The role of bicarbonate anions in methotrexate degradation via UV/TiO<inf>2</inf>: Mechanisms, reactivity and increased toxicity

Bicarbonate anion (HCO3 −) 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/TiO2. A comprehensive investigation of reaction mechanisms was performed by conducting scavenger experiments and substructure reactivity and Microtox®toxicity tests. In the presence of HCO3 −, the methotrexate degradation rate substantially increased, indicating the involvement of [rad]CO3 −. The estimated second-order rate constants of methotrexate with [rad]CO3 −and [rad]OH were 1.4 × 107 M−1 s−1and 8.7 × 109 M−1 s−1, respectively. Both the valence hole (hvb +) and [rad]OH resulted in the generation of [rad]CO3 −. 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]CO3 −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/TiO2system. The reactivity of the three methotrexate substructures decreased in the following order in the presence of HCO3 −: ABZ ≫ DHP ≫ LG∼0; however, without HCO3 −, the following order was observed: ABZ ∼ DHP > LG. The results of this work suggest that the increase in toxicity induced by the presence of HCO3 −likely occurs in many other [rad]OH-based advanced oxidation processes in wastewater containing pharmaceutical cocktails with ABZ moieties. © 2017 Elsevier Ltd