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.

No abstract

Aromatic polyamide-based membranes are widely used for reverse osmosis (RO) and nanofiltration (NF) treatment but degrade when exposed to free chlorine (HOCl/OCl–). The reaction mechanisms with free chlorine were previously explored, but less is known about the role of bromide (Br–) in these processes. Br– may impact these reactions by reacting with HOCl to form HOBr, which then triggers other brominating agents (Br2O, Br2, BrOCl, and BrCl) to form. This study examined the reactivities of these brominating agents with a polyamide monomer model compound, benzanilide (BA), and a modified version of it, N-CH3-BA. The results indicated that all these brominating agents only attacked the aromatic ring adjacent to the amide N, rather than the amide N, different from the previously examined chlorinating agents (HOCl, OCl–, and Cl2) that attacked both sites. Orton rearrangement was not observed. Species-specific rate constants (k i , M–1 s–1) between BA and HOBr, Br2O, Br2, BrOCl, and BrCl were determined to be (5.3 ± 1.2) × 10–2, (1.2 ± 0.4) × 101, (3.7 ± 0.2) × 102, (2.2 ± 0.6) × 104, and (6.6 ± 0.9) × 104 M–1 s–1, respectively, such that k BrCl > k BrOCl > k Br2 > k Br2O > k HOBr. N-CH3-BA exhibited lower reactivity than BA. Model predictions of BA loss during chlorination with varied Br– and/or Cl– concentrations were established. These findings will ultimately enable membrane degradation and performance loss following chlorination in mixed halide solutions to be better predicted during pilot- and full-scale NF and RO treatment.

Barium titanate nanoparticles embedded in flexible polymer films were synthesized using hydrothermal processing methods. The resulting films were characterized with respect to material composition, size distribution of nanoparticles, and spatial location of particles within the polymer film. Synthesis conditions were varied based on the mechanical properties of the polymer films, ratio of polymer to barium titanate precursors, and length of aging time between initial formulations of the solution to final processing of nanoparticles. Block copolymers of poly(styrene-co-maleic anhydride) (SMAh) were used to spatially separate titanium precursors based on specific chemical interactions with the maleic anhydride moiety. However, the glassy nature of this copolymer restricted mobility of the titanium precursors during hydrothermal processing. The addition of rubbery butadiene moieties, through mixing of the SMAh with poly(styrene-butadiene-styrene) (SBS) copolymer, increased the nanoparticle dispersion as a result of greater diffusivity of the titanium precursor via higher mobility of the polymer matrix. Additionally, an aminosilane was used as a means to retard cross-linking in polymer-metalorganic solutions, as the titanium precursor molecules were shown to react and form networks prior to hydrothermal processing. By adding small amounts of competing aminosilane, excessive cross-linking was prevented without significantly impacting the quality and composition of the final barium titanate nanoparticles. X-ray diffraction and X-ray photoelectron spectroscopy were used to verify nanoparticle compositions. Particle sizes within the polymer films were measured to be 108 ± 5 nm, 100 ± 6 nm, and 60 ± 5 nm under different synthetic conditions using electron microscopy. Flexibility of the films was assessed through measurement of the glass transition temperature using dynamic mechanical analysis. Dielectric permittivity was measured using an impedance analyzer.

No abstract