Interfacial films, mass transfer and biodegradation in coal tar (NAPL)-water systems

This study evaluates the potential for slurry biotreatment of coal tar-contaminated soils and sediments, focusing on the impact of physico-chemical mass transfer phenomena on the bioavailability of polynuclear aromatic hydrocarbons (PAHs) derived from non-aqueous phase liquid (NAPL) coal tar. The impact of mass transfer phenomena on coal tar biotreatment is evaluated by means of mathematical modeling techniques and controlled laboratory experiments.;For liquid coal tar globules, and for microporous media with a large residual saturation of coal tar, a simplified dissolution-degradation model is developed which focuses primarily on bulk-phase transport phenomena. Model results indicate that the rate of biotransformation of PAHs from coal tar NAPL depends on equilibrium partitioning between tar and water, and on the relative rates of aqueous phase PAH mass transfer and biodegradation. Using Raoult's law, the effect of equilibrium partitioning on bioavailability is quantified by a solubility factor. The relative importance of dissolution kinetics to biological processes is quantified by the dimensionless Damkohler number that is predicted to depend on the size and number density of the coal tar globules or solid aggregates, and on the degree of mixing in the bioreactors.;Abiotic mass transfer experiments are conducted with two coal tar samples in order to verify the assumptions and predictions of the dissolution-degradation model. The experiments evaluate equilibrium partitioning and dissolution kinetics for two PAH solutes, naphthalene and phenanthrene, dissolving from coal tar in solid-slurry and tar-water dispersion systems. Experimental data indicate that the theoretical models effectively describe dissolution equilibrium and kinetic phenomena, and may be used for a priori predictions of the solubility factor and the Damkohler number in coal tar bioslurry systems. A priori estimates of the solubility factor and the Damkohler number, determined from the mass transfer studies, are used to analyze initial naphthalene biotransformation rates obtained from companion coal tar biomineralization experiments (Ghoshal et al., 1994). Analysis of the biomineralization profiles confirms that the Damkohler number can be used to distinguish between coal tar bioslurry systems constrained by mass transfer phenomena and those controlled by biokinetic processes. Further, the approximate rate of biotransformation can be estimated with a priori knowledge of the solubility factor and the Damkohler number. Thus, experimental data support model predictions, enabling quantitative assessment of physico-chemical parameters that control the rate of biotransformation of PAH compounds from coal tar NAPL.;A new facet of NAPL-water interactions was discovered in the course of the laboratory tests conducted with coal tar. A semi-rigid, skin-like film was observed to form at a tar-water interface that has aged for a period of several days. The latter part of this thesis investigates experimental protocols for measurement of the interfacial film resistance to mass transfer. A theoretical framework is developed to assess the relevance of film-resistances to solute transport and biodegradation in multi-phase environments.;A multi-step mass transport-degradation model couples sequentially occurring mass transport phenomena with first-order biokinetic processes occurring in the system. Model results indicate that dimensionless parameters, such as the Biot number, the Thiele number and the Damkohler number, may be developed for a pair-wise comparison of the intermediate rate-processes that lead to overall biotransformation in the NAPL-slurry system. The dimensionless parameters provide quantitative criteria for identifying dominant rate-limiting processes occurring in the slurry bioreactors, enabling the development of simpler models for solute biotransformation in NAPL-water systems.