Steven R. Higgins

The structure of hematite (α-Fe2O3) (001) surfaces in aqueous media: scanning tunneling microscopy and resonant tunneling calculations of coexisting O and Fe terminations

The iron oxide–water interface is of interest not only in geochemical and environmental processes, but also in fields ranging from corrosion to photocatalysis. The structure of α-Fe2O3 (001) surfaces is not fully understood, and questions have arisen recently concerning different terminations of (001) terraces; a so-called Fe-termination is expected, but under some conditions an O-termination may also be possible. Ultra-high vacuum (UHV) scanning tunneling microscope (STM) studies report evidence for an O-termination in coexistence with an Fe-termination, but other studies find no evidence for an O-termination. Molecular mechanics studies suggest that an O-termination should be possible in an aqueous environment. An O-termination could result from dissolution; if Fe atoms were to dissolve from an Fe-termination, an O-termination would presumably result (and vice-versa). We imaged hematite (001) surfaces in air and aqueous solution using STM. To aid interpretation of the images, we use a resonant tunneling model (RTM) parameterized using ab initio calculations. Our STM and RTM results are consistent with mixed O- and Fe-terminated (001) surfaces. For acid-etched surfaces we find evidence for a periodic (with wavelength of 2.2 ± 0.2 nm) arrangement of nominal O- and Fe-terminated domains. Two different borders between domains should occur, one in which the Fe-termination is high relative to the O-termination, and the reverse. The different domain borders have significantly different heights, consistent with RTM calculations. This agreement allows us to conclude that the Fe-termination is topographically high on most terraces. Surface domains are observed in aqueous solutions at the atomic scale, and appear to be very unreactive on tens-of-seconds time scales at pH 1.

Dissolution kinetics of magnesite in acidic aqueous solution, a hydrothermal atomic force microscopy (HAFM) study: Step orientation and kink dynamics

The dissolution kinetics of features on the magnesite (104) surface were studied in aqueous solutions from pH 4.2 to 2 and at temperatures between 60 and 90°C by hydrothermal atomic force microscopy (HAFM). At pH = 4.2, HAFM images showed magnesite step orientations that are comparable to the step orientations on calcite. Similar to calcite (104), there is anisotropy in the step velocity, but the magnitude of the anisotropy is much greater for magnesite. Furthermore, below pH = 4.2, changes in the dominant step orientation were observed. These results are discussed in terms of a nearest neighbor kink dynamic model, and the associated kink dynamics were tested with kinetic Monte Carlo (KMC) simulations. The KMC results suggest that the kink dynamic model does not account for the experimental observations and that further details such as second-nearest neighbor interactions or surface/edge diffusion cannot be excluded from the model. The dominant step orientations at low pH also point toward mechanisms stabilizing steps along periodic bond chain directions.

A hydrothermal atomic force microscope for imaging in aqueous solution up to 150 °C

We present the design of a contact atomic force microscope (AFM) that can be used to image solid surfaces in aqueous solution up to 150 °C and 6 atm. The main features of this unique AFM are: (1) an inert gas pressurized microscope base containing stepper motor for coarse advance and the piezoelectric tube scanner; (2) a chemically inert membrane separating these parts from the fluid cell; (3) a titanium fluid cell with fluid inlet–outlet ports, a thermocouple port, and a sapphire optical window; (4) a resistively heated ceramic booster heater for the fluid cell to maintain the temperature of solutions sourced from a hydrothermal bomb; and (5) mass flow control. The design overcomes current limitations on the temperature and pressure range accessible to AFM imaging in aqueous solutions. Images taken at temperature and pressure are presented, demonstrating the unit-cell scale (<1 nm) vertical resolution of the AFM under hydrothermal conditions.

