Complexation plutonium

The Table shows a great spread in Kd-values even at the same location. This is due to the fact that the environmental conditions influence the partition of plutonium species between different valency states and complexes. For the different actinides, it is found that the Kd-values under otherwise identical conditions (e.g. for the uptake of plutonium on geologic materials or in organisms) decrease in the order Pu>Am>U>Np (15). Because neptunium is usually pentavalent, uranium hexavalent and americium trivalent, while plutonium in natural systems is mainly tetravalent, it is clear from the actinide homologue properties that the oxidation state of plutonium will affect the observed Kd-value. The oxidation state of plutonium depends on the redox potential (Eh-value) of the ground water and its content of oxidants or reductants. It is also found that natural ligands like C032- and fulvic acids, which complex plutonium (see next section), also influence the Kd-value. 

The first step is diffusion controlled while the second represents the formation of an outer sphere complex in which the metal ion and the ligand are separated by at least one molecule to water. In the final step, this outer sphere complex ejects the water and forms an inner sphere complex in which the metal and ligand are directly associated. Some ligands cannot displace the water and complexation apparently terminates with the formation of the outer sphere complex. Plutonium cations form both inner and outer sphere complexes, depending on the ligand pK. For trivalent plutonium, we can assign a predominant outer sphere character to the halide, nitrate, sulfonate and trichloroacetate complexes and an inner sphere character to the fluoride, iodate, sulfate and acetate complexes (23). A study of Am , Th and complexation... 

Carbonate Complexes. Of the many ligands which are known to complex plutonium, only those of primary environmental concern, that is, carbonate, sulfate, fluoride, chloride, nitrate, phosphate, citrate, tributyl phosphate (TBP), and ethylenediaminetet-raacetic acid (EDTA), will be discussed. Of these, none is more important in natural systems than carbonate, but data on its reactions with plutonium are meager, primarily because of competitive hydrolysis at the low acidities that must be used. No stability constants have been published on the carbonate complexes of plutonium(III) and plutonyl(V), and the data for the plutoni-um(IV) species are not credible. Results from studies on the solubility of plutonium(IV) oxalate in K2CO3 solutions of various concentrations have been interpreted to indicate the existence of complexes as high as Pu(C03) , a species that is most unlikely from both electrostatic and steric considerations. From the influence of K2CO3 concentration on the solubility of PuCOH) at an ionic strength of 10 M, the stability constant of the complex Pu(C03) was calculated (10) to be 9.1 X 10 at 20°. This value... 

Fluoride Complexes. Fluoride is known to complex plutonium strongly, but quantitative data on these environmentally important complexes are limited. Cation exchange studies (17) yielded values of 4.5 X 10 at I = 1 M and 7.9 X 10 at I = 2 M for -the stability constant of the monofluoro complex of plutonium(IV), which are in satisfactory agreement with the value 1.2 X 10 obtained from... 

The plutonium(V) analog of the neptunium(V) [22]hexaphyrin( 1.0.1.0.0.0) was confirmed by UV-vis spectroscopy. In this complex, plutonium(VI) is exposed to the ligand and is reduced to pluto-nium(V). It is postulated that the coordination is the same as the neptunium(V) complex. ... 

Although the redox potentials for aqueous solution indicate that uranium(IV) should reduce plutonium(IV), anions and other complexing agents can change the potentials sufficiently that uranium(IV) and plutonium(IV) can coexist in solution (25). Since one of the products of photochemical reduction of uranyl by TBP is dibutyl phosphate (DBP), which complexes plutonium(IV) strongly, experiments were done to test the photochemically produced urani-um(IV) solutions as plutonium(IV) reductants (26). Bench-scale stationary tests showed these solutions to be equivalent to hydroxylamine nitrate solutions stabilized with hydrazine (27). 

Plutonium complexes. Plutonium ions form complexes with many anions. The most important of the complexes are those that form with Pu, some of which are listed in Table 9.20, in order of increasing stability. 

