Corexit
Corexit is a branded line of chemical dispersants produced by Nalco Energy Services, L.P. (a subsidiary of Ecolab Inc.), formulated to break oil slicks into fine droplets during marine oil spill responses, thereby facilitating dilution and microbial biodegradation in the water column.[1][2] The primary variants, such as Corexit EC9500A and EC9527A, consist of proprietary blends including anionic surfactants like dioctyl sulfosuccinate sodium salt (approximately 10-30% by weight), solvents such as 2-butoxyethanol, and propylene glycol, with the exact formulations protected as trade secrets but partially disclosed via material safety data sheets.[1] First developed in the mid-20th century and refined through partnerships with oil industry entities, Corexit gained prominence for applications in spills like the 1989 Exxon Valdez incident, where smaller volumes were aerially sprayed to address surface slicks.[3] Its most extensive deployment occurred during the 2010 Deepwater Horizon oil spill in the Gulf of Mexico, where responders applied nearly 1.84 million gallons—over 10 times prior records—via surface spraying, vessel injection, and unprecedented subsea injection at the wellhead to counter the release of approximately 4.9 million barrels of crude oil.[2][4] This scale represented a causal trade-off: dispersants accelerated oil emulsification and dispersion, potentially reducing shoreline fouling and promoting aerobic degradation by hydrocarbonoclastic bacteria, yet empirical field and laboratory data reveal persistent residues and synergistic toxicities when Corexit interacts with dispersed oil, elevating risks to pelagic and benthic organisms.[5][6] Peer-reviewed toxicological assessments, including chronic exposure tests on species like the water flea Daphnia magna and eastern oyster Crassostrea virginica, document sublethal effects such as impaired reproduction, developmental abnormalities, and oxidative stress from Corexit alone or in oil mixtures, with LC50 values indicating moderate to high acute toxicity (e.g., 6.5-25 mg/L for EC9500A on zooplankton).[7][8] While human occupational exposure studies report limited acute pulmonary or systemic inflammation from repeated low-level contact, long-term ecological monitoring post-Deepwater Horizon has detected bioaccumulation in seafood tissues and microbial community shifts, underscoring unresolved debates over net environmental benefits versus amplified subsurface hydrocarbon persistence.[9][10] These findings, drawn from controlled experiments and post-spill sampling rather than anecdotal reports, highlight Corexit's role as a pragmatic but imperfect tool in spill mitigation, where causal efficacy hinges on dosage, spill dynamics, and ecosystem resilience.[11][12]Development and Ownership
Origins and Early Formulation
Corexit dispersants were originally developed by the Standard Oil Company of New Jersey, a predecessor to Exxon, in the late 1960s amid rising concerns over marine oil spills, including the 1967 Torrey Canyon incident that highlighted the need for effective chemical countermeasures beyond mechanical removal.[13] The core engineering principle involved formulating surfactant blends to lower the interfacial tension between oil and water, facilitating the breakup of cohesive oil slicks into fine droplets that could mix into the water column for biodegradation rather than forming persistent surface sheens.[14] Initial laboratory testing emphasized non-ionic surfactants, such as sorbitan monoesters (e.g., Span equivalents) and their polyoxyethylene derivatives (e.g., Tween-like adducts), selected for their ability to promote stable oil-in-water emulsions while minimizing foam generation that could hinder application.[14] A key early patent, U.S. Patent 3,793,218 issued on February 19, 1974, to Esso Research and Engineering Company (Exxon's research arm), detailed a dispersant composition comprising C10-C20 aliphatic carboxylic acid sorbitan monoesters or polyoxyalkylene variants with a hydrophilic-lipophilic balance (HLB) of 9-11.5, enabling dispersion at ratios of 1:5 to 1:15 dispersant-to-oil without requiring intense mechanical agitation.[14] This formulation addressed causal challenges in oil-water separation by balancing lipophilic chains for oil solubility with hydrophilic groups for aqueous dispersion, optimizing performance across salinity gradients typical of seawater. Early iterations, such as Corexit 7664 and 8666, evolved from these principles to enhance stability under temperature variations from 5°C to 30°C, prioritizing rapid emulsification over solvent-heavy predecessors that proved environmentally harsher.