Novel approach to measurement of rhizosphere effect in phytoremediation of petroleum contaminated soils

Rhizodegradation uses plant root supported microorganisms to enhance degradation of immobile contaminants in soil. The rhizosphere is considered as an important region or zone that defines the extent of contaminant degradation. By utilizing the inherent luminescent property of the polycyclic aromatic hydrocarbons (PAHs) under UV light, a non-destructive and real time methodology was developed to quantify the development of the rhizosphere associated with a single root and the degradation kinetics of the contaminants in that region. The approach employed techniques including thin film physical vapor deposition of the contaminants, UV illuminated photography and digital image processing. The feasibility of the approach was demonstrated with Sudan grass, Bermuda grass, and soybean while pyrene and phenanthrene were used as representative immobile PAH contaminants. The approach achieved a spatial resolution of 0.695 mum which is finer than the resolution of 3--10 mum achieved with an FR-FTIR method. The measured rhizosphere thicknesses ranged from 0.14--0.37 mm for young soybean and Bermuda roots. The calculated first-order pyrene degradation rate constant in Bermuda grass rhizosphere ranged from 0.009--0.044 day -1, which is believed more representative of the true rhizosphere kinetics than the rate constant of 0.001 day-1 obtained with a conventional soil sampling and gas chromatographic analysis method. The results also showed that dimension of the rhizosphere and the contaminant degradation rates were specific to each type of plant and contaminant. With further study of rhizosphere dimension and contaminant degradation kinetics, a better mathematical model can be constructed in order to more confidently predict the degradation of immobile PAH contaminants in soil.