OIL SPILLAGE INTO WATER—TREATMENT This latter incident represented a singular achievement in light of the weather conditions encountered during early March in Nova Scotia Over 6000 tons of viscous Bunker C oil were recovered from the sunken wreck The salvage team used a hot tap technique to penetrate the tanker cargo tanks and then used a steam traced pumping system to transfer the oil to a barge at the surface A more recent and massive removal of oil was the EXXON VALDEZ in March 1989 after its grounding on a reef in Prince William Sound Although approx 250,000 Bbls of North Slope Crude oil was spilled from the grounded vessel, 80 percent of its cargo was still in the tanker This offloading was a significant marine engineering feat since care must be taken to offload such a large vessel in the correct sequence since otherwise hull stresses could cause the vessel to break up The general subject of treatment of oil spillage represents a relatively new area of technology that is unique in that it encompasses chemical, mechanical, and biological disciplines There are three major aspects to the problem of oil spills 1) PREVENTION OF THE SPILL; 2) CONTAINMENT AND RECOVERY OF THE SPILL; 3) TREATMENT OF THE SURFACE OIL Although this discussion is mainly directed toward the treatment of the spilled oil, the related areas will also be considered in order to put the overall subject in the correct perspective PREVENTION OF THE SPILL The Prevention of Oil Spillages is the Primary Consideration CONTAINMENT, RECOVERY OR REMOVAL OF THE SPILLED OIL It should be emphasized that prevention is the first consideration and, of course, the most complete solution In the industrial and governmental communities, the major effort has been directed toward this area There is extensive ongoing research for example, ranging from operational areas such as collision avoidance techniques and training to more novel approaches such as the gellation of crude oil In this latter approach several chemical systems have been developed to gel the oil cargo This in-situ solidification thereby prevents the release of oil from a damaged cargo compartment that may be in danger of failure Details of this gellation system can be found in US Patent 3,634,050.1 Other details, such as the effects of mixing, crude oil type, chemical concentration, and so on, on the strength of the gel have been outlined by Corino.2 Gellation is a novel approach to prevent the release of oil However the fact that there have been no commercial uses of this method since its conception twenty five years ago raises questions regarding its practicality Finally, the removal of the oil cargo from a grounded tanker is another area where the threat of the release of a fluid and mobile oil cargo to the marine environment has been mitigated by advances in salvage techniques The offloading of the grounded SS General Colocotronis on a reef off Eleuthera Island in March–April 1969 and the well documented recovery of Bunker C oil from the sunken tanker SS Arrow in Chedabucto Bay, Nova Scotia during the winter of 19703 are two outstanding examples of this prevention technique If a spill has occurred, it is universally agreed that the recommended procedure is to contain and physically recover it with or without the use of adsorbents It is obviously the most direct solution to spill incident, if conditions permit its execution This approach may entail three processes: 1) Confinement of the spill by spill booms 2) Recovery of the spill by sorbing agents In this area, more recent advancements have been solidifying agents (Solidifiers) 3) Physical removal of the contained oil by oil pickup devices 4) Controlled burning of spilled oil These aspects of the recovery approach are interrelated as will be appreciated by the following discussion Confinement of the Spill by Spill Booms There are many oil spill booms commercially available today Unfortunately they are significantly limited by the velocity of the surface current and wave height Although there are variations in the materials of construction, strength, geometry, etc., of these various boom designs, as evidenced by the number available and the range of costs, their general forms are quite similar Almost any type of floating barrier will hold back and contain some amount of oil under quiescent 802 © 2006 by Taylor & Francis Group, LLC C015_002_r03.indd 802 11/18/2005 10:56:37 AM OIL SPILLAGE INTO WATER—TREATMENT Flotation Member 803 Oil Contained Under Quiescent Conditions Water Flotation Member Oil Oil Containment Capability Improved Oil Draw Down Due To Water Current Weighted Skirt Water FIGURE Mechanical boom principle conditions Indeed, telephone poles have been employed in more than one spill instance as a jury rig emergency measure To improve the capacity of such a floating barrier, a weighted skirt is from the flotating member illustrated in Figure Design requirements for spill booms have been published by Lehr and Scherer4 and Hoult,5 among others By a rather cursory inspection of Figure 1, we may now appreciate some of these requirements such as: Sufficient freeboard to prevent overtopping by waves; Adequate skirt length below water surface to confined a sufficient quantity of oil; Adequate flexibility to permit the boom to bend under wave action and maintain its retention of the oil spill; Sufficient mechanical strength to withstand the forces imposed by the environment Some of the difficulties of oil retention against the action of a steady current are illustrated in Figure A discussion of draw down phenomena by Hoult6 outlines that a gradient in oil thickness, h, is established by the stress imposed by the current flow There is, based on the fluid dynamics of the contained volume of oil in the presence of a water current, a limiting water velocity above which oil droplets are entrained and flow underneath the barrier The deployment of the above described mechanical booms is also an important consideration In the event of a spill, the speed of response is, of course, most critical Hence, © 2006 by Taylor & Francis Group, LLC C015_002_r03.indd 803 11/18/2005 10:56:37 AM 804 OIL SPILLAGE INTO WATER—TREATMENT TABLE Boom Classification Hs Maximum Environment ft meters Calm Water Harbor Offshore Freeboard inches centimeters 0.3 4–10 0.9 10–18 1.8 Ͼ18 easily deployed lightweight booms are desirable However, these desired properties are not necessarily consistent with making booms stronger and more capable of withstanding severe sea conditions These are the criteria and mechanism of operation of oil spill booms It is beyond the scope of this chapter to present the many commercial and changing commercial products In the world Catalog of Oil Spill Response Products, booms have been divided into three categories based on maximum operating significant wave height (Hs) Table shows the ranges of freeboard and draft corresponding to the expected maximum waves A boom size can thus be selected based on the expected environment In the World Catalog of Oil Spill Response Products, booms have been divided into three categories based on maximum operating significant wave height (Hs) Table shows the ranges of freeboard and draft corresponding to the expected maximum waves A boom size can thus be selected based on the expected environment Boom Selection Matrix The selection of a boom depends on how rapidly it is needed and how readily it can be utilized Deployment speed and ease relate to the number of people, the amount of time, and any special equipment (Winches, etc.