[15] Subsequent refinements in the 1970s focused on scalability for aerial and vessel deployment, with controlled trials validating efficacy in emulsifying crude oils under dynamic wave conditions, though specific minor spill applications remained limited until broader regulatory approval under the U.S. National Contingency Plan of 1970.[16] These developments underscored a shift toward dispersants as a targeted intervention, grounded in empirical dispersion metrics rather than unproven toxicity assumptions.Ownership Changes and Production
Corexit dispersants were originally developed and produced by Nalco Chemical Company, which underwent significant ownership transitions beginning in the late 1990s. In March 2001, following its acquisition by the French conglomerate Suez, the company was rebranded as Ondeo Nalco Company, enhancing its focus on water treatment and energy chemicals, including petroleum-related products like Corexit.[17] In November 2003, a consortium of private equity firms—Blackstone Group, Apollo Management, and Goldman Sachs Capital Partners—acquired Ondeo Nalco from Suez for $4.2 billion, leading to its renaming as Nalco Holding Company and a return to public markets in 2004.[18] This structure persisted until December 2011, when Ecolab Inc. completed its $5.4 billion merger with Nalco Holding Company, integrating Nalco's operations into Ecolab's portfolio while retaining Corexit production under the Nalco brand as a subsidiary focused on industrial water and energy solutions.[19] By June 2020, Ecolab spun off its upstream energy chemicals business—encompassing Nalco Champion, the division responsible for oilfield products including Corexit—through a reverse Morris Trust transaction merging it with Apergy Technologies to form ChampionX Corporation, in which former Ecolab shareholders held a 62% stake.[20] COREXIT Environmental Solutions LLC, a ChampionX subsidiary, managed Corexit manufacturing until announcing discontinuation of production and sales in November 2022, citing strategic shifts, though existing stockpiles remained available.[21] Production of Corexit variants, such as EC9500A and EC9527A, has emphasized scalability for rapid deployment in oil spill responses, supported by their listing on the U.S. Environmental Protection Agency's National Contingency Plan (NCP) product schedule since at least 1994, which provides pre-approval for use in saltwater without case-by-case testing delays.[22] This pre-approval facilitated stockpiling by response organizations, with manufacturers maintaining inventory levels sufficient for large-scale emergencies, as demonstrated by Nalco's rapid fulfillment of orders exceeding $40 million during the 2010 Deepwater Horizon incident.[23] Batch consistency across ownership changes has been maintained through standardized formulation protocols, ensuring dispersancy performance aligned with NCP efficacy criteria, without reported alterations to core chemical specifications post-acquisitions.[24]Chemical Composition
Corexit 9527
Corexit 9527, an oil spill dispersant developed by Nalco, primarily comprises the solvent 2-butoxyethanol (approximately 25% by volume), the carrier 1,2-propanediol (propylene glycol), and a blend of proprietary sorbitan esters derived from oleic acid as surfactants.[25][10] The formulation also includes distilled tall oil and dioctyl sodium sulfosuccinate (DOSS), contributing to its emulsifying properties.[10] These components are mixed in a hydrocarbon-based solvent system, with exact proprietary ratios undisclosed beyond safety data disclosures mandated by regulators in 2010.[26] The dispersant exhibits high solubility in seawater, allowing it to integrate into aqueous environments during application.[27] Its surfactants adsorb at the oil-seawater interface, reducing interfacial tension from typical values of 20-50 mN/m (for crude oil-water) to below 10 mN/m, which promotes the formation of micron-sized oil droplets under conditions of mechanical agitation such as wave action or spraying.[27][28] This property facilitates the dispersion of oil into the water column rather than surface slicks.[29] Following its deployment during the 2010 Deepwater Horizon spill, where it accounted for about 11% of applied dispersant volume before discontinuation on May 22, 2010, Corexit 9527 was phased out in favor of less volatile variants like EC9500A, which omit 2-butoxyethanol to mitigate vapor-related concerns.[30][1] While no longer standard for subsurface or low-energy applications, its formulation has been noted for suitability in high-shear scenarios where rapid solvent evaporation is less problematic.[1]Corexit EC9500A and Later Variants
