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Title: Mechanisms of CCl4 Retention and Slow Release in Model Porous Solids and Sediments

Abstract

This work is part of a larger collaborative project of the same title led by Robert Riley at PNNL. Our task goal is to use a state of the art microbalance and well-defined mesoporous silica particles to characterize the effects of pore size distribution on carbon tetrachloride release rate and sequestration.

Authors:
Publication Date:
Research Org.:
Washington State University, Pullman, WA
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
885263
Report Number(s):
EMSP-86729-2004
R&D Project: EMSP 86729; TRN: US200616%%478
DOE Contract Number:
FG07-02ER63503
Resource Type:
Technical Report
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; 54 ENVIRONMENTAL SCIENCES; CARBON TETRACHLORIDE; DISTRIBUTION; MICROBALANCES; RETENTION; SEDIMENTS; SILICA

Citation Formats

Peyton, Brent M. Mechanisms of CCl4 Retention and Slow Release in Model Porous Solids and Sediments. United States: N. p., 2004. Web. doi:10.2172/885263.
Peyton, Brent M. Mechanisms of CCl4 Retention and Slow Release in Model Porous Solids and Sediments. United States. doi:10.2172/885263.
Peyton, Brent M. Wed . "Mechanisms of CCl4 Retention and Slow Release in Model Porous Solids and Sediments". United States. doi:10.2172/885263. https://www.osti.gov/servlets/purl/885263.
@article{osti_885263,
title = {Mechanisms of CCl4 Retention and Slow Release in Model Porous Solids and Sediments},
author = {Peyton, Brent M.},
abstractNote = {This work is part of a larger collaborative project of the same title led by Robert Riley at PNNL. Our task goal is to use a state of the art microbalance and well-defined mesoporous silica particles to characterize the effects of pore size distribution on carbon tetrachloride release rate and sequestration.},
doi = {10.2172/885263},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Wed Dec 01 00:00:00 EST 2004},
month = {Wed Dec 01 00:00:00 EST 2004}
}

Technical Report:

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  • This work is part of a larger collaborative project of the same title led by Robert Riley at PNNL. Our task goal is to use a state of the art microbalance and well-defined mesoporous silica particles to characterize the effects of pore size distribution on carbon tetrachloride release rate and sequestration. Mesoporous silicon dioxide (SiO2) particles with pore size distribution in the range of 20 to 70 Angstroms were synthesized under acidic conditions using nonionic surfactants as structure directing agents (Zhao et al., 1998). Siliceous particles (primarily silica dioxide) containing the template molecules are precipitated from solution, dried and themore » template is removed by combustion at 550 C. The particles obtained by this procedure were characterized using standard nitrogen adsorption at 77 K. Surface area and pore size distribution were calculated from the data coming from the nitrogen experiments. The particle size distribution was determined by using a centrifugal automatic particle size distribution analyzer, which employs a non-contact measuring method based on liquid-phase sedimentation and the change in particle concentrations on the basis of light transmission. To examine the effects of the pore size on CCl4 sequestration and release, a state-of-the-art microbalance was used to measure real-time desorption profiles of CCl4 released from these mesoporous materials. In addition, a gas chromatograph with flammable ionization detector (FID) and electron capture detector (ECD) was used to close the mass balances and confirm the observations made with the microbalance. Experiments with the microbalance include helium calibration (critical to the measurement of small amounts of CCl4 release), and CCl4 desorption rate experiments, to quantify release rates and residual sequestration of CCl4 in the mesoporous silica particles. Finally, a fully automated accelerated solvent extraction system was used to determine the actual mass of CT remaining in the samples immediately after finishing the desorption experiments.« less
  • A migration-resistant fraction (MRF) is a portion of polluted sediment's contaminant inventory that exhibits slow release. MRF's are formed as a result pollutant aging. Slow release is a key process that controls organic contaminant transport and fate in a plume long after the major portion of the contaminant inventory of a source term has been depleted or removed. Slow release rates are not well understood nor are they commonly accounted for in subsurface numerical transport models. In this project, we propose to study the accumulation and slow-release behavior of carbon tetrachloride (CCl4) MRF as a function of time, contaminant concentrationmore » and different physicochemical properties of sediments. Experiments will be conducted at macro- and microscopic scales under saturated conditions. The results will be used to (1) develop a mechanistic description of slow release of CCl4 in the subsurface environment and (2) lay the groundwork for improving the robustness of numerical models that predict organic contaminant transport and fate under natural conditions. The outcomes of this study are expected to improve the conceptual model of CCl4 subsurface transport at different physical scales and have an impact on remediation and site closure decision-making at DOE sites, especially in situations involving the potential application of natural attenuation.« less
  • Provide a better description of the processes by which non-polar compounds are retained by sediments and subsequently released. The objective will be reached through a combination of theory and experimentation with model porous materials and natural sediments. Focus is on the behavior of carbon tetrachloride in aquifer sediments.
  • 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 allmore » 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. To 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
  • The flammable gas hazard in Hanford waste tanks was made an issue by the behavior of double-shell Tank (DST) 241-SY-101 (SY-101). Shortly after SY-101 was filled in 1980, the waste level began rising periodically, due to the generation and retention of gases within the slurry, and then suddenly dropping as the gases were released. An intensive study of the tank's behavior revealed that these episodic releases posed a safety hazard because the released gas was flammable, and, in some cases, the volume of gas released was sufficient to exceed the lower flammability limit (LFL) in the tank headspace (Allemann etmore » al. 1993). A mixer pump was installed in SY-101 in late 1993 to prevent gases from building up in the settled solids layer, and the large episodic gas releases have since ceased (Allemann et al. 1994; Stewart et al. 1994; Brewster et al. 1995). However, the surface level of SY-101 has been increasing since at least 1995, and in recent months the level growth has shown significant and unexpected acceleration. Based on a number of observations and measurements, including data from the void fraction instrument (VFI), we have concluded that the level growth is caused largely by increased gas retention in the floating crust. In September 1998, the crust contained between about 21 and 43% void based on VFI measurements (Stewart et al. 1998). Accordingly, it is important to understand the dominant mechanisms of gas retention, why the gas retention is increasing, and whether the accelerating level increase will continue, diminish or even reverse. It is expected that the retained gas in the crust is flammable, with hydrogen as a major constituent. This gas inventory would pose a flammable gas hazard if it were to release suddenly. In May 1997, the mechanisms of bubble retention and release from crust material were the subject of a workshop. The evaluation of the crust and potential hazards assumed a more typical void of roughly 15% gas. It could be similar to percolati on in single-shell tank (SST) waste forms. The much higher void being currently observed in SY-101 represents essentially a new crust configuration, and the mechanisms for sudden gas release need to be evaluated. The purpose of this study is to evaluate the situation of gas bubbles in crust based on the previous work on gas bubble retention, migration, and release in simulants and actual waste. We have also conducted some visual observations of bubble migration through simulated crusts to help understand the interaction of the various mechanisms.« less