Dissolution and Precipitation Dynamics at Environmental Mineral Interfaces Imaged by In Situ Atomic Force Microscopy

Chemical reactions at the mineral-solution interface control important interfacial processes, such as geochemical element cycling, nutrient recovery from eutrophicated waters, sequestration of toxic contaminants, and geological carbon storage by mineral carbonation. By time-resolved in situ imaging of nanoscale mineral interfacial reactions, it is possible to clarify the mechanisms governing mineral-fluid reactions. In this Account, we present a concise summary of this topic that addresses a current challenge at the frontier of understanding mineral interfaces and their importance to a wide range of mineral re-equilibration processes in the presence of a fluid aqueous phase. We have used real-time nanoscale imaging of liquid-cell atomic force microscopy (AFM) to observe the in situ coupling of the dissolution-precipitation process, whereby the dissolution of a parent mineral phase is coupled at mineral interfaces with the precipitation of another product phase, chemically different from the parent. These nanoscale observations allow for the identification of dissolution and growth rates through systematically investigating various minerals, including calcite (CaCO3), siderite (FeCO3), cerussite (PbCO3), magnesite (MgCO3), dolomite (CaMg(CO3)(2)), brushite (CaHPO4 center dot 2H(2)O), brucite (Mg(OH)(2)), portlandite (Ca(OH)(2)), and goethite (alpha-FeOOH), in various reacting aqueous fluids containing solution species, such as arsenic, phosphate, organo- or pyrophosphate, CO2, selenium, lead, cadmium, iron, chromium, and antimony. We detected the in situ replacement of these parent mineral phases by product phases, identified through a variety of analytical methods such as Raman spectroscopy, high-resolution transmission electron microscopy, and various X-ray techniques, as well as modeling by geochemical simulation using PHREEQC. As a consequence of the coupled processes, sequestration of toxic elements and hazardous species and inorganic and organic carbon, and limiting or promoting recovery of nutrients can be achieved at nano- and macroscopic scales. We also used in situ AFM to quantitatively measure the retreat rates of molecular steps and directly observe the morphology changes of dissolution etch pits on calcium phosphates in organic acid solutions present in most rhizosphere environments. By molecular modeling using density functional theory (DFT), we explain the origin of dissolution etch pit evolution through specific stereochemistry and molecular recognition and provide an energetic basis by calculating the binding energies of chemical functionalities on organic acids to direction-specific steps on mineral surfaces. In addition, we further quantified precipitation kinetics of calcium phosphates (Ca-P's) on typical mineral surfaces at the nanoscale in environmentally relevant solutions with various organic molecules, by measurements obtained from sequential images obtained by liquid-cell AFM. In situ dynamic force spectroscopy (DFS) was used to determine binding energies of single-molecules with different chemical functionalities found in natural organic matter at mineral-fluid interfaces. Quantifying molecular organo-mineral bonding or binding energies mechanistically explains phosphate precipitation and transformation. From DFS measurements, molecular-scale interactions of mineral-natural organic matter (DNA, proteins, and polysaccharides) associations were determined. With this powerful tool, singlemolecule determinations of polysaccharide-amorphous iron oxide or hematite interactions provided the mechanistic origin of the phase- or facet-dependent adsorption. These systematic investigations and findings significantly contribute to a more quantitative prediction of the fate of nutrients and contaminants, chemical element cycling, and potential geological carbon capture and nuclear waste storage in aqueous environments.