Dispersants and why they can be harmful to the environment

Dispersants are chemicals that are often used as a response during oil spills, to reduce the IFT (interfacial tension) of oil slicks on the water’s surface to allow the oils to disperse more quickly. Its subsequent diffusion and dilution by ocean currents make it more available for biodegradation, therefore decreasing its danger to coastal areas, aquatic animals, and birds (Fink, 2021; Seidel et al., 2016). The use of dispersants as a response to oil spills has been attractive, as they are able to treat a large amount of oil quickly, under various environmental conditions (Chen et al., 2021). An example is during the Deepwater Horizon oil spill when large quantities of the dispersant Corexit was used– both sprayed onto the surface slick, as well as through subsea injection into the broken wellhead under the surface (Jones et al., 2017). Figure 1 depicts the use of chemical dispersants during an oil spill.  

 

Figure 1: Use of Chemical Dispersants during a Subsurface Oil Spill. (Source: Government accountability office., 2021).

However, a growing literature has raised concerns about the use of dispersants such as Corexit. Firstly, there are concerns that the toxicity of oil that has been dispersed in the water column may be higher compared to oil that has not been dispersed (Lewis & Pryor, 2013; Vignier et al., 2016). This concern arises from the fact that the dispersant causes an increase in the oil’s surface-to-volume ratio, therefore increasing its bioavailability to aquatic organisms, while also retaining its phytotoxic properties (Frometa et al., 2017). Moreover, there has been apprehension about the potential inherent toxicity of dispersants, in which the long-term persistence of these dispersants may result in additional negative externalities to the environment that they were intended to safeguard (Goodbody-Gringley et al., 2013; White et al., 2014). Notably, a study by Rico-Martínez et al. (2013) found toxicity to B. manjavacas (marine zooplankton commonly used in ecotoxicological studies), to increase by 52 times, when the dispersant Corexit 9500A was mixed with crude oil. Additionally, in a recent review of knowledge of the toxicity of the dispersant Corexit 9500A by Stroski et al. (2019), they concluded that while Corexit 9500A dispersant has limited to no effect on higher-level organisms, it has significant negative impacts on certain lower trophic level organisms such as microzooplankton and small crustaceans, as well as on some larval form of higher trophic species. 

Possible solution moving forward: Sustainable dispersants? 

With regard to the second potential concern of dispersants being toxic, a possible solution moving forward is the switch to the use of sustainable dispersants. For this next section, I refer to a review of recent developments in new sustainable dispersants by Zhu et al. (2022). According to Zhu et al. (2022), sustainable dispersants are dispersants that are made out of environmentally friendly ingredients (e.g., food-grade chemical surfactants, biosurfactants, ionic liquids, and additives). Of the various sustainable dispersants that have been evaluated as dispersant ingredients (shown in Table 1), Lecithin and Tween 80, when used in combination, have been found to have a very high dispersant efficiency (89%), where it is able to reduce the interfacial tension between seawater and oil to 0.03-0.4 mN/m. Additionally, according to Owoseni et al. (2018), they are able to reduce the average oil droplet size to 7.81 mm. This is comparable to that of commercial dispersants (E.g. Corexit 9500A).

Table 1: Summary of evaluation of recent developments in sustainable dispersants. (Source: Zhu et al., 2022). 

These biosurfactant-based dispersants, as the biological equivalent of chemical alternatives, have the key environmental benefit of having a higher biodegradability (Zhu et al., 2022). For instance, Tween 80 and Tween 85 had a notably rapid rate of biodegradation, with their levels becoming undetectable after 4-8 days even in cold seawater environments (5°C). In contrast to Tween 80 and Tween 85, chemical surfactants have a significantly longer delay before they start to biodegrade, with reported half-lives of 20, 28, and 24 days in Corexit, Dasic, and Finasol, respectively (Brakstad et al., 2018). Additionally, biosurfactant-based dispersants have also been found to have lower than that of chemical surfactants (Zhu et al., 2022). 

However, there remain challenges. According to Zhu et al. (2022), low yield, and high production costs are the main obstacles barring the widespread adoption of such sustainable biosurfactant-based dispersants. Moving forward, one potential option is to consider using waste/by-products as substrates/raw materials for production. One suggestion has been to extract peptone from tuna fish waste, which can be used for the subsequent production of biosurfactants (Hu et al., 2021). 

Conclusion

Overall, sustainable dispersants, as a possible response to oil spills, provide a useful alternative that can potentially bring about lower ecological impacts. However, there is still an ongoing debate with regard to whether or not the dispersion of oil brings more or less negative externalities to the environment (Merlin et al., 2021; Prince, 2015). Therefore, more research still needs to be conducted in that area. Such research would inform policymakers’ response to future oil spills. 

