Patricia Stoffyn-Egli

Formation and Characterization of Oil–Mineral Aggregates

Abstract Oil associates with fine mineral particles in an aqueous medium not only as molecules adsorbed onto mineral surfaces, but also as a discrete phase to form microscopic oil–mineral aggregates (OMA). As it promotes the dispersion of stranded oil, this process is now believed to be instrumental in the natural recovery of oiled shorelines and in the efficacy of spill countermeasure techniques such as surf-washing (relocation of oiled sediment into the zone of wave action). To predict the fate of residual oil following spills and the effectiveness of such countermeasures, an improved understanding of the nature and properties of OMA and of the factors influencing their formation is required. Laboratory protocols and microscopical methods have been refined for the detection and classification of OMA. Three OMA types have been identified: droplet, solid and flake aggregates. Droplet aggregates are oil droplets (usually a few μm in diameter) surrounded by individual or flocculated mineral particles. Solid aggregates are mixed oil and mineral bodies of various shapes (scale of tens of μm). Flake aggregates are thin sheets that can reach the mm size range in which mineral and oil are arranged in an ordered configuration. The parameters controlling the quantity, type and size of OMA include mineral type and surface properties, quantity, viscosity and composition of the oil, and oil/mineral ratio. It is also evident that water turbulence (i.e. breaking waves, strong flood currents) greatly enhances OMA formation. Once formed, OMA appear to be very stable structures the buoyancy of which depends on the ratio of oil to mineral in each individual aggregate. OMA being on average less dense than mineral-only aggregates and even buoyant, they will be kept in suspension longer and be dispersed further than unoiled sediment. Mineral particles in OMA act as a surfactant preventing the oil from recoalescing. As OMA formation increases the surface to volume ratio of spilled oil, it prolongs and enhances oil weathering processes such as dissolution, evaporation and biodegradation.

SHORELINE CLEANUP BY ACCELERATION OF CLAY-OIL FLOCCULATION PROCESSES

ABSTRACT During the cleanup operations following the Sea Empress oil spill, it was observed that the oil emulsion did not adhere strongly to the shoreline and that fine mineral particles present in the surf waters interacted with oil to form clay-oil floes. In an attempt to enhance clay-oil flocculation, Amroth beach was subjected to repeated “surf washing”: the oiled cobbles from the high water mark were moved down to the intertidal zone using an excavator at low tide. After 4 days of treatment, most of the oil emulsion was removed from the cobbles. We estimate that the majority of the oil was removed as clay-oil flocs and that the remainder was released from the cobbles as a broken surface slick. Microscopic and chemical analysis of samples of flocs and oiled sediments showed that energy imparted to the surf zone resulted in clay-oil flocculation, which increased biodegradation rates of the residual oil. Surf washing increased the availability of fine mineral particles, which (1) minimized the contact o...

Oil–Mineral Aggregate Formation on Oiled Beaches: Natural Attenuation and Sediment Relocation

Abstract The significance of oil–mineral aggregate (OMA) formation on the effectiveness of the in situ shoreline treatment options of natural attenuation (natural recovery) and sediment relocation (surf washing) was examined during field trials on two mixed-sediment (sand and pebble) beaches experimentally oiled with IF-30 oil. At both sites, the amount of oil remaining in the experimental plots was dramatically reduced within five days after sediment relocation treatments. Time-series microscopy and image analysis of breaker-zone water samples demonstrate that OMA formation occurred naturally on the oiled beaches at both sites and was accelerated by the sediment relocation procedure. Lower concentrations of OMA in the breaker zone at Site 3 are attributed to the higher wave-energy levels at this site that presumably facilitated more rapid OMA dispersion. The granulometry and mineralogy of beach sediment and of subtidal sediment trap samples indicate that the material settling in nearshore waters originated from the relocated sediment and that a portion of the finer sediment was probably transported out of the study region before settling. Gas chromatography/mass spectrometry analysis demonstrated that a significant fraction of the oil dispersed into nearshore waters and sediments by interaction with mineral fines was biodegraded. The fact that little or no residual oil was found stranded on the shore in areas adjacent to the experimental plots and that only small amounts of oil were found in nearshore subtidal sediments and sediment trap samples suggests that a large fraction of the oil lost from the experimental plots may have been dispersed in the form of relatively buoyant OMA.