—even wrenches) necessary to move the required amount of boom from storage to the launch site, to deploy it, and to position it around the spill For example, self-inflatable booms can be deployed very rapidly either from reels or bundles Experience has shown, however, that this rapid response boom should be replaced by a more rugged boom if extended deployment is required Thus, deployment ease must often be traded off against ruggedness and durability The matrix shown in Table can be used to select the optimum boom for a specific application since it indicates: • • • Draft Generic types of boom that are most suitable in a given environment Selected booms that have the most needed performance characteristics Choices with the most desirable convenience features Excess or reserve buoyancy is the surplus of flotation over boom weight as deployed, and is a measure of resistance to boom submergence Wave response is a measure of conformance to the water surface and is usually improved by inches centimeters 10–25 6–12 15–30 25–46 12–24 30–61 Ͼ46 Ͼ24 Ͼ61 increasing boom water-plane area and flexibility Other characteristics should be evident from the headings To use the matrix correctly, follow these steps: Identify the most probable environmental conditions in which the boom will be used Note those types of booms with an acceptable rating (1 or 2) Identify the most needed performance characteristics for the intended application From the booms chosen above, select the ones that have an acceptable rating (1 or 2) in the most important performance characteristics Identify the most desirable convenience features With booms from steps and above, select the boom with the best rating in the convenience features of interest These data (Table and 2) were extracted and used with the permission of EXXON from a very informative OIL SPILL RESPONSE FIELD MANUAL by Exxon Production Research Company published in 1992 Recovery of the Spill by Sorbing Agents A most direct manner of physically removing the spilled oil is by use of sorbents These materials are buoyant, and preferentially wetted by and adsorb oil In essence, they permit this sorbed oil to be physically “picked up” from the water In addition to making the collection of oil an easier task, the oil is prevented from spreading and remains as a more congealed mass Materials that have been found useful for this service vary from simple, naturally occurring materials such as straw, sawdust, and peat to synthetic agents, such as polyurethane foam and polystyrene powder The oil pickup capability varies greatly For example, values of oil pickup, i.e., weight of oil sorbed per weight of adsorption material, have been reported by Struzeski and Dewling7 for straw as to 5, although higher values have been reported Polyurethane foam, by comparison, is capable of oil pick up values of 80 A complete investigation of sorbents for oil spill removal has been published by Schatzberg and Nagy.8 Of interest is the variation in the oil pickup capability of a given sorbent based on the type of spilled oil For example, in Schatzberg’s controlled tests, oil pickup by straw was 6.4 for heavy crude oil and 2.4 for light crude oil For urea formaldehyde foam, however, oil pickup was 52.4 for heavy crude and 50.3 for light crude Also some © 2006 by Taylor & Francis Group, LLC C015_002_r03.indd 804 11/18/2005 10:56:38 AM OIL SPILLAGE INTO WATER—TREATMENT 805 TABLE Boom Selection Matrix Type of Boom Legend 1—Good 2—Fair 3—Poor Internal Foam Flotation Self-Inflatable Pressure-Inflatable External Tension Member Fence Environmental Offshore Conditions Hs Ͼ ft; 2 1 V Ͼ kt — — — — — Harbor — — — — — Hs Ͻ ft; 1 2 V Ͻ kt — — — — — Calm Water — — — — — Hs Ͻ ft; 1 V Ͻ kt — High Currents — — — — V Ͼ kt 2* Shallow Water — — — — (Depth Ͻ ft) 2 3 Performance Operation 3 Characteristics in Debris — — — — — Excess 1 Buoyancy — — — — — Wave 2 1 Response — — — — — Strength 1 Convenience Ease of 2 Characteristics Handling — — — — — Ease of 1 Cleaning — — — — — Compactability 1 Cost/Ft — — — — — 1—Low — — — — — 2—Medium 3 3—High — — — — — Notes: * Hs ϭ Significant Wave Height * V ϭ Velocity of Surface Current Not all the booms of a particular generic type have the rating shown in the matrix But at least one or more commercially available booms of the generic type in question have the rating shown * Specially-designed high-current models may be available (river boom) © 2006 by Taylor & Francis Group, LLC C015_002_r03.indd 805 11/18/2005 10:56:38 AM 806 OIL SPILLAGE INTO WATER—TREATMENT sorbents are much less effective for oil adsorption if contacted by water prior to application to the spill Although highly effective sorbents are available as noted above, techniques for harvesting (recovering) the oil soaked sorbent have been limiting For example, there have been prior instances of oil soaked straw recovery by manual pickup with pitchforks However, there is development work underway to mechanize this step as well as the application procedure In this regard, some very practical observations on the use of sorbents have been made by an IMCO subcommittee on Marine Pollution This guidance manual outlined that “the use9 of absorbents involves six basic operations, the supply, storage, and transportation of the material and then the application, harvesting and disposal of the contaminated absorbent.” The manual further observes that some of the early applications of sorbents such as the Torrey Canyon and Santa Barbara suffered because of the lack of effective and efficient harvesting techniques More recently, since the early 1990s a new approach to oil pickup was conceived by the use of SOLIDIFIERS Solidifiers are products which, when mixed with oil, turn the oil into a coherent mass They are usually available in dry granular form Unlike sorbents that physically soak up liquid, solidifiers bond the liquid into a solid carpet-like mass with minimal volume increase, and retain the liquid for easy removal The bonded material also eliminates drippingsponge effect by not allowing the material to be squeezed out, minimizing residue or contamination Some polymers, in sufficient quantity or of high molecular weight, can actually convert the oil to a rubber-like substance Solidifiers are most commonly used during very small oil spills on land or restricted waterways to immobilize the oil and enhance manual recovery There has been little documented use of solidifiers on large spills or open water However, the possibility that they may reduce the spread of waterborne oil by solidifying it and increase recovery and removal rates is a concept with significant potential benefit The effectiveness