The influence of particle size and structure on the sorption and oxidation behaviour of birnessite: II. Adsorption and oxidation of four polycyclic aromatic hydrocarbons

Environmental context Sorption and oxidation reactions at mineral surfaces can substantially influence the mobility and toxicity of environmental contaminants. An understanding of the factors that control these reactions is crucial for predicting the fate of contaminant species. We investigate the reactivity of manganese oxides towards polycyclic aromatic hydrocarbons, persistent organic compounds of environmental concern. Abstract Birnessites are ubiquitous components of natural systems and may exert a significant influence on the mobility and toxicity of different types of contaminants, including organic species. Their small particle sizes and internal structure provide them with high sorption capacities and oxidising abilities for redox sensitive species. In the present work, the interactions of two MnIV birnessites (δ-MnO2 and acid birnessite) of different particle sizes and layer vacancy contents were investigated with four hydrophobic polycyclic aromatic hydrocarbons (PAHs) of three and four rings. Fluorene and anthracene were oxidised to produce the corresponding and less toxic quinones by both birnessites, but at a higher rate and extent by δ-MnO2. Phenanthrene and fluoranthene only adsorbed to δ-MnO2 but not to acid birnessite. The higher reactivity of δ-MnO2 is only partly explained by its higher specific surface area (114 v. 39m2g–1), i.e. by its smaller particle size. The repulsive effect of water molecules from hydrated cations sorbed on layer vacant sites is most likely decisive, because acid birnessite shows a considerably larger content of these vacancies. The results presented provide a fundamental understanding of the potential influence of birnessite minerals on the attenuation of low molecular weight PAHs in environments with low organic matter content, such as deep aquifers.