Helmholtz-Zentrum Dresden - Rossendorf, Institute of Resource Ecology, Bautzner Landstraße 400, 01328 Dresden, Germany.
Helmholtz-Zentrum Dresden - Rossendorf, Institute of Resource Ecology, Bautzner Landstraße 400, 01328 Dresden, Germany; The Rossendorf Beamline at ESRF - The European Synchrotron, CS40220, 38043 Grenoble Cedex 9, France.
Sci Total Environ. 2021 May 20;770:145334. doi: 10.1016/j.scitotenv.2021.145334. Epub 2021 Jan 22.
The uptake of the fission product technetium (Tc) by chukanovite, an Fe hydroxy carbonate mineral formed as a carbon steel corrosion product in anoxic and carbonate-rich environments, was studied under anoxic, alkaline to hyperalkaline conditions representative for nuclear waste repositories in deep geological formations with cement-based inner linings. The retention potential of chukanovite towards Tc is high in the pH range 7.8 to 12.6, evidenced by high solid-water distribution coefficients, log R ~ 6, and independent of ionic strength (0.1 or 1 M NaCl). Using Tc K-edge X-ray absorption spectroscopy (XAS) two series of samples were investigated, Tc chukanovite sorption samples and coprecipitates, prepared with varying Tc loadings, pH values and contact times. From the resulting 37 XAS spectra, spectral endmembers and their dependence on chemical parameters were derived by self-organizing (Kohonen) maps (SOM), a neural network-based approach of machine learning. X-ray absorption near-edge structure (XANES) data confirmed the complete reduction of Tc to Tc by chukanovite under all experimental conditions. Consistent with mineralogical phases identified by X-ray diffraction (XRD), SOM analysis of the extended X-ray absorption fine-structure (EXAFS) spectra revealed the presence of three species in the sorption samples, the speciation predominately controlled by pH: Between pH 7.8 and 11.8, TcO-dimers form inner-sphere sorption complexes at the surface of the initial chukanovite as well as on the surface of secondary magnetite formed due to redox reaction. At pH ≥ 11.9, Tc is incorporated in a mixed, chukanovite-like, Fe/Tc hydroxy carbonate precipitate. The same species formed when using the coprecipitation approach. Reoxidation of sorption samples resulted in a small remobilization of Tc, demonstrating that both the original chukanovite mineral and its oxidative transformation products, magnetite and goethite, contribute to the immobilization of Tc in the long term, thus strongly attenuating its environmental transport.
在无氧、碱性至强碱性条件下，研究了铁羟碳酸矿物 Chukanovite 对锝（Tc）的吸收，Chukanovite 是在缺氧和富含碳酸盐的环境中作为碳钢腐蚀产物形成的，这种条件代表了深部地质构造中核废料处置库的条件，这些处置库具有基于水泥的内部衬里。在 pH 值为 7.8 至 12.6 的范围内，Chukanovite 对 Tc 的保留潜力很高，这一点可以通过高固-水分配系数（log R~6）来证明，并且与离子强度（0.1 或 1 M NaCl）无关。使用 Tc K 边 X 射线吸收光谱（XAS）研究了两个系列的样品，分别是 Tc 负载量、pH 值和接触时间不同的 Chukanovite 吸附样品和共沉淀样品。根据由此产生的 37 个 XAS 光谱，通过自组织（Kohonen）映射（SOM），即基于机器学习的神经网络方法，推导了谱端元及其对化学参数的依赖性。X 射线吸收近边结构（XANES）数据证实，在所有实验条件下，Chukanovite 将 Tc 完全还原为 Tc。与 X 射线衍射（XRD）鉴定的矿物相一致，EXAFS 光谱的 SOM 分析表明，吸附样品中存在三种物种，其形态主要由 pH 控制：在 pH 为 7.8 至 11.8 之间，TcO-二聚体在初始 Chukanovite 表面以及由于氧化还原反应形成的次生磁铁矿表面形成内球吸附络合物。在 pH 大于等于 11.9 时，Tc 被纳入混合的、类似 Chukanovite 的 Fe/Tc 羟碳酸盐沉淀中。当使用共沉淀方法时，也形成了相同的物种。吸附样品的再氧化导致 Tc 的少量再迁移，这表明原始的 Chukanovite 矿物及其氧化转化产物磁铁矿和针铁矿都有助于 Tc 的长期固定，从而强烈削弱了 Tc 的环境迁移。
