FINAL REPORT - Mechanisms of CCl4 Retention and Slow Release in Model Porous Solids and Sediments

Abstract

A magnetically coupled microbalance system has been used to measure adsorption and desorption isotherms and rates of desorption for carbon tetrachloride on dry prepared porous silica particles with narrow pore size distributions in the mesoporous range. Pore size distributions estimated from the carbon tetrachloride isotherms were found to be in close agreement with those determined using standard low temperature nitrogen adsorption. Three different types of particles were studied, with average pore diameters of 2.7 nm, 4.6 nm, and 5.9 nm. Prior to desorption rate studies, evacuated particulate samples were charged with volatile organic vapor at pressures sufficient to fill all mesopores with condensed fluid. Desorption rates into dry flowing helium were determined at 25 °C and atmospheric pressure, using the microbalance system combined with chromatographic analysis of the exit helium stream. Initial rates were found to decrease significantly, as mass adsorbed decreased. This residual mass was desorbing at such a low rate, that it can be considered a migration resistant fraction of the original mass adsorbed. Attempts to remove this residual mass at higher temperatures were partially successful; however, differences between the microbalance and gas chromatograph responses leave open uncertainty about whether the residual mass was pure carbon tetrachloride. Tomore » date, attempts at analysis of the residual mass using solvent extraction have not removed completely this uncertainty. For particles prepared using the same template surfactant, but with different average pore sizes, desorption rates were higher for the larger-pore particles, with correspondingly lower residual mass. Particles prepared with another template surfactant did not follow this pattern, exhibiting intermediate desorption rates and slightly lower residual mass, even though these particles had the smallest pores. These particles exhibited desorption isotherm behavior characteristic of larger pores connected by smaller openings. Except for peculiar behavior in the very early part of desorption experiments for one type of particles, the carbon tetrachloride desorption curves could be fit by a two-part model, employing a diffusion model for the bulk of the desorption, followed by a deactivation model as the mass adsorbed approached residual values. Simultaneous microbalance and gas chromatograph measurements were used to determine carbon tetrachloride and water desorption rates from silica particles initially containing both volatile components. Varying water to carbon tetrachloride ratios were loaded on two types of particles with different pore sizes, with water always loaded first. With water on the particles, pore volumes were significantly reduced. When compared at the same mass adsorbed values, total desorption rates consistently decreased with increasing water content. Total residual mass was found to be a strong function of initial water content, increasing nonlinearly from as initial water content increased from 0 % to 100 %. As expected, during the first few hours of all desorption rate experiments, the rates of carbon tetrachloride desorption were larger than for water. At low initial water contents, total desorption rates were controlled throughout by the carbon tetrachloride rates. For higher water contents, the water rates became larger than the carbon tetrachloride rates for at least some period of intermediate times, after the bulk of the carbon tetrachloride had been desorbed. Although the compositions of the residual mass have not been independently measured, there is evidence that both components were retained, but that water was the major component when there was significant initial water on the particles.« less