Photodegradation of selected β-blockers in aqueous fulvic acid solutions: Kinetics, mechanism, and product analysis
Graphical abstract
Highlights
► Selected β-blockers undergo photosensitized degradation in aqueous FA solutions. ► Electron transfer interaction between β-blockers and 3FA∗ occurs. ► FA triplet quenching constants by substrates are calculated. ► Magnetic property of metal ions exhibited significant impact on photodegradation. ► Deisopropyl-blockers are main photosensitized products.
Introduction
Pharmaceuticals and personal care products (PPCPs) are the subject of increasing concern and scientific interest (Halling-Sørensen et al., 1998, Kolpin et al., 2002). As an important subclass of PPCPs, β-blockers are extensively used for the treatment of hypertension and cardiac arrhythmias (Liu and Williams, 2007). β-blockers cannot be totally eliminated in sewage treatment plants and they have been widely detected in surface waters (Hugger et al., 2003, Ashton et al., 2004, Kim and Aga, 2007). The presence of β-blockers is detrimental to aquatic organisms (Huggett et al., 2002). It has been demonstrated that most β-blockers had a specific toxicity towards the green algae Desmodesmus subspicatus (Cleuvers, 2005, Fent et al., 2006, Escher et al., 2006). Therefore, knowledge of their environmental fate is essential before an assessment of their ecological risk.
β-blockers undergo direct photodegradation under simulated sunlight in aqueous solution (Liu and Williams, 2007). Nevertheless, the quantum yields of direct photodegradation of the β-blockers are very low. Humic substances (HS) are ubiquitous in natural waters and exhibit a significant impact on indirect photodegradation of organic pollutants (Boreen et al., 2003, Boreen et al., 2005, Canonica et al., 2005, Guerard et al., 2009, Wenk et al., 2011, Caupos et al., 2011, Jacobs et al., 2011, Razavi et al., 2011, Xu et al., 2011). It is well known that nonselective hydroxyl radicals (•OH) are photochemically formed in HS solutions (Vaughan and Blough, 1998), which limit the persistence of many pollutants that are degraded relatively slowly by direct photolysis. In addition, photolysis of HS leads to the formation of other reactive species, including 1O2 (Haag and Hoigné, 1986), O2•−/HO2• (Goldstone and Voelker, 2000), H2O2 (Draper and Crosby, 1983), eaq− (Zepp et al., 1987), and triplet excited states of HS (3HS∗), of which O2•−/HO2•, H2O2, and eaq− may play a minor role in the sunlit surface waters (Boule et al., 1999).
In our previous study, it was found that the direct reaction between 3HS∗ and amine drugs occurs via electron transfer path (Chen et al., 2009). While numerous pharmaceuticals are nitrogen-containing compounds, very limited information is available on the photosensitized degradation mediated by 3HS∗. Metal ions are ubiquitous in natural waters. The metal-ion complexation with HS may affect the indirect photodegradation of substrates. It is documented that certain metal ions are capable of quenching the triplet states of organic compounds (Linschitz and Pekkarinen, 1960). However, until recently few studies are focused on the role of metal ions in photosensitized degradation of HS (Lang et al., 1997, Kamiya and Kameyama, 2001).
This study, therefore, will investigate the indirect photodegradation of amine drugs in the presence of FA. Two β-blockers atenolol and metoprolol were chosen as model compounds. The quenching experiments involving reactive oxygen species (ROS) were performed to probe the reaction mechanism. A kinetic model is developed to calculate the reaction constants between β-blockers and 3FA∗. The effects of metal ions on the photosensitized degradation of atenolol and metoprolol were systematically examined. The photoproducts were determined by LC-ESI-MS/MS and the degradation pathway was proposed.
Section snippets
Chemicals
Atenolol (99%) and metoprolol tartrate salt (≥98%) were purchased from TCI (Japan) and Sigma (USA), respectively. Fulvic acid (C, 51.04%; H, 5.22%; O, 39.13%; N, 4.62%) was extracted from weathered coal and obtained from Henan ChangSheng Corporation. 2-propanol, NaN3, NaCl, CaCl2, ZnCl2, MgCl2⋅6H2O, MnCl2, CuCl2⋅2H2O, FeCl3⋅6H2O, CrCl3⋅6H2O, and AlCl3⋅6H2O were supplied by Wuhan chemicals corporation. All chemicals used were of at least analytical-reagent grade.
Photolysis experiments
The photolysis experiments were
Indirect photodegradation
Direct photolysis of atenolol and metoprolol hardly occurred under simulated sunlight in this experiment since their predominant absorption wavelengths are lower than 300 nm (Fig. 1 and Fig. S2). Therefore, indirect photodegradation was investigated in the presence of FA. As shown in Table 1, both atenolol and metoprolol were degraded in FA solutions under simulated sunlight. The pseudo-first-order rate constants for the photodegradation of atenolol and metoprolol in air-saturated FA solutions
Conclusion
Atenolol and metoprolol undergo photosensitized degradation in aqueous FA solutions under simulated sunlight. The indirect photodegradation increased with increasing pH within the range of 6.0–10.0. Magnetic property of metal ions was first found to exhibit significant impact on photosensitized degradation in FA solutions. Diamagnetic metal ions including Mg2+, Ca2+, Zn2+, and Al3+ did not affect on the degradation of atenolol and metoprolol. In contrast, the paramagnetic metal ions markedly
Acknowledgements
This work was supported by the National Natural Science Foundation of China (21007018 and 51078161), Natural Science Foundation of Hubei Province (2010CDB01104), Chenguang Youth Found of Wuhan (201050231074), and Research Fund for the Doctoral Program of Higher Education of China (20100142120004).
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