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Iodination of Dimethenamid in Chloraminated Water: Active Iodinating Agents and Distinctions between Chlorination, Bromination, and Iodination.

Few studies have elucidated the agent(s) that generate iodinated disinfection byproducts during drinking water treatment. We present a kinetic investigation of iodination of dimethenamid (DM), a model compound lacking acid-base speciation. Water chemistry parameters (pH, [Cl-], [Br-], [I-], and [pH buffer]) were systematically varied. As pH increased (4-9), DM iodination rate decreased. Conventional wisdom considers hypoiodous acid (HOI) as the predominant iodinating agent; nevertheless, HOI (pKHOI = 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, [Cl-] was added at constant pH and ionic strength. Although chloride addition did increase the iodination rate, the reaction order in [Cl-] was fractional (≤0.36). The contribution of ICl to DM iodination remained below 47% under typical drinking water conditions ([Cl-] ≤ 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 and molecular halogen (X2), and a more muted influence of XCl, leading to lesser susceptibility to catalysis by chloride.