Sorption kinetics of PAHs in methanol–water systems

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Abstract

The objectives of this study were to evaluate the relationships between the equilibrium sorption constant (Kp), the first-order desorption rate coefficient (k2), and the volumetric fraction of water miscible solvent (fc); and to utilize SPARC-calculated (SPARC Performs Automatic Reasoning in Chemistry) solubility and partitioning parameters with empirical measurements for a priori prediction of solute sorption kinetics in aqueous and mixed solvent systems. Soil column miscible displacement experiments were performed using naphthalene, phenanthrene, anthracene, pyrene and benzo(a)pyrene over an fc range from 0.0 to 0.8. Data from the miscible displacement studies were analyzed using a two-domain first-order mass transfer model to obtain Kp and k2. Results corroborated and extended the range of the validity of the log–log linear relationship between k2 and Kp and log–linear relationship between k2 and fc for mixed solvent systems. In addition, the slope of the log k2–log Kp relationship was found to be relatively solute independent. Predicted values of k2 in aqueous systems agreed well with experimentally measured values, thus, indicating the general applicability of the predictive approaches for predicting the sorption kinetics of hydrophobic organic compounds.

Introduction

Hazardous waste disposal sites and accidental spill sites often contain significant amounts of organic solvents that can effect the near-field sorption and transport of other co-occurring contaminants. The net result of this cosolvent effect is often an increase in contaminant flux through the unsaturated zone to ground water. Intelligent management of these complex waste sites requires a quantitative understanding of contaminant sorption and transport in the presence of water-miscible cosolvents. In addition to furthering our understanding of contaminant transport in complex wastes, these mixed solvent systems may also provide a valuable technique for studying the sorption and transport of very hydrophobic contaminants that are difficult to study in aqueous systems.

The influence of water-miscible organic solvents on organic chemical sorption was initially described by Rao et al. (1985)with subsequent investigations into differing aspects of the effects of mixed solvents on contaminant sorption and transport (Wood et al., 1990; Brusseau et al., 1991; Nzengung et al., 1996). The organic cosolvent used in most of these studies has been methanol. These studies have established that the log–linear relationship between chemical aqueous solubility and the volumetric fraction of water miscible solvent (fc) can be extended to describe the phase distribution of solutes between the water-cosolvent phase and the organic carbon phase of soils, sediments and aquifer materials. Furthermore, limited sorption kinetics studies (Brusseau et al., 1991) have indicated that the desorption rate coefficient (k2) increases log–linearly with increasing fc. It is on this latter aspect, sorption kinetics, that this paper focuses.

The objectives of this study were to evaluate the relationships between the equilibrium sorption constant (Kp), k2 and fc and to utilize SPARC-calculated (SPARC Performs Automatic Reasoning in Chemistry) (Hilal et al., 1994) solubility and partitioning parameters with empirical measurements for a priori prediction of solute sorption kinetics in aqueous and mixed solvent systems. Data from miscible displacement studies with soil columns were analyzed using a two-domain, or bicontinuum, first-order mass transfer model to obtain Kp and k2.

Section snippets

Materials

The naphthalene, phenanthrene, anthracene, pyrene and benzo(a)pyrene (Aldrich Chemical, Milwaukee, WI) used as solutes in the miscible displacement experiments were reagent grade. These polynuclear aromatic hydrocarbons (PAHs) were selected because they exhibit a wide range of hydrophobicity, are of environmental concern, and because they have been used frequently as model solutes in solute transport and environmental fate studies. All methanol (Fisher Scientific, Pittsburgh, PA) used in the

Methanol effects on column hydrodynamics and solute retardation

Fig. 1 contains the BTCs for 3H2O at fc=0.0, 0.2, 0.5 and 0.8. These 3H2O BTCs were all symmetric, and the data were described well by the advective–dispersive local equilibrium solute transport model. Such symmetry and equilibrium model fit is indicative of hydrodynamic equilibrium during transport; that is, that diffusional mass transfer of the 3H2O solute into and out of the microporous structure of the soil occurred rapidly enough relative to solute resident times in the column to be at

Summary and conclusions

The data and analysis presented here corroborates and extends the range of the validity of the log–log linear relationship between k2 and Kp and log–linear relationship between k2 and fc for mixed solvent systems. In addition, the nature of the log k2–log Kp relationship, i.e., the slope, was found to be relatively constant across the suite of PAHs studied. While this constancy may well apply for many neutral hydrophobic compounds, its validity for more polar, hydrophilic compounds is

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