Title: Western Region Hazardous Substance Research Center Project 2-SU-05
Sorption and Hydrolysis of Halogenated Hydrocarbons in Soil Nanopores

Investigator: Martin Reinhard

Institution: Stanford University

Research Category: Groundwater, transport

Project Period: January 2003- August 2007

 

Objectives: The overall goal of this project was to develop a better understanding of the impact of soil nanopores on the fate and transport of halogenated hydrocarbon contaminants. Specific project objectives were to: (1) study the kinetics of slow sorption and desorption of halogenated hydrocarbons in aquifer sediment, and (2) determine effect of sorption on contaminant reactivity. Results allow us to better predict natural attenuation of hydrocarbon compounds in aquifers and assess the risks associated with groundwater aquifers contaminated by halogenated hydrocarbons.

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.

Summary of Findings: A novel analytical system was developed that allowed us to simultaneously study sorption and transformation of volatile organics in geological sorbents. The system consists of the previously developed soil column chromatograph (Project 1-SU-03), 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.

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 mm) 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 real sediment from a previously studied aquifer has 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 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 mL/g, while its hydrophobic micropore volume was only 0.022 mL/g. In a microporous silica gel, a hydrophobic micropore volume of 0.038 mL/g was measured.

Dehydrohalogenation 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 showed that reactive, i.e., hydrolysable contaminants sorbed in slow desorbing sites of geological solids reacted 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 hindered exchange between the sorption sites and bulk solution, thus preventing hydrolysis. As a result, the halogenated hydrocarbon molecules in hydrophobic nanopores were less exposed to water molecules and were prevented from hydrolysis.

Publications: 

Journal Articles

Cheng, H. and M. Reinhard (2006). Quantifying the Volume of Hydrophobic Micropores from Trichloroethylene Desorption. Environmental Science and Technology, 40 (11), 3595-3602.

Cheng, H. and M. Reinhard (2006). Sorption of Trichloroethylene in Hydrophobic Micropores of Dealuminated Y Zeolites and Natural Minerals, Environmental Science and Technology, 40 (24), 7694-7701.

Cheng, H. and M. Reinhard (2007). Sorption and Inhibited Dehydrohalogenation of 2,2-Dichloropropane in Micropores of Dealuminated Y Zeolites. Environmental Science and Technology, 41 (6), 1934-1941.

Cunningham, J.A., J.J. Deitsch, J.A. Smith and M. Reinhard (2005). Quantification of Contaminant Sorption-Desorption Time-Scales from Batch Experiments. Environmental Toxicology and Chemistry, 24 (9), 2160-2166.

 

Abstracts and Posters

Cheng, H. and M. Reinhard (2005). Inhibition of 2,2-dichloropropane dehydrohalogenation by micropore sorption. The 230th ACS National Meeting, Washington, DC (Aug 28-Sept 1).

 

Theses

Cheng, H. (2006). Sorption and Hydrolysis of Chlorinated Aliphatic Hydrocarbons in Hydrophobic Micropores, Ph.D., Stanford University.

 

Supplemental Keywords: groundwater; soil; characterization; chlorinated solvent; VOCs; environmental chemistry