Sunlight-induced compositional alterations in the seawater-soluble fraction of a crude oil

Abstract

Oil released into the sea undergoes immediate compositional changes as a result of processes of volatilization, dissolution, particle adsorption, and microbial and photochemical decomposition. Based on our observation of the molecular composition of oil residues dissolved in subtropical ocean waters, we hypothesized photo-oxidation of alkyl-substituted polycyclic aromatic hydrocarbons (PAH) and heterocyclic aromatics in preference to their unsubstituted parent compounds. As the ratios of alkyl to parent PAH are used to assign sources to contaminant residues in environmental samples, we tested the hypothesis in a controlled experiment: membrane-filtered seawater collected near the island of Bermuda was saturated with a Nigerian crude oil and exposed to natural sunlight in a quartz flask. Comparison with a dark control under otherwise identical conditions served to differentiate between sunlight-induced and microbially mediated decreases in concentration and changes in composition. We conclude on the basis of UV fluorescence, GC-MS, and microbiological data that sunlight-induced oxidation is responsible for the rapid loss of the UV fluorescence signal for total aromatic hydrocarbons and for the preferential depletion of the alkyl-substituted PAH and heterocyclic aromatics. Structure-dependent selectivity in the photo-oxidation of dissolved oil residues may thus result in residual hydrocarbon blends which could be mistaken as originating from incomplete combustion processes.