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CITATION John, V., C. Arnosti, J. Field, E. Kujawinski, and A. McCormick. 2016. The role of in remediation: Fundamental concepts, rationale for use, fate, and transport issues. Oceanography 29(3):108–117, http://dx.doi.org/10.5670/ oceanog.2016.75.

DOI http://dx.doi.org/10.5670/oceanog.2016.75

COPYRIGHT This article has been published in Oceanography, Volume 29, Number 3, a quarterly journal of The Oceanography Society. Copyright 2016 by The Oceanography Society. All rights reserved.

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The Role of Dispersants in Oil Spill Remediation Fundamental Concepts, Rationale for Use, Fate, and Transport Issues

By Vijay John, Carol Arnosti, Jennifer Field, Elizabeth Kujawinski, and Alon McCormick

The crew of a Basler BT-67 fixed-wing aircraft releases oil over the Deepwater Horizon oil spill, offshore Louisiana. US Coast Guard photo by Petty Officer 3rd Class Stephen Lehmann

108 Oceanography | Vol.29, No.3 ABSTRACT. Offering a scientific perspective, this paper provides a rationale for the and their ecological impacts are found use of dispersants in oil spill remediation by discussing their formulations and modes both in earlier research, as summarized of action and connecting their physics and chemistry to a their environmental fates in a National Research Council (2005) and impacts. With the first use of dispersants at the source of the oil release during the report, and in a more recent book sum- Deepwater Horizon incident, there is a new great need for understanding the efficiency marizing dispersant application for the and the environmental impacts of their use. The paper concludes with some cautionary DWH incident (Judson et al., 2010), with recommendations on dispersant research. details on dispersant testing for efficacy and , especially as designed by the INTRODUCTION AND are typically solutions containing one or US Environmental Protection Agency PHYSICOCHEMICAL ASPECTS more (amphiphilic molecules (EPA). In addition, recent research spon- OF DISPERSANT ACTION with hydrophilic polar head groups and sored by the Gulf of Mexico Research have been shown to break hydrophobic hydrocarbon-based tails) Initiative (GoMRI; http://www.gomri. up surface oil slicks when applied under that partition to the oil-water interface. org) has generated a significant body of the appropriate conditions, leading to The surfactants are dissolved in . literature on the ecological and biological reduced oiling of beaches and shorelines The role of the is to reduce impacts of dispersants. (National Research Council, 2005). The the interfacial tension between oil and The most widely used and bench- DWH spill was unprecedented in that oil water. The interfacial tension is a mea- marked dispersant is the Corexit class of was being released ~1,600 m below the sure of the energy needed to increase dispersants, particularly EC9500A (here- ocean’s surface. Nevertheless, concerns the oil-water interfacial area by one unit, after referred to as Corexit 9500), devel- about surface worker safety and oil on Gulf defined in units of J –2m or N m–1. Thus, oped by ExxonMobil and manufactured of Mexico beaches and marshes led to the the interfacial tension, γ, can be related to by EcoLab. Corexit 9500 was used in decision to apply dispersants at the sur- the energy input (W) through W = γΔA, the DWH incident and represented the face as well as at depth during the DWH where ΔA is the increase in the oil-water​ first instance of dispersants applied to spill. Over the course of the spill, 2.9 mil- interfacial area. This equation shows that a deep-sea spill at the source of the oil lion liters of dispersant were applied at a reduction in the interfacial tension has release. Other major classes of disper- depth, and 4.1 million liters were applied the effect of increasing the interfacial area sants include the Dasic class (Slickgone) at the surface (Place et al., 2016). Early for a given amount of energy input (wave made by the Dasic Corporation in the in the spill, the dispersant Corexit 9527 action for surface spills and turbulence UK, and the Finasol class of disper- was the primary formulation, but it was associated with deep-sea release). The sants made by Total SA (France). In replaced with Corexit 9500 once suffi- objective of dispersant systems, there- this paper, we focus on Corexit 9500, cient supplies were available (National fore, is to lower interfacial tension suf- which typically contains three surfactant Commission on the BP Deepwater ficiently to create droplets that have a groups, Span 80 (sorbitan monooleate), Horizon Oil Spill and Offshore Drilling, diameter of less than about 70 µm. At this Tween 80 and Tween 85 (polyoxyethylene 2011). At the surface, dispersants were size range, the oil droplets stay suspended (20) sorbitan monooleate), and DOSS (bis applied sporadically in space and time, in the water column (colloidal stability) (2-ethyl­hexyl) sodium sulfosuccinate). based on real-time response to oil slicks. and do not rise to the surface. The sig- DOSS (imprecisely expanded as dioctyl At depth, dispersants were applied nificant increase in surface area of dis- sodium sulfosuccinate) is also referred to directly to the damaged wellhead through persed oil compared to the flat interface as AOT or Aerosol OT in the literature a wand inserted into the primary flow of of a surface oil slick is thought to allow on colloidal science, but this is trademark oil and gas. Although the flow of disper- easier access to oil by oil-degrading bac- nomenclature, and we will use the term sants was not constant, the variability in teria, thus enhancing rates of biodegrada- DOSS in this paper. The surfactants are flow rate was not as large as for the appli- tion (Lee et al., 2013; Prince et al., 2013; dissolved in , typically propylene cation rate on the surface, and thus aver- Aeppli et al., 2014; McFarlin et al., 2014; glycol and distillates. age flow rates could reasonably approxi- Prince and Parkerton, 2014; Prince and Figure 1 shows structures of the mate an input of dispersant to the deep Butler, 2014). However, recent work indi- primary surfactant components of ocean (Kujawinski et al., 2011). cates that dispersants may suppress the Corexit 9500. The Span and the Tweens With these introductory comments, activity of oil-degrading microorganisms are nonionic saccharide-based surfac- we start with the basics of the physics and (Kleindienst et al., 2015a,b). Excellent tants that are considered to be easily bio- chemistry of dispersants. Oil dispersants reviews on dispersants, their effectiveness, degraded due to their easily metabolized

