No abstract

Surfactants above their critical micelle concentration can solubilize hydrophobic contaminants into their micelles. This process enhances the apparent solubility of contaminants such as hydrocarbons and, therefore, also their desorption from soils. Conceivably, in the absence of any inhibitory effects, such surfactants may enhance the biodegradation of the hydrocarbon. Through a set of screening experiments, a series of nonionic surfactants were identified that do not inhibit the biodegradation of phenanthrene. A mathematical model was formulated to describe the interaction of the biomass−contaminant−water−surfactant system. Assumptions that the model formulation is based on are that the phenanthrene in solution, partitioned into the micellar phase and sorbed onto the biomass and other solid surfaces,is at equilibrium and that these equilibria can be described by simple partition coefficients. It was also assumed that the presence of the surfactant does not affect the biochemical characteristics of the biomass. An effective bioavailable micellar-phase concentration of phenanthrene was defined. The model simulates experimental data well, indicating that a fraction of the micellar-phase phenanthrene is directly bioavailable. For three of the surfactants tested (Triton N101, Triton X100, and Brij 30), the micellar-phase bioavailable fraction of phenanthrene decreased with an increasing surfactant concentration. For Brij 35, it was found that the fraction of the phenanthrene associated with the micellar phase was not directly bioavailable.

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 micellarphase 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.

Enhanced solubilization of naphthalene, phenanthrene, and pyrene in micellar solutions of Triton X-100 was studied for single compounds and their binary and ternary mixtures. Experimental results were obtained for the following conditions:  (i) naphthalene, phenanthrene, and pyrene as single compounds at saturation; (ii) naphthalene, phenanthrene, and pyrene as binary and ternary mixtures at satura tion; (iii) phenanthrene at saturation in a range of naphthalene concentrations; (iv) pyrene at saturation in a range of naphthalene concentrations. The solubility enhancement of naphthalene was slightly reduced in the presence of phenanthrene and/or pyrene. A synergistic effect on the solubilization of phenanthrene was observed in the presence of different amounts of naphthalene. The solubility of phenanthrene was greatly enhanced in both binary mixtures and in the ternary mixture. The solubility of pyrene was slightly reduced in the presence of naphthalene, remained unaffected in the presence of phenanthrene, and increased significantly in the ternary mixture. The increase in the partition coefficient values is explained by the partitioning of the solubilzate at the micellar core−water interface, which changes the interfacial free energy and effectively increases the volume of the core leading to a higher solubilization potential.

Biodegradation kinetics of naphthalene, phenanthrene and pyrene were studied in sole‐substrate systems, and in binary and ternary mixtures to examine substrate interactions. The experiments were conducted in aerobic batch aqueous systems inoculated with a mixed culture that had been isolated from soils contaminated with polycyclic aromatic hydrocarbons (PAHs). Monod kinetic parameters and yield coefficients for the individual compounds were estimated from substrate depletion and CO2 evolution rate data in sole‐substrate experiments. In all three binary mixture experiments, biodegradation kinetics were comparable to the sole‐substrate kinetics. In the ternary mixture, biodegradation of naphthalene was inhibited and the biodegradation rates of phenanthrene and pyrene were enhanced. A multisubstrate form of the Monod kinetic model was found to adequately predict substrate interactions in the binary and ternary mixtures using only the parameters derived from sole‐substrate experiments. Numerical simulations of biomass growth kinetics explain the observed range of behaviors in PAH mixtures. In general, the biodegradation rates of the more degradable and abundant compounds are reduced due to competitive inhibition, but enhanced biodegradation of the more recalcitrant PAHs occurs due to simultaneous biomass growth on multiple substrates. In PAH‐contaminated environments, substrate interactions may be very large due to additive effects from the large number of compounds present. © 1999 John Wiley & Sons, Inc. Biotechnol Bioeng 65:491–499, 1999.