Bioavailability of hydrophobic compounds partitioned into the micellar phase of nonionic surfactants

The apparent solubility of polycyclic aromatic hydrocarbon compounds such as phenanthrene can be increased in the presence of surfactants above their critical micelle concentration. A fraction of the phenanthrene partitioned into the micellar phase of some nonionic surfactants can be directly bioavailable to phenanthrene-degrading microorganisms. A model describing the biodegradation of the directly bioavailable micellar-phase substrate is presented. The hypothesis on which the model is based considers the following steps: (a)the contaminant is transported by filled micelles from the bulk solution to the proximity of the cells; (b) the exchange of the filled micelle with the hemimicellar layer around the cell delivers the contaminant to the cell; (c) the contaminant diffuses into the cell and is biodegraded. The biodegradation kinetics were explained in terms of a series of mass-transfer processes, which lead to a similar equation as the Monod kinetics. The theoretically derived expression, describing the micellar-phase substrate that is directly bioavailable, includes a series of surfactant dynamics and mass transfer rate parameters that are not readily available or easily determined. A simplified formulation, which can be used to estimate the direct bioavailability of the micellar-phase substrate was therefore obtained and was used to explain experimental observations. The bioavailable fraction of the micellar-phase substrate was independent of the biomass concentration and was a function of the surfactant concentration, the polyoxyethylene chain length of the surfactant, and the biomass surface characteristics.