Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.
OCR for page 28
2 Chemistry and Physics of Dispersants and Dispersed Oil Most oils spilled on water rapidly spread into a slick, with thick- ness from several millimeters down to one micrometer depending on the oil type and the area available for spreading. Wind-driven waves and other turbulence can break up the slick, producing more or less spherical droplets ranging in size from a few micrometers to a few millimeters. Sometimes, these droplets can be stabilized by natural surface-active agents (surfactants) present in the of] or contributed by the sea-surface microlayer in the region where the oil was spiked. These surfactants stabilize the droplets by orienting in the oil-water interface with the hydrophobic part of the surfactant molecule in the oil phase and the hydrophilic part in the water phase, thereby diminishing the interfacial tension. Applying chemical dispersants to an of} slick greatly increases the amount of surfactant available and can reduce oil-water interfacial tension to very low values it therefore tales only a small amount of mixing energy to increase the surface area and break the slick into droplets (Figure 2-1~. Dispersants also tend to prevent coalescence of of! droplets. The interface, stabilized by the surfactant, permits droplets to survive despite frequent collisions with adjacent droplets. The same stabi- lizing factors reduce adherence to hydrophilic solid particles, such as sediments, as well as other solid surfaces (discussed later in Chapter 4~. 28
OCR for page 29
CHEMISTRY AND PHYSICS OF OIL DISPERSANTS A 29 Application By_ B Hydrophilic Group Hydrophil Portion of Dispersant Prevents Droplet Coalescence Water - Lipophilic a~;g Surfactant-Stabilized c~°il Droplets FIGURE 2-1 Medh~sm of chemical dispersion. A. Surfactant locates at oil-water interface. B. Oil sliclc is readily dispersed into micelles or surfactant-stabilized droplets with mirumal energy. Source: Derived from C~evari (1969~. During the past 20 years, significant reviews and descriptions of dispersant chemistry include those by Poliakoff (1969), Dodd (1974), Canevari (1971, 1985), Wells (1984), Pastorak et al. (1985), Wells et al. (1985), API Task Force (1986), and Brochu et al. (1987~. COMPOSITION OF DISPERSANTS The key components of a chemical dispersant are one or more surface-active agents, or surfactants sometimes loosely caned "de- tergents." They contain molecules with both water-compatible (hy- drophilic) and oil-compatible (lipophilic or hydrophobic) portions. Most formulations also contain a solvent to reduce viscosity and facilitate dispersal. Chemistry of Surfactants The behavior of a surfactant is strongly affected by the balance betweeen the hydrophilic and lipophilic groups in the molecule. Grif- fin (1954) defined the hydrophile-lipophile balance. The useful range of this parameter is from 1 (most lipophilic) to 20 (most hydrophilic). Many organic compounds, like hexane, with no hydrophilic groups could have HLB as low as zero, and would not be surface active. In
OCR for page 30
30 USING OIL SPILL DISPERSANTS ON THE SEA the HLB range of 1 to 4, the surfactant does not mix in water; above 13, a clear solution in water Is obtained tHosen, 1978~. Bancroft's rule states that the dominant group of a surfactant tends to be oriented in the external phase (Bancroft, 1913~. Thus, a predominantly lipophilic surfactant (HLB, 3 to 6) would stabilize a water-in-oi} emulsion, and a predominantly hydrophilic surfactant (HLB, ~ to 18) would stabilize an oil-in-water emulsion. Surfact ant s used in of} spin dispersants tend to be of the latter type. Natural surfactants, which promote mousse (water-in-oil emulsion), tend to be predominately lipophilic. HLB is important in determining the effect of salinity on dis- persant performance, since hydrophobic portions of the surfactant molecule tend to be salted out. Laboratory measurements on weath- ered crude of} with a dispersant sensitive to salinity showed that, at a comparable treatment rate and mixing energy, the amount of oil dispersed is approximately 58 percent in seawater compared to 1 percent in fresh water. This formulation, balanced for effective performance in seawater, is too hydrophilic for freshwater service (Canevari, 1985~. Surfactants are also classed by charge type, as noted below (a list of formulations is given in Appendix A): Anionic. Examples include sulfosuccinate esters, such as sodium diocty! sulfosuccinate (e.g., Aerosol OT). Other examples are oxyalkylated Cue to Ci5 alcohols and their sulfonates. · Cationic. An example is the quaternary ammonium salt RN(CH3~3+CI-, but such compounds are often toxic to many or- ganisms and are not currently used in commercial dispersant formu- lations (I`ewis and Wee, 1983~. · Nonionic. These are the most common surfactants used in commercial dispersant formulations. Examples are sorbitan mono- oleate (HLB, 4.3), sold as Span 80, and ethoxylated sorbitan mono- oleate (HUB, 15), sold as Tween 80. In addition, polyethylene glyco! esters of unsaturated fatty acids and ethoxylated or propoxylated fatty alcohols are used. ~ Zwitterionic or amphoteric. These molecules contain both positively and negatively charged groups, which may balance each other to produce a net uncharged species. An example would be a molecule with both a quaternary ammonium group and a sulfonic acid group (refer to Appendix A), but such compounds are not found in current commercial formulations.