Oceanography | September 2016 109 saccharide-containing structures (Place including cosmetics, textiles, paints, and stockpiled at various locations along the et al., 2016). DOSS is an anionic double- medicine, but had not been previously US coastline and is currently considered tailed surfactant that could persist for sig- applied in large volumes to seawater as the dispersant of choice. nificantly longer periods in the marine dispersants. Place et al. (2016) also found In the remediation of surface spills, environment (Kujawinski et al., 2011; that Corexit 9500 contains 0.28% w/w dispersant is sprayed down onto the sur- Place et al., 2016). Span is insoluble in α- and β-ethylhexyl sulfosucccinate face of the oil slick, preferably from an water while DOSS is sparingly soluble, (EHSS); EHSS likely occurs as a syn- airplane (for fast response, and to avoid but both surfactants are fully soluble in thesis impurity in Corexit 9500, but it spreading the slick with boats; see title hydrocarbons. The twin-tailed DOSS eas- is also reported as an abiotic hydroly- page photo). In application, it is import- ily forms reverse micelles, which transi- sis product and biodegradation prod- ant for the dispersant to be injected into tion to water-in-oil microemulsions with uct of DOSS (Hales, 1993; Campo et al., the oil to enhance surfactant attachment the addition of small amounts of water. 2013). The invention of the Corexit class at the oil-water interface. Dispersant Tweens are fully soluble in water. The of dispersants is attributed to Gerard P. application to bulk seawater leads to dilu- roles of the two solvents are not entirely Canevari at Exxon (McAuliffe et al., 1980; tion of the surfactants and a loss of effi- clear, but it is generally accepted that Canevari, 1982; Nedwed et al., 2008) cacy. After delivery to the oil in surface both solvents are used to ensure com- and is a landmark in oil spill remedia- spills, surfactants must be able to assist patibility with the oil-soluble and the tion technologies. Canevari (1973) devel- dispersion into small droplets with only water-soluble surfactants, and are mutu- oped the idea of blending these three wave action for mechanical agitation. ally miscible, leading to a single-phase types of surfactants in an organic sol- Dispersants are only effective in situa- system. Although the exact composi- vent to apply to the surface of an oil slick tions where weather situations are condu- tion of Corexit 9500 was not disclosed on the open sea. This development was cive to wave action. In the DWH incident, by EcoLab (Nedwed et al., 2008), Place stimulated by the need for more effec- dispersant was directly introduced into et al. (2016) reported the composition tive and more environmentally friendly the jet of oil emanating from the seafloor (% w/w) of Corexit 9500 as 18% DOSS, dispersants after poor experience with as well as onto surface slicks. The turbu- 4.4% Span 80, 18% Tween 80, and 4.6% detergents (notably, in the Torrey lence generated by fluid dynamics of such Tween 85. All of these surfactants are Canyon oil spill off the Cornwall coast jets is sufficient to cause mixing and to used in other commercial applications, in 1968; Canevari, 1973). Corexit 9500 is break oil globules into smaller droplets.

