The science of dispersants
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By Daniel Cressey
Since the Deepwater Horizon oil rig exploded and sank in April, vast quantities of dispersant chemicals have been sprayed onto the resulting slick. The chemicals--alongside corralling floating oil with booms and setting it aflame in "controlled burns"--are an attempt to reduce the environmental damage of oil still spewing out of a ruptured well in the Gulf of Mexico at the rate of 5,000 barrels (about 800,000 liters) per day.
What do dispersants do?
Dispersants do not remove oil from the area. Rather, they help large globs of oil "disperse" into smaller pieces--hence their name--which are easier for sea-living microbes to break down.
They work on the same principle as kitchen washing-up liquids. Both are made up mainly of surfactant molecules, which have heads that are attracted to water and tails that are repelled by water.
The molecules embed themselves at interfaces between oil and water. This lowers surface tension at the interface--that is, the difficulty of disrupting molecules of oil and molecules of water from clinging to their own kind rather than mixing.
"Lowering the interfacial tension reduces the energy required to mix the oil as discrete droplets into the water phase," says David Horsup, division vice-president for research and development at Nalco Energy Services, which manufactures the dispersant products currently in use in the Gulf of Mexico. "Dispersing the oil into small droplets allows naturally present microbes to then consume those oil droplets, and the dispersant rapidly to biodegrade."
Which dispersants are being used in the Gulf of Mexico?
So far, at least 325,000 gallons (over 1 million liters) of two types of dispersants have been used: Corexit 9500 and Corexit EC9527A.
"One is designed for light, fresh oil or oil which has been released very early on and hasn't been weathered," explains Horsup. "The other is designed for heavier oils and those that have been weathered for a few days."
What are the environmental impacts of dispersants?
Dispersants are themselves unpleasant chemical concoctions, although today's products are less damaging than the toxic solvents used in earlier spills such as the Torrey Canyon disaster. The main problem with dispersants relates to the fact that they help to spread oil more widely into the environment.
This helps to prevent large amounts of oil fouling beaches and coating animals, but although surface-dwelling animals may benefit from a reduction in oil exposure, animals in other parts of the oceans and those at the sea floor will experience more pollution.
"It's a trade-off, and no one will tell you using dispersants won't have an effect. You're trading one species for another," says Carys Mitchelmore, an environmental chemist at the University of Maryland Center for Environmental Science in Solomons and a co-author on a 2005 U.S. National Academies report on dispersants.
In one study, not yet published, Mitchelmore and her colleagues exposed soft corals to crude oil and the Corexit 9500 dispersant currently in use. Growth rates fell significantly.
Overall, however, there is still a dearth of knowledge about the impact of dispersants, especially if used in the volumes now being applied in the Gulf.
"The long-term effects are really unknown," says Mitchelmore. "The dispersant has inherent toxicity. And these oil droplets tend to be the same sort of size as food particles for filter-feeding organisms."
So far, dispersants have mainly been applied to surface slicks by spraying from planes. Could they be targeted to the source of the oil leak?
Underwater robots are now being used in the Gulf to apply dispersants directly to oil as it leaks from the sea floor. Such underwater use has never been attempted before at the depth of the current spill.
Eric Adams, an environmental engineer at the Massachusetts Institute of Technology and another author on the 2005 report, notes that, at the surface, wind and waves help dispersants to mix with and break up oil. Underwater, it is harder to ensure mixing.
"In some sense it's easier to apply it [dispersant] at the surface," says Adams, who has done theoretical work on deep-sea application systems for dispersants. But the advantage of underwater application, which can reach oil at its source, makes the experiment worth trying, he thinks.
Some have criticized test runs of deep-sea dispersant use at the Deepwater Horizon site by the U.S. Environmental Protection Agency (EPA), on the grounds that research so far has focused on the impact of dispersants in the upper parts of the ocean.
But the attempt does appear to be working. "Aerial observations indicated that the slick on the surface was significantly reduced once you inject the chemical right at the well head," Horsup says. "You get very rapid dispersion of the oil into the water column."
The EPA admits that the impact of underwater use on the environment is "still widely unknown." However, after the initial test runs, the go-ahead was given this week for further underwater applications.
What does the future hold?
In the short term, chemical companies worldwide are scrambling to keep up with the massive demand for dispersants resulting from the accident. Horsup says that suppliers are currently keeping up with demand, but that if demand increases markedly stockpiles may become depleted.
In the longer term, the Deepwater Horizon accident response has begun what is, to all intents and purposes, a giant experiment on dispersant use.
The 2005 National Academies report raised numerous questions about dispersant toxicity and the final fate of dispersed oil that need answers. Both Adams and Mitchelmore say not much progress has yet been made on those questions. The events in the Gulf of Mexico may push this research forwards
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6 Comments
Add CommentSurfactants just disperse mineral oils and make them less visible to our species.
Reply | Report Abuse | Link to thisDispersing oil increases its bio-availability. It will enter the respiratory system and digestive tracts of all the fauna that use this water, it will also effect the flora and the bacterial life in the contaminated waters. The author is hoping for certain bacterial life forms to consume the oil, but most bacterial life forms do not. The dispersed oil will give the life forms that consume oil a Darwinian advantage over the rest, their numbers will increase and can you guess what will be the outcome?
Dispersing oil is a public relations exercise.
"Dispersants are themselves unpleasant chemical concoctions, although today's products are less damaging than the toxic solvents used in earlier spills such as the Torrey Canyon disaster"
Reply | Report Abuse | Link to thisSo today's disperpersants are less toxic than dispersants used in the past. But how toxic are they, and if their composition is secret, how can we know?
We don't
Reply | Report Abuse | Link to thisew Don't
Reply | Report Abuse | Link to thisExcuse my scientific ignorance on this issue but why would it not be possible to do the opposite and conglomerate the oil to be pumped back into tankers?
Reply | Report Abuse | Link to thisSinking Agents
Reply | Report Abuse | Link to thisThe National Oil and Hazardous Substances Pollution Contingency Plan, more commonly called the National Contingency Plan or NCP, is the federal governments blueprint for responding to both oil spills and hazardous substance releases. The National Contingency Plan is the result of our countrys efforts to develop a national response capability and promote overall coordination among the hierarchy of responders and contingency plans.
Pursuant to NCP Section 300.310, As appropriate, actions shall be taken to recover the oil or mitigate its effects. Of the numerous chemical or physical methods that may be used, the chosen methods shall be the most consistent with protecting public health and welfare and the environment. Sinking agents shall not be used.
Sinking agents means those additives applied to oil discharges to sink floating pollutants below the water surface.
The question is whether BPs dispersants are sinking agents when they are applied a mile underwater at the source of the well leak.
For a clear understanding of the issues involved, visit:
http://renergie.wordpress.com/2010/05/25/bp-is-not-the-only-responsible-party/
and
http://donovanlawgroup.wordpress.com/2010/06/05/the-oil-pollution-act-provides-for-the-federalization-of-the-bp-oil-spill/
and
http://donovanlawgroup.wordpress.com/2010/06/14/why-bp-does-not-want-an-accurate-measurement-of-the-gulf-oil-spill/