of a solidifier is based on the amount of product and time it takes to “fix” a given volume of oil The less effective products require larger amounts to solidify oil Fingas et al (1994) presented results from effectiveness tests on various solidifiers and found that generally between 13–44 percent by weight of the product to oil was required to solidify Alberta Sweet Crude over a 30-minute period The entire treatment of solidifiers as an aid to oil spill response is well covered in an MSRC publication.10 Physical Removal of the Contained Oil by Oil Pickup Devices Since oil containment booms have a fixed capacity for oil spill containment, it is important to consider means to physically remove the contained oil from the surface The use of sorbents has been discussed An alternate approach is to remove the fluid oil by means of skimming devices Oil skimmers have been divided into five categories:11 • Oleophilic surfaces (belts, disc, ropes, and brushes, either acting independently, mounted on a vessel or used in combination with a boom) • • • • Weirs (simple, self-leveling, vortex assisted, auger assisted, vessel-mounted, and weir/boom systems) Vacuum units (portable units and truck-mounted units) Hydrodynamic devices (hydrocyclone and water jet types) Other methods (including paddle belt and net trawl) The selection of the optimum skimmer for a particular spill is based on site conditions such as the sea state and characteristics of the spilled oil e.g viscosity and emulsionforming tendency There are over 100 commercially available skimmers on the market that fall within the generic types previously mentioned These are summarized in publications such as the WORLD CATALOG OF OIL SPILL RESPONSE PRODUCTS For example, for the principle of oleophilic surfaces, these can comprise either a sorbent belt, an oleophilic rope or a solid oleophilic disc that rotates through the surface oil film In heavy sea conditions this type would be more effective than a wier type that is more suited to protected in-shore areas Controlled Burning of the Spilled Oil Burning represents a surface treatment of an oil spill that is attractive in that the oil is essentially removed from the water However, some of the negative aspects of this approach that have hampered its widespread acceptance and use may be summarized as follows: In many spill instances, there is an obvious concern regarding the combustion of the oil for safety reasons Spills near harbors, tankers, offshore platforms would create an obvious hazard if set afire A minimum thickness of oil is required to establish combustion Air pollution is a concern in some instances There is continuing evaluation and development burning of agents As reported by Alan Allen,12 there are fire retardant booms and ignition methods available to burn the oil under proper conditions e.g oil film thickness and amount of emulsified water in the oil An effective burn after the EXXON VALDEZ spill on Sat March 25, 1989 was reported by Allen in this publication The very encouraging burn rate statistics suggest that only 2% of the original relatively fresh oil remained as residue In this regard, it is relevant to quote the author of this publication in its entirety because of its concise and sufficient analysis of this technique by one well recognized in this method “It should be recognized that the elimination of spilled oil using in-situ burning must be considered in light of the full range of potential impacts (safety, air quality, etc.) associated with the burning of oil an water The mechanical removal of spilled oil is by far the preferred cleanup technique whenever possible Burning, on the other hand, may provide a safe, © 2006 by Taylor & Francis Group, LLC C015_002_r03.indd 806 11/18/2005 10:56:38 AM OIL SPILLAGE INTO WATER—TREATMENT efficient and logistically simple method for eliminating oil under certain conditions As a backup for mechanical cleanup techniques, in-situ burning can provide a useful means of eliminating large quantities of oil quickly, while avoiding the need for recovered oil storage containers Anyone considering the use of burning should be sure that all regulatory controls have been satisfied, that the ignition and burning operations can be carried out safety, and that the temporary reductions in local air quality represent the lower of all other environmental impacts should the spilled oil not be burned.” TREATMENT OF THE SURFACE OIL Chemical Treatment of Surface Oil Should Be Considered as an Alternate Solution It is generally agreed, as indicated above, that situations can arise where the spill cannot be contained and recovered because sea conditions, weather state, and so on, are beyond the current operating capability of containment devices There are also instances wherein the logistics of containment and recovery equipment, that is, containment boom availability and/or deployment time and effort, could indicate chemical treatment as the most practical and expedient handling technique When physical recovery of the oil pollutant is impractical, there are, in effect, two courses of action possible In one case, the oil may be permitted to remain as intact cohesive slick on the surface of the water and possibly reach shore The alternate course is to “treat” this surface oil—such treatment essentially directed toward the removal of the oil from the water surface and the enhancement of its ultimate removal from the environment This many be accomplished by chemical dispersion The Ecological and Economic Damage Caused by an Untreated Oil Spill Can Be Extensive The damage resulting from an untreated oil spill is both visually apparent and extensive It encompasses both biological as well as property damage The potential damage may be summarized as follows: Marine fowl, particularly diving birds, are particularly vulnerable to an oil spill As reported by Nelson-Smith,13 sea birds are most obvious victims of an oil spill due to “mechanical damage.” The oil penetrates and clogs the plumage which the bird depends upon for waterproofing and heat insulation For example, a duck with oil-impregnated plumage is under the same stress at a moderate temperature of ϩ59ЊF as a normal bird would be under a more severe temperature condition at −4ЊF Some statistics regarding bird damage have been cited by McCaull.14 More than 25,000 birds, mostly guillemots and razorbills, were killed after the Torrey Canyon grounding The guillemot casualties equaled the entire breeding stock between the Isle of Wright and Cardigan Bay Bird losses in the Santa Barbara spill, according to the state Department of Fish and Game, totaled 3500 Shore contamination by beached oil represents biological, as well as property damage The tendency of oil to cling 807 to shore surfaces, such as beach sand, sea walls, and the resultant property damage, are well established This is perhaps the most apparent and widely publicized damaging aspect as attested by lawsuits on the part of tourist interests, property owners, etc There is also, in a biological sense, a physical smothering effect on some attached, intertidal