Environmental photochemistry of fenamate NSAIDs and their radical intermediates

Fenamates are a class of nonsteroidal anti-inflammatory drugs (NSAIDs) that are not fully removed during wastewater treatment and can be released to surface waters. Here, near-surface photochemical half-lives were evaluated to range from minutes to hours of four fenamates and the closely related diclofenac. While quantum yields for direct photochemical reactions at the water surface vary widely from 0.071 for diclofenac to <0.001 for mefenamic acid, all fenamates showed significant reactivity towards singlet oxygen and hydroxyl radical with bimolecular reaction rate constants of 1.3-2.8 × 10⁷ M^-1 s^-1 and 1.1-2.7 × 10¹⁰ M^-1 s^-1, respectively. Photodecay rates increased in the presence of dissolved organic matter (DOM) for diclofenac (+19%), tolfenamic acid (+9%), and mefenamic acid (+95%), but decreased for flufenamic acid (-2%) and meclofenamic acid (-14%) after accounting for light screening effects. Fast reaction rate constants of all NSAIDs with model triplet sensitizers were quantified by laser flash photolysis. Here, the direct observation of diphenylamine radical intermediates by transient absorption spectroscopy demonstrates one-electron oxidation of all fenamates. Quenching rate constants of these radical intermediates by ascorbic acid, a model antioxidant, were also quantified. These observations suggest that the balance of oxidation by photoexcited triplet DOM and quenching of the formed radical intermediates by antioxidant moieties determines whether net sensitization or net quenching by DOM occurs in the photochemical degradation of fenamates.