Temperature dependent lithium isotope fractionation during glass dissolution
Temperature dependent lithium isotope fractionation during glass dissolution
复制标题
DOI：
10.1016/j.gca.2021.09.005
复制
发表时间：
2021-11
影响因子：
5
通讯作者：
Thomas L. Goût;Madeleine Bohlin;E. Tipper;G. Lampronti;I. Farnan
中科院分区：
文献类型：
--
作者：
Thomas L. Goût;Madeleine Bohlin;E. Tipper;G. Lampronti;I. Farnan
了解硼硅酸盐玻璃在与水溶液接触时腐蚀的机理仍然是对玻璃化高放射性核废料地质处置的安全案例的挑战。在这里，锂同位素指纹技术被应用于模拟Magnox废玻璃的渗滤液，以探索短时间尺度和长时间尺度(6小时至464天)的水腐蚀机理。实验在40℃和90℃下进行，以评估在一系列常用温度下溶解机制的一致性，以及应用较高温度的实验数据集来了解较低温度下处置设施内的玻璃腐蚀的合法性。观察到锂的两种竞争释放机制(扩散和水解)，这些机制的相对比例随着时间的变化而变化。由于锂的扩散过程不一致，在这两个温度下，渗滤液最初的δ7Li值都低于原始玻璃(相对于原始玻璃，40℃时的−为2.7‰，90℃时的−为1.1‰)。在较低温度下，溶液和固体之间的偏移量越大，表明扩散速率(不一致溶解)相对于较低温度下的水解率(一致溶解)更大。通过扩散释放的锂相对于通过水解释放的锂的比例，在两种温度下都有所增加，最高可达126天，在40℃和90℃时，分别从6小时的0.47和0.22增加到126天的0.66和0.41。随后，通过扩散释放的锂的分数在464天后急剧下降到40℃时的0.36和90℃时的0.22，这与网络水解和后来的第二相沉淀相一致，控制了这两种温度下Li的长期释放。在整个实验期间(464日)，由于在40℃形成滑石和蒙脱石相以及在90℃形成额外的蒙脱石相，溶液中的δ7Li值在40℃时增加到9.0‰，在90℃时增加到10.0‰。此外，在这两种温度下的后期溶解阶段，都没有明显的证据表明在蚀变层内形成了扩散障碍来阻碍锂的迁移。然而，在溶解的所有阶段，通过扩散浸出的锂的比例仍然很大。溶液中的锂同位素比值与由锂扩散为主的体系转变成由长时间的二次相沉淀结合的水解控制的体系有关。除了溶液中的元素比例，这些结果与在所研究的温度范围内控制溶解的同一套机制一致。
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.