Search

This study explores the potential contribution of secondary production of OH radicals in aerosols and cloud/fog conditions arising from brown carbon (BrC) triplet state chemistry. For this purpose, extracts of brown carbon from atmospheric aerosols from Grenoble, France, were analyzed for their ability to produce triplet states from the degradation of a common triplet state probe, 2,4,6-trimethylphenol (TMP). This ability of brown carbon to produce triplet states was compared to that of three photosensitizers, where it was found that vanillin (VL) showed a similar rate of degradation of the probe and was hence chosen as an alternative to BrC in aqueous aerosols to investigate OH formation from triplet states. The rates of OH formation from the triplet states were compared to those from nitrate anions (NO3−) and hydrogen peroxide (H2O2), which are well-known sources of OH radicals in the aqueous phase, and a species that is structurally similar to VL, 4-hydroxybenzaldehyde (4HB). VL and 4HB both showed a 1–2 orders of magnitude higher rate of secondary OH formation than NO3−, while it was similar or one order of magnitude smaller than H2O2. To evaluate the influence of the different OH radical sources in aqueous aerosols and cloud/fog conditions, the concentrations of the species were summarized from the literature. Considering the concentrations of HULISs in aerosols, the rates of secondary OH formation from BrC triplet states could potentially represent a significant source of OH in the atmospheric aqueous phase under some circumstances. This study shows the relevance of further investigations into the role of triplet states in impacting atmospheric oxidative capacity and studying other effects of triplet states in aerosols, a field that is, until now, still not fully understood.

While the use of transition metal oxides in catalyzing advanced oxidation reactions has been widely investigated, very few reports have focused on how the preliminary contact of oxides with target compounds may affect the succession of reaction. In this study, we examined the adsorption and electron transfer reactions of two fluoroquinolones, flumequine (FLU), and norfloxacin (NOR), with goethite (α-FeOOH) or manganese (Mn) oxide, and their impact on the subsequent mineralization of target compounds using H2O2or S2O82–under UVA irradiation. Intriguingly, higher total organic carbon (TOC) removal was achieved when antibiotics and metal oxides were allowed for preequilibration before starting the oxidation reaction. The rate and extent of TOC removal are strongly dependent on the molecule structure and the redox-active mineral used, and much less on the preequilibration time. This high efficiency can be ascribed to the presence of reduced metal ions, chemically or photochemically generated during the first stage, onto oxide minerals. Oxide-bound MnIIplays a crucial role in catalyzing oxidant decomposition and then producing greater amounts of radical species through a photoassisted redox cycle, regardless of the underlying surface, MnIVO2or MnIIIOOH. This finding will be of fundamental and practical significance to Mn-based oxidation reactions and wastewater treatment processes.