Applicability of the DSPM with dielectric exclusion to a high rejection nanofiltration membrane in the separation of nitrate solutions

This work discusses the applicability of the DSPM with dielectric exclusion (DSPM-DE) to describe nitrate rejection of a high rejection nanofiltration membrane (NF90 from Dow-Filmtec). In particular, the limitations of the application of the model to relatively high feed concentration and different pH are discussed.The model parameters were obtained through direct fitting to the experimental results, rather than performing separate experiments for each one. The experimental data were flux and rejection results for two ionic systems: solutions containing different ratios of sulphate to nitrate and solutions containing different ratios of chloride to nitrate. Both ionic systems were studied at two concentration levels and two pH values. Once the model parameters were obtained for every experimental set, it was observed that the values of the parameters related with membrane charge density significantly differed. The model showed better agreement with experimental rejections for solutions containing sulphate ion than those containing chloride ion. The results confirm that developments must be done to include the dependence of the parameters on pH and composition for extending the model to multi-ionic solutions.