Unraveling the coordination chemistry will improve plutonium separation by the precipitation processes

Structural characterization of compounds targeted in plutonium separation for the past 50 years

Since the early days of its discovery, the chemical separation and purification of plutonium has been directly correlated with its applications in weapons and nuclear fuel technologies. However, large-scale processing schemes were often dictated by historically required chemical conditions that were found to be the most convenient for precipitation, ion exchange, or solvent extraction. The pragmatic focus on developing a separation scheme outweighed the need for a more fundamental understanding of the technical basis for plutonium separation. In more modern times, changing regulatory requirements for lower levels of contaminants in waste waters requires process optimization that, without a fundamental understanding of how the process works, will likely not be achieved. Examples are the precipitation of plutonium oxalate (C2O42–) and iodate (IO3–) compounds, which were recognized early on as excellent candidates for plutonium separation owing to their remarkably low solubility in slightly acidic solutions.

Blue Pu(III) oxalate crystals.

Oxalate forms strong complexes with the lower oxidation states, Pu(III) and (IV), whereas iodate can effectively precipitate Pu(III), (IV), and (VI). In fact, the use of oxalate to separate plutonium from solution has long been implemented into plutonium purification and waste-treatment processes, while plutonium iodate precipitation has been used less abundantly for analytical purposes. Despite the fact that oxalate and iodate compounds have been applied for about 50 years in large-scale separations, the nature and composition of the actual precipitates are still under debate. Unraveling the uncertainties in the chemistry of these compounds is necessary to improve and optimize the precipitation processes currently used.

The solid plutonium oxalate system is dominated by two hydrates (crystals incorporating water molecules into their structures), Pu₂(C₂O₄)₃•10H₂O and Pu(C₂O₄)₂•6H₂O, for which only powder x-ray diffraction data have been reported. Several different hydrates are suggested to exist at different temperatures, without strong experimental evidence regarding how the water molecules are actually bound within the respective crystals.

To gain better insight into the oxalates, we obtained single crystals of blue Pu(III) oxalate by slowly reducing Pu(IV) oxalate in 1 M HCl acid at ambient temperature. The x-ray diffraction analysis revealed this compound to be the hydrated Pu(III) oxalate of formula Pu₂(C₂O₄)₃(H₂O)6•3H₂O. (We write the formula this way to indicate that three H₂O molecules are directly bound to each Pu atom, while three H₂O molecules are incorporated in the crystalline lattice.)

Ball and stick illustration of the Pu(C₂O₄)₃(H₂O)₃^3- unit in the solid Pu(III) oxalate Pu₂(C₂O₄)₃(H₂O)6•3H₂O. The dark lines represent the trigonal prism with the Pu atom (green) in the center, which is coordinated to nine O atoms (red). The prism is capped by three O atoms contributed from three oxalate (C₂O₄^2-) groups. C atom (black).

Crystal packing in Pu₂(C₂O₄)₃(H₂O)6•3H^{2O viewed down the crystallographic b axis. The two-dimensional structure contains layers that orient themselves in the ac plane. The PuO₉ units are represented as green polyhedra; the planar oxalate groups are black.}

The two-dimensional compound consists of [PuO₉] polyhedra that are linked by [C₂O₄] groups and a network of interstitial water molecules. The plutonium atom is coordinated to six oxygen atoms from three bidentate oxalate ligands and three oxygen atoms from coordinated waters in a distorted tricapped prismatic geometry. In this geometry, six oxygen atoms form the vertices of a trigonal antiprism and each of the three faces is capped by water. The Pu–O distances range between 2.48 Å and 2.57 Å.

In the complex mixtures of actual plutonium process streams and the presence of high concentrations of pyrochemical salt waste such as CaCl₂ or Na/KCl it is expected that the binary Pu oxalates incorporate cations (Ca²⁺, Na⁺, or K⁺ ions) in exchange for lattice waters. This exchange would increase the distance between the plutonium oxalate layers as has been observed in many-layered mineral phases. In the presence of K⁺ cations and under hydrothermal conditions at 180 °C, green crystalline needles of the ternary Pu(IV) oxalate, KPu(C₂O₄)₂OH•2H₂O, were obtained.

