Thermal Remediation: Enhancing Ecosystem Recovery for Soils Contaminated with Heavy Hydrocarbons

Terrestrial spills of crude oil and refined petroleum products have substantial environmental, economic, and public health consequences. Thermal treatment technologies hold an important niche in the remediation of hydrocarbon-contaminated soils and sediments due to their ability to quickly and reliably meet cleanup standards. However, sustained high temperature can be energy intensive and can damage soil properties. Pyrolysis of hydrocarbon-contaminated soils offers the potential for rapid remediation with fewer effects on soil fertility than existing thermal technologies. Pyrolysis of contaminated soils at 420ºC converted recalcitrant heavy hydrocarbons into “char” (a carbonaceous material like petroleum coke) and enhanced soil fertility. Pyrolytic treatment reduced total petroleum hydrocarbons (TPH) to below regulatory standards (typically < 1% by weight) within 3 h using only 40-60% of the energy required for incineration at 600-1200ºC. Formation of polycyclic aromatic hydrocarbons (PAHs) was not observed, with post-pyrolysis levels well below applicable standards. Plant growth studies showed higher biomass production of Arabidopsis thaliana and Lactuca sativa (Simpson black-seeded lettuce) (80-900% heavier) in pyrolyzed soils than in contaminated or incinerated soils. Elemental analysis showed that pyrolyzed soils contained more carbon than incinerated soils (1.4-3.2% versus 0.3-0.4%). Using thermogravimetry and evolved gas analysis, we identified the two stages of pyrolytic remediation. Desorption of light hydrocarbons is the dominant process for temperatures between 150 and 350oC. Pyrolysis reactions dominate in the 400-500oC range releasing gaseous products (hydrogen, methane, higher alkanes and olefins) and forming a solid char. XPS analysis and partial combustion revealed that the char forms a layer coating the particles of treated soils. Since pyrolysis can effectively reduce the TPH of contaminated soils at temperatures below 500oC, it avoids carbonate decomposition reactions that may lead to large soil pH increases and severe loss of fertility. This is a significant potential advantage over competing thermal processes that expose contaminated soil to temperatures above 500oC. Overall, the convergence of treatment process engineering with soil science, ecosystem ecology, and plant biology research is essential to fill critical knowledge gaps and improve both the removal efficiency and sustainability of thermal technologies.