Tween 85 HO O O OH O O O O OH a Span 80 O b O O c O O O d OH n = a + b + c + d

Tween 80

Na+ O O– y O DOSS O O O O SO HO OH O x O O O x + y + z = 20 O O z OH

FIGURE 1. Structures of the surfactants used in the formulation of Corexit 9500.

110 Oceanography | Vol.29, No.3 Effectiveness of the dispersant depends the time period to complete dispersion. adsorption at the interface. This result is strongly on its surfactant composition Although Span (being very oil soluble) rather unexpected, because surfactants (Brochu et al., 1986; Brandvik et al. 1998; will tend to stay in the oil, too much Span have finite partition coefficients between Riehm and McCormick, 2014), with the actually degrades dispersion effectiveness the interface and the continuous bulk optimal mixture of Tween, Span, and (Riehm and McCormick, 2014), even water phase where they exist as mono- DOSS needed for maximum effectiveness when the interfacial tension is quite low. mers at concentrations lower than the defined as accomplishing the greatest This behavior might reflect the fact that critical micellar concentration. Reducing reduction in oil-water interfacial tension. sufficient hydrophilic components are the bulk concentration to infinitely dilute Blending surfactants, which each have needed to maintain a low interfacial ten- levels is expected to lead to surfactant different hydrophilic-lipophilic-balance sion. Rates of diffusion of different com- partitioning away from the interface and (HLB) values, results in an interfacial ten- ponents in the oil phase onto the surface into the bulk to maintain the value of the sion less than 10–3 mN m–1, much lower of a droplet can also affect the dispersion partition coefficient. Molecular simula- than the interfacial tension achieved using process. As new droplets are being cre- tion by Tang et al. (2014) indicates that the components of Corexit 9500 individ- ated, the breakup rate can become limited the large hydrophilic head groups of ually. The molecular mechanisms of such to being only as fast as the rate at which Tween may lead to entanglements at the synergistic effects of the surfactant mix- surfactants are able to populate the new oil-​water interface, preventing desorption ture on interfacial tension remain to be surfaces. (Riehm and McCormick, 2014). back into seawater, thus explaining the fully understood. However, in dynamics Packing of surfactants at the oil- irreversibility of adsorption observed by of the dispersion process, low interfacial water interface is of significant relevance Reichert and Walker (2013). Implications tension alone may not fully explain effec- in achieving low interfacial tensions and of this finding include the possibility that tive dispersion composition. One com- in maintaining such tensions. A very the cooperative interaction of this irre- plication is that water-soluble Tween interesting set of dynamic interfacial ten- versible monolayer with Span and DOSS and some DOSS can separate from the sion experiments by Reichert and Walker might also play a role in the composi- interface and leach into the seawater. If (2013) showed that the Tweens appeared tional dependence of dispersant effec- this occurs too quickly, then the disper- to create a stable monolayer at the inter- tiveness. Figure 2 illustrates the various sant-treated oil slick might not maintain face even under bulk dilution, lead- surfactant components at the oil-water a sufficiently low interfacial tension over ing to a thermodynamically irreversible interface (Riehm and McCormick, 2014).

FIGURE 2. Representation of the surfactant components of Corexit 9500 at the oil-water interface. Reproduced from Riehm and McCormick (2014)