OCR for page 31
CHEMISTRY AND PHYSICS OF OIL DISPERSANTS 31 As surfactants become more concentrated, the interfacial tension between of] and water decreases until a critical miceDe concentration (CMC) is reached. Micelles are ordered aggregates of surfactant molecules, with the hydrophobic portions of the molecules together at the interior of the micelle and the hydrophilic portions facing the aqueous phase. Above the CMC level, there is little change in inter- facial tension, and additional surfactant molecules form new micelles. Below the CMC, additional surfactant molecules accumulate at the water-air or oil-water interfaces. The CMC can be estimated from the concentration at which a change in slope of a plot of interfacial tension (as measured by the drop weight technique) versus dispersant concentration occurs (Figure 2-2~. Some formulations can reduce interfacial tension to a few percent of the value without surfactant addition. For example, in a specially adapted Wilhelmy Plate instrument, the initial oil-water interfacial tension of 18 dye/cm was reduced to the minimum detectable value, approximately 0.05 dye/cm, within ~ min after dispersant was added (Ross anti Kvita, unpublished! data). Current Dispersant Formulations Early dispersant formulations were derived from engine room degreasers, and some were highly toxic (Chapter 1~. To reduce toxicity, nonaromatic hydrocarbons (or water-miscible solvents such as ethylene glyco! or glycol ethers), as well as less toxic surfactants, have been used in more recent formulations (Chapter 4 and Appendix A). Figure 2-3 illustrates how different surfactants become oriented at the oil-water interface. Compound A is sorbitan monooleate (HLB, 4.3), predominantly lipophilic. Compound B is Compound A that has been ethoxylated with 20 mol of ethylene oxide, rendering it more hydrophilic (HLB, 15~. A dispersant containing both A and B. with a larger amount of B. can stabilize an oil-in-water emulsion.* A blend of surfactants with different HLB, giving a resultant HLB of 12, wiD be more effective than a similar quantity of a sin- gle surfactant with HLB of 12. This is shown in Figure 2-3: the hydrophilic groups of B penetrate farther into the water phase, per- *As discussed earlier, the dominant group of a surfactant tends to be oriented in the external phase (Bancroft, 1913~.
OCR for page 32
32 USING OIL SPILL DISPERSANTS ON THE SEA 22 21 20 19 18 a, 16 to- 15 '.1~3 1 4 ,_ 13 J to 1 ~ O '' 11 10 9 8 7 6 ~ ~ ~ Dispersant C If\ \\ cmc = 5.3 + 2 ppm Dispersant F cmc = 38.2 + 6 ppm 1 ;\ Dispersant B cmc = 58.0 + 5 ppm I: 0 10 20 30 40 50 60 70 80 90 100 DISPERSANTIN SEAWATER(ppm) FIGURE 2-2 Interfacial tension as a function of Dispersant concentration showing the discontinuity in slope at the critical micelle concentration (cmc). Light Arabian crude with three dispersant formulations at 28°C aIld 38 percent salinity; interracial tension is measured by the dro~weight method. Source: Rewidlc et al., 1984. Reprinted, with permission, from the American Society for Testing and Matenals. ~ 1984 by ASTM. milting closer physical interaction between the lipophiles of both A and B. The overall result is to provide a stronger interfacial surfactant film and resistance to coalescence of dispersed of! droplets. A review of the patent literature (Appendix A), combined with discussions with several major suppliers of dispersants, indicates that a limited number of surfactant chemicals are used in the dispersant formulations most widely available today. The exact details of disper- sent formulations are proprietary, but the chemical characteristics of these formulations are broadly known (Canevari, 1986; Brochu et al., 1987; Wells et al., 1985; Fraser, private communication). Thus,
OCR for page 33
33 m ~ m ~ Z ~ to\ ~ ~ '. — _ ._ m Cal U ~ 0 ~ on
OCR for page 34
34 USING OIL SPILL DISPERSANTS ON THE SEA modern dispersant formulations containing one or more nonionic sur- factants (15 to 75 percent of the formulation) may also contain an anionic surfactant (5 to 25 percent of the formulation) and include one or more solvents. The surfactants used include the following: nonionic surfactants, such as sorbitan esters of oleic or lauric acid, ethoxylated sorbitan esters of oleic or lauric acid, polyethy- lene glyco} esters of oleic acid, ethoxylated and propoxylated fatty alcohols, and ethoxylated octy~phenol; and ~ anionic surfactants, such as sodium dioctyl sulfosuccinate and sodium ditridecanoyl sulfosuccinate. Dispersant formulations also contain a solvent to dissolve solid surfactant and reduce viscosity so that the dispersant can be sprayed uniformly. A solvent may be chosen to promote rapid solubility of the surfactant in the of} and to depress the freezing point of the dispersant mixture so that it can be used at lower temperatures. The three main cIasses of solvents are: (~) water, (2) water- miscible hydroxy compounds, and (3) hydrocarbons. Aqueous sol- vents permit surfactants to be applied by eduction into a water stream. Hydrocarbon solvents enhance mix~ng and penetration of surfactant into more viscous oils. Examples of hydroxy-compounc! solvents are ethylene glyco} monobuty] ether, diethylene glyco] mono- methy} ether, and diethylene glyco! monobuty! ether. An example of a hydrocarbon solvent is a low-aromatic kerosene. High-boiling solvents containing branched saturated hydrocarbons are also used since they are less toxic than aromatics. Appendix A gives a list of chemical formulations for use on oiT discharges listed in 1987 by the U.S. Environmental Protection Agency in its National Contingency Plan Product Schedule (Flaherty and Riley, 1987~. Composition of some of these formulations have been disclosed in patents, but all are proprietary. Only U.S. EPA listed formulations may be used to treat of} discharges in U.S. waters (Chapter 6~. Matching Dispersant Formulations With Oil Type for Increased Effectiveness Because oils vary widely in composition, it is reasonable to hy- pothesize that a particular dispersant formulation could be more effective with one of} than another; indeed that a dispersant could