The many possible oxidation states of the actinides up to americium make the chemistry of their compounds rather extensive and complicated. Taking plutonium as an example, it exhibits oxidation states of -E 3, -E 4, +5 and -E 6, four being the most stable oxidation state. These states are all known in solution, for example Pu" as Pu ", and Pu as PuOj. PuOl" is analogous to UO , which is the stable uranium ion in solution. Each oxidation state is characterised by a different colour, for example PuOj is pink, but change of oxidation state and disproportionation can occur very readily between the various states. The chemistry in solution is also complicated by the ease of complex formation. However, plutonium can also form compounds such as oxides, carbides, nitrides and anhydrous halides which do not involve reactions in solution. Hence for example, it forms a violet fluoride, PuFj. and a brown fluoride. Pup4 a monoxide, PuO (probably an interstitial compound), and a stable dioxide, PUO2. The dioxide was the first compound of an artificial element to be separated in a weighable amount and the first to be identified by X-ray diffraction methods. 

Table 6 presents a summary of the oxidation—reduction characteristics of actinide ions (12—14,17,20). The disproportionation reactions of UO2, Pu , PUO2, and AmO are very compHcated and have been studied extensively. In the case of plutonium, the situation is especially complex four oxidation states of plutonium [(111), (IV), (V), and (VI) ] can exist together ia aqueous solution ia equiUbrium with each other at appreciable concentrations. 

A number of organic compounds, eg, acetylacetone [123-54-6] and cupferron [135-20-6] form compounds with aqueous actinide ions (IV state for reagents mentioned) that can be extracted from aqueous solution by organic solvents (12). The chelate complexes are especially noteworthy and, among these, the ones formed with diketones, such as 3-(2-thiophenoyl)-l,l,l-trifluoroacetone [326-91-0] (C4H2SCOCH2COCF2), are of importance in separation procedures for plutonium. 

Historically, ferrous sulfamate, Fe(NH2S02)2, was added to the HNO scmbbing solution in sufficient excess to ensure the destmction of nitrite ions and the resulting reduction of the Pu to the less extractable Pu . However, the sulfate ion is undesirable because sulfate complexes with the plutonium to compHcate the subsequent plutonium purification step, adds to corrosion problems, and as SO2 is an off-gas pollutant during any subsequent high temperature waste solidification operations. The associated ferric ion contributes significantly to the solidified waste volume. 

Uranium Purification. Subsequent uranium cycles provide additional separation from residual plutonium and fission products, particularly zirconium— niobium and mthenium (30). This is accompHshed by repeating the extraction/stripping cycle. Decontamination factors greater than 10 at losses of less than 0.1 wt % are routinely attainable. However, mthenium can exist in several valence states simultaneously and can form several nitrosyl—nitrate complexes, some for which are extracted readily by TBP. Under certain conditions, the nitrates of zirconium and niobium form soluble compounds or hydrous coUoids that compHcate the Hquid—Hquid extraction. SiUca-gel adsorption or one of the similar Hquid—soHd techniques may also be used to further purify the product streams. 

Plutonium was the first element to be synthesized in weighable amounts (6,7). Technetium, discovered in 1937, was not isolated until 1946 and not named until 1947 (8). Since the discovery of plutonium in 1940, production has increased from submicrogram to metric ton quantities. Because of its great importance, more is known about plutonium and its chemistry than is known about many of the more common elements. The metallurgy and chemistry are complex. MetaUic plutonium exhibits seven aUotropic modifications. Five different oxidation states are known to exist in compounds and in solution. 

Plutonium(III) in aqueous solution, Pu " ( 4)> is pale blue. Aqueous plutonium(IV) is tan or brown the nitrate complex is green. Pu(V) is pale red-violet or pink in aqueous solution and is beUeved to be the ion PuO Pu(VI) is tan or orange in acid solution, and exists as the ion PuO. In neutral or basic solution Pu(VI) is yellow cationic and anionic hydrolysis complexes form. Pu(VII) has been described as blue-black. Its stmcture is unknown but may be the same as the six-coordinate NpO (OH) (91). Aqueous solutions of each oxidation state can be prepared by chemical oxidants or reductants... 