Arrangement of the oxalate ligands in the Pu(C₂O₄)₄(H₂O)^5- units that build up the Pu(IV) oxalate, KPu(C₂O₄)₂(H₂O)•2H₂O. The nine-coordinate Pu atom (green) is surrounded by eight O atoms (red) from four oxalate ligands and one water molecule. C atom (black).

View of the three-dimensional structure of the Pu(IV) oxalate KPu(C₂O₄)₂(H₂O)•2H₂O along the ac plane. Water molecules (red) and K atoms (purple) are aligned down the b axis. The PuO₉ units are represented as green polyhedra; the planar oxalate groups are black.

Surprisingly, this compound consists of a three-dimensional framework built up from oxalate-linked [PuO₉] polyhedra. Although plutonium again exhibits a coordination number of nine, the local coordination geometry is quite different. Eight oxygen atoms from four chelating oxalate (Pu–O = 2.48–2.54 Å) ligands are arranged around the plutonium atom while one hydroxide ligand (Pu–OH = 2.48 Å) lies above the plutonium atom. This coordination geometry is similar to that found for the limiting complex of trivalent neodymium in carbonate solution, Nd(CO₃)4(H₂O)^5-. It is very likely that Pu(C₂O₄)₄(OH)^5- is the limiting solution complex at high oxalate concentrations and assumes very similar coordination geometry as found in the extended structure of KPu(C₂O₄)₂OH•2H₂O.

Illustration of the PuO₂(IO₃)₅^3- building block in the Pu(VI) iodate PuO2(IO3)2•H2O. The Pu atom (green) is coordinated in a pentagonal bipyramidal geometry to five iodate anions, IO₃^- (I = purple, O = red) and to two O atoms within the characteristic linear plutonyl, [O=Pu=O]²⁺, moiety.

Crystal packing in PuO₂(IO₃)₂•H₂O. The two-dimensional solid (viewed down the a axis) contains infinite staggered Pu(VI) iodate layers that are separated by water molecules. The PuO₇ units are represented as green polyhedra; the trigonal iodate groups are purple. O atoms from lattice H₂O molecules are red.

Iodate precipitation was used for oxidation state determination of plutonium in October 1942 when Pu(IV) was precipitated from HNO₃ solutions upon addition of HIO₃ or KIO₃. The calculated molecular weight did not match the suggested formula, Pu(IO3)₄, indicating the possible presence of KIO₃ or HIO₃ in the solid or even the presence of other plutonium oxidation states. Due to the oxidizing nature of iodate, synthesizing a crystalline Pu(III) or (IV) iodate has proved an intractable task, raising some doubt as to the validity of a Pu(IV) iodate precipitate. The use of KIO₄, I2O₅, or H₅IO₆ (E°(IO₄^-/IO₃^-) = 1.65 V) led to the complete oxidation of lower plutonium oxidation states to Pu(VI) (i.e., E°(Pu₄₊/PuO₂²⁺) = 0.98 V) and the crystallization of a Pu(VI) iodate of formula PuO₂(IO₃)₂•H₂O .

The binary plutonyl(VI) iodate, PuO₂(IO₃)₂•H₂O , is unique in its structure and displays previously unknown actinyl-iodate coordination. This plutonyl compound is isostructural with that of NpO₂(IO₃)₂•H₂O and consists of infinite staggered layers of PuO₂(IO₃)₂ with water molecules arranged between the layers. The layers are made up of pentagonal bipyramidal [PuO₇] polyhedra that are connected by bridging pyramidal IO₃ units.

Coordination environment of Am(III) (green) in the pseudo-tricapped trigonal prismatic coordination in K₃Am₃(IO₃)₁₂•HIO₃. All oxygen atoms (red) originate from eight iodate (IO₃^-) anions, which link the Am polyhedra in a three-dimensional framework. The longer bond between the Am center and the O atom (dashed line) from the neutral HIO₃ molecule completes the ninefold coordination of the Am atom.

