Transport and Reaction Processes in Bioremediation of Organic Contaminants. 2. Role of Aggregate Size in Soil Remediation in a Slurry Bioreactor

Abstract

Transport processes in the soil matrix can control the rates of bioremediation of low-solubility contaminants. In this study, experiments were designed to test the hypothesis that diffusion of contaminants within soil aggregates of diameter 40 - 1000 micron was the limiting factor in bioremediation of creosote-contaminated soil. The concentrations of 6 different PAHs (acenaphthene, anthracene, chrysene, fluoranthene, phenanthrene, and pyrene) were monitored during the course of bioremediation of sonicated and non-sonicated soil by an active mixed culture in slurry bioreactors. Sonication of the soil to disperse soil aggregates 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 independence of the residual concentrations of PAHs on sonication treatment. The soil aggregates were modelled as porous materials, with pores filled with non-aqueous phase liquid and with films of non-aqueous phase liquid on particle surfaces. As the soil aggregates were dispersed, either by sonication or mixing, then the fraction of contaminants in exposed films increased. A diffusion-controlled mass transfer model was developed to represent release of PAHs from the soil, based on this physical model. The estimated diffusion coefficients of four of the PAHs (acenaphthene, phenanthrene, fluoranthene and pyrene ) in the residual creosote phase were in the range 4.4-4.8 x 10-14 cm2/s, while the diffusion coefficients for anthracene and chrysene were lower by a factor of 2. The similar values of diffusion coefficient between the components was consistent with release by diffusion through a viscous residual creosote. The magnitude of the diffusion coefficients was intermediate between the transport properties in high-viscosity oils, and polymers.