Transport of Organic Compounds With Saturated Groundwater Flow: Model Development and Parameter Sensitivity

Abstract

A model which includes the transport and retardation mechanisms of advective flow, axial dispersion, liquid-phase mass transfer, diffusion into immobile liquid, and local adsorption equilibrium was developed to describe the migration of nondegradable, organic chemicals through a column of saturated• aggregated soil. A range of simplifying assumptions were explored to assess the relative importance of the various mechanisms. Solutions to the model were either adapted from the literature or derived from mass balances and mass transfer principles. The most general form of the model required the development of numerical solutions which employed orthogonal collocation. Soil column breakthrough predictions in terms of relative concentration as a function of total column pøre volumes fed can be characterized by seven independent dimensionless parameters: the Peclet number, the Stanton number, a pore diffusion modulus, a surface diffusion modulus, an adsorbed solute distribution ratio, an immobile fluid solute distribution ratio, and the Freundlich parameter 1In. For a strongly adsorbed chemical in long soil columns, a fifteen fold decrease of the Peclet number, a fivefold decrease of the Stanton number, or a onefold decrease in either the pore diffusion modulus or the surface diffusion modulus have an equivalent effect on the spreading of the breakthrough curve. The breakthrough curve tends to sharpen for favorably adsorbed chemical species (1In < 1.0) and spread when adsorption is unfavorable (1In > 1.0). The movement of chemical is retarded as the solute distribution ratios increase. A sensitivity analysis of model parameters, which were derived from literature correlations, column geometry, soil adsorption isotherms, and breakthrough curves, showed that adsorption capacity, adsorption intensity, and aggregate geometry have the greatest effect on chemical retardation and spreading, while liquid-phase mass transfer has little effect. INTRODUCTION The evaluation of the extent of groundwater pollution requires an understanding of the transport mechanisms which are involved and the ability to incorporate these mechanisms into a predictive mathematical model. A predictive model for the transport of organic chemicals may also be useful in designing landfills or wastewater land application systems. Many of the hazardous chemicals that originate from waste disposal and are subsequently found in groundwater are slightly degradable, weakly hydrophobic, organic compounds which are not readily removed as they travel with the groundwater {'Roberts et al., 1982-1. Many of these compounds are also volatile i-Bouwer and Rice, 1984-1, posing an additional problem because they may diffuse through the soil atmosphere at a much faster rate than with advective transport in the aqueous phase. Much of the understanding of the behavior of organic chemicals in soil and groundwater has been derived from work performed on pesticides •Bailey and White, 1970; Goring and Hamaker, 1972; Helling et al., 197!•. Several mechanisms are responsible for ...

Mathematically modeling fixed-bed adsorption in aqueous systems

“…Note that several parameters in the general rate models can be determined both theoretically and experimentally. The theoretical method is based on some basic correlations (Roberts et al, 1985;Crittenden et al, 1986;Weber and Smith, 1987;Wolborska, 1989b;Tien, 1994;Sperlich et al, 2005;Worch, 2008) and part of them are listed in Table 3, whereas the accuracy of estimation values of the parameters is not satisfactory. Especially, to our knowledge, there is no reliable method to theoretically predict the tortuosity τ and surface diffusion coefficient D s , which are indispensable when using the general rate models.…”

“…To model a liquid-solid column adsorption, it is necessary to divide it into four basic steps ( Fig. 1): (1) liquid phase mass transfer including convective mass transfer and molecular diffusion; (2) interface diffusion between liquid phase and the exterior surface of the adsorbent (i.e., film diffusion); (3) intrapellet mass transfer involving pore diffusion and surface diffusion; and, (4) the adsorption-desorption reaction (Crittenden and Weber, 1978;Crittenden et al, 1986;Helfferich, 1995).…”

Capturing Lithium from Wastewater Using a Fixed Bed Packed with 3-D MnO2 Ion Cages

“…Thus, in actuality, we used the pore diffusion model (PDM) to predict the lithium breakthrough curve. 26,32,33 PDM simulations were conducted using the AdDesignS software (Michigan Technological University). 34 The wall effect on mass transfer can be neglected for d column / d p ratios >20.…”

“…However, they are empirical models, which limits their ability to predict the performance of full-scale adsorption treatment systems. Accordingly, we used the pore diffusion model (PDM) 26 to predict and validate small column breakthrough data. Thus, we can use the model to predict lithium breakthrough in full-scale adsorption systems.…”

Hydrodynamic dispersion and mass transfer in unsaturated flow

“…Hydrodynamic dispersion coefficients for the unsaturated zone have not bccn sufficiently investigated, although practical values arc nccdcd for exploring NAPL (non-aqueous phase liquid) transport in an aquifer [Crittenden et al, 1986 [Wilson, 1995;Anderson, 1994Anderson, , 1995 and soil vapor extraction [Gierke et al, 1990[Gierke et al, , 1992; Armstrong et al, 1994; Conant et al, 1996;Fischer et al, 1996]. In particular, there is little information available for cases in the range of high water saturation.…”