Understanding the chlorination mechanism and the chlorine-induced separation performance evolution of polypiperazine-amide nanofiltration membrane

Although the tertiary amide bonds are quite stable to oxidants, the typical polypiperazine-amide (PPA) nano-filtration membranes (NFMs) were still reported to be susceptible to chlorine degradation. However, the understanding of the chlorination mechanism and the chlorine-induced separation performance evolution of the PPA NFMs still remains incomplete, significantly limiting the development of chlorine-resistant NFMs. In this work, two types of self-made PPA NFMs with different physicochemical structures and separation performance were employed to investigate the chlorination processes. The regeneration behaviour of the chlorinated PPA NFMs was studied for the first time. Our results indicated that the deterioration of the PPA separating layer upon chlorine exposure follows two pathways: reversible chlorine substitution of the -NH group to -NCl group, and oxidation of the -NH group to imine group. Part of the -NCl groups could be reduced to -NH groups during the regeneration process. No cleavage of tertiary amide bonds in the PPA polymer occurred. Separation performance evolution of the PPA NFMs after chlorination or regeneration was mainly induced by the variation of -NH group content in the PPA separating layer. The loss of -NH groups after chlorination and the reformation of -NH groups after regeneration could not only affect the density of the PPA separating layer through hydrogen bonds but also the electrostatic interaction between the PPA separating layer and ions. Additionally, the regenerated PPA NFMs showed good stability under elevated pressure or continuous filtration.