Impact of Polymer Chain Rearrangements in the PA Structure of RO Membranes on Water Permeability and N-Nitrosamine Rejection

The use of solvents is overall recognized as an efficient method to improve the water permeability of polyamide thin film composite membranes (PA-TFC). The objective of this work was to test the performance of the membranes after exposing them to n-propanol (n-PrOH) to improve the permeability of the membranes while maintaining the rejection factor for small uncharged organic molecules, namely N-nitrosamines (NTRs). After the membranes were exposed to n-PrOH, the water permeability of the UTC73AC membrane increased by 98%, with minimal change in rejection. N-nitrosodiethylamine (NDEA) rejection decreased (3.4%), while N-nitrosodi-n-propylamine (NDPA) and N-nitrosodi-n-butylamine (NDBA) rejection increased by 0.9% and 2.8%, respectively. In contrast, for the BW30LE membrane, water permeability decreased (by 38.7%), while rejection factors increased by 14.5% for NDEA, 6.2% for NDPA, and 15.0% for NDBA. In addition, the morphology of the membrane surface before and after exposure to n-PrOH was analyzed. This result and the pore size distribution (PSD) curves obtained indicate that the rearrangement of polymer chains affects the network or aggregate pores in the PA layer, implying that a change in pore size or a change in pore size distribution could improve the permeability of water molecules, while the rejection factor for NTRs is not significantly affected.

Automated summary

The aim of this study was to evaluate the effects of treating polyamide thin film composite membranes (PA-TFC) with n-propanol (n-PrOH) on water permeability and rejection factors for N-nitrosamines (NTRs). The results showed that exposure to n-PrOH increased water permeability by 98% for the UTC73AC membrane, while rejection factors remained relatively unchanged. For the BW30LE membrane, water permeability decreased by 38.7%, but rejection factors for NTRs increased. Furthermore, analysis of the membrane surface morphology and pore size distribution indicated that the rearrangement of polymer chains affected the network or aggregate pores in the PA layer, resulting in improved water permeability without significant impact on rejection factors for NTRs.