Colloidal plutonium

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. 

Although it is important to understand the mechanisms in the formation of the distribution of sizes among the colloidal plutonium hydroxide particles, the distributions themselves will influence the behavior... 

There have been several laboratory and field studies concerned with the uptake of aqueous plutonium by plants, marine biota, soils, minerals, and glass. These have been discussed in the first paper of this series (I), which shows that several solution variables, as they influence the particle size distribution of the aqueous plutonium, greatly affect its interaction with silica surfaces. Studies were conducted to determine the rates, equilibria, and mechanism of the sorption and desorption of aqueous, colloidal plutonium-239 onto the surfaces of quartz silica. The orientation of these studies is the understanding of the likely behavior and fate of plutonium in environmental waters, particularly as related to its interaction with suspended and bottom sediments. 

In order to determine if bicarbonate may similarly affect colloidal plutonium, a comparison was made of sorption from solutions containing bicarbonate only from atmospheric CO2 and from those with added 10"2Af bicarbonate. Two different ionic strengths were used in order to distinguish adequately between and rule out the possibility of an ionic strength effect. The results of the study are given in Table IV. When the bicarbonate concentration was increased from the equilibrium atmospheric value to 10 2M, the sorption coefficient greatly decreased at both ionic strengths. 

Pu(HP04)44 in 2M nitric acid. The formation of the phosphate complex, in effect, increases the average negative charge of the ionic or colloidal plutonium species. As in the case of the bicarbonate complex, this should reduce sorption onto the negatively charged silica surface. However, as is shown in Table V, the sorption increased. In another experiment in which the phosphate concentration was varied between zero and 1.25 X 10 2M, there was no significant difference in the sorption constants. 

Natural colloid particles in aqueous systems, such as clay particles, silica, etc. may serve as carriers of ionic species that are being sorbed on the particulates (pseudocolloids). It seems evident that the formation and transport properties of plutonium pseudocolloids can not yet be described in quantitative terms or be well predicted. This is an important area for further studies, since the pseudocolloidal transport might be the dominating plutonium migration mechanism in many environmental waters. 

Similar affinity of polonium and plutonium for marine surfaces implies that studies of the more easily measured polonium might be valuable in predicting some consequences of plutonium disposal in die oceans [8-11]. Rates at which plutonium and polonium deposit out of seawater onto surfaces of giant brown algae and inert surfaces, such as glass and cellulose, suggest that both nuclides are associated in coastal seawater with colloidal sized species having diffusivities of about 3 x 10"7 cm2/s. The parallel behaviour possibly... 

Schell et al. [ 57] have described a sorption technique for sampling plutonium and americium, from up to 4000 litres of water in 3 h. Battelle large-volume water samples consisting of 0.3 xm Millipore filters and sorption beds of aluminium oxide were used. Particulate, soluble, and presumed colloidal fractions are collected and analysed separately. The technique has been used in fresh and saline waters, and has proved to be reliable and comparatively simple. 

With the aid of ultrafiltration techniques Lindenbaum and Westfall (15) showed that 92% of the plutonium solution (1.96 x 10 sm) was ultrafilterable in the presence of 3.4 x 10 2m citrate over a pH range of pH 4.0 to 10.0. However, when the citrate concentration was lowered to 1.96 x 10-SM the ultrafilterability was only 77 % for the range pH 4.0 to 8.5 and at pH 11.0 only 5 %. Lindenbaum and Westfall (14) have also demonstrated that citrate could bring about a resolubilisation of the colloid. Equimolar concentrations of Pu(IV) and citrate were adjusted to pH 11.0 and then one hour later the solution was adjusted to either pH 7.8 or pH 4.0. Within... 

As trivalent americium has a smaller ionic potential than the ions of plutonium it hydrolyses to a much lesser extent than the various plutonium ions. However, like Pu3+, hydrolytic reactions and complex formation are still an important feature of the aqueous chemistry of Am3+. Starik and Ginzberg (25) have shown that Am(III) exists in its ionic form from pH 1.0 to pH 4.5 but above pH 4.5 hydrolysis commences and at pH 7.0 colloidal species are formed. The hydrolytic behaviour of Cm(III) resembles that of Am(III). 

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... 

There have been many investigations into the colloidal behaviour of plutonium and much of these data have been thoroughly summarised by Andelman and Rozzell... 

Lu, N., Cotter, C. R., Kitten, H. D., Bentley, J. TRIAY, I. R. 1998. Reversibility of sorption of plutonium-239 onto hematite and goethite colloids. Radiochimica Acta, 83, 167-173. 

Figure 3 illustrates the distributions found for Pu and Am when a mixed sample of these tracers was infiltrated into a large (30 cm x 30 cm) block of Bandolier tuff (6). The nuclide activities were determined simultaneously by coring sections of the tuff and represent the activity distributions in the rock. It is obvious that although the activities are both normalized at 100% at the surface the increased dispersion of the plutonium concentration during elution leads to an increase of almost an order of magnitude in its activity relative to Am at the 5-6 cm depth. It is highly unlikely that abnormal flow paths or movement of colloidal clay particles would discriminate between americium and plutonium therefore this experimental result tends to discount these possible types of mechanisms. However, a pure Pu polymer could carry the Pu more rapidly downstream. 

Occasionally, granular silica (supplied either by the Ottawa Silica Co. or Fisher Scientific Co.) was added to the plutonium solution for various periods before centrifugation to determine its effect on the colloidal system. The silica was first dry sieved, washed thoroughly, then oven dried at 120 °C. the particle sizes ranged from 280 to 390p. Other chemicals used were reagent grade. 

Further evidence of the effect of ionic strength on the plutonium colloidal system is shown in Figure 4, in which the size distribution of a 43-day old solution with an ionic strength of 0.002 is compared with a... 

The bicarbonate ion, HC03, is a prevalent species in natural waters, ranging in concentrations up to 0.8 X 10 3. As was indicated previously, carbonate ions have the ability to form complexes with plutonium. Starik (39) mentions that, in an investigation of the adsorption of uranium, there was a decrease in the adsorption after reaching a maximum, which was explained by the formation of negative carbonate complexes. Kurbatov and co-workers (20) found that increasing the bicarbonate ion concentration in a UXi (thorium) solution decreased the amount of thorium which formed a colloid and became filterable. This again was believed to be caused by the formation of a soluble complex with the bicarbonate. 

Although the general effect of the addition of bicarbonate was to increase the size of the colloidal species, Lindenbaum and Westfall obtained the opposite effect with citrate addition over the pH range 4-11, as measured by the percent of plutonium (IV) that was ultrafilterable (22). However, their plutonium concentrations were 2 X 10 5Af, and the solutions probably contained true colloids, rather than pseudocolloids, if one accepts Davydovs analysis. Lindenbaum and Westfall concluded that the mechanism of the citrate action was the complexation of plutonium, thereby preventing the formation of hydrolytic polymers. It should be noted, however, that even with a citrate-plutonium molar ratio of 1800 (3.4 X 10 4Af citrate), about 10% of the plutonium still could not pass through the ultrafilter for solutions aged up to four days (22). 

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