Effect of manganese on the speciation of neptunium(V) on manganese doped magnetites

• Published in: Colloids and surfaces. A, Physicochemical and engineering aspects Physicochemical and engineering aspects, 2022-02, Vol.635, p.128105, Article 128105
• Main Authors: Kumar, Sumit, Rothe, Jörg, Finck, Nicolas, Vitova, Tonya, Dardenne, Kathy, Beck, Aaron, Schild, Dieter, Geckeis, Horst
• Format: Article
• Language: English
• Subjects: HR-XANES; Magnetite; Manganese doping; Neptunium(V); Redox process
• ISSN: 0927-7757; 1873-4359
• DOI: 10.1016/j.colsurfa.2021.128105

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. [Display omitted] •Single phase manganese-doped (3–12 wt%) magnetites were synthesized.•Neptunium(V) interaction with Mn-doped magnetites in neutral pH condition.•HR-XANES confirmed single speciation of Np on undoped and doped magnetites.•Mn-O sites participates through Fe(II)/Mn(III) redox couple in reducing Np(V).