Kinetic model of biosurfactant-enhanced hexadecane biodegradation by Pseudomonas aeruginosa

Many sites of environmental concern contain groundwater contaminated with nonaqueous phase liquids (NAPL). In such sites interfacial processes may affect both the equilibrium and kinetic behavior of the system. In particular, insoluble hydrocarbon partitioning and microbial biodegradation of insoluble hydrocarbon are influenced by the physicochemical and interfacial characteristics of the system. A mechanistic model describing the influence of biological surfactants on microbial biodegradation of liquid-phase insoluble hydrocarbon and subsequent reduction of nonaqueous-phase liquid hydrocarbon is presented. The model consists of six coupled differential equations which use lumped kinetic parameters to describe surfactant micelle formation and diffusion to the microbial cell, nonlinear kinetic expressions for microbial growth and degradation of insoluble hydrocarbon, kinetic spatial descriptions of the change in NAPL-phase droplet size and the organic phase volume fraction with time, as well as equilibrium partitioning expressions for hydrophobic organic contaminant partioning into the surfactant micelle. The model is validated by comparison to data obtained for hexadecane degradation in a well-mixed batch system by the biosurfactant producing microorganism Pseudomonas aeruginosa strain PG201 as well as for nonproducing mutants' growth and hexadecane biodegradation in the presence of exogenously added biosurfactant. Experimentally determined biological growth parameters, as well as physical parameters such as hydrocarbon droplet size, were applied in the kinetic model. Parameter sensitivity analysis was performed on the physical and biological parameters in the model. The parameter sensitivity analysis indicates that for the biological system examined the rate of hydrocarbon solubilization and micellar transport to the cell controls the rate at which cellular uptake and biodegradation of insoluble hydrocarbon occurs. Practical aspects relating to use of the model for support of surfactant-based bioremediation efforts are discussed.