Dr. Borrok’s research examines the fate and transport of metals in geologic and biologic systems. This includes understanding and quantifying the role of microorganisms and natural organic molecules in the fate and transport of trace metals, and the application of transition metal isotope chemistry to biogeochemical and environmental problems. Specific research projects include:

• The biogeochemistry of stable Cu, Zn, and Fe isotopes. Recent work in this area has focused on the development of isotopic fractionation factors for bacteria-metal interactions, including reversible cell-surface adsorption and irreversible cellular uptake. Students are performing laboratory experiments to isolate these reactions. Subsequent analysis of the metal isotopes of the experimental components will allow us to rigorously define isotopic fractionation factors for these reactions (NSF EAR 0745345; 0820986).
• The use of Zn and Fe isotopes to understand the watershed-scale biogeochemistry of streams (and groundwater) impacted by metal contamination. Part of this focus involves the use of these metal isotopes to help resolve anthropogenic and natural contamination sources. This work is being conducted with the U.S. Geological Survey in several Alpine watersheds in the Rocky Mountains in Colorado (NSF EAR 0838120; 0820986).
• Corrosivity of crushed rock/soil materials used for construction. Here we develop laboratory testing protocols and models for the quantification (and prediction) of parameters like resistivity, pH, and the SO4/Cl contents of waters in contact with construction materials. These models will be used to determine the likelihood of corrosive failure for metallic reinforcements. This work is being done with collaborators in Civil Engineering at UTEP.
• The incorporation of trace elements into bacterially mediated mineral precipitates. Understanding the mobility of trace metals and metal-radionuclides is critical in assessing the environmental impact of contaminants and evaluating nuclear waste disposal options.  One pathway for the immobilization of trace elements is their incorporation into mineral phases. Trace element-substituted minerals may also have important materials science applications and/or might be used to trace historic bacterial processes. This work is being done with collaborators in Materials Science and Engineering at UTEP.
• Thermodynamic modeling of bacterial-metal adsorption (and natural organic matter-metal complexation) reactions.  Both bacteria and NOM can affect the speciation of metal cations in near-surface environments through complexation reactions, impacting the chemistry, fate, toxicity, and mobility of metals. For example, bacteria attached to mineral surfaces can immobilize metals, while bacterial cells and dissolved NOM molecules in the water column can enhance metal mobilities through complexation reactions.
• The link between metal adsorption onto bacterial cell surfaces and metal toxicity.  Heavy metals are the most common contaminants in soil and water environments, and can be remarkably toxic to plants and animals.  Bacteria are perhaps the most sensitive and important indicators of metal bioavailability and toxicity in natural settings, but current bioavailability models cannot be applied to bacteria because of limitations in model framework that stem from a lack of quantitative information regarding metal binding and toxicity processes within bacterial cells.

David received his B.S. degree from the University of Missouri at Rolla (1995), M.S. degree from the University of Michigan (1997), and Ph.D. from the University of Notre Dame (2005).  David also has extensive work experience in ore deposit exploration (Cu, Zn, Pb, Ag, and Au in the U.S., Europe, and South America), and hazardous waste investigation and remediation.

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