Use of δ-manganese dioxide for the removal of acetaminophen from aquatic environment: Kinetic - thermodynamic analysis and transformation products identification

High delta-manganese dioxide's initial concentration (800 mg/L), temperature (50 degrees C) and acidic pH (3.0-3.5) were found to be the optimum conditions for the removal of acetaminophen. Additionally, acetaminophen was found to be removed more rapidly at high initial concentrations (70-100 mg/L). The kinetic analysis of the experiments revealed that the two tested kinetic models, the one assuming the electron transfer and the other assuming the complex formation as the rate-limiting step, provided a good fit. The positive value of activation energy estimated by the Arrhenius equation indicated that the reaction is endothermic. The transformation products analysis by means of RPLC-QToF, revealed the formation of acetaminophen's dimer and trimer in the early stages of the reaction, followed by their complete removal in the latter stages of the reaction. Furthermore, four new compounds were detected. Three isomeric compounds fit the formulas C14H11NO4, attributed to aggregates of 1,4-benzoquinone and N-Acetyl-p-benzoquinone Imine, and one additional transformation product was attributed to C17H8N3O5. The abundance of the isomeric compounds was increasing after 60 min of the reaction, and decreasing during the last stages of the reaction, while the latter's abundance was increasing during the reaction process.