ABSTRACT
We investigated the influence of natural organic matter (NOM) on the behavior of Pu(V) in the vadose zone through a combination of the field lysimeter and laboratory studies. Well-defined solid sources of NH4Pu(V)O2CO3(s) were placed in two 5-L lysimeters containing NOM-amended soil collected from the Savannah River Site (SRS) or unamended vadose zone soil and exposed to 3 years of natural South Carolina, USA, meteorological conditions. Lysimeter soil cores were removed from the field, used in desorption experiments, and characterized using wet chemistry methods and X-ray absorption spectroscopy. For both lysimeters, Pu migrated slowly with the majority (>95%) remaining within 2 cm of the source. However, without the NOM amendment, Pu was transported significantly farther than in the presence of NOM. Downward Pu migration appears to be influenced by the initial source oxidation state and composition. These Pu(V) sources exhibited significantly greater migration than previous studies using Pu(IV) or Pu(III) sources. However, batch laboratory experiments demonstrated that Pu(V) is reduced by the lysimeter soil in the order of hours, indicating that downward migration of Pu may be due to cycling between Pu(V) and Pu(IV). Under the conditions of these experiments, NOM appeared to both enhance reduction of the Pu(V) source as well as Pu sorption to soils. This indicates that NOM will tend to have a stabilizing effect on Pu migration under SRS vadose zone field conditions.
Subject(s)
Plutonium , Soil Pollutants, Radioactive , Water Pollutants, Radioactive , Rivers , South CarolinaABSTRACT
Due to its radiotoxicity, long half-life, and potentially high environmental mobility, neptunium transport is of paramount importance for risk assessment and safety. Environmental transport of neptunium through field lysimeters at the Savannah River Site was observed from both oxidized (Np(V)) and reduced (Np(IV)) source materials. While transport from oxidized neptunium sources was expected, the unexpected transport from reduced neptunium sources spurred further investigation into transport mechanisms. Partial oxidation of the reduced neptunium source resulted in significant release and transport into the mobile aqueous phase, though a reduced colloidal neptunium species appears to have also been present, enhancing neptunium mobility over shorter distances. These field and laboratory experiments demonstrate the multiple controls on neptunium vadose zone transport and chemical behavior, as well as the need for thorough understanding of radionuclide source terms for long-term risk prediction.
Subject(s)
Neptunium , Oxidation-ReductionABSTRACT
The preparation and structure of three trivalent plutonium chloride compounds from aqueous solution is reported. Two of the three are plutonium tetraaquatetrachloro complexes exhibiting a cis and a trans arrangement of Cl about the Pu. The identification of the coordination number of 4 with respect to Cl and the isomerism are both unprecedented in actinide solution chemistry. The third complex is a hexaaquadichloro complex of Pu(III), predicted by available thermodynamic data.
ABSTRACT
The reaction of NpO(2) with SeO(2) in the presence of CsCl at 180 degrees C results in the formation of Np(NpO(2))(2)(SeO(3))(3) (1). The structure of 1 consists of three crystallographically unique Np centers with three different coordination environments in two different oxidation states. Np(1) is found in a neptunyl(V), O[double bond]Np[double bond]O(+), unit that is further ligated in the equatorial plane by three chelating SeO(3)(2-) anions to create a hexagonal bipyramidal NpO(8) unit. A second neptunyl(V) cation also occurs for Np(2); it is bound by four bridging selenite anions and by the oxo atom from the Np(1) neptunyl cation to form a pentagonal bipyramidal, NpO(7), unit. The third neptunium center, Np(3), which contains Np(IV), is found in a distorted NpO(8) dodecahedron. Np(3) is bound by five bridging selenite anions and by three neptunyl units via cation-cation interactions. The NpO(7) pentagonal bipyramids and NpO(8) hexagonal bipyramids share both corners and edges. Both of these polyhedra share corners via cation-cation interactions with the NpO(8) dodecahedra creating a three-dimensional structure with small channels that house the stereochemically active lone pair of electrons on the selenite anions. Magnetic susceptibility data follow Curie-Weiss behavior over the entire temperature range measured (5 < or = T < or = 320 K). The effective moment, mu(eff) = 2.28 mu(B), which represents an average over the three crystallographically inequivalent Np atoms, is within the expected range of values. There is no evidence of long-range ordering of the Np moments at temperatures down to 5 K, consistent with the negligible Weiss constant determined from fitting the susceptibility data. Crystallographic data: 1, orthorhombic, space group Pbca, a = 10.6216(5), b = 11.9695(6), and c = 17.8084(8) A and Z = 8 (T = 193 K).