OCR for page 35
CHEMISTRY AND PHYSICS OF OIL DISPERSANTS 35 be matched to a particular of! for increased, possibly optimal, ef- fectiveness. This is not possible at present for a number of reasons. Only a limited number of different dispersants have been used or are available. Such a narrow subset of all possible formulations has resulted from a convergent evolution in the industry. Furthermore, limited use has been made of chemical dispersants during accidental spins in the United States, and only a few research spills have been conducted using different dispersants with the same oil. Because it is not possible to conduct field tests of all dispersants with all oils, a more practical approach has been taken: formulations that perform best in laboratory studies are used on oil spills. If a dispersant works, it continues to be used. It should be noted, however, that the effect of dispersant appli- cation may be delayed, as was observed in Lichtenthaler and Daling's (1985) Norwegian offshore research studies. Effectiveness also may be reduced by of] resurfacing later (Bocard et al., 1987~. Temperature can affect dispersant performance in ways other than by changing the viscosity of the oil. The solubility of ethoxylated surfactants in water increases at lower temperature. For example, at similar dispersant- oil ratios, water salinity, and wave energy, dispersant OSD-1 was found to be 100 percent effective (aD of! was removed from the water surface) in laboratory tests at 15°C, but only 56 percent effective at 5°C. In contrast, dispersant EXP-A was 100 percent effective at both temperatures. This difference was explained by changes in water solubility and HLB of the dispersant (Becker and Lindblom, 1983~. Fate of Surfactants and Solvents in the Aqueous Environment Surfactants are used for many purposes other than treating of} spills, and their degradation in the aqueous environment has been a concern since the 1950s, when synthetic alky~benzene sulfonate (ABS) detergents were found to be resistant to biodegradation and produced persistent foam on waters receiving domestic sewage efflu- ent. This problem was solved by replacing the ABS detergents with more readily biodegradable surfactants. Manufactured surfactants in 1983 totalled about 24 m~lion metric tons worIdwide, most of which were employed as household detergents and industrial cleaners (Lay- man, 1984:49~. It is recognized that surfactants used in detergents may contribute to stream poDutant discharges that are continuous and often affect a large area over many years. In contrast, disper- sants and dispersed of] inputs to the sea are usually rare or infrequent
OCR for page 36
36 USING OIL SPILL DISPERSANTS ON THE SEA events that could cause temporary effects in the open sea, but cause large, short-term disruptions in restricted areas. Therefore, a direct comparison of continuous surfactant discharge from industrial and household use and dispersant surfactant loading is tenuous, at best, in regard to environmental effects of the two sources. Some discharged surfactants are biodegraded in sewage treat- ment plants, but many are not because much of the sewage is not treated. Some of the linear alky~benzenes (LAB), used in the man- ufacture of linear alky} sulfonates (LAS) remain as an impurity in LAS, and are found in suspended particles and sediments surround- ing municipal waste discharges. Eganhouse et al. (1983) used LAB as tracers and stated that they appear to be preserved in sediments for 10 to 20 years. Some work has been done on the rate of breakdown and environ- mental concentrations of surfactants in the water column (Kozarac et al., 1983; I'acaze, 1973, 1974; Penrose et al., 1976; Una and Gar- cia, 1983~. Surfactants are also transferred from water to air via sea spray, and increase the production of marine aerosol. They may thus encourage the transfer of of] slick components into the atmo- sphere (Fontana, 1976~. Adsorption of surfactn~nts onto sediments is discussed later in this chapter (see also Inoue et al., 1978~. Surfactants can also become bioconcentrated and metabolized in the tissue of fishes and invertebrates (Comotto et al., 1979; Kimerie et al., 1981; Payne, 1982; Schmidt and KimerIe, 1981~. Metabolic breakdown of the surfactants is rapid (85 percent in 4 days). The fate of solvents used in dispersant formulations might also be of concern. Hydrocarbon solvents are similar to portions of the of! being dispersed and tend to suffer a similar fate. Glyco] ether solvents are likely to be more readily biodegradable than the of] being dispersed, but nothing appears in the literature about the toxicity of their degradation products. However, the concentrations are usually small and decrease rapidly owing to dilution and mixing. FATE OF Off SPILLED ON OPEN WATER Slick Thickness Slick thickness is an important parameter in predicting optimum dispersant dosage (Chapter 5), but the thickness of an of} slick at sea cannot be readily determined. Reliable measurements of thickness over the whole area of a-slick have rarely been made (HoDinger and
OCR for page 37
