William L. Bourcier

Aqueous alteration on the hydrous asteroids: Results of EQ3/6 computer simulations

Abstract In order to understand the course of aqueous alteration of the hydrous asteroids we constructed two models for carbonaceous chondrite precursor material, one (model A) based upon the anhydrous mineralogy of the abundant CM chondrites and the other (model B) based upon that of the equally abundant CV chondrites. We then proceeded to calculate the likely course of aqueous alteration on asteroids composed of these materials through the use of the EQ3/6 computer algorithm. We find that the alteration mineralogy of the CM chondrites (and by extension the CM parent asteroids) may best be produced by starting with the anhydrous mineralogy of the same CM chondrites, at temperatures of 1 to approximately 25°C, and at total solution carbon concentrations which vary from as little as 10−8m up to at least 10−2m. A wide range of rock/fluid ratios is permitted by this alteration mineralogy. Under these conditions solution pH is calculated to vary from 7 to just above 12. Solution Eh is calculated to vary from −0.5 to −0.75 V. We calculate that the mineralogy of the important CI chondrites can be well-reproduced by alteration of either CM or CV anhydrous material. This alteration is calculated to occur best for temperatures of 50 to (at least) 150°C, total solution carbon concentrations varying from approximately 10−3 to (at least) 10−2m, and a wide range of rock/fluid ratios. Under these conditions solution pH is calculated to vary from 7 to between 9 and 10, and Eh from −0.3 to −0.8 V, in the direction of increasing alteration. We therefore conclude that these were the principal conditions of aqueous alteration on the CI parent asteroids. The alteration assemblage observed for the CM chondrites is not produced by alteration of the CV chondrites under the modeling conditions imposed by this study, which suggests that the CM chondrites do not necessarily share the same parent asteroids with the CV chondrites. On a purely mineralogical basis, howerver, the CI chondrites could have been produced from either (or both) CM or CV chondrite material, and therefore be present on either type of parent asteroid.

Capacitive desalination with flow-through electrodes

Capacitive desalination (CD) is a promising desalination technique as, relative to reverse osmosis (RO), it requires no membrane components, can operate at low (sub-osmotic) pressures, and can potentially utilize less energy for brackish water desalination. In a typical CD cell, the feed water flows through the separator layer between two electrically charged, nanoporous carbon electrodes. This architecture results in significant performance limitations, including an inability to easily (in a single charge) desalinate moderate brackish water feeds and slow, diffusion-limited desalination. We here describe an alternative architecture, where the feed flows directly through electrodes along the primary electric field direction, which we term flow-through electrode (FTE) capacitive desalination. Using macroscopic porous electrode theory, we show that FTE CD enables significant reductions in desalination time and can desalinate higher salinity feeds per charge. We then demonstrate these benefits using a custom-built FTE CD cell containing novel hierarchical carbon aerogel monoliths as an electrode material. The pore structure of our electrodes includes both micron-scale and sub-10 nm pores, allowing our electrodes to exhibit both low flow resistance and very high specific capacitance (>100 F g−1). Our cell demonstrates feed concentration reductions of up to 70 mM NaCl per charge and a mean sorption rate of nearly 1 mg NaCl per g aerogel per min, 4 to 10 times higher than that demonstrated by the typical CD cell architecture. We also show that, as predicted by our model, our cell desalinates the feed at the cells RC timescale rather than the significantly longer diffusive timescale characteristic of typical CD cells.

Direct observation of reactive flow in a single fracture

We carried out a laboratory experiment to examine the relationship between local rate of dissolution and local aperture during flow of a slightly acidic aqueous solution through a rough fracture in Carrara marble under a confining pressure of 0.2 MPa. Fracture surfaces were digitized in three dimensions before the fluid flow tests and after the tests. Digital reconstruction of the aperture then allowed numerical simulation of flow patterns, and digital comparison of surfaces before and after dissolution allowed mapping of patterns of dissolution. We observed that both mean aperture and fracture permeability decreased as a result of dissolution. Despite the low confining pressure, the experiments thus simulate dissolution in deeply buried formations, in contrast to the gaping and karst formation that occur under vanishingly low confining pressure in the shallow crust. We observed that the growth of new pathways for flow changed from stable to unstable as length scale increased. At the millimeter scale the fracture aperture evolved in stable fashion from a strongly heterogeneous arrangement of tortuous flow channels to a smoother topography, while at the scale of the full rock (50 mm), the aperture developed a single, broad flow channel. The scale dependence of the dissolution pattern may be the result of changes with scale in extent of reaction (i.e., the Damkohler number) and in the relative importance of diffusion (the Peclet number). Finally, we also see evidence of a negative relationship between local fluid flux and local rate of dissolution in some locations in the fracture.