Characteristics of oil droplets stabilized by mineral particles: Effects of oil type and temperature

Abstract The relative influence of oil type and temperature on the characteristics of oil droplets stabilized by mineral particles (oil–mineral aggregates––OMA) was studied in the laboratory. OMA were generated by shaking eight different oils under two temperatures with natural mineral fines in seawater at a pre-defined energy level. Shape, mean and maximum sizes, size distribution and concentration of oil droplets forming negatively buoyant OMA were measured by image analysis using epi-fluorescence microscopy. Results showed that oil droplets are, on average, spherical regardless of oil composition and temperature. Non-spherical “elongated” oil droplets form more at 20 °C than at 0 °C. Droplet shape and size were not correlated to oil viscosity. The concentration of oil droplets decreased rapidly with oil viscosity, temperature and asphaltenes–resins content (ARC). When normalized with ARC, mass concentration of oil droplets correlates well with oil viscosity, regardless of experimental temperature. A model was proposed to calculate mass of oil dispersed by OMA as a function of oil viscosity and ARC. Size distributions of oil droplets follow similar trends, but their magnitudes depend on oil type and temperature. A function was derived that describes all the data when size distributions were presented in a normalized form N / N t = f ( D / D 50 ), where N is number of droplets of diameter D , N t is the total number of droplets and D 50 the mean size of the droplets.

The Influence of Salinity on Oil–Mineral Aggregate Formation

Abstract The formation of microscopic oil–mineral aggregates (OMA) has been linked to the natural removal of stranded oil in coastal marine environments and to the efficacy of surf washing (relocation of oiled sediment into the zone of wave action) as an oil spill remediation procedure. To predict the significance of this process in estuarine and freshwater environments, OMA formation was tested in the laboratory with seawater diluted to obtain a salinity range of 0–35. Quantification of the amount of oil incorporated into OMA shows that the extent of OMA formation is not significantly different from that of seawater for salinity values as low as 1.5–0.15 (1/20 to 1/200 of pure seawater). The precise value of this threshold depends on parameters including oil type and the nature of the mineral present. Below this salinity threshold, there is a linear decrease in the amount of oil incorporated in OMA, to practically zero in distilled water. It is concluded that oil spill remediation by natural or induced OMA formation (i.e. surf washing) is applicable to marine, estuarine and possibly inland hyper-saline environments.

Application of Ultraviolet Fluorescence Spectroscopy to Monitor Oil–Mineral Aggregate Formation

Abstract At an excitation wavelength of 320 nm, the ultraviolet fluorescence (UVF) spectra emitted by reference oils dispersed in seawater with mineral fines yielded two important results: (1) Resuspended negatively-buoyant oil–mineral aggregates (OMAs) exhibited maximum fluorescence at an emission wavelength of 450 nm and, (2) the hydrocarbons dispersed and/or dissolved in the seawater that remained after the aggregates had settled out exhibited maximum fluorescence at 355 nm. Data from UVF analysis (450 nm emission) and microscopical observations of seven reference oils suggest that higher-viscosity oils are less likely to form aggregates with mineral fines. This decline in OMA formation with increased oil viscosity could be predicted from a decrease in the ratio of emission at 450–355 nm. The data suggest that direct UVF spectroscopy of dispersed/dissolved oil and OMAs in seawater can be used to predict and verify the extent of OMA formation.

CHARACTERIZATION OF OIL-MINERAL AGGREGATES

ABSTRACT Oil associates with fine mineral particles in an aqueous medium not only as molecules adsorbed on mineral surfaces, but also as a discrete phase to form microscopic oil-mineral aggregates ...

NATURAL DISPERSION OF OIL IN A FRESHWATER ECOSYSTEM: DESAGUADERO PIPELINE SPILL, BOLIVIA

ABSTRACT During a flood event in January 2000, approximately 29,000 barrels of mixed crude oil and condensate was accidentally released from a fracture in the OSSA II pipeline at a crossing point o...

Characteristics of Oil Droplets Stabilized by Mineral Particles: The Effect of Salinity

ABSTRACT The influence of salinity on the characteristics of oil droplets stabilized by mineral particles (oil-mineral aggregates – OMA) was studied in the laboratory using three different oils and a natural sediment. Size and concentration of oil droplets associated with negatively and positively buoyant OMA were measured by image analysis using epi-fluorescence microscopy. Results showed that the median droplet size increases rapidly from about 5 μm at zero salinity to double at salinity close to 1.2 ppt; decreases dramatically to about 5 μm at salinity 3.5 ppt and then increases slightly to 6 μm at the seawater salinity. The concentration of oil droplets also increases sharply when the salinity increases from zero to a critical aggregation salinity Scas, after which it stabilizes at its maximum value. The concentration of mineral-stabilized droplets is strongly affected by oil type at any salinity. When normalized to its maximum value, the concentration of droplets correlates well with normalized salin...