CHEMISTRY AND PHYSICS OF OIL DISPERSANTS 37 Menella, 1973; Lehr et al., 1984; 0'Neill et al., 1983~. Infrared remote sensing provides an image of thick slicks, 10 to 50 I'm or greater (O'NeiD et al., 1983), and ultraviolet sensing can measure slicks down to the submicron range. Combined, infrared and ultraviolet remote sensors can be used to calculate areas and ratios of thin to thick slicks. The limits to infrared detection are unknown, however, and certainly vary with environmental conditions and of} type. Varying intensity levels in the infrared have been processed to yield additional contours, but assignment of thickness to such contours, although attempted, has been only relative (Ross, 1982~. Actual slicks at sea are nonuniform in thickness and distribution on the surface. The thickest portion of a slick can be as great as several millimeters and the "sheen" (m~crolayer) only 1 to 10 ~m. In one experiment, infrared thermography showed that thin areas of the slick were 10 to 20 Am and thick areas were 150 to 200 ~m. The thicker portion contained 28 of the 42 bb! of spired oil (Bocard et al., 1984), but covered only about one-fifth of the stick's surface area. Most estimates of slick thickness are averages based on the visual appearance of the of} or calculated by dividing the total volume of of} by its observed area (International Tanker-Owners Pollution Federation [ITOPF], 1982~. Some investigators believe that using average thickness, although formally consistent, is misleading and obscures one of the most important aspects of an oil slick from the cleanup team's point of view its nonuniformity. Also, from a biological point of view, exposures under a nonuniform stick are likely to be patchy. Nevertheless, many spills of widely varying size tend to reach a similar average thickness of about 0.1 mm rather quickly and this rule of thumb is widely employed by dispersant application specialists. Over several days, as the slick spreads, average thickness may decrease to 0.01 mm (API Task Force, 1986; McAuliffe, 1986~. The following is some evidence for such a generalization: · The Chevron Main Pass Block 41 C blowout, released of! at 1,680 to 6,650 bbl/day. The slick size varied, but was about 1-km wide and 10-km long, with thicker of] near the platform. The average thickness was 0.02 to 0.09 mm (McAuliffe et al., 1975; Murray, 1975~. · The Hasbah 6 of] well blowout in the Arabian Gulf released a viscous oil slick that extended for many miles. After 2 weeks, 9,930 bb} of of! were skimmed from an area of 11.3 km2. The slick was thus estimated to be at least 0.13-mm thick (Cuddeback, 1981~.
OCR for page 38
38 USING OIL SPILL DISPERSANTS ON THE SEA · In the API-EPA research spills, 20 bb} were released over 5 to 10 min. corresponding to rates of 3,000 to 6,000 bbl/day. After 15 to 30 min. before dispersant spraying began, the slicks covered 20,000 to 30,000 m2. Average thickness was therefore 0.1 to 0.2 mm (Johnson et al., 1978; McAuliffe et al., 1980, 1981~. · In a Norwegian test spill of 700 bb} over 2 hr (corresponding to 8,400 bbl/day), the thick part of the slick, containing 90 percent of the oil, covered 1 km2 after ~ hr and remained at 1.5 to 2 km2 for the next 5 days. The average spin thickness decreased from 0.06 to 0.013 mm (Audunson et al., 1984~. · Data from the 1983 Halifax trials showed average thicknesses of untreated slicks was of the order of 47 Em after ~ hr. decreasing to 40 ,um after 2 hr. The thin portion of the slicks was estimated to be of the order of 1-pm thick (Canadian Offshore Aerial Applications Task Force [COAATF], 1986~. A large release over a short time, such as would occur from a tanker accident, initially produces much thicker slicks near the release point. For example, release of 200,000 bb} into an area 100 m in diameter could create an average thickness of 20 mm, but if the slick spreads, the average thickness decreases. In cold climates and waters, however, the higher viscosity of of! can cause a stable slick to be thicker than 0.! mm. In addition, slick thickness could increase as the of! becomes emulsified to form mousse, which occurred during the Amoco Cadiz disaster and many other incidents. Slick Spreading Oil slicks are usually nonuniform in thickness because of the in- teraction of interfacial tension, gravity, and viscosity in spreading processes, the accumulation of oil at downwehing convergence zones procluced by water movement, and the formation of high-viscosity water-in-oi} emulsions (mousse). Furthermore, oils have different spreading tendencies, particularly on cold water (Tramier et al., 1981~. The work of Fay (1971) provided a mathematical prediction for spreading under the influence of interracial tension and gravity. His model predicts an area increasing to a maximum value propor- tional to the 3/4 power of the volume of oil spilled. Although the model included water viscosity, it did not include the effects of of! viscosity, emulsification, and evaporation, and considered only a calm water surface. Nevertheless, the model was successful in predicting
OCR for page 70
70 USING OIL SPILL DISPERSANTS ON THE SEA LABORATORY STUDIES OF EFFECTIVENESS Purpose of Laboratory Testing The general objectives of laboratory testing of dispersants in- clude the following: . testing a variety of dispersants to rank their relative effective- ness (e.g., Doe and Wells, 1978; Mackay and Szeto, 1981; Mackay et al., 1984; Martinelli, 1984; Rewick et al., 1981, 1984~; and · testing effectiveness of dispersants under carefully controlled conditions to assess the role of oil type, weathering state, dispersant- oil ratio, mixing energy, salinity, temperature, and application meth- ods (e.g., Byford et al., 1983; I,ebtinen and Vesala, 1984; Mackay et al., 1984; Payne et al., 1985; U.S. EPA, 1984~. Laboratory tests are also used to screen dispersant types prior to more expensive field testing (Meeks, 1981; Nichols and Parker, 1985~. They provide data for contingency planning; for stockpiling specific dispersants for particular environments, oil types, or deployment methods (Byford et al., 1983; U.S. EPA, 1984~; and ultimately for deciding whether