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2004 Progress Report: The Role of Micropore Structure in Contaminant Sorption and Desorption
EPA Grant Number: R828772C014Subproject: this is subproject number 014 , established and managed by the Center Director under grant R828772
(EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
Center: HSRC (2001) - Western Region Hazardous Substance Research Center for Developing In-Situ Processes for VOC Remediation in Groundwater and Soils
Center Director: Semprini, Lewis
Title: The Role of Micropore Structure in Contaminant Sorption and Desorption
Investigators: Reinhard, Martin
Institution: Stanford University
EPA Project Officer: Lasat, Mitch
Project Period: September 1, 2001 through August 31, 2006
Project Period Covered by this Report: September 1, 2003 through August 31, 2004
RFA: Hazardous Substance Research Centers - HSRC (2001)
Research Category: Hazardous Waste/Remediation
Description:
Objective:The overall goal of this project is to develop a better understanding of the impact of soil nanopores on the fate and transport of halogenated hydrocarbon contaminants. Specific project goals are to: (1) study the kinetics of slow sorption and desorption of halogenated hydrocarbons in aquifer sediment, and (2) determine the effect of sorption on contaminant reactivity. Results will allow us to better predict natural attenuation of hydrocarbon compounds in aquifers and assess the risks associated with groundwater aquifers contaminated by halogenated hydrocarbons.
Progress Summary:Rationale
Geological solids contain nanopores because of material imperfections or weathering, cracking, or turbostratic stacking. Previous work has demonstrated that sorption of hydrophobic organic compounds in nanopores can be a significant sequestering process. Sorption in nanopores is reversible but rates are very slow (weeks to months) and difficult to quantify, especially in the field. Our understanding of geosorbent nanoporosity and how it affects the sorption and chemical transformations of organic contaminant is very limited. The fundamental hypothesis is that water is unable to compete for sorption sites in hydrophobic nanopores and unable to displace sorbed hydrophobic contaminants. We hypothesize that inside such nanopores, halogenated hydrocarbon compounds are prevented from reacting with water and that this phenomena leads to long residence times of reactive contaminants in soils and aquifers.
Approach
A novel analytical system has been developed that allows us to study simultaneously sorption and transformation of volatile organics in geological sorbents. The system consists of the previously (Project 1-SU-03, Grant No. R828772C005) developed soil column chromatograph, which is directly coupled to a chromatograph for the analysis of the sorbate and transformation products. The procedure involves first loading contaminant onto the soil column by passing a stream of contaminant vapor through the column until breakthrough using helium (1.00 mL/min) as the carrier gas. The column is then disconnected, sealed and equilibrated for weeks to months. Following equilibration, the columns are purged with a helium stream (1.00 mL/min) that is fed directly to the on-line gas chromatograph (GC), which quantifies the concentrations of the sorbate and the transformation products. Desorption and transformation concentration-time profiles are obtained as a function of temperature, humidity, and competitive cosorbates or cosolvents. The procedure can be calibrated using sorbents with known porosity (silica gel) and sorbates with known hydrolysis rates—trichloroethylene (TCE), benzyl chloride (BzC), and 2,2-dichloropropene (DCP).
Status
Initial studies focused on the non-reactive (TCE) and the two reactive model substrates (BzC and DCP) as the sorbates, and the clay and silt fraction (< 50 μm) of soil from a site at the Lawrence Livermore National Laboratory (LLNL) as the sorbent. DCP sorption data obtained at different soil moisture contents confirmed that the sorption capacity decreases significantly as the moisture content increases, indicating that water displaces DCP from sorption sites as the moisture content increases. However, water did not completely eliminate the sorption capacity for DCP, and a small but significant amount of DCP (~0.1 mg/g dry soil) could still be sorbed when the soil was wet. Most of this fraction was desorbing very slowly, which is consistent with sorption in hydrophobic nanopores.
The effect of competitive cosorbates on desorption of nanopore-sorbed contaminant was studied by adding water, methanol, or methane to the dry helium stream during TCE desorption. TCE desorption fluxes increased significantly with water (1.6 times) and methanol (8.2 times), whereas methane showed no effect, indicating that small relatively polar molecules can dislodge TCE from the sorption sites. More data are needed to conclusively interpret these results but a preliminary analysis suggests that sorption in hydrophobic nanopores could play a role. Hydrophobic nanopores are pores of the size of a TCE molecule in which water sorption is energetically and entropically unfavored. The fact that the impact of water and methanol on desorption flux is evident within a relatively short time (~42 min) suggests that the diffusion from such nanopores into larger pores is a relatively rapid process.
Hydrolytic transformation of DCP in the nanopores of LLNL soil was studied by measuring DCP and its hydrolysis product desorbed from soil columns. In bulk solution, DCP hydrolyzes mainly via dehydrohalogenation:
CH3CCl2CH3 = CH3C(Cl) = CH2+ H++ Cl-
At 25°C, the rate in bulk water is 3.18 × 10-4 min-1 with an activation energy (Ea) of 111.1 ± 2.0 kJ/mol. The compound has been chosen because the rate is pH independent and the half-life (t½) of 45.9 hrs at 25°C is within the range of interest of these experiments. The rate increases with temperature and t½ at 50°C is 1.06 hrs. With a 10-hour reaction time at 50°C (equivalent to 9.4 times t½), DCP is expected to transform nearly completely (to > 99.99%). In contrast to this prediction, desorption profiles indicate that the conversion of DCP to 2-chloropropene is significantly slowed. In the fully wet (5.10% water) and semi-wet (1.94% water) soil columns, only 15.5% and 21.3% of the total DCP is converted, respectively. The fact that the DCP desorption profiles show unhydrolyzed DCP being released from slow desorbing sites is consistent with the hypothesis of DCP sorption in water-free nanopores.
