Smith Lecture: Mineral-Aqueous Solution Interfacial Reactions and Their Impact on the Environment
My talk will focus on an area of geochemistry that involves minerals, their surfaces, and the interactions of these surfaces with water and the ions and molecules present in water, some of which, like arsenic, are highly toxic to organisms. These interactions, together with those between microorganisms and mineral surfaces, control the composition of our natural environment and mitigate some of the anthropogenic perturbations that are changing our environment in ways that are often unpredictable and sometimes detrimental. I will highlight some of the scientific contributions of Victor Goldschmidt, Irving Langmuir, Linus Pauling, Konrad Krauskopf, and Werner Stumm that led directly or indirectly to the evolution of this field. In all fields of science, advances are made when new experimental methods, new characterization and computational tools, and new theories become available. The field of mineral-water interface geochemistry is no different and has advanced significantly over the past 30-40 years due to enormous changes in molecular-level experimental methods, particularly those involving the extremely intense X-ray sources known as synchrotrons, in digital computers, and in molecular-level theories. I will discuss some of these new methods and theories and their applications to mineral-water interface processes through various examples, including (1) experimental and theoretical studies of the reaction of water with metal-oxide surfaces, (2) the structure of hydrated mineral surfaces, (3) the uptake of cations and anions on metal-oxide surfaces, (4) X-ray absorption spectroscopy studies of lead and arsenic adsorption complexes at mineral-water interfaces in simplified model systems, and (5) the effect of organic and microbial biofilm coatings on the reactivity of mineral surfaces. To put these model system studies in context I will discuss selected applications of mineral-water interface geochemistry to environmental problems, including sorption reactions in real environmental systems that were anthropogenically perturbed, focusing on mercury-polluted sites in California, lead polluted sites in Colorado and central France, and As-polluted areas in southern Asia and France. I will conclude with thoughts on what has and has not been learned about mineral-water interface processes over the past 30 years and offer my opinion about some of the exciting research opportunities in this field that await the next generation of geochemists.