Halogenation and Degradation Mechanisms and Modification Strategy of the Polyamide Membrane Monomer during Chlorination: A Computational Study

Aromatic polyamide thin film composite membranes have been extensively utilized in water treatment and desalination. However, unintended degradation of these membranes occurs when free chlorine is added to control biofouling. In this study, halogenation and degradation mechanisms of the polyamide monomer model compound, benzanilide (BA), during chlorination in the presence of halides Cl^-, Br^-, and I^- were systematically investigated by a quantum chemical computational method. The results indicate that the reactive amide N and <i>ortho</i>/<i>para</i>-C in the anilide ring of BA undergo the respective concerted and classic S_EAr mechanisms, and their reactivity depends on not only the chlorinating agents but also the speciation and tautomer of substrate BA, which could explain the experimental results observed at different pH. Comparing halogenation of BA by HOX (X = Cl/Br/I) at pH 7.0, notably, the kinetic reactivity order follows Br > I > Cl and I > Br > Cl for the amide N and C sites in the anilide ring, respectively, which can be explained by the "like-attracts-like" principle in the hard-soft-acid-base theory. The degradation of the membrane occurs in the hydrolysis of the chlorinated-BA with the C-N bond cleavage, in which <i>N</i>-halogenated products exhibit remarkably higher reactivity than BA and <i>C</i>-halogenated ones, and hydrolysis catalyzed by acid/base is definitely accelerated. Additionally, some modification strategies to improve the chlorine resistance of the polyamide membrane were proposed. The findings of this work are helpful in further understanding the degradation mechanisms and designing chlorine resistance of polyamide membrane during chlorination.