7 3 Comparison of the Relative Oil Concentrations of Oil Droplets into the Water Column Over Time Resulting From Natural Dispersion and Chemical Dispersion.. Chemically dispersed oil d
Oil Chemistry Review
This section reviews the chemical components and characteristics of crude and refined oils, highlighting how an oil's chemistry affects dispersion processes and the effectiveness of dispersants Understanding oil chemistry is crucial for interpreting specific data that informs response decisions General properties are summarized, while a more detailed discussion on physical oil properties and classification is available in the first booklet of this series, titled "Fate of Spilled Oil in Marine Waters: Where Does It Go, What Does It Do, and How Do Dispersants Affect It?"
Oil is a complex and variable mixture of compounds, primarily consisting of hydrocarbons and, to a lesser extent, trace metals The most abundant compounds in crude oil include asphaltenes and waxes Generally, crude oil can be categorized into three main groups of hydrocarbons.
Light-weight components (low molecular weight)
- 1 to 10 carbon atoms (Ci to C10);
- small number of atoms in each molecule;
- evaporate and dissolve rapidly (hours) and leave little or no residue because they are simple in molecular structure;
- many of these components (e.g., benzene) are thought to be readily absorbed by animals through the skin or through inhalation; and
- potentially flammable and readily inhaled by people, and so are of concern for human health and safety
Medium-weight components (medium molecular weight)
- 11 to 22 carbon atoms (Ci 1 to C22);
- evaporate or dissolve more slowly, over several days, with some residue remaining;
- sometimes regarded as more toxic than the light-weight components (Clark, pers c o m ) ; and
Examples Of H w - w e i g h t Molecules Found in Oil
- not as bioavailable as lower-weight components, so less likely to affect animals
Heavy-weight components (high molecular weight)
- undergo little to no evaporation or dissolution; and
- can cause chronic (long-term) effects via smothering or coating as residue in the water column and sediments Persistence refers to an oil orrefinedproducrs tendency
(tar balls, etc.) (Helton, 1996) to remain in the environment following a discharge for a long period of time
Persistent o i l s are those crude and refined oiiproducts
Crude oils consist of different combinations of three categories of hydrocarbons When analyzing crude oils, it is essential to consider the concentration of larger molecular compounds, specifically medium- and heavy-weight hydrocarbons.
amount of light-weight components) within the oil affects PERSIS-
Oils with higher concentrations of medium- and heavy-weight components tend to exhibit increased persistence in affected environments due to weathering processes In contrast, residues primarily made up of light-weight components are generally classified as non-persistent Typically, oil with a weathering half-life ranging from months to years is regarded as persistent.
Refined products are typically composed of a narrow range of processed components, usually containing the lighter-weight components (e.g., gasoline, condensates, and diesel-like products) RESIDUAL or LOW
N m - p m n f oils are re fined oil products that wiùl be removed from the
API Gravity oil products consist mainly of heavy components that can affect the environment, often blended with agents like No 2 fuel oil during their development The refined products are anticipated to have only short-term environmental impacts.
According to US 33 CFR, Section 155.1029, an oil is classified as non-persistent if natural weathering processes take days to weeks to reduce its volume to half of its original amount.
Category Persistence Specific Gravity Typical Examples
Group I Non-pasistent* N/A Gasoline p d c t s , condvnsates
Group II Persistent ** < 0.85 Diesel-like prociictS and light
Group rn Persistent 0.85 I 0.95 Medium-@ cru& &
Group IV Persistent 0.95 I 1.00 Heavy cru& oils and mi&al
Group V Persistent > 1.00 Low API gravity p d c t s
CN& oils intemedate pmhcts products
Non-persistent petroleum-based oil is characterized by its hydrocarbon fractions, with a minimum of 50% by volume distilling at a temperature of 340°C (645°F) and at least 95% by volume distilling at 370°C (700°F) during shipment.
** Persistent: a petroleum-basedoil that &es not meet the dstiilation criteria for a non-persistent oil
Residuals are compounds that remain after crude oil is processed at refineries to extract gasoline, diesel fuel, and other oil products These residues are frequently mixed with lighter-weight refined products to create residual fuels, commonly known as low API and medium petroleum oil (MPO), or Group V oil, which are sold to utilities for electricity generation (Schotz et al., 1994).
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To discuss chernical dlspers- ants and to answer the fol- lowing questions:
- why would you want to use them?
- what happens to the chemical dispersant once it is applied to the
- what are ihe differences1 similarities between chemical dispersants and natural dispersion and dssSolution weather- ing processes?
