Iodination of Dimethenamid in Chloraminated Water

Iodinated disinfection byproducts (I-DBPs) are potent toxins found in drinking water, yet few have studied iodination mechanisms that initiate I-DBP formation under oxidative aqueous conditions. This kinetic investigation utilizes an oxidant (monochlormaine) residual to maintain the iodine speciation expected in disinfected water, and dimethenamid (DM), a model compound lacking acid-base speciation. Water chemistry parameters (pH, [chloride], [bromide], [iodide], and [pH buffer]) were systematically varied. As pH increased (4 – 9), DM iodination rate decreased. Conventional wisdom considers HOI as the predominant iodinating agent; nevertheless, HOI (pK_HOI = 10.4) could not have produced this result, as its concentration is essentially invariant from pH 4 – 9. In contrast, [H2OI+] and [ICl] both decrease as pH increases. To distinguish their contributions to DM iodination, [chloride] was added at constant pH and ionic strength. Chloride addition increased iodination rates, although the reaction order in [chloride] was fractional (≤0.36) across pH 5.8 – 7.7, indicating a complex mechanism. Our results indicate that contribution from ICl to DM iodination remains below 47% under drinking water conditions ([chloride] ≤ 250 mg/L), implicating H2OI+ as the predominant iodinating agent. Distinctions between DM iodination versus chlorination or bromination include a more pronounced role for the hypohalous acidium ion (H2OX+), negligible contributions by hypohalous acid (HOX) and molecular halogen (X2), and a more muted influence of XCl, leading to lesser susceptibility to catalysis by chloride. The rate-determining step (RDS) of DM halogenation was investigated using deuterium kinetic isotope effect (KIE) analysis. DM chlorination and bromination did not exhibit KIEs (kH/kD ≈ 1), consistent with rate-determining halogen addition. In contrast, iodination always exhibited primary KIEs (>1.4), indicating proton removal during the RDS. Varying pH (5–9) and [iodide] (10 – 100 µM) did not substantially affect the magnitude of KIEs (1.47 to 2.06), suggesting that iodine addition also occurs in the RDS. Adding 1 mM [chloride] enhanced iodination rates without affecting the KIE appreciably (1.71), whereas chloride addition above 25 mM elevated the KIE (2.9 – 3.4) but not iodination rates further, consistent with a stepwise mechanism in which kinetic control shifts from iodine addition to proton removal