title: Amide Link Scission in the Polyamide Active Layers of Thin-Film Composite Membranes upon Exposure to Free Chlorine: Kinetics and Mechanisms.
abstract: The volume-averaged amide link scission in the aromatic polyamide active layer of a reverse osmosis membrane upon exposure to free chlorine was quantified at a variety of free chlorine exposure times, concentrations, and pH and rinsing conditions. The results showed that (i) hydroxyl ions are needed for scission to occur, (ii) hydroxide-induced amide link scission is a strong function of exposure to hypochlorous acid, (iii) the ratio between amide links broken and chlorine atoms taken up increased with the chlorination pH and reached a maximum of ∼25%, (iv) polyamide disintegration occurs when high free chlorine concentrations, alkaline conditions, and high exposure times are combined, (v) amide link scission promotes further chlorine uptake, and (vi) scission at the membrane surface is unrepresentative of volume-averaged scission in the active layer. Our observations are consistent with previously proposed mechanisms describing amide link scission as a result of the hydrolysis of the N-chlorinated amidic N-C bond due to nucleophilic attack by hydroxyl ions. This study increases the understanding of the physicochemical changes that could occur for membranes in treatment plants using chlorine as an upstream disinfectant and the extent and rate at which those changes would occur.
authors.affiliation: Department of Environmental Sciences and Engineering, Gillings School of Global Public Health, University of North Carolina at Chapel Hill , Chapel Hill, North Carolina 27599, United States.
authors.forename: Joshua
authors.initials: J
authors.lastname: Powell
authors.affiliation: Department of Environmental Sciences and Engineering, Gillings School of Global Public Health, University of North Carolina at Chapel Hill , Chapel Hill, North Carolina 27599, United States.
authors.forename: Jeanne
authors.initials: J
authors.lastname: Luh
authors.affiliation: Department of Environmental Sciences and Engineering, Gillings School of Global Public Health, University of North Carolina at Chapel Hill , Chapel Hill, North Carolina 27599, United States.
authors.forename: Orlando
authors.initials: O
authors.lastname: Coronell
mesh_terms: D000577:Amides / Q000737:chemistry; D002713:Chlorine / Q000737:chemistry*; D004202:Disinfectants / Q000737:chemistry; D005374:Filtration / Q000295:instrumentation / Q000379:methods; D054879:Halogenation; D006863:Hydrogen-Ion Concentration; D006868:Hydrolysis; D006997:Hypochlorous Acid / Q000737:chemistry; D007700:Kinetics; D008567:Membranes, Artificial; D009757:Nylons / Q000737:chemistry*; D009995:Osmosis; D018508:Water Purification / Q000295:instrumentation*
chemical_list: D000577:Amides; D004202:Disinfectants; D008567:Membranes, Artificial; D009757:Nylons; D002713:Chlorine; D006997:Hypochlorous Acid