ABSTRACT
Two new layered uranyl selenites, [C(4)H(12)N(2)](0.5)[UO(2)(HSeO(3))(SeO(3))] (1) and [C(6)H(14)N(2)](0.5)[UO(2)(HSeO(3))(SeO(3))].0.5H(2)O.0.5CH(3)CO(2)H (2), have been isolated from mild hydrothermal reactions. The preparation of 1 was achieved by reacting UO(2)(C(2)H(3)O(2))(2).2H(2)O with H(2)SeO(4) in the presence of piperazine at 130 degrees C for 2 d. Crystals of 2 were synthesized by reacting UO(2)(C(2)H(3)O(2))(2).2H(2)O, H(2)SeO(4), and 1,4-diazabicyclo[2.2.2]octane at 150 degrees C for 2 d. The structure of 1 consists of UO(2)(2+) cations that are bound by bridging HSeO(3)(-) anions and chelating/bridging SeO(3)(2)(-) anions to yield UO(7) pentagonal bipyramids. The joining of the uranyl moieties by the hydrogen selenite and selenite anions creates two-dimensional 2(infinity) [UO(2)(HSeO(3))(SeO(3))](-) layers that extend in the bc-plane. The stereochemically active lone pair of electrons on the HSeO(3)(-) and SeO(3)(2)(-) anions align along the a-axis making each layer polar. The 2(infinity)[UO(2)(HSeO(3))(SeO(3))](-) layers and piperazinium cations stack in a AA'BAA'B sequence where two layers stack on one another without intervening piperazinium cations. While each 2(infinity)[UO(2)(HSeO(3))(SeO(3))](-) layer is polar, in the AA' stacking, the polarity of the second sheet is reversed with respect to the first, yielding an overall structure that is centrosymmetric. The structure of 2 is constructed from uranyl cations that are bound by three bridging SeO(3)(2)(-) and two bridging HSeO(3)(-) anions to create UO(7) pentagonal bipyramids. The linking of the uranyl cations by the HSeO(3)(-) and SeO(3)(2-) anions creates 2(infinity)[UO(2)(HSeO(3))(SeO(3))](-) layers that extend in the ac-plane. In 1 and 2, the organic ammonium cations form hydrogen bonds with the anionic uranyl selenite layers. Crystallographic data: 1, monoclinic, space group P2(1)/c, a = 10.9378(5) A, b = 8.6903(4) A, c = 9.9913(5) A, beta = 90.3040(8) degrees, Z = 4; 2, orthorhombic, space group Pnma, a = 13.0858(8) A, b = 17.555(1) A, c = 10.5984(7) A, Z = 8.
ABSTRACT
The hydrothermal reaction of NpO(2) with IO(3)(-) in the presence of nitrate results in the formation of NpO(2)(IO(3)) (1). Under similar conditions, NpO(2) reacts with AgNO(3) and SeO(2) to yield alpha-AgNpO(2)(SeO(3)) (2) and beta-AgNpO(2)(SeO(3)) (3). The structure of 1 consists of distorted pentagonal bipyramidal Np(V) centers that are bridged by iodate anions. In addition, the oxo atoms of the neptunyl(V) cations coordinate adjacent Np(V) centers creating layers that are linked into a three-dimensional network structure by the iodate anions. The structure is polar owing to the alignment of the stereochemically active lone pair of electrons on the iodate anions along the c-axis. alpha-AgNpO(2)(SeO(3)) (2) forms a layered structure consisting of hexagonal bipyramidal NpO(8) polyhedra that are bound by chelating and bridging selenite anions. The primary and secondary structures of 3 are similar to those of 1, and neptunyl-neptunyl interactions are partially responsible for the creation of a three-dimensional network structure. However, the selenite anions in 3 are rotated with respect to the iodate anions found in 1, and the structure is centrosymmetric. The network found in 3 consists of interconnecting, approximately square channels that house the Ag(+) cations. A bond-valance sum parameter of 2.036 A for Np(V) bound exclusively to oxygen has been developed with b = 0.37 A. Crystallographic data: 1, orthorhombic, space group Pna2(1), a = 13.816(2) A, b = 5.8949(8) A, c = 5.5852(8) A, Z = 4; 2, monoclinic, space group P2(1)/n, a = 4.3007(3) A, b = 9.5003(7) A, c = 11.5877(9) A, beta = 95.855(1) degrees, Z = 4; 3, triclinic, space group Ponemacr;, a = 7.1066(6) A, b = 8.3503(7) A, c = 8.3554(7) A, alpha = 89.349(1) degrees, beta = 77.034(1) degrees, gamma = 76.561(1) degrees, Z = 2.
