccharides? Experimental assessment of the dehydration assumption

Abstract

AbstractThe aim of the work was to improve the understanding of the role of ion hydration on the transfer during nanofiltration of saccharides. More precisely, it consisted in evaluating the physical relevance of the dehydration assumption to explain the increased transfer of saccharides in presence of electrolytes. Experiments were carried out using saccharides (xylose, glucose) and electrolytes containing ions of different hydration levels (NaCl, Na2SO4). The Filmtec NF membrane (previously referred to as NF45) was used. Unlike saccharide/NaCl solutions, Na2SO4 had a weak influence on the saccharide transfer. This means that the impact of the electrolyte on the transfer of saccharide was strongly affected by the ions retention. Indeed, SO42‐ ions were much more often retained than Cl‐. Thus, an enhanced saccharide transfer was expected as the electrolyte concentration in the permeate was high. The physical relevance of the assumed dehydration was evaluated. The decrease of the saccharide radius was determined from the mass transfer modelling. It was shown that the decrease was physically relevant with the dehydration phenomenon since this decrease corresponded to a fraction of the water molecule (2–3 %). Next, the increased transfer was compared with the variation of the apparent molar volume, characterizing the hydration state of solutes, to better understand the mechanisms involved. For a given electrolyte, the transfer increased continuously with saccharide dehydration. This result could confirm that the increase of the transfer could be attributed to the saccharide dehydration caused by the electrolyte.