Concentrations of petroleum hydrocarbons at different depths of soil following phytoremediation

Petroleum hydrocarbons are among the major organic environmental pollutants whose toxicity and carcinogenicity effects have raised great concerns while the numerous available physical and chemical methods to remove petroleum hydrocarbons are rarely employed due to their extreme costs and harmful side effects, biological methods, including phytoremediation, have attracted wide attention during the recent years. The current study used sorghum (Sorghum vulgare) and barley (Hordeum vulgare) to decrease petroleum hydrocarbon content of contaminated soil around Isfahan Oil Refinery (Isfahan, Iran). It also assessed the concentrations of petroleum hydrocarbons and petroleum-degrading bacteria at different depths of soil following phytoremediation. Polyvinyl chloride (PVC) pipes (20 cm in diameter and 130 cm long) were employed for phytoremediation. In each pot, hordeum or sorghum seeds were separately sown approximately 1-2 cm below the soil surface. Thirteen weeks after sowing of the plants, soil columns were sampled at 25, 50, 75, and 100 cm depths and concentration of petroleum hydrocarbons and number of oil-degrading bacteria was determined. Statistical analyses indicated the two plants to reduce the concentration of petroleum hydrocarbons to a significantly higher extent (23%-35%) than the control treatment Obviously, such an effect was only detected in depths where plant roots penetrated (especially 0-50 cm), i.e. the absence of roots in deeper parts of the soil column (50-100 cm) was accompanied by lower number of oil-degrading bacteria and higher concentration of petroleum hydrocarbons. Accordingly, future studies are recommended to investigate the efficacy of landfarming and stimulation and injection of oil-degrading microorganisms in eliminating deep soil contamination.