Degradation of N-nitrosamines and 1,4-dioxane using vacuum ultraviolet irradiation (UV254+185 nm or UV172 nm).

Advanced oxidation processes (AOPs) play a vital role in attenuating contaminants of emerging concern (CECs) during potable water reuse. AOPs are conventionally performed by irradiating with a 254-nm low-pressure (LP) mercury-vapor (Hg) ultraviolet (UV) lamp along with chemical treatment. Compared with UV-C light treatment (200-280 nm), vacuum-UV (V-UV) light treatment (100-200 nm) is advantageous in terms of hydroxyl radical generation without the requirement for chemical treatment. This study assessed the potential of V-UV (172-nm Xe2 excimer or 185 + 254-nm LP-Hg) lamps on the destruction of two major CECs in potable water reuse, namely N-nitrosodimethylamine (NDMA) and 1,4-dioxane. Direct irradiation using UV254 nm or UV185+254 nm lamps achieved ≥94% removal of N-nitrosamines, including NDMA, at a UV dose of 900 mJ/cm2. In contrast, the Xe2 excimer lamp (UV172 nm) was less effective for N-nitrosamine removal, achieving up to 82% removal of NDMA. The removal of 1,4-dioxane by V-UV lamps at a UV dose of 900 mJ/cm2 reached 51% (UV172 nm) and 28% (UV185+254 nm), both of which results were superior to that obtained using a conventional UV254 nm lamp (10%). The addition of hydrogen peroxide during UV254 nm or UV185+254 nm irradiation was found to enhance the removal of 1,4-dioxane, while UV172 nm irradiation without hydrogen peroxide addition still exhibited greater efficiencies than those UV254 nm lamps-based AOPs. Overall, this study demonstrated that the removal of both NDMA and 1,4-dioxane can be successfully achieved using either a UV254+185 nm lamp with hydrogen peroxide or a UV172 nm Xe2 excimer lamp without hydrogen peroxide.

Degradation of dibromophenols by UV irradiation.

We examined the degradation of dibromophenols (DBPs), i.e. 2,4-DBP, 2,6-DBP and 3,5-DBP by ultraviolet (UV) irradiation and estimated the relationship between degradability and molecular orbital properties of each dibromophenol. The removal of DBPs under a UV lamp system was successfully performed in an aqueous solution. After 5 min of irradiation, the initial DBPs concentration of 20 mg/L was decreased to below 1 mg/L, and about 60% of bromide ion was released. A decrease in the concentration of dissolved organic carbon (DOC) suggested the mineralization of DBPs. The mineralization may occur after release of bromide ions because the decrease of DOC was slower than the release of bromide ions. The degradability of 3,5-DBP was slightly lower than 2,6-DBP and 2,4-DBP. Molecular orbital calculation suggested that the electrophilic frontier density and the highest occupied molecular orbital (HOMO) energy may be related to the degradability of DBPs.