The effect of NaCl and glucose concentration on retentions for nanofiltration membranes processing concentrated solutions

•Glucose retention for NF membranes decreases when NaCl is added to the solution.•Above 100 g L−1 increasing NaCl concentration has minor effect on glucose retention.•Changing glucose concentration does not affect NaCl retention at the same flux.•Changes in NF pore over solute radius explain changes in observed retention.•Tight NF membranes show negative NaCl retention at high glucose concentration.

Most nanofiltration studies deal with solutions containing less than 60 g L−1 salts and low sugar concentrations. For concentrated solutions limited information is available about interactions between neutral and charged solutes during nanofiltration, and their effect on flux and retention. This study reports results for nanofiltration of solutions containing NaCl in concentrations between 1 g L−1 and 300 g L−1 and glucose concentrations as high as 80 g L−1. Five membranes were characterized and evaluated. For mixed glucose/NaCl solutions with NaCl concentrations higher than 90 g L−1, a glucose concentration increase from 1 g L−1 to 80 g L−1 only affected glucose retention at the same flux for NTR 7450. For the tightest membranes slightly negative NaCl retentions were obtained for high glucose concentrations in the feed. Glucose retention for solutions containing 1 g L−1 glucose decreased strongly when the NaCl concentration was increased from 0 g L−1 to 100 g L−1. The observed glucose retention reductions were not caused by flux changes. A change in the ratio of glucose radius over membrane effective pore radius could explain the obtained results. Determined pore radii and effective thickness can be used for development of nanofiltration applications for desalination of concentrated glucose solutions.

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