BIOAVAILABILITY OF HYDROPHOBIC COMPOUNDS PARTITIONED INTO THE MICELLAR PHASE OF NONIONIC SURFACTANTS

The apparent solubility of polycyclic aromatic hydrocarbon compounds s uch as phenanthrene can be increased in the presence of surfactants ab ove their critical micelle concentration. A fraction of the phenanthre ne partitioned into the micellar phase of some nonionic surfactants ca n be directly bioavailable to phenanthrene-degrading microorganisms. A model describing the biodegradation of the directly bioavailable mice llar-phase substrate is presented. The hypothesis on which the model i s based considers the following steps: (a)the contaminant is transport ed by filled micelles from the bulk solution to the proximity of the c ells; (b) the exchange of the filled micelle with the hemimicellar lay er around the cell delivers the contaminant to the cell; (c) the conta minant diffuses into the cell and is biodegraded. The biodegradation k inetics were explained in terms of a series of mass-transfer processes , which lead to a similar equation as the Monod kinetics. The theoreti cally derived expression, describing the micellar-phase substrate that is directly bioavailable, includes a series of surfactant dynamics an d mass transfer rate parameters that are not readily available or easi ly determined. A simplified formulation, which can be used to estimate the direct bioavailability of the micellar-phase substrate was theref ore obtained and was used to explain experimental observations. The bi oavailable fraction of the micellar-phase substrate was independent of the biomass concentration and was a function of the surfactant concen tration, the polyoxyethylene chain length of the surfactant, and the b iomass surface characteristics.