Likwan Cheng

Resolving orthoclase dissolution processes with atomic force microscopy and X-ray reflectivity

Abstract Direct measuremens of orthoclase (001) were performed using in situ atomic force microscopy (AFM) and synchrotron X-ray reflectivity to reveal the A-scale dissolution process as a function of pH and temperature. Distinct processes were observed, involving mainly terrace roughening at pH = 1.1 and step motion at pH = 12.9. A gel-like surface coating was observed to form at acidic pH under slow fluid flow-rate conditions. No coating was observed either at alkaline pH or at acidic pH under high fluid flow-rate conditions. The corresponding dissolution rates were measured directly at pH = 1.1 and 12.9 at ∼50°C using real-time X-ray reflectivity measurements, and reacted interface structures were derived from crystal truncation rod measurements after reaction at both acidic and alkaline pH. Our observations reveal, under these experimental conditions, that 1) orthoclase dissolution is controlled by at least two separate surface reactions having distinct reactive sites; 2) dissolution is stoichiometric at alkaline pH and only minimally nonstoichiometric (limited to one unit-cell depth) at acidic pH; previously identified nonstoichiometric layer thicknesses derived from macroscopic measurements are associated with the formation of the gel-like coatings; 3) dissolution rates measured at freshly cleaved (001) surfaces are comparable to those derived from steady-state powder dissolution rates for both alkaline and acidic pH; and 4) elevated transient dissolution rates are not observed for freshly cleaved surfaces but are obtained under alkaline conditions after reacting the orthoclase (001) surface at acidic pH. These observations clarify differences in orthoclase dissolution mechanisms as a function of pH, demonstrate the utility of AFM and X-ray scattering methods for measuring A-scale structures and face-specific dissolution rates on single crystals and place new constraints on the understanding of alkali feldspar weathering processes.

Lead adsorption at the calcite-water interface: Synchrotron x-ray standing wave and x-ray reflectivity studies

Abstract By combining synchrotron X-ray standing wave (XSW) measurements with synchrotron X-ray reflectivity measurements, we have determined: (1) the precise three-dimensional location within the calcite unit cell of submonolayer Pb ions adsorbed at the calcite (104) surface from dilute aqueous solutions, and (2) the precise one-dimensional location of these unit cells relative to the calcite surface. Our XSW measurements, using three separate calcite Bragg reflections for triangulation, show that most adsorbed Pb ions occupy Ca sites in the calcite lattice with an ordered coverage of 0.05 equivalent monolayers, while the remaining Pb ions are disordered with a coverage of 0.03 equivalent monolayers. Our X-ray reflectivity measurements show that the ordered Pb ions occur primarily (>70%) in the surface atomic layer of calcite. Atomic force microscopy (AFM) was used to characterize the topography of the calcite (104) surface under conditions similar to the X-ray experiments. The quantitative morphological information obtained by AFM was used to develop realistic models of the calcite surface. The calculated X-ray reflectivities for these model surfaces were compared with the measured X-ray reflectivities. The new combined X-ray method that we have developed can be used to determine the atomic-scale structure of other metals adsorbed at mineral-water interfaces. Such high-resolution structural determinations are essential before detailed conceptual and theoretical models can be further developed to understand and predict the behavior of dissolved metals in mineral-water systems.

X-RAY STANDING WAVE STUDY OF ARSENITE INCORPORATION AT THE CALCITE SURFACE

Abstract The location and orientation of arsenite incorporated at the CaCO3 (10 1 4) cleavage surface from a dilute aqueous solution was examined with the X-ray standing wave (XSW) technique. The high coherent fractions measured for As on the (10 1 4) and (0006) Bragg reflections indicate that the arsenite was well ordered in registration with the calcite surface lattice. The As coherent positions show that arsenite was located at the carbonate site. The XSW analysis is consistent with a structural model in which the pyramidal-shaped arsenite ion was oriented with its oxygen base coincident with the carbonate plane, and its As apex pointing outward from the (10 1 4) surface. The structure observed here for arsenite, and previously for selenite, suggests that other pyramidal trioxyanions could be incorporated at the calcite (10 1 4) surface with the same geometry.

