Mineral-aqueous solution interfaces and their impact on the environment

This Perspective describes 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. The importance of these interactions, together with those between microorganisms and mineral surfaces, cannot be overestimated, for they 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. Following overviews of our scientific careers to date, including acknowledgments of our former and current students and research collaborators, we highlight some of the scientific contributions of Victor Goldschmidt, Irving Langmuir, Linus Pauling, Konrad Krauskopf, Werner Stumm, and others that led directly or indirectly to the evolution of this field, recalling personal interactions with some of these pioneers. In all fields of science, advances are made when new experimental methods, new characterisation 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. We (GB and GC) offer our perspectives on the development of synchrotron radiation sources and their applications to mineral-water interface processes, based on our personal experiences starting in the early days of these major user facilities. We discuss some of these new methods and theories and their applications to mineral-water interface processes through various examples. Because of the complexity of mineral-water interfaces, particularly in natural Earth surface environments, where natural organic matter and microorganisms play very important roles, we adopt a reductionist approach and consider simple model systems of increasing complexity in our quest to understand the chemical processes occurring at these interfaces at the molecular level. We start with a discussion of the acid-base chemistry of metal-oxide surfaces in contact with bulk water and empirical models of the electrical double layer (EDL) that is thought to develop at solid-water interfaces. We then consider experimental and theoretical studies of the EDL, which show that the classical Helmholtz-Gouy-Chapman-Stern-Grahame model is qualitatively correct. Following a story about some of the new, controversial research on the structure of water, we discuss (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, (5) the adsorption of organic molecules at mineral-water interfaces, (6) the effect of organic and microbial biofilm coatings on the reactivity of mineral surfaces, and (7) the effect of particle size on the structure and properties of nanoparticles, using ferrihydrite as an example of a natural nanomineral and silver nanoparticles as an example of an engineered nanoparticle. In keeping with the spirit of Geochemical Perspectives, we interspersed personal experiences resulting from our involvement in most of these research areas. To put the results of this more basic research in context we end by discussing selected applications of mineral-water interface geochemistry to environmental and Earth science problems, including (1) sorption reactions in real environmental systems that were anthropogenically perturbed, focusing on lead-polluted sites in the USA and France and As-polluted areas in southern Asia and France, (2) dissolution and weathering mechanisms of silicate minerals and zircon, (3) the interaction of aluminum with diatom surfaces, (4) the interaction of cobalt with manganese oxides, (5) the role of mineral-surface reactions in isotope fractionation, (6) the role of mineral surfaces in mineral-carbonation reactions, and (7) the surface chemistry associated with alteration of nuclear waste glasses. We finish with our thoughts on what has and has not been learned about mineral-water interface processes over the past 30 years and offer our opinions about some of the exciting research opportunities in this field that await the next generation of geochemists.