We studied the formation of photochemically produced reactive intermediates (RI) from dissolved organic matter (DOM). Specifically, we focused on the effects of variable molecular weight and chemical reduction on the optical properties of DOM (absorbance and fluorescence) and the formation of singlet oxygen ((1)O2), DOM triplet excited states ((3)DOM*), and the hydroxyl radical ((•)OH). The data are largely evaluated in terms of a charge-transfer (CT) model, but deficiencies in the model to explain the data are pointed out when evident. A total of two sets of samples were studied that were subjected to different treatments; the first set included secondary-treated wastewaters and a wastewater-impacted stream, and the second was a DOM isolate. Treatments included size fractionation and chemical reduction using sodium borohydride. Taken as a whole, the results demonstrate that decreasing molecular weight and borohydride reduction work in opposition regarding quantum efficiencies for (1)O2 and (3)DOM* production but in concert for fluorescence and (•)OH production. The optical and photochemical data provide evidence for a limited role of CT interactions occurring in lower-molecular-weight DOM molecules. In addition, the data suggest that the observed optical and photochemical properties of DOM are a result of multiple populations of chromophores and that their relative contribution is changed by molecular-weight fractionation and borohydride reduction.

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 (λ = 254 nm). These results showed that the UV fluence-normalized rate of 1,4-dioxane degradation in UV/NH2Cl ranged between 1.1 × 10–4 and 2.9 × 10–4 cm2·mJ–1. The photolysis of NH2Cl produced NH2 • and Cl•, 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 Cl2 •– and HO• contributed significantly to 1,4-dioxane removal, while the NH2 • radical reacted slowly with 1,4-dioxane. The presence of dissolved oxygen further decreased NH2 • 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.

Dissolved organic matter (DOM) is ubiquitous in natural waters and can facilitate the chemi-cal transformation of many contaminants through the photochemical production of reactive inter-mediates, such as singlet oxygen (1O2),...