Photocatalytic Oxidation of Paracetamol: Dominant Reactants, Intermediates, and Reaction Mechanisms

The role of primary active species (ecb −, hvb +, •OH, HO2 •, O2 • −, and H2O2) during photocatalytic degradation of paracetamol (acetaminophen) using TiO2 catalyst was systematically investigated. Hydroxyl radicals (•OH) are responsible for the major degradation of paracetamol with a second-order rate constant (1.7 × 109 M−1 s−1) for an •OH−paracetamol reaction. A total of 13 intermediates was identified and classified into four categories: (i) aromatic compounds, (ii) carboxylic acids, (iii) nitrogen-containing straight chain compounds, and (iv) inorganic species (ammonium and nitrate ions). Concentration profiles of identified intermediates indicate that paracetamol initially undergoes hydroxylation through •OH addition onto the aromatic ring at ortho (predominantly), meta, and para positions with respect to the −OH position of paracetamol. This initial •OH hydroxylation is followed by further oxidation generating carboxylic acids. Subsequent mineralization of smaller intermediates eventually increases ammonium and nitrate concentration in the system.