The role of bicarbonate anions in methotrexate degradation via UV/TiO2: 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 CO3-. The estimated second-order rate constants of methotrexate with CO3- and OH were 1.4 × 107 M-1 s-1 and 8.7 × 109 M-1 s-1, respectively. Both the valence hole (hvb+) and OH resulted in the generation of CO3-. Initial transformation pathways of methotrexate were proposed, including the addition of atomic oxygen, hydroxylation, deamination, CC cleavage and CN cleavage. CN cleavage at the aniline moiety (the N(13) position) is the primary decomposition pathway, leading to an aminopterin yield of 43%. 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/TiO2 system. 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 OH-based advanced oxidation processes in wastewater containing pharmaceutical cocktails with ABZ moieties.