Oceanography | September 2016 111 DISPERSANT FATE AND the impact of Corexit 9500 on the biodeg- surfactants Span 80 and Tweens 80 and 85 ECOSYSTEM IMPACT radation of oil (Kleindienst et al., 2015b). showed little long-term presence of these Mitigating the impact of oil on nearby During DWH and afterward, anal- molecules in deep waters (McAuliffe coastal areas was a primary concern yses of surfactants from the dispersant et al., 1980). This is likely due to analyt- during the DWH oil spill. Once disper- focused on DOSS in the deepwater plume ical difficulties and rapid degradation in sants were applied, it was imperative to rather than on surface samples. The few non-fixed seawater samples (Place et al., assess the fate of these mixtures in order studies that included surface samples 2016). In summary, work to date suggests to estimate their impact on water quality showed nearly universal low (or non- that the Corexit 9500 component DOSS and environmental health. Unfortunately, detectable) concentrations of DOSS in was transferred to the deepwater plume. appropriate sample analysis protocols surface waters (Gray et al., 2014). In con- Once in the plume, DOSS was trans- were not in place during the DWH inci- trast, samples collected in the oil-derived ported along deepwater currents until dilution reduced concentrations of DOSS below the detection limits. Most of the work on dispersants used in the DWH spill focused on the water The observation that microbially mediated column because it appeared that the bulk aggregation and degradation processes of the DOSS measured occurred in this affected the dynamics of oil at the sea surface large reservoir. However, there is now evi- dence that DOSS was entrained in oil that “ raises questions about effects of dispersant rose to the surface or sank into sediments application on microbial activities and microbially near the damaged wellhead (White et al., 2014). White et al. (2014) detected DOSS catalyzed degradation of oil. . in surface sediments and on corals near the damaged wellhead. Concentrations were spatially heterogeneous, suggest- ing stochastic delivery of dispersants to dent to quantify the components of dis- plume contained appreciable DOSS. Near the ocean floor, perhaps as part of marine persants in seawater (Place et al., 2010). ”the wellhead during the blowout, con- oil snow, as discussed below. These sam- However, the two Corexit formulations centrations of DOSS were correlated ples were collected within six months of were chemically similar in their surfac- with dissolved methane concentrations capping of the wellhead, but a more com- tant compositions, enabling the rapid and fluorescence-based measurements of prehensive survey shows that DOSS con- development of protocols that would hydrocarbons (Kujawinski et al., 2011). centrations remained high for the follow- track surfactants from both dispersants. Samples from shallower depths contained ing four years (Perkins and Field, 2014). Place et al. (2016) developed a method no detectable DOSS, suggesting that Nearly four years after the spill, DOSS to analyze the surfactant components in DOSS was transferred quantitatively into was detected within oil-sand patties col- seawater, based on large volume injec- the dissolved oil plume after injection at lected from beaches along the Gulf of tion (which takes minimal sample prepa- the wellhead. As the plume traveled far- Mexico (White et al., 2014). Composition ration) liquid chromatography coupled ther from the wellhead, DOSS concen- of oil in the inner portion of the pat- to mass spectrometry. This method is trations decreased primarily by dilu- ties confirmed the source of the patties advantageous because it is able to quan- tion (Kujawinski et al., 2011) rather than as DWH-released Macondo oil. Because tify all four components of Corexit 9500 by biological degradation. Subsequent DOSS is more water-soluble than petro- in one analysis. In the case of DOSS, more experiments showed that DOSS does leum hydrocarbons, its persistence within targeted methods were also developed for not degrade appreciably under the cold these patties suggests that at least part of higher sensitivity analyses (Kujawinski and dark conditions of the deep ocean each patty was not in contact with the et al., 2011; Gray et al., 2014). All methods (Campo et al., 2013). Measurements of surrounding water (White et al., 2014). were able to detect DOSS below the EPA α-EHSS and β-EHSS in open sea­water Although DOSS concentrations in these limit of concern of 20 µg L–1 (Kujawinski could not be interpreted as unambig- matrices are exceedingly small, they high- et al., 2011; White et al., 2014). In the uous evidence of DOSS biodegrada- light the pervasive contamination of the years since the DWH spill itself, these and tion because α-EHSS and β-EHSS occur Gulf of Mexico by the DWH oil spill and other complementary methods have been in Corexit 9500 as a synthesis impurity the challenges associated with removal of applied in order to monitor the fate of (Barsamian et al., 2014; Place et al., 2016). all oil and the surfactants from disper- surfactants in Corexit 9500 and to assess Complementary analyses of the nonionic sants from the ecosystem.

