Plutonium carbonate complexes

The partitioning of plutonium from surface water to sediments in freshwater and marine environments depends on the equilibrium between plutonium(IV) and plutonium(V), and the interaction between plutonium(IV) in solution and plutonium sorbed onto sediment particle surfaces (NCRP 1984). Sorption onto marine clays was found to be largely irreversible (Higgo and Rees 1986). Higgo and Rees (1986) also found that the initial sorption of plutonium onto clays was effective in removing most of the plutonium species that would be able to sorb onto the clay. When sorption to carbonate marine sediments was investigated, it was found that some desorption from the surface would also occur. This behavior was due to the presence of plutonium carbonate complexes on the sediment surfaces which were sorbed less strongly than plutonium dioxide... 

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). 

Considering the anion concentration ranges in natural waters (Table II) and the magnitude of the corresponding plutonium stability constants (Table III), the chemistry of plutonium, as well as that of uranium and neptunium, is almost entirely dominated by hydroxide and carbonate complexation, considering inorganic complexes only (41, 48, 49). ... 

In Figure 2 the solubility and speciation of plutonium have been calculated, using stability data for the hydroxy and carbonate complexes in Table III and standard potentials from Table IV, for the waters indicted in Figure 2. Here, the various carbonate concentrations would correspond to an open system in equilibrium with air (b) and closed systems with a total carbonate concentration of 30 mg/liter (c,e) and 485 mg/liter (d,f), respectively. The two redox potentials would roughly correspond to water in equilibrium wit air (a-d cf 50) and systems buffered by an Fe(III)(s)/Fe(II)(s)-equilibrium (e,f), respectively. Thus, the natural span of carbonate concentrations and redox conditions is illustrated. 

In the presence of mineral phases containing anions that would form sparingly soluble compounds (e.g. POt - and F for the lower oxidation states) an enhanced plutonium uptake due to chemisorption can be expected (57). For plutonium in the higher oxidation states the formation of anionic carbonate complexes would drastically reduce the sorption on e.g oxide and silicate surfaces. 

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. 

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... 

Several areas of research seem to merit top priority attempt to verify published stability constants of environmental interest at lower metal concentrations and higher pH determine stability constants that are not currently known, the prime example being the plutonium-carbonate system assess the interplay of complexation, hydrolysis, and polymerization at environmental pH values, as these factors are important but not well understood under neutral conditions study the complex chemistry of plutonyl(V), which some workers believe to be an important species in ground waters attempt to elucidate the nature and behavior of polymeric species with the ultimate objective of developing quantitative, reproducible expressions for dispersion, precipitation. 

Moskvin, A. I., and Gel man, A. D. Determination of the composition and instability constants of oxalate and carbonate complexes of plutonium(IV), Russ. J. Inorg. Chem., 2>... 

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. 

Preliminary thermodynamic data for plutonium from Puigdomenech and Bruno (1991) are given in Table A13.8. Unfortunately, the identities and stabilities of the important carbonate complexes of Pu(III) and Pu(IV) remain in dispute. Puigdomenech and Bruno (1991) suggest 1 1 Pu -CO " and 1 1 and 1 5 Pu -CO complexes, whereas Nitsche (1991) proposes, in addition, 1 2 and 1 3, Pu -COl" complexes and 1 2, 1 3, and 1 4 Pu + COl" complexes (see also Adloff and Guillau-mont 1993). Stability constants for the complexes recognized by both sources, half of which have been estimated, are in fair agreement. 

Figure 5 shows plutonium adsorption Kd values for three anion exchange resins over a pH range of 4 to 8. Plutonium adsorption on each of the three resins, Lewatit MP-500-FK, Dowex 2IK, and Dowex M-41, increased significantly as the pH increased. This was expected since anionic hydoxyl and carbonate complexes are more stable at higher pH. Since these resins are not effective plutonium adsorbents below a pH of 7 or 8, they are not useful in wastewater treatment (at least when used alone). 

Research into the aquatic chemistry of plutonium has produced information showing how this radioelement is mobilized and transported in the environment. Field studies revealed that the sorption of plutonium onto sediments is an equilibrium process which influences the concentration in natural waters. This equilibrium process is modified by the oxidation state of the soluble plutonium and by the presence of dissolved organic carbon (DOC). Higher concentrations of fallout plutonium in natural waters are associated with higher DOC. Laboratory experiments confirm the correlation. In waters low in DOC oxidized plutonium, Pu(V), is the dominant oxidation state while reduced plutonium, Pu(III+IV), is more prevalent where high concentrations of DOC exist. Laboratory and field experiments have provided some information on the possible chemical processes which lead to changes in the oxidation state of plutonium and to its complexation by natural ligands. 

The early field studies revealed that elevated concentrations of fallout plutonium correlated with Increased concentrations of dissolved organic carbon. Experiments at Argonne National Laboratory corroborate this correlation the explanation Is probably that the organic compounds complex Pu(IV), and, hence, decrease the distribution ratio between water and sedlments(27). In these experiments the distribution ratio (Kj) between sediment and natural waters was measured as a function of DOC. Measurements of Kj In both field and laboratory experiments show an unmistakable effect of DOC upon the distribution ratio. Figure 4 shows the Inverse correlation between the K, of plutonium and concentration of DOC. 

Mathew and Pillai observed a threefold increase in plutonium concentration at low, normal, and high carbonate concentrations when 20mg/liter of organic matter were added to sea water samples (29). Again this indicates the effect of organic complexation upon plutonium solubility in natural waters. 

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