Degradation of metoprolol by UV/sulfite as an advanced oxidation or reduction process: The significant role of oxygen

Abstract

The degradation of metoprolol (MTP) by the UV/sulfite with oxygen as an advanced reduction process (ARP) and that without oxygen as an advanced oxidation process (AOP) was comparatively studied herein. The degradation of MTP by both processes followed the first-order rate law with comparable reaction rate constants of $1.50\times {10}^{-3}\text{se}{c}^{-1}$ and $1.20\times {10}^{-3}\text{se}{c}^{-1}$, respectively. Scavenging experiments demonstrated that both $e_{\text{aq}}^{-}$ and $H•$ played a crucial role in MTP degradation by the UV/sulfite as an ARP, while ${\text{SO}}_4^{•-}$ was the dominant oxidant in the UV/sulfite AOP. The degradation kinetics of MTP by the UV/sulfite as an ARP and AOP shared a similar pH dependence with a minimum rate obtained around pH 8. The results could be well explained by the pH impacts on the MTP speciation and sulfite species. Totally six transformation products (TPs) were identified from MTP degradation by the UV/sulfite ARP, and two additional ones were detected in the UV/sulfite AOP. The benzene ring and ether groups of MTP were proposed as the major reactive sites for both processes based on molecular orbital calculations by density functional theory (DFT). The similar degradation products of MTP by the UV/sulfite process as an ARP and AOP indicated that $e_{\text{aq}}^{-}$/$H•$ and ${\text{SO}}_4^{•-}$ might share similar reaction mechanisms, primarily including hydroxylation, dealkylation, and H abstraction. The toxicity of MTP solution treated by the UV/sulfite AOP was calculated to be higher than that in the ARP by the Ecological Structure Activity Relationships (ECOSAR) software, due to the accumulation of TPs with higher toxicity.