Uptake of hydrophobic organic compounds into organisms is often limited by the diffusive transport through a thin boundary layer. Therefore, a microscale diffusion technique was applied to determine the diffusive mass transfer of 12 polycyclic aromatic hydrocarbons through water, air, surfactant solutions, humic acid solutions, aqueous soil and horse manure extracts, digestive fluid of a deposit-feeding worm, and root exudates from willow plants. In most cases the diffusive mass transfer of PAHs was much higher through the tested media than through water, and the enhancement factors increased with increasing hydrophobicity of the PAHs. The diffusive flux of benzo[a]pyrene was for instance enhanced 74 times through gut fluid of a deposit-feeding worm when compared to water. These findings demonstrate that a wide variety of dissolved organic carbon (DOC) at environmental levels can enhance diffusive mass transfer in various transport scenarios. The diffusive uptake of PAHs into sediment dwelling organisms is particularly efficient within the gut and at direct contract with the sediment matrix. Bioremediation might be enhanced bythe addition of auxiliary agents that enhance diffusive mass transfer. Enhanced diffusion needs also to be considered in dynamic transport models and for the operation and calibration of passive sampling techniques.

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Biological in situ methods are options for the remediation of contaminated sites. An approach to quantify biodegradation by soil bacteria was developed, combining experiment with mathematical modelling. We performed in vitro assays to investigate the potential and kinetics of the wild-type degrader, Burkholderia sp. strain LB400 (expressing bph) and the genetically modified Pseudomonas fluorescens strains F113pcb and F113L::1180 (expressing bph under different promoters) to metabolise individual congeners of polychlorinated biphenyls (PCBs). Kinetics of metabolism was analysed using the Monod model.Results revealed similar patterns of degradable PCB congeners for LB400 and F113L::1180. The degree of PCB degradation was comparable for LB400 and F113L::1180, but was much lower for F113rifpcb. In additional mesocosm experiments with PCB contaminated soil, the F113 derivatives demonstrated a good survival ability in willow (Salix sp.) rhizosphere.Strain F113L::1180 in combination with willow plants is expected to degrade a large spectrum of PCB congeners in soil. The data from the experiments were used to calculate the time scale of the degradation process in a PCB-contaminated soil. The uncertainty of the model predictions due to the uncertainties of experimental removal velocities and bacterial cell density in soil was quantified.