organisms such as mussels and barnacles The effects of untreated oil coming ashore is well illustrated by Blumer et al.15 regarding a No diesel fuel spill from the barge Florida in Buzzards Bay, Massachusetts in September 1969 Oil was incorporated into the bottom sediment to at least 10 meters of water depth, testifying to the wetting effect of untreated oil in this instance, the oil was physically dispersed by the heavy seas but retained its adhesive characteristics Therefore, it is deduced that the oil droplets probably came into contact with and wetted and upswept, suspended particulates which later settled again to the bottom Other spill instances depicting the importance of this aspect that of the incorporation of oil into the sediment-have been reported by Murphy.16 In the Buzzards Bay and several other spill incidents of distillate fuels cited by Murphy, there has been a significant kill of all marine life in the area since these highly aromatic products are known to be much more toxic than whole crude oil Persistent tarry agglomerates are formed as the spilled oil weathers at sea There has been increasing attention directed to the presence of tar-like globules ranging up to 10 cm in diameter in the open sea As reported by Baker17 during the voyage of Thor Heyerdahl’s papyrus boat, Ra, during five separate days, they sailed through masses of these agglomerates whose age could be substantiated by the growth of goose barnacles adhering to them There have been other incidents reported recently by the International Oceanographic Foundation18 and a well documented survey was made by the research craft, R.V Atlantis, as reported by Horn et al.19 In this latter investigation, tarry agglomerates were present in 75% of over 700 hauls with a surface skimming (neuston) net in the Mediterranean Sea and eastern North Atlantic The amount of tar in some areas was estimated at 0.5 milliliter in volume per square meter of sea surface The Behavior of Spilled Oil at Sea Before consideration of the mechanism of dispersing oil and its associated effects, an understanding of the behavior of spilled oil at sea will be useful When a volume of oil is spilled onto the surface of water, the oil has a driving force to film out or spread-in essence, a spreading pressure usually expressed as a Spreading Coefficient This Spreading So/w, is readily quantified and is determined by a balance of the surface tension forces as follows So/w ϭ g w Ϫ go/w Ϫ go, (1) wherein: So/w is the spreading coefficient for oil on water ergs/cm2 or dynes/cm © 2006 by Taylor & Francis Group, LLC C015_002_r03.indd 807 11/18/2005 10:56:38 AM 808 OIL SPILLAGE INTO WATER—TREATMENT γo γw Oil so/w γ o/w Water so/w , Spreading Coefficient For Oil On Water, - γ - γ w o o/w Measured Value For Kuwait Crude Oil On Sea Water = so/w FIGURE Film Thickness Inches x 10–6 γ = 61 - 28 - 22 = 11 Dynes/cm The spreading behavior of spilled oil Appearance Of Film Approx Gals./Sq Mile 1.5 Barely Visible 25 3.0 Silver Sheen 50 6.0 First Trace Of Color 100 Bright Bands Of Color 200 12.0 80.0+ Dark Colors 1330+ Oil Spill Water Column FIGURE Oil slick appearance during spreading © 2006 by Taylor & Francis Group, LLC C015_002_r03.indd 808 11/18/2005 10:56:38 AM OIL SPILLAGE INTO WATER—TREATMENT 809 HYDROPHILIC-LIPOPHILIC BALANCE (HLB) 10 Increase In Oil Solubility 15 20 Increase In Water Solubility Hydrophilic Group (Water Compatable) SCHEMATIC OF SURFACTANT Lipophilic Group (Oil Compatable) Oil Soluble Surfactant Favors Water-In-Oil Dispersion TYPE OF EMULSION FORMED DISPERSE WATER DROPLETS OIL Water Soluble Surfactant Favors Oil-In-Water Dispersion DISPERSE OIL DROPLETS WATER FIGURE Influences of surfactant structure on type of dispersion gw is surface tension of water, dynes/cm go is surface tension of oil, dynes/cm go/w interfacial tension of oil and water, dynes/cm By an examination of the force balance shown in Figure it can be seen that if So/w—the resultant spreading force is positive, the oil will spread on the water; if negative, it will not spread but remain a “lens” of Liquid For example, spreading coefficient values for Kuwait Crude on sea water, reported by Canevari20 are positive and confirm that for this system the oil readily spreads on the water phase Garrett21 has summarized spreading pressures of various oils on sea water that vary from 25 to 33 dynes/cm Cochran22 has also published values that generally agree with these level on sea water As one can see from Figure 3, for positive spreading coefficients, the oil is capable of filming out to very thin films A film thickness of only 3.0 ϫ 10−6 inches representing a spill of 50 gallons of oil distributed over a surface area of one mile will be quite visible as a “flat” silver sheen on the surface of the water However, the initial spreading rate of a large volume of spilled oil is based on the volume and density of the oil in essence, sort of static head that overcomes other factors such as interfacial tension The Mechanism of Dispersing Surface Oil Slicks by Chemical Dispersants The dispersion of surface oil films as fine oil droplets into the water column is promoted by the use of a chemical dispersant This oil spill dispersant consists primarily of a surface active agent (surfactant) and a solvent The solvent is added as a diluent or vehicle for the surfactant It also reduces the viscosity and aids in the uniform distribution of the surfactant to the oil film A surfactant is a compound that actually contains both water compatible (hydrophilic) and oil compatible (lipophilic) groups Due to this amphiphatic nature, a surfactant locates and arranges itself at an oil–water interface as schematically shown in Figure The surfactant’s molecular structure, e.g ratio of hydrophilic to lipophilic portion, determines the type of dispersion (oil droplets dispersed in water phase or water droplets dispersed in oil phase), as well as stability of the dispersion In essence, a surfactant that is principally water soluble disperses oil-in-water and established water as the continuous phase; a surfactant that is principally oil soluble, the converse This is Bancroft’s Law,23 which has been tested and proven empirically true over the years A convenient classification for surfactants therefore, is based on the ratio or balance of the water © 2006 by Taylor & Francis Group, LLC C015_002_r03.indd 809 11/18/2005 10:56:39 AM 810 OIL SPILLAGE INTO WATER—TREATMENT compatible portion to the oil compatible portion-sometimes referred to as HLB (Hydrophilic–Lipophilic Balance).24 This relationship between the molecular structure of the surfactant and the emulsion type is also shown in Figure and the physical concept behind Bancroft’s Law may be appreciated For example, it can be visualized that for a more water compatible surfactant, the physical location of the larger hydrophilic group on the outside of the dispersed oil droplets results in a more effective “fender” to parry droplet collisions and prevent droplet coalescence The converse, location and the larger portion of the surfactant in the dispersed rather than the