Chlorine degradation of semi-aromatic polypiperazine-amide membranes and the mechanisms

Abstract

Semi-aromatic polypiperazine-amide (PPA) nanofiltration (NF) membranes are widely used in water treatment applications, but the resistance to free chlorine and other biocidal agents need to be improved. Understanding the chlorination mechanisms of PPA NF membranes is of great significance for avoiding membrane degradation and developing new chlorine-resistant membranes. In this paper, the effects of chlorination on the rejection selectivity as well as the surface charge, hydrophilicity and roughness, and pore size distribution of PPA NF membranes are presented. In addition, the chlorination mechanisms of the membranes are proposed through functional groups characterization of the stripped PPA active layer, element composition analysis, quantum mechanical calculations based on density-functional theory, and use of model compound ((3,5-dimethylphenyl)(4-methylpiperazin-1-yl)methanone) for PPA material. The molecular active chlorine species, HOCl, predominantly induces the cleavage of C–N bonds, whereas the primary reaction pathway caused by the ionic active chlorine species, ClO−, involves the ring-opening of benzene. Under the same chlorination conditions, HClO has a more severe impact on membrane properties and performance than ClO−. NF membranes with a looser polyamide layer are more susceptible to the attack by active chlorine species.