The presence of mineral surfaces and of microorganisms significantly impacts the nature and composition of mineral phases formed in biogeochemical systems. Despite the importance of these heterogeneous reactions, predictions of the fate of biogeochemical systems relevant to DOE often rely solely on bulk thermodynamics due to a lack of robust theoretical tools that can predict heterogeneous mineral nucleation and growth. Our understanding of the molecular-scale processes of contaminant immobilization in mineral phases formed at mineral/water and mineral/microbe interfaces remains limited as a result. The long-term vision of this research is therefore to develop a quantitative understanding of contaminant immobilization in mineral precipitates at mineral/water and mineral/microbe interfaces to enable accurate predictions of the fate of metal contaminants in heterogeneous biogeochemical systems.
Immobilization of divalent metal cations in carbonate precipitates formed at carbonate, silicate, and bacterial cell surfaces will be used as a model system due to the ubiquitous presence of carbonates in and major impact on soils, marine sediments, and suspended matter in fresh and sea waters. A combination of state-of-the-art experimental capabilities and molecular simulations will be deployed to overcome current limitations and quantify the effects of several thermodynamic and kinetic controlling factors (kinetics of water exchange, structural mismatch, and ability to form amorphous intermediates). Specifically, this research will investigate: (1) the effects of the kinetics of water exchange around cations on their ability to incorporate and intermix in epitaxial carbonate coatings; (2) any potential correlation between the ionic radius of metal contaminants and their ability to form carbonate coatings on non-isostructural mineral substrates; and (3) any enhancement in metal contaminant incorporation afforded by the formation of an amorphous calcium carbonate intermediate during microbially-induced calcite (CaCO3) precipitation.