A Kinetic Model for Borosilicate Glass Dissolution Based on the Dissolution Affinity of a Surface Alteration Layer

A kinetic model for the dissolution of borosilicate glass, incorporated into the EQ3/6 geochemical modeling code, is used to predict the dissolution rate of a nuclear waste glass. In the model, the glass dissolution rate is controlled by the rate of dissolution of an alkalidepleted amorphous surface (gel) layer. Assuming that the gel layer dissolution affinity controls glass dissolution rates is similar to the silica saturation concept of Grambow [1] except that our model predicts that all components concentrated in the surface layer, not just silica, affect glass dissolution rates. The good agreement between predicted and observed elemental dissolution rates suggests that the dissolution rate of the gel layer limits the overall rate of glass dissolution. The model predicts that the long-term rate of glass dissolution will depend mainly on ion concentrations in solution, and therefore on the secondary phases which precipitate and control ion concentrations.

Experimental observations of fracture dissolution: The role of Peclet number on evolving aperture variability

[1] Dissolution of the surfaces of rock fractures can cause significant alteration of the fracture void space (aperture) and fracture permeability (k). Both surface reaction rates and transport of reactants within the fracture can limit local dissolution. We investigated the role of Peclet number (Pe), a measure of the relative importance of advective and diffusive transport of reactants, on fracture dissolution in two identical transparent analog fractures with different initial values of Pe (Peo). High-resolution light-transmission techniques provided direct measurements of the evolving aperture field during each experiment. For Peo = 54 distinct dissolution channels formed, while for Peo = 216 we measured minimal channeling and a reduction in short wavelength aperture variability. The nature of the dissolution patterns strongly influenced the relative increase in k. A 110% increase in the mean aperture due to dissolution resulted in estimated permeability increases of 440% and 640% for the Peo =5 4 and Peo = 216 experiments, respectively. INDEX TERMS: 5104 Physical Properties of Rocks: Fracture and flow; 5114 Physical Properties of Rocks: Permeability and porosity; 1829 Hydrology: Groundwater hydrology; 1832 Hydrology: Groundwater transport. Citation: Detwiler, R. L., R. J. Glass, and W. L. Bourcier, Experimental observations of fracture dissolution: The role of Peclet number on evolving aperture variability, Geophys. Res. Lett., 30(12), 1648, doi:10.1029/ 2003GL017396, 2003.

Aluminum hydrolysis constants to 250°C from boehmite solubility measurements

Abstract Boehmite solubilities were measured at 150, 200, and 250°C at pH values from 1 to 10 at 100 bars total pressure and used to determine the stability constants for the mononuclear aluminum hydroxide complexes (Al(OH)2+, Al(OH)2+, Al(OH)30, Al(OH)4−),and the solubility product of boehmite. Buffer solutions of HCl-KCl, acetic acid-sodium acetate, sodium bicarbonate-carbonic acid, and boric acid-potassium hydroxide were used to control pH. Our solubility data are in good agreement with boehmite solubility measurements in perchloric acid and sodium hydroxide solutions reported by Kuyunko et al. (1983). The stability constants for the aluminum hydroxide species were determined from the solubility data using a Ridge regression technique. The results indicate that aluminum ion hydrolysis becomes stronger at higher temperatures, and the stability field of the neutral complex Al(OH)30 becomes larger. The results are used to provide a set of equilibrium constants for aluminum hydroxide complex formation and boehmite hydrolysis from 0–300°C.

Encapsulated liquid sorbents for carbon dioxide capture

Drawbacks of current carbon dioxide capture methods include corrosivity, evaporative losses and fouling. Separating the capture solvent from infrastructure and effluent gases via microencapsulation provides possible solutions to these issues. Here we report carbon capture materials that may enable low-cost and energy-efficient capture of carbon dioxide from flue gas. Polymer microcapsules composed of liquid carbonate cores and highly permeable silicone shells are produced by microfluidic assembly. This motif couples the capacity and selectivity of liquid sorbents with high surface area to facilitate rapid and controlled carbon dioxide uptake and release over repeated cycles. While mass transport across the capsule shell is slightly lower relative to neat liquid sorbents, the surface area enhancement gained via encapsulation provides an order-of-magnitude increase in carbon dioxide absorption rates for a given sorbent mass. The microcapsules are stable under typical industrial operating conditions and may be used in supported packing and fluidized beds for large-scale carbon capture.

