Structure of Water in Unsaturated Geologic Media and Its Influence on Solute Chemical InteractionsEPA Grant Number: U916171
Title: Structure of Water in Unsaturated Geologic Media and Its Influence on Solute Chemical Interactions
Investigators: Hay, Michael B.
Institution: Princeton University
EPA Project Officer: Michaud, Jayne
Project Period: January 1, 2003 through January 1, 2006
Project Amount: $102,000
RFA: STAR Graduate Fellowships (2003) Recipients Lists
Research Category: Fellowship - Geology , Academic Fellowships , Ecological Indicators/Assessment/Restoration
The objective of this research project is to probe water structure and solute behavior within water films and bulk-water contact lines ("menisci") on a variety of solid substrates to answer the following questions: (1) How does the molecular-scale structure of water in the form of films and lenses on mineral surfaces differ from that in bulk water? (2) How does the concentration of chemical species change as a function of position within a water meniscus? (3) To what degree are surface complexation and solute adsorption affected by water-layer thickness?
Understanding the fate of contaminants in soil water systems necessitates an indepth knowledge of both the physical properties (e.g., soil structure, hydraulic conductivity) and chemical phenomena (sorption/phase partitioning, speciation, transformation) governing solute transport in geologic media. Of particular interest (and difficulty) is an understanding of the behavior of solutes in the unsaturated zone, which serves not only as the spatial intermediate between the soil surface and underlying groundwater, but also as host to many important plant and microbial processes. Traditionally, studies on chemical behavior of solutes in solution and at mineral-water/air-water interfaces have involved speciation and kinetic behavior in bulk solution. Application of these results to soil solutions in the vadose zone, where water exists in the form of thin films and pendular rings joining mineral grains, is highly questionable. Many properties of aqueous solutions differ close to an interface, including dielectric constant1, hydrogen-bonding dynamics2, and ion concentration (i.e., electric double-layer formation). There is much to be understood, however, regarding the effects of these properties on thermodynamic equilibria and kinetics, particularly in the case of thin films and menisci (where this "interfacial water" composes a significant fraction of the water present). Extremely thin water films (~10 nm) have the added complexity of overlapping interfaces (e.g., air-water-mineral), which affects both water thermodynamic activity and solute mobility by diffusion in ways that are not entirely understood.
Experimental techniques will primarily involve infrared (IR), ultraviolet-visible, and x-ray spectroscopic/spectromicroscopic methods for obtaining spatially resolved chemical information, coupled with ellipsometric and interferometric methods for determining water-layer thickness. Previous researchers have probed the instantaneous2,3 and time-averaged4 hydrogen-bonding dynamics of bulk water using soft x-ray and IR absorption spectroscopy, respectively. Instrumentation for the analysis of thin-film water using these methods is under development for use with existing synchrotron light source facilities. Specifically, a high-vacuum sample chamber for temperature- and humidity- (film thickness) controlled x-ray absorption measurements has been designed and is under construction. Future studies also will involve x-ray and IR microscopy to assess water structure with position at a meniscus. Studies on solute behavior also can be performed using these techniques; future investigations will utilize the x-ray spectroscopy vacuum chamber, as well as the spectromicroscopy facilities, in the determination of solute concentrations, speciation, diffusion and reaction rates, and surface complex formation in controlled thin-film experiments (for comparison with bulk observations).
1. Sverjensky DA. Interpretation and prediction of triple-layer model capacitances and the structure of the oxide-electrolyte-water interface. Geochimica et Cosmochimica Acta 2001;65:3643-3655.
2. Wilson KR, et al. Characterization of hydrogen bond acceptor molecules at the water surface using near-edge x-ray absorption fine-structure spectroscopy and density functional theory. Journal of Physics: Condensed Matter 2002;14:L221-L226.
3. Myneni S, et al. Spectroscopic probing of local hydrogen-bonding structures in liquid water. Journal of Physics: Condensed Matter 2002;14:L213-L219.
4. Libnau FO, Toft J, Christy AA, Kvalheim OM. Structure of liquid water determined from infrared temperature profiling and evolutionary curve resolution. Journal of the American Chemical Society 1994;116:8311-8316.