Oil spill dispersant chemicals may need to be stored for long periods awaiting their use in an emergency. It is not uncommon to find stocks being stored for more than five years. Dispersants can lose much of their efficiency or deteriorate in other ways during storage so accelerated storage and corrosion tests have been performed (Albone et al., 1990), compiled background information which have about eight typical high-performance oil spill dispersants.

Crude oils contain various amounts of indigenous surface active agents that stabilize water-in-oil emulsions. It has been shown that the effectiveness of a dispersant is dependent on both the dispersant type and the specific crude oil (Canevari, 1987), but there is no apparent correlation between the emulsion-forming tendency of the crude oil, which is a function of its indigenous surfactant content, and the effectiveness of the dispersant. In general, indigenous surfactants in crude oil reduce the effectiveness of the dispersant, but to an unpredictable level.

Dispersants are widely used in many parts of the world to deal with oil spills on the ocean. The objective of adding the dispersant is to emulsify the oil slick into the water column, which prevents wind forces from moving the slick to shore. This may also increase the bioavailability of the oil because of the large increase in surface area caused by emulsification. Dispersants are surface active agents whose behavior can be understood through the application of surface chemical principles (Christopher, 1993).

Modern oil spill-dispersant formulations are concentrated blends of surface active agents in a solvent carrier system. The solvent system has two key functions: (1) to reduce the viscosity of the surfactant blend in order to allow efficient dispersant application, and (2) to promote mixing and diffusion of the surfactant blend into the oil film (Fiocco et al., 1994).

A method for treating an oil film floating on water has two parts (Heller and Brock, 1995):

The particles consist of a bead with an exterior surface that is at least partially coated with a material capable of accelerating the oxidation of organic compounds floating on water, under illumination, and in the presence of air. The coated bead floats in water and has a diameter of less than 2 mm. It has an intermediate layer of a material that prevents the oxidation of the plastic material.

A dispersant fan sprayer has been built and tested statically on land and demonstrated offshore on a supply vessel while spraying water. Coverage rates of 4 miles²d⁻¹ are possible, using high-speed fans that create a focused air stream with maximal velocities of 90 miles h⁻¹ (Allen, 1985). The dispersant is injected into and propelled by the air stream, which acts as a carrier for the dispersant. This makes possible the spraying of smaller volumes of concentrated or dilute dispersant over a wide swath. The water surface is gently agitated by the air stream and liquid impact.

Corexit^® 9527 is a dispersant, used in a solution of water and ethylene glycol monobutyl ether. The nature of the surface active agent has not been disclosed. Laboratory tests were conducted using 0.5 mm thick, fresh Alberta Sweet-Mixed Blend crude oil, treated with Corexit^® 9527 dispersant applied from an overhead spray boom (Belore, 1987). The effects on dispersion efficiency of mixing jet pressure, mixing jet flow rate, jet standoff distance, and vessel speed were evaluated. The system operates with a nozzle pressure of 7000 kPa, a flow rate of 55 liter/min per nozzle, and nozzles positioned approximately 0.6 m from the water surface. In laboratory tests, such a system was shown to be capable of dispersing 80–100% of the surface slick.

Ships are considered best for applying a dispersant with spray booms, because of their large carrying capacity, and their ability to navigate and operate under bad weather conditions and at night. Experiments have shown that clean-up at a speed greater than 10 knots is unadvisable, however, because the bow wave breaks up the oil film on the water. A high-speed craft such as a hydrofoil, when flying foilborne, solves this problem (Vacca-Torelli et al., 1987).

The hydrofoil has a special stability because it is kept above the water by the foil lift. This avoids creating a disturbing wave motion, and thus long spray booms can be used.

A portable spray unit has been developed for the application of dispersants by large airplanes, such as the Hercules C-130. This spray unit can be rapidly placed in the cargo aircraft without any mechanical alterations. Tests spraying a dispersant concentrate have been performed (Lindblom, 1987).

Campaigns of dispersant offshore trials were conducted from 1979 to 1985 off the French Mediterranean and Brittany coasts. Approximately 30 slicks were treated with several dispersants applied from ships, helicopters, and an aircraft by different spraying systems (Bocard et al., 1987). The experiments identified different effects of dispersants such as short-term dispersion of oil, delayed dissemination, and limiting parameters such as minimal energy of sea surface, ratio of dispersant to oil needed, and the negative herding effect. Various techniques were tested to optimize the application of dispersants in different situations, including the use of a variable flow rate system to spray neat concentrates from ships, and a range of ways of operating ships and aircraft to reach a selective distribution of dispersant and get good coverage of slicks.

A field test was conducted by spraying a commercial oil spill-dispersant (Corexit^® 9527) from aircraft (Geyer et al., 1992). The objectives of the test were to determine the efficiency of delivering the dispersant to a selected target, and to compare various measurement systems for droplet size and spray pattern distribution. The results indicated that aerial flights up to 46 m can produce droplet sizes and swath widths that would be operationally effective for an oil spill.

