Phytoremediation of hydrocarbon-contaminated soil

Removal of organic contaminants aged in soil is often limited by their availability for microbial degradation. Phytoremediation, the use of plants to clean up contaminated soil, has been suggested as a remediation strategy since soil systems with plants often show greater remediation than implanted systems. This research was undertaken to determine the effects of plants, fertilization, and soil saturation on removal of hydrocarbons from an aged refinery soil and to gauge oxygen release from roots as a mechanism by which plants may enhance microbial degradation of contaminants in soil. One part involved following the removal of polycyclic aromatic hydrocarbons (PAHs) and total petroleum hydrocarbons (TPH) over time in soil in the field with planted (fertilized wetland grass mix, switchgrass, and New England aster) and implanted (fertilized and unfertilized) treatments and in the greenhouse with planted (fertilized switchgrass: and aster) and implanted treatments under saturated and unsaturated soil conditions. Plant and soil samples were harvested over time and analyzed for levels of hydrocarbons by gas chromatography/mass spectrometry. The most significant effect on hydrocarbon removal was that of time; however, there were significant effects of soil saturation and fertilization over time. There was a significant interaction between fertilization and presence of switchgrass on concentrations of several PAHs at the last harvest, suggesting that plants may compete with microbes for nutrients in soil, but are also enhancing microbial degradation when sufficient nutrients are present. There was a significant interaction between saturation and presence of switchgrass on concentrations of several PAHs at the last harvest, which suggests that plant roots provide oxygen to saturated soil and enhance aerobic degradation. The second part measured oxygen loss from the roots of switchgrass and aster plants. Oxygen loss was measured by oxidation of leucomethylene blue in a colorimetric assay. Switchgrass plants grown under saturated conditions released more oxygen than those grown under unsaturated conditions and more oxygen than aster plants (both saturation regimes). Based on the greater biomass and greater oxygen release of switchgrass grown under saturated conditions than that of aster, switchgrass has the greatest potential to deliver oxygen to a saturated contaminated soil.