Significant changes in the photo-reactivity of TiO2 in the presence of a capped natural dissolved organic matter layer

Natural dissolved organic matter (NDOM) in surface waters has a high sorption affinity for TiO2 during long contact. An attached NDOM layer can act as a conduction band electron and/or valance band hole acceptor, and NDOM can also decrease the concentration of hydroxyl radicals ((OH)-O-center dot) in the bulk phase. Therefore, the degradation kinetics and mechanism for degradation of acetaminophen on NDOM capped TiO2 (NDOM-TiO2) are significantly different from those on raw TiO2. Quantum calculation results suggest that hydroxylation to the ortho position in relation to the acetamide group is more favorable. Although (OH)-O-center dot induced hydroxylation is the predominant pathway for degradation of acetaminophen on TiO2, one-electron oxidation of acetaminophen by a valance band hole, excited triplet NDOM or NDOM radical cation is the major degradation pathway on NDOM TiO2. This study is the first to detect and confirm APAP oligomers as intermediates during the degradation of acetaminophen by TiO2 photo catalysis, especially when using NDOM TiO2 as a catalyst. The results suggest the reactivity of TiO2 could change significantly after long exposure to natural water, which need to be concerned about for removal of micropollutants in surface water by TiO2 photocatalysis. (C) 2016 Elsevier Ltd. All rights reserved.