Reactivity of the Polyamide Membrane Monomer with Free Chlorine: Reaction Kinetics, Mechanisms, and the Role of Chloride

Aromatic polyamide thin-film composite membranes are widely used in reverse osmosis (RO) and nanofiltration (NF) due to their high water permeability and selectivity. However, these membranes undergo biofouling and can degrade and eventually fail during free chlorine exposure. To better understand this effect, the reactivity of the polyamide monomer (benzanilide (BA)) with free chlorine was tested under varying pH and chloride (Cl-) conditions. The kinetic results indicated that the current existing mechanisms, especially the Orton rearrangement, were invalid. Revised reaction pathways were proposed where BA chlorination was driven by two independent pathways involving the anilide ring and amide nitrogen moieties. The ability for one moiety to be chosen over the other was highly dependent on the pH, Cl- concentration, and the resulting chlorinating agents (e.g., Cl-2, HOCl, OCl-, and Cl2O) generated. Species-specific rate constants for BA with Cl-2, OCl-, and HOCl equaled (7.6 +/- 0.19) x 10(1), (1.7 +/- 1.5) X 10(1), (2.1 +/- 0.71) x 10(-2) M-1 s(-1), respectively. A similar value for Cl2O could not be accurately estimated under the tested conditions. The behavior of these chlorinating agents differed for each reactive site such that OCl- > HOCl for N-chlorination and Cl-2 > HOCl > OCl- for anilide ring chlorination. Experiments with modified monomers indicated that substituent placement largely affected which reactive site was kinetically favorable. Overall, such findings provide a predictive model of how the polyamide monomer degrades during chlorine exposure and guidance on how chlorine-resistant polyamide membranes should be designed.