Kathryn L. Nagy

Structures of quartz (100)- and (101)-water interfaces determined by x-ray reflectivity and atomic force microscopy of natural growth surfaces

Abstract The structures of prismatic (100) and pyramidal (101) growth faces of natural quartz crystals, and their modification upon annealing at T ≤ 400°C were investigated ex situ by atomic force microscopy (AFM) and in water by high-resolution X-ray reflectivity. AFM images revealed the presence of ∼ 0.1 to 1 μm-wide flat terraces delimited by steps of one to several unit cells in height. These steps follow approximately directions given by the intersection of growth faces. Modeling of X-ray reflectivity data indicates that surface silica groups on flat terraces have only one free Si-O bond each (presumably hydroxylated), except for some having two free Si-O bonds observed on a single (100) surface. Vertical relaxation of atomic positions (

Atomic-scale structure of the orthoclase (001)-water interface measured with high-resolution x-ray reflectivity.

In situ X-ray specular reflectivity and atomic force microscopy were used to determine the structure of the orthoclase (001) cleavage surface in contact with deionized water at 25°C. These are the first in situ measurements of the orthoclase–water interface structure performed to Angstrom-scale resolution. The orthoclase (001) cleavage surface has minimal roughness, and only one of two possible surface terminations is exposed. The X-ray data show that (1) the silica network at the orthoclase surface is terminated by an oxygen-containing species (e.g., O or OH) having a coverage of 1.9 ± 0.25 ML (the expected coverage is 2.0 ML, where 1 ML = 1 atom/55.76 A2), (2) the outermost layer of K+ ions have been removed with a derived coverage of 0.0 ± 0.08 ML (the bulk truncated K+ coverage is 1.0 ML), and (3) a complex relaxation profile affecting the near-surface structure propagates ∼26 A into the orthoclase with a maximum relaxation of ∼0.15 A near the surface. These data are inconsistent with K+ ion depletion below the topmost K+ layer. These results provide a new baseline for understanding the initial steps of the feldspar dissolution process, demonstrate the power of combining X-ray scattering techniques with scanning probe microscopies for understanding the intrinsic characteristics of complex mineral–water interface systems, and suggest a new approach for understanding feldspar dissolution mechanisms.

Quantifying surface areas of clays by atomic force microscopy

Abstract Rapid and accurate determination of the surface area of three kaolinite clay standards, taking into account the complex microtopography of the particles, was achieved using atomic force microscopy images and computerized image analysis. All surface areas were determined to within 3%. Edge surface area is 18.2-30.0% of the total surface area depending on the particular kaolinite standard. Specific surface areas agree to within 4% of published values determined by the BET method. The approach can be applied to clay and nanoparticle samples too small in quantity for BET analysis, since it requires ~11 orders of magnitude less material.

All-atom ab initio energy minimization of the kaolinite crystal structure

Abstract Calculations that minimize the energy and optimize the geometry of all atomic coordinates for two proposed kaolinite crystal structures were performed using a first-principles, quantum chemical code based on local density functional theory. All calculations were performed using published unit-cell parameters. Inner- and interlayer H atom positions agree well with those determined by Bish (1993) from neutron diffraction data and confirm a unit cell with C1 symmetry.

Gibbsite growth kinetics on gibbsite, kaolinite, and muscovite substrates: atomic force microscopy evidence for epitaxy and an assessment of reactive surface area

Abstract New experimental data for gibbsite growth on powdered kaolinite and single crystal muscovite and published data for gibbsite growth on gibbsite powders at 80°C in pH3 solutions show that all growth rates obey the same linear function of saturation state provided that reactive surface area is evaluated for each mineral substrate. Growth rate (mol m−2 s−1) is expressed by Rateppt = (1.9 ± 0.2) × 10−10|ΔGr|/RT(0.90±0.01), which applies to the range of saturation states from ΔGr = 0 to 8.8 kJ mol−1, where ΔGr = RT[ln(Q/K)] for the reaction Al3+ + 3H2O = Al(OH)3 + 3H+, and equilibrium defined as ΔGr = 0 was previously determined. Identification of the growth phase as gibbsite was confirmed by rotating anode powder x-ray diffraction. Rates on kaolinite were determined using steady-state measured changes between inlet and outlet solutions in single-pass stirred-flow experiments. Rates on muscovite were determined by measuring the volume of precipitated crystals in images obtained by Tapping Mode™ atomic force microscopy (TMAFM). In deriving the single growth rate law, reactive surface area was evaluated for each substrate mineral. Total BET surface area was used for normalizing rates of gibbsite growth onto powdered gibbsite. Eight percent of the BET surface area, representing the approximate amount occupied by the aluminum octahedral sheet exposed at crystal edges, was used for powdered kaolinite. The x - y area of the TMAFM images of the basal surface was used for single crystal muscovite. Linearity of growth rates with saturation state suggests that the dominant nucleation and growth mechanisms are two dimensional. Such mechanisms are supported by observations of the morphologies of gibbsite crystals grown on muscovite at ΔGr = 8.8 kJ mol−1. The morphologies include (1) apparent epitaxial films as determined by hexagonal outlines of edges and thicknesses of 30 to 40 A, (2) elongate crystals 30 to 40 A thick aligned with the structure of the distorted Si-tetrahedral sheet of the 2M1 muscovite, and (3) micrometer-scale three-dimensional clumps of intergrown crystals. Reactive surface area as defined now for heterogeneous crystal growth in reactive-transport models must be modified to include substrates other than that of the growing mineral and to account for the role of structural and chemical controls on epitaxial nucleation and growth.

