(452h) Mechanistic Insights into the Catalytic Removal of Polyaromatic Hydrocarbons (PAHs) from Contaminated Soils

Polycyclic-aromatic-hydrocarbons (PAHs) are common toxic soil contaminants that pose significant risks to public and environmental health. We recently demonstrated that bentonite containing Fe³⁺ or Cu²⁺ cations introduced by ion exchange is an effective catalyst for pyrolyzing pyrene (a 4-ring PAH) at the unprecedented low temperature of 150 ^oC) [S. B. Denison et al., Environ. Sci. Technol., vol. 57, no. 38, pp. 14373–14383, Sep. 2023]. Using density functional theory (DFT), we show that the strong adsorption of pyrene on the Fe³⁺ or Cu²⁺ cation sites is attributed to favorable electron transfer from the highest occupied molecular orbital of pyrene (pi bonds) to the cation site. Accordingly, Fe³⁺ and Cu²⁺ cations serve as reduction centers during the pyrolytic process, facilitating the initial activation of the C-H bonds in the PAH molecule at the onset of pyrolysis. Our calculations demonstrate that the oxidation state of the cation site is critical, as the cation must be reducible to facilitate electron transfer from the PAH. For example, ion-exchanged bentonite with Fe²⁺or Zn²⁺ is ineffective because Fe⁺ and Zn⁺ oxidation states are unstable. Thus, the redox behavior of the cation site dictates reactivity toward PAHs and is a key descriptor for designing active catalysts for PAH pyrolysis. Our research elucidates the PAH pyrolysis reaction mechanism on ion-exchanged clay catalysts that significantly decrease energy and contact time requirements for effective soil remediation, thus contributing to the advancement of sustainable solutions for environmental cleanup.