Removal of Aqueous Uranyl and Arsenate Mixtures after Reaction with Limestone, PO43-, and Ca2.

The co-occurrence of uranyl and arsenate in contaminated water caused by natural processes and mining is a concern for impacted communities, including in Native American lands in the U.S. Southwest. We investigated the simultaneous removal of aqueous uranyl and arsenate after the reaction with limestone and precipitated hydroxyapatite (HAp, Ca10(PO4)6(OH)2). In benchtop experiments with an initial pH of 3.0 and initial concentrations of 1 mM U and As, uranyl and arsenate coprecipitated in the presence of 1 g L-1 limestone. However, related experiments initiated under circumneutral pH conditions showed that uranyl and arsenate remained soluble. Upon addition of 1 mM PO43- and 3 mM Ca2+ in solution (initial concentration of 0.05 mM U and As) resulted in the rapid removal of over 97% of U via Ca-U-P precipitation. In experiments with 2 mM PO43- and 10 mM Ca2+ at pH rising from 7.0 to 11.0, aqueous concentrations of As decreased (between 30 and 98%) circa pH 9. HAp precipitation in solids was confirmed by powder X-ray diffraction and scanning electron microscopy/energy dispersive X-ray. Electron microprobe analysis indicated U was coprecipitated with Ca and P, while As was mainly immobilized through HAp adsorption. The results indicate that natural materials, such as HAp and limestone, can effectively remove uranyl and arsenate mixtures.

Photochemical water oxidation and origin of nonaqueous uranyl peroxide complexes.

Sunlight photolysis of uranyl nitrate and uranyl acetate solutions in pyridine produces uranyl peroxide complexes. To answer longstanding questions about the origin of these complexes, we conducted a series of mechanistic studies and demonstrate that these complexes arise from photochemical oxidation of water. The peroxo ligands are easily removed by protonolysis, allowing regeneration of the initial uranyl complexes for potential use in catalysis.

Series of uranyl-4,4'-biphenyldicarboxylates and an occurrence of a cation-cation interaction: hydrothermal synthesis and in situ Raman studies.

Three uranium(VI)-bearing materials were synthesized hydrothermally using the organic ligand 4,4'-biphenyldicarboxylic acid: (UO2)(C14O4H8) (1); [(UO2)2(C14O4H8)2(OH)]·(NH4)(H2O) (2); (UO2)2(C14O4H8)(OH)2 (3). Compound 1 was formed after 1 day at 180 °C in an acidic environment (pH(i) = 4.03), and compounds 2 and 3 coformed after 3 days under basic conditions (pH(i) = 7.95). Coformation of all three compounds was observed at higher pH(i) (9.00). Ex situ Raman spectra of single crystals of 1-3 were collected and analyzed for signature peaks. In situ hydrothermal Raman data were also obtained and compared to the ex situ Raman spectra of the title compounds in an effort to acquire formation mechanism details. At pH(i) = 4.00, the formation of 1 was suggested by in situ Raman spectra. At an increased pH(i) (7.90), the in situ data implied the formation of compounds 1 and 3. The most basic conditions (pH(i) = 9.00) yielded a complex mixture of phases consistent with that of increased uranyl hydrolysis.

Uranyl peroxide oxalate cage and core-shell clusters containing 50 and 120 uranyl ions.

Cage clusters built from uranyl hexagonal bipyramids and oxalate ligands crystallize from slightly acidic aqueous solution under ambient conditions, facilitating structure analysis. Each cluster contains uranyl ions coordinated by peroxo ligands in a bidentate configuration. Uranyl ions are bridged by shared peroxo ligands, oxalate ligands, or through hydroxyl groups. U(50)Ox(20) contains 50 uranyl ions and 20 oxalate groups and is a topological derivative of the U(50) cage cluster that has a fullerene topology. U(120)Ox(90) contains 120 uranyl ions and 90 oxalate groups and is the largest and highest mass cluster containing uranyl ions that has been reported. It has a core-shell structure, in which the inner shell (core) consists of a cluster of 60 uranyl ions and 30 oxalate groups, identical to U(60)Ox(30), with a fullerene topology. The outer shell contains 12 identical units that each consist of five uranyl hexagonal bipyramids that are linked to form a ring (topological pentagon), with each uranyl ion also coordinated by a side-on nonbridging oxalate group. The five-membered rings of the inner and outer shells (the topological pentagons) are in correspondence and are linked through K cations. The inner shell topology has therefore templated the location of the outer shell rings, and the K counterions assume a structure-directing role. Small-angle X-ray scattering data demonstrated U(50)Ox(20) remains intact in aqueous solution upon dissolution. In the case of clusters of U(120)Ox(90), the scattering data for dissolved crystals indicates the U(60)Ox(30) core persists in solution, although the outer rings of uranyl bipyramids contained in the U(120)Ox(90) core-shell cluster appear to detach from the cluster when crystals are dissolved in water.