The role of reactive oxygen species in sulfamethazine degradation using UV-based technologies and products identification

• Photo-oxidation processes were compared in terms of sulfamethazine loss rates.
• Photolysis and photosensitization were investigated in photo-oxidation processes.
• Contribution of photosensitization and reactive oxygen species was investigated.
• The cleavage of CS and NS bonds from sulfamethazine was proposed.
• Hydroxylated aromatic compounds were detected in the H2O2/UV system.

Sulfamethazine (SMT) is an emerging pollutant continuously delivered into water bodies that has attracted growing attention due to its recalcitrant behavior in the environment. Although UV-based technologies have been investigated for the treatment of both water and wastewater containing SMT, photosensitization of the target compound is not usually examined during photo-induced oxidation processes. It is not our intention to propose processes for water treatment, only to understand the phenomena induced by the interactions between UV radiation and pollutant molecule. In this way, the photo-initiated and photo-induced oxidation processes are compared in terms of changes in SMT degradation rates and photoproducts formed. A tubular photochemical reactor with a concentric low pressure mercury vapor lamp emitting at 254 nm was used and SMT concentrations were determined by ultra-fast liquid chromatography analysis. Hydrogen peroxide consumption was monitored spectrophotometrically and the photoproducts were analyzed by LCMS-IT-TOF. To characterize samples we used three spectroscopic methods based on: (1) UV–Vis absorption; (2) excitation-emission matrix (EEM) fluorescence; and (3) phosphorescence. Our results suggest that photolysis and photosensitization played an important role in sulfamethazine degradation during irradiation at 254 nm. The reaction with singlet oxygen and radical species promotes sulfamethazine degradation during a photosensitized process occurring simultaneously with direct photolysis. The photo-initiated oxidation process was advantageous over the photo-induced oxidation in terms of SMT degradation rates, achieving concentration below the detection limit after 47 min of irradiation. Compounds originated from the cleavage of the CS and NS bonds of the SMT molecule were detected as common transformation products during photo-initiated and photo-induced oxidation processes, whereas a degradation product resulting from the electrophilic addition of hydroxyl radicals to the aromatic ring was detected for the H2O2/UV system. The high efficiency of the photo-initiated oxidation process on the photodegradation of the recalcitrant sulfamethazine was verified, suggesting the formation of acidic subproducts probably from the oxidative cleavage of hydroxylated aromatic rings.

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