Structure Property Relations Of Mixed Alkali And Ion Exchange Silicate Glasses

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ISBN 13 :
Total Pages : 60 pages
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Download or read book Ion-Exchange Processes and Mechanisms in Glasses written by and published by . This book was released on 2001 with total page 60 pages. Available in PDF, EPUB and Kindle. Book excerpt: Leaching of alkalis from glass is widely recognized as an important mechanism in the initial stages of glass-water interactions. Pioneering experimental studies [1-3] nearly thirty-five years ago established that alkali (designated as M) are lost to solution more rapidly than network-forming cations. The overall chemical reaction describing the process can be written as: {triple_bond}Si-O-M + H --> {triple_bond}Si-OH + M+ (1) or {triple_bond}Si-O-M + H3O+ --> {triple_bond}Si-OH + M+ + H2O. (2) Doremus and coworkers [4-7] fashioned a quantitative model where M+ ions in the glass are exchanged for counter-diffusing H3O+ or H+. Subsequent investigations [8], which have relied heavily on reaction layer analysis, recognized the role of H2O molecules in the alkali-exchange process, without minimizing the importance of charged hydrogen species. Beginning in the 1980s, however, interest in M+-H+ exchange reactions in silicate glasses diminished considerably because important experimental observations showed that network hydrolysis and dissolution rates were principally controlled by the chemical potential difference between the glass and solution (chemical affinity) [9]. For nuclear waste glasses, formation of alteration products or secondary phases that remove important elements from solution, particularly Si, was found to have very large impacts on glass dissolution rates [10,11]. Consequently, recent work on glass/water interactions has focused on understanding this process and incorporating it into models [12]. The ion-exchange process has been largely ignored because it has been thought to be a short duration, secondary or tertiary process that had little or no bearing on long-term corrosion or radionuclide release rates from glasses [13]. The only significant effect identified in the literature that is attributed to alkali ion exchange is an increase in solution pH in static laboratory tests conducted at high surface area-to-volume ratios [14,15]. Renewed interest in alkali ion exchange reactions has come about because of interest in development of durable Na-rich silicate glasses for immobilization of low-activity waste (LAW) at Hanford, Washington [16] and high-level wastes in China [17]. In reactive transport simulations of a LAW glass disposed in a shallow subsurface facility, Chen, McGrail, and Engel [18] showed that ion-exchange reactions increased the radionuclide release rate by over two orders of magnitude when compared with simulations where ion exchange was excluded. Sheng, Luo, and Tang [17] conducted static tests in a simulated groundwater and showed that alkali ion exchange was the dominant release mechanism over a large temperature range. Although the significance of alkali ion exchange reactions in long-term disposal system performance has now been recognized, the fundamental processes and mechanisms controlling the exchange reactions are still remarkably poorly understood, especially with regard to how glass structure affects alkali ion exchange kinetics. Experimental studies of Na release from various simple silicate glasses are numerous [19-23]. However, in all previous studies of which we are aware, no attempt was made to distinguish between M+ release through alkali exchange versus matrix dissolution. The release rate of alkali in all of the early work was convoluted by contributions from matrix dissolution, which dominates in dilute solutions. Also, none of the previous studies attempted to define the relationship, if any, between glass structure (composition) and the kinetics of the ion exchange reaction. The motivation behind this Environmental Management Science Project (EMSP) is to develop a better understanding of how glass structure impacts sodium ion exchange so that improved glasses can be developed. Development of low ion-exchange rate glasses may also permit engineers to use higher loadings in nuclear waste glasses, which would result in substantial savings in production and disposal costs. This report summarizes the experimental data, and the interpretation and analysis of this data that was collected over the duration of the project from 1997 to 2001. Three silicate glass systems were investigated: (1) Na2O-Al2O3-SiO2 system, (2) Na2O-B2O3-SiO2 system, and the (3) Na2O-Al2O3-B2O3-HfO2-SiO2 system.