The Environmental Chemical sciences (ECS) program of the Division of Chemistry will support the research program of Prof. Franz Geiger of Northwestern University. Prof. Geiger's research program focuses on the application for the first time of a unique spectroscopic method, named the Eisenthal chi(3) method, to study the adsorption of metal cations on mineral surfaces. The Eisenthal chi(3)method is uniquely sensitive to the interfacial potential and can be viewed as an Optical Voltmeter. If successful, the study will provide environmental scientists with a new and widely applicable tool for studying ion adsorption on mineral surfaces. The project has the potential to transform our understanding of interfacial phenomena in complex environmental systems. The project will provide excellent educational opportunities for trainees desiring to work at the forefront of environmental science.
798 words Problem Statement. Just like two equally poled magnets repel one another, charge-repulsion among positively charged toxic species may lead to changes in the charge state of the surface bound species, which would yield less repulsion and allow for a orders of magnitude larger surface excess of the metal species. Despite the large effects that charge-charge interactions are predicted to have on how far a toxic metal species may travel in soil, concrete experimental evidence for this effect has not yet been provided. Intellectual Merit: The experiments carried out under this grant provide the first concrete, as opposed to hypothetical, data into how a variety of doubly and triple charged metal contaminants change their charge state, as predicted, while other do not. Broader Impacts. This project has provided important new benchmarks for existing theoretical and mechanistic geochemical frameworks and computer simulations. The quantitative results also improve the reliability of predicting the impact of human activities on the environment. By directly and indirectly interacting with students and policy makers, our integrated research, education, and public outreach activities are powerful tools for communicating our scientific efforts to the general public with the goal of educating the general public about the scientific method. Brief Summary of Outcomes (13 publications to date, a few select summarized here) Second harmonic generation (SHG) and the χ(3) technique were used to investigate the effect of oxalic acid on the adsorption of aluminum [Al(III)] to the fused silica/water interface. Al(III) adsorption isotherms were measured in the presence and absence of oxalic acid and analyzed to quantify thermodynamic binding parameters. These data revealed important information about the surface activity and speciation of Al(III) in the presence of oxalic acid that have direct implications for the mobility, toxicity, and ultimate environmental fate of this metal pollutant. In related work, we studied chromium remediation by iron films in the environment. Atomic layer deposition was used to deposit a 10 nm film of polycrystalline α-Fe2O3 (hematite) onto a fused silica substrate which was analyzed using SHG. Specifically, the χ(3) technique was used to investigate the adsorption of Cr(III) and Cr(VI) to the hematite/water interface under flow conditions at pH 4 with 10 mM NaCl. The results of this investigation suggest that the use of hematite in permeable reactive barriers, for cost-effective chromium remediation, allows for Cr(III) remediation at very low concentrations through adsorptive and redox processes but quickly renders the barriers ineffective at high chromium concentrations due to surface saturation. Preceding this work was the following: Iron oxides are a ubiquitous class of compounds that are involved in many biological, geological, and technological processes, and the Fe(III)/Fe(II) redox couple is a fundamental transformation pathway; however, the study of iron oxide surfaces in aqueous solution by powerful spectroscopic techniques has been limited due to "strong absorber problem". In this work, atomic layer deposition (ALD) thin films of polycrystalline α-Fe2O3 were analyzed using the Eisenthal χ(3) technique. By determining the surface charge densities at multiple pH values, the point of zero charge was found to be 5.5 ± 0.3. We were also interested in pushing our NLO methods to materials others than oxides. To this end, we recorded uranyl adsorption at the muscovite (mica)/water interface. Using the nonresonant χ3 technique and the Gouy–Chapman model, the initial surface charge density of the mica surface was determined to be −0.022(1) C/m2 at pH 6 and in the presence of dissolved carbonate. This work provides benchmark measurements to be used in the improvement of contaminant transport modeling. Functionalized surfaces were studied as well: The interaction of Zn2+ ions with undecanol-functionalized fused silica/water interfaces was studied directly at the aqueous/solid interface. We characterized the surface functionalization using vibrational sum frequency generation (SFG) and X-ray photoelectron spectroscopy (XPS). We then employed the SHG χ(3) technique to determine the degree of silane functionalization, track Zn2+ adsorption directly at the hydroxyl-terminated undecanol silane-functionalized fused silica/aqueous interface at pH 7 and 10 mM NaCl concentration, determine the electrostatic and thermodynamic binding parameters, quantify the change in interfacial potential upon zinc ion adsorption, and compare these values to our previous work with glucosamine-functionalized and bare fused silica/water interfaces, with direct implications for controlling and predicting coordination chemistry. Fundamental studies regarding charge states at interfaces include the following: The binding constants, adsorption free energies, absolute adsorbate number densities, and interfacial charge densities of Al(III), Sc(III), Y(III), La(III), and Gd(III) interacting with fused silica/water interfaces held at pH 4 were determined using SHG and the Eisenthal χ(3) technique. By quantifying the exponential sensitivity of the surface coverage of the adsorbed ions to their charge state directly at the fused silica/water interface, we provided benchmarks for theory calculations describing the interactions of metal ions with oxide interfaces in geochemistry and hope to improve the prediction of trivalent metal ion transport through groundwater environments.