Quantification of minor phases in growth kinetics experiments with powder X-ray diffraction

Abstract Minor amounts of clay minerals precipitated from aqueous solution can be rapidly identified and quantified in a mineral mixture with powder X-ray diffraction using a rotating-anode source and a position-sensitive detector. For the case of gibbsite precipitated on a kaolinite powder substrate we demonstrate a simple method having a minimum detection limit of 0.1 wt%, using pure gibbsite as the intensity reference in mechanical mixtures of gibbsite and kaolinite. The amount of gibbsite precipitated onto kaolinite at 80 °C, pH 3 is higher when determined from solution chemistry than from the X-ray method, and the difference in amounts increases with increasing Al concentration in solution. This discrepancy can be explained by assuming that a fraction of the precipitated material is effectively invisible to the X-ray diffraction technique, either due to a small diffracting domain size along the gibbsite [001] direction or formation of an Al-phase that is amorphous to X-rays. This method should be generally useful for a range of mineral mixtures where at least one intense reflection for the phase of interest is not obscured. The ability to identify, characterize, and quantify trace phases by X-ray diffraction, especially when combined with surface analysis by electron or atomic force imaging, is an important complement to the conventional approach of monitoring solution composition in growth kinetics experiments.

Protective coatings for concrete

The new two-layer protective coating developed for monuments constructed of limestone or marble was applied to highway cement and to tobermorite, a component of cement, and tested in batch dissolution tests. The goal was to determine the suitability of the protective coating in retarding the weathering rate of concrete construction. The two-layer coating consists of an inner layer of aminoethylaminopropylsilane (AEAPS) applied as a 25% solution in methanol and an outer layer of A2** sol-gel. In previous work, this product when applied to calcite powders, had resulted in a lowering of the rate of dissolution by a factor of ten and was shown through molecular modeling to bind strongly to the calcite surface, but not too strongly so as to accelerate dissolution. Batch dissolution tests at 22 C of coated and uncoated tobermorite (1.1 nm phase) and powdered cement from Gibson Blvd. in Albuquerque indicated that the coating exhibits some protective behavior, at least on short time scales. However, the data suggest that the outer layer of sol-gel dissolves in the high-pH environment of the closed system of cement plus water. Calculated binding configuration and energy of AEAPS to the tobermorite surface suggests that AEAPS is well-suited as the inner layer binder for protecting tobermorite.

Phase Chemistry of Tank Sludge Residual Components

We are attempting to understand the solid phase chemistry of the high level nuclear waste (HLW) stored in tanks at Hanford. Because this waste is compositionally complex, our approach is to study experimentally the aging dynamics of simplified systems whose bulk chemistry approximates that of the tank sludges. After a basic understanding of these dynamics has been attained we plan to increase the compositional complexities one component at a time, in order to assess the influence of each component. Results will allow for reliable prediction of sludge phase chemistry over a range of sludge compositions. Iron and aluminum comprise the bulk of most HLW sludges, so we chose to begin by studying the behavior of iron-aluminum systems. Fe/Al ratios were chosen to approximate those relevant to the solutions that produced the sludge. Aluminum and iron concentrations in the various process fluids are summarized and compared to our experimental starting solutions in Table 1 (process solution data from Krumhansl, personal communication, 1998). Our low aluminum experiments serve as direct analogues to both Bismuth Phosphate and low-Fe PUREX waste. Cornell and Giovanoli (1985) found that, in a pure iron system at 70 C, a 10-fold or even 50-fold increase in suspension concentration had only very slight effects on the final aged products. Since our experiments have similar Al/Fe ratios to some high Fe-PUREX process solutions our results are probably relevant to those wastes as well. However, our results may not apply to the high-Fe and high-Al PUREX wastes, as discussed below. The high Al experiments were designed specifically to simulate REDOX waste.

Phase chemistry of tank sludge residual components. 1998 annual progress report

The proposed research will provide a scientific basis for predicting the long-term fate of radionuclides remaining with the sludge in decommissioned waste tanks. Nuclear activities in the United States and elsewhere produce substantial volumes of highly radioactive semi-liquid slurries that traditionally are stored in large underground tanks while final waste disposal strategies are established. Although most of this waste will eventually be reprocessed a contaminated structure will remain which must either be removed or decommissioned in place. To accrue the substantial savings associated with in-place disposal will require a performance assessment which, in turn, means predicting the leach behavior of the radionuclides associated with the residual sludges. The phase chemistry of these materials is poorly known so a credible source term cannot presently be formulated. Further, handling of actual radioactive sludges is exceedingly cumbersome and expensive. This proposal is directed at: (1) developing synthetic nonradioactive sludges that match wastes produced by the various fuel processing steps, (2) monitoring the changes in phase chemistry of these sludges as they age, and (3) relating the mobility of trace amounts of radionuclides (or surrogates) in the sludge to the phase changes in the aging wastes. This report summarizes work carried out during the first year of a three year project. A prerequisite to performing a meaningful study was to learn in considerable detail about the chemistry of waste streams produced by fuel reprocessing. At Hanford this is not a simple task since over the last five decades four different reprocessing schemes were used: the early BiPO{sup 4} separation for just Pu, the U recovery activity to further treat wastes left by the BiPO{sup 4} activities, the REDOX process and most recently, the PUREX processes. Savannah River fuel reprocessing started later and only PUREX wastes were generated. It is the working premise of this proposal that most of the phase chemistry in the wastes was defined when the acidic process fluids were first neutralized prior to storage. The only notable exception being that some silicates obviously formed later under highly caustic conditions. Waste stream chemistries for each process have been established and surrogate sludges prepared. Aging of these different recipes have begun at 25, 60, and 90 C and the phase chemistry of the different mixes is being monitored.