2005 Progress Report: The Role of Micropore Structure in Contaminant Sorption and Desorption

EPA Grant Number: R828772C014
Subproject: 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, 2004 through August 31, 2005
RFA: Hazardous Substance Research Centers - HSRC (2001) RFA Text |  Recipients Lists
Research Category: Hazardous Waste/Remediation , Land and Waste Management

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, equilibrated, and incubated for weeks to months at predetermined temperatures. 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 has been calibrated using sorbents with known porosity (silica gel), zeolites with surface properties ranging from polar to hydrophobic, and sorbates with known hydrolysis rates—trichloroethylene (TCE), which is practically unreactive, and 2,2-dichloropropene (2,2-DCP) which reacts with water to 2-chloropropane.

Status

Initial studies focused on the non-reactive (TCE) and the one reactive model substrates (2,2-DCP) as the sorbates, (synthetic) silica, zeolites, and the clay and silt fraction (< 50 μm) of soil from a site at the Lawrence Livermore National Laboratory (LLNL), as the sorbent. 2,2-DCP sorption data obtained at different soil moisture contents confirmed that the sorption capacity decreases significantly as the moisture content increases. Data indicate that water displaces 2,2-DCP from sorption sites in micropores as the moisture content increases. However, water did not completely eliminate the sorption capacity for 2,2-DCP, and a small but significant amount of 2,2-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. More recent sorption data obtained using zeolites and TCE shows that hydrophobic compounds displace water from hydrophobic micropores.

Method development and system evaluation using a model silica gel and a real sediment from a previously studied aquifer ha ve been completed and reported (submitted). It was confirmed that hydrophobic micropores play a significant role in controlling the long-term release of hydrophobic organic contaminants. This is a significant factor affecting the times it takes to remediate sites. We have developed a technique for quantifying the total and the hydrophobic micropore volumes based on the mass of TCE sorbed in the slow-releasing pores under dry and wet conditions. The micropore environment in which organic molecules were sorbed in the presence of water was probed by studying the transformation of a water-reactive compound (2,2-DCP). For sediment from an alluvial aquifer, the total micropore volume was estimated to be between 1.56 and 3.75 μL/g, while its hydrophobic micropore volume was only 0.022 μL/g. In a microporous silica gel, a hydrophobic micropore volume of 0.038 μL/g was measured. The d ehydrohalogenation rate of 2,2-DCP sorbed in hydrophobic micropores was slower than that reported in bulk water, which is indicative of an environment of low water activity. The results suggest that hydrolyzable organic contaminants sorbed in hydrophobic micropores may be preserved for many times longer than their half-lives in water, consistent with the reported persistence of reactive contaminants in natural soils. Although the hydrophobic micropores represent a small fraction of the total micropore volume, the significant amounts of hydrophobic contaminants stored in them may pose long-term risk to groundwater quality.

More recent work focused on sorption of TCE in zeolites with a range of hydrophobic surface properties. We have elucidated the mechanism of hydrophobic organic compound sorption in mineral micropores by studying the water sorption and thermal dehydration behaviors of three dealuminated Y zeolites, and sorption of TCE in partially dehydrated zeolites and wet zeolites (equilibrated with saturated water vapor). Zeolites of higher Si/Al ratios exhibited lower affinity for water sorption and lost water more easily during dehydration. It was also observed that the high silica zeolites, both partially dehydrated and wet, could sorb more TCE than the low Si/Al zeolite under the same conditions. Experimental results suggest that the density of hydrophilic centers (surface cations and hydrogen bonding sites) on the pore wall surface of micropores plays a key role in water sorption and determines their hydrophobicity. The enhanced dispersion interactions of TCE molecules are only strong enough to displace the loosely bound water molecules from the hydrophobic micropores, while water molecules coordinated to surface cations and the hydrogen bonded water molecules are unaffected. The results indicate that sorption of hydrophobic organic molecules in hydrophobic micropores occurs through displacing the weakly sorbed water molecules in them and organic molecules co-exist with the strongly sorbed water molecules in them.

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 to some extent, 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 less exposed to water molecules and are prevented from hydrolysis.

Future Activities:

With method development completed and reported, and results of sorption in hydrophobic micropores from model solids and sediments evaluated and reported, we are now moving towards the study of redox reactions in model systems.


Journal Articles on this Report : 3 Displayed | Download in RIS Format

Other subproject views: All 3 publications 3 publications in selected types All 3 journal articles
Other center views: All 158 publications 63 publications in selected types All 60 journal articles
Type Citation Sub Project Document Sources
Journal Article Cheng H, Reinhard M. Sorption of trichloroethylene in hydrophobic micropores of dealuminated Y zeolites and natural minerals. Environmental Science & Technology 2006;40(24):7694-7701. R828772 (Final)
R828772C014 (2005)
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  • Journal Article Cheng H, Reinhard M. Measuring hydrophobic micropore volumes in geosorbents from trichloroethylene desorption data. Environmental Science & Technology 2006;40(11):3595-3602. R828772 (Final)
    R828772C014 (2005)
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  • Journal Article Cunningham JA, Deitsch JJ, Smith JA, Reinhard M. Quantification of contaminant sorption-desorption time-scales from batch experiments. Environmental Toxicology and Chemistry 2005;24(9):2160-2166. R828772 (Final)
    R828772C014 (2005)
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  • Relevant Websites:

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    Progress and Final Reports:

    Original Abstract
  • 2002
  • 2003
  • 2004 Progress Report
  • Final

  • 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