PHOTOCHEMICAL OXIDATION OF PHENANTHRENE SORBED ON SILICA-GEL

There have been relatively few detailed studies of PAH photochemical d egradation mechanisms and products at solid/air interfaces under contr olled conditions. Results from mechanistic studies on particulate simu lants are important in understanding the fates of PAH sorbed on simila r materials in natural settings. In this study, the photolysis of phen anthrene (PH) on silica gel, in the presence of air, has been carefull y 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 n m) of PH leads to the formation of 2,2'-biformylbiphenyl (1), 9,10-phe nanthrenequinone (2), cis-9,10-dihydrodihydroxyphenanthrene (3), benzo coumarin (4), 2,2'-biphenyldicarboxytic acid (5), 2-formyl-2'-biphenyl carboxylic 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 wavel ength (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 photochem ical reactions of the initially formed product. Singlet molecular oxyg en is produced through quenching of the lowest triplet state of PH at the silica gel/air interface, The high material balance and detailed m echanistic information provided by this study serve as a standard for comparisons with the products and mechanism of PH photochemical oxidat ion on environmentally derived inorganic oxide particulates.