Effect of Anions on the Photochemical Oxidation of Amoxicillin in Aqueous Solutions

The effect of anions commonly found in natural and wastewaters (Cl–, NO₃⁻ , SO₄^2 - , HCO₃⁻ ) on the oxidative degradation efficiency of amoxicillin and the mineralization of total organic carbon (TOC) in the {UV-C/S2O ₈^2 - } and {UV-C/S2O ₈^2 - /Fe2+} systems activated by UV radiation from a KrCl* excilamp (222 nm) has been investigated. It has been established that chlorides (1–10 mM) promote the degradation and mineralization of the target compound in both systems. However, in the {UV-C/S2O ₈^2 - } system at low chloride concentrations (1 mM), the formation and accumulation of toxic intermediates are observed. Sulfates exhibit a pronounced promoting effect. Nitrates inhibit the oxidation process in the {UV/S2O ₈^2 - } system, whereas in the Fenton-like oxidative system {UV/S2O ₈^2 - /Fe2+}, a promoting effect is observed at low concentrations (1 mM). Bicarbonates exhibit a significant inhibitory effect in the {UV-C/S2O ₈^2 - /Fe2+} system due to the deactivation of Fe2+. In a model mixture of anions and in tap water, the efficiency of the processes, especially in the {UV-C/S2O ₈^2 - /Fe2+} system, is substantially reduced; however, the preliminary removal of bicarbonates allows it to be restored. The results demonstrate the critical dependence of the oxidative degradation efficiency of amoxicillin on the composition of the aqueous matrix and the necessity of accounting for this factor in practical implementation.