Newly recognized hosts for uranium in the Hanford Site vadose zone

Abstract

Uranium contaminated sediments from the U.S. Department of Energy's Hanford Site have been investigated using electron microscopy. Six classes of solid hosts for uranium were identified. Preliminary sediment characterization was carried out using optical petrography, and electron microprobe analysis (EMPA) was used to locate materials that host uranium. All of the hosts are fine-grained and intergrown with other materials at spatial scales smaller than the analytical volume of the electron microprobe. A focused ion beam (FIB) was used to prepare electron-transparent specimens of each host for the transmission electron microscope (TEM). The hosts were identified as: (1) metatorbernite [Cu(UO 2 ) 2 (PO 4 ) 2 Á8H 2 O]; (2) coatings on sediment clasts comprised mainly of phyllosilicates; (3) an amorphous zirconium (oxyhydr)oxide found in clast coatings; (4) amorphous and poorly crystalline materials that line voids within basalt lithic fragments; (5) amorphous palagonite surrounding fragments of basaltic glass; and (6) Fe-and Mn-oxides. These findings demonstrate the effectiveness of combining EMPA, FIB, and TEM to identify solid-phase contaminant hosts. Furthermore, they highlight the complexity of U geochemistry in the Hanford vadose zone, and illustrate the importance of microscopic transport in controlling the fate of contaminant metals in the environment.

Multi-scale investigation of uranium attenuation by arsenic at an abandoned uranium mine, South Terras

“…[14][15][16][17] Owing to the higher solubility of uranyl-phosphate minerals compared with uranyl-arsenates (with solubility products of 10 −12.8 and 10 −49.2 for metatorbernite and metazeunerite, respectively), 24 it might be expected that arsenate end-members of this mineral family would dominate over the phosphate end-members, which is indeed observed in the Rietveld analysis of data shown in Fig. 3b, and composition inferred from interpolation of the unit cell volume of the end members.…”

“…Assuming the solid solution to follow Vegards Law, our unit cell volume suggests 18% solid solution of metatorbenite in metazeunerite. Metatorbernite has previously been reported to sequester uranium at other uranium-contaminated sites such as Hanford (USA) [14][15][16][17] and has also been reported to occur at South Terras 3 . Although the occurrence of metazeunerite at South Terras has been previously inferred, 18 to our knowledge, this study provides conclusive and verifiable evidence for this phase at the site, and indeed for any studied uranium-contaminated vadose sediments.…”

“…Although the occurrence of metazeunerite at South Terras has been previously inferred, 18 to our knowledge, this study provides conclusive and verifiable evidence for this phase at the site, and indeed for any studied uranium-contaminated vadose sediments. [14][15][16][17] In the µ-XANES analysis of the copper, uranium, arsenic particle shown in Fig. 3a, the reduced intensity of the white line, and the presence of a shoulder feature (labelled 'A', Fig.…”

An integrated study of uranyl mineral dissolution processes: etch pit formation, effects of cations in solution, and secondary precipitation

“…For example, Stubbs et al [2] identified metatorbernite, Cu(H 2 O)[(UO 2 ) 2 (PO 4 ) 2 ], and five other hosts for U 6+ in contaminated sediments at the Hanford site. The above results suggest that interactions of these hosts with the local pore water (pH 7.2, Um et al [60]) result in the formation of nanometer-thick coatings of uranyl minerals on the surface of each host.…”

“…These minerals are important in controlling the mobility of uranium in the contaminated subsurface. For example, the uranyl silicate, boltwoodite, and the uranyl phosphate, metatorbernite, have formed in contaminated sediments of the Hanford site [2][3][4], uranyl phosphates occur in contaminated soils of the Fernald site [5], and uranyl sulfates are common in altered mine tailings [6]. The interactions of these minerals with aqueous systems, and the details of how they dissolve and precipitate, are integral to understanding and predicting the mobility of uranium.…”