Catechol degradation on hematite/silica–gas interface as affected by gas composition and the formation of environmentally persistent free radicals

Environmentally persistent free radicals (EPFRs) formed on a solid particle surface have received increasing attention because of their toxic effects. However, organic chemical fate regulated by EPFRs has rarely been investigated, and this information may provide the missing link in understanding their environmental behavior. Previous studies have suggested that the reduction of transition metals is involved in EPFRs formation. We thus hypothesize that an oxidative environment may inhibit EPFRs formation in particle-gas interface, which will consequently release free radicals and accelerate organic chemical degradation. Our result indicates that a 1% hematite coating on a silica surface inhibited catechol degradation in N(2), especially at low catechol loadings on solid particles (S(CT)). However, under an O(2) environment, catechol degradation decreased when S(CT) was <1 μg/mg but increased when S(CT) was >1 μg/mg. Stable organic free radicals were observed in the N(2) system with g factors in the 2.0035–2.0050 range, suggesting the dominance of oxygen-centered free radicals. The introduction of O(2) into the catechol degradation system substantially decreased the free radical signals and decreased the Fe(II) content. These results were observed in both dark and light irradiation systems, indicating the ubiquitous presence of EPFRs in regulating the fate of organic chemicals.