Oil-Contaminated Soil Finds New Fertility

Oil spills are unfortunately a fact of life in our industrialized world. Until we find a renewable resource that can fulfill our energy demands, the transportation of oil and other petroleum products is a requisite and no method is entirely foolproof. Scientists at Rice University have addressed this problem by developing a new technique that not only rids contaminated soil of oil, but restores its fertility. Salad anyone?

Though marine oil spills tend to garner the most attention with their heart-breaking images of oil-coated sea birds and mammals, the vast majority (98%) of spills occur on land with thousands of acres of contaminated soil a result. Oil spilled on land coats the soil such that water can no longer penetrate, rendering the land useless for agriculture. Furthermore, land-based spills must be contained rapidly to avoid contaminant leakage into groundwater sources. The Rice University team, led by professor of Environmental Engineering Dr. Pedro J. J. Alvarez, set out to find a method to increase the response speed to land-based oil spills and the ensuing enhanced soil fertility was just a happy side effect.

Oiled bird: this bird, photographed after the Black Sea oil spill, shows the consequences of these environmental disasters.Oiled bird: this bird, photographed after the Black Sea oil spill, shows the consequences of these environmental disasters.

Their strategy was to employ a technique called pyrolysis in which the contaminated soil is heated to high temperatures in the absence of oxygen. Petroleum products are made up of hydrocarbons – chains of carbon and hydrogen atoms of varying lengths. Upon heating, the shorter chains are vaporized leading to an initial decontamination step which, after just three hours, reduces the amount of hydrocarbons in the soil to well below regulatory standards. At higher temperatures, above 350 °C, the remaining longer chain hydrocarbons break down to form solid char, an environmentally benign carbonaceous contaminant that actually enhances soil fertility. Though formed by heating oil, the char produced is not classified as hazardous waste by the Environmental Protection Agency. The char-enhanced soil was evaluated using lettuce as a test plant. Lettuce is known to be particularly susceptible to environmental toxins making it an ideal agricultural guinea pig. The result was healthy and robust plants from soil that was only recently too filled with oily muck to sustain life. A remarkable achievement and one with incredible relevance in our oil-hungry world!

The process: through pyrolysis, contaminated soil is fit to grow even delicate plants like lettuce. Image from Julia Vidonish/Rice University.The process: through pyrolysis, contaminated soil is fit to grow even delicate plants like lettuce. Image from Julia Vidonish/Rice University.

Intense heating has been used in the past as a means of rapid response to oil spills, but typically even higher heats are employed and oxygen is introduced to incinerate ALL the hydrocarbons, including those that lead to char. This method is much more energy intensive thanks to the higher temperatures required and leaves the soil completely depleted in regards to future plant growth. Conveniently, however, the new technique requires the same equipment and expertise as the older more energy-demanding strategy making it easy to deploy in the field.

While oil spills are absolutely devastating environmentally, we will have to accept them as a necessary evil so long as we continue to rely on non-renewable energy to power our increasing demand for vehicular transportation and electronic gadgetry. No oil transportation method is fool proof – pipes leak or rupture, trains and trucks crash, and boats run aground. Even with myriad precautions, uncontrollable natural events like earthquakes throw a wrench in the best-laid preventative plans. For this reason, research like that reported here is invaluable. We may not be able to avoid an awful situation, but we now have the wherewithal to turn it into something positive.

Via Science Daily and Environmental Science and Technology.