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

Abstract

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 Cl 2 O) generated. Species-specific rate constants for BA with Cl 2 , OCl − , and HOCl equaled (7.6 ± 0.19) × 10 1 , (1.7 ± 1.5) × 10 1 , (2.1 ± 0.71) × 10 −2 M −1 s −1 , respectively. A similar value for Cl 2 O 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.

Reactivity of the Polyamide Membrane Monomer with Free Chlorine: Role of Bromide

“…In the end, this procedure was the primary one used to measure these compounds since no N-halogenated byproducts (e.g., N-Br-BA or N-Cl-BA) were present in any of the experiments. These N-halogenated byproducts were of concern since N-Cl-BA was previously observed to revert back to its unhalogenated form when quenched by phenol or SO 3 2− , 20 and a similar outcome was expected for N-Br-BA. Their absence was confirmed by crossvalidating byproduct formation through two experiments, which are described in detail in Text S4.…”

“…20 Our previous results provided evidence that the Orton rearrangement (Scheme 1a) did not occur during chlorination, which along with these additional studies casts a further doubt as to whether it is involved during bromination as well. 20 In contrast, one study found that N-Br-acetanilide directly transformed to para-Br-acetanilide more easily than its corresponding chlorinated analogue (e.g., N-Cl-acetanilide) 35 (eq 12), initially suggesting that the Orton rearrangement could occur (Scheme 1a). Notably, this reaction took place in non-dissociating solvents (e.g., benzene) (eq 12) 35 that likely prohibited the formation of ionized species, such as Br − , which might be required for the Orton rearrangement to occur, as observed for its analogous Cl − (Scheme 1a).…”

“…12,32,33 However, the exact nature of how Br uptake occurs and how these brominating agents (eqs 4−9) react with the polyamide monomer still remains unclear, especially when considering the recently revised mechanisms proposed for polyamide monomer chlorination (Scheme 1b). 20 In one previous study, various model compounds representing the polyamide monomer (e.g., acetanilide or BA; eq 10) and an N-substituted version of this monomer (e.g., N-CH 3 -acetanilide or N-CH 3 -BA; eq 11) were directly exposed to Br 2 . 22,34 Direct ring bromination occurred for all these compounds and did so preferentially at the para position (eqs 10 and 11).…”

“…These studies include our own in which a revised mechanism was proposed based on findings obtained when benzanilide (BA), a model compound for the polyamide-based monomer (structure provided in Table S1 of the Supporting Information), reacted with free chlorine in the absence or presence of Cl − . 20 This revised mechanism suggests that the N-chlorination and ring chlorination pathways are independent of each other and driven by the potential for specific chlorinating agents (e.g., Cl 2 , HOCl, and OCl − ) to react with each reaction site (Scheme 1b). 20 The presence of halides, such as Cl − and Br − , is important for membrane degradation because they can participate in the reaction chemistry and are present at relatively high concentrations in NF-and RO-treated waters [e.g., [Cl − ] = 19.2 and ∼2.4 g/L in seawater 23 and brackish groundwater (one type which specifically serves as a source for desalination in the US), 24 respectively; [Br − ] = 67 and ∼4.4 mg/L in seawater 23 and brackish groundwater (one type which specifically serves as a source for desalination in the US), 24 respectively].…”

“…20 This revised mechanism suggests that the N-chlorination and ring chlorination pathways are independent of each other and driven by the potential for specific chlorinating agents (e.g., Cl 2 , HOCl, and OCl − ) to react with each reaction site (Scheme 1b). 20 The presence of halides, such as Cl − and Br − , is important for membrane degradation because they can participate in the reaction chemistry and are present at relatively high concentrations in NF-and RO-treated waters [e.g., [Cl − ] = 19.2 and ∼2.4 g/L in seawater 23 and brackish groundwater (one type which specifically serves as a source for desalination in the US), 24 respectively; [Br − ] = 67 and ∼4.4 mg/L in seawater 23 and brackish groundwater (one type which specifically serves as a source for desalination in the US), 24 respectively]. For example, [Cl − ] and pH can significantly alter the types of chlorinating agents formed, including HOCl and OCl − (eq 1), Cl 2 O (eq 2), and Cl 2 (eq 3).…”