or not to use a particular dispersant (Mackay and Wells, 1983~. Mathematical models for dispersal of oil can be partially vali- dated in the laboratory (Mackay, 1985~. Appropriate concentrations for toxicity testing can also be determined in laboratory tests (An- derson et al., 1985; Bocard et al., 1984; Mackay and Wells, 1983; Wells et al., 1984b). There are three generic types of laboratory tests in use as of 1987: 1. Tank tests with water volumes of 6 to 150 liters, including test vessels agitated using circulated seawater, and tests that employ breaking or nonbreaking waves to generate more realistic turbulent mixing energy. Examples are the EPA test (U.S. EPA, 1984), MNS test (Mackay et al., 1984), and the French Institute of Petroleum (IFB) test. 2. Shake-flask or rotating flask tests that are conducted on a 1-liter scale. Examples include the WSL Labofina test (Martinelli I984~. 3. Interfacial tension tests that measure properties of the treated oil instead of degree of dispersal in a system with given energy input (Rewick et al., 1984~. L7
OCR for page 71
CHEMISTRY AND PHYSICS OF OIL DISPERSANTS 71 The most common of these tests are compared later in this chapter. All tests reviewed establish an oil slick and then apply disper- sant in a defined dispersant-oi} ratio. Dispersant may be applied by spraying neat or mixed with seawater, by pouring into a ring on the water surface that contains the of} slick, by adding dispersant to seawater, or by premixing the dispersant with the oil. Mixing energy is applied by a high-speed propeller, by rotating a separatory funned containing the oil-dispersant mixture (Labofina), by a spray hose and circulation pump (EPA), or by a high-velocity air stream (MNS). Dispersant effectiveness is determined by one of the following four criteria: I. The amount of of! dispersed in the water. This can be mea- sured by visual observation, or by solvent extraction and spectropho- tometric analysis. The amount of dispersed oil may be determined under dynamic conditions (e.g., MNS and EPA tests) or after mixing has terminated (e.g., Labofina and MNS tests). It is also desirable to measure the amount of of! remaining in the surface slick to obtain a mass balance, but standard methods for doing so have not yet been developed (Nichols and Parker, 1985~. 2. Dispersed of} droplet size (Byford et al., 1984; Lewis et al., 1985~. This is another important criterion since larger droplets resur- face some time after dispersal in the water column. The volume mean diameter in the MNS test was 14 to 226 ,um depending on the dispersant-to-oi} ratio (DOR) and the dispersant used (Byford et al., 1984; Lewis et al., 1985~; in the Labofina test it was less than 154 am. 3. Dispersed droplet stability as a function of time and turbu- lence in both static and dynamic systems (Mackay et al., 1984; U.S. EPA, 1984). 4. Interfacial tension (Mackay and Hossain, 1982; Rewick et al., 1981, 1984) has been used to rank dispersant formulations, but the static character of the measurement makes correlation with dispersal under turbulent conditions unrealistic. Critical Factors In all tests, oil-water ratio, dispersant-oi] ratio, dispersant appli- cation method, mixing energy application, and methods of sampling and analysis were found to be critical factors in determining the pre- cision of results. Oil-water ratio is most important for the relatively
OCR for page 72
72 i: USING OIL SPILL DISPERSANTS ON THE SEA hydrophilic dispersant formulations (i.e., those with relatively high HLB) since greater dispersion occurs with higher concentration of surfactant; this will be the case if the volume of water is smaller for the same volume of oil and surfact ant. The nearly infinite capacity of the open ocean for diluting hydrophilic dispersant is not normally ac- counted for in laboratory tests. Typical oil-t~water ratios are 0.02:1 for the Labofina test, 0.0017:! for the MNS test, and 0.00077:1 for the EPA test. At high oil-water ratios, collisions (and possible coalescence) of dispersed droplets are more frequent; this too is an unrealistic simulation of the open ocean. Dispersant-oil ratio is especially important below 0.2:l, where a steep dependence of effectiveness on dispersant-oil ratio is observed (Rewick et al., 1981~. The method of applying the dispersant to the oil is a key factor In an effectiveness test. Types of application used include: dispersant premixed with water; dispersant premixed with oil; · neat (undiluted) dispersant, poured on slick; and · neat dispersant sprayed on slick (this is the only test that has direct field relevance). For dispersant sprayed on the slick, droplet diameters in the 200 to 700 Am range are desirable. These diameters are similar to those produced by dispersant spray systems used in practice, which is a fortunate match of circumstances (Chapter 5~. Dispersal tends to be more efficient with smaller drop sizes. Other factors affecting results include amount (volume) of dispersant in the slick surface, oil slick thickness, and drop-to-drop distance in the sprayed slick (Mackay et al., 1984~. These are areas that continue to be important in laboratory research. How mixing energy is applied in the laboratory is also a major factor. Recognizing that premixing dispersant with of} or water does not realistically represent field conditions, various methods have been employed to mix dispersant with oil: swirling flasks, water jetting, and air streams, for example. The mixing energy can be affected by the materials under study. In the MNS test, wave amplitude is reduced by No. 6 fuel oil and other viscous oils resulting in less dispersal (Mackay and Szeto, 1981; Mackay and Wells, 19S3; Mackay et al., 1984~. Oil-water ratio appears to be at least as important as viscosity and mixing energy.
