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Desorption hysteresis has been reported frequently in the literature, with several theories advanced to explain the cause of this phenomenon. Several of these theories hypothesize that desorption hysteresis is an experimental artifact. Batch experiments were performed to observe sorption-desorption and transformation of the solute lindane in systems that included a subsurface sand material. Results of these experiments were interpreted with a diffusion and reaction model. Methods were developed to isolate and determine model parameters from experimental data. Rates of dehydrochlorination were determined for lindane in solution and solid phases. A pseudo sorption-desorption equilibrium experiment was performed by allowing 720 h for attainment of sorption equilibrium and 384 h for attainment of equilibrium for each of three desorption steps-longer than most experiments of a similar nature have been allowed to equilibrate. The pseudoequilibrium data showed marked hysteresis. A surface-diffusion model, which accounted for solute degradation from the solution and solid phases, was calibrated with sorption rate data. The model was verified with desorption rate data. Model predictions of the pseudo sorption-desorption experiment showed that slow sorption-desorption rates explained most of the apparent hysteresis.

A mathematical model was developed to quantify chemotaxis to naphthalene by Pseudomonas putida G7 (PpG7) and its influence on naphthalene degradation. The model was first used to estimate the three transport parameters (coefficients for naphthalene diffusion, random motility, and chemotactic sensitivity) by fitting it to experimental data on naphthalene removal from a discrete source in an aqueous system. The best-fit value of naphthalene diffusivity was close to the value estimated from molecular properties with the Wilke-Chang equation. Simulations applied to a non-chemotactic mutant strain only fit the experimental data well if random motility was negligible, suggesting that motility may be lost rapidly in the absence of substrate or that gravity may influence net random motion in a vertically oriented experimental system. For the chemotactic wild-type strain, random motility and gravity were predicted to have a negligible impact on naphthalene removal relative to the impact of chemotaxis. Based on simulations using the best-fit value of the chemotactic sensitivity coefficient, initial cell concentrations for a non-chemotactic strain would have to be several orders of magnitude higher than for a chemotactic strain to achieve similar rates of naphthalene removal under the experimental conditions we evaluated. The model was also applied to an experimental system representing an adaptation of the conventional capillary assay to evaluate chemotaxis in porous media. Our analysis suggests that it may be possible to quantify chemotaxis in porous media systems by simply adjusting the model's transport parameters to account for tortuosity, as has been suggested by others.