Effect of manganese on the speciation of neptunium(V) on manganese doped magnetites
2022
Sumit Kumar, | Rothe, Jörg | Finck, Nicolas | Vitova, Tonya | Dardenne, Kathy | Beck, Aaron | Schild, Dieter | Geckeis, Horst
Speciation and fate of radionuclides in environment are strongly dependent on their interaction with various mineral surfaces. Presence of impurity atoms in minerals modifies these interactions. ²³⁷Np, a long-lived component of nuclear high-level waste, is highly mobile in environment as neptunyl (NpO₂⁺) cation. Role of impurities in defining its interaction with mineral surfaces present in natural and engineered systems is yet to be completely understood. Manganese is a ubiquitous impurity present in naturally occurring iron corrosion product, magnetite. Here, a series of manganese (Mn)-doped magnetites (FM) was synthesized and the evolution of solid-sorbed speciation of Np(V) was investigated as a function of Mn doping (3–11 wt%) in near neutral pH condition. Combining Mn K-edge and Np L₃-edge X-ray absorption fine structure spectra with Np M₅-edge high energy-resolution X-ray absorption near-edge structure spectra, we found nanoparticulate NpᴵⱽO₂ solid as the single speciation of Np sorbed on both doped and undoped magnetites. Without changing the surface speciation of Np, Mn-O sites did show preference for sorbing Np(V) vis-á-vis Fe-O sites. Mn participates through Fe(II)/Mn(III) redox couple in reducing sorbed Np(V). Its participation, however, becomes significant only when the doped-magnetite contains a critical minimum concentration (> 6 wt%) of Mn. This study highlights the significant role of impurity atom wherein it interacts with sorbing species through redox process.
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