ReviewA review of oil, dispersed oil and sediment interactions in the aquatic environment: Influence on the fate, transport and remediation of oil spills
Introduction
Oil may be released into the marine environment from routine or accidental releases as a result of human activities including drilling, manufacturing, storing, transporting, and waste management (NRC, 2003). For example, an offshore oil well blowout or pipe line ruptures can release large amounts of oil into the marine environment. The 2010 Deepwater Horizon (DWH) oil spill released an estimated 4.9 million barrels of South Louisiana sweet crude oil into the Gulf of Mexico, resulting in the largest marine oil spill in U.S. history and perhaps the second largest in the world, after the first Gulf War oil spill from Kuwait (Abbriano et al., 2011, Graham et al., 2010, Hemmer et al., 2011). Oil tankers carrying millions of gallons of oil can also pose a significant threat to the marine environment in the event of ship collisions or grounding (Bandara et al., 2011, Lytle and Peckarsky, 2001). The 1989 Exxon Valdez oil spill discharged 11 million gallons of Alaskan North Slope crude oil through the ruptured hull in Prince William Sound, Alaska, resulting in a contamination of ∼2000 km of intertidal shorelines within the Sound and the Gulf of Alaska (Bragg et al., 1994). It is noteworthy that most oil spilled into the environment from well blowouts or pipe line ruptures is a mixture of oil and natural gas, and is known as “live oil” for its very high vapor pressures. On the other hand, oil from tankers or some pipelines has been separated from the natural gas, and is called “dead oil” for its low vapor pressure (Reddy et al., 2012).
Application of oil dispersants has been a critical response measure to mitigate impacts of marine oil spill for decades (Franklin and Warner, 2011). Following 1989 Exxon Valdez oil spill, an estimated 5500 gallons of the dispersant known as Corexit 9527 was applied to the oil slick (ADEC, 1993). The 1999 M/V Blue Master Spill resulted in a release of roughly 100 barrels of Intermediate Fuel Oil (IFO) 180 from the M/V Blue Master following a collision with a fishing vessel 55 km south of Galveston, Texas. Twelve hours after the spill, approximate 700 gallons of Corexit 9500 were applied (Kaser, 2001). In 2000, around 2000 barrels of South Louisiana crude oil were discharged into the Gulf of Mexico from a 24 in. pipeline 65 miles south of Houma, Louisiana. In response to the oil spill, 3000 gallons of Corexit 9527 were employed (Stoermer et al., 2001). During the DWH response, BP applied approximately 2.1 million gallons of oil dispersants Corexit 9500 and Corexit 9527, of which 1.4 million gallons of the dispersants were applied at the surface and 0.77 million gallons at the wellhead (Kujawinski et al., 2011).
Generally, oil dispersants are complex mixtures containing three types of chemicals: solvents, additives and surfactants. Solvents are added primarily to promote the dissolution of surfactants, reduce the dispersant’s viscosity and affect its solubility in spilled oil. Additives may improve the dissolution of the surfactants into an oil slick and increase the long-term stability of the dispersant (NRC, 2005). Commercial chemical dispersants usually consist of two or more surfactants. These surfactants in fixed ratios can emulsify oil, and hydrocarbon-based solvents can help break up large clumps of high molecular weight, more viscous oil (Kujawinski et al., 2011, Ramachandran et al., 2004). Oil dispersants lower the oil–water interfacial tension, thereby breaking oil slicks into fine droplets and accelerating dispersal into the water column and dissolution of the hydrophobic oil components (NRC, 2005).
Once oil is released into the marine environment, it undergoes complex physical, chemical and biological transformations, including spreading, drifting, dispersion, stranding, and weathering. The important weathering processes include evaporation, dissolution, biodegradation, emulsification (i.e., “mouse” formation), and photo-oxidation (NRC, 2003). Whole oil droplets may be dispersed into the water column while monocyclic compounds (e.g., benzene and alkyl-substituted benzenes) with log Kow values between 2.1 and 3.7 and selected lower molecular weight, 2–3 ring polycyclic aromatic hydrocarbons (PAHs) with log Kow values between 3.7 and 4.8 may undergo partial dissolution (National Research Council, 2005, Payne et al., 2003). These environmental processes can be strongly affected by interactions between dissolved and dispersed oil components and sediment particles.
