Bioavailability of mixtures of PAHs partitioned into the micellar phase of a nonionic surfactant

Recent work has shown that a fraction of a contaminant solubilized in the micellar phase of some nonionic surfactants is directly available for biodegradation, meaning that the contaminant can be transferred directly from the core of the micelle to cell without having to transfer to the water phase first. This study extends the understanding of the bioavailability of the micellar phase for a single compound to a multicomponent system of contaminants. Biodegradation experiments were conducted with binary and ternary mixtures of naphthalene, phenanthrene, and pyrene in the presence of a nonionic surfactant, Triton X-100. A mixed bacterial culture, isolated and enriched from a PAH-contaminated soil at the Wurstsmith Air Force Base, MI, was used for the biodegradation experiments. In the absence of the surfactant and at surfactant concentrations below cmc, the multisubstrate Monod kinetics adequately simulated the biodegradation kinetics of the binary and ternary mixtures. In the multicomponent systems, as in single solute systems, the solutes in the micelle were found to be directly bioavailable, and the bioavailability of each compound in the micellar phase decreased with increasing surfactant concentration. For a given surfactant concentration, the bioavailability was higher for the lower molecular weight PAHs. There was little difference in the bioavailability of the same compound as a single solute or in different binary and ternary mixtures. To predict the bioavailability of the micellar phase substrates, a mass transfer-based model was formulated that describes the transfer of substrate from the micellar phase to the microorganisms. The predictions matched the experimental observations well, indicating the validity of the model and its potential for applications in remediation designs.