Biodegradation kinetics of phenanthrene partitioned into the micellar phase of nonionic surfactants

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 unto 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.