Estimation of direct-contact fraction for phenanthrene in surfactant solutions by toxicity measurement

The toxicity of solutions containing nonionic surfactants Tween 80, Brij 35 and/or phenanthrene to Pseudomonas putida ATCC 17484 was investigated. The fraction of direct contact between micellar-phase phenanthrene and bacterial cell surface was estimated by using the toxicity data and a mathematical model. The mathematical model was used to calculate phenanthrene concentration in the micellar phase and aqueous pseudophase separately. The first-order death rate constant increased from 0.088 ± 0.016 to 0.25 ± 0.067 h^-1 when the phenanthrene concentration was increased from 0 to 5.17 × 10^-6 M (equals water solubility). The intrinsic toxicity of surfactant was higher in Brij 35 than in Tween 80. When phenanthrene concentration was increased to 9.7 × 10^-5 M in surfactant solutions, the death rate constant increased to 1.8 ± 0.024 and 0.41 ± 0.088 h^-1 for 8.4 × 10^-4 M Brij 35 and 7.6 × 10^-4 M Tween 80. The direct-contact fraction was 0.083 and 0.044 for Brij 35 and Tween 80, respectively, under these conditions using exponential model. The toxicity increased with increasing phenanthrene concentration at a fixed surfactant concentration. The toxicity decreased with increasing the surfactant concentration at a fixed phenanthrene concentration due to decreased contact of bacteria with phenanthrene present in the interior of surfactant micelles.