Surfucfunfs are naturally occurring and chemically manufactured molecules of- ten referred to as surface ac- tive agents or ‘detergents.‘
Surfactant molecules contain both water compatible (h y- drophilic) and oil compatible
Molecules with oleophilic or hydrophobic sections align at the oil-water interface, where the oil-compatible part attaches to the oil, while the water-compatible portion extends outward into the surrounding water.
SohrenfS are chemical compounds found in dispersants that help surfactants penetrate oil Oleophilic refers to a chemical structure that has an affinity for oil or oil-like substances.
Hydrophilic refers to a chemical structure that has an affinity for water, naturally repelling oil and oil-like substances Additionally, there are several naturally occurring crude oils classified as low API gravity oil products (Scholz et al., 1994).
Chemical Dispersants
What Are They?
Chemical dispersants are composed of surfactants and solvents, with surfactants playing a crucial role in breaking down oil into tiny droplets that remain suspended in water due to wave action These surfactant molecules have a dual nature, featuring an oleophilic end that attracts oil and a hydrophilic end that interacts with water When applied to oil spills, dispersants facilitate the formation of oil droplets and minimize the oil's tendency to adhere to other droplets or surfaces, such as beaches and wildlife.
In 1967, the first large-scale application of dispersants for oil spill cleanup utilized heavy-duty detergent and degreaser solutions, originally designed for cleaning oily residues in industrial settings These early dispersants, composed of toxic solvents and surfactants, effectively penetrated hardened oil and maintained it in suspension as an emulsion in water, facilitating the cleaning process.
Concerns regarding the toxicity of solvents and the natural degradation of surfactants were largely overlooked, as these substances were not intended for direct environmental application (Lindblom, 1978; Lewis and Aurand, 1997) The use of these products during the 1967 Torrey Canyon spill proved detrimental, resulting in greater environmental harm than if they had not been employed.
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MOLECULE OIL DROPLET iydrophilic (water-seeking) headgroup
Figure 1 Surfactants and how they align themselves in oil Adapted from P E C A (1993)
Modern dispersant formulations consist of a blend of solvents and surfactants, specifically engineered for environmental use to disperse oil while minimizing toxicity These formulations can be categorized into three types, depending on the solvent classes used in their production.
Table 2 Dispersants listed on the NCP Product Schedule as of January 1999
1 water-based solvents - dilutable with water for application; de- veloped for use on light-distillate fuels and low-viscosity crude and products; least effective type;
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NEOS AB 3000 Hydrocaban- based based based
Surface tension occurs at the interface of two liquids or substances, as noted by Moms (1992) For example, when trying to remove oil or grease from a shirt, water alone is ineffective due to the significant interfacial tension between the oil and water However, by adding soap or surfactants, which contain both water-loving and oil-loving components, the oil or grease is loosened and suspended in the water, allowing for effective cleaning.
The soap has broken the inter- facial tension between the oil/ grease and the water
Surface Tension is the stretching force required to form a liquid ủlm (Mo&, 1992)
The surface tension of water causes it to bead up on surfaces, which slows down the wetting process and hinders effective cleaning For example, when a drop of water is placed on a waxed car, it retains its shape and does not spread due to this surface tension.
Hydrocarbon-based solvents improve the mixing and penetration of surfactants into thicker oils These solvents are typically used in their undiluted form, although they can also be diluted with water for application Most dispersants are classified within this category.
3 solvent-based containing lower concentrations of surfactants
(less than 20 to 25%) - only a limited number of these dispers- ants are in general use
Surfactants commonly found in household products differ from those used in chemical dispersants Everyday items such as all-purpose grease cleaners like Fantast&@, various glass cleaners, dishwashing soap, laundry pre-treatment products, and certain FDA-approved food additives contain these surfactants Additionally, household solvents like turpentine, nail polish remover, and lighter fluid are also prevalent.
How Do Chemical Dispersants Work?
Sea energy plays a crucial role in mixing spilled oil into the water column, especially during heavy wind and wave activity However, as conditions calm, the oil can coalesce and resurface as a slick The application of oil-spill dispersants enhances this natural dispersion process by using surfactants, which prevent the formation of large droplets that would quickly resurface These surfactants break the oil slick into smaller droplets, allowing for more effective mixing into the water column, where the oil can remain until it is naturally degraded.
Canevari (1978; 1985) provides a brief, but concise summary of how the surfactant mechanism works (Figure 2)
Dispersion occurs when a chemical dispersant penetrates the oil and settles at the oil-water interface, leading to a significant reduction in interfacial tension This reduction creates a driving force for some surfactant molecules to diffuse into the water As these surfactants diffuse into the water column, they carry with them fine oil droplets, facilitating the dispersion process.