continuous phase, would be geometrically awkward and unstable.25 The mechanism of oil slick dispersion by the application of chemical dispersants has been covered in some detail by Poliakoff26 and Canevari,27,28,29 among others From the above discussion, one can see that the chemical dispersant (surfactant) will locate at the oil–water interfaced reduce interfacial tension This will then act to increase the spreading tendency of the oil film as shown by Eq (1) More important, it promotes fine droplet formation which can be expressed as: Wk ϭ Ao/wgo/w, , (2) where: Wk Ao/w γo/w mixing energy, ergs interfacial area, cm2 interfacial tension, dynes/cm Thus, for the same amount of mixing energy, a reduction of γo/w will result in a corresponding increase in Ao/w It is important to emphasize that, as can be realized from the above discussion, the chemically dispersed oil does not sink Rather, the surfactant merely enhances small droplet formation for a given amount of mixing energy Smaller diameter oil droplets have a much lower rise velocity per the familiar Stokes Law Hence, once the oil is chemically treated, and placed to feet below the surface of the water by the mixing process, it does not rise to the surface as readily, as illustrated by Figure There are many surfactants that will aid the formation of fine droplets in the above manner It has already been noted that the surfactant structure (Hydrophilic–Lipophilic Balance) influences the efficiency of the emulsifier However, a more subtle and less tractable requirement for an effective dispersant is the prevention of droplet coalescence once the fine oil droplets are formed This is illustrated by Figure wherein a volume of oil has been dispersed by a chemical surfactant and maintained in suspension by gentle bubbling of air After 24 hours, there has been no coalescence or separation of these fine oil droplets In the control sample, with similar volume of oil and mixing energy, the oil separated almost immediately and reformed an intact, cohesive film of oil In essence then, an effective dispersant must parry droplet collisions physically For example, dispersed oil may separate in a sample bottle but even though there may be a “creaming” effect, i.e oil droplets concentrate near the surface, the droplets should not coalesce to reform an intact slick It is this same “fendering” action that reduces the tendency of the droplets to stick to a solid surface The Physical and Environmental Incentives for Dispersing Oil Slicks Consideration of the previous summary of the potential damaging aspects of an untreated and unrecoverable oil spill indicates that the removal of the intact, cohesive mass of oil from the surface of the water yields more than a cosmetic effect as is often claimed For this alternate approach when conditions not permit the recovery of the spilled oil, the removal of oil from the surface by dispersing it into fine droplets yields established benefits that can be summarized by the following discussion: 1) Oil properly dispersed with a chemical dispersant will not stick to a solid surface As previously outlined, the physical fending action of a properly selected surface-active agent prevents the oil droplets from coalescing after dispersion This same property also inhibits the oil from wetting out on a solid surface This has become a controversial point and it has actually been claimed that the converse is true For example, in the First Report of the President’s Panel on Oil Spills,30 it has been stated that such agents cause the oil to “spread into the sand-surfaces which untreated oil would not wet.” A laboratory experiment was conducted to evaluate this aspect A mixture of 256 cc of sea water, 95 cc of beach sand (New Jersey shore area), and 20 cc Kuwait Crude, were placed in a graduate This represented a vertical cross section of the marine environment after an oil spill The mixture was then agitated to simulate the possible contact of sediment by the oil when turbulent conditions existed After mixing, the sample was settled to separate the oil–sand water phases In a body of water, either the oil may be driven down into contact with the sandy bottom or the sand may be suspended in the body of water by wave action, such as deduced from the previously cited Buzzards Bay spill The graduate was then purged with clean water to simulate the return of the environment to a non-contaminated condition The experiment was then repeated using 20 cc of Kuwait Crude Oil and cc of a chemical dispersant (5 parts oil/1 part dispersant) Virtually no “treated” oil impregnated the sand For the experiment with the untreated crude oil, an analysis of the oil content of the sand bed indicated that 11.20 cc of oil remained of the initial 20 cc 2) Oil removed from surface water prevents bird damage The aforementioned hazard to marine fowl that is presented by the surface oil film is © 2006 by Taylor & Francis Group, LLC C015_002_r03.indd 810 11/18/2005 10:56:40 AM OIL SPILLAGE INTO WATER—TREATMENT 811 a) Oil Spill γo Oil Water γw γ o/w so/w b) Dispersant Reduces Interfacial Tension Surfactant Water Water Soluble Oil Soluble c) Agitation Readily Forms Oil Droplets Mixing Prop Fine Oil Droplets FIGURE Water Dispersant enhances droplet formation Dispersant Prevents Coalescence Of Droplets Oil Droplets Dispersant FIGURE Dispersant maintains oil droplets in suspension with mild agitation © 2006 by Taylor & Francis Group, LLC C015_002_r03.indd 811 11/18/2005 10:56:40 AM 812 OIL SPILLAGE INTO WATER—TREATMENT clearly eliminated when the oil is dispersed as fine droplets into the water column These dispersed droplets are placed several feet below the water surface by the mixing process 3) The fire hazard from the spilled oil is reduced by dispersion of the oil several feet into the water column The removal of this combustible material from the water’s surface and from contact with the atmosphere prevent possible combustion of the spilled oil This is perhaps the most accepted benefit accruing from the use of dispersants It has provided the motivation for many past instances of dispersant applications 4) The rate of biodegradation of the oil is enhanced This is the historical basis for the dispersion of oil It is perhaps the most significant contribution of dispersants The order of magnitude increases in interfacial area that are generated by the dispersant greatly increases the rate of biodegradation of the oil ZoBell31 has reported biodegradation rates that are one or two orders of magnitude higher in laboratory experiments in which the oil is emulsified Not only is the physical state of the oil, that is, small droplets, more conducive to bacterial action, but it is also made available to a much larger population of microbial organisms This particular reference has been one of the most complete treatments of the subject of oil biodegradation to date A study by Robichaux and Myrick32 presented the results of a study of the effects of chemical dispersing agents on the rate of microbial destruction of crude oil in aqueous environments Increased destruction rates of