Utilities incorporating the potable reuse of municipal wastewater are interested in converting from the UV/H₂O₂ to the UV/free chlorine advanced oxidation process (AOP). The AOP treatment of reverse osmosis (RO) permeate often includes the de facto UV/chloramine AOP because chloramines applied upstream permeate RO membranes. Models are needed that accurately predict oxidant photolysis and subsequent radical reactions. By combining radical scavengers and kinetic modeling, we have derived quantum yields for radical generation by the UV photolysis of HOCl, OCl^-, and NH₂Cl of 0.62, 0.55, and 0.20, respectively, far below previous estimates that incorporated subsequent free chlorine or chloramine scavenging by the ^•Cl and ^•OH daughter radicals. The observed quantum yield for free chlorine loss actually decreased with increasing free chlorine concentration, suggesting scavenging of radicals participating in free chlorine chain decomposition and even free chlorine reformation. Consideration of reactions of ^•ClO and its daughter products (e.g., ClO₂^-), not included in previous models, were critical for modeling free chlorine loss. Radical reactions (indirect photolysis) accounted for ∼50% of chloramine decay and ∼80% of free chlorine loss or reformation. The performance of the UV/chloramine AOP was comparable to the UV/H₂O₂ AOP for degradation of 1,4-dioxane, benzoate and carbamazepine across pH 5.5-8.3. The UV/free chlorine AOP was more efficient at pH 5.5, but only by 30% for 1,4-dioxane. At pH 7.0-8.3, the UV/free chlorine AOP was less efficient. ^•Cl converts to ^•OH. The modeled ^•Cl:^•OH ratio was ∼20% for the UV/free chlorine AOP and ∼35% for the UV/chloramine AOP such that ^•OH was generally more important for contaminant degradation.

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