Corexit^® 9527, dyed with Rhodamine WT, was applied by aircrafts at a target dose rate of 5 gal/acre over a collection grid of metal trays, Kromekote cards, oil-sensitive cards, and a continuous trough (Fay et al., 1993). Analysis of the collected dispersant was done colorimetrically, fluorometrically, and by image analysis. Correlations of the different methodologies demonstrated that high-speed, moderate-altitude application of oil dispersant could deliver the dispersant to the surface at an effective concentration and appropriate drop size. Environmental studies of the test area showed no residual dispersant in the soil following cessation of spraying treatment.

Biodegradable oil spill dispersants with high efficiency and low toxicity have been prepared and tested. They consist of non-ionic and surfactants with a low toxicity with different molecular weights (Abdel-Moghny and Gharieb, 1995). The relationship between IFT, efficiency, and chemical structure of the prepared oil spill dispersants was also studied.

A test to determine the biodegradation rate of the dispersant and the biodegradation rate of the dispersant-oil mixture has been proposed (Mulyono et al., 1993). It is intended to supplement the toxicity and effectiveness tests, which are currently used to evaluate the performance of oil spill dispersants.

The number and variety of both toxicological and analytical methodologies that have generated the available data on this topic are numerous, making it virtually impossible to compare data sets and arrive at a coherent conclusion.

In 1994, the Chemical Response to Oil Spills Ecological Effects Research Forum (CROSERF) was formed. This is a working group composed of representatives from industry, academia, and government, whose goals are to standardize and improve the quality and usefulness of laboratory and mesocosmos research into the ecologic effects of oil spill treating agents (Aurand et al., 2001; Singer et al., 1995).

Ideally, decisions regarding the use of a dispersant use should take place before an emergency, to reach a timely decision (Cunningham et al., 1989). Several states and regional response teams have active programs that address the planning and technical and environmental considerations affecting dispersant use. In several states where the use of dispersants is an emerging issue, there appears to be a willingness to consider their use on a case-by-case basis and a genuine interest in learning more about their effectiveness and toxicity.

A decision concerning the use of a specific dispersant involves several components, including considerations of operational feasibility, regulatory policy, and environmental concerns. Eleven examples of major published procedures for making oil spill-response decisions, including decisions for or against the use of chemical dispersants, have been summarized and compared in a study (Fraser, 1989).

Several guidelines have been given for the use of oil spill dispersants, among them, ASTM guidelines (Corbin and Ott, 1985; Flaherty et al., 1987; Fraser, 1985; Fraser et al., 1989; Manen et al, 1987; Merlin et al., 1991; Wiechert et al., 1991). The guidelines cover a variety of environments such as fresh water ponds, lakes, and streams, as well as land. The laboratory tests to measure the effectiveness of the dispersant that are specified in federal regulations are not easy to perform, nor inexpensive, and generate a large quantity of oily waste water.

A computerized model has been developed for planning and implementing an effective dispersant application program (Allen and Dale, 1995). This makes it possible to conduct a rapid assessment of specific oil-dispersant relationships, oil slick configurations, equipment types, and staging locations, as well as a broad range of dosages achievable within realistic operating constraints. Such constraints are provided for vessel, helicopter, and fixed-wing application systems. For a given spill scenario, the user can determine the amount of dispersant needed, the number of sorties required, the area and potential volume of oil treated per sortie, and the time required to treat a specified percentage of the slick.

Many sea trials of dispersant chemicals have been undertaken to demonstrate the effectiveness of specific products, or to elucidate the processes of oil dispersion into the water column. Most have proved inconclusive, leading many to believe that dispersant chemicals are only marginally effective.

Tests in a wave basin have now been conducted to measure the effectiveness of the dispersant under closely controlled conditions (Brown et al., 1987). These tests show that dispersed oil plumes may be irregular and concentrated over small volumes, so extensive plume sampling was required to obtain accurate measurements. In large-scale sea trials, dispersants have been shown effective, but only when sufficient sampling of the water column was done to detect small concentrated dispersed oil plumes and when it was known that the dispersant was applied primarily to the thick floating oil.

Experiments have been conducted in a wave basin to determine the effectiveness of dispersants when oil was spilled onto a mixture of broken ice and water. Forty-liter portions of a light crude oil were spilled into containment booms that had been frozen into ice in a salt water-filled wave tank (Brown and Goodman, 1996). The spills were treated with either Corexit^® 9527 or Corexit^® 9500, and then low-amplitude waves were generated for 2 h. In a short time, the spills were dispersed by 90% or better.

The oil-in-water dispersion was monitored by fluorometry, video, and still photography, and by measuring the oil remaining on the water and ice surface after the experiment. The size distribution of the ice floes had little effect on the amount of dispersion. The dispersion of oil spilled into a single straight lead in the ice sheet was also studied. It was found that oil spilled into a lead filled with slush ice and treated with dispersant rapidly dispersed into the water column.

Finite difference models to simulate the diffusion and advection of oil in water have been developed and tested in wave basins (To et al., 1987).