up to 15 times the rate of untreated oil/water mixtures were reported 5) The formation of persistent tar lumps from an untreated oil spill is prevented The tarry agglomerates (up to 10 cc dia.) found on the ocean surface, as mentioned previously represent a small percent residue of the crude oil If the crude oil had been dispersed into 10 µ to mm diameter droplets, these large residue agglomerates would not have formed and their persistence in the marine environment would have been greatly reduced The Concern Regarding the Chemical Dispersion of Oil Spills Clearly then, from a consideration of the foregoing, the removal of oil from the surface of the sea has merits in mitigating the damage resulting from a spill It is more than a cosmetic, hide-it-from view effect What then are the negative aspects to their use? What is the ecological price for introducing the chemical dispersant and dispersed oil into the water column? The major concern regarding the use of dispersants are twofold as covered in the following discussion The Toxic Effects of the Chemical Dispersants This has been an area of great concern since their use has become significant There is a basis for this concern It was highlighted by the investigation by Smith et al.,33 after the Torrey Canyon that indicated that in some areas, particularly in the intertidal zone, the chemicals used were more toxic to the marine life than the oil itself During this period (1967–1968), the chemical formulations available to disperse spilled oil were derived mainly from cleaning agents, hence the term “detergent” was used quite commonly To permit these agents to dissolve tar-like residues and perform their cleaning function, an aromatic solvent, such as heavy aromatic naphtha, was generally employed The short term acute toxicity of aromatic hydrocarbon to marine life is well-known Blumer34 states that low boiling aromatics are toxic to man as well as all other organisms and that it was the great tragedy of the Torrey Canyon that the detergents used were dissolved low boiling aromatics The Toxicity of these aromatic solvent constituents were extensively studied by the Marine Biological Laboratory of the UK Their acute toxicity was evident since ppm of kerosene extract solvent killed 50% of the Elminius nauplius larvae in 21 minutes Their analyses of the more common detergents (dispersants) used during the Torrey Canyon indicated that they contained some portion of aromatics In addition to these toxic aromatic solvents, the surfactants were typically selected from the class compounds formed by the reaction of hydroxy-containing compounds (e.g phenol or alcohol) with ethylene oxide A typical surfactant might be ethoxylated nonylphenol The number of ethylene oxide groups added to the nonylphenol hydro-phobe may be controlled to any desired extent to adjust the degree of water solubility of the material These types of surfactants, although effective emulsifiers, were quite detrimental to marine life However, there has been research directed toward formulating dispersants that would have little effect on marine life For example, water is now used as the solvent in products where it is compatible with the particular surfactants High boiling saturated hydrocarbons which are similar to the type of hydrocarbon that occur naturally in the marine environment have a low order of toxicity and are also employed as solvents in some of the more recent dispersants This modification of the solvent, and the selection of generic types of surface-active agents that are not considered to be chemically toxic, have resulted in the development of dispersants that have greatly reduced toxicity This can be illustrated by the study of J.E Portmann,35 summarized in Table For example, three dispersant products used during the Torrey Canyon spill and identified as Torrey Canyon Dispersants A, B, C, have 48 hr LC50 values of 8.8, 5.8, and 6.6 ppm, respectively These concentrations represent the amount of the specific agent to kill 50% of the test species (Crangon crangon) in 48 hours The toxicity of a typical Torrey Canyon surfactant, ethoxylated nonlphenol is shown at 89.5 ppm By © 2006 by Taylor & Francis Group, LLC C015_002_r03.indd 812 11/18/2005 10:56:41 AM OIL SPILLAGE INTO WATER—TREATMENT TABLE Development of low toxicity dispersants illustrated by Portmann Study 48 hour LC50, ppm brown shrimp (Crangon Crangon) Chemical Torrey Canyon Dispersant “A” 8.8 Torrey Canyon Dispersant “B” 5.8 Torrey Canyon Dispersant “C” 6.6 Post Torrey Canyon Dispersant “D” 7,500–10,000 Post Torrey Canyon Dispersant “E” 3,300–10,000 Post Torrey Canyon Dispersant “F” 3,300 Nonyl Phenol-Ethylene Oxide 89.5 TABLE Summary of Canadian Fish Res Bd evaluation of 10 dispersants Classification Numbers of dispersants 48 hour LC50, ppm Salmon (Salmo Salar L) Toxic 1–100 Moderately toxic 100–1000 Slightly toxic 1000–10,000 Practically non toxic Ͼ10,000 TABLE Toxicity of dispersants with and without crude oil Chemical Dispersant A Dispersant A ϩ oil 96 hour TLM, ppm Fathead minnow (Pimephales promelas) 5.6 14.0 Dispersant B 14.0 Dispersant B ϩ oil 27.0 Dispersant C 25.0 Dispersant C ϩ oil 42.0 Dispersant D 32.0 Dispersant D ϩ oil 44.0 Dispersant E 56.0 Dispersant E ϩ oil 75.0 Dispersant F 3200ϩ Dispersant F ϩ oil 1800ϩ comparison, the Toxicity levels of three dispersant products developed since the Torrey Canyon, identified as Post Torrey Canyon, Dispersants D, E, F, are 7500–10,000; 3300–10,000; and >3300 ppm, respectively These concentrations are orders of magnitude greater than the level applied by conventional application in the field Other agencies have confirmed this finding Table illustrates results of a recent study by the Fisheries Research Board of Canada entitled, “Toxicity Tests with 813 Oil Dispersants in Connection with Oil Spill at Chedabucto Bay N.S.”36 Again, the large difference in toxicity due to the surfactant-solvent recipe can be noted in the summary of results (Table 4) These values represent day LC50 values in fresh water to Salmon (Salmo salar L) and vary from “Toxic” (1–100 ppm) to “Practically non-toxic” (>10,000 ppm) Over 25 research institutions are known to have conducted studies on these lower toxicity chemicals Testing by Dr Molly Spooner,37,38 among others, has encompassed juvenile species, planktonic life and other very sensitive forms of marine life Clearly then, the concern and conclusion that all chemical dispersants are in themselves inherently toxic is incorrect Some of the most effective emulsifiers/dispersants available are those derived from and found in the natural environment The Toxic Effects of the Dispersed Oil When the surface film of oil is dispersed several feet or more into the water column, it is unfortunately made available to other forms of marine life in addition to the hydrocarbonoxidizing bacteria Necton and other filer feeder many now come into contact with dispersed oil droplets that they otherwise may have escaped as surface oil This is, effect, the “ecological price” for the cited benefits of dispersing oil There are published data on the acute toxicity