Removing the pollutants
UAF research focuses on the use of microbes to eradicate spilled hydrocarbons
To find novel ways of dealing with spills of hydrocarbon fluids such as crude oil and diesel fuel, a professor in the University of Alaska Fairbanks has been leading a research project, investigating the harnessing of naturally occurring, oil devouring microbes.
Mary Beth Leigh, associate professor of microbiology in the Institute of Arctic Biology, is particularly focusing on how this spill response technique, referred to as bioremediation, may be applicable in the Arctic. Her research team is looking at bioremediation both for oil spills on land and for offshore spills in Arctic seas, Leigh told Petroleum News in a March 7 interview.
Using plant rootsLeigh said that she has been working on one form of bioremediation, the use of plant roots to encourage microbes to devour oil, since she was a graduate student. Currently her team is running an experiment to use the technique, known as phytoremediation, to clean up diesel fuel spilled from a tank at the village school in Kaltag, on the Yukon River in western Alaska. Diesel spills of this type are common in rural Alaska because of aging tanks, Leigh said.
The workings of phytoremediation depend on the fact that plants tend to secrete chemical compounds for deterring the attentions of herbivores, but which have similar chemical structures to those of petroleum hydrocarbons. These compounds tend to leak from the plant roots into the soil, where they attract naturally occurring microbes that feed on the compounds. Thus, by growing plants in soil contaminated by petroleum products, the roots stimulate the microbial community. Then the microscopic organisms will chomp on, not just the plant-based chemicals, but also the polluting petroleum materials.
In Alaska, in particular, one of the microbe attracting compounds, salicylic acid, is particularly prevalent in willows and birch, plants that are commonly seen in the state. Scientists know that this chemical especially switches on genes in bacteria for aromatic hydrocarbon degradation, Leigh said.
Study in FairbanksAbout 20 years ago the U.S. Army Corps of Engineers initiated a phytoremediation study in Fairbanks, growing plants on a crude oil contaminated gravel pad and on diesel contaminated soil. The Corps found higher rates of petroleum degradation in the planted areas than in unplanted sections but abandoned the experiment after two or three years when funding of the project ended. Leigh’s team went to the site 15 years later and found that, by then, the petroleum contamination had dropped to below regulatory levels.
The university researchers found that shrubs and trees had been more effective than grass in hydrocarbon removal, although it appears that the cultivation of grass is a useful first step, to have a quick, short term impact on the pollution and to stabilize the soil.
Kaltag experimentSo, in their experiment at Kaltag, the researchers are testing different plant combinations: willows alone, grasses alone, and a combination of willows and grasses. The researchers are also testing the use of fertilizer, to determine if that impacts the effectiveness of the remediation technique.
At this stage the plants are faring well, but it will probably be another year or two before the impact on the contamination can be determined, Leigh said.
The established means of dealing with this type of soil contamination is to remove the soil to a suitable site and then repeatedly till it, to stimulate the petroleum consuming microbes. But this technique is expensive, given the need for a local person with heavy machinery to conduct the repeated tilling operations. And, in the event of high rainfall, the tilling site can turn into an unproductive mud pit.
The phytoremediation technique, while relatively slow, would be cheaper, could perhaps be conducted in situ at the spill site and stabilizes the soil. And the willow, a suitable plant for the treatment, is easy to cultivate and is ubiquitous in Interior Alaska, Leigh said.
Arctic offshore spillsLeigh’s team has also been investigating the bioremediation of oil spilled offshore Arctic Alaska. There is heightened interest in this topic because of offshore oil exploration and increased Arctic shipping, she commented. Her team has collected Arctic seawater samples and conducted experiments using North Slope crude oil, incubating the oil with the water in bottles to assess the impact of oil consuming microbes that are known to exist in the Arctic seas. The tests have been conducted using water at a temperature of around minus 1 to minus 2 C, Leigh explained.
Perhaps surprisingly, the researchers found that at these low temperatures the biodegradation of the oil was almost as rapid as has been observed at the temperatures of more temperate regions. However, in those temperate regions oil does tend to disappear more quickly from the ocean because of higher rates of evaporation, Leigh commented.
Dispersant effectivenessIn a collaborative effort with analytical chemists at Oregon State University who developed methods for analyzing the Corexit 9500 oil dispersant used in the Deepwater Horizon response in the Gulf of Mexico, the University of Alaska Fairbanks team has been investigating the fate and effectiveness of Corexit in Arctic conditions. Through incubations in bottles at Arctic temperatures the researchers have found that the Corexit itself undergoes biodegradation, as it does in warmer climes.
Dispersants work by breaking up the spilled oil into tiny droplets which spread through the water column, where they are consumed by microbes more rapidly than in a surface slick. Despite questions that have been raised over whether the dispersants may also tend to slow the biodegradation, the Fairbanks research team has found that simulations of dispersed oil in Arctic conditions demonstrate that, if anything, the dispersant speeds up the biodegradation process, Leigh said.
Leigh also said that one of her students had conducted some research in conjunction with a joint industry project and had found that the toxicity of Corexit 9500 is fairly low, comparable to the toxicity of domestic dish soap. However, a major toxicity concern with oil dispersal is the toxicity of the oil itself, as the oil droplets permeate the seawater. The use of dispersants thus becomes a question of what presents the bigger risk: dispersing the oil for a faster rate of bioremediation, or leaving the oil as a surface slick.
The University of Alaska Fairbanks team now has funding from the Oil Spill Recovery Institute and the federal Bureau of Ocean Energy Management to investigate the bioremediation of spilled marine diesel in seawater, using a variety of commercially available dispersants. The team is about to launch an experiment using Prince William Sound seawater and also anticipates using Arctic water.
Remediating sulfolaneAnother intriguing potential use of bioremediation is in the treatment of sulfolane, a water-soluble solvent sometimes used in oil refining. Sulfolane has achieved a certain level of notoriety in Alaska because of major groundwater sulfolane contamination around a now closed refinery at North Pole, near Fairbanks.
There are naturally occurring microbes that will rapidly biodegrade sulfolane, but the biodegradation process requires oxygen, Leigh said. Unfortunately at North Pole the oxygen levels in the subsurface are very low and, with the contamination having now spread wide and deep, bioremediation is not an option. However, sulfolane bioremediation could prove effective in a situation where the pollution is caught early. And it may be possible to use the bioremediation technique to treat sulfolane contaminated water recovered from construction sites, Leigh suggested. The water would need to be aerated, and lab tests suggest that nutrients may be needed to speed up the process, she said.