Validation of a Hybrid “Two-Site Gamma” Model for Naphthalene Desorption Kinetics

Abstract

Three models for sorption/desorption of polycyclic aromatic hydrocarbon (PAH) contaminants from soil were compared for their ability to predict the transport of PAH in soil: a "gamma" model, a "two-site/two-region" nonequilibrium model, and a "hybrid" model. In the "hybrid" model, soil organic matter was conceptually divided into two compartments; a fraction with rapid sorption/desorption kinetics and a compartment with mass-transfer-limited kinetics. Contaminant sorbed in the rapid compartment was assumed to be in instantaneous equilibrium with the aqueous phase, while the release of contaminant from the slow fraction was assumed to be governed by a gamma distribution of rate coefficients. The "hybrid" model successfully described the initial rapid release of a model PAH contaminant, naphthalene, from a sieved soil sample of moderate organic content (=2.3%) as well as the following slow release observed over 25 days in batch desorption experiments. Other necessary model parameters, such as the hydrodynamic dispersion coefficient of naphthalene and the macropore porosity, were evaluated in separate experiments. A transport model incorporating the "hybrid" model for naphthalene sorption/desorption successfully predicted the elution profile of naphthalene in independent soil-column experiments with no adjustable parameters. The success of the hybrid model suggests that a wide array of rate controls govern PAH desorption. This conclusion is consistent with the view of soils as consisting of a mix of different sorptive constituents and heterogeneous physical constraints on PAH release.

Characterizing and modeling of extensive atrazine elution tailing for stable manure-amended agricultural soil

“…Previous studies revealed that soil organic matter (SOM) fraction played a major role for the sorption and transport of atrazine in subsurface environmental systems (Barriuso et al, 1992;Dousset et al, 1994;Lesan and Bhandari, 2003;Park et al, 2004;Boivin et al, 2005;Kempf and Brusseau, 2009;Yang et al, 2009). A number of research papers indicated that the presence of organic matter fraction in the soil led to non-ideal behavior such as rate-limited desorption and extensive low-concentration elution tailing phenomena for some contaminants (Pignatello et al, 1993;Piatt and Brusseau, 1998;Ahn et al, 1999;Johnson et al, 2003Johnson et al, , 2009Deng et al, 2008;Kempf and Brusseau, 2009;Russo et al, 2010;Akyol et al, 2011;Brusseau et al, 2012). However, few studies have examined low-concentration tailing phenomena associated with nonlinear and/or rate-limited sorption/desorption for atrazine specifically.…”

Quantitation of bound pesticide residues in field and wetland soils

“…The gamma model was slightly better than the model with a lognormal distribution of rates in being able to describe the elution profile of TCE from the soil columns. Unlike Ahn et al ( 1999) who used data determined in batch experiments to predict elution profiles from soil columns, the parameters in this study determined in batch desorption experiments were not transferable in describing the elution profile of TCE from soil columns. The distribution of rate constants in the batch desorption experiments were significantly different from those fitted to the column elution profiles.…”

“…This study showed that sorption/desorption of QCB to soil could be described with a two compartment kinetic model with a slow-desorbing fraction being attributed to a limited number of high affinity sites. Ahn et al (1999) developed and compared three models for their ability to predict the sorption/desorption behavior of polycyclic aromatic hydrocarbons (PAH) to soil. The three models were a gamma model, a two-site nonequilibrium model, and a hybrid model.…”

“…The hybrid model was developed in batch sorption experiments with naphthalene and incorporated into a transport model that was validated in independent soil-column experiments with no adjustable parameters. Ahn et al (1999) also discuss the intraparticle diffusion model where tortuous paths and small pores within soil aggregates are envisioned as the controlling mechanism in the release and uptake of contaminants. Intraparticle diffusion models based on this mechanism have been unable to completely describe the extensive tailing exhibited in some breakthrough curves and elution profiles of organic contaminants from soil columns.…”

“…PAH's and pesticides. Ahn et al (1999) notes that this model has been unable to explain the extensive tailing of some breakthrough curves of HOC's from soil columns. In this model, desorption rates are dependent on the diffusion coefficients which are correlated with the diffusion length scale which is the particle radius.…”