Photochemical fate of ß-blocker pindolol in riverine and its downstream coastal waters.

Pindolol (PIN) is a commonly used ß-blocker drug and has been frequently detected in various natural waters. Comprehensive understanding of its environmental photochemical transformation is necessary to assess its environmental risk. In this study, the photodegradation kinetics and mechanisms of PIN in both freshwater and coastal water were investigated for the first time. The photodegradation experiments were carried out by steady-state photochemical experiment under simulated sunlight irradiation. The results showed that the photodegradation rate of PIN in the freshwater of the Pearl River estuary was significantly faster than that in its downstream coastal water. In river water, PIN can undergo both direct photolysis and indirect photolysis induced by riverine dissolved organic matter (DOM) mainly through excited triplet-state of DOM and singlet oxygen, while direct photolysis dominated its degradation in coastal water. The promotion effect was found to be much greater for Suwannee River Natural Organic Matter (SRNOM) than that of the sampled riverine DOM, due to its high steady-state concentrations of reactive species. Interestingly, coastal DOM in northern and southern China were found to have similar promotion effects on PIN photodegradation for the first time, but both less than that of riverine DOM. A total of seven degradation products of PIN resulting from hydroxylation, hydrogen abstraction and cleavage of ether bond were identified. Biological toxicity of one products were found to be higher than that of PIN. These results are of significance for knowing the persistence and ecological risk of PIN in natural waters.

Photodegradation of propranolol in surface waters: An important role of carbonate radical and enhancing toxicity phenomenon.

Antihypertensive propranolol (PRO) is frequently detected in surface waters and has adverse effects on aquatic organisms. In this study, its photochemical fate in surface water with the aspect of kinetics, products and toxicity were investigated employing steady-state photochemistry experiments and ecotoxicity tests. The results showed that photodegradation of PRO was enhanced in river water than that in phosphate buffer where dissolved organic matter (DOM), NO3-, and HCO3- played important roles. DOM accelerated the photodegradation mainly through generation of excited triplet-state DOM while NO3- played dual roles in the photodegradation. The reaction between excited triplet-state PRO and HCO3- can generate carbonate radical (CO3·-) to promote the photodegradation. The second-order reaction rate constant between PRO and CO3·- was determined to be (3.4 ± 0.8) × 108 M-1 s-1. Eight photodegradation products were identified in the studied river water sample. Finally, the toxicity evaluated by Vibrio fischeri increased after photodegradation and three photodegradation products were responsible for the increasing toxicity, which was concluded from the significant correlation between toxicity parameters and quantity of the photodegradation products.

Photodegradation of three antidepressants in natural waters: Important roles of dissolved organic matter and nitrate.

Antidepressants have become ubiquitous emerging organic pollutants. Therefore, it is essential to investigate photodegradation of the antidepressants in environment waters for their ecological risk assessment. However, photodegradation behavior of antidepressants varied from different structures and photodegradation mechanism was rarely known for most antidepressants. Herein, citalopram (CIT), paroxetine (PAR) and fluvoxamine (FLUVO) were employed to study the photodegradation behavior of antidepressants in lake water. Results show that direct photolysis of CIT decreased when pH increased from 6 to 9 while the pH effect was not obvious on direct photolysis of FLUVO and PAR. Photodegradation of CIT occurred from its triplet-state and can undergo self-photosensitization through reaction with the generated hydroxyl radical (·OH). In lake water, PAR and FLUVO are degraded mainly via direct photolysis, while CIT is transformed mainly via indirect photolysis. Dissolved organic matter (DOM) and NO3- was proved to be the main factors affecting antidepressants photodegradation in lake water. DOM and NO3- showed inhibition effect on photodegradation of FLUVO and PAR, while promoted CIT degradation. The promotion effect of CIT was mainly through reaction with ·OH and excited triplet-state of DOM while singlet oxygen played a minor role. Based on the photodegradation products identified by MS/MS, the photodegradation pathways were proposed for CIT and PAR, respectively. For FLUVO, one (Z)-isomer degradation product was also found. Understanding the photodegradation behavior of antidepressants is vital for providing data to do ecological risk assessment.

Photodegradation of acebutolol in natural waters: Important roles of carbonate radical and hydroxyl radical.

Acebutolol (ACE) has been widely used for the treatment of cardiovascular disorders, and its photochemical fate in natural waters is a matter of concern due to its ubiquitous occurrence and its toxicity to aquatic organisms. In this study, the photodegradation of ACE in river water and synthetic waters were investigated under simulated sunlight irradiation. The results demonstrated that ACE photodegradation rate in river water was 3.2 times higher than that in pure water. Then the influences of HCO3-, NO3- and DOM on ACE photolysis were investigated under their concentrations similar with the ones in river water. ACE photodegradation was significantly enhanced in the presence of HCO3- alone, and the scavenging experiments and the electron paramagnetic resonance experiments together proved that HCO3- could be oxidized by triplet-excited state of ACE to generate CO3•-, which subsequently played a key role in ACE degradation. The presence of both NO3- and DOM also increased the ACE photodegradation rates, and •OH and 3DOM* were found to be involved in the degradation. In addition, when DOM was added to a solution with HCO3-, the enhancement effect of HCO3- on ACE photodegradation was weakened due to the scavenging of CO3•- by DOM combined with the light screening effect of DOM.

Mechanism of bicarbonate enhancing the photodegradation of ß-blockers in natural waters.

The impact of HCO3- on the photodegradation of ß-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 ß-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.

Dichlorine radicals (Cl2•-) promote the photodegradation of propranolol in estuarine and coastal waters.

Propranolol (PRO) is frequently detected in estuarine and coastal waters, which has adverse effects on estuarine and coastal ecosystems. In this study, the effects of halide ions and DOM from estuarine and coastal waters on the photochemical transformation of PRO were investigated. The results demonstrated that the presence of Br- alone exhibited slight effect on photochemical transformation of PRO, while photodegradation rates of PRO increased with the addition of 0.1-0.54 M Cl-. The quenching experiments and the laser flash photolysis experiments together demonstrated the generation of Cl2•- in the photolytic systems. Cl2•- is possibly produced through the charge separation of exciplex of 3PRO* and Cl- rather than via direct oxidation of Cl-. Additional experiments indicated that addition of seawater DOM inhibited the halide ions-sensitized photodegradation rates of PRO, which may be due to the quenching of Cl2•- by phenolic substances in DOM molecules. Compared with pure water, three new photochemical intermediates were identified in the presence of DOM or Cl-. The direct photolysis of PRO mainly reacted by hydroxyl additions, hydroxyl elimination and de-propylation, whereas electron transfer coupled with H-abstraction by Cl2•- and 3DOM* was proposed as the primary role for PRO degradation in the presence of Cl- or DOM.