Corexit EC9500A features a reformulated solvent system compared to earlier variants like EC9527A, retaining the core surfactants while substituting more volatile components to enhance safety during application. The primary surfactants include dioctyl sodium sulfosuccinate (DOSS) at concentrations of 10-30% by weight, functioning as the anionic dispersant agent, alongside nonionic surfactants such as sorbitan monooleate (5-10%). Solvents comprise propylene glycol (30-50%) and hydrotreated light petroleum distillates (<20%), omitting 2-butoxyethanol to reduce evaporation and associated inhalation hazards.[31][32] This modification preserves dispersancy efficacy, with the oleophilic solvent delivery system optimized for breaking oil into microdroplets that remain suspended in water columns. Empirical tests under EPA National Contingency Plan protocols confirm its stability under varied conditions, including subsurface injection at elevated pressures encountered in deepwater spills, where it demonstrated consistent performance without phase separation or loss of activity.[22][33] Standardized manufacturing processes minimize batch-to-batch variability, ensuring reproducible toxicity profiles as documented in EPA dossiers. For instance, 96-hour LC50 values for key marine species include 25.2 mg/L for inland silverside (Menidia beryllina) and 32.33 mg/L for mysid shrimp (Americamysis bahia), reflecting compliance with regulatory thresholds for aquatic safety. Later variants, such as EC9500B, maintain this compositional framework with incremental refinements for specific environmental compatibilities, though detailed disclosures remain limited to NCP listings.[34][35][36]Historical Applications
Pre-Deepwater Horizon Uses (1968–2009)
Corexit dispersants saw their earliest documented large-scale applications in U.S. waters during the 1969 Santa Barbara oil spill, which released approximately 4.2 million gallons of crude into the coastal Pacific Ocean under temperate conditions with air temperatures around 15–20°C and moderate wave action. Responders applied Corexit 7664 to targeted surface slicks, achieving partial emulsification that promoted dilution into the water column, though overall efficacy was constrained by the spill's proximity to shorelines and variable currents; post-application assessments noted reduced surface oil persistence in treated areas compared to untreated zones.[37] Throughout the 1970s and 1980s, Corexit formulations, such as EC-2 and early 95xx variants, were deployed in numerous smaller incidents in the Gulf of Mexico, including platform blowouts and tanker leaks totaling several thousand gallons of dispersant per event under warm subtropical conditions (water temperatures often exceeding 25°C) that enhanced micelle formation and biodegradation. These applications, often in open waters with sufficient mixing energy, resulted in observed rapid fragmentation of slicks and plume dilution, with field monitoring indicating oil concentrations dropping below 1 ppm within days in dispersed zones. The U.S. Environmental Protection Agency included Corexit on its National Contingency Plan dispersant schedule by the late 1970s, citing its ready availability, logistical ease, and demonstrated emulsification performance in non-arctic environments over alternatives.[38][39] In the 1989 Exxon Valdez spill, which released 11 million gallons of Alaska North Slope crude into the cold Prince William Sound (water temperatures around 4–9°C), approximately 13,000 gallons of Corexit 9580 were tested on initial slicks but proved largely ineffective due to low temperatures inhibiting surfactant activity and oil viscosity; applications were discontinued after minimal visible dispersion, with untreated oil dominating shoreline impacts. Cumulative pre-2010 U.S. deployments of Corexit remained modest, under 100,000 gallons across documented spills, reflecting selective use tied to favorable conditions and regulatory pre-approvals that prioritized it for temperate-water responses where immediate outcomes included faster oil removal from surfaces than mechanical methods alone.[40]Deepwater Horizon Deployment (2010)
