Phyto Availability And Rhizospheric Gradient Distribution Of Bound-PAH Residues In Soils

Soil organic pollution has became increasingly serious in recent years. Binding organic pollution into soil matrices to form bound residues is considered as an important soil detoxification process, and traditionally thought that bound residues is the final endpoint for risk assessment and safe management of organic contaminants in the soil environment. However, there is little information available on the distribution of their bound residues in the rhizosphere. Understanding the phytoavailability and rhizospheric gradient distribution of bound-PAH residues are of crucial importance for risk assessment of PAH-contaminated areas.In this work, the progress of the bound-PAH residues and rhizosphere remediation organic pollutants were clarified. The temporal and gradient distribution of bound-PAH residues (phenanthrene and pyrene) in rhizosphere soils were studied. Accumulation bound-PAH residues in plant in contaminanted field were evaluated. The main findings are shown as follows:(1) The temporal and gradient distribution of bound-PAH residues in the rhizosphere soil of ryegrass (Lolium perenne L.) was investigated using a greenhouse pot experiment. Rhizosphere soil from the root surface (0-9 mm) was divided into three fractions:the rhizoplane, and strongly and loosely adhering soil. The concentrations of bound-PAH residues increased with distance away from the root surface after plants were cultivated for 15 to 75 d. In general, the concentrations of bound-PAH residues decreased over time in all three components of rhizosphere soils; 23.57 to 39.50% and -13.79% to 36.21% of the bound residues of phenanthrene and pyrene were dissipated in the three rhizosphere soil fractions within 15 to 75 d, respectively. Dissipation ratios decreased in rhizosphere soils with distance away from the root surface. The bound-PAH residues were more conducive to degrade in rhizosphere soil than non-rhizosphere soil (unplanted soil). Extractable PAHs are the primary fractions in both rhizosphere and non-rhizosphere. In addition, different forms of PAHs can be transformed into each other.(2) The gradient distribution of bound-PAH residues (reference to parent compounds) in rhizospheric soils of Moleplant (Euphorbia lathyris L.) and wood sorrel (Oxalis corniculata L.) grown in soil contaminated by PAHs were investigated. Results show that bound residues of the ten EPA-priority PAHs were detected in both the rhizospheric and non-rhizospheric soils, about 3.31 mg·kg-1 in total concentration in the latter, much higher than in the former (1.07-1.82 mg·kg1). The concentration of bound-PAH residues increased with increasing distance (0-9 mm) from the root surface. It is feasible to use rhizosphere effect (R, in percent) to measure the proportion of the decrement of bound residues in the rhizosphere as against that in the non-rhizosphere soil, R values decreased with increasing distance from the root. R of the total bound-PAH residues in three continuous rhizosphere layers were 45.15%～67.66%. R of two-ringed PAHs was the highest (61.18%～93.50%), while R of four- and five-ringed PAHs the lowest (2.39%～6.31%), which indicates that the bound residues of PAHs with fewer rings are more liable to transformation in the rhizosphere. R of the bound residues in the rhizosphere of moleplants was found to be relatively higher than that of wood sorrels suggests that the rhizosphere of moleplants was more favorable to transform bound-PAH residues.(3) The soil containing the bound residues of 11 EPA-priority PAHs were used to grow ryegrass(Lolium perenne L.). After 50 days growth, ryegrass could accumulate all 11 kinds of bound-PAH residues. Different organs of ryegrass uptaked different levels of bound-PAH residues. Under the same conditions, the enrichment capacity of shoot was lower than root. Bound-PAH residues could transform into extractable fraction due plausibly to the contribution of plants.