Iron Vacancies Accommodate Uranyl Incorporation into Hematite.
Iron Vacancies Accommodate Uranyl Incorporation into Hematite.
复制标题
DOI：
10.1021/acs.est.8b00297
复制
发表时间：
2018-05
影响因子：
11.4
通讯作者：
M. McBriarty;S. Kerisit;E. Bylaska;S. Shaw;K. Morris;E. Ilton
中科院分区：
文献类型：
--
作者：
M. McBriarty;S. Kerisit;E. Bylaska;S. Shaw;K. Morris;E. Ilton
在矿物生长过程中，自然系统和核废料容器中的放射性毒性铀污染可以通过掺入天然丰富的铁（氧合）氧化物（如赤铁矿（α-Fe2O3））来隔离。掺入铀的局部配位环境影响了掺铀材料的稳定性和性能。虽然已经尝试使用扩展x射线吸收精细结构（EXAFS）分析来测量赤铁矿中的铀配位，但传统的逐壳EXAFS拟合结果模糊不清。利用杂化泛函数从头算分子动力学（AIMD）模拟各种缺陷构型，生成合成的EXAFS光谱，并结合吸附铀酰谱拟合U6+掺杂赤铁矿的实验U l3边缘EXAFS。我们发现赤铁矿晶体结构具有类似U6+的反式二氧基铀酰结构，取代了Fe3+结构，这需要两个部分质子化的铁空位位于相对的角共享位点。令人惊讶的是，与实验最匹配的是除了最小能量配置之外的空位配置的显著比例，这表明掺入机制和动力学在确定低温水热条件下杂质掺入宿主相的状态方面的重要性。
Radiotoxic uranium contamination in natural systems and nuclear waste containment can be sequestered by incorporation into naturally abundant iron (oxyhydr)oxides such as hematite (α-Fe2O3) during mineral growth. The stability and properties of the resulting uranium-doped material are impacted by the local coordination environment of incorporated uranium. While measurements of uranium coordination in hematite have been attempted using extended X-ray absorption fine structure (EXAFS) analysis, traditional shell-by-shell EXAFS fitting yields ambiguous results. We used hybrid functional ab initio molecular dynamics (AIMD) simulations for various defect configurations to generate synthetic EXAFS spectra which were combined with adsorbed uranyl spectra to fit experimental U L3-edge EXAFS for U6+-doped hematite. We discovered that the hematite crystal structure accommodates a trans-dioxo uranyl-like configuration for U6+ that substitutes for structural Fe3+, which requires two partially protonated Fe vacancies situated at opposing corner-sharing sites. Surprisingly, the best match to experiment included significant proportions of vacancy configurations other than the minimum-energy configuration, pointing to the importance of incorporation mechanisms and kinetics in determining the state of an impurity incorporated into a host phase under low temperature hydrothermal conditions.