Acidic dissolution of plagioclase: In-situ observations by hydrothermal atomic force microscopy

Hydrothermal atomic force microscopy (HAFM) provides in situ access to the surfaces of dissolving crystals at temperatures above the ambient boiling point of water. Here, we applied HAFM to the (001) surfaces of labradorite and anorthite at temperatures up to 125°C. In HCl solutions (pH 2) we observed the formation of a rough and soft surface layer on both minerals. By applying high loading forces to the scanning tip, the soft layer can be removed and the underlying interface (between the fresh solid and the altered layer) can be observed. In this way, in situ information about the thickness of the altered layer on plagioclase and the morphology of the underlying interface can be obtained. On labradorite, the thickness of this layer does not exceed about 30 nm within the first 5 hr of exposure to acidic solution at 125°C, but on anorthite thicknesses of up to about 300 nm were observed. The uncovered interface on anorthite shows a nonuniform morphology and either appears rough in AFM images or shows a step-like pattern. On anorthite, etch pits spread underneath the altered layer. This suggests that material must be released and transported through the layer without obvious changes in morphology of the layer’s surface. Based on the rate of spreading of etch pits, the dissolution rate was calculated to be about 2 × 10−6 mol m−2 s−1 at 125°C. This value agrees reasonably well with literature data and supports the suggestion that dissolution mainly takes place underneath the altered layer and not on its surface.

Point of zero charge of a corundum-water interface probed with optical second harmonic generation (SHG) and atomic force microscopy (AFM): New approaches to oxide surface charge

Abstract The p H and ionic strength dependence of light generated at a corundum-solution interface by the nonlinear optical process of second harmonic generation (SHG) is reported. A point of zero salt effect occurs in the p H range 5 to 6. The p H and ionic strength dependence of the SHG is qualitatively consistent with a model describing SHG from a charged mineral/water interface from Ong et al. (1992) and Zhao et al. (1993a, 1993b) , but certain aspects of the model appear inadequate to describe the full range of our data. Atomic force microscopy (AFM) force-distance measurements, though imprecise, were consistent with a point of zero charge (p.z.c.) for the interface also in the p H range 5 to 6. The SHG (and AFM) results are different from expectation; the observed p.z.s.e. (and presumably also the p.z.c.) is considerably lower than the accepted point of zero charge of clean alumina powders ( p H 8–9.4; Parks, 1965; Sverjenksy and Sahai, 1996) . Although the reasons for this are unclear, SHG holds promise as a probe of oxide-water interfaces that is independent of interpretation of acid-base titration stoichiometry.

Dissolution kinetics of magnesite in acidic aqueous solution: a hydrothermal atomic force microscopy study assessing step kinetics and dissolution flux

Magnesite (104) dissolution kinetics were studied in acidic aqueous solutions (2.0 < pH < 4.2) at temperatures between 60 and 90°C by atomic force microscopy (AFM). Comparison of dissolution fluxes obtained by AFM and chemical methods revealed six to seven times larger dissolution fluxes obtained by chemical analysis. Corresponding empirical activation energies were found to be 74 ±22 kJ/mol and 41 ± 4 kJ/mol (at pH 4.2) for the AFM and chemical methods, respectively. The empirical reaction order with respect to proton concentration was 0.36 ± 0.13 and 0.47 ± 0.03 for AFM and chemical methods, respectively. These comparisons suggest that the two experimental measurement methods differ as a result of the different sampling length scales associated with the methods. Negligible changes in step dissolution velocity with changes in bulk pH were found, suggesting that the principal source of increasing dissolution flux with decreasing pH is an increase in step density. However, the observed stable step orientation, which is dependent on pH, suggests that more than one proton adsorption equilibrium should be used to describe the surface chemistry of magnesite in acidic solution.

Dissolution of the periclase (001) surface: A scanning force microscope study

Abstract In contrast to most ionic minerals studied by SFM, the periclase (001) surface dissolves not by retreat of monolayer steps parallel to (001), but by retreat of a rough surface perpendicular to (001). At pH < 2, dissolution has an additional contribution from retreating macro-steps at the edges of nearly square pits. The macro-steps have heights up to 120 nm. In general, step direction is parallel to [110] and equivalent directions. During dissolution at low pH, a soft near-surface region is formed. Other investigators have shown that the near-surface region is protonated. Protonation is supposed to stabilize the (111) surface of periclase. Due to the structural similarities between periclase (111) and brucite (001), and similar dissolution rates of periclase and brucite at pH < 5, we conclude that during dissolution the periclase (001) surface is restructured into ‘‘(111) nano-facets’’ representing brucite (001)-like layers and appearing as a rough and soft surface in SFM images. The most probable reasons that the slopes of these macro-steps (up to 508) are lower than the slopes of perfect (111) facets are the likely poorly ordered structure of these layers, microtopography on these surface facets, and tip-surface convolution in SFM. By measuring the vertical position of the surface vs. time, we calculated the dissolution rate. At pH 1 and pH 2 we found the rates to be 17.1 ± 5.8 × 10-6 and 5.7 ± 3.7 × 10-6 mol/m2·s, respectively. These rates are in reasonable agreement with previously reported rates of periclase and brucite (001) dissolution, and are consistent with the idea that the MgO (001) surface consists of Mg(OH)2 (001)-like layers.

