Sonolytic degradation of benzophenone-3 in water matrices: Reaction mechanism, transformation products, ecotoxicological implications & microplastic interaction

The degradation of the UV filter benzophenone-3 (BP-3) by low-frequency ultrasound was investigated in different aqueous matrices. BP-3 sonodegradation followed pseudo-first-order kinetics, achieving >97 % removal of 500 μg L⁻¹ of BP-3 within 120 min in ultrapure water (UPW), using a 20 kHz ultrasound horn at 71 W L⁻¹. Varying the initial pH from 3 to 6 and 9 had only a slight effect on the process efficacy, with corresponding kinetic constant rates of 0.029, 0.030, and 0.041 min⁻¹, respectively. Experiments conducted in different water matrices showed a decrease in the apparent rate constant from 0.030 min⁻¹ in ultrapure water to 0.027 min⁻¹ in drinking water and 0.015 min⁻¹ in secondary effluent. Similarly, the presence of 250 mg L⁻¹ of chlorides or 10 mg L⁻¹ of humic acid reduced the degradation rate to 0.016 and 0.020 min⁻¹, respectively, while 250 mg L⁻¹ of hydrogen carbonate had no significant effect. The presence of nano- or microplastics led to a moderate decrease in BP-3 removal, particularly with smaller particles. Electron paramagnetic resonance (EPR) spectroscopy confirmed that fewer hydroxyl radicals were available in the presence of plastics. Twelve transformation products were identified by UHPLC-TOF/MS, resulting mainly from hydroxylation, demethylation, and ring-cleavage. According to the ecotoxicity analysis using the ECOSAR software, most of the transformation products were less toxic than the parent compound, enhancing the environmental feasibility of the process.