Pu IV Polymer

The polymer can be formed from many solutions. The formation of PuQV) polymer is favored by an increase in the Pu concentration and temperature or by a decrease in the acidity. As the total nitrate concentration increases the polymer 

Depolymerization is very slow at room temperature in moderate acid concentrations. The rate is increased by heating, stronger acid concentration, and addition of strong complexing agents such as fluoride and sulfate. 

Care must be taken to prevent the formation of Pu(IV) polymer in radiochemical separations because of the very different chemical properties of the polymer. The example of non-adsorption on cation exchange resin has already been given. The 

Pu(lV) polymer does not extract into tri- n-butyl phosphate, (TBP), for example, but 

Costanzo and Biggers have determined the rate of polymerization and depolymerization as a function of acidity, temperature, Pu concentration, and salt con-278 

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Plutonium(IV) polymer has been examined by infrared spectroscopy (26). One of the prominent features in the infrared spectrum of the polymer is an intense band in the OH stretching region at 3400 cm 1. Upon deuteration, this band shifts to 2400 cm 1. However, it could not be positively assigned to OH vibrations in the polymer due to absorption of water by the KBr pellet. In view of the broad band observed in this same region for I, it now seems likely that the bands observed previously for Pu(IV) polymer are actually due to OH in the polymer. Indeed, we have observed a similar shift in the sharp absorption of U(0H)2S0ir upon deuteration (28). This absorption shifts from 3500 cm 1 to 2600 cm 1. 

Near-IR and visible spectra of the Pu(IV) polymer have been published (27). Although the spectra generally are very broad, the absorptions correspond well to the families of peaks seen for I. Especially notable are features between 1000-1200 nm and 600-700 nm. In each case, the spectrum of I resembles that of... 

The only crystalline phase which has been isolated has the formula Pu2(OH)2(SO )3(HaO). The appearance of this phase is quite remarkable because under similar conditions the other actinides which have been examined form phases of different composition (M(OH)2SOit, M=Th,U,Np). Thus, plutonium apparently lies at that point in the actinide series where the actinide contraction influences the chemistry such that elements in identical oxidation states will behave differently. The chemistry of plutonium in this system resembles that of zirconium and hafnium more than that of the lighter tetravalent actinides. Structural studies do reveal a common feature among the various hydroxysulfate compounds, however, i.e., the existence of double hydroxide bridges between metal atoms. This structural feature persists from zirconium through plutonium for compounds of stoichiometry M(OH)2SOit to M2 (OH) 2 (S0O 3 (H20) i,. Spectroscopic studies show similarities between Pu2 (OH) 2 (SOO 3 (H20) i, and the Pu(IV) polymer and suggest that common structural features may be present. 

The reflux of aqueous Pu(IV) solutions containing <6 M HNO3 produces polymer precipitates that are resistant to subsequent dissociation and dissolution in nitric acid. Eapid aging of the Pu(IV) polymer to form a PuC -like structure is responsible for the unusually stable polymer. Comparative studies under nonreflux conditions show that polymer does not form at concentrations of HNO3 >3 M. 

Figure 1. Percent Pu(IV) polymer vs. time for 0.05 M Pu solutions at 50°C. Solid/dashed lines—solutions with/without 0.05 M U02(N0o)2 added. Makeup HNO concentrations for solutions are indicated. (Reprinted with permission from Ref. 2.)...

Chemical analyses reveal that measurable amounts of uranyl ion are actually present in Pu(IV) polymers grown in mixtures of Pu(IV) and uranyl nitrate suggesting that uranyl ion is being taken up in the polymer network and consequently hampers the growth through a chain termination process as suggested in Fig. 3. The uranyl serves to terminate active sites because it does not typically form extensive polymeric aggregates as does Pu(IV) instead it tends only to dimerize and, at most, tri-merize (4). 

Carbon Dioxide Adsorption on Dried Polymer. Other unexpected interactions of these hydrolytic polymers have been noted previously during the measurement of infrared spectra of dried Pu(IV) polymers (like those used for diffraction studies). Vibrational bands first attributed to nitrate ion were observed in samples exposed to room air however, these bands were not present in samples prepared under nitrogen atmospheres (see Fig. 4) (6). Chemical analyses established enough carbon in the exposed samples to confirm the assignment of the extraneous bands to the carbonate functional group... 

