Microscale-based modeling of polynuclear aromatic hydrocarbon transport and biodegradation in soil

A mathematical model to describe polynuclear aromatic hydrocarbon (PAH) desorption, transport, and biodegradation in saturated soil was constructed by describing kinetics at a microscopic level and incorporating this description into macroscale transport equations. This approach is novel in that the macroscale predictions are made independently from a knowledge of microscale kinetics and macroscopic fluid dynamics and no adjustable parameters are used to fit the macroscopic response. It was assumed that soil organic matter, the principal site of PAH sorption, was composed of a continuum of compartments with a gamma distribution of desorption rate coefficients. The mass transport of substrates and microorganisms in a mesopore was described by diffusion and that in a macropore by one- dimensional advection and dispersion. Naphthalene was considered as a test PAH compound for initial model simulations. Three mechanisms of naphthalene biodegradation were considered: growth-associated degradation as a carbon and energy source for microbial growth; degradation for maintenance energy; and growth-independent degradation. The Haldane modification of the Monod equation was used to describe microbial growth rates and to account for possible growth inhibition by naphthalene. Multisubstrate interactions were considered and described with a noninteractive model for specific growth rates. The sensitivity of selected model parameters was analyzed under conditions when naphthalene was the sole growth rate-limiting substrate. The time necessary to achieve a specific degree of naphthalene biodegradation was found to be proportional to the initial concentration of naphthalene in soil organic matter. The biodegradation rate of naphthalene increased when the sorption equilibrium constant of naphthalene was reduced. The presence of an alternative carbon source inhibited naphthalene biodegradation in spite of the calculated increase in biomass.