Relationship between Micellar and Hemi-Micellar Processes and the Bioavailability of Surfactant-Solubilized Hydrophobic Organic Compounds

In a landmark study on surfactant-enhanced biodegradation of hydrophobic organic compounds (HOCs), Guha and Jaffé demonstrated that a fraction (f) of micellar-phase HOC is directly bioavailable to bacterial cells. They developed a theoretical description of f which provided an excellent model of the experimental results. However, a mass transfer term describing the transport of the HOC through the cell wall (m_c) was found to vary over an order of magnitude for the different surfactants examined, and a theoretical description of it remained elusive. This elusivity also resulted in the model not being able to describe a priori why f was zero for the non-ionic surfactant C₁₂E₂₃. Here, the results of a recent study on surfactant sorption are used to develop an alternative mechanism describing m_c, where hemi-micellar formation on the cell surface is incorporated into the pathway describing micellar HOC bioavailability. The revised model is validated against HOC bioavailability data for five different C₁₂E_y surfactants, and it is shown that a single value for the mass transfer coefficient describing transfer of the hemi-micellar HOC into the bacterial cell is able to replicate the complete C₁₂E_y dataset, including that of C₁₂E₂₃ which eluded the original model. Overall, the results indicate that surfactant sorption and hemi-micelle formation are important parameters governing surfactant-enhanced bioavailability.