There are various testing procedures available, such as the Warren Spring Rotating Flask test (WSL test, Labofina test), Institute Francais du Petrole flow test (IFP test), Mackay-Nadeau-Steelman test (MNS test), EXDET, and other procedures.

A solution of a surfactant mixture in liquid paraffin, containing an oil-soluble, oxyethylated alkyl phenol, with a C₈–C₁₂ alkyl group, an alkyl phosphate of a higher fatty acid alcohol (RO)₂>PO-OH where R is C₁₀ to C₂₀, and a fatty acid amide of diethanol amine, was found to be suitable for removing oils and petroleum products from water surfaces (Chaplanov et al., 1992). The composition has low toxicity, is not inactivated by freezing, and has high biological activity, stimulating the growth of microflora and giving 80–83% dispersion in 5 min.

Dispersant compositions for the treatment of oil spills at the surface of the water consist of a mixture of water, a hydrocarbon solvent, and a mixture of surfactants consisting of 55–65% of emulsifiers and 35–45% of dioctyl sodium sulfosuccinate. The emulsifying agents consist of a mixture of various sorbitan oleates (Charlier, 1988, Charlier, 1989, Charlier, 1990 and Charlier, 1991).

Petroleum spillages can be removed from water surfaces more efficiently with the following detergent mixture, which contains mainly oxyethylate fatty C₁₀ to C₂₀ alcohols and additional oxyethylated fatty C₁₁ to C₁₇ acids with an oxyethylene chain length of one to two units (Sulejmanov et al., 1993). It is used in the form of an aqueous 20–25% emulsion, which is sprayed onto a contaminated surface.

A proteinaceous particulate material has been described, which is effective as an oil spill-dispersant composition (Potter, 1994). The material is a product of grain, such as oats, from which the lipids are removed through organic solvent extraction. When such compositions are applied to an oil spill, they will absorb oil, emulsify it, and finally, disperse it. The compositions are also substantially non-toxic.

Functionalized copolymers of dienes and p-alkylstyrenes can serve as dispersants and viscosity index improvers. The functionalities are introduced via the aromatic units (Brandes and Loveless, 1996a and Brandes and Loveless, 1996b). The polymers are selectively hydrogenated to produce polymers with highly controlled levels of unsaturation, permitting a highly selective functionalization. The dispersant substances may also include a carrier fluid to provide concentrates of the dispersant.

The recovery of sludging oil crudes from hydrocarbon-bearing formations during acid stimulation treatments can be enhanced using an antisludging agent that is basically a dispersant. Such an antisludging agent consists of an admixture of dicyclopentadiene and a mixture of naturally occurring cyclic monoterpenes isolated from Pinus species (Ford and Hollenbeak, 1987 and Ford and Hollenbeak, 1991). The agent is added to the acid used for the well stimulation treatment. Another dispersing agent that is active under these conditions is ethoxylated alkyl phenol dissolved in a mixture of ethylene glycol, methanol, and water (Ford, 1989, Ford, 1991 and Ford, 1993).

The use of chemical dispersant for oiled shorelines is one of the most controversial, complex, and time-critical issues facing officials responsible for making decisions about the response methods used on coastal oil spills (Walker and Henne, 1991).

In general, the clean-up of oiled shorelines has been performed by mechanical, labor-intensive means. The use of surfactants to lift the oil from the surface results in more complete and rapid cleaning. Not only is this cleaning process more efficient, but it can also be less environmentally damaging, because potentially less human intrusion and stress on the biological community occurs, and also the chemicals can make the washing more effective at a lower temperature.

Chemical beach cleaners can facilitate the clean-up of oiled shorelines by improving the efficiency of washing with water. A dispersant has been developed that reduces the adhesion of the oil coating, which makes it easier to remove from shoreline surfaces, thereby reducing washing time and lowering the temperature of the wash water needed to clean a given area (Fiocco, 1991).

These experiences resulted in the development of Corexit^® 9580 (Canevari et al., 1994a; Fiocco et al., 1996), which consists of two surfactants and a solvent. It exhibits low fish toxicity, low dispersiveness, and effective rock cleaning capability. Experiments on mangroves aimed at exploring the potential use of Corexit^® 9580 to save and restore oiled vegetation have been considered.

Such a dispersant formulation contains a mixture of a sorbitan monoester of an aliphatic monocarboxylic acid, a polyoxyethylene adduct of a sorbitan monoester of an aliphatic monocarboxylic acid, a water-dispersible salt of a dialkyl sulfosuccinate, a polyoxyethylene adduct of a sorbitan triester or a sorbital hexaester of an aliphatic monocarboxylic acid, and propylene glycol ether as solvent (Canevari et al., 1994b and Canevari et al., 1997).

Linseed oil, fatty acids, alkenes, and polyisobutylmethacrylate, are treated in a thermal process to prepare an oil coagulant, which floats on the water surface and coagulates oil independent of both agitation and temperature and can be used in both salt water and fresh water. After coagulation at least 99.9% of the floating coagulated oil can readily be removed from the water by mechanical methods.