Hydrolysis of DCP sorbed on a silica gel (nanoporous Davisil 60 Å) with known pore characteristics was studied under fully wet condition, which was achieved by draining the water for 2 hours from a column that has been previously saturated with water for two days. Water content in the silica gel column is relatively high (0.48 g/g dry silica), and small water droplets and meniscus were visible in the column. After 10 hrs’ reaction time at 50°C, 10.5% DCP was recovered from this water-rich system. This observation also confirms that DCP hydrolysis in the columns was not limited by the availability of liquid phase water. Hydrolysis of DCP in LLNL soil (previously equilibrated with water vapor for more than 3 months) at room temperature (for 2 months, which is equivalent to 31.2 times t½) and at elevated temperature (50°C for 10 hrs, which is equivalent to 9.4 times t½) was also compared. The mass balances of DCP recovered from the two columns are comparable (81.0% and 82.6%). A greater fraction of DCP was recovered as unhydrolyzed from the column set at the room temperature (30.2%) than from the one that had been heated to 50°C (6.1%), although DCP under the former condition should have undergone a greater extent of hydrolytic transformation. The greater extent of DCP hydrolysis observed in the heated columns results from the experimental procedure of heating the column to 50°C immediately after DCP sorption. There was not sufficient time for DCP molecules to diffuse into nanopores when the heating started, which accelerated the reaction rate of DCP by ~43 times. Although the diffusivity of DCP molecules is also higher at 50°C, more ended up undergoing dehydrochlorination before diffusing into the nanopores, where they could be preserved.
The sorption and hydrolytic transformation of BzC on LLNL soil was also studied. BzC desorption fluxes at 50°C and 90°C were higher than expected based on hydrolysis data, which is consistent with BzC sorption in nanopores.
In summary, our experimental data show that reactive (i.e., hydrolysable) contaminants sorbed in slow desorbing sites of geological solids react significantly slower than in bulk solution, suggesting that the contaminants reside in an environment that is essentially excluded from water. Conversely, steric and energetic factors hinder exchange between the sorption sites and bulk solution, thus preventing hydrolysis. As a result, the halogenated hydrocarbon molecules in hydrophobic nanopores are not in contact with water molecules and are prevented from hydrolysis.
Students Working on the Project
Hefa Chang, Ph.D. candidate, Department of Civil and Environmental Engineering, Stanford University.
Future Activities:We plan to substantiate and quantify these findings by conducting further column experiments using a wider range of materials and conditions.
Journal Articles:No journal articles submitted with this report: View all 3 publications for this subproject
Relevant Websites:
http://wrhsrc.oregonstate.edu/ 
Progress and Final Reports:
Original Abstract
2005 Progress Report
Main Center Abstract and Reports:
R828772 HSRC (2001) - Western Region Hazardous Substance Research Center for Developing In-Situ Processes for VOC Remediation in Groundwater and Soils
Subprojects under this Center:
(EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
R828772C001 Developing and Optimizing Biotransformation Kinetics for the Bio- remediation of Trichloroethylene at NAPL Source Zone Concentrations
R828772C002 Strategies for Cost-Effective In-situ Mixing of Contaminants
and Additives in Bioremediation
R828772C003 Aerobic Cometabolism of Chlorinated Aliphatic Hydrocarbon Compounds with Butane-Grown Microorganisms
R828772C004 Chemical, Physical, and Biological Processes at the Surface of Palladium Catalysts Under Groundwater Treatment Conditions
R828772C005 Effects of Sorbent Microporosity on Multicomponent Fate
and Transport in Contaminated Groundwater Aquifers
R828772C006 Development of the Push-Pull Test to Monitor Bioaugmentation
with Dehalogenating Cultures
R828772C007 Development and Evaluation of Field Sensors for Monitoring
Bioaugmentation with Anaerobic Dehalogenating Cultures for In-Situ Treatment of
TCE
R828772C008 Training and Technology Transfer
R828772C009 Technical Outreach Services for Communities (TOSC) and Technical Assistance to Brownfields Communities (TAB) Programs
R828772C010 Aerobic Cometabolism of Chlorinated Ethenes by Microorganisms that Grow on Organic Acids and Alcohols
R828772C011 Development and Evaluation of Field Sensors for Monitoring Anaerobic Dehalogenation after Bioaugmentation for In Situ Treatment of PCE and TCE
R828772C012 Continuous-Flow Column Studies of Reductive Dehalogenation with Two Different Enriched Cultures: Kinetics, Inhibition, and Monitoring of Microbial Activity
R828772C013 Novel Methods for Laboratory Measurement of Transverse Dispersion in Porous Media
R828772C014 The Role of Micropore Structure in Contaminant Sorption and Desorption