Phototransformation of mefenamic acid induced by nitrite ions in water: mechanism, toxicity, and degradation pathways.

Here, we evidenced the photo-induced degradation of mefenamic acid, a nonsteroidal anti-inflammatory drug, through the 254-nm light excitation of nitrite. The results demonstrated that the photodegradation of mefenamic acid was enhanced, and the mefenamic acid photodegradation rate significantly increased, from 0.00627 to 0.0350 min(-1) as the nitrite was increased from 0 to 0.5 mmol L(-1). The photodegradation rate increased from 0.0287 to 0.0512 min(-1) as the pH was elevated, from 5.0 to 10.0. The actual second-order rate constant for the reaction of mefenamic acid with ·OH was investigated to 1.079 × 10(10) M(-1) s(-1) according to steady-state ·OH concentration of 3.5 × 10(-14) mmol L(-1) and the contribution to the rate of ·OH of 67.1%. The photoproducts were identified using HPLC/MS/MS, and possible nitrite-induced photodegradation pathways were proposed by hydroxylation, dehydrogenation, hydration, nitrosylation, and ketonized reactions. The toxicity of the phototransformation products was evaluated using the Microtox test, which revealed that the photoproducts were more toxic than mefenamic acid for the generation of nitrosation aromatic compounds.