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Oil dispersants
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    Oil dispersants

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    Color illustration of how oil dispersants work
    Oil dispersant mechanism of action

    An oil dispersant is a mixture of emulsifiers and solvents that helps break oil into small droplets following an oil spill. Small droplets are easier to disperse throughout a water volume, and small droplets may be more readily biodegraded by microbes in the water. Dispersant use involves a trade-off between exposing coastal life to surface oil and exposing aquatic life to dispersed oil. While submerging the oil with dispersant may lessen exposure to marine life on the surface, it increases exposure for animals dwelling underwater, who may be harmed by toxicity of both dispersed oil and dispersant. Although dispersant reduces the amount of oil that lands ashore, it may allow faster, deeper penetration of oil into coastal terrain, where it is not easily biodegraded.

    History

    Plane spraying dispersants over an oil spill
    U.S. Air Force C-130 plane releases dispersants over the Deepwater Horizon oil spill.

    Torrey Canyon

    In 1967, the supertanker Torrey Canyon leaked oil onto the English coastline.Alkylphenol surfactants were primarily used to break up the oil, but proved very toxic in the marine environment; all types of marine life were killed. This led to a reformulation of dispersants to be more environmentally sensitive. After the Torrey Canyon spill, new boat-spraying systems were developed. Later reformulations allowed more dispersant to be contained (at a higher concentration) to be aerosolized.

    Exxon Valdez

    Alaska had fewer than 4,000 gallons of dispersants available at the time of the Exxon Valdez oil spill, and no aircraft with which to dispense them. The dispersants introduced were relatively ineffective due to insufficient wave action to mix the oil and water, and their use was shortly abandoned.

    A report by David Kirby for TakePart found that the main component of the Corexit 9527 formulation used during Exxon Valdez cleanup, 2-butoxyethanol, was identified as "one of the agents that caused liver, kidney, lung, nervous system, and blood disorders among cleanup crews in Alaska following the 1989 Exxon Valdez spill."

    Early use (by volume)

    Dispersants were applied to a number of oil spills between the years 1967 and 1989.

    Year Spill Country Oil volume (L) Dispersant volume (L)
    1967 Torrey Canyon England 119,000,000 10,000,000
    1968 Ocean Eagle Puerto Rico 12,000,000 6,000
    1969 Santa Barbara USA 1,000,000 3,200
    1970 Arrow Canada 5,000,000 1,200
    1970 Pacific Glory England 6,300,000
    1975 Showa Maru Singapore 15,000,000 500,000
    1975 Jakob Maersk Portugal 88,000,000 110,000
    1976 Urquiola Spain 100,000,000 2,400,000
    1978 Amoco Cadiz France 200,000,000 2,500,000
    1978 Eleni V England 7,500,000 900,000
    1978 Christos Bitas England 3,000,000 280,000
    1979 Betelgeuse Ireland 10,000,000 35,000
    1979 Ixtoc I Mexico 500,000,000 5,000,000
    1983 Sivand England 6,000,000 110,000
    1984 SS Puerto Rican USA 7,570
    1989 Exxon Valdez USA 50,000,000 8,000

    Deepwater Horizon

    During the Deep water Horizon oil spill, an estimated 1.84 million gallons of Corexit was used in an attempt to increase the amount of surface oil and mitigate the damage to coastal habitat. BP purchased all of the world's supply of Corexit soon after the spill began. Nearly half (771,000 gallons) of the dispersants were applied directly at the wellhead. The primary dispersant used were Corexit 9527 and 9500, which were controversial due to toxicity.

    In 2012, a study found that Corexit made the oil up to 52 times more toxic than oil alone, and that the dispersant's emulsifying effect makes oil droplets more bio-available to plankton. The Georgia Institute of Technology found that "Mixing oil with dispersant increased toxicity to ecosystems" and made the gulf oil spill worse.

    In 2013, in response to the growing body of laboratory-derived toxicity data, some researchers address the scrutiny that should be used when evaluating laboratory test results that have been extrapolated using procedures that are not fully reliable for environmental assessments. Since then, guidance has been published that improves the comparability and relevance of oil toxicity tests.

    Rena oil spill

    Maritime New Zealand used the oil dispersant Corexit 9500 to help in the cleanup process. The dispersant was applied for only a week, after results proved inconclusive.

    Theory

    Overview

    Surfactants reduce oil-water interfacial tension, which helps waves break oil into small droplets. A mixture of oil and water is normally unstable, but can be stabilized with the addition of surfactants; these surfactants can prevent coalescence of dispersed oil droplets. The effectiveness of the dispersant depends on the weathering of the oil, sea energy (waves), salinity of the water, temperature and the type of oil. Dispersion is unlikely to occur if the oil spreads into a thin layer, because the dispersant requires a particular thickness to work; otherwise, the dispersant will interact with both the water and the oil. More dispersant may be required if the sea energy is low. The salinity of the water is more important for ionic-surfactant dispersants, as salt screens electrostatic interactions between molecules. The viscosity of the oil is another important factor; viscosity can retard dispersant migration to the oil-water interface and also increase the energy required to shear a drop from the slick. Viscosities below 2,000 centipoise are optimal for dispersants. If the viscosity is above 10,000 centipoise, no dispersion is possible.

