Scientists assessed the use of clay particles in experiments to develop a new class of dispersant that is effective and less toxic than those used in the Deepwater Horizon response.
They found that clay particles adhered to the interface where oil and water meet and acted as emulsifiers, combining the oil and water into droplets. The tiny hollow tube-like structures in the clay lay flat and formed networks that cover oil droplets, providing a barrier that stabilized the droplets and could prevent a slick from reforming. A synergistic effect from clay tubes filled with solvent-free dispersant components (surfactants) produced smaller droplets and further enhanced oil dispersion. This technology could serve as an effective, cost-efficient, and environmentally benign formulation of dispersant materials. The researchers published their findings in the October 2014 issue of Langmuir: Release of Surfactant Cargo from Interfacially-Active Halloysite Clay Nanotubes for Oil Spill Remediation.
Dispersants have an important role in reducing shoreline oiling and promoting biodegradation. But, the unprecedented amount used in response to the Deepwater Horizon oil spill raised concerns that toxicity from petroleum-derived solvents in dispersants could negatively impact the environment. Prior research observed that sediment particles attach to dispersed oil droplets, giving rise to the concept of oil-mineral aggregates. This study’s scientists looked to halloysite clay, an abundant mineral-rich fine-grained sediment with the novel internal structure of tubular air-filled spaces, as a potential natural vehicle to deliver a less toxic and more effective dispersing agent. They saw clay nanotubes filled with surfactants for oil spill remediation being analogous to particle-based carriers in drug delivery technologies.
The team tested the ability of halloysite tubes in their natural state and filled with surfactant to disperse oil and stabilize droplets under various conditions. For surfactant loading, researchers suctioned the air out of the tubes and filled them with Corexit 9500 surfactant components (dioctyl sodium sulfosuccinate or DOSS, Tween 80, and Span 80) dissolved in liquid solvent (methanol) and then used a rotary evaporator to remove the solvent. In separate experiments, they released the natural-state and surfactant-filled clay tubes in mixtures of saline water and Louisiana sweet crude oil. The team analyzed the resulting oil droplets, the dynamics of surfactant release, and oil-water interfacial tension (reduced tension makes it easier for oil to break up and stay in the water column).
The natural-state clay tubes were effective in breaking up oil into drops and stabilizing them. As clay concentrations rose, their effectiveness improved in reducing droplet size and maintaining stable emulsions. A ten-fold increase in clay particles decreased the average droplet size by more than 50% and the reappearance of oil from broken droplets decreased to zero. The emulsions remained stable for more than three months. Using cryo-scanning electron microscope imagery, the team observed that the nanotubes assembled end-to-end and formed a rigid, protective film that prevented oil droplets from coming back together. These barrier-forming linkages are strong, especially at the oil-water interface. It took eleven times the energy to break apart ten linked tubes as it did to detach a single clay tube from the interface. However, there was no improvement in reducing oil-water interfacial tension.
The synergistic effect of surfactants’ ability to reduce interfacial tension and the clay tubes’ tiny pores provided for a slow release of the surfactants and produced smaller droplets than the natural-state tubes alone. At constant clay concentration, the average droplet size decreased as surfactant loading increased. The combination also reduced the oil-water interfacial tension to levels appropriate for oil spill dispersion. A blend of all three surfactants reduced interfacial tension beyond levels obtained with individual surfactants.
Dr. Vijay John, one of the study’s authors, describes additional bioremediation benefits, “The halloysite particles may enable bacteria to anchor to the droplet, allowing the bacteria to more rapidly colonize and consume the oil.” John also noted that because halloysites are solid particles, they can be easily granulated and used in a spray for targeted application to surface sheens or as a slurry for direct injection into an oil jet.
This study showed that surfactant-loaded hallyosite clay particles, with no hydrocarbon solvent, are an advancement towards a more safe and effective oil remediation technology compared to commercial liquid dispersants. The abundant availability of halloysite clay makes translation to large-scale industrial applications feasible. The team is investigating additional applications for oil spill mitigation such as including fluorescent or magnetic markers in the clay tubes to improve identifying oil spills.
The study’s authors are Olasehinde Owoseni, Emmanuel Nyankson, Yueheng Zhang, Samantha J. Adams, Jibao He, Gary L. McPherson, Arijit Bose, Ram B. Gupta, and Vijay T. John (Langmuir, 2014, 30 (45), 13533 – 13541).
This research was made possible in part by a grant from the Gulf of Mexico Research Initiative (GoMRI) to theConsortium for the Molecular Engineering of Dispersant Systems (C-MEDS). Additional funding sources included the National Science Foundation (1049330).