INHIBITION OF PHENANTHRENE MINERALIZATION BY NONIONIC SURFACTANTS IN SOIL-WATER SYSTEMS

The biodegradation of [C-14]phenanthrene by a polycyclic aromatic hydrocarbon (PAH) degrading inoculum was studied in water and in soil-water systems with nonionic surfactants. The purpose of surfactant addition was to assess the effect on biodegradation of liquid-phase PAH solubility enhancement via surfactant micellization. The nonionic surfactants were selected on the basis of their demonstrated PAH-solubilizing capacities in soil-water systems, viz., an alkylethoxylate, C12E4, and alkylphenol ethoxylate surfactants, C8PE9.5 and C9PE10.5. In the presence of surfactants at concentrations that resulted in aqueous-phase cmc or micelle formation, the mineralization of [C-14]phenanthrene was substantially inhibited. This inhibition was reversible on diluting the surfactant to a concentration below that resulting in micellization. Sub-cmc levels of surfactant in soil-water systems did not appear to have an inhibitory effect on phenanthrene mineralization, but neither did such doses serve to enhance the rate of degradation. Companion studies with [C-14]glucose, phenanthrene, and surfactants suggest that the supra-cmc inhibitory effect on biodegradation is not a toxicity phenomenon, per se, of surfactant or micellized PAH, or a consequence of the nonionic surfactant being used as preferential substrate. Chemical modeling of the phase partitioning of phenanthrene between water, soil, and micelle shows that micellization results in a decrease in the equilibrium free aqueous phenanthrene concentration insufficient to explain the microbial inhibition observed. Additional tests are required to assess the role of surfactants as competitive substrates, the interaction of nonionic surfactant with membrane protein, and the bioavailability of micellized PAH and to determine whether the inhibitory effect is specific to the nonionic surfactants employed in this work.