Photochemical oxidation of phenanthrene sorbed on silica gel

There have been relatively few detailed studies of PAH photochemical degradation mechanisms and products at solid/air interfaces under controlled conditions. Results from mechanistic studies on particulate simulants are important in understanding the fates of PAH sorbed on similar materials in natural settings. In this study, the photolysis of phenanthrene (PH) on silica gel, in the presence of air, has been carefully examined. Once sorbed onto the silica surface, PH is not observed to repartition into the gas phase, even under vacuum, and dark reactions of PH are not observed at the silica/air interface. Photolysis (254 nm) of PH leads to the formation of 2,2'-biformylbiphenyl (1), 9,10-phenanthrenequinone (2), cis-9,10-dihydrodihydroxyphenanthrene (3), benzocoumarin (4), 2,2'-biphenyldicarboxytic acid (5), 2-formyl-2'-biphenylcarboxylic acid (6), 2-formylbiphenyl (7), 1,2-naphthalenedicarboxylic acid (8), and phthalic acid (9). These products account for 85-90% of the reacted PH. The photoproducts are independent of excitation wavelength (254 and 350 nm), and the reaction proceeds entirely through an initial step involving the addition of singlet molecular oxygen to the ground state of phenanthrene with subsequent thermal and/or photochemical reactions of the initially formed product. Singlet molecular oxygen is produced through quenching of the lowest triplet state of PH at the silica gel/air interface, The high material balance and detailed mechanistic information provided by this study serve as a standard for comparisons with the products and mechanism of PH photochemical oxidation on environmentally derived inorganic oxide particulates.