Growth and dissolution kinetics at the dolomite-water interface: An in-situ scanning probe microscopy study

Abstract Dissolution and epitaxial growth on dolomite [CaMg(CO3)2] cleavage surfaces under well-defined aqueous solution conditions were studied in real-time by atomic force microscopy (AFM). Step-retreat speeds in undersaturated solutions were used to predict the growth speed for ordered dolomite utilizing a theoretical step-growth model. Monolayer growth on dolomite under highly supersaturated conditions was observed, but multilayer growth was inhibited due to film disorder enhanced solubility. Ordered dolomite film growth is predicted in supersaturated solutions close to equilibrium, but observed step speeds were immeasurably low under the current laboratory low-temperature conditions.

Spatially-resolved electrochemistry of the lead sulfide (galena) (001) surface by electrochemical scanning tunneling microscopy

Abstract Electrochemical scanning tunneling microscopy (STM) has been used to investigate the chemical and electrochemical reactions of naturally-occurring lead sulfide (galena) (001) surfaces. Both oxidation and reduction reactions are shown to be spatially anisotropic, involving almost exclusively atoms located at step edges. STM images show that the oxidation reaction PbS → Pb 2+ + S + 2e − occurs by removal of atoms from step edges, and that impurity-related defects on the surface lead to etch pit formation under oxidizing conditions. STM images at ambient lead concentrations reveal that the electrochemical reduction PbS + 2H + + 2e − → Pb 0 + H 2 S occurs exclusively by removal of material from step edges, with simultaneous formation of Pb(111) islands on the PbS terraces. Re-oxidation of the Pb islands causes regrowth of PbS at the step edges and the simultaneous disappearance of Pb islands from the terraces, indicating that this reaction is partially reversible at the atomic level. Using atomic-resolution images to establish the crystallographic orientation, it is shown that the rate of removal of material from step edges is directionally anisotropic and is fastest along the 〈100〉 directions, leaving behind steps with outward normal directed along the equivalent 〈110〉 directions.

Bioaugmentation with Distinct Dehalobacter Strains Achieves Chloroform Detoxification in Microcosms

Chloroform (CF) is a widespread groundwater contaminant not susceptible to aerobic degradation. Under anoxic conditions, CF can undergo abiotic and cometabolic transformation but detoxification is generally not achieved. The recent discovery of distinct Dehalobacter strains that respire CF to dichloromethane (DCM) and ferment DCM to nonchlorinated products promises that bioremediation of CF plumes is feasible. To track both strains, 16S rRNA gene-based qPCR assays specific for either Dehalobacter strain were designed and validated. A laboratory treatability study explored the value of bioaugmentation and biostimulation to achieve CF detoxification using anoxic microcosms established with aquifer material from a CF-contaminated site. Microcosms that received 6% (v/v) of the CF-to-DCM-dechlorinating culture Dhb-CF to achieve an initial Dehalobacter cell titer of 1.6 ± 0.9 × 10(4) mL(-1) dechlorinated CF to stoichiometric amounts of DCM. Subsequent augmentation with 3% (v/v) of the DCM-degrading consortium RM to an initial Dehalobacter cell abundance of 1.2 ± 0.2 × 10(2) mL(-1) achieved complete DCM degradation in microcosms amended with 10 mM bicarbonate. Growth of the CF-respiring and the DCM-degrading Dehalobacter populations and detoxification were also observed in microcosms that received both inocula simultaneously. These findings suggest that anaerobic bioremediation (e.g., bioaugmentation) is a possible remedy at CF- and DCM-contaminated sites without CT, which strongly inhibited CF organohalide respiration and DCM organohalide fermentation.