A green approach to cleaning oil spills

Dr. Neil Canter, Contributing Editor | TLT Tech Beat December 2010

Developed from sugar-based derivatives, a new solidifier has potential to officially clean up oil spills.

KEY CONCEPTS
• The current dispersants, sorbents and solidifiers used to clean up oil spills have limitations.
• A new type of green solidifier developed from sugar-based derivatives is effective in treating a number of different hydrocarbons including diesel fuel and vegetable oils.
• The solidifier is a molecular gelator that can be recovered from the oil and reused many times through a thermoreversibility process.

Those of us working in the lubricant industry are very aware of the problems in dealing with oil spills. Oil is the most widely used raw material in our industry. It is present either in its neat form or as part of a lubricant in manufacturing, distribution and end-user facilities. Unfortunately, this means that the odds of dealing with oil spills are rather high.

In a previous TLT article, new technology that is very useful in extracting oil from water-based emulsions was introduced (1). A nanowire membrane based on potassium manganese oxide coated with polydimethylsiloxane will readily absorb up to 20 times its weight in oil through the use of capillary action.

Current technologies that can be used to clean up oil spills are divided into the categories of dispersants, sorbents and solidifiers. The former has been widely publicized because of its use in cleaning up the recent blowout of an oil well in the Gulf of Mexico.

George John, associate professor of chemistry at The City College of New York in New York City, says, “The three types of materials used to clean up oil all work in different fashions. Dispersants tend to emulsify the oil, sorbents are solid powders that absorb oil and solidifiers are usually polymers that tend to gel oil even in the presence of water.”

All of these materials have limitations. For example, the polymers used as solidifiers do not work well in cleaning up viscous oils. John adds, “In cleaning up oil spills, users cannot rely just on one technique but need to use all of them.”

This means that there is a need for developing materials that can more effectively clean up oil spills. For solidifiers, John lists five characteristics needed for an ideal solidifier. He says, “A solidifier should efficiently gel the oil phase in the presence of water at room temperature, be manufactured in a cost-effective fashion, facilitate the recycling of the oil from the gel for further use and be environmentally friendly and reusable.”

A new type of solidifier that exhibits these characteristics would be welcomed. Such a material has just been developed.

SUGAR-BASED MOLECULAR GELATORS
John’s research team includes his graduate student Swapnil Jadhav, former postdoctoral fellow Dr. Praveen Kumar Vemula and professor Srinivasa Raghavan of the University of Maryland, who have developed a new type of solidifier that is known as a molecular gelator. He explains, “Molecular gelators are compounds with molecular weights below 1,000 that organize into a three-dimensional fiber network similar to a sponge above a minimum gelation concentration. The network contains fibers that are 20-30 nanometers in diameter, which form a kind of crosslink that holds the oil.”

The molecular gelators are weakly polar molecules that interact through hydrogen bonding and van der Waals forces. For this type of molecule to work, it must act in a similar function to a surfactant by incorporating hydrophilic and hydrophobic components. Oil tends to be trapped in the interior of the fiber network and is surrounded by the hydrophobic portion of the molecular gelator.

In looking for a green candidate, the researchers decided to evaluate sugar-based derivatives. John says, “Sugars are readily water soluble, so our objective was to incorporate hydrophobic character to render them water insoluble but compatible with a suitable solvent.”

A series of ester derivatives were prepared from the sugars mannitol and sorbitol. Diesters based on butanoic and octanoic acids were prepared under relatively mild conditions (heating at 45 C at 48 hours) using regiospecific enzyme catalysts. The resulting molecules are designated by the type of sugar and the number of carbons contributed by the carboxylic acid. For example, Man-8 is a molecular gelator prepared from mannitol and octanoic acid.

The gelation capabilities of several of these sugar derivatives were evaluated by using a number of different hydrocarbons and vegetable oils. John says, “Most of these sugar esters are solid structures, so we needed to find a solvent to efficiently add the molecular gelator to the oil.”

The researchers found that mixing the molecular gelator in ethanol was an effective strategy. Addition of an ethanol aliquot to a 1:1 mixture of an oil (such as diesel fuel) and water led to the instantaneous partitioning of the solvent into water and the molecular gelator in the oil. Gelation of the oil phase then readily occurred, as seen in Figure 3.


Figure 3. The sugar-based molecular gelator readily isolates oil in the top layer from water by forming a reversible crosslink. (Courtesy of The City College of New York)

John notes that the molecular gelator represents less than 5% of the total solvent. In fact, the oil gel formed is not only strong enough to hold its own weight but also the weight of an aqueous solution.

The additional appeal is that the gelation process can be reversed at elevated temperature. John says, “In a process known as themoreversibility, we can recover the gelled oil from the fiber network through simple distillation.” The thermoreversible temperatures of the sugar diesters ranged from 38 C to 125 C.

The ability of molecular gelators to reversibly crosslink distinguishes them from polymers that form irreversible chemical crosslinks. Advantageously, the reversibility property enables isolation and subsequent reuse of the molecular gelator many times.

The researchers have filed a patent application on their process. Future work involves development of molecular gelators with improved properties. John is hopeful that this technology can be commercialized. He says, “We have only prepared gram quantities of the molecular gelators and we are in the process of scaling up this process.”

Additional information can be found in a recent publication (2) or by contacting John at john@ccny.cuny.edu. 

REFERENCES
1. Canter, N. (2008), “More Efficient Cleanup of Oil Spills,” TLT, 64 (12), pp. 12–13.
2. Jadhav, S., Vemula, P., Kumar, R., Raghavan, S. and John. G. (2010), “Sugar-Derived Phase-Selective Molecular Gelators as Model Solidifiers for Oil Spills,” Angew. Chem. Int. Ed., 49 (42), pp. 7695–7698.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.