Degradation of DEET and Caffeine under UV/Chlorine and Simulated Sunlight/Chlorine Conditions

Photoactivation of aqueous chlorine could promote degradation of chlorine-resistant and photochemically stable chemicals accumulated in swimming pools. This study investigated the degradation of two such chemicals, <i>N</i>,<i>N</i>-diethyl-3-methylbenzamide (DEET) and caffeine, by low pressure ultraviolet (UV) light and simulated sunlight (SS) activated free chlorine (FC) in different water matrices. Both DEET and caffeine were rapidly degraded by UV/FC and SS/FC but exhibited different kinetic behaviors. The degradation of DEET followed pseudo-first-order kinetics, whereas the degradation of caffeine accelerated with reaction. Mechanistic study revealed that, under UV/FC, ·OH and Cl· were responsible for degradation of DEET, whereas ClO· related reactive species (ClOrrs), generated by the reaction between FC and ·OH/Cl·, played a major role in addition to ·OH and Cl· in degrading caffeine. Reaction rate constants of DEET and caffeine with the respective radical species were estimated. The imidazole moiety of caffeine was critical for the special reactivity with ClOrrs. Water matrix such as pH had a stronger impact on the UV/FC process than the SS/FC process. In saltwater matrix under UV/FC and SS/FC, the degradation of DEET was significantly inhibited, but the degradation of caffeine was much faster than that in nonsalty solutions. The interaction between Br^– and Cl^– may play an important role in the degradation of caffeine by UV/FC in saltwater. Reaction product analysis showed similar product patterns by UV/FC and SS/FC and minimal formation of chlorinated intermediates and disinfection byproducts

Degradation of Pharmaceuticals and Metabolite in Synthetic Human Urine by UV, UV/H₂O₂, and UV/PDS

To minimize environmental pharmaceutical micropollutants, treatment of human urine could be an efficient approach due to the high pharmaceutical concentration and toxic potential excreted in urine. This study investigated the degradation kinetics and mechanisms of sulfamethoxazole (SMX), trimethoprim (TMP) and N₄-acetyl-sulfamethoxazole (acetyl-SMX) in synthetic fresh and hydrolyzed human urines by low-pressure UV, and UV combined with H₂O₂ and peroxydisulfate (PDS). The objective was to compare the two advanced oxidation processes (AOPs) and assess the impact of urine matrices. All three compounds reacted quickly in the AOPs, exhibiting rate constants of (6.09–8.53) × 10⁹ M^–1·s^–1 with hydroxyl radical, and (2.35–16.1) × 10⁹ M^–1·s^–1 with sulfate radical. In fresh urine matrix, the pharmaceuticals’ indirect photolysis was significantly suppressed by the scavenging effect of urine citrate and urea. In hydrolyzed urine matrix, the indirect photolysis was strongly affected by inorganic urine constituents. Chloride had no apparent impact on UV/H₂O₂, but significantly raised the hydroxyl radical concentration in UV/PDS. Carbonate species reacted with hydroxyl or sulfate radical to generate carbonate radical, which degraded SMX and TMP, primarily due to the presence of aromatic amino group(s) (<i>k</i> = 2.68 × 10⁸ and 3.45 × 10⁷ M^–1·s^–1) but reacted slowly with acetyl-SMX. Ammonia reacted with hydroxyl or sulfate radical to generate reactive nitrogen species that could react appreciably only with SMX. Kinetic simulation of radical concentrations, along with products analysis, helped elucidate the major reactive species in the pharmaceuticals’ degradation. Overall, the AOPs’ performance was higher in the hydrolyzed urine than fresh urine matrix with UV/PDS better than UV/H₂O₂, and varied significantly depending on pharmaceutical’s structure

Rapid Disinfection by Peracetic Acid Combined with UV Irradiation

This study proposes a novel disinfection process by sequential application of peracetic acid (PAA) and ultraviolet light (UV), on the basis of elucidation of disinfection mechanisms under UV/PAA. Results show that hydroxyl radicals, generated by UV-activated PAA, contribute to the enhanced inactivation of Escherichia coli under UV/PAA compared to PAA alone or UV alone. Furthermore, the location of hydroxyl radical generation is a critical factor. Unlike UV/H₂O₂, which generates hydroxyl radicals mainly in the bulk solution, the hydroxyl radicals under UV/PAA are produced close to or inside E. coli cells, due to PAA diffusion. Therefore, hydroxyl radicals exert significantly stronger disinfection power under UV/PAA than under UV/H₂O₂ conditions. Pre-exposing E. coli to PAA in the dark followed by application of UV (i.e., a PAA-UV/PAA process) promotes diffusion of PAA to the cells and achieves excellent disinfection efficiency while saving more than half of the energy cost associated with UV compared to simultaneous application of UV and PAA. The effectiveness of this new disinfection strategy has been demonstrated not only in lab water but also in wastewater matrices

PPCP Degradation by Chlorine–UV Processes in Ammoniacal Water: New Reaction Insights, Kinetic Modeling, and DBP Formation

The combination of chlorine and UV (i.e., chlorine–UV process) has been attracting more attention in recent years due to its ready incorporation into existing water treatment facilities to remove PPCPs. However, limited information is available on the impact of total ammonia nitrogen (TAN). This study investigated two model PPCPs, <i>N,N</i>-diethyl-3-toluamide (DEET) and caffeine (CAF), in the two stages of the chlorine–UV process (i.e., chlorination and UV/chlor­(am)­ine) to elucidate the impact of TAN. During chlorination, the degradation of DEET and CAF was positively correlated with the overall consumption of total chlorine by TAN. Reactive nitrogen intermediates, including HNO/NO^– and ONOOH/ONOO^–, along with ^•OH were identified as major contributors to the removal of DEET and CAF. During UV irradiation, DEET and CAF were degraded under UV/chlorine or UV/NH₂Cl conditions. ^•OH and ^•Cl were the major reactive species to degrade DEET and CAF under UV/NH₂Cl conditions, whereas ^•OCl played a major role for degrading CAF under UV/chlorine conditions. Numerical models were developed to predict the removal of DEET and CAF under chlorination–UV process. Chlorinated disinfection byproducts were detected. Overall, this study presented kinetic features and mechanistic insights on the degradation of PPCPs under the chlorine–UV process in ammoniacal water