112 Oceanography | Vol.29, No.3 During the course of the DWH oil (Lu et al., 2012). Bacteria enriched at 5°C of the natural microbial community, as spill, abundant marine oil snow formed on Corexit 9500 and Macondo oil, using suggested by experiments conducted at the sea surface. These macroaggregates uncontaminated water samples collected using deep water from a natural oil seep contain organic matter, oil, and bacteri- at a depth of 1,100 m near the spill site, in the Gulf of Mexico incubated with ally produced EPS (extracellular poly- were found to remove 25% and 60% of the the water-accommodated fraction (con- meric substances; Gutierrez et al., 2013) dissolved oil without and with addition of taining no dispersant) and chemically that help bind the particles together Corexit 9500, respectively (Bælum et al., enhanced water-accommodated frac- (Passow et al., 2012; Ziervogel et al., 2012). Straight-chain alkanes, as well as tion (with dispersant) of Louisiana sweet 2012). Bacterially derived EPS facili- Corexit 9500, were readily degraded in crude oil. The composition of the micro- tates access to oil components and con- these incubations. Large oil-snow flocs bial community at the end of the incu- currently serves as a metabolic substrate also were observed; the microbial com- bations differed: incubations includ- for the diverse community of γ- and munity associated with the flocs were ing dispersant had high abundance of α-Proteobacteria, Bacteroidetes, and dominated by Colwelliaceae (Bælum et al., Colwellia, whereas incubations with- Planktomycetales associated with these 2012). The Colwellia strain RC25 isolated out dispersant were dominated by marine oil snow aggregates (Arnosti from these incubations proved capable of Marinobacter (Kleindienst et al., 2015b). et al., 2016). Much of the oil that even- degrading 75% of the Macondo oil in the Dispersant-only incubations (lacking the tually vanished from the surface ocean is presence of Corexit 9500 (Bælum et al., water-​accommodated oil fraction) also hypothesized to have been removed via 2012). The effect of droplet size was also showed enhanced Colwellia abundance, sinking of the marine oil snow once its found to affect biodegradation of oil, as with differentColwellia taxa dominat- density exceeded that of seawater (Passow demonstrated using oil collected from the ing the dispersant-​only and the disper- et al., 2012; Passow and Ziervogel, 2016, riser pipe in May 2010 and Corexit 9500. sant and water-accommodated oil exper- in this issue). Figure 3 illustrates a labo- These experiments, carried out with iments. Moreover, degradation rates of ratory experiment demonstrating the for- water collected from a Norwegian fjord, hexadecane and naphthalene were more mation of marine oil snow (Ziervogel showed that smaller oil droplet size cor- rapid in the absence of dispersants, as et al., 2012). related with faster biodegradation rates was the overall removal of the water-​ The observation that microbially (Brakstad et al., 2015). accommodated oil fraction (Kleindienst mediated aggregation and degrada- Application of dispersants, however, et al., 2015b). Selective effects of dispersant tion processes affected the dynamics may affect the growth and composition on degradation of specific components of of oil at the sea surface raises questions about effects of dispersant application on microbial activities and microbially cat- FIGURE 3. Marine oil snow formed after incubation of alyzed degradation of oil. Application of uncontaminated surface sea- dispersant at the broken riser pipe at a water from the Gulf of Mexico depth of 1,500 m likely contributed to the with spilled Deepwater Horizon oil collected at the sea surface formation of a plume of dispersed oil at near the spill site in early May depths of ~1,100–1,200 m (Camilli et al., 2010. This photo shows the 2010). This plume harbored a distinct marine oil snow hanging sus- pended from the surface inter- microbial community enriched in the face after 21 days of incubation γ-Proteobacterial order Oceanospirillales on a roller table. The marine oil (Hazen et al., 2010). After the riser pipe snow contains active bacteria, sticky bacterially derived extra- was plugged, the plume was no longer cellular polymeric substances, detectable; the specific fate of the plume, and distinct droplets of oil (dark and the role of microbial communities in spheres). Photo from Ziervogel et al. (2012) processing the escaped oil, are a focus of considerable study. Efforts to understand specific effects of dispersant application, particularly at depth, has spurred further investi- gations of microbial communities and 10 mm their activities. Microbial communities deep in the Gulf of Mexico metabolized a considerable fraction of the spilled oil

