Oxidation of β-blockers by birnessite: Kinetics, mechanism and effect of metal ions

Manganese dioxides are ubiquitous in natural waters, soils, and sediments and play an important role in oxidative transformation of organic pollutants. This work presents the kinetics of the oxidation of selected β-blockers, betaxolol, metoprolol, and atenolol by birnessite (δ-MnO2) as a function of concentration of the β-blocker, dosage of δ-MnO2, and solution pH. The values of pseudo-first-order rate constants (kobs) of β-blockers decreased in the order betaxolol > atenolol > metoprolol, which was positively correlated with their acid dissociation constants (Ka). Effect of series of metal ions (Fe3+, Cr3+, Al3+, Pb2+, Cu2+, Zn2+, Ni2+, Cd2+, Mg2+, and Ca2+) on the degradation of β-blockers by δ-MnO2 was systematically examined. All of these metal ions inhibited the oxidation reaction under the same constant ionic strength. The inhibition efficiency was positively correlated with the logarithm of stability constant of metal ions in aqueous solution (logKMeOH). By LC-ESI-MS/MS analyses, the oxidation of β-blockers primarily involved hydroxylation and cleavage of the parent molecules to the short branched chain compounds. An electron transfer mechanism for the oxidation of β-blockers by δ-MnO2 was proposed. The oxidation was initiated by the electron transfer from the nonbonding electrons on nitrogen (N-electrons) of β-blockers to δ-MnO2, followed by transformation of radical intermediates. These findings will help to understand the oxidation processes of β-blockers and predict the effect of metal ions on the removal of pollutants by δ-MnO2 in the environment.