Environ. Sci. Technol. 2000, 34, 2966-2972
Structural Evaluation of Slow Desorbing Sites in Model and Natural Solids Using Temperature Stepped Desorption Profiles. 2. Column Results HUMBERTO J. CASTILLA,† CHARLES J. WERTH,* AND SCOTT A. MCMILLAN‡ Department of Civil and Environmental Engineering, University of Illinois at UrbanasChampaign, Urbana, Illinois 61801
Results from temperature stepped desorption (TSD) experiments are presented and compared with simulations from the TSD model presented in the first of this twopaper series. TSD columns were filled with a sand, a sediment, a soil, or a silica gel, all at 100% relative humidity. Next, TSD columns were equilibrated with trichloroethene (TCE), initially purged at 30 °C, and then heated to 60 °C after 100, 1000, or 10 000 min of slow desorption. One γ distribution of diffusion rate constants at 30 °C and one γ distribution of diffusion rate constants at 60 °C were used to simulate column results at all three heating times for a single solid. At each heating time, diffusion rate constants of the γ distributions at 30 °C and 60 °C were used to calculated an effective activation energy, Eact,eff. Values of Eact,eff for all solids were between 47 and 94 kJ/mol, on the order of activation energy values found for diffusion in microporous solids. Between 100 and 10 000 min heating times, the value of Eact,eff increased by a factor of 1.7 for the sand and by a factor of ∼1.1 for the sediment and the soil. This suggests that diffusion occurs from micropores with a wider distribution of widths in the sand than in the other solids and that with decreasing mass remaining diffusion occurs from successively smaller width micropores. For the sediment, values of Eact,eff and 〈D/lm2〉 were lower than those in the other solids. For a given sorbate, larger width micropores are associated with smaller values of Eact,eff and larger values of D. Hence, it is likely that micropores in the sediment are both wider and longer (i.e. larger value of lm2) than those in the other solids. These results suggest that micropore geometry varies between natural solids, and it is an important parameter that must be quantified to predict rates of slow desorption.
Introduction During remediation, contaminants in the subsurface may be subject to perturbations in concentration, temperature, * Corresponding author phone: (217)333-3822; fax: (217)333-6968; e-mail:
[email protected]. † Present address: Bradburne, Briller, and Johnson, LLC, 208 South LaSalle Street, Suite 1440, Chicago, IL 60604. ‡ Present address: Department of Chemical Engineering, Northwestern University, Evanston, IL 60208-3120. 2966
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 34, NO. 14, 2000
and/or pressure. During such perturbations, the release of contaminants from a sorbent (i.e. desorption) may vary depending on the mechanism controlling this process (1). Previous works (2-14) have primarily examined sorption kinetics under conditions where system parameters and boundary conditions were held constant over the course of an experiment (e.g. constant temperature). As discussed by Haggerty and Gorelick (13), various rate models (that assume different mechanistic processes) can be used to simulate contaminant uptake and release under such conditions. Hence, the effects of system perturbations, and the mechanisms controlling desorption, are not well understood. In the first (1) of this two-paper series, a temperature stepped desorption (TSD) model was developed which describes the desorption of organic chemicals from slow desorbing sites subject to a temperature perturbation. The model assumes slow desorption is controlled by onedimensional diffusion from a single micropore or many micropores and that the micropores of a geosorbent are defined by a γ distribution of diffusion rate constants. The model simulates (1) isothermal desorption at temperature T1 until time ts and (2) isothermal desorption at temperature T2 after ts. Application of the model requires defining the initial slow desorbing mass, Mi,slow (defined as M0,tot in paper 1 (1)), a γ distribution of diffusion rate constants at T1, and a γ distribution of diffusion rate constants at T2. In this work, results from TSD columns are presented and compared with simulations from the TSD model to test the hypothesis that slow desorption can be described by diffusion in one-dimension. Columns for TSD experiments were filled with a sorbent and equilibrated with trichloroethylene (TCE) at 30 °C. All columns were initially desorbed at 30 °C and then heated to 60 °C after approximately 100, 1000, or 10000 min of slow desorption. The increase in flux upon heating was simulated and used to quantify the effective activation energy, Eact,eff, a new parameter defined in paper 1 (1) that indicates the energy required for desorption to occur. Values of Eact,eff are compared to published values of activation energy to test the hypothesis that slow desorption is controlled by diffusion from micropores. For each solid, values of Eact,eff are compared at different heating times to determine if diffusion occurs from micropores with uniform or variable widths. Values of D/lm2 and Eact,eff are compared between solids to evaluate differences in micropore geometry. Results from TSD columns heated at 1000 min were presented in a previous paper (15). However, this is the first time that TSD desorption profiles have been measured or simulated as a function of mass remaining.
Experimental Section Materials. TCE was the only sorbate used in this work. TCE has characteristics typical of many hydrophobic volatile organic chemicals (VOCs), and it is a common soil and groundwater contaminant (16-18). One model and three natural solids were used in this study. Silica gel was the model solid, and it was used because prior works (15, 19) indicated it contains relatively uniform micropore properties and no organic matter. The natural solids were a Livermore sand fraction, a Norwood soil, and a Santa Clara sediment. These sorbents were selected because prior works (15, 19) suggested that they have different micropore properties and variations in organic matter content. Table 1 documents the properties of these solids. Methods. Experimental methods are the same as those used previously (15). Briefly, all stainless steel columns (25 cm × 9 mm i.d.) with stainless steel Swagelok fittings were 10.1021/es990430t CCC: $19.00
2000 American Chemical Society Published on Web 06/10/2000
TABLE 1. Physical Properties of Adsorbents
solid
N2 surface areaa [m2/g]
silica gel Santa Clara sediment Norwood soil Livermore sand fraction
297 12 55 13
foc [%]
particle diameterb [µm]
mesoporosity [mL/g]
median mesopore diameterc [Å]
microporosityd [mL/g]
