Phytoremediation of soil contaminated with used motor oil: II. Greenhouse studies

The decomposition of used motor oil in soil as influenced by plant treatment was monitored in a greenhouse study. Soil contaminated with used motor oil (1.5% w/w) was seeded with soybean (Glycine max)/green bean (Phaseolus vulgaris); sunflower (Helianthus annus)/Indian mustard (Brassica juncea); mixed grasses/maize (Zea mays); and mixed clover (red clover, Trifolium pratense/ladino clover, Trifolium repens) and incubated. Soxhlet-extractable oil and grease remaining in the soil was monitored after 100 and 150 days. After 150 days in the clover treatment, the added oil was no longer detected. A total of 67% of the oil was removed in sunflower/mustard, and with addition of NPK fertilizer, the oil was completely removed. The grass/maize treatment resulted in a 38% oil reduction, which increased to 67% with fertilizer application. The control treatment reduced oil in soil by 82% when fertilizer was added. At 150 days the sunflower/mustard and wheat/oats treatments produced the greatest biomass in the presence of used oil. Gas chromatography/mass spectroscopy (GC/MS) spectra of oil/grease extracts revealed the presence of new peaks associated with hydrocarbon decomposition. The presence of new hydrocarbons was corroborated by changes in Fourier-transformed infrared spectrometry (FTIR) spectra. Fertilizer additions to treatments resulted in negligible changes to FTIR bands. Based on oil/grease residues and biomass results, the clover and sunflower/mustard treatments are considered superior to the other plant treatments in terms of overall phytodegradation of used oil hydrocarbons.