Oceanography | September 2016 113 oil have also been reported for Alaskan sweet crude oil, and Corexit 9500 yielded NEW DEVELOPMENTS AND North Slope oil and Corexit 9500: hexa- marine oil snow, also in the presence of IMPLICATIONS IN DISPERSANT decane and phenanthrene were miner- dispersant; the marine oil snow formed in DESIGN AND IMPLEMENTATION alized less in the presence of dispersant, the presence of dispersant that contained Although Corexit 9500 is currently the but dodecane and naphthalene deg- more n-alkanes than the oil-only incu- dispersant of choice, there are ongoing radation did not show any effect from bations (Fu et al., 2014). Other experi- issues regarding its use, and there is the the presence of dispersant (Lindstrom ments with plume and non-plume deep potential for improvement. First, liq- et al., 2002). Corexit 9500 itself was also water, however, showed variable forma- uid phase dispersants may not work well found to support considerable micro- tion of marine oil snow, potentially due on weathered oil and are easily washed bial growth, but it did not increase min- to differences in deepwater microbial away by wave action when applied onto weathered oil. Second, the dispersant that does not directly fall on an oil slick is not effective, and may have unintended del- eterious effects on microbial commu- nities. There is also concern that sol- [V]ariable effects of Corexit 9500 on microbial vents used in dispersant formulations, isolates and microbial communities and their although relatively innocuous in com- parison to the polycyclic aromatic hydro- oil-degrading capabilities have been observed, carbons (PAHs) in the oil phase, consti- “ depending in large part on the nature and state tute an additional and perhaps significant insertion of hydrocarbons to the environ- of the oil (weathered or not), relative ratios of ment. Lastly, the degree to which micro- oil and dispersant, and the composition of the organism attachment to oil droplets and in situ microbial community. . the degree to which formation of bio- films are facilitated or hindered by sur- factants at the oil-water interface both remain poorly constrained. However, replacement of Corexit 9500 and its vari- ants from Dasic and Total with entirely eralization of either fresh or weathered ”community composition, which changed different dispersants is unlikely due to Alaskan North Slope oil (Lindstrom et al., with time and space (Passow et al., 2012). the logistics of large-scale manufacture, 2002). Moreover, Corexit 9500 was found Systematic investigations using seawater rapid deployment, toxicity testing, and to be toxic to specific bacteria, including from above natural oil seeps in the Gulf of approval procedures from the appropriate Marinobacter and Acinetobacter isolates Mexico as well as from the Santa Barbara federal agencies, including the EPA and obtained from DWH oil-​contaminated Basin highlighted the effects of oil weath- the National Oceanic and Atmospheric beach sands (Hamdan and Fulmer, ering (in particular, exposure to UV light) Administration (NOAA). 2011). Further study into the fundamen- as a factor enhancing marine oil snow There are opportunities, however, for tal mechanisms of surfactant-marine formation (Passow, 2014). In these inves- new dispersants that work in synergy bacteria interactions is clearly required to tigations, the presence of Corexit 9500— with current dispersants and mitigate understand such toxicity. particularly at low concentrations— some of their disadvantages. For exam- Whether marine oil snow formed reduced or inhibited marine oil snow ple, dispersants that adhere to weathered at depth in the plume at the time of the formation, although high Corexit 9500 oil and cause the oil to disperse, perhaps oil spill is not clear (Passow et al., 2012), concentrations (1 mL L–1) led to aggre- by breaking up the water-in-oil emul- although Hazen et al. (2010) reported gate formation (Passow, 2014). In sum- sions present in such systems, would be “cellular flocs” in the deepwater plume. mary, variable effects of Corexit 9500 on relevant. Indeed, there is recent work by Experimental investigations using deep microbial isolates and microbial commu- ExxonMobil to develop gel-type disper- Gulf of Mexico water have reported for- nities and their oil-degrading capabili- sants for such applications (Nedwed et al., mation of marine oil snow in treatments ties have been observed, depending in 2008). Key beneficial characteristics of with oil, whether or not dispersant is large part on the nature and state of the gel-type dispersants over traditional liquid present (Bælum et al., 2012; Kleindienst oil (weathered or not), relative ratios of dispersants include (a) close adherence to et al., 2015b). Likewise, investigations oil and dispersant, and the composition weathered oils without being washed off, with coastal surface seawater, Louisiana of the in situ microbial community. (b) extended buoyancy periods that allow

