Deciphering the transformation mechanism of substituted polycyclic aromatic hydrocarbons on Al(III)-montmorillonite: An experimental and density functional theory study

Highlights: • Anthracene with electron-donating group has higher degradation rate. • Electron transfer occurs between anthracene derivatives and clay surface. • OH and O₂⁻ are associated with anthracene transformation. • The reactivity of PAH derivatives is characterized by DFT calculations. The researches on transformation of polycyclic aromatic hydrocarbons (PAHs) on clay minerals modified by metal ions have received increasing attention. However, the transformation of PAHs with electron-withdrawing or electron-donating substitutional groups on clay minerals is not well understood currently. In this study, the degradation of anthracene (ANT) with different substituents (including -CH₃, -CHO, -Br, -OMe, and -NO₂) on Al(III)-montmorillonite (MMT) was investigated in the dark. The results showed that aromatic compounds were degraded with the rate constants (k_obs) of 0.004–0.141 d⁻¹. Moreover, ANT with electron-donating substituents (e.g., -CH₃, -OMe) had a higher transformation rate than that with electron-withdrawing substituents (e.g., -Br, -NO₂). The reactive oxygen species (ROS) quenching experiments indicated that ROS played a significant role in the transformation of ANT and ANT derivatives. Density functional theory (DFT) calculations revealed that the reactivity of single substituted PAHs was highly correlated with their ionization potential (IP), the energy of highest occupied molecular orbital (E_HOMO), the energy of lowest unoccupied molecular orbital (E_LUMO), and electronegativity (ζ), while independent of hardness (η). This study provides novel insights into predicting the reactivity of PAHs derivatives, and lays a fundamental basis for better understanding the fate of substituted PAHs in soils.