Figure 3. Termination of Pu(IV) polymer network propagation by attachment of V02 r to active -OH sites.

Figure 4. Infrared spectra of KBr pellets containing Pu(IV) polymer precipitates, (A) prepared in -purged glove hag free of CO2 (B) prepared in laboratory atmosphere. (Reprinted with permission from Ref. 6.)...

The implication of these two examples is that the medium in which the Pu(IV) hydrolysis chemistry is studied has a strong bearing on the outcome of the results. In the past, we were content to treat the pure systems and either ignore external interferences (such as the atmosphere) or infer the behavior of mixtures (such as Pu + and U02 " ") based on the known chemistries of the individual species. The example of U02 + interactions with Pu(IV) polymer demonstrates that neither of these approaches is accurate. Therefore, future research efforts will necessarily have to consider plutonium hydrolysis reactions in more detail than has previously been done. 

Recent work in this laboratory has sought to provide some of this information. Unfortunately, absorption spectrophotometry is not suitable for monitoring the formation of Pu(IV) polymer... 

Aqueous plutonium photochemistry is briefly reviewed. Photochemical reactions of plutonium in several acid media have been indicated, and detailed information for such reactions has been reported for perchlorate systems. Photochemical reductions of Pu(VI) to Pu(V) and Pu(IV) to Pu(III) are discussed and are compared to the U(VI)/(V) and Ce(IV)/(III) systems respectively. The reversible photoshift in the Pu(IV) disproportionation reaction is highlighted, and the unique features of this reaction are stressed. The results for photoenhancement of Pu(IV) polymer degradation are presented and an explanation of the post-irradiation effect is offered. 

The effects of four 1 h, low intensity, UV light exposures to enhance the degradation of freshly prepared Pu(IV) polymer. The darkened line represents depolymerization under dark conditions. The solution contained 0.0093 M total Pu and 0.47 11 HClOi at 22°C (3). 

The effects of UV irradiation to enhance the degradation of aged Pu(IV) polymer in HClOi. Depolymerization under dark conditions for each experiment is shown by a data point directly above the last light sample point (4). 

Silver was critical of the lack of use by plutonium chemists of a-coefficients. Assuming that Silver was referring to a-coeffi-cients defined as the fraction of the total concentration of a substance that exists as a particular species, he was wrong to say that plutonium chemists have not used them. Phil Horwitz at ANL has used them. Publications from ORNL have reported them to easily show relative concentrations of plutonium species, and L. M. Toth used such a-coefficients as percent of Pu(IV) polymer in his symposium talk Tuesday. Alpha coefficients are a commonly used, simple concept - certainly since Ringbom s article in the Journal of Chemical Education in 1958."... 

In the presence of complexing agents at a sufficiently high concentration it is possible to prevent the formation of the Pu(IV) polymer. Acetate at a concentration of 5 x 10 2m facilitated the retention of 90% of Pu(IV) as themonomerin concentrations of 10 sm Pu(IV) when the pH was raised from pH 2.8 to pH 5.7. However, citric acid and diethylenetriaminepentacetic acid (DTPA), both at 5 x 10 4m, were even more efficient at preventing polymer formation (14). 

In view of the results obtained by Bell and Friedman (136) it is possible that sunlight could bring about a photochemical solubilisation of Pu(IV) polymers. These workers obtained 13 % degradation of polymeric plutonium in the presence of organic materials in one hour when the polymer (0.0093m Pu in 0.47m HC104) was irradiated at 260—280 nm. This degradation rate was four times faster than that observed over a similar period of time in the absence of the UV source. 

Lloyd, M. H. and Haire, R. G., "Studies on the Chemical and Colloidal Nature of Pu (IV) Polymer" the XXIV th International Union of Pure and Applied Chemistry Congress", Hamburg, Germany, September, 1973. 

Although the effect of reflux on pol3rmer formation has been recognized for many years, little detailed information is available concerning the extent to which changes in temperature, acidity, and plutonium concentration affect it. Recent work in this laboratory has sought to provide some of this information. Unfortunately, absorption spectrophotometry is not suitable for monitoring the formation of Pu(IV) polymer... 

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