Structure of the orthoclase (001)- and (010)-water interfaces by high-resolution x-ray reflectivity.

High-resolution in situ X-ray specular reflectivity was used to measure the structures of orthoclase (001) and (010) cleavage surfaces in contact with deionized water at 25°C. X-ray reflectivity data demonstrate a high degree of structural similarity between these two orthoclase-water interfaces. Both interfacial structures include cleavage along the plane of minimal bond breakage resulting in surfaces terminated by non-bridging oxygens; structured water within 5 A of the orthoclase surface (consisting of adsorbed species at the surface and layered water above the surface), with a featureless water profile beyond 5 A; substitution of outermost K+ ions by an oxygen containing species (presumably H3O+); and small structural displacements of the near surface atoms. The interfacial water structure, in comparison with recent results for other mineral-water interfaces, is intermediate between the minimal structure found at calcite-, barite-, and quartz-water interfaces and the more extensive structure found at the muscovite-water interface.

Otavite-calcite solid-solution formation at the calcite-water interface studied in situ by synchrotron X-ray scattering

Abstract Synchrotron X-ray scattering measurements were performed in situ during the formation of thin (50–600 A) overgrowths of otavite-calcite solid-solutions at the (10 1 4) cleavage surface of single-crystal calcite. These solid-solutions were precipitated from EDTA-bearing aqueous solutions having varied initial saturation states of otavite and calcite. From repetitive X-ray diffraction scans, the Cd/(Ca + Cd) ratios and the effective thicknesses (average domain size perpendicular to the calcite cleavage surface) of the solid-solutions were determined as a function of time. Additional in-plane X-ray diffraction scans were done to further characterize the relationship between the solid-solutions and the calcite cleavage surface. The solid-solution phase grew epitaxially with a (1014) growth plane oriented parallel to the calcite (10 1 4) cleavage surface. The compositions of the solid-solutions evolved with time, while their growth rates (increases in effective thickness) remained fairly constant (10–54 A/hr). In each experiment, the coverage of the initial surface by the. solid-solution (calculated from the difference between the initial and final Cd concentrations in the aqueous solution) was about 20%. Glancing-incidence X-ray reflectivity scans were also monitored as a function of time. From these scans, we determined that the solid-water interface did not become significantly rougher during the nucleation and growth of the solid-solution phase. These observations indicate that the solid-solution grew by layer spreading and that most growth may have occurred preferentially at macrostep faces produced during cleavage.

Orthoclase dissolution kinetics probed by in situ X-ray reflectivity: Effects of temperature, pH, and crystal orientation

Initial dissolution kinetics at orthoclase (001) and (010) cleavage surfaces were measured for ∼2 to 7 monolayers as a function of temperature using in situ X-ray reflectivity. The sensitivity of X-ray reflectivity to probe mineral dissolution is discussed, including the applicability of this approach for different dissolution processes and the range of dissolution rates (∼10−12 to 10−6 mol/m2/sec) that can be measured. Measurements were performed at pH 12.9 for the (001) surface and at pH 1.1 for the (001) and (010) surfaces at temperatures between 46 and 83°C. Dissolution at pH 12.9 showed a temperature-invariant process with an apparent activation energy of 65 ± 7 kJ/mol for the (001) cleavage surface consistent with previous powder dissolution results. Dissolution at pH 1.1 of the (001) and (010) surfaces revealed a similar process for both surfaces, with apparent activation energies of 87 ± 7 and 41 ± 7 kJ/mol, respectively, but with systematic differences in the dissolution process as a function of temperature. Longer-term measurements (five monolayers) show that the initial rates reported here at acidic pH are greater than steady-state rates by a factor of 2. Apparent activation energies at acidic pH differ substantially from powder dissolution results for K-feldspar; the present results bracket the value derived from powder dissolution measurements. The difference in apparent activation energies for the (001) and (010) faces at pH 1.1 reveals an anisotropy in dissolution kinetics that depends strongly on temperature. Our results imply a projected ∼25-fold change in the ratio of dissolution rates for the (001) and (010) surfaces between 25 and 90°C. The dissolution rate of the (001) surface is higher than that of the (010) surface above 51°C and is projected to be lower below this temperature. These results indicate clearly that the kinetics and energetics of orthoclase dissolution at acidic pH depend on crystal orientation. This dependence may reflect the different manifestation of the Al-Si ordering between the T1 and T2 tetrahedral sites at these two crystal faces and can be rationalized in terms of recent theoretical models of mineral dissolution.

