| As the population of the world increases and industrialization processes speed up, the demand for energy keeps growing.The worldwide exploration, extraction, marine transportation, and consumption of petroleum and petroleum products inevitably result in the spill and release of oil to the environment, particularly into the aquatic ecosystem. The highly toxic nature of oil to the aquatic environment and the costly removal of spilled oil from shorelines have motivated the oil spill response community to seek cost-effective and environmentally friendly oil spill remediation technologies.It is well established that when oil spills occur in the aquatic environment, dispersed oil droplets aggregate readily with suspended particulate matter (SPM) present in the water column to form oil-SPM aggregates (OSAs).The oil spill research community has recognized for decades that this process can enhance natural cleansing of oiled shorelines as well as oil dispersion in the water column.Several studies have been conducted over the last two decades to understand the formation and characteristics of OSAs.Quantitative studies were conducted on the effects of SPM size and concentration, oil type, water salinity, and temperature on OSA formation. However, most of the studies were conducted either under a certain level of mixing energy for quite a long time or under a very high mixing energy for a short period of time. Few studies have been conducted on the kinetic aspects of OSA formation. Operationally, this has left decision makers lacking information on the time scale of this process and its significance to oil dispersion following oil spills in aquatic environments.Mixing energy has strong influence on the OSA formation as it controls the splitting of an oil slick into small oil droplets, dispersion and suspension of particulate matter into the water column, and the interaction between the oil and suspended particulate matter. However, very little has been done to quantify the effect of mixing energy on the kinetics of OSA formation.In this regard, a laboratory study was designed and conducted to investigate the kinetics of OSA formation and its variations under various conditions. The effects of concentrations of suspended particulate matter, different mixing energy levels, and different crude oils with different chemical composition and physical properties were examined.Oil-SPM aggregates (OSAs) were prepared using a natural sediment referred as Standard Reference Material 1941b prepared by the U.S.National Institute of Standards and Technology (NIST), three types of crude oil (Arabian light, Arabian medium, and Arabian heavy crude oil), artificial seawater (salinity:33‰), and various mixing energy levels.A reciprocating shaker was used to simulate various mixing energy levels controlled by the shaking rate.The variance of oil trapping efficiency (the percentage of oil mass trapped in negatively buoyant OSAs) with shaking time was used to quantify the OSA formation process.Total Petroleum Hydrocarbons (TPHs) trapped in negatively buoyant OSAs was used to determine oil trapping efficiency. TPH was determined using an Agilent 6890 Gas Chromatograph system equipped with a Flame Ionization Detector (GC-FID) and an Agilent 7683 auto-sampler. Both the oil trapping efficiency in negatively buoyant OSAs and the mass of settled particulate matter were tracked with shaking time.The best curve fit method was used to analyze the data and get the model to predict the OSA formation process.The UV epi-fluorescence microscopy technology and Matlab-based image analysis were used for investigation of the structure of OSAs and the size distribution and concentration of oil droplets in the OSAs formed. The size distribution of OSAs formed under different mixing conditions was also measured by a Malvern laser diffraction particle size analyzer.The following are the main results and findings obtained from this series of studies. (1)The mixing energy level has a strong control on the time scale of OSA formation. The results showed that a minimum mixing energy level exists, under which OSAs will not form within a short time period, even if there is an adequate concentration of suspended particulate matter. When the mixing energy is equivalent to the breaking wave conditions, OSAs form within 7 hours.The variance of oil trapping efficiency in OSAs with the shaking time could be described by Equation 3-1, which indicates that the oil trapping efficiency in OSAs increases exponentially with the increase in shaking time until it reaches a maximum and then stabilizes.By increasing the mixing energy, the oil trapping efficiency in OSAs was largely increased, and the OS A formation time was much shortened.(2) The concentration of suspended particulate matter has a strong control on the kinetics of OSA formation. By increasing the concentration of suspended particulate matter, the OSA formation was greatly enhanced and the OSA formation time was largely decreased. Compared to the mixing energy level, the concentration of suspended particulate matter has a stronger control on OSA formation when OSAs could form.(3) Compared to the concentration of suspended particulate matter and the mixing energy level, the crude oil properties have a weaker influence on OSA formation in this study. The results indicated that a much higher oil trapping efficiency was observed in OSAs formed with Arabian heavy crude oil, which has a much higher asphaltenes-resins content than the other two oils.Therefore, it may be inferred that a higher content of asphaltenes-resins in crude oil samples could play an important role in promoting the OSA formation.(4) The ratios of the oil to suspended particulate matter in formed OSAs were also investigated in this study. Results showed that the variance of the oil/suspended particulate matter ratio in OSAs with shaking time could be described by an equation similar to Equation 3-1.That is, the oil/suspended particulate matter ratio in OSAs increased exponentially with the shaking time,until a maximum was reached and then it stabilized. The results also indicated that the concentrations of the suspended particulate matter have strong controlling effects on the variance of oil/suspended particulate matter ratio with time. When the sediment concentration was at 100 mg/L, the oil/suspended particulate matter ratio in OSAs was determined to be in the range of 0.59 to 1.13 mg oil/mg particulate matter; when the concentration of the suspended particulate matter was at 200 mg/L, the oil-suspended particulate matter ratio in OSAs was decreased to range from 0.28 to 0.45 mg oil/mg particulate matter; when the concentration of the suspended particulate matter was further increased to 400 mg/L, the oil-suspended particulate matter ratio in OSAs was further reduced to 0.21 to 0.24 mg oil/mg particulate matter.(5) The structures of OSAs formed under various mixing energy levels were studied using the UV epi-fluorescence technique.The study results show that OSAs formed under the low mixing energy level were dominated by solid OSAs and single-droplet OSAs, whereas most of the OSAs formed under the high mixing energy level were multi-droplet OSAs.The results also showed that with the increase of the mixing energy, more droplets were incorporated into OSAs, and the size of the oil droplets in OSAs was greatly reduced. The floating flake OSAs were observed when the sediment concentration was at 400 mg/L.When the mixing energy level was low, however, more solid OSAs were formed with the Arabian heavy crude oil, which has the highest content of asphaltenes-resins among 3 different oils studied.(6) The size distribution of OSAs under different mixing conditions was studied using a laser diffraction particle size analyzer. The study results showed that the size of OSAs increased with the increase in shaking time.However, the higher mixing energy has a breakage effect on formed OSAs and sediment flocks, thus inhibiting the growth of the OSAs.
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