Hydroxyl radical-initiated degradation kinetics of organic pollutants in surfactant-rich environments

Abstract

Surfactant-rich aqueous media are common in natural environments. The sea surface microlayer and sea spray droplets are good examples and are also frequently markedly enriched in organic pollutants. This study focuses on the degradation kinetics of organic pollutants initiated by the hydroxyl radical in such surfactant-rich environments. The apparent second-order rate constants of the reaction of the hydroxyl radical with two model pollutants, carbamazepine and phenanthrene, were determined in the presence of sodium dodecyl sulfate or rhamnolipids at concentrations below and above their critical micelle concentration (CMC). The results show that the apparent rate constant remains unaffected below the CMC, unless additional reactive processes are induced. From the results obtained above the CMC, second-order rate constants were derived for the reaction of the hydroxyl radical with the pollutant trapped in the surfactant micelles. A rather similar decrease of about a factor of 3 in comparison to pre-micellar conditions was observed for all three experimental conditions tested. This work appears to provide a suitable foundation for evaluating the impact of such surfactant-rich environments on the half-lives and fates of organic pollutants and a preliminary indication of how reaction kinetics may be modified in various organized surfactant structures.