Plutonium , complex fluorides

In the case of the monofluorocomplexes of quadrivalent plutonium, it is obvious that the lower values obtained in chloride and nitrate media are due to complexing by these ions these results will not be discussed further. In HCIO4 media the data for the first two fluoride complexes are quite self-consistent and well within the same order of magnitude as these reported for the other quadrivalent actinides (12, 89). An extensive comparison would extend beyond the scope oT tKTs paper. In the case of PuF3+, extrapolation of bi to zero ionic strength is not warranted as such in view of the limited number of data. However, in the case of ThF3+ where the data extend over a very wide range of ionic... 

The effect of fluoride was further demonstrated by the increase in plutonium solubility in deionized water from about 11 percent to essentially 100 percent by addition of sufficient NaF to raise the fluoride concentration to that of basalt ground water. It is likely that the enhanced solubility of plutonium in waters containing high fluoride concentrations is the result of stabilization of Pu(IV) in solution by formation of fluoride complexes. Normally Pu(IV) is the least soluble of the four... 

Swanson, (31 ) Smith,(32) Phillips, ( 33 ) and others have described many aspects of "EHe Zirflex process but, unfortunately, not the form or charge of plutonium and other actinides in spent decladding solution. Presumably, soluble amounts of these elements are present as fluoride complexes, but this remains to be proven. 

Hydrofluoric acid is best known for its ability to dissolve silica, but even small quantities exert a marked catalytic effect on the nitric acid dissolution of many refractory oxides, including ignited plutonium oxide, due to the high stability of the fluoride complexes. Consequently, use of hydrofluoric acid in leaching procedures generally gives more accurate results. However, erratic... 

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... 

Table IV. Some Alkali Fluoride Protactinium ( V ), Uranium ( V ), Neptunium(V), Plutonium(V) Fluoride Complexes"...

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. 

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]... 

On the basis of these facts, it was speculated that plutonium in its highest oxidation state is similar to uranium (VI) and in a lower state is similar to thorium (IV) and uranium (IV). It was reasoned that if plutonium existed normally as a stable plutonium (IV) ion, it would probably form insoluble compounds or stable complex ions analogous to those of similar ions, and that it would be desirable (as soon as sufficient plutonium became available) to determine the solubilities of such compounds as the fluoride, oxalate, phosphate, iodate, and peroxide. Such data were needed to confirm deductions based on the tracer experiments. 

Only two complex fluorides of pentavalent plutonium are known, both having been prepared by Penneman et al. ( 1). One of these, Rb2PuF7, appeared to be stable its crystal structure ( 1) and its electronic spectrum (2) have been reported. The other, CsPuF6, appeared to decompose after a few days (J ) and only its crystal structure was reported. Our interest in the bonding and electronic structure of Pu(V) and particularly in Pu(V) fluorides prompted the present study of CsPuFg. 

Dissolution. Plutonium is solubilized in nitric acid solutions at Rocky Flats. The feed material consists of oxide, metal and glass, dissolution heels, incinerator ash and sand, slag, and crucible from reduction operations. The residues are contacted with 12M HNO3 containing CaF2 or HF to hasten dissolution. Following dissolution, aluminum nitrate is added to these solutions to complex the excess fluoride ion. 

At pH 7 33% of the plutonium was not readily desorbed from particles. However, it is possible that complexing agents could alter the affinity of silicates for different cations. The natural affinity of kaolins is Th > La > Ca > K, but in the presence of citrate or fluoride the order is reversed (156). Presumably these complexing agents will also alter the binding of plutonium to the silica particles. 

Ammonium plutonium(iv) fluorides were obtainedby reaction of Pu with NH F in HF. The stability constants for the formation of BkCF and EsCP are very similar to those reported for AmCP", and it was concluded that the monochloro-complexes of tervalent actinides up to Es are weak and of the outer-sphere type. " ... 

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