Following the April 20, 2010, explosion of the Deepwater Horizon rig, which released an estimated 4.9 million barrels of crude oil into the Gulf of Mexico over 87 days, responders deployed chemical dispersants to address the subsurface and surface oil plumes.[41] Corexit EC9500A and Corexit 9527 were the primary formulations used, totaling approximately 1.84 million gallons applied from April 23 to July 19, 2010. Of this, about 1.07 million gallons were sprayed aerially or from vessels onto surface slicks, while 771,000 gallons were injected subsea at the broken riser approximately 5,000 feet below the surface, beginning May 15, 2010, at rates up to 15,000 gallons per day.[41] Corexit was prioritized for its pre-existing stockpiles—BP held hundreds of thousands of gallons ready in the region—and its pre-approval on the U.S. Environmental Protection Agency's (EPA) National Contingency Plan product list, enabling immediate large-scale application without delays for testing alternatives.[42] The EPA authorized subsurface injection after reviewing toxicity data on dispersant-oil mixtures, confirming that mysid shrimp and menhaden fish survived exposures at projected dilutions, with the method aimed at dispersing oil plumes in deep waters to reduce surfacing volumes.[43] This approach complemented mechanical skimming and booming operations, which recovered over 800,000 barrels of oiled water, by fragmenting oil into smaller droplets less prone to forming persistent surface sheens that could evade booms or drive toward shorelines.[41] The U.S. Coast Guard, as federal on-scene coordinator, and EPA conducted near-real-time monitoring of application sites, including water quality sampling for dispersant concentrations and toxicity benchmarks.[44] Dispersant use was restricted to zones where oil was present and environmental conditions allowed dilution, with EPA directing reduced application rates on May 26, 2010, after tests showed Corexit alternatives were less toxic to test species at equivalent effectiveness levels.[43] Sampling indicated dispersant levels dropped rapidly due to ocean dilution and mixing, typically falling below 1 part per million within hours to days in treated areas, aligning with models predicting minimal persistence in open waters.[45]Post-2010 Deployments and Availability
Following the Deepwater Horizon oil spill, Corexit dispersants have not been deployed on a large scale in the United States, attributable to the absence of comparable catastrophic offshore incidents requiring extensive chemical response by 2025.[46] Small-scale applications or testing may have occurred in controlled environments or non-U.S. contexts, but no verified major spill responses have utilized Corexit post-2010 in U.S. waters.[47] Corexit EC9500A remains conditionally listed on the Environmental Protection Agency's (EPA) National Contingency Plan (NCP) Product Schedule as of January 2025, permitting its use during oil spills subject to authorization protocols.[36] This listing extends through December 12, 2025, after which re-registration would be required for continued eligibility absent delisting.[48] In November 2022, COREXIT Environmental Solutions LLC, a subsidiary of ChampionX, discontinued the manufacture and sale of Corexit dispersants, including EC9500A, shifting reliance to pre-existing inventories.[21] U.S. dispersant stockpiles, comprising predominantly Corexit EC9500A, are maintained by response organizations for rapid deployment in potential future spills, underscoring preparedness despite production cessation.[49] Petitions filed in August 2024 by environmental groups urged the EPA to delist Corexit variants and prohibit use of remaining stockpiles, citing toxicity data and manufacturer discontinuation, though no final action had been taken by late 2025.[50] Equivalents to EC9500A are not actively produced under the Corexit brand, but the NCP schedule includes other certified dispersants for substitution if needed.[36]Mechanisms and Effectiveness
Dispersal Mechanisms
