Effects of polyether-polyamide block copolymer coating on multilayer thin film composite membranes for nanofiltration applications

The lack of potable water poses a great challenge as population growth and urbanization continues. Nanofiltration (NF) as membrane technology shows promise in cutting freshwater demands by treating wastewater and pretreating reverse osmosis feed. Despite the advantages, there is a lack of understanding of how NF membrane operate. This project studied the effects of coating a water-permeable polyether-polyamide block copolymer (Pebax 1657) on three commercially available membranes, namely, NF1, NF2, and NF6. The aim was to analyze its influence over the active polyamide layer and investigate its effect on water permeance, salt rejection, and gas separation to examine the degree to which the membrane defects were plugged. Effects of Pebax were studied before on RO membranes but were never thoroughly investigated for NF membranes. Pebax-coated NF membranes exhibited a decreased permeance, most noticeable for the NF1 membrane. Desalination tests revealed that all Pebax-coated membranes successfully enhanced the rejection of divalent ions and had no significant effect on the rejection of monovalent ions like NaCl. The resistance model calculations deviated slightly for NF2 and NF6 membranes, indicating that plugging was more prominent in the polyamide layer of the membranes. Several characterization techniques were carried out to understand the effect of Pebax on NF membranes. AFM tests showed a rough “leaf-like” surface for NF1, whereas a flat surface was observed for NF2 and NF6. The results were further confirmed by SEM pictures. In addition, the effect of pretreatment of NF membranes with and without IPA was studied. It was observed that IPA treatment destroyed the Pebax coating and showed a large thickness variance, as confirmed by ellipsometry and SEM images. Furthermore, ellipsometry also showed that the coating layer was not uniform across the membrane. Surface zeta potential measurements found that all coated membranes exhibited lower surface charge, and the salt rejections contradicted the theory behind the Donnan exclusion mechanism. Gas permeation tests were conducted to study the degree of pore plugging of the Pebax coating on NF membranes. However, no conclusive results were obtained as Knudsen diffusion was observed for noncoated NF membranes and low gas permeance of Pebax-coated NF membranes.