The transport processes in soil bioremediation

The availability of contaminants is a major issue in soil bioremediation. Numerous studies have shown that even in the presence of active culture, bioremediation does not achieve the desired level of clean up. This study considers the physical structure of oil and clay as the key factor in bioremediation, and reviews the transport processes that limit the bioavailability of polynuclear aromatic hydrocarbons (PAHs). This research focuses on diffusion of contaminants in non-aqueous phase liquid (NAPL) and through porous structure of soil aggregates. Preliminary experiments on pristine soil, spiked with anthracene, suggested that reducing the size of the soil aggregates enhanced the rate of degradation. This hypothesis was tested on a creosote-contaminated soil. The aggregate size was reduced by sonication, and degradation of 6 target PAHs was monitored during the course of bioremediation. Sonication increased the rate of microbial degradation up to 5 fold, however there was no significant difference in the final residual concentrations between the two soil treatments. The aggregate size distribution after three weeks of treatment in a slurry bioreactor was comparable in both the sonicated and non-sonicated soils which was consistent with the equivalent residual concentrations of PAHs.; Two microscopy methods, confocal scanning laser microscopy (CSLM) and cryogenic scanning electron microscopy (cryo-SEM), were employed to investigate the microstructure of contaminated soils and to examine the distribution of non-aqueous phase liquid (NAPL) in clay aggregates. Based on the information obtained, a physical model was proposed for the distribution of NAPL in soil microstructure. This model suggested that the distribution of NAPL change from liquid-filled pores for large aggregates, to a surface film for small aggregates.; Based on the physical model of the distribution of NAPL in soil aggregates and the data obtained in bioremediation experiments, a mathematical model was developed. This model suggested a diffusion-controlled mass transfer mechanism in soil aggregates. The effective diffusivity of PAHs in NAPL was estimated to be in the range of 0.5 x 10-14--5 x 10-14 cm2/s. A model based on competitive inhibition of the substrates was developed and partially accounted for the initial lag phase in degradation of higher ring PAHs.