Oil-mineral Aggregates Research Articles

The Influence of Salinity on Oil–Mineral Aggregate Formation

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

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.

Mechanisms of Crude Oil–Mineral Interactions

The formation of oil–mineral aggregates (OMA) in water is initiated by a variety of physical and chemical factors that are not readily amenable to differentiation in the field or in the laboratory. This study illustrates a method that may be used to isolate the limiting parameter responsible for the formation and stabilization of OMA based on the microstructure and sedimentation characteristics of the aggregates. A flocculation index based on the sedimentation behavior of a sheared crude oil–mineral–water mixture was used to quantify the degree of interaction of oil and minerals in water. The degree of crude oil–mineral interaction was found to be dependent on the viscosity of the crude oil and the type of mineral present. Hydrophilic quartz and kaolinite interact more strongly with low-viscosity oils than with high-viscosity oils, whereas calcite (an oleophilic mineral) interacts strongly with crude oils irrespective of their viscosities. It was observed that the water chemistry must favor flocculation before the minerals can effectively stabilize oil–mineral flocs. In a fresh water analogue, quartz and kaolin interact more strongly with crude oils than montmorillonite but the reverse is true in seawater. Calcite flocculates crude oils in fresh or seawater more strongly than the hydrophilic minerals tested. In all the minerals tested, the degree of oil–mineral interaction increases and then plateaus as mixing energy is increased. Confocal laser images of the oil–mineral structures reveal two types of OMA. In low-viscosity oils with hydrophilic minerals, negatively buoyant flocs comprising minerals stabilizing oil droplets in a water-continuous phase are predominant. OMA with calcite show mineral-stabilized oil droplets and mineral-rich positively buoyant oil-continuous phase.

Effect of Suspended Mineral Load, Water Salinity and Oil Type on the Size of Oil–Mineral Aggregates in the Presence of Chemical Dispersant

When spilled in the environment, especially in coastal systems such as estuaries, oil frequently interacts with fine mineral particles to form aggregates. This phenomenon may be enhanced in the case of chemical dispersion and influence the behavior and fate of the pollutant in the environment. Understanding this process will help decide whether chemical dispersion is a good oil clean-up option in a particular environment. This study investigated the formation of oil–mineral aggregates (OMA) when the oil was chemically dispersed, focussing on the size distribution of these structures. Results of laboratory experiments show that aggregate size is correlated to its relative composition in oil and clay, and that for a given concentration of mineral, the average size presents a maximum. Other highlights include the influence of oil type and salinity on the clay concentration corresponding to maximum size. The behavior of a particular oil as OMA depends on the size and buoyancy of its aggregates which will vary with the local of salinity, suspended mineral load and hydrodynamics conditions.

Characteristics of Oil Droplets Stabilized by Mineral Particles: Effects of Oil Type and Temperature

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.

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 (OMA). On the basis of recent studies, OMA formation now is believed to be instrumental in the natural recovery of oil spill impacted shorelines and in the efficacy of cleanup techniques such as surf washing. A better understanding of the nature and properties of OMA will help predict the fate of oil spilled in the aquatic environment. This work describes the various instruments and methods currently available for the detection and identification of OMA. Three types of OMA have been characterized by microscopy techniques: droplet, solid, and flake aggregates. Droplet aggregates are oil droplets (usually a few μm in diameter) surrounded by individual or aggregated mineral particles. Solid aggregates are a mixture of oil and minerals blended into microscopic bodies of various shapes. Flake aggregates are thin sheets reaching several millimeters across in which mineral and oil are arranged in a regular pattern. Energy from breaking waves facilitates 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.

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 on the Rio Desaguadero, Bolivia. The resultant downstream impact zone included over 400 km of river banks and 500 km2 of flooded lowlands within the Bolivian Altiplano. Analysis of stranded oil samples recovered from the riverbanks suggested that significant oil loss occurred by evaporation (>40%). With additional data on the amount of surface oil remaining (<0.2%) and oil recovered (3–13%) at the end of cleanup operations, it became apparent that a large fraction of the spilled oil remained unaccountable (27–37%). Laboratory tests were undertaken to assess if oil-mineral aggregate (OMA) formation—a natural process believed to promote rapid oil dispersion—may provide part of the explanation for this unaccounted loss. OMA were generated from local oil, sediment and water samples under simulated field conditions. Epi-fluorescence microscopy analysis verified the presence of flake aggregates that would be formed typically by the interaction of oil with smectite minerals (e.g. montmorillonite). On the basis of these microscopy observations and of mineral analysis of suspended material in water from the spill site, which revealed abundant smectite, it is hypothesized that OMA formation occurred, and that this process facilitated significant removal of residual oil from the ecosystem by enhancing biodegradation rates.

PRELIMINARY FINDINGS OF OIL-SOLIDS INTERACTION IN EIGHT ALASKAN RIVERS

ABSTRACT The formation of oil-mineral aggregates in rivers with high concentrations of suspended solids could be an important mechanism for transferring oil from a surface slick into the water column. Seven glacier-fed rivers in south-central Alaska were sampled three times in the summer of 1999 and tested to see how suspended solids in the water would interact with crude oil. Oil-mineral aggregates formed with all samples, which lead to the conclusion that similar oil-solids interaction should occur if oil is spilled into other glacier-fed rivers. The amount of oil associated with the solids after mixing was roughly proportional to the concentration of suspended solids in the samples. The oil-loading results ranged between 0.01 and 0.45 g oil/g solid, with most of the measurements falling between 0.03 and 0.24 g oil/g solid. The average loading for all tests was 0.13 g oil/g solid, which is consistent with the findings of a similar study done in 1998 on the Yukon River. This result indicates that the suspended solids in the streams studied—especially the larger Yukon, Copper, and Lowe Rivers—have a large oil-holding capacity and would be capable of scavenging a considerable volume of oil. The exact quantity would depend on several factors, including the volume and dimensions of the slick, concentration of suspended solids, and current velocity of the river. Historical data on stream flow, suspended-solids concentration, and particle-size distribution were gathered and used in conjunction with the laboratory oil-solids interaction results to identify the times of the year when such interaction could occur to a significant degree. This data indicated that oil-mineral aggregates are expected to form in all rivers tested, with the most significant periods being during the summer months.

Anaerobic Biodegradation of Vegetable Oil Spills

ABSTRACT Vegetable oil spills are common in the United States and Canada. Although vegetable oils usually are not toxic in the classic sense, at least not to the extent normally associated with crude oil and refined petroleum products, they can cause severe harmful effects in contaminated ecosystems. Since most of the adverse environmental effects of vegetable oils appear to be a result of the presence of the oil on the water surface, on shoreline sediments, or (in emulsified form) in the water column, a prudent spill response is to remove floating and suspended oil from the contaminated water body as quickly as possible. The goal of this research is to investigate a response alternative that is based on sedimentation of floating and suspended oil followed by anaerobic biodegradation in the sediments. The authors' research demonstrates that sedimentation of floating oil by formation of oil-mineral aggregates (OMAs) is possible, and that the interaction between oil and dry clay is crucial to the successful formation of negatively buoyant floes. The rate and extent of vegetable oil biodegradation under methanogenic and iron-reducing conditions in freshwater sediments have also been investigated. Anaerobic biodegradation of vegetable oil occurred in all sediments that were examined, including sediments from a river, a lake, and a wetland. Carbon and electron balances indicate that anaerobic mineralization of the added vegetable oil was complete.