levels of dispersed oil such as that from the State of Michigan39 presented as Table This does indicate an approximate tolerance level of a thousand ppm or more for dispersed oil It can also be noted that the toxicity of the chemical is reflected in the toxicity level of 1000 ppm or so for dispersed oil, however it should be noted that (1) it is unlikely that fish would remain in this inhospitable environment for 96 hours and (2) the dispersed oil has a driving force to dilute itself Of greater concern than these short term acute effects is the possibility that the finely dispersed oil droplets represent a more subtle contaminant and may cause long-range detrimental effects However, it should also be noted that crude oil is a natural rather than man-synthesized material Wheeler North40 reported after extensive research into several spill incidents, “Unlike many of the products man liberates into the environment, crude oil is a naturally occurring substance From time to time it appears on the earth’s crust by natural processes of exudation.” More Recent Dispersant Research Has Involved Improvement in Effectiveness The previous discussion regarding the dispersion mechanism cited the need for mixing energy, Wk This is normally supplied by means of a work boat applying the chemical However, consider the rate by which this work is accomplished by the boat’s wake and propeller A typical work boat may apply energy to swath 50 ft wide at a speed of knots thereby only mixing 35 acres per hour of ocean © 2006 by Taylor & Francis Group, LLC C015_002_r03.indd 813 11/18/2005 10:56:41 AM 814 OIL SPILLAGE INTO WATER—TREATMENT Therefore, in recent years, research has been directed at eliminating the need for the tedious, time consuming mixing process In essence, a “self-mix” dispersant formulation has been developed that requires essentially no energy to be applied to the oil-water interface in order to generate a dispersion of fine oil droplets This has greatly enhanced the scope and potential of chemical dispersion particularly for large spills For example, since mixing is no longer needed, aerial application alone would be feasible Some aircraft uniquely adapted for this service, such as the canadiar CL-215, carries 1500 gallons of dispersant and covers 3000 acres per hour based on a 150 knot speed and treated swath width of 150 feet Extensive use has already been made of commercial DC-4’s and DC-6’s for this purpose A very novel development of a load on tank and spray system for even larger aircraft is now in place The Mechanism of More Recently Developed Self-Mix Dispersants The mechanism of the self-mix chemical dispersants goes beyond the simple thesis represented by Eq (2) In an ideal no-mixing system true spontaneous emulsification (or “selfmixing”) is postulated to occur in the following manner The chemical surfactant formulation is made compatible with the bulk oil However, when the oil phase comes into contact with a water boundary rather than air, part of the surfactant Application Oil Layer A) Sea Water B) Diffusion C) Oil Associated With Self-Mix Dispersant Transported Into Water Phase As Fine Droplets FIGURE Mechanism of self-mix dispersion © 2006 by Taylor & Francis Group, LLC C015_002_r03.indd 814 11/18/2005 10:56:41 AM OIL SPILLAGE INTO WATER—TREATMENT has a strong driving force to diffuse into the water phase In this transport process, a small amount of oil “associated” with the surfactant is carried into the water phase A continuation of this process produces a series of fine oil droplets migrating from the oil phase into the water phase as schematically shown by Figure In the graphical presentation of Figure 7, the surfactant formulation can be seen to be compatible with the crude oil phase as shown in (A) However, due to the nature of the specific compounds, there is a driving force for part of the formulation of diffuse into the water phase when it contacts an oil/water interface (B) During this diffusion, some oil associated with the surfactant as fine oil droplets is carried along with the surfactant into the water column as shown in (C) In essence, a three component system—oil ϩ water ϩ surfactant is formed at the interface As the surfactant diffuses into the water phase, the associated oil is thrown out of solution The migration of the surfactant from the oil into the water phase-in essence, the source of energy for spontaneous emulsification comes from the redistribution of materials It can be seen that for this system to work in the field as an oil slick dispersant, the surfactant must be brought into contact with the oil phase initially It is also interesting to observe that as the surfactant diffuses through the interface, a reduction in interfacial tension occurs Over the entire oil/water interface, there are dissimilar values of interfacial tension due to the somewhat random diffusion of the surfactant at varying sites along the interface Any difference in interfacial tension produces a spreading pressure, II, which causes rapid movement of the interface This interfacial turbulence also aids in the dispersion of the oil into the water phase Field Tests Support the Role of Chemical Dispersants to Minimize Oil Spill Impact In summary, there is an increased awareness and recognition that there is a role for chemical dispersants in minimizing damage from oil spills The improved effectiveness afforded by the self-mix dispersant system has been demonstrated Over the past 10 years, there have been a number of major field tests that have demonstrated under real life conditions the effectiveness and biological safety of this approach These have been reviewed and summarized in a study by the National Research Council.41 In order to establish that the transient, rapidly diluting concentrations of dispersed oil are not harmful, actual measurements of the biological effects were made during several controlled oil spills For examples, on August 19, 1981 a field experiment was carried out in Long Cove, Searsport, Maine, which simulated the dispersal of oil slicks in the nearshore zone.42 The object of this experiment was to obtain quantitative information on the fate and effects of dispersed and non-dispersed oil in the nearshore area An upper and lower intertidal sampling are within a 60 × 100 meter test plot were exposed to dispersed oil in water resulting from the discharge of 250 gallons of 815 oil premixed with 25 gallons of COREXIT 9527 dispersant Release of treated oil was around high-water slack tide on the surface of the water The maximum water depth over the test areas was 3.5 meters Untreated crude oil (250 gallons) was released on an ebbing tide within a separate, boomedoff 60 × 100 meter test plot A third test plot served as an oilfree reference plot To evaluate the effects on the intertidal infaunal community structure, chemical and biological analyses were carried out concurrently throughout the pre- and post-spill periods The conclusions reached by the Bowdoin College scientists are quoted as follows: • • • No evidence of any adverse effects was observed on infaunal