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

The role of primary active species (e(cb)(-), h(vb)(+), (OH)-O-center dot, HO2 center dot, O-2(center dot-), and H2O2) during photocatalytic degradation of paracetamol (acetaminophen) using TiO2 catalyst was systematically investigated. Hydroxyl radicals ((OH)-O-center dot) are responsible for the major degradation of paracetamol with a second-order rate constant (1.7 x 10(9) M-1 s(-1)) for an (OH)-O-center dot-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)-O-center dot addition onto the aromatic ring at ortho (predominantly), meta, and para positions with respect to the (OH)-O-center dot position of paracetamol. This initial (OH)-O-center dot hydroxylation is followed by further oxidation generating carboxylic acids. Subsequent mineralization of smaller intermediates eventually increases ammonium and nitrate concentration in the system.