    Requirements

    There are five requirements for surfactants to successfully disperse oil:

    • Dispersant must be on the oil's surface in the proper concentration
    • Dispersant must penetrate (mix with) the oil
    • Surfactant molecules must orient at the oil-water interface (hydrophobic in oil and hydrophilic in water)
    • Oil-water interfacial tension must be lowered (so the oil can be broken up).
    • Energy must be applied to the mix (for example, by waves)

    Effectiveness

    The effectiveness of a dispersant may be analyzed with the following equations. The Area refers to the area under the absorbance/wavelength curve, which is determined using the trapezoidal rule. The absorbances are measured at 340, 370, and 400 nm.

    Area = 30(Abs340 + Abs370)/2 + 30(Abs340 + Abs400)/2 (1)

    The dispersant effectiveness may then be calculated using the equation below.

    Effectiveness (%) = Total oil dispersed x 100/(ρoilVoil)

    • ρoil = density of the test oil (g/L)
    • Voil = volume of oil added to test flask (L)
    • Total oil dispersed = mass of oil x 120mL/30mL
    • Mass of oil = concentration oil x VDCM
    • VDCM = final volume of DCM-extract of water sample (0.020 L)
    • Concentration of oil = area determined by Equation (1) / slope of calibration curve

    Dispersion models

    Developing well-constructed models (accounting for variables such as oil type, salinity and surfactant) are necessary to select the appropriate dispersant in a given situation. Two models exist which integrate the use of dispersants: Mackay's model and Johansen's model. There are several parameters which must be considered when creating a dispersion model, including oil-slick thickness, advection, resurfacing and wave action. A general problem in modeling dispersants is that they change several of these parameters; surfactants lower the thickness of the film, increase the amount of diffusion into the water column and increase the amount of breakup caused by wave action. This causes the oil slick's behavior to be more dominated by vertical diffusion than horizontal advection.

    One equation for the modeling of oil spills is:

    where

    • h is the oil-slick thickness
    • is the velocity of ocean currents in the mixing layer of the water column (where oil and water mix together)
    • is the wind-driven shear stress
    • f is the oil-water friction coefficient
    • E is the relative difference in densities between the oil and water
    • R is the rate of spill propagation

    Mackay's model predicts an increasing dispersion rate, as the slick becomes thinner in one dimension. The model predicts that thin slicks will disperse faster than thick slicks for several reasons. Thin slicks are less effective at dampening waves and other sources of turbidity. Additionally, droplets formed upon dispersion are expected to be smaller in a thin slick and thus easier to disperse in water. The model also includes:

    • An expression for the diameter of the oil drop
    • Temperature dependence of oil movement
    • An expression for the resurfacing of oil
    • Calibrations based on data from experimental spills

    The model is lacking in several areas: it does not account for evaporation, the topography of the ocean floor or the geography of the spill zone.

    Johansen's model is more complex than Mackay's model. It considers particles to be in one of three states: at the surface, entrained in the water column or evaporated. The empirically based model uses probabilistic variables to determine where the dispersant will move and where it will go after it breaks up oil slicks. The drift of each particle is determined by the state of that particle; this means that a particle in the vapor state will travel much further than a particle on the surface (or under the surface) of the ocean. This model improves on Mackay's model in several key areas, including terms for:

    • Probability of entrainment – depends on wind
    • Probability of resurfacing – depends on density, droplet size, time submerged and wind
    • Probability of evaporation – matched with empirical data

    Oil dispersants are modeled by Johansen using a different set of entrainment and resurfacing parameters for treated versus untreated oil. This allows areas of the oil slick to be modeled differently, to better understand how oil spreads along the water's surface.

    Surfactants

    Surfactants are classified into four main types, each with different properties and applications: anionic, cationic, nonionic and zwitterionic (or amphoteric). Anionic surfactants are compounds that contain an anionic polar group. Examples of anionic surfactants include sodium dodecyl sulfate and dioctyl sodium sulfosuccinate. Included in this class of surfactants are sodium alkylcarboxylates (soaps). Cationic surfactats are similar in nature to anionic surfactants, except the surfactant molecules carry a positive charge at the hydrophilic portion. Many of these compounds are quaternary ammonium salts, as well as cetrimonium bromide (CTAB). Non-ionic surfactants are non-charged and together with anionic surfactants make up the majority of oil-dispersant formulations. The hydrophilic portion of the surfactant contains polar functional groups, such as -OH or -NH. Zwitterionic surfactants are the most expensive, and are used for specific applications. These compounds have both positively and negatively charged components. An example of a zwitterionic compound is phosphatidylcholine, which as a lipid is largely insoluble in water.

    HLB values

    Surfactant behavior is highly dependent on the hydrophilic-lipophilic balance (HLB) value. The HLB is a coding scale from 0 to 20 for non-ionic surfactants, and takes into account the chemical structure of the surfactant molecule. A zero value corresponds to the most lipophilic and a value of 20 is the most hydrophilic for a non-ionic surfactant. In general, compounds with an HLB between one and four will not mix with water. Compounds with an HLB value above 13 will form a clear solution in water. Oil dispersants usually have HLB values from 8–18.