OCR for page 73
CHEMISTRY AND PHYSICS OF OIL DISPERSANTS 73 Salinity and temperature are environmental factors that affect the results of all effectiveness tests. Salinity affects the hydrophilic- lipophilic balance, and salting-out effects diminish water solubility of ethoxylated surfactants. Lower temperatures tend to increase viscosity of both of} and dispersant as weD as changing solubility of various components. In some cases the effect of temperature can be so great that an of] dispersible at 15°C may not be dispersible at 5°C (Lehtinen and Vesala, 1984; Weds and Harris, 1979~. Sampling and analysis are the last factors to be considered. Be- cause of} drops resurface, the most reproducible results are obtained by sampling while mung is proceeding, or at predetermined times immediately after mixing is stopped (Welis et al., 1984a). Contami- nation of surfaces with of! is frequently a cause for major errors. Need for Standard Testing Oils A number of investigators (Canevari, 1985; Mackay et al., 1986; Fingas, private communication) have expressed the need for a sur- rogate or standard of} for dispersant testing, which would improve reproducibility of product testing and provide international intercal- ibration of methods and products. Canevari (1985) recommended that tetradecane be used. However, Mackay et al. (1986) have shown that long-chain paraffins are a primary inhibitor of effectiveness. Short-chain alkalies also reduce dispersant effectiveness, but not as dramatically, and aromatic compounds increase effectiveness. In view of these findings, a single compound as surrogate of] (e.g., te- tradecane) would not be representative of the dispersion properties of real oils. A multicomponent mixture containing aLkanes and aromatics might be more representative. In earlier work Mackay and Leinonen (1977) constructed such a synthetic oil, consisting of 10 components, to test evaporation rate modeling. Their mixture anticipated chemi- cal and natural dispersion. The surrogate oil consisted of the normal isomers of butane, hexane, octane, decane, dodecane, and hexade- cane, as well as benzene, toluene, naptthalene, phenathrene, and an "inert" component. From 1980 to 1982 the American Petroleum Institute in coop- eration with the U.S. Environmental Protection Agency set aside a number of oils for of] spill testing. They include Pru~hoe Bay and Arabian light crudes and No. 6 and No. 2 fuel oils. The U.S. EPA has analytical data on all of the stored oils. The actual samples 19
OCR for page 74
74 USING OIL SPILL DISPERSANTS ON THE SEA 55-gal (208-liter) drums of each of six oils are stored* at EPA's Oil and Hazardous Materials Simulated Environmental Test Tank (OHMSETT) facility at Leonardo, New Jersey (Kolde, private com- munication). At the same time, Canada's Environmental Protection Service set aside a reference oil (Environment Canada, 1984; Fingas, private communication). These oils have been available upon request to investigators in Canada, the United States, and other countries. Advantages and Disadvantages of Testing Methods To be useful, a laboratory test should be fairly easy and quick to perform. This is satisfied by shake-flask tests (Abbott, 1983), but not by the more sophisticated tests requiring specialized apparatus and trained operators. On the other hand, these more complex tests are designed to mimic actual field conditions more closely. A test should also be repeatable (within the same laboratory), reproducible (from one laboratory to another), and precise (variation coefficient of less than 20 percent). This is not always easy to achieve. Most significantly, the test should show a good correlation with real dispersant operations at sea. Although serious attempts have been made to mimic sea surface turbulence, so far no test satisfies this · ~ criterion. For example, the French Institute of Petroleum test, which uses a beating hoop just under the surface and changes the water contin- uously, was compared with the WSL Labofina test, using a variety of oil types and dispersants (Bocard et al., 1984; GiDot and Charlier, 1986~. No correlation on the basis of hydrophile-lipophile balance was observed, but rank order of effectiveness was often the same for both tests. Two primary differences are continuous washout of soluble materials in the IFP test, and greater mixing energy of the Labofina test (500 W/m3 versus 1.5 W/m3), although the longer run time of the IFP test (1 to 5 hr) brings total energy input into the same range. The IFP test process, by its continuous removal of water con- taining dispersed oil, tends to simulate at-sea flow conditions while other test processes use a fixed volume of water. This feature of the IFP process can be a disadvantage since the test system is more *Test oil stored at OHMSETT is kept under positive pressure and normal NO2 atmosphere, and stored at the ambient temperature of the facility. Oil is checked for indications of aging or deterioration before testing (Tennyson, private communication).
