Efficient photodegradation of phenanthrene under visible light irradiation via photosensitized electron transfer

Photoinduced electron transfer between transition metal ions and polycyclic aromatic hydrocarbons (PAHs) was utilized to induce the photodegradation of phenanthrene (PHE), a tricyclic aromatic hydrocarbon, under visible light irradiation (λ > 420 nm). Experimental results indicate that Fe ³⁺ ions possess excellent ability of sensitization in the photodegradation of PHE. In solution, PHE could be oxidated completely by oxygen after 3 h irradiation. The intermediates of PHE photodegradation were detected by gas chromatography-mass spectrometer. Eleven kinds of intermediates were determined, including one decomposition product, five ring-opening products, and other oxidation derivatives. Phenanthrenequinone and (1,1′-biphenyl)-2, 2′-dicarboxaldehyde were the two main products. Based on the experiment results, a possible mechanism was proposed. In alcohols and acetic acid, the photodegradation efficiency of PHE was low, probably because that active PHE⁺ was quenched in these solvents. The impacts of variable factors including solvent component, irradiation wavelength, presence or lack of oxygen, and reaction time on the photodegradation of PHE were investigated in detail. In addition, the geometry optimization of the complex of PHE and Fe³⁺ ions and the calculation of the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap were carried out using the B3LYP functional and 6-31 G* basis set in the Gaussian 98 program.