Impact of the Ultraviolet Photolysis of Monochloramine on 1,4-Dioxane Removal: New Insights into Potable Water Reuse

Although chloramines are ubiquitously present during ultraviolet-driven advanced oxidation processes (UV/AOP) that are becoming increasingly important for potable water reuse, the photochemistry of chloramines in treated wastewater, and the associated effects on trace chemical contaminant degradation, are unknown. This study investigated the fundamental radical chemistry involved in monochloramine (NH2Cl) photolysis and its efficiency in degrading 1,4-dioxane using a low-pressure Hg lamp (lambda = 254 nm). These results showed that the UV fluence-normalized rate of 1,4-dioxane degradation in UV/NH2Cl ranged between 1.1 X 10(-4) and 2.9 X 10(-4) cm(2).mJ(-1). The photolysis of NH2Cl produced NH2 center dot and Cl-center dot, which further transformed to a series of reactive radical species. An optimal NH2Cl dosage for 1,4-dioxane degradation was observed at a NH2Cl/1,4-dioxane concentration ratio of 8.0, while excess NH2Cl scavenged reactive radicals and decreased the treatment efficiency. Scavenging experiments and probe compound calculations showed that both Cl-2(center dot-) and HO center dot contributed significantly to 1,4-dioxane removal, while the NH2 center dot radical reacted slowly with 1,4-dioxane. The presence of dissolved oxygen further decreased NH2 center dot reactivity. This study generated critical knowledge of the photochemistry of NH2Cl and will allow for future optimization of the UV/AOP for more efficient water reuse treatments.