Plutonium hydrolysis products

It has been established that plutonium hydrolysis products exhibit colloidal behaviour (147-151) and may adsorb onto minerals and other surfaces to form radiocolloids. However, it is difficult to determine whether a radiocolloid is a true colloid or a pseudocolloid formed by adsorption of the plutonium species onto other colloidal impurities in the solution (152). In some cases both forms may be present... 

Even though the solubility product of Pu(OH)4 is 1 X 10 56, some Pu4+ must remain in solution as the equilibrium is established. The monomeric Pu(OH)4 and the very low molecular weight polymeric species are able to pass through an ultrafilter, and Lindenbaum and West-fall (22) found that as much as 5% of the hydrolysis species remained ultrafilterable after 72 hours at pH 11. These unfilterable species may be either true radiocolloids or pseudocolloids. The latter likely occur as a result of minute impurities in the solutions which act as nuclei on which the polymeric or ionic species adsorb (14). However, this point has been the subject of extensive debate (36, 37, 39), and opinions vary as to whether pseudocolloids form in this manner, or in fact whether there are such species at all. In general the term colloidal plutonium will be used throughout this paper to indicate all of the insoluble plutonium hydrolysis products and polymeric species of colloidal size. 

Many reports on the hydrolysis of Pu(IV) and polymerization (aggregation) of the primary hydrolysis products exist in one form or another. The validity of some of the earlier data may be subject to question because the experimental conditions were not properly controlled. Therefore, these systems deserve further consideration for the sake of refinements. Nevertheless, the major area of interest for the future will remain with interactions between Pu(IV) hydrolysis products and other reactive species present in the solution. There is not only considerable promise of elucidating novel chemical interactions, but there is also a great practical need to fully understand the extent of these interactions in order to ensure the most complete control of plutonium in reprocessing operations. 

As indicated earlier Kraus (9) has recognised three types of hydrolysis products of tetravalent thorium, uranium, plutonium and americium. These products are ... 

Trivalent plutonium, Pu(III), appears in many compounds. It also occurs as the Pu3+ ion which is relatively stable in solutions of medium acidity (40). It has been shown (II) that acid solution hydrolysis of Pu3+ is negligible. At a pH of about 7 the first small quantities of the hydrolysis product Pu(OH)2+ appear,... 

Rai and Serne, 1977 Amacher and Baker, 1982). Both elements can exist in multiple oxidation states in aqueous environments. Chromium can exist in trivalent and hexavelent states, while Pu can occur in trivalent, quadrivalent, pentavalent, and hexavalent states. Additionally, both elements can exist as cationic or anionic species in aqueous systems. Trivalent Cr exists as the cation Cr3+ and its hydrolysis products, or as the anion CrO at very low concentrations. Hexavalent Cr occurs as the dichromate Cr207 or chromate HCrO or CrO anions, depending on pH. Plutonium exists in cationic states such as Pu3+ and Pu02 and anionic forms such as Pu02(C030H ). 

The polynuclear species would not be expected at environmental plutonium concentrations, at which the most common hydrolysis product is Pu02(0H)" ". The cited hydrolysis constants for this species are in satisfactory agreement. 

Hydroxides. The hydrolysis and carbonate complexation of the actinides has been recently reviewed." Plutonium(III) hydrolysis is not well known because Pu is readily oxidized to Pu in aqueous solutions, particularly at near-neutral and basic pH. The first hydrolysis product, Pu(OH) ", has been identified in acid solution up to pH 3 (where it is about 70% formed) before oxidation to Pu prevents further study." The first hydrolysis product of Np has been similarly studied." The hydroxide solids, Pu(0H)3 xH20 and Np(0H)3 xH20, are prepared by precipitation and presumed to be isostuctural with Am(OH)3. 

The sorption process and the attainment of apparent equilibrium may be regarded then as involving essentially two kinds of sorbing species. There are a very small number of ionic plutonium species, including monomeric and low-molecular-weight polymeric hydrolysis products (1) which sorb relatively quickly and perhaps are involved in a true equilibrium, such as by ion exchange with silanol sites at the silica surface. There is evidence of such sorption of various types of univalent and multivalent cations on silica, and both chemisorption and physical adsorption processes have been deduced (13, 14, 15). Filtration of the desorbing plutonium with a 15-40-micron porous silica disc indicated that the very first material to desorb was essentially small, unfilterable Pu(IV). 

Previous studies of the hydrothermal hydrolysis of tetravalent Th, U and Np (1-4) have shown a remarkable similarity in the behavior of these elements. In each case compounds of stoichiometry M(0H)2S0i, represent the major product. In order to extend our knowledge of the hydrolytic behavior of the actinides and to elucidate similarities and differences among this group of elements, we have investigated the behavior of tetravalent plutonium under similar conditions. The relationships between the major product of the hydrothermal hydrolysis of Pu(IV), Pu2(OH)2(SO.,)3 (H20) t, (I)> and other tetravalent actinide, lanthanide and Group IVB hydroxysulfates are the subject of this re-... 

Plutonium(IV) polymer is a product of Pu(IV) hydrolysis and is formed in aqueous solutions at low acid concentrations. Depolymerization generally is accomplished by acid reaction to form ionic Pu(IV), but acid degradation of polymer is strongly dependent on the age of the polymer and the conditions under which the polymer was formed (12). Photoenhancement of Pu(IV) depolymerization was first observed with a freshly prepared polymer material in 0.5 HClOh, Fig. 3 (3 ). Depolymerization proceeded in dark conditions until after 140 h, 18% of the polymer remained. Four rather mild 1-h illuminations of identical samples at 5, 25, 52, and 76 h enhanced the depolymerization rates so that only 1% polymer remained after the fourth light exposure (Fig. 3). 

Plutonium chemists use reaction (1) as the net reaction for reactions (2), (3), and (4). This is clearly documented in the Plutonium Handbook edited by Wick, and in Cleveland s book. The Chemistry of Plutonium. Reaction (1) is an accurate representation of an equilibrium and the equilibrium concentration quotient is the product of the quotients for reactions (2), (3), and (4). Therefore, it is correct to discuss equilibrium concentrations of Pu(IV), Pu(VI), and Pu(III), without Pu(V). Circumstances where the net reaction has not been properly considered are those where concentrations of oxidation states in solutions with low acidity are calculated without consideration of Pu(IV) hydrolysis and polymerization. The distributions of Pu oxidation states (including Pu(V) and Pu(IV) polymer in nitric acid systems) were reported in JINC 609 (1973), and Silver has not included... 

The tendency toward hydroly of some of these elements can be used to advantage in separation processes. For example, in the Redox process for separating uranium and plutonium from fission products, the aqueous feed to the separation plant is made acid-deficient to promote hydrolysis of zirconium to a less extractable species, probably a colloidal hydrate [B5]. 

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