2 SURFACTANT LOCATES AT OIL-W ATER INTERFACE
3 O I L SLICK DISPERSES INTO DROPLETS WITH MINIMAL ENERGY
Figure 2 Mechanism of surfactant action in oil spilled on the water Adapted from
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The effective use of chemical dispersants on oil slicks allows surfactants to penetrate the oil and reach the oil-water interface Even minimal wave or current energy can effectively distribute the dispersant and promote droplet formation Modern dispersant formulations often need little to no mixing energy to reduce the interfacial tension at the oil-water boundary (Canevari et al., 1989).
Chemical dispersants play a crucial role in preventing the re-coalescence of small oil droplets after their formation These surfactants effectively remain at the oil-water interface, serving as a barrier that inhibits the random collision of dispersed oil droplets.
Chemically dispersed oil droplets are less likely to re-coalesce, reducing the formation of tar balls and patties Additionally, these droplets possess a greater surface area per volume compared to the original oil slick, which facilitates a larger population of indigenous bacteria that naturally biodegrade the oil.
SECTION 111: WHY WOULD You WANT TO CONSIDER USE OF CHEMICAL DISPERSANTS?
In an oil spill response at sea, the main goal is to reduce the environmental impacts of the spilled oil The five most frequently employed strategies for addressing on-water oil spills include various methods aimed at effective containment and cleanup.
1 no response (i.e., no action or monitor only);
2 mechanical clean-up and physical removal;
3 the addition of chemical dispersant agents;
5 bioremediation with nutrient or microbial additions
There are various pros and cons associated with each (adapted from Payne, 1994):
The no response option, which involves allowing oil spills to naturally weather and break up, is often suitable when spills occur far from shore or when winds are pushing the oil away from sensitive habitats However, the public may find the idea of leaving the oil untreated politically unacceptable.
Mechanical clean-up and physical removal methods achieve only a 10-15 percent recovery rate for oil spills at sea, leaving 85-90 percent of the oil in the environment or altered by natural weathering processes such as evaporation, photo-oxidation, biodegradation, and sedimentation Additionally, the effectiveness of mechanical recovery is often limited by challenging ambient conditions, including currents exceeding 1 knot and waves taller than 3 feet.
In-situ burning is highly effective, with an estimated 95 percent efficiency in removing oil contained within fire-resistant booms on water surfaces However, the process of corralling oil for burning using boats and towed booms faces similar challenges as mechanical removal, particularly due to weather conditions Previous burn events have raised concerns about emissions, worker safety, and the integrity of the spill source This method is generally suitable only for fresh oils that have experienced minimal weathering, specifically those in slicks of 2 to 10 mm thickness, and oils that have not formed an emulsion or have an emulsion with less than 50 percent water content.
50 percent water incorporated (Buist et al., 1994) Experienced personnel are required for proper deployment and maintenance of the fire-resistant boom during the bum
Bioremediation is a long-term solution for oil spill clean-up, relying on naturally occurring microbes to break down oil into carbon dioxide and water over extended periods, ranging from months to decades While it is an effective shore-based response, it does not prevent shoreline contamination To date, there have been no scientifically validated enhanced bioremediation programs implemented at sea Public pressure for immediate pollution response often leads to the neglect of this method, reserving it as a last resort.
Chemical dispersants are formulated to permanently break up surface oil slicks and disperse oil into fine droplets within the water column This process facilitates natural mixing, which dilutes the concentration of subsurface oil, allowing for biodegradation to take place By transferring oil from the water's surface to a more dispersed state, these dispersants help to quickly reduce both acute and chronic toxicity risks to aquatic organisms.
Weathering changes the physical and chemical characteristics of spilled oil over time The removal of spilled oil and refined products occurs from the water's surface to the atmosphere, water column, sediments, and shorelines, a process known as weathering.
Evaporation is the primary weathering process responsible for the removal of oil from the sea surface, which varies depending on the type of oil This process alters the relative abundance of oil components, often resulting in remaining materials that are more challenging to manage during response operations.
Photo-oxidation occurs when oil components undergo chemical transformation due to a photochemical reaction involving sunlight and oxygen, resulting in the formation of new compounds that are often more water-soluble and toxic in the short term than the original substances However, this process contributes only minimally to the overall weathering of oil spills on the water's surface.
Biodegradation occurs when naturally occurring bacteria and fungi utilize petroleum hydrocarbons as a food source, converting these molecules into oxidized by-products These by-products are subsequently further degraded through oxidation into carbon dioxide and water.
Sedimentation transfers oil from the surface and water column to the seafloor bot- tom through: I ) direct sinking;
2) adhering to suspended sediments that eventually settle-out; and 3) as fecal maffer following ingestion
A /hot (Kt) is a unit ofspeed equal to 1 nautical mile per hour, approximately 1.7 feet per second (5 1 centimeters per second or i 15 mph)
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