OCR for page 75
CHEMISTRYAND PHYSICS OF OIL DISPERSANTS 75 complex to set up and transport. The test has been used in France and Norway, but few researchers are familiar with the process. In addition, the control of turbulence generated by the IFP agitation device is significantly affected by the levier of the water surface. As a result, reproducibility of tests between laboratories is uncertain. Labofina Test The WSL Labofina test uses a standard separatory funnel in which the test fluids are mixed by a mechanical rotator (Martinelli, 1984~. It has one major advantage: it is simple and fast- 16 tests per day can be conducted. It is as reproducible as other tests (variation coefficient of 10 to 14 percent). The Labofina test shows a general bias toward lower effectiveness ratings for many oils owing to the relatively high oil-to-water ratio in this test (Daring, 1988~. However, this test tends to give a relatively high efficiency rating for high-density oils, which do not rise rapidly after agitation has stopped (Daring, 1988~. Byford and Green (1984) report good agreement between the MNS and Labofina tests, when used to identify optimum surfactant combinations. The parameter that caused the greatest effect on the Labofina test was the shape of the conical separatory funnel. Precision of timing was also important: the stopcock orifice diameter affected results since a narrower orifice lengthened the time to collect a 50-m} aliquot of dispersed of] for analysis. Oil adhering to the flask walls can also produce major errors. The major disadvantages of the Labofina test are that it uses an unreaTisticaBy high oil-water ratio, and mixing does not simulate the turbulence of an ocean surface. The samples are collected un- der static conditions, and the results depend on precisely when the samples are collected after mixing stops. Mackay-Nadeau-Steelman Test The MNS test uses a 20-liter closed glass, temperature-controlled tank, with a stream of air blowing tangentially on the water surface to generate reproducible waves and turbulence (Mackay and Szeto, 1981; Mackay et al., 197S, 1984; U.S. EPA, 1984; Wells and Harris, 1979~. It reproduces turbulence that closely simulates actual mix- ing conditions. In this dynamic measurement, effectiveness can be assessed as a function of time, and of} film thickness can be indepen- dently controlled. Airflow rate is critical, however, and satisfactory
OCR for page 76
76 USING OIL SPILL DISPERSANTS ON THE SEA waves cannot be generated unless the apparatus is precisely level (Byford and Green, 1984~. Reproducibility is good (variation coeffi- cient of 10 to 15 percent), and six tests can be completed per day by one operator. The disadvantages of the MNS test are that its mixing energy, while reproducible, is difficult to quantify, and wave dampening by the materials under study can affect the results. It also gives anoma- lous results at 0°C, because of viscous shearing (Fingas et al., 1987~. The airflow rate and direction must be critically adjusted, the appa- ratus is complex, and since it is hand-made tends to be expensive. The MNS and WSL tests were compared in a study involving 13 dispersants and two of} types (Daring, 1986~. Low-performance products were rated similarly by both tests, but the ranking of more effective products was generally different. The MNS test seems to be more reliable for low-viscosity oils, while the WSE test gives information for the higher-viscosity range. A recent comparison of effectiveness for 10 oils using the MNS, Labofina, and swirling flask methods demonstrated a poor correlation between results (Fingas et al., 1987~. U.S. Environmental Protection Agency Test The EPA test uses a 130-liter stainless steel tank,* that, with its larger water volume, allows lower oil-water ratios (U.S. EPA, 1984~. Use of a spray simulates application in the field from a boat. In this test also, effectiveness can be measured as a function of time, and samples can be obtained under dynamic conditions. Reproducibility is good for one operator using a consistent procedure (variation coefficient of less than 10 percent). Critical factors include rate of application of dispersant to the oil surface, height above the surface from which it was poured, exact height of the jet spray nozzle, and need for care to avoid splashing of} onto the tank wads. Disadvantages of the test are that mixing energy (from the water jet) is not only Circuit to quantify, it is difficult to reproduce. The test is sensitive to technique (Payne et al., 1985~; some operators produce more reproducible results than others. High-shear turbulence is generated by the circulation pump, *One reviewer recommends using a glass test container since it has hydrophilic properties; use of Lass would remove oil adherence problems that are common to stain- less steel containers.
OCR for page 77
CHEMISTRY AND PHYSICS OF OIL DISPERSANTS 77 and this affects results. The apparatus requires a skilled operator, is difficult to clean, and generates a large volume of contaminated water that must be disposed of. Only two tests per day are feasible. Flume or Wave-Tank Tests Flume or wave-tank tests do the best job of simulating turbu- lence from breaking or nonbreaking waves and currents even in ice-covered waters (Brown et al., 1987; Delvigne, 1985; To et al., 1987~. A flume, with its large volume, permits low oil-water ratios and greatly reduces wall effects. In a flume, the resurfacing of dis- persed of} droplets can be studied, and droplet size distributions in the water column can be measured in the course of the test. The disadvantages of a flume are obvious: it is expensive, com- plicated, and not portable. Large volumes of water are required, and the contaminated water must be heated a.nCI disposed of. Bocard et al. (1984) and Bocard and Castaing (1986), noting the dissolution processes in at-sea trials, suggested that laboratory tests should incorporate a flow-through seawater system because closed vessels cannot duplicate the dilution process that occurs in the field. It would be desirable to separate the rate of dispersion from the advective loss of dispersed droplets. The IFP dilution test was designed to do this. Four grams of of! are agitated on the surface of water in a 4-liter reactor; the water is replaced continuously at 0.5 liter/hr, and the percentage of of! washed out is measured with time (Bardot et al., 1984; Demarquest et al., 1985~. Deivigne (1985) concluded that one shortcoming of most labo- ratory tests is that evaporation, photooxidation, emulsification, and nonhomogeneity of of} layer thickness cannot be modeled. Therefore, in the Delft flume experiments, he evaluated the following parame- ters: evaporation (0, 12, and 30 percent) in a pan outside the flume; · photooxidation by ultraviolet exposure for 0.2 and 10 hr; · emulsification by premixing the of} and water in laboratory beakers to a total water content of 70 percent; and · layer thickness of 0.1, 0.5, and 2.5 mm. Visual observations during the flume experiments showed that dispersant droplets barely incorporating of! had moved into the water column after fading or slowly sinking through the of! layer. The oil layer contracted into small slicks with open areas (herding) immedi- ately after the dispersant droplets hit the of} and water surface.
