Geochemical Modeling of ILAW Lysimeter Water Extracts
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Geochemical modeling results of water extracts from simulated immobilized low-activity waste (ILAW) glasses, placed in lysimeters for eight years suggest that the secondary phase reaction network developed using product consistency test (PCT) results at 90°C may need to be modified for field conditions. For sediment samples that had been collected from near the glass samples, the impact of glass corrosion could be readily observed based upon the pH of their water extracts. For unimpacted sediments the pH ranged from 7.88 to 8.11 with an average of 8.04. Sediments that had observable impacts from glass corrosion exhibited elevated pH values (as high as 9.97). For lysimeter sediment samples that appear to have been impacted by glass corrosion to the greatest extent, saturation indices determined for analcime, calcite, and chalcedony in the 1:1 water extracts were near equilibrium and were consistent with the secondary phase reaction network developed using PCT results at 90°C. Fe(OH)3(s) also appears to be essentially at equilibrium in extracts impacted by glass corrosion, but with a solubility product (log Ksp) that is approximately 2.13 units lower than that used in the secondary phase reaction network developed using PCT results at 90°C. The solubilities of TiO2(am) and ZrO2(am) also appear to be much lower than that assumed in the secondary phase reaction network developed using PCT results at 90°C. The extent that the solubility of TiO2(am) and ZrO2(am) were reduced relative to that assumed in the secondary phase reaction network developed using PCT results at 90°C could not be quantified because the concentrations of Ti and Zr in the extracts were below the estimated quantification limit. Gibbsite was consistently highly oversaturated in the extract while dawsonite was at or near equilibrium. This suggests that dawsonite might be a more suitable phase for the secondary phase reaction network than gibbsite under field conditions. This may be due to the availability of carbonate that exists in the Hanford sediments as calcite. A significant source of carbonate was not available in the PCTs and this may account for why this phase did not appear in the PCTs. Sepiolite was consistently highly undersaturated, suggesting that another phase controls the solubility of magnesium. For samples that were most impacted by the effects of glass corrosion, magnesite appears to control glass corrosion. For samples that show less impacts from glass corrosion, clinochlore-7A or saponite-Mg appears to control the magnesium concentrations. For zinc, it appears that zincite is a better candidate than Zn(OH)2-γ for controlling zinc concentrations in the extracts; however, in some samples all zinc phases considered were highly oversaturated. As a result the phase that controls zinc concentrations in the lysimeter extracts remains uncertain.
- Research Organization:
- Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 1166880
- Report Number(s):
- PNNL-23693; 830403000
- Country of Publication:
- United States
- Language:
- English
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