Solubility Effects in Waste Glass/Demineralized Water Systems
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by Elsevier Science Publishing Company, BASIS FOR NUCLEAR WASTE MANAGEMENT
Inc.
173
Stephen V. Topp, editor SOLUBILITY EFFECTS IN WASTE GLASS/DEMINERALIZED WATER SYSTEMS
HAROLD T. FULLAM Pacific Northwest Laboratory,
P. 0. Box 999,
Richland, WA 99352,
USA
ABSTRACT A study was carried out to determine the solubility limits of various elements found in waste glasses in demineralized water as a function of temperature. The work was sponsored by the Office of Nuclear Waste Isolation under contract to the Department of Energy. 0 Solubility measurements were carried out at 350, 650, 950, and 150 C using three nonradioactive waste glass compositions. Subsaturation and supersaturation methods were used to determine the solubility limits. The two methods gave markedly different values for most glass components. The results obtained indicate that it is difficult to assign solubility limits to most glass components without thoroughly describing the glass-water system. This includes not only defining the glass type, and system temperature, but also the glass surface area-towater volume (S/V) ratio of the system and its thermal history.
INTRODUCTION The Waste/Rock Interaction Technology (WRIT) program is one of several programs underway at the Pacific Northwest Laboratory (PNL) that are designed to help obtain the data needed to show that the nuclear wastes can be safely stored in geologic repositories. The WRIT program is funded by the Office of Nuclear Waste Isolation (ONWI) under contract to the Department of Energy. As part of the WRIT program a study was carried out at PNL to determine the solubility limits of various elements present in waste glasses in demineralized water as a function of temperature. The work was limited to a study of aqueous systems involving demineralized water and three waste glass compositions at temperatures up to 1500C. Complete details of the solubility studies are presented in PNL-3614 [1] and the discussion below is limited to a single waste glass composition (76-68). Results obtained with the other waste glasses were similar to those obtained with the 76-68 glass. EXPERIMENTAL Two methods were used to determine the solubility of glass components in demineralized water. Using the first method, equilibrium was approached from subsaturation, while, with the second method, equilibrium was approached from supersaturation. The first method simply involved contacting crushed glass with demineralized water at a constant temperature and monitoring the solution pH and composition as a function of time. Typically, a weighed amount of glass, of known particle size and surface area, was placed in a flask and a known volume of demineralized water added. The flask was sealed, placed in a constant temperature bath, and maintained at a constant temperature for an extended time. Filtered samples of the aqueous solutions were taken at periodic intervals for chemical analysis and pH determination. The pH was determined at the test temperature and at room temperature. All of the tests were carried out at
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