Photochemical Transformation of Monochloramine Induced by Triplet State Dissolved Organic Matter.

The photoexcited dissolved organic matter (DOM) could produce reactive intermediates, affecting chemical oxidant transformation in UV based advanced oxidation processes (AOPs). This study confirmed the critical role of triplet state DOM ( ³ DOM*), generated from DOM photoexcitation, in the transformation of monochloramine (NH ₂ Cl), a commonly used chemical oxidant and disinfectant in water treatment. NH ₂ Cl (42.25 μM, as Cl ₂ ) was decayed by 17.4-73.4 % within 60 min, primarily due to ³ DOM* , in DOM (2-30 mg _C L ^-1 ) solutions irradiated by 365 nm, where NH ₂ Cl has no absorption. The second-order quenching rate constants of triplet state model photosensitizers by NH ₂ Cl were determined to be 0.95(± 0.04)-4.49(± 0.04)× 10 ⁸ M ^-1 s ^-1 by using laser flash photolysis. As a reductant, ³ DOM* reacted with NH ₂ Cl through one-transfer mechanism, leading to amino radical (NH ₂ ^• ) generation, which then transferred to ammonia (NH ₄ ⁺ , pKa 9.25) through H-abstraction by the phenolic moieties in DOM. Additionally, the intermediate product of ³ DOM* oxidized by NH ₂ Cl or those triplet state quinones can hydrolyze to form phenolic moieties, elevating NH ₄ ⁺ yield to higher than 99% upon 365 nm irradiation. These findings suggest that the widespread DOM can be applied to convert NH ₂ Cl via ³ DOM* with minimal toxic risks.
Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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