ABSTRACT
The reactions of UO(2)(C(2)H(3)O(2))(2).2H(2)O with K(2)TeO(3).H(2)O, Na(2)TeO(3) and TlCl, or Na(2)TeO(3) and Sr(OH)(2).8H(2)O under mild hydrothermal conditions yield K[UO(2)Te(2)O(5)(OH)] (1), Tl(3)[(UO(2))(2)[Te(2)O(5)(OH)](Te(2)O(6))].2H(2)O (2) and beta-Tl(2)[UO(2)(TeO(3))(2)] (3), or Sr(3)[UO(2)(TeO(3))(2)](TeO(3))(2) (4), respectively. The structure of 1 consists of tetragonal bipyramidal U(VI) centers that are bound by terminal oxo groups and tellurite anions. These UO(6) units span between one-dimensional chains of corner-sharing, square pyramidal TeO(4) polyhedra to create two-dimensional layers. Alternating corner-shared oxygen atoms in the tellurium oxide chains are protonated to create short/long bonding patterns. The one-dimensional chains of corner-sharing TeO(4) units found in 1 are also present in 2. However, in 2 there are two distinct chains present, one where alternating corner-shared oxygen atoms are protonated, and one where the chains are unprotonated. The uranyl moieties in 2 are bound by five oxygen atoms from the tellurite chains to create seven-coordinate pentagonal bipyramidal U(VI). The structures of 3 and 4 both contain one-dimensional [UO(2)(TeO(3))(2)](2-) chains constructed from tetragonal bipyramidal U(VI) centers that are bridged by tellurite anions. The chains differ between 3 and 4 in that all of the pyramidal tellurite anions in 3 have the same orientation, whereas the tellurite anions in 4 have opposite orientations on each side of the chain. In 4, there are also additional isolated TeO(3)(2-) anions present. Crystallographic data: 1, orthorhombic, space group Cmcm, a = 7.9993(5) A, b = 8.7416(6) A, c = 11.4413(8) A, Z = 4; 2, orthorhombic, space group Pbam, a = 10.0623(8) A, b = 23.024(2) A, c = 7.9389(6) A, Z = 4; 3, monoclinic, space group P2(1)/n, a = 5.4766(4) A, b = 8.2348(6) A, c = 20.849(3) A, beta = 92.329(1) degrees, Z = 4; 4, monoclinic, space group C2/c, a = 20.546(1) A, b = 5.6571(3) A, c = 13.0979(8) A, beta = 94.416(1) degrees, Z = 4.
ABSTRACT
The transition metal, alkali metal, and main group uranyl selenites, Ag(2)(UO(2))(SeO(3))(2) (1), K[(UO(2))(HSeO(3))(SeO(3))] (2), Rb[(UO(2))(HSeO(3))(SeO(3))] (3), Cs[(UO(2))(HSeO(3))(SeO(3))] (4), Tl[(UO(2))(HSeO(3))(SeO(3))] (5), and Pb(UO(2))(SeO(3))(2) (6), have been prepared from the hydrothermal reactions of AgNO(3), KCl, RbCl, CsCl, TlCl, or Pb(NO(3))(2) with UO(3) and SeO(2) at 180 degrees C for 3 d. The structures of 1-5 contain similar [(UO(2))(SeO(3))(2)](2-) sheets constructed from pentagonal bipyramidal UO(7) units that are joined by bridging SeO(3)(2-) anions. In 1, the selenite oxo ligands that are not utilized within the layers coordinate the Ag(+) cations to create a three-dimensional network structure. In 2-5, half of the selenite ligands are monoprotonated to yield a layer composition of [(UO(2))(HSeO(3))(SeO(3))](1-), and coordination of the K(+), Rb(+), Cs(+), and Tl(+) cations occurs through long ionic contacts. The structure of 6 contains a uranyl selenite layered substructure that differs substantially from those in 1-5 because the selenite anions adopt both bridging and chelating binding modes to the uranyl centers. Furthermore, the Pb(2+) cations form strong covalent bonds with these anions creating a three-dimensional framework. These cations occur as distorted square pyramidal PbO(5) units with stereochemically active lone pairs of electrons. These polyhedra align along the c-axis to create a polar structure. Second-harmonic generation (SHG) measurements revealed a response of 5x alpha-quartz for 6. The diffuse reflectance spectrum of 6 shows optical transitions at 330 and 440 nm. The trailing off of the 440 nm transition to longer wavelengths is responsible for the orange coloration of 6.