114 Oceanography | Vol.29, No.3 more contact with oil, (c) high surfactant oil-water interfacial tension significantly halloysite nanotubes on oil droplets. concentrations, (d) reduced solvent levels, and are therefore ineffective as disper- Finally, there is a need to develop safe and (e) possibility of the gels providing sants. Owoseni et al. (2014) used a tubu- materials for efficient chemical herding, a some degree of visible feedback to oil spill lar particulate system made of halloysite, related process that uses surfactants at the responders (Nedwed et al., 2008). a natural clay type mineral where the air-water interface to compact oil slicks Athas et al. (2014) pioneered a sys- tube lumen is loaded with dispersant into layers (at least 2–3 mm) that are suf- tem containing soybean lecithin and the surfactant components (Tween, Span, ficiently thick to enable burning of the oil surfactant Tween 80 dissolved in etha- and DOSS) (see also Athas et al., 2014; or facilitate surface skimming (Buist et al., nol, showing that this formulation leads Nyankson et al., 2015b). If targeted cor- 2011). Herding is especially relevant in to effective emulsification. Substitution rectly, these particles attach to and deliver the Arctic where the presence of ice floes of the double-tailed phospholipids in lec- the surfactant to the oil-water interface, dampens wave action that aids in dispers- ithin for DOSS, which is the persistent lowering the oil-water interfacial ten- ing oil, and dispersant application is inef- component in Corexit 9500, is a signifi- sion and breaking up the oil into smaller fective due to indiscriminate spraying on cant finding, as naturally occurring phos- droplets. Figure 4 illustrates attachment of ice. The development of new plant-based pholipids are likely to be environmentally benign. Additional research on the use of lecithin as a substitute for DOSS has been Halloysite nanotube (HNT) carried out by Riehm et al. (2015) and by Nyankson et al. (2015a). Oil Use of particle-stabilized emulsions Droplet is also being studied intensively as a way to stabilize emulsion droplets against coalescence, thus complementing the role of dispersants. The ability of parti- cles to partition to the oil-water inter- face and stabilize emulsions is a much studied phenomenon often referred to as Pickering emulsions, following early pio- neering studies by Ramsden (1903) and Pickering (1907). Several experimen- Surfactant-loaded HNT tal and theoretical studies have been car- ried out on solids-​stabilized emulsions in order to understand factors that affect sta- bility and structure of the interface (Binks Aqueous and Lumsdon, 2000; Ingram et al., 2010). Phase Examples include use of carbon-based particles and silicas (Creighton et al., Surfactant molecules 2014; Rodd et al., 2014). We also note the released from HNTs relevance to oil-mineral aggregates, a sig- nificant topic related to understanding the fate of oil droplets when they come into contact with sediment particles (Passow et al., 2012; Gong et al., 2014). The attach- ment of sediments to such droplets at the oil-water interface is clearly a conse- quence of the thermodynamics of parti- cle-stabilized droplets, where the energy to remove particles from the oil-water​ 50 µm 1 µm interface is several orders of magnitude above the thermal energy. Although par- FIGURE 4. (top) Schematic of halloysite clay nanotube (HNT) stabilization of an emulsion droplet and the release of surfactant cargo. (bottom) Cryo scanning electron microscope image showing an ticulate systems are effective at stabiliz- oil droplet stabilized with HNT on the left, and a magnified view on the right showing the network of ing emulsions, they do not decrease the HNT on the oil-water interface. Reproduced from Owoseni et al. (2014)

Oceanography | September 2016 115 amphiphiles for herding (Gupta et al., water interface is an important determi- plume transport and biodegradation at Deepwater Horizon. Science 330:201–204, http://dx.doi.org/​ 2015) may be extended to use of such nant of exposure. Future research that 10.1126/science.1195223. materials in the development of environ- couples greater insights into microbial Campo, P., A.D. Venosa, and M.T. Suidan. 2013. Biodegradability of Corexit mentally benign surfactants that may also capabilities and responses with higher- 9500 and dispersed South Louisiana have applications as dispersants. resolution physicochemical defini- crude oil at 5 and 25°C. Environmental Science & Technology 47:1,960–1,967, tion of interfaces and dispersant gradi- http://dx.doi.org/10.1021/es303881h. CONCLUDING REMARKS ents will greatly contribute to our under- Canevari, G.P. 1973. Development of the “next gen- eration” chemical dispersants. 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