Mechanism of bicarbonate enhancing the photodegradation of β-blockers in natural waters

Abstract

The impact of HCO3− on the photodegradation of β-blockers was investigated under simulated sunlight irradiation. The results show that in the presence of HCO3−, the photodegradation rates increase significantly for sotalol (SOT), whereas no effects on the degradation of carvedilol and arotinolol are observed. Using quenching experiments, electron paramagnetic resonance spectra and degradation product determination, we demonstrate that carbonate radical (CO3•−) is formed by direct oxidation of HCO3− by triplet-excited SOT (3SOT*) and plays a significant role in SOT photodegradation. Competition kinetics experiments show that the three β-blockers all have high second-order rate constants (107–108 M−1 s−1) for the reaction with CO3•−. However, only 3SOT* has a higher reduction potential that can oxidize HCO3− to produce CO3•−. Thus, enhanced SOT removal rates in the presence of HCO3− were observed. In addition, the results show that seawater DOM could increase HCO3−-induced photodegradation of SOT, whereas SRNOM mainly behaves as a CO3•− quencher and decreases the removal rate of SOT. The results underscore the role of HCO3− in limiting the persistence of organic pollutants like SOT in sunlit natural waters, and especially in marine and coastal waters.