This paper describes recent advances in microscopic analysis for quantitative measurement of oil droplets. Integration of a microscope with bright-field and ultraviolet epi-fluorescence illumination (excitation wavelengths 340-380 nm; emission wavelengths 400-430 nm) fitted with a computer-controlled motorized stage, a high resolution digital camera, and new image-analysis software, enables automatic acquisition of multiple images and facilitates efficient counting and sizing of oil droplets. Laboratory experiments were conducted with this system to investigate the size distribution of chemically dispersed oil droplets and oil-mineral aggregates in baffled flasks that have been developed for testing chemical dispersant effectiveness. Image acquisition and data processing methods were developed to illustrate the size distribution of chemically dispersed oil droplets, as a function of energy dissipation rate in the baffled flasks, and the time-dependent change of the morphology and size distribution of oil-mineral aggregates. As a quantitative analytical tool, epifluorescence microscopy shows promise for application in research on oil spill response technologies, such as evaluating the effectiveness of chemical dispersant and characterizing the natural interaction between oil and mineral fines and other suspended particulate matters.
Crude oils dispersed in seawater produce distinct emission spectra when exposed to ultraviolet (UV) light. The spectra can be used to estimate how effectively oil is dispersed by chemical methods. Oil dispersants (such as Corexit 9500) have a pronounced effect on water-based UV spectra, strongly enhancing emission at 445 nm. This enhancement of fluorescence over the 455 nm bandwidth is the result of dispersant breaking up higher molecular weight (> 3 ring) polycyclic aromatic hydrocarbons (PAHs) into stable suspensions of small droplets. Ultraviolet fluorescence spectroscopy (UVFS) has been tested as a rapid analytical tool in the laboratory and in a wave tank designed to investigate the response of crude oils to dispersants and a range of energy dissipation rates. The results obtained with UVFS are consistent with standard chemical analyses, confirming that the method can be employed as a rapid, quantitative measure of dispersed oil concentration. Given that higher molecular weight PAHs are associated with many of the persistent toxic effects of crude oils on marine organisms, UVFS may also prove to be a useful tool for tracking these fractions during dispersed oil toxicity assays.
In 2005, the National Research Council (NRC) published a comprehensive treatise on oil spill dispersants. Among other things, it concluded that research on dispersion effectiveness as a function of energy dissipation rate and particle size distribution was a high priority. Energy dissipation rate (turbulence and existence of breaking waves) is important to initiate and promote effective dispersion, and the particle size distribution of dispersed oil droplets affects dispersion and the ultimate fate of oil in the water column. In this paper, we discuss the use of a wave tank built on the premises of the Bedford Institute of Oceanography, Dartmouth, Nova Scotia, Canada as part of collaborative research begun in 2003 by the U.S. Environmental Protection Agency (EPA) and Fisheries and Oceans Canada (DFO). This tank is able to produce breaking waves of various energy levels at precise locations in the tank. We studied the effects of 2 commercial dispersants (Corexit 9500 and SPC 1000) and a no dispersant control on two different crude oils (unweathered Alaska North Slope and weathered MESA Light) at 3 different energy dissipation rates (regular non-breaking waves, spilling breakers, and plunging breakers), amounting to 18 different treatments. We quantified the energy dissipation rates under those 3 wave conditions and measured oil dispersion in a factorial experiment involving 3 replicates of the 18 treatments over the course of the summer of 2006. Results clearly showed the importance of wave energy and the presence of a chemical dispersant on the ability to produce effective dispersion of oil into the water column. The presence of dispersants at increasing wave energies produced significantly better dispersion (p <0.05) than the no-dispersant controls. This study was conducted under batch conditions. Future work will be done under continuous flow conditions.
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