Abstract
The dissolution kinetics of five glasses along the NaAlSiO4-NaBSiO4 join were used to evaluate how the structural variations associated with boron-aluminum substitution affect the rate of dissolution. The composition of each glass varied inversely in mol% of Al2O3 (5-25mol%) and B2O3 (20-0mol%) with Na2O (25mol%) and SiO2 (50mol%) making up the remaining amount, in every case Na/(Al+B)=1.0. Single-pass flow-through experiments (SPFT) were conducted under dilute conditions as a function of solution pH (from 7.0 to 12.0) and temperature (from 23 to 90°C). Analysis of unreacted glass samples by 27Al and 29Si MAS-NMR suggests Al (∼98% [4]Al) and Si-atoms (∼100% [4]Si) occupy a tetrahedral coordination whereas, B-atoms occupy both tetrahedral ([4]B) and trigonal ([3]B) coordination. The distribution of [3]B fractionated between [3]B(ring) and [3]B(non-ring) moieties, with the [3]B(ring)/[3]B(non-ring) ratio increases with an increase in the B/Al ratio. The MAS-NMR results also indicated an increase in the fraction of [4]B with an increase in the B/Al ratio. The 27Al peak maxima shift to lesser values with an increase in the B/Al ratio which suggests mixing between the [4]Al and [3]B sites, assuming avoidance between tetrahedral trivalent cations ([4]Al-O-[4]B avoidance). Unlike the 27Al and 11B spectra, the 29Si spectra illustrate a subtle shift to more negative chemical shift (chemical shift range between -88 and -84ppm) and increases in the spectral widths as the B/Al ratio increases. Raman spectroscopy of unreacted glass samples was also used to cross-check the results collected from MAS-NMR and suggested that NeB4 (the glass sample with the highest B content) may consist of B-Na enriched and Al-Si enriched micro-domains, which affected the measured dissolution rates. Results from SPFT experiments suggest a forward rate of reaction and pH power-law coefficients, η, that are independent of B/Al under these neutral to alkaline test conditions for all homogeneous glasses. The temperature dependence shows an order of magnitude increase in the dissolution rate with a 67°C increase in temperature and suggests dissolution is controlled by a surface-mediated reaction, as indicated by the activation energy, Ea, being between 44±8 and 48±7kJ/mol. Forward dissolution rates, based on Na and Si release, for homogeneous glasses are independent of the B/Al ratio, whereas dissolution rates based on Al and B release are not. Normalized dissolution rates, based on B release, increase with the molar fraction of [3]B(ring). Finally in accord with previous studies, the data discussed in this manuscript suggest rupture of either the Al-O or Si-O bonds as the rate-limiting step controlling the dissolution of these glasses.
| Original language | English |
|---|---|
| Pages (from-to) | 2634-2654 |
| Number of pages | 21 |
| Journal | Geochimica et Cosmochimica Acta |
| Volume | 74 |
| Issue number | 9 |
| DOIs | |
| State | Published - May 2010 |
| Externally published | Yes |
Funding
The authors would like to extend a special thanks to Jonathan Stebbins for his insightful comments on an earlier version of this manuscript. The authors would like to express gratitude to S.R. Baum, of Pacific Northwest National Laboratory (PNNL), for providing high quality analytical data from sample solutions. We also thank J.L. Steele (PNNL) and J.V. Crum (PNNL) for their assistance with various aspects of this work. L.R. Reed, and J. Broady acknowledge the student funding obtained from the Department of Energy’s Community College Initiative Program and the Summer Research Apprenticeship Program, respectively, being administered at PNNL. This work was supported by the U.S. Department of Energy’s (DOE) Office of Science and Technology under the Environmental Management Science Program (proposal number 42400). The authors acknowledge W.J. Shaw, J.J. Ford, S.D. Burton, and J.A. Sears for providing instrument time and helping with the collection of MAS-NMR spectra and C.F. Windisch for collecting the Raman spectra. A portion of this research was performed in part with the Nuclear Magnetic Resonance Spectrometers in the William R. Wiley Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research and located at PNNL. PNNL is operated by Battelle for the DOE under Contract DE-AC05-76RL01830.