Although major oil spills at sea might grab the headlines, up to 98 percent of all oil spills actually occur on land – with around 25,000 cases being reported every year. Often, these spills have long term consequences for the ground on which they occur, including contaminating groundwater and reducing the fertility of soil.
Traditionally, removing oil contaminants from soil has been a difficult task, with most methods cleaning the soil, but also damaging its long term health. Now, researchers at Rice University in Houston, Texas have developed a new method which rectifies these issues by not only cleaning the soil, but returning it to near its original fertility.
The method relies on the process of pyrolysis, which involves heating the soil to burn off the oil’s petroleum hydrocarbons. However, usually, the heating process also dehydrates the water held in the clays of soil, effectively rendering the soil ‘dead’. Rice University engineers were able to fine-tune the process and more importantly locate the sweet spot at which the hydrocarbons are removed, but the soil is not irreparably damaged.
Finding The Oil Cleaning Sweet Spot
Their method, as outlined in the American Chemical Society journal Environmental Science and Technology, heats the soil in a rotating drum in an oxygen-less environment. This allows them to avoid the temperature spikes which often occur when burning off petroleum hydrocarbons, and better regulate the temperature. Once this system had been established, they conducted various experiments with contaminated soil in a kiln, to best narrow down the ideal temperature for pyrolysis.
They discovered that heating the soil to 420 C for 15 minutes removed 99.9 percent of total petroleum hydrocarbons (TPH) and 94.5 percent of polycyclic aromatic hydrocarbons (PAH). As a result, the soil was roughly of the same fertility as ordinary, non-contaminated soil. Researcher Kyriacos Zygourakis explained the process:
“Between 200 and 300 C (392-572 F), the light volatile compounds evaporate. When you get to 350 to 400 C (662-752 F), you start breaking first the heteroatom bonds, and then carbon-carbon and carbon-hydrogen bonds triggering a sequence of radical reactions that convert heavier hydrocarbons to stable, low-reactivity char.”
Heating the soil to 470 C did a slightly better job of removing the pollutants, but also began permanently diminishing the quality of the soil. By 500 C the soil is essentially useless.
To test their newly “detoxified” soil, the team attempted to grow Simpson black-seeded lettuce – a variety of the plant which is highly vulnerable to oil contamination – in their lab. Although the lettuce was slower to initially take hold, after 21 days they had arrived at a healthy plant with the same germination rates and weight as lettuce grown in non-contaminated oil. Alvarez concluded:
”One important lesson we learned is that different treatment objectives for regulatory compliance, detoxification and soil-fertility restoration need not be mutually exclusive and can be simultaneously achieved.”
Of course, how this method would be practically used in a large scale ground contamination scenario remains to be seen. Additionally, it might be cheaper and more effective to counter the causes of ground contamination – such as increasing the regulation and monitoring of major oil companies – rather than to deal with the symptoms of their activities. Indeed, it is worth pointing out that the Chevron Corporation – one of the world’s largest oil companies with a fair share of contamination controversies to its name – was a major funder of the Rice University project.