Metal-Site Reducibility Governs C-H Activation and PAH Degradation in Ion-Exchanged Bentonite

Polycyclic aromatic hydrocarbons (PAHs) are persistent soil contaminants commonly associated with terrestrial oil spills and creosote sites, requiring remediation to protect human health and restore environmental quality. Previous work showed that transition-metal cations introduced into clays can catalyze PAH pyrolysis, lowering the temperatures required for thermal remediation and reducing treatment costs. However, the catalytic role of these ion-exchanged metals remains poorly understood. Here, we investigate how transition-metal cations function as active sites for PAH degradation by enhancing adsorption and initiating catalytic pyrolysis. Density functional theory calculations were used to examine PAH adsorption on bentonite surfaces doped with transition-metal cations spanning different regions of the periodic table and exhibiting varied oxidation states and redox flexibility. The results show that site reactivity toward PAHs is governed by metal-site reducibility. Reducible cations such as Fe³⁺ and Cu²⁺ promote pyrolysis by accepting electron density from aromatic π-bonds, facilitating C-H bond activation. Adsorption and activation energies scale linearly with hydrogen adsorption energies on doped bentonite surfaces, indicating that PAH pyrolysis activity correlates directly with site reducibility. Pyrolysis experiments confirm these trends, with Fe³⁺ and Cu²⁺ sites showing higher pyrene removal efficiency than Co²⁺ and Zn²⁺. These findings provide guidance for selecting ion-exchanged cations to enhance catalytic remediation of PAH-contaminated soils.