Binary solute desorption isotherms of trichloroethylene (TCE) and tetrachloroethylene (PCE) at 100% relative humidity from silica gel and two well-characterized natural solids were investigated. Results indicated that the ideal adsorbed solution theory (IAST) was able to describe desorption isotherms for the silica gel. For the natural solids, IAST was not able to describe desorption isotherms for the full concentration range examined. Failure of IAST was greatest for the most heterogeneous sorbent, even when considering multiple sorption domains. In addition, IAST predictions worsened as nonlinear uptake mechanisms began to dominate. Several possible explanations for the failure of the IAST are given, including the possibility that complex interactions between the sorbing solutes and the sorbent may exist, causing deviations from ideal sorption behavior.
Results from temperature stepped desorption (TSD) experiments are presented and compared with simulations from the TSD model presented in the first of this two-paper 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.
In the first of this two-paper series, a new model is presented that simulates the effects of a temperature perturbation on the rate of slow desorption as a function of mass remaining. The model assumes slow desorption is controlled by one-dimensional diffusion from a single or many hydrophobic micropores and that the micropores of a geosorbent are defined by a γ distribution of diffusion rate constants. Simulation results indicate that during slow desorption the relative increase in flux upon heating increases with decreasing micropore width. Simulation results also indicate that the relative increase in flux upon heating increases with desorption time when diffusion occurs from successively smaller width micropores with decreasing mass remaining. In paper 2, the model is tested and used to examine micropore geometry in natural and model solids by simulating results from temperature stepped desorption (TSD) experiments.