The physical-chemical property changes of oil in water are in sum total referred to as weathering, and include evaporation, photooxidation, water-in-oil emulsification, and biodegradation.
Evaporation (volatilization) is the most important and rapid step among the weathering processes. For a given amount of oil spilled into the ocean, evaporation can potentially remove almost half of crude oil and more than 75% of refined products, but less than 10% of residual fuel oils. Small molecular weight compounds evaporate immediately, while compounds with vapor pressures greater than »-C11 (b.p. < 204 °C) are generally lost within the first 20 days. The evaporation of low molecular weight compounds, including MAHs as well as two- and three-ring PAHs, can greatly reduce the toxicity of oil. The residual high molecular weight components become more viscous, precipitating to coat entrained water droplets into an emulsion. Speed of evaporation can be altered by temperature, wind speed, and sea conditions. When oil is mixed with water, forming a water-in-oil emulsion, or when a thick slick appears, the speed of evaporation decreases.
Sunlight irradiation can change the physical-chemical properties of oil in the environment. Photooxidation can generate more polar compounds including aliphatic and aromatic ketones, carboxylic acids, aldehydes, fatty acids, esters, epoxides, sulfoxides, sulfones, quinine, phenols, and alcohols. Some of these products significantly contribute to the toxicity of oil to marine organisms. In general, aromatic compounds are most sensitive to photochemical oxidation, and alkyl substitution increases their reactivity. PAHs can be oxidized to more stable metabolites like quinones. Aliphatic sulfur compounds are oxidized more easily than the aromatic thiophenes; aliphatic sulfur is commonly oxidized to its corresponding sulfoxide, sulfonate, sulfone, and sulfate.
Sunlight wavelength greatly affects the rate of photo-oxidation; higher-energy light (wavelengths < 300 nm) increases the speed of photooxidation. In many cases, chemicals become more bioactive after photooxidation, which leads to greater toxicological effects in marine organisms. In addition to photooxidation, photosensitiza-tion is another important type of phototoxicity mechanism. While photooxidation modifies chemical structures to become more reactive, photosensitization transfers light energy to endogenous chemicals in tissues and can result in tissue damage. Both mechanisms are important pathways for cellular damage.
High molecular weight molecules with high boiling points (>350-400 °C), such as asphaltenes, resins, and waxes, are recognized as emulsification agents that can stabilize water-in-oil emulsification or 'mousse' formation. Those molecules orient within the oil phase at the oil-water interface and retard water droplets from forming separate water and oil phases. High viscosities of starting oil will stabilize the emulsions. The properties of starting oil, especially asphaltene, resin, and wax content, and the viscosity, greatly influence the stability of resultant emulsions. Unstable emulsions are those that separate to oil and water somewhat rapidly after the mixing energy is removed.
The change in oil composition under certain conditions is often determined after analytical measurement. Besides physical-chemical factors that change oil composition, biodegradation also usually occurs. Several studies have utilized isotopically labelled substrates to study the biodegradation ofspecific components. The data provide insight regarding common biodegradation products and possible metabolic actions. However, it is difficult to predict the biodegradation of the remaining thousands of chemicals in most oils. Metabolic pathways commonly involved in oil degradation in the ocean include hydroxylation and oxidation by microorganisms (usually bacteria). For instance, Alcanivorax spp. are mainly responsible for alkane degradation, while Cycloclasticus spp. commonly contribute to MAH and PAH degradation.
Some dissolved oil components may evaporate from water to air when droplets resurface by buoyant or mixing energy, while others enter in the water column. The amount and distribution of oil droplets entering the water column will determine the areas and organisms that are affected; entry rate is very sensitive to droplet size. Larger droplets resurface after a reduction in wind or wave turbulence, while smaller droplets (<60-80 mm) usually sink deeper into the water column and eventually dissolve. Soluble hydrocarbon content, viscosity, surface tension, and physical energy (i.e., wave and wind action) will affect droplet size and content. Evaporation and water-in-oil emulsification can increase the viscosity of a surface slick, preventing both wind and wave forces from forming oil droplets.
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