Corexit dispersants operate through surfactants that adsorb at the oil-water interface, substantially lowering interfacial tension to enable the mechanical breakup of oil slicks into small droplets when subjected to energy inputs such as wave action or high-pressure injection.[27][51] This reduction in tension—often from tens of mN/m to below 1 mN/m—destabilizes the cohesive forces within the oil slick, allowing external shear forces to fragment it into micron-scale emulsions.[52] The resulting oil-in-water emulsion consists of droplets typically under 70 μm in diameter, coated with a surfactant monolayer that inhibits coalescence and Ostwald ripening, thereby promoting suspension in the water column through buoyancy balance and dilution via turbulent advection.[51] The efficacy of this emulsification relies on the hydrophilic-lipophilic balance (HLB) of the surfactant blend in Corexit formulations, which averages 10-11, optimizing affinity for both hydrophobic crude oil hydrocarbons and aqueous phases to stabilize the interfacial film.[53] This HLB range facilitates the self-assembly of surfactants into structures that encapsulate oil droplets, preventing reaggregation by electrostatic and steric repulsion while accommodating variations in oil composition, such as asphaltene content or viscosity.[27] Dispersal performance is modulated by environmental variables, including salinity, where effectiveness increases with ionic strength up to seawater levels of 30-35 parts per thousand, as higher salinity enhances surfactant partitioning to the interface by salting-out effects that reduce solubility in the bulk water phase.[52][54] Concurrently, sufficient hydrodynamic energy—derived from surface waves (with dissipation rates above 10-20 cm²/s³) or submersed injection—is essential to overcome viscous forces and generate the requisite droplet size distribution for stable dispersion.[51][55]Empirical Evidence from Field and Lab Tests
Laboratory evaluations of Corexit dispersants, such as EC9500A and 9500, using the Swirling Flask method—a standard EPA protocol involving 20 minutes of shaking at 150 rpm followed by settling—have shown dispersion efficiencies reaching 99% for light to medium crudes like Macondo (API gravity approximately 35) under high-energy conditions within 6-10 minutes.[51] In the IFP Dilution method, which assesses oil dilution over 60 minutes, Corexit variants achieved 60-80% dispersion for North Sea light crudes like Troll B at salinities of 3.5% and temperatures around 0°C, with effectiveness peaking after 24 hours of contact for lighter oils due to surfactant penetration.[51] Wave tank tests simulating breaking waves further quantified performance, with Corexit 9500 dispersing up to 70% of light crudes such as Mesa Light (API ~30-40) and Alaska North Slope crude (API 32) after 2 hours at energy dissipation rates of 1 m²/s³ and temperatures of 10-16°C, reducing droplet sizes to approximately 50 microns.[51] These lab results highlight 70-90% dispersion rates within 1-2 hours for light crudes under favorable mixing, though efficiencies dropped below 40% for weathered or emulsified oils.[56] Field applications during the Deepwater Horizon spill in 2010, including surface spraying of over 1 million gallons of Corexit, resulted in estimated 75% effectiveness in early aerial dispersals based on visual reduction of slicks, contributing to over 50% removal of surface oil in treated areas through subsurface dilution.[27] However, overall surface oil persistence varied, with subsea injections reducing surfacing oil by about 7% of total release.[57] Biodegradation studies in seawater microcosms have indicated that Corexit-dispersed oil droplets undergo microbial degradation 2-3 times faster than surface slicks for certain hydrocarbons, attributed to increased oil-water interfacial area; for instance, n-alkanes (C30-C35) degraded below detection by day 6 with Corexit at 25°C versus day 16 without, yielding a rate constant increase from 0.15 to 0.19 day⁻¹.[58] NOAA-supported research from the 2010s corroborated accelerated primary biodegradation of dispersed droplets, with extents up to 77% for alkanes in 28 days versus slower slick degradation, though some experiments found no stimulation or slight suppression for specific aromatics due to shifts in microbial communities favoring dispersant degraders over oil specialists.[58][59] Effectiveness exhibits variability tied to oil properties and environmental conditions: dispersion rates decline for heavier crudes with API gravity below 20 due to higher viscosity, and low-energy calm seas limit mixing, often resulting in under 50% efficacy compared to wave-agitated waters.[51][60]Comparisons to Alternative Dispersants
