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Temperature dependent lithium isotope fractionation during glass dissolution
Geochimica et Cosmochimica Acta ( IF 5 ) Pub Date : 2021-09-09 , DOI: 10.1016/j.gca.2021.09.005
Thomas L. Goût 1 , Madeleine S. Bohlin 1 , Edward T. Tipper 1 , Giulio I. Lampronti 1 , Ian Farnan 1
Affiliation  

Understanding the mechanisms by which borosilicate glasses corrode in contact with aqueous solutions remains a challenge to the safety case for the geological disposal of vitrified high-level nuclear waste. Here, lithium isotope fingerprinting techniques were applied to the leachates of a simulant Magnox waste glass to probe the mechanisms of aqueous corrosion at both short and long timescales (6 hours to 464 days). Experiments took place at 40 and 90 °C to assess the consistency of the dissolution mechanisms across a range of commonly employed temperatures and the legitimacy of applying higher temperature experimental datasets to understand glass corrosion within a disposal facility at lower temperatures.

Two competing release mechanisms were observed for lithium (diffusion and hydrolysis), and the relative proportions of these mechanisms changed through time. Leachates initially had lower δ7Li values than the pristine glass (−2.7‰ at 40 °C and −1.1‰ at 90 °C relative to the pristine glass) at both temperatures due to lithium leaching incongruently through diffusive processes. The greater offset between solution and solid at lower temperatures indicates a larger rate of diffusion (incongruent dissolution) relative to the rate of hydrolysis (congruent dissolution) at lower temperatures. The fraction of lithium released through diffusion relative to the fraction of lithium released through hydrolysis then increased at both temperatures with time up to 126 days, increasing from 0.47 and 0.22 at 6 hours to 0.66 and 0.41 at 126 days at 40 and 90 °C respectively. Subsequently, the fractions of lithium released through diffusion sharply decreased to 0.36 at 40 °C and 0.22 at 90 °C after 464 days, consistent with network hydrolysis coupled with secondary phase precipitation later controlling the long-term release of Li at both temperatures. Throughout the duration of the experiments (464 days) the δ7Li values in solution increased to 9.0‰ at 40 °C and 10.0‰ at 90 °C due to the formation of talc and montmorillonite phases at 40 °C and additional smectite phases at 90 °C. Further, no evidence for the formation of a diffusive barrier to the transport of Li within the alteration layers became apparent during the later stages of dissolution at either temperature. However, the fraction of lithium leached through diffusion was still significant throughout all stages of dissolution. Lithium isotope ratios in solution were correlated with the transition from a system which was increasingly dominated by lithium diffusion as the dissolution rate slowed to one which was controlled by hydrolysis coupled with secondary phase precipitation at long durations. Alongside elemental ratios in solution, these results were consistent with the same set of mechanisms governing dissolution across the temperature range studied.



中文翻译:

玻璃溶解过程中的温度依赖性锂同位素分馏

了解硼硅酸盐玻璃在与水溶液接触时腐蚀的机制仍然是玻璃化高放核废料地质处置安全案例的挑战。在这里,锂同位素指纹技术被应用于模拟 Magnox 废玻璃的浸出液,以探测短期和长期(6 小时到 464 天)的水腐蚀机制。实验在 40 和 90 °C 下进行,以评估溶解机制在一系列常用温度下的一致性,以及应用更高温度的实验数据集来了解较低温度下处置设施内玻璃腐蚀的合法性。

观察到锂的两种竞争性释放机制(扩散和水解),并且这些机制的相对比例随时间而变化。渗滤液最初具有较低的 δ 7由于锂通过扩散过程不一致地浸出,因此在两个温度下 Li 的值都比原始玻璃(40°C 时为 -2.7‰,90°C 时相对于原始玻璃为 -1.1‰)。较低温度下溶液和固体之间的较大偏移表明相对于较低温度下的水解速率(一致溶解)更大的扩散速率(不一致溶解)。通过扩散释放的锂的比例相对于通过水解释放的锂的比例然后在两个温度下随着时间的推移而增加,最多 126 天,分别从 6 小时时的 0.47 和 0.22 增加到 40 和 90°C 时的 126 天时的 0.66 和 0.41 . 随后,464 天后,通过扩散释放的锂比例在 40°C 时急剧下降至 0.36,在 90°C 时降至 0.22,与网络水解和二次相沉淀相一致,随后控制了两种温度下锂的长期释放。在整个实验期间(464 天),δ7由于在 40 °C 下形成滑石和蒙脱石相以及在 90 °C 下形成额外的蒙脱石相,溶液中的锂值在 40 °C 下增加到 9.0‰,在 90 °C 下增加到 10.0‰。此外,在任一温度下溶解的后期阶段,都没有证据表明在蚀变层内形成了对 Li 传输的扩散势垒。然而,在溶解的所有阶段,通过扩散浸出的锂的比例仍然很重要。溶液中的锂同位素比率与从越来越多地由锂扩散主导的系统的转变相关联,因为溶解速率减慢到由水解和长持续时间的第二相沉淀控制的系统。除了溶液中的元素比,

更新日期:2021-09-24
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