Competitive Sorption of Pb(II) and Zn(II) on Polyacrylic Acid-Coated Hydrated Aluminum-Oxide Surfaces

Natural organic matter (NOM) often forms coatings on minerals. Such coatings are expected to affect metal–ion sorption due to abundant sorption sites in NOM and potential modifications to mineral surfaces, but such effects are poorly understood in complex multicomponent systems. Using poly­(acrylic acid) (PAA), a simplified analog of NOM containing only carboxylic groups, Pb­(II) and Zn­(II) partitioning between PAA coatings and α-Al₂O₃ (1–102) and (0001) surfaces was investigated using long-period X-ray standing wave-florescence yield spectroscopy. In the single-metal–ion systems, PAA was the dominant sink for Pb­(II) and Zn­(II) for α-Al₂O₃(1–102) (63% and 69%, respectively, at 0.5 μM metal ions and pH 6.0). In equi-molar mixed-Pb­(II)–Zn­(II) systems, partitioning of both ions onto α-Al₂O₃(1–102) decreased compared with the single-metal–ion systems; however, Zn­(II) decreased Pb­(II) sorption to a greater extent than vice versa, suggesting that Zn­(II) outcompeted Pb­(II) for α-Al₂O₃(1–102) sorption sites. In contrast, >99% of both metal ions sorbed to PAA when equi-molar Pb­(II) and Zn­(II) were added simultaneously to PAA/α-Al₂O₃(0001). PAA outcompeted both α-Al₂O₃ surfaces for metal sorption but did not alter their intrinsic order of reactivity. This study suggests that single-metal–ion sorption results cannot be used to predict multimetal–ion sorption at NOM/metal–oxide interfaces when NOM is dominated by carboxylic groups

Evolution of Strain in Heteroepitaxial Cadmium Carbonate Overgrowths on Dolomite

The evolution and accommodation of lattice strain in an epitaxial mineral film grown on an isostructural substrate were observed as a function of film thickness. Cadmium carbonate films (approximately CdCO₃ (otavite) in composition) were grown on the (104) surface of CaMg­(CO₃)₂ (dolomite) from aqueous solutions that were supersaturated with respect to both pure otavite and Cd-rich (Cd_1–<i>x</i>Ca<i>_x</i>)­CO₃. Specular and nonspecular X-ray reflectivity (XR) revealed that the structure and strain of the otavite overgrowths evolved in a manner that is fully consistent with a Stranski-Krastanov growth mode. Otavite films initially grew as coherently strained films, up to an average thickness of ∼15 Å, with lateral compressive strains and an expansion of the vertical film lattice spacing resulting in a unit cell volume consistent with pure otavite. Thicker films (>15 Å) became incommensurate with the substrate, having lattice parameters that are indistinguishable from pure otavite. These results indicate that the evolution of these mineral films is controlled by epitaxy and are consistent with the growth of essentially pure otavite films. These results provide a foundation for understanding the stability of thin-film overgrowths in the natural environment

Dynamic Stabilization of Metal Oxide–Water Interfaces

The interaction of water with metal oxide surfaces plays a crucial role in the catalytic and geochemical behavior of metal oxides. In a vast majority of studies, the interfacial structure is assumed to arise from a relatively static lowest energy configuration of atoms, even at room temperature. Using hematite (α-Fe₂O₃) as a model oxide, we show through a direct comparison of <i>in situ</i> synchrotron X-ray scattering with density functional theory-based molecular dynamics simulations that the structure of the (11̅02) termination is dynamically stabilized by picosecond water exchange. Simulations show frequent exchanges between terminal aquo groups and adsorbed water in locations and with partial residence times consistent with experimentally determined atomic sites and fractional occupancies. Frequent water exchange occurs even for an ultrathin adsorbed water film persisting on the surface under a dry atmosphere. The resulting time-averaged interfacial structure consists of a ridged lateral arrangement of adsorbed water molecules hydrogen bonded to terminal aquo groups. Surface p<i>K</i>_a prediction based on bond valence analysis suggests that water exchange will influence the proton-transfer reactions underlying the acid/base reactivity at the interface. Our findings provide important new insights for understanding complex interfacial chemical processes at metal oxide–water interfaces