References 

Brakstad, O. G., Størseth, T. R., Brunsvik, A., Bonaunet, K., & Faksness, L.-G. (2018). Biodegradation of oil spill dispersant surfactants in cold seawater. Chemosphere, 204, 290–293. https://doi.org/10.1016/j.chemosphere.2018.04.051

Chen, B., Lee, K., Merlin, F., Yang, M.,Ye, X., Zhang, B., & Zhu, Z. (2021). Ecological impact analysis of dispersants and dispersed oil: An overview. Journal of Environmental Informatics Letters, 5(2), 120-133. https://doi.org/10.3808/jeil.202100058

Fink, J. (2021). Oil spill treating agents. In Petroleum Engineer’s Guide to Oil Field Chemicals and Fluids (pp. 859–879). Elsevier. https://doi.org/10.1016/B978-0-323-85438-2.00019-0

Frometa, J., DeLorenzo, M. E., Pisarski, E. C., & Etnoyer, P. J. (2017). Toxicity of oil and dispersant on the deep water gorgonian octocoral Swiftia exserta, with implications for the effects of the Deepwater Horizon oil spill. Marine Pollution Bulletin, 122, 91–99. https://doi.org/10.1016/j.marpolbul.2017.06.009

Goodbody-Gringley, G., Wetzel, D. L., Gillon, D., Pulster, E., Miller, A., & Ritchie, K. B. (2013). Toxicity of deepwater horizon source oil and the chemical dispersant, corexit® 9500, to coral larvae. PLoS ONE, 8(1), e45574. https://doi.org/10.1371/journal.pone.0045574

Government Accountability Office. (2021). Offshore oil spills: Additional information is needed to better understand the environmental tradeoffs of using chemical dispersants (GAO-22-104153). https://www.gao.gov/products/gao-22-104153 

Hu, J., Luo, J., Zhu, Z., Chen, B., Ye, X., Zhu, P., & Zhang, B. (2021). Multi-scale biosurfactant production by bacillus subtilis using tuna fish waste as substrate. Catalysts, 11(4), 456. https://doi.org/10.3390/catal11040456

Jones, E. R., Martyniuk, C. J., Morris, J. M., Krasnec, M. O., & Griffitt, R. J. (2017). Exposure to Deepwater Horizon oil and Corexit 9500 at low concentrations induces transcriptional changes and alters immune transcriptional pathways in sheepshead minnows. Comparative Biochemistry and Physiology Part D: Genomics and Proteomics, 23, 8–16. https://doi.org/10.1016/j.cbd.2017.05.001

Lewis, M., & Pryor, R. (2013). Toxicities of oils, dispersants and dispersed oils to algae and aquatic plants: Review and database value to resource sustainability. Environmental Pollution, 180, 345–367. https://doi.org/10.1016/j.envpol.2013.05.001

Merlin, F., Zhu, Z., Yang, M., Chen, B., Lee, K., Boufadel, M. C., Isaacman, L., & Zhang, B. (2021). Dispersants as marine oil spill treating agents: A review on mesoscale tests and field trials. Environmental Systems Research, 10(1), 37. https://doi.org/10.1186/s40068-021-00241-5

Prince, R. C. (2015). Oil spill dispersants: Boon or bane? Environmental Science & Technology, 49(11), 6376–6384. https://doi.org/10.1021/acs.est.5b00961

Rico-Martínez, R., Snell, T. W., & Shearer, T. L. (2013). Synergistic toxicity of Macondo crude oil and dispersant Corexit 9500A® to the Brachionus plicatilis species complex (Rotifera). Environmental Pollution, 173, 5–10. https://doi.org/10.1016/j.envpol.2012.09.024

Seidel, M., Kleindienst, S., Dittmar, T., Joye, S. B., & Medeiros, P. M. (2016). Biodegradation of crude oil and dispersants in deep seawater from the Gulf of Mexico: Insights from ultra-high resolution mass spectrometry. Deep Sea Research Part II Topical Studies in Oceanography, 129, 108–118. https://doi.org/10.1016/j.dsr2.2015.05.012

Stroski, K. M., Tomy, G., & Palace, V. (2019). The current state of knowledge for toxicity of corexit EC9500A dispersant: A review. Critical Reviews in Environmental Science and Technology, 49(2), 81–103. https://doi.org/10.1080/10643389.2018.1532256

Vignier, J., Soudant, P., Chu, F. L. E., Morris, J. M., Carney, M. W., Lay, C. R., Krasnec, M. O., Robert, R., & Volety, A. K. (2016). Lethal and sub-lethal effects of Deepwater Horizon slick oil and dispersant on oyster (Crassostrea virginica) larvae. Marine Environmental Research, 120, 20–31. https://doi.org/10.1016/j.marenvres.2016.07.006

White, H. K., Lyons, S. L., Harrison, S. J., Findley, D. M., Liu, Y., & Kujawinski, E. B. (2014). Long-term persistence of dispersants following the deepwater horizon oil spill. Environmental Science & Technology Letters, 1(7), 295–299. https://doi.org/10.1021/ez500168r

Zhu, Z., Song, X., Cao, Y., Chen, B., Lee, K., & Zhang, B. (2022). Recent advancement in the development of new dispersants as oil spill treating agents. Current Opinion in Chemical Engineering, 36, 100770. https://doi.org/10.1016/j.coche.2021.100770