Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352, United States.
Physical and Computational Sciences Directorate, Physical Sciences Division, Pacific Northwest National Laboratory, Richland, Washington 99352, United States.
Environ Sci Technol. 2021 Jan 19;55(2):1057-1066. doi: 10.1021/acs.est.0c06707. Epub 2020 Dec 28.
Ettringite is a naturally occurring mineral found in cementitious matrices that is known for its ability to incorporate environmentally mobile oxyanion contaminants. To better assess this immobilization mechanism for contaminants within cementitious waste forms intended for nuclear waste storage, this work explores how mixed oxyanion contaminants compete for ettringite incorporation and influence the evolving mineralogy. Ettringite was precipitated in the presence of TcO, IO, and/or CrO, known contaminants of concern to nuclear waste treatment, over pre-determined precipitation periods. Solution analyses quantified contaminant removal, and the collected solid was characterized using bulk and microprobe X-ray diffraction coupled with pair distribution function and microprobe X-ray fluorescence analyses. Results suggest that ≥96% IO is removed from solution, regardless of ettringite precipitation time or the presence of TcO or CrO. However, TcO removal remained <20%, was not significantly improved with longer ettringite precipitation times, and decreased to zero in the presence of IO. When IO is co-mingled with CrO, calcite and gypsum are formed as secondary mineral phases, which allows for oxyanion partitioning, e.g., IO incorporation into ettringite, and CrO incorporation into calcite. Results from this work exemplify the importance of competitive immobilization when assessing waste form performance and environmental risk of contaminant release.
钙矾石是一种在水泥基质中自然存在的矿物，以其能够容纳环境中移动的含氧阴离子污染物的能力而闻名。为了更好地评估水泥固化废物形式中污染物的这种固定化机制，以期用于核废料储存，这项工作探讨了混合含氧阴离子污染物如何竞争容纳钙矾石以及影响不断演变的矿物学。在预先确定的沉淀期内，在存在 TcO、IO 和/或 CrO 的情况下沉淀出钙矾石，TcO、IO 和 CrO 是核废料处理中已知的关注污染物。溶液分析定量了污染物的去除，收集的固体使用体相和微探针 X 射线衍射结合配对分布函数和微探针 X 射线荧光分析进行了表征。结果表明，无论沉淀钙矾石的时间长短或是否存在 TcO 或 CrO，都可以从溶液中去除≥96%的 IO。然而，TcO 的去除率仍低于 20%，随着沉淀钙矾石时间的延长，去除率没有明显提高，而在存在 IO 的情况下，去除率降为零。当 IO 与 CrO 混合时，会形成方解石和石膏等次生矿物相，从而实现含氧阴离子的分配，例如，IO 被容纳到钙矾石中，CrO 被容纳到方解石中。这项工作的结果说明了在评估废物形式性能和污染物释放的环境风险时，竞争固定化的重要性。
