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 Jaffe 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 C12Ey 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 C12Ey 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.