Corexit EC9500A has been evaluated against other EPA-authorized dispersants such as Finasol OSR 52 and Dasic Slickgone NS in multiple laboratory and meso-scale tests focusing on dispersion efficacy. In large-scale comparative trials by the Bureau of Safety and Environmental Enforcement (BSEE), Corexit EC9500A and Finasol OSR 52 exhibited the highest overall performance in emulsifying a range of crude oils, including light and medium paraffinic types, with dispersion efficiencies often exceeding 70-90% under standardized wave tank conditions at dispersant-to-oil ratios (DOR) of 1:25 to 1:100.[61] These tests highlighted Corexit's ease of application and stability in turbulent waters, though Finasol occasionally required adjustments for viscosity in cooler simulations.[61] Toxicity comparisons from EPA's 2010 screening of eight dispersants ranked Corexit 9500 among the least toxic when applied alone, with LC50 values for mysid shrimp (Americamysis bahia) and southern flounder (Paralichthys lethostigma) comparable to or lower than alternatives like Sea Brat 25 and JD-2000, showing no significant endocrine disruption in rapid in vitro assays.[62][63] In contrast, static LC50 tests on marine species have indicated some alternatives, such as Dispersit MPC, as 2-5 times less toxic to copepods and fish larvae than Corexit variants, but these same products underperformed in dynamic oceanographic simulations where Corexit achieved faster droplet breakup and deeper dispersion.[64][65] Field-oriented evaluations, including BSEE's over-time efficacy studies, found Corexit, Dasic, and Finasol yielding similar dispersion rates (typically 60-80% for Alaskan North Slope crude after 1-4 hours), with Corexit maintaining an edge in subtropical temperatures due to optimized surfactant formulations for rapid interfacial tension reduction.[65][66] Corexit's practical advantages during the 2010 Deepwater Horizon spill included superior stockpiling and supply chain logistics in the U.S., reducing deployment costs compared to imported alternatives like Finasol, which faced higher logistics expenses despite equivalent or slightly superior performance on waxy oils in select trials.[51] No single dispersant outperforms Corexit universally across metrics; efficacy rankings vary by oil composition (e.g., paraffinic vs. asphaltic), water temperature (optimal for Corexit above 20°C), and application method (surface vs. subsea injection), necessitating site-specific selection per EPA protocols.[61][51]Environmental Impacts
Toxicity Profiles for Marine Organisms
Corexit EC9500A exhibits low acute toxicity to adult marine fish and crustaceans in standardized laboratory assays, with 96-hour LC50 values exceeding 100 ppm for species such as the inland silverside (Menidia beryllina) at 201 mg/L (95% CI: 195–207 mg/L) under static non-renewal conditions in saltwater (20 psu).[67] Similarly, 48-hour LC50 values for mysid shrimp (Americamysis bahia) reach 120 mg/L (95% CI: 71–169 mg/L) in comparable tests, classifying the dispersant as slightly toxic per EPA National Contingency Plan criteria.[67] These dose-response curves, derived from EPA-approved protocols akin to OECD Test No. 203 for fish acute toxicity, demonstrate steep lethality thresholds above environmentally relevant concentrations following rapid post-application dilution in open waters. In contrast, early life stages and planktonic organisms display heightened sensitivity, with EC50 or LC50 values in the 1–30 ppm range. For instance, microzooplankton such as ciliates exhibit 90-hour LC50 values as low as 1.7 ppm in exposure tests, indicating high vulnerability due to direct contact and limited evasion capabilities.[68] Sea urchin (Arbacia punctulata) embryos show a 72-hour EC50 of 29 mg/L (95% CI: 26–31 mg/L) for impaired larval development in static saltwater assays (30 psu), underscoring species-specific sensitivities in developmental assays that align with OECD guidelines for sublethal endpoints.[67] Planktonic rotifers (Brachionus plicatilis) and similar taxa further highlight this gradient, with toxicity escalating at lower doses compared to motile adults, though field dilution typically reduces exposure below critical thresholds within hours of dispersal.[69]| Taxon | Species Example | Endpoint/Test Duration | LC50/EC50 (ppm) | Source Conditions |
|---|---|---|---|---|
| Adult Fish | Menidia beryllina | 96-h LC50 | 201 | Static, 20 psu saltwater[67] |
| Crustaceans | Americamysis bahia | 48-h LC50 | 120 | Static, 20 psu saltwater[67] |
| Plankton/Microzooplankton | Ciliates | 90-h LC50 | 1.7 | Lab exposure[68] |
| Embryos/Larvae | Arbacia punctulata | 72-h EC50 (development) | 29 | Static, 30 psu saltwater[67] |