Comparative evaluation of metoprolol degradation by UV/chlorine and UV/H2O2 processes.

The degradation of metoprolol (MTP), a β-blocker commonly used for cardiovascular diseases, by UV/chlorine and UV/H2O2 processes was comparatively evaluated. MTP direct photolysis at 254 nm could be neglected, but remarkable MTP degradation was observed in both the UV/chlorine and UV/H2O2 systems. Compared with UV/H2O2, UV/chlorine has a more pronounced MTP degradation efficiency. In addition to primary radicals (OH and Cl), secondary radicals (ClO and Cl2-) played a pivotal role in degrading MTP by UV/chlorine process. The relative contributions of hydroxyl radicals (OH) and reactive chlorine species (RCS) in the UV/chlorine system varied at different solution pH values (i.e., the contribution of RCS increased from 57.7% to 75.1% as the pH increased from 6 to 8). The degradation rate rose as the oxidant dosage increased in the UV/chlorine and UV/H2O2 processes. The presence of Cl- slightly affected MTP degradation in both processes, while the existence of HCO3- and HA inhibited MTP degradation to different extents in both processes. In terms of the overall cost of electrical energy, UV/chlorine is more cost efficient than UV/H2O2. The degradation products during the two processes were identified and compared, and the degradation pathways were proposed accordingly. Compared with the direct chlorination of MTP, pre-oxidation with UV/chlorine and UV/H2O2 significantly enhanced the formation of commonly known DBPs. Therefore, when using UV/chlorine and UV/H2O2 in real waters to remove organic pollutants, the possible risk of enhanced DBP formation resulting from the degradation of certain pollutants during post-chlorination should be carefully considered.