Transformation of Polycyclic Aromatic Hydrocarbons and Formation of Environmentally Persistent Free Radicals on Modified Montmorillonite: The Role of Surface Metal Ions and Polycyclic Aromatic Hydrocarbon Molecular Properties

This paper presents the transformation of PAHs (phenanthrene, anthracene, benzo[a]anthracene, pyrene, and benzo[a]pyrene) on montmorillonite clays that are modified by transition-metal ions [Fe(III), Cu(II), Ni(II), Co(II), or Zn(II)] at room temperature (∼23 °C). The decay of these PAHs follows first-order kinetics, and the dependence of the observed rate constants ( k obs, day –1 ) on the presence of metal ions follows the order Fe(III) > Cu(II) > Ni(II) > Co(II) > Zn(II). The values of k obs show reasonable linear relationships with the oxidation potentials of the PAHs and the redox potentials of the metal ions. Notably, transformation of these PAHs results in the formation of environmentally persistent free radicals (EPFRs), which are of major concern due to their adverse effects on human health. The potential energy surface (PES) calculations using density functional theory were performed to understand the trends in k obs and the plausible mechanisms for radical formation from the PAHs on modified clays. The yields and stability of these EPFRs from anthracene and benzo[a]pyrene on clay surfaces varies with both the parent PAH and the metal ion. The results demonstrated the potential role of metals in the formation and fate of PAH-induced EPFR at co-contaminated sites.