Hydrogen Speciation in Hydrated Layers on Nuclear Waste Glass

Abstract

DE87 005836The hydration of an outer layer on nuclear waste glasses Is known to occur during leaching, but the actual speclatlon of hydrogen (as water or hydroxyl groups) in these layers has not been determined. As part of the Nevada Nuclear Waste Storage Investigations Project, we have used infrared spectroscopy to determine hydrogen spedatlons n three nuclear waste glass compositions (SRL-131 & 165, and PNL 76-6 1, which were leached at 90'C (all glasses) or hydrated In a vapor-satu ated atmosphere at 202*C (SRL-131 only). Hydroxyl groups were found in the surface layers of all the glasses. In addition, molecular wat~>r was found in the surface of SRL-131 and PNL 76-68 glasses that had i.een leached for several months 1n deionlzed water, and 1n the vapor-hydrated sample. The water/hydroxyl ratio Increases with increasing reaction time; molecular water makes up most of the hydrogen In the thick reaction layers on vapor-phase hydrated glass while only hydroxyl occurs In the least reacted samples. Using the known molar absorptlvltle; of water and hydroxyl 1n silica-rich glass the vapor-phase layer contained 4.8 moles/liter of molecular water, and 0.6 moles water in the forr hydroxyl. A 15 micrometer layer on SRL-131 glass formed by leachln at 90°C contained a total of 4.9 moles/liter of water, 2/3 of which wa: as hydroxyl. The unreacted bulk glass contains about 0.018 moles/liter water, all as hydroxyl.The amount of hydrogen added to the SRL-131 glass was about 70% of the original Na + Li content, not the 300% that would result from alkali-hydronium 1on (HgO-1 -) interdiffusion. If all the hydrogen Is then assumed to be added as the result of alkali-H + Interdiffusion, the molecular water observed nay have formed from condensation of the original hydroxyl groups according to: 20H -H2O molecular + 0°where 0° refers to a bridging oxygen, and OH refers to a hydroxyl group attached to a silicate polymer. The hydrated layer on the nuclear waste glasses appears to be of relatively low water content (4 to 7% by weight) and Is not substantially hydroxylated. Thus, these layers do not have many of the properties associated with "gel" layers.

Effect of calcium on dissolution and precipitation reactions of amorphous silica at high alkalinity

“…(2) [17,18]. In fact, the surface of silicate glass that is exposed to water is almost always hydrolyzed, and`Si\ \OH groups are abundant [19,20]. The reverse reaction in Eq.…”

Corrosion of Glass

“…If sufficient network bonds are hydrolyzed, a transformed layer [83] (gel) is formed with a clear phase boundary to the glassy phase. The transformed surface layers are porous [84], containing molecular water [85,86] and allowing for high ionic mobility [87] as well as high water mobility. Molecular water may result from inward diffusion of water carrying H þ ions or may alternatively, result from the condensation reaction [88,89], 2:SiÀOH !…”

“…Metal ions from solution may become incorporated into the gel layer (e.g., Mg). The content of molecular water is significantly higher than the content of silanol groups [100] and water may occur phase separated in the structure of the layer [101]. The interface between the diffusion layer and the gel layer often has the character of a phase boundary.…”

Surface Layer Effects on Waste Glass Corrosion

“…Water molecules must be present before ion-exchange reactions take place. However, water diffusion without accompanying dealkalization has been reported for natural obsidian [15][16][17][18] and tektites [19]. thus, a region of glass can, therefore, be hydrated but not alkali depleted.…”