Ryegrass-Accelerating Degradation of Polycyclic Aromatic Hydrocarbons(PAHs) in Soils

The prospect of using vegetation to enhance the degradation of organic contaminants such as PAHs in soil systems is an attractive cost-effective alternative to traditional engineering approaches. The principle contributions of plants to phytoremediation are either stimulation of soil microbial activity and degradation of contaminants, or plant direct uptake and accumulation of the PAHs. However, information is scant on plant uptake of these chemicals from soils. As a consequence, there are difficulties to evaluate the plant contribution to phytoremediation process. Thus, the mechanisms of phytoremediation for soil PAH pollutants were elucidated in this study based on investigation of plant uptake of these chemicals in soils. Phytoremediation for soil phenanthrene and pyrene with ryegrass (Lolium multiflorum Lam) was investigated using a greenhouse study. The initial concentrations of phenanthrene and pyrene in soils were in a range of 0~456.5 and 0~488.7 mg·kg-1, respectively. It has been found that ryegrass significantly promoted the degradation of these PAHs in the soils. At the end of the experiment after 45 days, about 85.80%~90.79% and 44.32%~89.21% of added respective phenanthrene and pyrene were disappeared from soils with ryegrass, which were higher than those in unplanted control soils. While the residual concentrations of phenanthrene and pyrene in the soils were in a range of 53.6% and 78.3%, lower than those in unplanted soils. Although uptake and accumulation of these PAHs by ryegrass were obvious, and the root or shoot concentrations monotonically increased with the increase of the soil PAH concentrations, plant off-take only accounted for less than 0.54% of dissipation enhancement for phenanthrene and pyrene in planted versus unplanted control soils, whereas plant-promoting biodegradation was the predominant contribution of remediation enhancement of these soil PAHs in the presence of ryegrass.