The dissolution and photodegradation of Kuwaiti crude oil in seawater. Part 2: A laboratory photodegradation apparatus and photodegradation kinetics of a model seawater soluble hydrocarbon (phenanthrene)

A laboratory photodegradation apparatus, incorporating a calibrated xenon lamp, controlled temperature unit and quartz reaction cells designed to simulate natural irradiation by sunlight, and which is suitable for kinetic studies of hydrocarbon degradation in seawater, is described. The apparatus and experimental method was used to determine the pseudo-first order photodegradation constant (kp) of phenanthrene (a component present in the seawater-soluble fraction of crude oil) in seawater at 25 °C. The kp values determined in duplicate experiments were −0.342 ± 0.003 and −0.317 ± 0.029 h−1 with half-lives (t1/2) of 2.03 and 2.19 h. Extrapolation to natural sunlight at latitude 30 °N (Florida midday, midsummer assuming 12 h daylight) gave t1/2 values of 8.6–9.5 h, suggesting that photodegradation is a significantly fast reaction for removal of phenanthrene from seawater under some conditions. Gas chromatography-mass spectrometry (GC-MS) and ultraviolet fluorescence spectrophotometry were both suitable methods for monitoring the degradation, but GC-MS was most suitable for the determination of photoproducts. The identifiable products amounted to some 12% of the total products after 7 h irradiation, and their distribution was consistent with a known mechanism of phenanthrene degradation involving formation of an unstable and reactive 9,10-epoxide. The photolysis method should prove valuable for the determination of kp and t1/2 values for a wide range of petroleum hydrocarbons in seawater.