Sediments have long been recognized as important vectors in the transport of oil from one environmental compartment (i.e., phase) to another following oil spill events in the aquatic environment. Studies have shown that interactions between oil and sediments play an important role in dispersion and degradation of spilled oil (Muschenheim and Lee, 2002). In nearshore waters, naturally dispersed oil droplets may aggregate readily with suspended particulate material (SPM) such as clay minerals or organic matter to form oil–SPM aggregates (OSAs). Terminologies such as oil–clay flocculation, oil–SPM interactions, oil–mineral aggregation, and oil–fines interactions have been used to describe this natural process (Khelifa et al., 2002, Khelifa et al., 2005, Muschenheim and Lee, 2002, Omotoso et al., 2002, Owens, 1999, Owens and Lee, 2003, Payne et al., 2003). Interactions between oil and SPM have been documented in several laboratory and field studies. Poirier and Thiel (1941) reported that oil dispersed in a mixture of sediments and seawater settled and was trapped on the bottom by the sediments. Detailed laboratory experiments suggested that mineral–oil interactions may have been instrumental in the natural cleansing of shorelines oiled in Prince William Sound, Alaska, following the Exxon Valdez spill (Bragg and Owens, 1994, Bragg and Owens, 1995, Bragg et al., 1994, Bragg and Yang, 1993, Jahns et al., 1991).
The overall goal of this review is to gauge the state of science pertaining to interactions of physically and chemically dispersed oil with sediments, and how these interactions affect fate and transport of spilled oil. In specific, this review aimed to: (1) elucidate the mechanism governing OSAs formation and the controlling parameters including oil and sediments characteristics, Pickering emulsions, salinity (or ionic strength) and mixing energy, (2) illustrate the effects of environmental conditions, such as sediment organic matter (SOM), mineralogy, temperature, pressure, and salinity, on sorption of dissolved/dispersed oil components onto sediments, (3) interpret effects of deepwater conditions and oil dispersants on the oil–sediment interactions, (4) provide an overview of analytical methods for characterization of interactions between dispersed oil and sediments, and (5) discuss future research needs.
Section snippets
Interactions between oil and sediments
Oil can be present in water in two physical forms: dispersed oil droplets and dissolved oil components. Likewise, sediments in water can be in the forms of SPM and settled aggregates. Dispersed oil droplets or dissolved oil components can interact with sediments through (1) direct aggregation to form OSAs, and (2) adsorption on or incorporation in the sediment phase (Lee, 2002, Sterling et al., 2005). These interactions can change the fate and transport of oil by removing oil from the aqueous
Effects of deepwater conditions on oil–sediment interactions
There is no strict definition to deepwater. Considering approximate 10 m of water depth produces a pressure of 1 atm, hydraulic and coastal engineers define the 100 m of water depth as deepwater (Yapa and Li, 1997). However, in the oil industry, drilling depth can often reach 1000 m or more. As a result, the oil industry tends to refer the deepwater/deep-sea to the lower layer in the ocean, i.e., below the thermocline and above the sea-bed at depths of at least 700–1000 m (Camilli et al., 2010,
Effects of oil dispersants on oil–sediment interactions
Oil dispersants are composed of surfactants (surface active agents) dissolved in one or more organic solvents. Commonly used dispersants contain a mixture of two nonionic surfactants, sorbitan monooleate (Span 80) and ethoxylated sorbitan monooleate (Tween 80), together with an anionic surfactant sodium dioctyl sulfosuccinate (SDSS) (Thibodeaux et al., 2011). For example, Corexit 9527 consists of two nonionic surfactants (48%) including ethoxylated sorbitan mono- and trioleates (Tween 80 and
Analytical methods for studying oil–sediment interactions
Many analytical methods have been applied for the characterization of oil/dispersed oil–sediment interactions, such as microscopic analysis of particle morphologies, particle size distribution, and spectroscopic characterization.
Future research needs
This review reveals some key knowledge gaps in the area of particle, oil, and/or dispersant interactions: (1) interactions between oil dispersants, oil and sediments and various SPMs, (2) how oil dispersants affect sorption/desorption and physical, chemical and biological availabilities of persistent oil components, (3) how dispersant-oil–sediment interactions alter the transport and weathering of oil, and (4) how deepwater conditions (e.g., P = 160 atm, T = 4 °C, and salinity >3.5%) alter the
Summary
Understanding the interactions between oil, dispersants and sediments has been considered an important component in developing oil spill countermeasures. Yet, only limited knowledge is available pertaining to sorption and desorption of oil droplets by sediment particles and how dispersants affect the sorption/desorption processes and the formation of OSAs. Understanding of the factors controlling these interactions is crucial for sounder evaluation of the environmental fate and impacts of oil
Acknowledgement
This work was partially supported by the U.S. Department of the Interior, Bureau of Ocean Energy Management.
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