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

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. Np-237, 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 L-3-edge X-ray absorption fine structure spectra with Np M 5 -edge high energy-resolution X-ray absorption near-edge structure spectra, we found nanoparticulate (NpO2)-O-IV 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-a-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.

Automated summary

The study reveals that a nanoparticulate (NpO2)-O-IV solid is the only speciation of Np sorbed on both doped and undoped magnetites. The Mn-O sites preferentially sorb Np(V) compared to Fe-O sites, indicating the significant role of impurity atom interacting with sorbing species through redox process. The participation of Mn through Fe(II)/Mn (III) redox couple in reducing sorbed Np(V) becomes significant only when the doped-magnetite contains a critical minimum concentration of Mn (>6 wt%).