Understanding the steric and charge contributions in NF membranes using increasing MWCO polyamide membranes

Various studies have shown the importance of steric and charge effects in NF membranes. Most of the studies, however, were done using NF membranes of different polymeric materials. In this study we have used three membranes made of polyamide with MWCO ranging from 200-2000 to investigate the significance of steric and charge effects upon the separation performance of NF membranes. The characterization of the membranes using uncharged solutes and salt solution shows that the membrane characteristics did not follow the expected pattern based on the increasing MWCO. The rejection performance of the membranes against various categories of salts/charged organic solute was then compared to the model prediction using DSPM. Membrane BM02D was found to have a combination of high steric and charge effects, which explain why the rejections of all solutes were in the range above 80% irrespective of concentrations. For BM20D and BM05D, the pattern of rejections was quite varied and inconsistent, which made it quite difficult to assess the relative contribution-of steric and charge effect between the two membranes.