community structure from the exposure of intertidal sediments to dispersed oil under real spill treatment conditions There is clear evidence that the undispersed oil treatment caused some mortality of a commercially important bivalve and increased densities of opportunistic polychaetes The results seen in the test plot that received untreated oil, are consistent with studies of realworld oil spills REFERENCES Corino, E.R., E.F Broderick and G.P Canevari, Method of gelling tanker cargoes, US Patent 3,634,050 Issued January 11, 1992 Corino, E.R., Chemical gelling agents and dispersants Paper presented to the Third Joint Meeting of the American Institute of Chemical Engineers and Puerto Rican Institute of Chemical Engineers, May 20, 1970 Department of US Navy, The recovery of bunker C fuel oil from the sunken tanker, SS ARROW, Navships 0994–008–1010, March 1970 Lehr, W.E and J.O Scheren, Jr., Design requirements for booms, Proc of API and FWPCA Joint Conference on Control of Oil Spills, NYC, New York, December 1969 Hoult, David P., Containment and collection devices for Oil slicks, Oil on the Sea, Plenum Press, 1969 Hoult, David P., Containment of Oil Spills by Physical and Air barriers, paper presented on the Third Joint Meeting of the American Institute of Chemical Engineers and the Puerto Rican Institute of Chemical Engineers, May 20, 1970 Struzeski, E.J., Jr and R.T Dewling, Chemical treatment of oil spills, Proc of API and FWPCA Joint Conference on Control of Oil Spills NYC, December 15–17, 1969 Schatzbertg, Paul and K.V Nagy, Sorbents for oil spill removal Proc of API and EPA Joint Conference on Prevention and Control of Oil Spills, Washington, DC, June Subcommittee on Marine Pollution IMCO, National arrangements for dealing with oil pollution preparation of a manual for the guidance of governments, March 2, 1992 10 Chemical Oil Spill Treating Agents MSRC Technical Report Series 93–015, 1993 11 Oil Spill Response Manual, Exxon Production Research Co page 77, 1992 12 Allen, Alan, Comparison of Response Options for Offshore Oil Spills, 11th Annual AMOP Seminar Vancouver British Columbia, June 7–9, 1988 13 Nelson Smith, A., Effects of oil on plants and animals, Proc., Seminar on Water Pollution by Oil, Aviemore, Scotland, May 4–8, 1970 14 McCaull, Julian, The black tide, Environment, November 1969 15 Blumer, M., G Souza, and J Sass, Hydrocarbon pollution edible shellfish by an oil spill, Marine Biology, 1970 © 2006 by Taylor & Francis Group, LLC C015_002_r03.indd 815 11/18/2005 10:56:42 AM 816 OIL SPILLAGE INTO WATER—TREATMENT 16 Murphy, Thomas A., Environmental aspects of oil pollution, paper presented to the Session on Oil Pollution Control, ASCE, Boston, Massachusetts, July 13, 1970 17 Baker, Norman, The life and death of the good ship RA, Sports Illustrated, April 20, 1970 18 Sea Secrets International Oceanographic Foundation, 14, No 4, p 2, July–August 1970 19 Horn, Michael H., John M Teal, and Richard H Backus, Petroleum lumps on the surface of the sea, Science, 168, pp 245–246, April 10, 1970 20 Canevari, Gerard P., The role of chemical dispersants in oil cleanup, Oil on the Sea, Plenum Press, 1969 21 Garrett, William D., Confinement and control of oil pollution on water with monomolecular surface films, Proc of API and FWPCA Joint Conference on Control of Oil Spills, NYC, NY, December 15–17, 1969 22 Cochran, Robert A and Paul R Scott, The growth of oil slicks and their control by surface chemical agents, J Petroleum Technology, July 1971 23 Bancroft, W.D., J Phys Chem., 17, p 501, 1913; 19, p 275, 1915 24 Becker, P., Emulsions: Theory and Practice, Reinhold Publishing Corp., NY, 1957 25 Canevari, G.P., Some basic concepts regarding the separation of oily water mixtures, ASLE Transactions, pp., 190–198, July 1968 26 Poliakoff, M.Z., Oil dispersing chemicals, Water Pollution Control Research Series ORD-3, Washington, DC, May 1969 27 Canevari, Gerard P., The role of chemical dispersants in oil cleanup, Oil on the Sea, Plenum Press, pp 29–51, 1969 28 Canevari, Gerard P., General dispersant theory, Proceedings of Joint Conference on Prevention and Control of Oil Spills, API/FWQA, New York City, New York, Dec 1969 29 Canevari, Gerard P., Oil spill dispersants-Current status and future outlook, Proc of API and EPA Joint Conference on Prevention and Control of Oil Spills, Washington, DC, June 15–17, 1971 30 First Report of the President’s Panel on Oily Spills, Executive Office of the President, Office of Science and Technology, Washington, DC, 1970 31 ZoBell, Claude E., The occurrence, effects and fate of oil polluting the sea, Int Journal Air Water Pollution, pp 173–198, Pergamon Press, 1963 32 Robichaux, T.J and H.N Myrick, Chemical enhancement of the biodegradation of oil pollution, paper presented at the Offshore Technology Conference, Dallas, Texas, April 19–21, 1971 33 Smith, J.E., Torrey Canyon Pollution and Marine Life, Cambridge University Press, 1968 34 Blumer, Max, The extent of marine oil pollution, Oil on the sea, Plenum Press, pp 29–51, 1969 35 Portmann, J.E., The toxicity of 120 substances to marine organisms, Fisheries Laboratory, Burnham-on-Crouch, Essex, England, September 1970 36 Sprague, John B and W.G Carson, Toxicity tests with oil dispersants in connection with oil spill at Chedabucto Bay, NS Fisheries Research Board of Canada, St Andrews, NB, 1970 37 Spooner, M.F and G Malcolm Spooner, The problems of oil spills at sea, Marine Biological Association of the UK Plymouth, England 1968 38 Spooner, M.F., Preliminary work on the comparative toxicities of some oil spill dispersants and a few tests with oils and COREXIT, Marine Biological Association of the UK, Plymouth, England 1968 39 A biological evaluation of six chemicals used to disperse oil spills, Department of Natural Resources, State of Michigan (1969) 40 Mitchell, Charles T., Einar K Anderson, Lawrence G Jones, and Wheller J North, What oil does to ecology, journal WPCE, 42, No 5, Part 1, May 1970, pp 812–818 41 Marine Board Commission on Engineering and Technical Systems National Research Council “Using on Spill Dispersant on the Sea” National academy Press, 1989 42 Gilfillan, E.S., D Page, S.A Hanson, J.C Foster, J.R.P Gelber, and S.D Pratt 1983 Effect of spills of dispersed and non-dispersed oil on intertidal infaunal community structure Proc 1983 Oil Spill Conference Washington, D.C: API pp 457–463 GERARD P CANEVARI G.P Canevari Associates © 2006 by Taylor & Francis Group, LLC C015_002_r03.indd 816 11/18/2005 10:56:42 AM ... mechanism of dispersing oil and its associated effects, an understanding of the behavior of spilled oil at sea will be useful When a volume of oil is spilled onto the surface of water, the oil has... Group (Oil Compatable) Oil Soluble Surfactant Favors Water-In -Oil Dispersion TYPE OF EMULSION FORMED DISPERSE WATER DROPLETS OIL Water Soluble Surfactant Favors Oil- In-Water Dispersion DISPERSE OIL. .. “treated” oil impregnated the sand For the experiment with the untreated crude oil, an analysis of the oil content of the sand bed indicated that 11.20 cc of oil remained of the initial 20 cc 2) Oil