Evaluation of silica-water surface chemistry using NMR spectroscopy

We have combined traditional batch and flow-through dissolution experiments, multinuclear nuclear magnetic resonance (NMR) spectroscopy, and surface complexation modeling to re-evaluate amor- phous silica reactivity as a function of solution pH and reaction affinity in NaCl and CsCl solutions. The NMR data suggest that changes in surface speciation are driven by solution pH and to a lesser extent alkali concentrations, and not by reaction time or saturation state. The 29 Si cross-polarization NMR results show that the concentration of silanol surface complexes decreases with increasing pH, suggesting that silanol sites polymerize to form siloxane bonds with increasing pH. Increases in silica surface charge are offset by sorption of alkali cations to ionized sites with increasing pH. It is the increase in these ionized sites that appears to control silica polymorph dissolution rates as a function of pH. The 23 Na and 133 Cs NMR results show that the alkali cations form outersphere surface complexes and that the concentration of these complexes increases with increasing pH. Changes in surface chemistry cannot explain decreases in dissolution rates as amorphous silica saturation is approached. We find no evidence for repolymerization of the silanol surface complexes to siloxane complexes at longer reaction times and constant pH. Copyright

Kinetics of uranium release from Synroc phases

This paper presents experimental studies on the kinetics of U release from near single-phase zirconolite, pyrochlore, brannerite and pyrochlore-rich titanate ceramic materials. The dissolution tests were conducted at 20–75°C with initial pHs from 2 to 12, and flow rates from 10 to 80 ml d−1 in the open atmosphere. The U releases from these titanate materials are controlled by initial fast process and then followed by linear kinetics. The close-to-stoichiometric U release from zirconolite and pyrochlore-rich materials and preferential U release from brannerite are consistent with the alterations observed for the natural samples. The rate constants for U releases were determined and the effects of pH and temperature were examined. For each material, the U release vs. pH exhibits a V-shape with a minimum near pH 8. The measured activation energies suggest surface reaction controlled dissolution mechanism. Pyrochlore-rich materials and zirconolite demonstrated higher chemical durability and more resistance to aqueous attack than brannerite. However, impurities and minor brannerite inclusions do not appear to have a detrimental effect on U releases from pyrochlore-rich multi-phase ceramics.

The hydration of borosilicate waste glass in liquid water and steam at 200 °C☆

Abstract Simulated borosilicate waste glass was hydrated in steam at 200 °C for times up to 40 days to assess the effect of a very high glass surface area/leachant volume (SA/V) ratio on the reaction. The reactions in steam attained an SA/V in excess of 4000 m−1 due to the limited amount of water that was available to condense on the glass surface. Experiments in liquid water were performed at an SA/V of 40 m−1 for comparison. A solid reaction layer formed on the glass surface in both environments, and the thickness of this layer was used as a measure of the reaction progress. Other secondary phases formed on top of (and within) the layer on the steam-reacted samples after a few days of reaction but not on samples reacted in liquid water. The rate (layer thickness/time) measured in experiments with liquid water slows with time while the reaction in steam is very slow initially but then proceeds at a high rate after secondary phases form. The secondary phases are believed to increase the reaction rate by lowering the solution concentrations of glass species (probably most importantly silicon) which control the reaction affinity. The glass reaction is accelerated in a steam environment relative to liquid environment because, in steam, the small solution volume becomes saturated and precipitates are formed after much less glass has reacted. The experimental technique described allows secondary phases to be generated within short time periods at elevated temperatures in a steam environment. Knowledge of the phases formed is necessary to predict the long-term reaction rate. Precipitates formed on the steam-reacted samples were identified using SEM/EDS analysis and XRD. The EQ3/6 computer code was used to predict secondary phases formed at 200 °C for comparison to the observed phases. Differences in the assemblage predicted by the computer simulation and that produced in the experiments are attributed to the limited data base use by the simulation.