A great variety of fluorescing compounds - mainly polycyclic aromatic hydrocarbons (PAHs) which are of concern in environmental and public health risk assessment due to their carcinogenic properties - exists as a complex mixture in petroleum, which makes their separation and identification difficult, if not impossible. Ultraviolet (UV) fluorescence spectroscopy offers a relatively simple way to characterize and quantify these compounds in environmental samples. PAHs are known to be sensitive to photochemical reactions rather than to biological oxidation. Several studies carried out with pure PAH compounds demonstrated their fast photolysis, half-lives of a few minutes to a few hours have been observed, which depend on the irradiation light flux. The objective of the present study was to describe photolysis characteristics of mixtures of fluorescing petroleum compounds exposed to sunlight.
Crude oil and benzo(a)pyrene (BaP) were used as model mixture of fluorescing compounds in seawater. Solar irradiation conditions were selected to characterize maximum photolysis rates at 29 °N latitude. In addition to the characteristics of the irradiation light, reactor geometry, level of dispersion of the petroleum compounds and the presence of particulate matter influenced significantly the reaction rate coefficients. Photolysis of BaP was described with a first-order rate coefficient, whereas photolysis of the other compounds, quantified by fluorescence intensity at different wavelengths, was characterized with a multicomponent equation. BaP can be considered as an internal actinometer in photolysis experiments with PAH mixtures. The level of dispersion affected the photolysis rates, which were highest in the dissolved and chemically dispersed samples. Ratios between fluorescence intensities at different wavelengths allowed comparison with the characteristics of actual environmental samples, providing additional information on their weathering history.
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