Title: Western Region Hazardous Substance Research Center Project 1-SU-03
Effects of Sorbent Microporosity on Multicomponent Fate and Transport in Contaminated Groundwater Aquifers

Investigators: Martin Reinhard

Institution: Stanford University

Research Category: Groundwater, transport

Project Period: 2001-2003

Goal: The overall goal of this project was to develop a better understanding of organic contaminant sequestration by geosorbents. Specific project goals were to (1) develop a method to characterize microporosity in geological solids in the presence of moisture, (2) determine how micropore hydrophobicity/hydrophilicity and contaminant properties influences contaminant sequestration and desorption,and quantify the interactions among multiple contaminants during uptake in and release from micropores. Information gained from this research allows us to better predict contaminant bioavailability, the rate of natural attenuation processes, and the time scale of contaminant release from natural sorbents.

Rationale: Micropores are pores (less than 2 nm in diameter) that are comparable in size to small organic contaminant molecules, and the sorption potential inside these pores are significantly enhanced due to the proximity of the opposite pore walls. Understanding contaminant sequestration in micropores is essential for predicting the long-term fate of contaminants in groundwater aquifers, and for assessing the significance of natural attenuation processes. Most natural solids contain micropores that form due to weathering, cracking, material imperfections, or turbostratic stacking. Previous work has demonstrated that sorption of hydrophobic organic compounds in micropores can be a significant sequestering process. Sorption in micropores is reversible but rates are very slow and difficult to quantify, especially in the field. Our understanding of geosorbent microporosity and its effect on contaminant sorption is limited because conventional microporosity characterization methods used (vacuum piezometric and gravimetric techniques) employ only a single “model” sorbate and are not sensitive enough to detect the low volumes of micropores typically present in geological solids. Furthermore, our understanding of sorption in micropores is inadequate to predict solid-contaminant interactions based on pore volume and pore size distribution data obtained with these methods.

Approach: The first task was to develop and validate a methodology for measuring slow uptake and release rates of volatile organic compounds (VOCs) by microporous solids. To validate the method, the sorption and desorption of simple sorbates, such as methane, carbon tetrachloride, and trichloroethylene on model sorbents, such as silica gel were studied. Subsequently, the methodology was applied to study contaminant sorption and desorption on natural sorbents and characterize their microporosity under environmentally relevant conditions.

The focus of this investigation was study how contaminant properties (e.g., molecular size, structure, and polarity) and environmental variables (such as relative humidity and temperature) affect micropore sequestration. Sorption and desorption kinetics of model contaminants on microporous engineered solids and natural solids under different conditions were measured with an apparatus developed as part of this project. The interactions between contaminant molecules and microporous sorbents and the influence of the sorbate and sorbent properties were elucidated by comparing contaminant sorption and desorption kinetics under these conditions.

Summary of Findings: An apparatus has been developed for measuring slow sorption and desorption kinetics of VOCs on solid materials packed in columns. A HP 5890 II GC equipped with FID and ECD detectors is used to analyze gas phase compositions at the inlet and outlet of the column in rapid sequence. Samples of the gas stream that enters and leaves the column are alternatively injected into the GC column (through an valve injector), and the contaminant mixtures are subsequently separated in GC column and detected by both FID and ECD. This design has expanded our investigative capabilities in several ways: data are acquired in real-time with high temporal resolution over the entire contaminant desorption profile; contaminant detection is extremely sensitive (0.1 nmol/L), and sorption and desorption of multiple volatile organic contaminants can be studied. Only a relatively small amount of solid (packed in a column of 3.0 mm i.d. 304.8 mm length) is required because of the system’s high resolution, and gas flows through the column at 2.00 mL/min regulated by a digital mass flow controller. Constant vapor concentrations of organic contaminant and water in the flow line are achieved by bubbling the gas through organic liquid and water reservoirs submerged in a constant temperature water bath. Sorption of TCE on several solid materials as a function of temperature and moisture content were studied.

Data show that high concentrations of water vapor lead to rapid displacement of contaminant molecules sorbed/condensed in mesopores in a natural soil, a process that may be described as “chromatographic elution.” By contrast, desorption of contaminants sequestered in micropores is significantly slower. We hypothesized that narrow throats in pores restrict the exchange of background gas molecules with contaminant molecules in the deeper micropores. Data indicated that water molecules have very small effect on the desorption rate of TCE sequestered in micropores in a natural soil, and the kinetics of TCE sorption. In an engineered microporous sorbent (silica gel) desorption was barely influenced by the presence of water molecules. These results suggest that both hydrophilic and hydrophobic micropores exist in these solids, and that sorption and desorption of hydrophobic species (TCE) only occurs in hydrophobic micropores that are not accessible to water molecules. That is, hydrophobic and hydrophilic species are sequestered into two separate micropore-domains (hydrophobic micropores and hydrophilic micropores) independently, and there is no apparent competitive effect between them. This suggests that the affinity of micropores for water, rather than the total volume of the micropores, plays key role in controlling contaminant sequestration and desorption. Also, contaminants sequestered in hydrophobic micropores may not be in contact with water molecules, which prevents it from undergoing chemical and biological transformations (e.g., hydrolysis, biodegradation) during natural attenuation processes. This hypothesis was evaluated in the subsequent phase.

 

Publications:

Journal Articles

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

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.

 

Thesis

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

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