Incorporation of Np(V) and U(VI) in carbonate and sulfate minerals crystallized from aqueous solution

Abstract The neptunyl Np(V)O2+ and uranyl U(VI)O22+ ions are soluble in groundwater, although their interaction with minerals in the subsurface may impact their mobility. One mechanism for the immobilization of actinyl ions in the subsurface is co-precipitation in low-temperature minerals that form naturally, or that are induced to form as part of a remediation strategy. Important differences in the crystal-chemical behavior of the Np(V) neptunyl and U(VI) uranyl ions suggest their behavior towards incorporation into growing crystals may differ significantly. Using a selection of low-temperature minerals synthesized in aqueous systems under ambient conditions, this study examines the factors that impact the structural incorporation of the Np(V) neptunyl and U(VI) uranyl ions in carbonate and sulfate minerals. Calcite (CaCO3), aragonite (CaCO3), gypsum (CaSO4·2H2O), strontianite (SrCO3), cerussite (PbCO3), celestine (SrSO4), and anglesite (PbSO4) were synthesized from aqueous solutions containing either 400–1000 ppm of U(VI) or Np(V) relative to the divalent cation present in the system. The synthetic products were investigated by inductively coupled plasma mass spectrometry, luminescence and time resolved luminescence spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy. Amongst the carbonate minerals, calcite significantly favors Np(V) incorporation over U(VI). U(VI) and Np(V) are incorporated in aragonite and strontianite in similar amounts, whereas cerussite did not incorporate either U(VI) or Np(V) under the synthesis conditions. The sulfate minerals weakly interact with the actinyl ions, relative to the carbonate minerals. Incorporation of U(VI) and Np(V) in celestine was observed at the level of a few tens of ppm; anglesite and gypsum did not incorporate detectable U(VI) or Np(V). Luminescence spectra of the uranyl incorporated in aragonite and strontianite are consistent with a uranyl unit coordinated by three bidentate CO32− groups. The time-resolved spectra of calcite indicate multiple coordination environments about the uranyl unit, with the spectra of the longer-lived components displaying similarities with uranyl-incorporated aragonite. The luminescence spectrum of uranyl-bearing celestine is consistent with a uranyl unit coordinated by monodentate sulfate groups. Anglesite synthesized in the presence of uranyl shows no luminescence, whereas the spectra of gypsum and cerussite suggest uranyl surface adsorption or precipitation of secondary uranyl minerals on the mineral surfaces. Our findings indicate that geometrical constraints of the Np(V) and U(VI) species in solution, together with the crystallographic steric constraints of the host material, affect preferential uptake in the mineral structures studied. Calcium and strontium appear to be favorable incorporation sites for both U(VI) and Np(V) in aragonite and strontianite. In calcite, Np(V) incorporation is strongly favored over U(VI), whereas in gypsum incorporation of neither actinyl ion occurs. Substitution of actinyl ions was also not observed for lead, in either the carbonate or sulfate minerals studied.