Aerobic biodegradation kinetics of four anionic and nonionic surfactants at sub- and supra-critical micelle concentrations (CMCs)

Aerobic biodegradation of four representative commercial surfactants under a range of concentrations (i.e., sub- and supra-critical micelle concentrat ions, CMCs) beyond those ordinarily used in biodegradation studies was inve stigated. Of the four surfactants tested, sodium dodecylbenzene sulfonate ( SDBS) showed no biodegradation and foam degradation. At a sub-CMC (500 mg/l ), primary biodegradation, as measured by methylene blue active substances (MBAS) for an anionic surfactant sodium dodecyl sulphate (SDS) and by cobal t thiocyanate active substances (CTAS) for a nonionic surfactant Witconol, followed Monod kinetics, substrate depletion being a function of both micro bial growth and substrate concentrations. However, primary biodegradation a t supra-CMCs (500 for Tergitol and 2500 mg/l for SDS and Witconol) and ulti mate biodegradation (mineralization) at both suband supra-CMCs were best de scribed by first-order kinetics, the degradation rate being a sole function of substrate concentrations. Parameters for the kinetic models (Ks, Y, mu( max), and k) were estimated using non-linear least squares methods. Increas ing surfactant concentrations from sub- to supra-CMCs significantly decreas ed primary biodegradation, ultimate biodegradation, and foam degradation. T his decrease may be attributed to the limited bioavailability of surfactant s in the micellar phase as compared to the monomeric surfactants. The resul ts are of significance for surfactant-based remediation, since the concentr ations of surfactants employed in remediation are typically much higher tha n CMCs, while surfactant concentrations of primary interest in biodegradati on work are in the neighborhood of and below 10 mg/l, which is usually 10-1 00 times lower than CMCs. (C) 1998 Elsevier Science Ltd. All rights reserve d.