With an understanding of the different physical, chemical and biological weathering processes, a series of laboratory experiments were conducted to describe biological and chemical oxidation processes at realistically low concentrations of physically and chemically dispersed oil in seawater by incubating living and sterilized samples spiked with oil alone, oil-dispersant mixture and dispersant alone, under light and dark conditions. Biological oxygen demand and photosynthetic oxygen production were characterized by the variation in dissolved oxygen, whereas the degradation of oil and dispersant was followed by the variation in total extractable matter and surfactant concentration, respectively. Gas chromatography, infrared and fluorescence spectrophotometry were used for the characterization of the petroleum and its degradation compounds.
Biological oxidation was considered the major process in the living seawater sample incubated in the dark, and chemical oxidation, i.e., photo-oxidation, in the sterilized seawater sample exposed to light. In the early stages of each experiment, biodegradation of the physically dispersed oil progressed faster than the chemically dispersed one due to the concurrent degradation of the dispersant and the limited assimilative capacity of the seawater. The n-alkanes, mono- and dicyclic aromatic compounds showed significant biodegradation and limited photo-oxidation, whereas polycyclic aromatics were mainly photo-oxidized. Photo-oxidation was stimulated in the chemically dispersed oil. Synergistic degradation was observed when biological and chemical oxidation progressed simultaneously.