摘要
Bicarbonate anion (HCO <sub>3</sub> <sup>−</sup> ) is a major constituent in wastewater and natural water matrices, and the aim of this study was to investigate its roles in the degradation of the antineoplastic agent methotrexate via UV/TiO <sub>2</sub> . A comprehensive investigation of reaction mechanisms was performed by conducting scavenger experiments and substructure reactivity and Microtox <sup>®</sup> toxicity tests. In the presence of HCO <sub>3</sub> <sup>−</sup> , the methotrexate degradation rate substantially increased, indicating the involvement of [rad]CO <sub>3</sub> <sup>−</sup> . The estimated second-order rate constants of methotrexate with [rad]CO <sub>3</sub> <sup>−</sup> and [rad]OH were 1.4 × 10 <sup>7</sup> M <sup>−1</sup> s <sup>−1</sup> and 8.7 × 10 <sup>9</sup> M <sup>−1</sup> s <sup>−1</sup> , respectively. Both the valence hole (h <sub>vb</sub> <sup>+</sup> ) and [rad]OH resulted in the generation of [rad]CO <sub>3</sub> <sup>−</sup> . Initial transformation pathways of methotrexate were proposed, including the addition of atomic oxygen, hydroxylation, deamination, C[sbnd]C cleavage and C[sbnd]N cleavage. C[sbnd]N cleavage at the aniline moiety (the N(13) position) is the primary decomposition pathway, leading to an aminopterin yield of 43%. [rad]CO <sub>3</sub> <sup>−</sup> preferentially reacted with the 4-aminobenzamide (ABZ) moiety and generated toxic byproducts during the later stages of decomposition, which was not observed in the UV/TiO <sub>2</sub> system. The reactivity of the three methotrexate substructures decreased in the following order in the presence of HCO <sub>3</sub> <sup>−</sup> : ABZ ≫ DHP ≫ LG∼0; however, without HCO <sub>3</sub> <sup>−</sup> , the following order was observed: ABZ ∼ DHP > LG. The results of this work suggest that the increase in toxicity induced by the presence of HCO <sub>3</sub> <sup>−</sup> likely occurs in many other [rad]OH-based advanced oxidation processes in wastewater containing pharmaceutical cocktails with ABZ moieties.