Kinetic control of contaminant release from NAPLs e Information potential of concentration time profiles Markus Wehrer a, b, * , Juliane Mai c , Sabine Attinger c , Kai U. Totsche b a Department of Earth and Environmental Sciences, Rutgers, The State University of New Jersey,101 Warren Street, Newark 07102, United States b Lehrstuhl für Hydrogeologie, Institut für Geowissenschaften, Friedrich-Schiller-Universität Jena, Burgweg 11, 07749 Jena, Germany c Department Computational Hydrosystems CHS, Helmholtz Centre for Environmental Research GmbH e UFZ, Permoserstraße 15, 04318 Leipzig, Germany article info Article history: Received 31 October 2012 Received in revised form 13 March 2013 Accepted 15 April 2013 Keywords: Spherical film diffusion Contaminant release NAPL First order kinetics Dual domain abstract Release of contaminants from non-aqueous phase liquids (NAPLs) is often limited by the dynamic ex- change with aqueous solutions governed by a priori unknown kinetic laws. Release experiments require a thorough evaluation of the potential and limitations of kinetic models to reveal release processes. In this study, we investigated the characteristic concentration-time profiles of various models for the release of contaminants from an organic phase into an aqueous solution under no flow conditions. Criteria have been tested that allow for distinction of a first order one domain, a first order two domain, a spherical diffusion model, a spherical diffusion model with a time variable diffusion coefficient, a model for diffusion in a sphere with organic film, and a model for diffusion in a sphere with an aqueous film. The results can serve to evaluate the processes potentially governing release of organic contaminants from non-aqueous liquid phases. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction The release of organic compounds from multicomponent non- aqueous phase liquids (NAPLs) into aqueous phase is controlled by the kinetics of the release process. The time scale of the release process and the limiting release mechanism depend mainly on the contact time of aqueous solution and NAPL, NAPL properties and solution chemistry (Eberhardt and Grathwohl, 2002; Totsche et al., 2003; Wehrer and Totsche, 2009). A range of models is available illustrating different concepts of kinetic release of organic com- pounds (Heyse et al., 2002; Wehrer et al., 2011). Conceptually, release of an organic molecule from an organic into an aqueous phase comprises several mass transfer steps. First, the molecule diffuses inside the organic phase toward the water-NAPL interface. The interface may be covered with a (semi-)rigid film or emulsion, which restricts mobility of the molecule. Then, at the interface the molecule dissolves or desorbs and may move through an aqueous film until it reaches the “bulk” aqueous phase. The situation be- comes more complicated with a micro-porous organic matrix, as this may cause additional mass transfer limitations (Wehrer et al., 2011). In summary, bulk organic and bulk water phases might be spatially separated from each other by a series of films of finite thickness and with various properties. Accordingly, the mass transfer steps will act in series, and the mass transfer step with the largest resistance will control the overall contaminant release rate (Ortiz et al., 1999). For a wide range of natural organic sorbents as well as NAPLs, it is assumed that one of the mass transfer processes is limiting and not the desorption rate of the target compound from the interface (Benhabib et al., 2006; Ghosh et al., 2001; Heyse et al., 2002; Ortiz et al., 1999; Pignatello and Xing, 1996). Yet, which of the named mass transfer steps may be rate limiting, depends on the conditions. For example, whether the mass transfer is limited by an organic film or an aqueous film is determined by the diffusion co- efficients in both films, the film thicknesses and the partitioning coefficient (Heyse et al., 2002; Sardin et al., 1991). Mathematically, each mass transfer step may be described with a first order kinetic. First order models are convenient to handle and are thus frequently used (Imhoff et al., 1994; Lee et al., 1998; Powers et al., 1994; Priddle and MacQuarrie, 1994). First order models are often also applied within a two domain approach to explain findings of an initial fast and a subsequent slow release of organic compounds, in particular from soil organic matter (Cornelissen et al., 1998; Heyse et al., 2002). Mostly kinetic limi- tations of NAPL dissolution or organic phase desorption are attributed to diffusion mechanisms (Cornelissen et al., 2005; Delle Site, 2001; Heyse et al., 2002). A first order model can be construed as a pseudo-diffusion model, because it is mathematically equiva- lent under the prerequisite of a linear concentration gradient * Corresponding author. E-mail address: markus.wehrer@uni-jena.de (M. Wehrer). Contents lists available at SciVerse ScienceDirect Environmental Pollution journal homepage: www.elsevier.com/locate/envpol 0269-7491/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.envpol.2013.04.029 Environmental Pollution xxx (2013) 1e14 Please cite this article in press as: Wehrer, M., et al., Kinetic control of contaminant release from NAPLs e Information potential of concentration time profiles, Environmental Pollution (2013), http://dx.doi.org/10.1016/j.envpol.2013.04.029