Oxidative transformation kinetics and pathways of albendazole from reactions with manganese dioxide

Albendazole (ABZ) is a benzimidazole-based veterinary anthelmintic used extensively in the treatment of intestinal parasites. Due to its high hydrophobicity, ABZ tends to accumulate in soils and sediments in the environment. This study aims to investigate ABZ's possible degradation by manganese oxides. Minor effects from ionic strength and metal cations on ABZ degradation were observed. By contrast, decrease of pH greatly enhanced the reaction rate. Surface complexation between ABZ and MnO₂ was indicated to be the dominant control in the reaction kinetics. Suppression by the presence of co-solvents was negatively proportional to the solvent polarities (suppression from high to low: diethyl ether ～ n-butanol > ethanol > methanol > acetonitrile). Humic acid was found to cause significant inhibition due to the reductive dissolution of MnO₂. Four hydrolysis and six oxidative products were identified. ABZ and its hydrolysis products containing the propylthio side chain underwent the same oxidative transformation to form their corresponding sulfoxide compounds. Dehydrogenative coupling reaction between sulfoxide products and hydrolysis products could occur to generate dimers. All hydrolysis and oxidative products were eluted faster than ABZ in liquid chromatogram, suggesting that the spreading out of ABZ will be significantly enhanced if reacting with MnO₂.