Figure 11. Experimental rate constant for MTBE hydrolysis in HTW. (Reprinted with permission from ref 137.

Formic acid decomposes primarily to CO and H2 0 in the gas phase, but to CO, and H, in

We examined cyclohexanol dehydration in pure water at temperatures of 250, 275, 300, 350, and 380 °C with water densities ranging from 0.08 to 0.81 g/cm 3 . Under these conditions, cyclohexanol dehydrates readily in the absence of added catalysts to form cyclohexene as the major product. The most abundant minor products are 1-and 3-methyl cyclopentenes. The reaction rate and product distribution at 380 °C show a remarkable sensitivity to the water density. At low densities, the reaction is slow, and cyclohexene is the only product. At high densities, the reaction is nearly complete, and methyl cyclopentenes appear along with cyclohexene. The experimental results implied a reaction mechanism that comprises two pathways: (1) reversible cyclohexanol dehydration to form cyclohexene through an E2 mechanism, and (2) subsequent cyclohexene protonation to form the cyclohexyl cation, which rapidly rearranges to form methyl cyclopentyl cations, which then lose a proton to form methyl cyclopentenes. A kinetics model based on the proposed mechanism was able to predict the striking effect of the water density on the product yields at 380 °C and, thereby, to demonstrate that the proposed mechanism captures the trends in the experimental data. An analysis of mechanistic issues regarding cyclohexanol dehydration in high-temperature water (HTW) revealed three roles for water. Water participates in elementary reaction steps as a reactant and as a product, water is the source of the acid catalyst (H 3 O + ), and water also drives the mechanism toward E2 by favoring, through solvation, the oxonium ion rather than the carbocation as the reaction intermediate. This study provides further evidence that acid-catalyzed reactions can be accomplished readily in HTW in the absence of added acid and that HTW has potential applications in environmentally benign industrial chemistry.

A surfactant/foam process is described for the remediation of aquifers contaminated with dense nonaqueous phase liquid (DNAPL). Foam is used for mobility control to displace DNAPL from low permeability sands that are often unswept during a remediation process. Introduction An area where the technology developed for enhanced oil recovery can be applied to environmental remediation is the application of surfactant to remove nonaqueous phase liquid (NAPL) from aquifers. NAPL can be of two types, those which are less dense than water, called light nonaqueous phase liquid (LNAPL) and those which are more dense than water, called dense nonaqueous phase liquid (DNAPL). We concentrate on DNAPL because there are fewer viable alternatives to surfactant remediation. DNAPL will tend to migrate to the lowest accessible point in the aquifer and to enter lower permeability sediments if the capillary pressure becomes large enough. The challenge is to remove DNAPL from local depressions along the base of an aquifer and from low permeability layers in the presence of higher permeability layers. An approach to improve the sweep efficiency of a displacement process is to use mobility control so that the injected fluid is less mobile than the resident fluids. The common method of mobility control for surfactant flooding is through the generation of an inherently viscous microemulsion phase and through the addition of a polymer. However, Lawson and Reisberg introduced the concept of injecting gas with the surfactant solution to generate an in situ foam for mobility control. This approach has not been as popular because the mobility of foam is not as predictable as with polymers. However, much has been learned about the mobility of foam since that time and some publications on the use of foam for mobility control of surfactant flooding have appeared. Also foam has the potential of selectively reducing the mobility more in higher permeability layers in contact with lower permeability layers. Site Characterization The location for a field test of the surfactant/foam process for aquifer remediation is Hill Air Force Base near Ogden, Utah. This base has been the test site of many remediation technologies during 1996. The Operable Unit 2 (OU2) is a waste disposal site where unlined earthen trenches were used from 1967 to 1975 for the disposal of spent liquid degreasing solvents (primarily trichloroethylene). OU2 is currently being treated by "pump and treat" where the DNAPL and ground water are pumped out and the organic material removed by sedimentation and steam stripping. However, pump and treat treatment alone would have to continue for a very long time because of the low solubility of the contaminants in water and the large volume of DNAPL existing in pools and as a residual saturation. A surfactant flood without mobility control was conducted successfully by INTERA and the University of Texas at a site adjacent to where the surfactant/foam is to be tested. A steam flood test in an adjacent site is planned in the near future. Aquifer structure A structure map of the base of the unconfined aquifer is shown in Fig. 1. The aquifer consists of coarse-grained, unconsolidated sediments of recent alluvium and/or Provo Formation. It is about 50 ft thick and the water table is about 25 ft below ground level. The aquifer is underlain by more than 100 ft of the clay dominated Alpine Formation. This formation will be called the "aquitard". The structure of the aquitard and the water table helps to keep the aquifer confined in a trough or channel. Fig. 2 is a cross section along the long axis of the channel. The disposal trenches were located somewhere near the southern end of this cross-section. P. 471

Recent experiments showed that the rate of dissociation of H 2 O 2 in supercritical water (SCW) is density dependent and faster than its high-pressure limit rate in the gas phase. These observations suggest that water molecules play a role in this reaction in SCW. We performed density functional theory (DFT) calculations and molecular dynamics simulations to investigate the role of water in H 2 O 2 dissociation. We generated the potential energy surface for H 2 O 2 -water and OH-water complexes by DFT calculations to determine the parameters in an analytical intermolecular potential model, which was subsequently employed in the molecular dynamics simulations. These simulations were performed at different water densities. They provided the structural properties (pair correlation functions) of dilute mixtures of H 2 O 2 and OH in SCW, from which we were able to calculate the number of excess solvent molecules and partial molar volumes for each solute. We used the partial molar volumes for H 2 O 2 and OH to calculate the reaction volume for H 2 O 2 ) 2OH and thereby determined the density dependence of the equilibrium constant for this reaction. The results show that at the reduced temperature of T r ) 1.15 (695 K) the equilibrium constant for H 2 O 2 dissociation is a function of the water density. The mean value of the equilibrium constant changes by less than 5% between 0.25 < F r < 1, but it decreases by an order of magnitude between 1 < F r < 2.75. Knowing the density dependence of the equilibrium constant for this reaction will allow more accurate mechanism-based models of supercritical water oxidation chemistry to be developed. The computational approach applied herein for H 2 O 2 dissociation is general and can be profitably employed to discern the density dependence of the equilibrium constant of any elementary reaction in SCW. There is currently no experimental approach that will provide this information for reactions involving free radicals.