    HLB values for various surfactants
    Surfactant Structure Avg mol wt HLB
    Arkopal N-300 C9H19C6H4O(CH2CH2O)30H 1,550 17.0
    Brij 30 polyoxyethylenated straight chain alcohol 362 9.7
    Brij 35 C12H25O(CH2CH2O)23H 1,200 17.0
    Brij 56 C16H33O(CH2CH2O)10H 682 12.9
    Brij 58 C16H33O(CH2CH2O)20H 1122 15.7
    EGE Coco ethyl glucoside 415 10.6
    EGE no. 10 ethyl glucoside 362 12.5
    Genapol X-150 C13H27O(CH2CH2O)15H 860 15.0
    Tergitol NP-10 nonylphenolethoxylate 682 13.6
    Marlipal 013/90 C13H27O(CH2CH2O)9H 596 13.3
    Pluronic PE6400 HO(CH2CH2O)x(C2H4CH2O)30(CH2CH2O)28-xH 3000 N.A.
    Sapogenat T-300 (C4H9)3C6H2O(CH2CH2O)30H 1600 17.0
    T-Maz 60K ethoxylated sorbitan monostearate 1310 14.9
    T-Maz 20 ethoxylated sorbitan monolaurate 1226 16.7
    Triton X-45 C8H17C6H4O(CH2CH2O)5H 427 10.4
    Triton X-100 C8H17C6H4(OC2H4)10OH 625 13.6
    Triton X-102 C8H17C6H4O(CH2CH2O)12H 756 14.6
    Triton X-114 C8H17C6H4O(CH2CH2O)7.5H 537 12.4
    Triton X-165 C8H17C6H4O(CH2CH2O)16H 911 15.8
    Tween 80 C18H37-C6H9O5-(OC2H4)20OH 1309 13.4

    Comparative industrial formulations

    Two formulations of different dispersing agents for oil spills, Dispersit and Omni-Clean, are shown below. A key difference between the two is that Omni-Clean uses ionic surfactants and Dispersit uses entirely non-ionic surfactants. Omni-Clean was formulated for little or no toxicity toward the environment. Dispersit, however, was designed as a competitor with Corexit. Dispersit contains non-ionic surfactants, which permit both primarily oil-soluble and primarily water-soluble surfactants. The partitioning of surfactants between the phases allows for effective dispersion.

    Omni-Clean OSD Dispersit
    Category Ingredient Function Category Ingredient Function
    Surfactant Sodium laurylsulfate V.1.svg Sodium lauryl sulfate Charged ionic surfactant and thickener Emulsifying agent Oleic Acid Sorbitan Monoester Oleic acid sorbitan monoester Emulsifying agent
    Surfactant Cocamidopropyl betaine2.png Cocamidopropyl betaine Emulsifying agent Surfactant Coconut oil monoethanolamide.png Coconut oil monoethanolamide Dissolves oil and water into each other
    Surfactant Nonoxynol-9.png Ethoxylated nonylphenol Petroleum emulsifier & wetting agent Surfactant Poly(ethylene glycol) monooleate.png Poly(ethylene glycol) monooleate Oil-soluble surfactant
    Dispersant Lauramide DEA Structural Formula V1.svg Lauric acid diethanolamide Non-ionic viscosity booster & emulsifier Surfactant Polyethoxylated tallow amine.svg Polyethoxylated tallow amine Oil-soluble surfactant
    Detergent Diethanolamine.png Diethanolamine Water-soluble detergent for cutting oil Surfactant Polyethoxylated linear secondary alcohol.png Polyethoxylated linear secondary alcohol Oil-soluble surfactant
    Emulsifier Propylene glycol chemical structure.png Propylene glycol Solvent for oils, wetting agent, emulsifier Solvent Dipropylene glycol methyl ether.png Dipropylene glycol methyl ether Enhances solubility of surfactants in water and oil.
    Solvent H2O Water Reduces viscosity Solvent H2O Water Reduces viscosity

    Degradation and toxicity

    Concerns regarding the persistence in the environment and toxicity to various flora and fauna of oil dispersants date back to their early use in the 1960s and 1970s. Both the degradation and the toxicity of dispersants depend on the chemicals chosen within the formulation. Compounds which interact too harshly with oil dispersants should be tested to ensure that they meet three criteria:

    • They should be biodegradable.
    • In the presence of oil, they must not be preferentially utilized as a carbon source.
    • They must be nontoxic to indigenous bacteria.

    Methods of use

    Oil Spill Response Boeing 727 displaying its dispersants delivery system at the 2016 Farnborough Airshow

    Dispersants can be delivered in aerosolized form by an aircraft or boat. Sufficient dispersant with droplets in the proper size are necessary; this can be achieved with an appropriate pumping rate. Droplets larger than 1,000 µm are preferred, to ensure they are not blown away by the wind. The ratio of dispersant to oil is typically 1:20.

    See also

    Further reading


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