High-resolution structural study of zinc ion incorporation at the calcite cleavage surface

The atomic-scale structure of Zn{sup 2+} incorporated at the CaCO{sub 3} (10{ovr 1}4) surface by adsorption from solution was determined by X-ray standing wave triangulation and surface extended X-ray absorption fine structure spectroscopy. At low coverage (approximately 0.1 ML), Zn{sup 2+} substitutes for Ca{sup 2+} in the surface layer. Structural relaxation of the adjacent in-plane CO{sup 2-}{sub 3} ions in the host surface is shown by the reduced nearest-neighbor distance of Zn-O relative to Ca-O. Relaxation of the Zn{sup 2+} ion in the out-of-plane direction is shown by the displacement of its lattice position from the ideal Ca{sup 2+} position. These relaxations, resulting in a local lattice buckling feature at the Zn{sup 2+} adsorption site, can be fully explained as the combined effect of the electrostatic relaxation of the nearest-neighbor anions in response to the smaller size of Zn{sup 2+}, and the bonding asymmetry due to surface truncation.

X-ray standing wave investigation of the surface structure of selenite anions adsorbed on calcite.

The adsorption of selenite ions (SeO2−3) from a dilute aqueous solution onto a freshly-cleaved calcite (1014) surface was studied with the X-ray standing wave (XSW) technique. The complex ion SeO2−3 is found to selectively adsorb at the CO2−3 site via ionic exchange, forming a two-dimensional solid-solution of the form Ca(SeO3)x(CO3)1 − x at the interface. The calcite (1014), (0006) and (1120) Bragg reflections were used to triangulate the Se position with respect to the calcite lattice. The local surface structure at the SeO2−3 adsorbate site, derived from the XSW results, is consistent with a model in which the base of the SeO2−3 trigonal pyramid aligns with (and replaces) the CO2−3 equilateral triangular group. The SeO2−3 adsorption saturated at a coverage of 0.02 monolayers. Under identical chemical conditions, selenate (SeO2−4) adsorption was inhibited.

Structure of the fluorapatite (100)-water interface by high-resolution X-ray reflectivity

Abstract A complete understanding of the surface chemistry of the apatite-water system requires direct observation of the interfacial structure at the molecular scale. We report results for the structure of the apatite (100)-water interface obtained with high-resolution specular X-ray reflectivity from a natural growth surface of Durango fluorapatite. A uniform termination at the crystallographic unit-cell boundary was determined. An atomistic model of the interfacial structure is compared to the experimental results and optimized through non-linear least-squares fitting in which the structural parameters were selected to be both physically and chemically plausible. The best-fit structure includes a Ca- and/or F-deficient outermost surface, minimal structural relaxations of the near-surface apatite crystal, and the presence of a layered interfacial water structure exhibiting two distinct water layers. The height of the first water layer is 2.64(9) Å relative to the relaxed surface with 3.5(1.3) water molecules per surface unit-cell area (64.9 Å2). A second layer of adsorbed water is found 1.53(5) Å above the first layer, followed by a nearly featureless profile of the bulk liquid water. The layered structure of water is interpreted as being due to hydrogen bonding at the solid-water interface. The interfacial structure shows a strong similarity with the octacalcium phosphate structure projected along a surface normal direction.