Trimethoprim: Kinetic and mechanistic considerations in photochemical environmental fate and AOP treatment

摘要

Trimethoprim (TMP), a bacteriostatic antibiotic, has recently been detected in wastewater and surface waters. In this study the sunlight mediated photochemical fate, and treatment using advanced oxidation and reduction (free radical) processes, have been investigated with respect to their effect on TMP. Photochemical fate, in the presence of humic acid, and advanced oxidation treatment both involve the hydroxyl radical (~·OH) as one of the reactive species of interest. Another reactive oxygen species, singlet oxygen (~1O_2), may also be important in the photochemical fate of TMP. The bimolecular reaction rate constants of TMP with ~1O_2 and ·OH were evaluated to be (3.2 ± 0.2) × 10~6 M~(-1)s~(-1) and 8.66 × 10~9 M~(-1) s~(-1) respectively. The reaction kinetics for the sub-structural moieties of TMP, 1,2,3-trimethoxybenzene (TMBz) and 2,4-diaminoprimidine (DAP), was evaluated to facilitate an understanding of the loss mechanisms. For TMBz and DAP the reaction rate constants with ~1O_2 were <1.0 × 10~4 and (3.0 ± 0.1) × 10~6 M~(-1) s~(-1) while with ·OH they were 8.12 ×10~9 and 1.64 × 10~9 M~(-1) s~(-1), respectively. The data suggests that the ~1O_2 attacks the DAP and the •OH radical attacks the TMBz moiety. However, for TMP, ~1O_2 and ~·OH reactions accounted for only ～19% and ～6%, of its total photodegradation, respectively. Therefore, the reaction of TMP with excited state natural organic matter is postulated as a significant degradation pathway for the loss of TMP in sunlit waters containing natural organic matter. There was no effect of pH on the direct or indirect photolysis of TMP. To complete the study for reductive treatment processes, the solvated electron reaction rates for the destruction of TMP, TMBz and DAP were also evaluated. The absolute bimolecular reaction rates obtained were, (13.6 ± 0.01) × 10~9, (6.36 ± 0.11) × 10~7 and (10.1 ± 0.01) × 10~9 M~(-1) s~(-1) respectively.