Kinetic and mechanistic aspects of hydroxyl radical‒mediated degradation of naproxen and reaction intermediates

Abstract Hydroxyl radical ( • OH) based advanced oxidation technologies (AOTs) are effective for removing non‒steroidal anti−inflammatory drugs (NSAIDs) during water treatment. In this study, we systematically investigated the degradation kinetics of naproxen (NAP), a representative NSAID, with a combination of experimental and theoretical approaches. The second−order rate constant ( k ) of • OH oxidation of NAP was measured to be (4.32 ± 0.04) × 10 9  M −1  s −1 , which was in a reasonable agreement with transition state theory calculated k value (1.08 × 10 9  M −1  s −1 ) at SMD/M05–2X/6–311++G**//M05–2X/6–31+G** level of theory. The calculated result revealed that the dominant reaction intermediate is 2‒(5‒hydroxy‒6‒methoxynaphthalen‒2‒yl)propanoic acid (HMNPA) formed via radical adduct formation pathway, in which • OH addition onto the ortho site of the methoxy−substituted benzene ring is the most favorable pathway for the NAP oxidation. We further investigated the subsequent • OH oxidation of HMNPA via a kinetic modelling technique. The k value of the reaction of HMNPA and • OH was determined to be 2.22 × 10 9  M −1  s −1 , exhibiting a similar reactivity to the parent NAP. This is the first study on the kinetic and mechanistic aspects of NAP and its reaction intermediates. The current results are valuable in future study evaluating and extending the application of • OH based AOTs to degrade NAP and other NSAIDs of concern in water treatment plants.