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.

The parameters of the DSPM combined with dielectric exclusion were fitted to permselectivity data of ternary ionic solutions. In the fitting procedure, the previous determination of the membrane permeability coefficient was useful to reduce the number of parameters to be searched by minimization of the fitting error. The fitted values of the structural parameters (rp and Δx/Ak ) could be considered independent of feed conditions, but not the membrane charge density parameters that were very influenced by pH and composition. The membrane charge density was the determining factor of the membrane performance. Highest rejection values corresponded to high values of the membrane charge parameter q that were determined by pH. The inclusion of the dielectric effect to the model allows describing the performance of a high rejection membrane, but only for the ternary − ionic system NO3 /SO42−/Na+ . It could be due to the strong difference in the anion properties. However, by changing sulphate ion for chloride ion in the ternary mixture the fitted parameters of the model were not able to describe the differences in chloride and nitrate rejection experimentally observed. On the other hand, it is verified that fitted parameters are only suitable to describe membrane performance for the feed characteristics from which were obtained, but failed to describe the rejection if feed composition varies. The − parameters obtained with the NO3 /SO42−/Na+ − system were not applicable to predict the NO3/ − + Cl /Na system. Developments in the modeling of membrane charge are necessary to express the membrane charge density in such way that the model can work in a wide range of concentration or systems with pH changes. Further developments of the model should also be made in order to improve the rejection predictions for specific ions such as chloride.