Diffusion of PAH in Potato and Carrot Slices and Application for a Potato Model STEFAN TRAPP,* ,† ANITA CAMMARANO, †,‡ ETTORE CAPRI, ‡ FREDRIK REICHENBERG, § AND PHILIPP MAYER § Institute of Environment & Resources, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark, Istituto di Chimica Agraria ed Ambientale, Universita ` Cattolica del Sacro Cuore, I-29100 Piacenza, Italy, and National Environmental Research Institute, University of Aarhus, PO Box 358, DK-4000 Roskilde, Denmark A method for quantifying the effect of medium composition on the diffusive mass transfer of hydrophobic organic chemicals through thin layers was applied to plant tissue. The method employs two silicone disks, one serving as source and one as sink for a series of PAHs diffusing through thin layers of water, potato tissue, and carrot tissue. Naphthalene, phenanthrene, anthracene, and fluoranthene served as model substances. Their transfer from source to sink disk was measured by HPLC to determine a velocity rate constant proportional to the diffusive conductivity. The diffusive flux through the plant tissue was modeled using Fick’s first law of diffusion. Both the experimental results and the model suggest that mass transfer through plant tissue occurs predominantly through pore water and that, therefore, the mass transfer ratio between plant tissue and water is independent of the hydrophobicity of the chemical. The findings of this study provide a convenient method to estimate the diffusion of nonvolatile organic chemicals through various plant materials. The application to a radial diffusion model suggests that “growth dilution” renders the concentration of highly hydrophobic chemicals in potatoes below their equilibrium partitioning level. This is in agreement with field results for the bioconcentration of PAHs in potatoes. Introduction Potato is the fourth most important food crop in the world after wheat, rice and maize (1). The average daily consump- tion of potatoes and potato products in Denmark is 126 g, which is 58% of fruit and vegetable consumption (2). The situation is likely to be similar for many other countries, making potatoes the most important vegetable worldwide. Botanically, potato is a tuber and, as such, is a part of the stem (1). It is not connected to the root system and the transpiration stream. It is loaded via phloem from the leaves. For hydrophobic organic compounds, translocation down- ward in phloem is negligible (3). The uptake of hydrophobic organic contaminants into potatoes is, therefore, most likely to occur from soil and via diffusion through the peel. Diffusive mass transfer of chemicals in a multimedia- multiphase environment can involve the gas phase, aqueous phase, lipid phase, organic matter, and various solid inorganic phases. One way to approach such multimedia mass transfer are models that combine phase partitioning and Fick’s first law of diffusion. This requires knowledge of the diffusion coefficients and the partition coefficients within and between the various phases (4). Their determination requires con- siderable experimental effort, and the resulting equations for the prediction of transport can be complex. Another way to approach the diffusive transfer of a chemical in multiphase systems is the determination of the diffusive conductivity, integrating diffusion, binding, and partitioning. An experi- mental method to quantify the effect of medium composition on the diffusive conductivity for hydrophobic organic chemicals was recently described (5). The method employs two poly(dimethylsiloxane) (PDMS) silicone disks, one serv- ing as source and the other as sink for hydrophobic organic chemicals diffusing through thin layers. In the present study we modified this method to make it applicable to thin slices of plant tissue. The method was then used to determine the diffusive mass transfer of four PAHs through thin slices of potato and carrot. The experimental results were compared to the theory to develop a model for diffusive uptake of organic chemicals from soil into potatoes. The model was compared to field observations to validate the method. Materials and Methods Experimental Methods. Poly(dimethylsiloxane) (PDMS) sheets with a thickness of 600 μm((20 μm) were supplied by Rubber BV. Sodemann Industrifjedre A/S (Viby, Denmark) supplied 100-μm steel washers (ID 4 mm, OD 8 mm) to be used as spacers for the water experiments. Nickel-plated neodymium iron boron magnets with a diameter of 10 mm and a thickness of 5 mm were supplied by Farnell (Herlev, Denmark). Naphthalene (NAP, >99%, Sigma), phenanthrene (PHT, >96%, Sigma), anthracene (ANT, 97%, Aldrich), and fluoranthene (FLT, >97%, Fluka) were supplied by Sigma- Aldrich (Vallensbæk Strand, Denmark). The PAHs were extracted from the PDMS disks with 96% ethanol (De Danske Spritfabrikker, Aalborg, Denmark). Preparation of Thin Slices. Carrots (Daucus carota L. ssp. sativus) and potatoes (Solanum tuberosum L.) were pur- chased from a local supermarket. The carrots were purchased with green tops in order to ensure a fresh product. Potatoes were also selected for freshness. A large number of carrot and potato slices were cut using an electric ham slicer (model Euro 2560S from Graef, Arnsberg, Germany). The slice thickness was measured using a digital micrometer with a resolution of 10 μm. Slices with a thickness between 90 and 110 μm were selected. Circles with a diameter of 6 mm were cut from these slices and stored at 100% humidity. Experimental Apparatus. Disks with a diameter of 6 mm were cut out of the PDMS sheet and cleaned in three changes of >200 mL methanol with a total contact time of at least 24 h. Disks were contaminated according to ref 6 by placing them in a methanol:water solution (80:20, v/v) containing the PAHs at a concentration of 0.5 mmol/L each, with a minimum contact time of 16 h. The homogeneous distribu- tion of PAHs within the PDMS has been previously dem- onstrated (7). On the day of the experiment, contaminated disks were transferred to a small volume of water (=1 mL/ disk) to remove methanol. * Corresponding author phone: +45 4525 1622; fax +45 4593 2850; e-mail: stt@er.dtu.dk. † Technical University of Denmark. ‡ Universita ` Cattolica del Sacro Cuore. § University of Aarhus. Environ. Sci. Technol. 2007, 41, 3103-3108 10.1021/es062418o CCC: $37.00  2007 American Chemical Society VOL. 41, NO. 9, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 3103 Published on Web 03/24/2007