OCR for page 78
78 USING OIL SPILL DISPERSANTS ON THE SEA Evaporation did not seem to affect dispersed of] concentrations with either naturally or chemically dispersed systems. It appeared, however, that of} droplet size may have decreased slightly with some evaporation. Photochem~cal oxidation increased naturally dispersed oil con- centrations, with no change in the chemically dispersed concentra- tion. In both cases, however, droplet size distribution shifted to smaller volumes, presumably due to the formation of surface-active compounds in the of} slick, which lowered the oil-water interfacial surface tension (Payne and Phillips, 1985~. Emulsification (i.e., premixing the of} and water) decreased dis- persed of} concentration in naturally dispersed experiments with Statfjord crude but had no effect in the chemically dispersed tests (DeIvigne, 1985~. Oil droplet size increased with emulsification in the naturally dispersed slick, but the of! droplet size did not increase despite emulsification in the case of chemical dispersion. Layer thickness effects on both dispersed of! concentrations and of} droplet sizes were minor. Deivigne concluded that his flume ex- periments showed that the lack of dispersant effectiveness in field tests cannot be explained completely by the various parameters ma- nipulated in his study. Summary Different laboratory tests give consistent results in discriminat- ing broadly between high- and low-performance dispersants. Each test has special advantages and disadvantages; all give reproducible results, although some mimic field conditions better than others. Each test appears to measure different physical and chemical phe- nomena in the sense that the weight assigned to, or simulation of, processes and effects, such as oil-dispersant mixing, turbulence, drop size, distribution, en cl resurfacing tendency, are quite different. It is thus not surprising that they rank the most effective dispersants differently, corresponding to different performance criteria. There is a consensus that it is impossible to simulate, in a labo- ratory system measured in tens of centimeters, turbulence character- istics that exist at the oceanic air-water interface, with its turbulent eddies Bunt breaking waves. It is clearly necessary to introduce some turbulence in laboratory tests to promote and maintain dispersion, but an entirely satisfactory method of accomplishing this has not
OCR for page 79
CHEMISTRY AND PHYSICS OF OIL DISPERSANTS 79 yet been devised. Even sophisticated, large-scale, experimental wa- ter tank systems cannot claim to simulate closely the ocean surface turbulence (Bonwmeester and Wallace, 1986a; Brown et al., 1987~. No attempt is usually made in laboratory tests to simulate photoox- idation. Only in the IFP tests is diffusive dilution simulated. A recent evaluation (Anderson et al., 1985) has compared a num- ber of effectiveness and toxicity tests, and evaluated them. There is no strong correlation between laboratory and field tests (see Chapter 4~. A simple strategy for screening dispersants is to apply a reliable test such as the rotating flask test or the MNS test (see CONCAWE, 1986~. NEED FOR RESEARCH There has been a regrettable lack of input into dispersant re- search on the basic interactions of dispersants with of} by profes- sional surfactant scientists, with the obvious exception of the notable contribution by those who have formulated the products. Unfortu- nately, little of the commercially funded research has appeared in print. Had there been a complimentary program of research in this area, the state of knowledge could have been greatly advanced. As a manifestation of the absence of research, no references have been found to recent papers in the peer-reviewed surfactant litera- ture, for example, the Journal of CoZioid and Interfacial Science or Journal of Dispersion Science and Technology. A considerable liter- ature exists in journals and texts, such as that by Eicke and Parfitt (1987), on the behavior of surfactants in hydrocarbon and water sys- tems. The thrust of most oil spill related studies has apparently been to determine if existing commercially available products work in the laboratory and at sea when applied by conventional methods. This would have been appropriate if it had been found that dispersants work in an efficient, predictable manner. In reality, the performance is often in doubt (see Chapter 4~. It can be argued that more ef- fort should have been devoted to determining why dispersants do, or do not, work on different oils under different application conditions. This research did not occur. As a result, there is an inadequate understanding of dispersant phenomena. A program of research is needed to elucidate the mechanisms by which droplets of dispersant contact an of! film, mix and pene- trate into it, how the surfactant forms various phases with the oil and migrates to the oil-water interface, and the microscopic process
OCR for page 80
80 USING OIL SPILL DISPERSANTS ON THE SEA by which emulsions actually form. This appears to be a complex transient process and will be difficult to observe, but a knowledge of these phenomena is fundamental to determining why dispersants work, and at times why they do not work. For example, it appears that oil composition (as distinct from physical properties) affects dispersibility, but the reason for this is not known. It is still not too late to start a program to Fit this lack of understanding.
Representative terms from entire chapter: