Photolysis and photocatalytic decomposition of sulfamethazine antibiotics in an aqueous solution with TiO₂

Photo-decomposition of sulfamethazine (SMT) involves photolytic and photo-catalytic reactions, which occur simultaneously. The relative contributions of these two reactions to the overall SMT photo-decompositions by TiO₂ and the intermediates of SMT photo-decompositions were systematically examined with the effects of TiO₂ loading, and the pH and the initial SMT concentrations in the solutions. The apparent rate constants of SMT photo-decomposition reactions, which were well described by the pseudo-first-order kinetic model, ranged from 0.24 to 1.61 h^-1. The overall photo-decomposition efficiencies of 0.072 mM SMT were the highest at pH 5.5 with 0.5 g L^-1 TiO₂ due to the adsorption-induced photocatalytic decomposition of SMT on TiO₂. However, the SMT photolysis occurred more rapidly at pH 10. Two reactive species of holes and hydroxyl radicals concurrently participated in the photocatalytic decomposition of SMT, and the latter dominated the oxidative reactions of SMT on TiO₂. Eight intermediates of SMT photo-decomposition were determined using LC-MS. Their time-dependent distributions indicated that the photo-decomposition of SMT was triggered by hydroxylation on aniline and dimethylpyrimidinyl moieties, followed by the cleavage of the S-N bond of SMT. Our results illustrated that the intermediates with dimethylpyrimidinyl groups possess strong resistance to photo-decomposition and played a determinant role in the photo-decomposition of SMT.