Crystal packing in K₃Am₃(IO₃)₁₂•HIO₃. Views of the K (blue)-lined channels formed along the c axis, which are built up from alternating [AmO₈] polyhedra (green) and iodate (purple) ligands with the neutral HIO₃ (purple) molecules staggered in the center of the channel.

Axial Pu=O bond lengths are 1.75Å, characteristic for PuO₂²⁺, and equatorial Pu–O distances range between 2.33 and 2.42 Å. Two crystallographically unique iodate anions, IO₃^-, connect the plutonium atoms. The [I(1)O₃] groups join two plutonium atoms through bridging oxygen atoms, leaving one oxygen atom terminal; in contrast, the other iodate group, [I(2)O₃], coordinates three plutonyl units. This coordination has only been observed previously in the trivalent lanthanide iodate compounds.

To investigate the formation and coordination chemistry of Pu(III) iodates, the chemically analogous Am(III) was used because Am remains stable in its +3 oxidation state even in the presence of any of the iodate sources mentioned (E°(Am³⁺/AmO₂²⁺) = 1.69 V). Pink crystals were successfully synthesized by reacting 243Am(III) in 3 M HCl acid at 180 °C with KIO₄ solutions. X-ray diffraction analysis revealed the compound to be K₃Am₃(IO₃)₁₂•HIO₃ , a three-dimensional framework of [AmO₈] units bridged by corner-sharing [IO₃] pyramids. Eight oxygen atoms from eight iodate groups with Am–O distances between 2.42(3) and 2.60(3) Å form a distorted bicapped trigonal prismatic coordination polyhedron.

The most interesting aspect of this compound lies within the microporous channel framework. Three [AmO₈] and three [IO₃] groups form irregular hexagonal channels that are approximately 4.6 Å in diameter. Neutral HIO₃ molecules are staggered in the center of these channels and potassium cations line the cavity with close contacts to the HIO₃ molecule (K–O = 2.40(4) Å). The neutral HIO₃ molecules are anchored in the cavity center through a weak interaction between the oxygen atoms of the HIO₃ molecules and the americium atoms with a longer Am–O distance of 2.93(4) Å.

Although this is the first architecture of its kind among the f-element iodates, this structure suggests the possibility of the synthesis of new microporous lanthanide iodates with unique selectivity properties for ion-exchange, catalysis, and photochemical processes. It still must be shown if other molecules of varying size and shape can replace the trapped neutral molecules and if the replacement of potassium cations along the channel exterior will affect the retention strength of the trapped molecules.

From an academic perspective, structural characterization of transuranium complexes remains rare, and only about a dozen single crystal structures of americium compounds are known. As demonstrated in K₃Am₃(IO₃)₁₂•HIO₃, the coordination chemistry of americium may vary from its chemically analogous lanthanides and offers new insight into differences in bonding of 4f and 5f elements.

After more than 50 years of technological application of oxalate and iodate compounds, highly crystalline plutonium compounds have now been prepared using hydrothermal synthesis techniques, which has allowed for determination of their structural nature by x-ray diffraction. Chemical synthesis under hydrothermal conditions has proven to be a valuable method for targeting actinide compounds previously only known as amorphous or microcrystalline materials. Chemical conditions that mimic those used for large-scale plutonium separation and purification are used to trigger crystallization under process conditions, and the resulting solid precipitates can subsequently be structurally characterized. This work was supported by the Glenn T. Seaborg Institute Summer Student fellowship program and the Pu Stabilization and Scrap Recovery Program, under the direction of project leader Paul H. Smith.

This article was contributed by Los Alamos researchers Wolfgang Runde and Brian L. Scott of the Chemistry Division; and Amanda C. Bean, Kent Abney, and Paul H. Smith of the Nuclear Materials Technology Division.

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