Direct and nitrite-sensitized indirect photolysis of effluent-derived phenolic contaminants under UV 254 irradiation.

UV 254 photolysis has increasingly been utilized for disinfection of water-born pathogens in wastewater. During disinfection, wastewater-derived trace organic contaminants, such as pharmaceuticals and personal care products (PPCPs), may be subjected to direct photolysis and indirect photolysis sensitized by wastewater constituents such as nitrite (NO 2 - ). Herein, we reported the direct photolysis and NO 2 - -sensitized indirect photolysis of four phenolic contaminants commonly observed in wastewaters ( i.e. , bisphenol A (BPA), acetaminophen (ATP), salbutamol (SAL), and 2,4-dihydroxybenzophenone (BP1)). Spectroscopic characterization and quantum yield measurement were carried out to evaluate the photochemical reactivity of these phenolic compounds. In NO 2 - -sensitized photolysis, the relative contribution of direct and indirect photolysis was quantified by light screening factor calculation and radical quenching studies. The experimental results highlight the important roles of HO˙ and NO 2 ˙ in the NO 2 - -sensitized photolysis of phenolic compounds. A series of intermediate products, including hydroxylated, nitrated, nitrosated, dimerized, and alkyl chain cleavage products, were identified by solid phase extraction (SPE) combined with high-resolution mass spectrometry (HRMS) analyses. On the basis of identified products, the underlying mechanisms and transformation pathways for NO 2 - -sensitized photolysis of these phenolic compounds were elucidated. The second-order rate constants of BPA, SAL, BP1 with NO 2 ˙ were calculated to be 2.25 × 10 4 , 1.35 × 10 4 and 2.44 × 10 4 M -1 s -1 , respectively, by kinetic modeling. Suwanee River natural organic matter (SRNOM) played complex roles in the direct and NO 2 - -sensitized photolysis of phenolic compounds by serving as a photosensitizer, light screening and radical quenching agent. Wastewater constituents, such as NO 3 - and EfOM, could accelerate direct and NO 2 - -sensitized photolysis of BPA, SAL, and BP1 in the wastewater matrix. Our results suggest that NO 2 - at the WWTP effluent-relevant level can sensitize the photolysis of effluent-derived phenolic contaminants during the UV 254 disinfection process; however, the formation of potentially carcinogenic and mutagenic nitrated/nitrosated derivatives should be scrutinized.