Evidence foi the anionic complex PuCP is the precipitation of complex halides such as Cs2PuClg from concentrated HCl (aq). The ability of Pu(IV) to form stable nitrate complexes provides the basis for the Purex and ion-exchange (qv) process used in the chemical processing of Pu (107). Pu(VI) is similar to Pu(IV) in its abihty to form complex ions. Detailed reviews of complex ion formation by aqueous plutonium are available (23,94,105). 

Other Coordination Complexes. Because carbonate and bicarbonate are commonly found under environmental conditions in water, and because carbonate complexes Pu readily in most oxidation states, Pu carbonato complexes have been studied extensively. The reduction potentials vs the standard hydrogen electrode of Pu(VI)/(V) shifts from 0.916 to 0.33 V and the Pu(IV)/(III) potential shifts from 1.48 to -0.50 V in 1 Tf carbonate. These shifts indicate strong carbonate complexation. Electrochemistry, reaction kinetics, and spectroscopy of plutonium carbonates in solution have been reviewed (113). The solubiUty of Pu(IV) in aqueous carbonate solutions has been measured, and the stabiUty constants of hydroxycarbonato complexes have been calculated (Fig. 6b) (90). 

Plutonium(III), (IV), and (VI) complex stabiUty constants have been determined for some oxygen-donor (carboxylate) (114) and a few nitrogen-donor (115,116) ligands. Complexes of plutonium with natural complexants such as humic acids have also been studied extensively (89). 

Only slightly less accurate ( 0.3—0.5%) and more versatile in scale are other titration techniques. Plutonium maybe oxidized in aqueous solution to PuO " 2 using AgO, and then reduced to Pu" " by a known excess of Fe", which is back-titrated with Ce" ". Pu" " may be titrated complexometricaHy with EDTA and a colorimetric indicator such as Arsenazo(I), even in the presence of a large excess of UO " 2- Solution spectrophotometry (Figs. 4 and 5) can be utilized if the plutonium oxidation state is known or controlled. The spectrophotometric method is very sensitive if a colored complex such as Arsenazo(III) is used. Analytically usehil absorption maxima and molar absorption coefficients ( s) are given in Table 10. Laser photoacoustic spectroscopy has been developed for both elemental analysis and speciation (oxidation state) at concentrations of lO " — 10 M (118). Chemical extraction can also be used to enhance this technique. 

There are numerous complex (ternary and quaternary) plutonium oxides. Their properties have been reviewed (30). Plutonium oxidizes readily to Pu(VI) from binary oxides to complex oxides such as Ba PuO. The best way to oxidize Pu to Pu(VII) is to prepare complex oxides such as Li PuO from Li20 and PUO2 iu flowing oxygen (85). 

Plutonium trichloride, PUCI3, is the only known binary chloride of plutonium. No higher chloride in the soHd state has been prepared so far. The trichloride is a blue-green soHd and can be prepared from the reaction of Pu metal or PUO2, and HCl gas at elevated temperatures, and can be purified by sublimation and zone melting. The yellow complex chloride, CS2PUCI3, is stoichiometric, stable, and can be used as a primary analytical standard for plutonium (156). 

Plutonium(lV), Pu+Yagj, forms a complex ion with fluoride ion, PUF+3 ... 

The pattern of iridium halides resembles rhodium, with the higher oxidation states only represented by fluorides. The instability of iridium(IV) halides, compared with stable complexes IrCl4L2 and the ions IrX (X = Cl, Br, I), though unexpected, finds parallels with other metals, such as plutonium. Preparations of the halides include [19]... 

Ammonium ions, tetradecyldimethylbenzyl-liquid—Liquid extraction, 1, 548 Ammonium molybdate, 3,1257 Ammonium nitrate, hydroxyl-as plutonium(IV) reductant Purex process, 6, 949 Amphotericin B metal complexes, 2, 973 a-Amylase zinc, 6, 607 Anabaena spp. 

Hydrazine, bis(allylthiocarbamido)-in gravimetry, 1, 534 liquid-liquid extraction, 1,546 Hydrazines, 2, 101 macrocyclic synthesis, 2,904 metal complexes, 2, 30 in plutonium extraction, 6, 948 Hydrazoic acid... 

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