Plutonium, in aqueous solution

Plutonium is the most important transuranium element. Its two isotopes Pu-238 and Pu-239 have the widest applications among all plutonium isotopes. Plutonium-239 is the fuel for nuclear weapons. The detonation power of 1 kg of plutonium-239 is about 20,000 tons of chemical explosive. The critical mass for its fission is only a few pounds for a solid block depending on the shape of the mass and its proximity to neutron absorbing or reflecting substances. This critical mass is much lower for plutonium in aqueous solution. Also, it is used in nuclear power reactors to generate electricity. The energy output of 1 kg of plutonium is about 22 million kilowatt hours. Plutonium-238 has been used to generate power to run seismic and other lunar surface equipment. It also is used in radionuclide batteries for pacemakers and in various thermoelectric devices. 

Chemical and Physical Properties of Plutonium in Aqueous Solution... 

The reported solubility product of Pu(0H)i+, 7 X 10 as measured by pH titration ( 3, p. 300), is an exceedingly small number. If it were a true representation of the concentration of plutonium in solution, at pH 7 there would be only 7 X 10 mole of plutonium per liter thus the equilibrium concentration of plutonium in neutral water would be about one atom per 2400 liter and there would be no problem with plutonium-contaminated ground water. The solubility product does not accurately define the concentration of plutonium in aqueous solutions because it merely states the concentration of the Pu " " ion. At pH values of environmental interest, plutonium will be present not primarily as Pu " , but as species such as Pu(OH) " ", Pu(OH), unionized Pu(0H)4, colloidal polymeric forms to be discussed later, as well as other oxidation states formed by disproportionation at low acidities. Thus, the total plutonium concentration will be much higher than that described by the solubility product of Pu(0H)i. ... 

Bagawde, S. V., Ramakrishna, V. V., and Patil, S. K. Complex-ing of tetravalent plutonium in aqueous solutions, Inorg. Nucl. Chem., 38, 1339 (1976). 

Hydrolysis. Hydrolysis is one of the most important reactions in the chemistry of plutonium in aqueous solutions. The tendency of plutonium ions to hydrolyze decreases in the order ... 

Use of a single-parameter limit often leads to an inconveniently small size of batch or equipment. To permit safe operation on a larger scale, combinations of two parameters that together are safely subcritical are sometimes specified, provided that the simultaneous presence of both parameters can be assured. For example, if the maximum concentration of plutonium in aqueous solution can be limited to 20 g/liter, the maximum safe diameter of a cylinder may be increased from the single-parameter limit of 15.7 cm (Table 4.11 or 10.25) to 25 cm (Fig. 10.35). 

Plutonium in aqueous solution exists in the +3, -K, +5. or - 6 oxidation states, resembling both uranium (U), neptunium (Np) and americiiun (Am) in this respect. 

The hydrolysis of tetravalent plutonium in aqueous solution is dominated by the formation of colloids. Even in 1 Af HCIO4 polymer formation occurs, and reliable hydrolysis constants are found only for the first step with n = 1 (5) ... 

Nebel D (1966) Complex formation of plutonium in aqueous solution. Zeits Anal Chem 262 284-285... 

Fig. 6. Speciation diagram of plutonium as function of Eb and pH in aqueous solution at 25°C (a) carbonate-free (89) (b) 0.004 M total carbonate (90). Eb...

Plutonium(III) in aqueous solution, Pu " ( 4)> is pale blue. Aqueous plutonium(IV) is tan or brown the nitrate complex is green. Pu(V) is pale red-violet or pink in aqueous solution and is beUeved to be the ion PuO Pu(VI) is tan or orange in acid solution, and exists as the ion PuO. In neutral or basic solution Pu(VI) is yellow cationic and anionic hydrolysis complexes form. Pu(VII) has been described as blue-black. Its stmcture is unknown but may be the same as the six-coordinate NpO (OH) (91). Aqueous solutions of each oxidation state can be prepared by chemical oxidants or reductants... 

The chemistry of plutonium ions in solution has been thoroughly studied and reviewed (30,94—97). Thermodynamic properties of aqueous ions of Pu are given in Table 8 and in the Uterature (64—66). The formal reduction potentials in aqueous solutions of 1 Af HCIO or KOH at 25°C maybe summarized as follows (66,86,98—100) ... 

Only slightly less accurate ( 0.3—0.5%) and more versatile in scale are other titration techniques. Plutonium maybe oxidized in aqueous solution to PuO " 2 using AgO, and then reduced to Pu" " by a known excess of Fe", which is back-titrated with Ce" ". Pu" " may be titrated complexometricaHy with EDTA and a colorimetric indicator such as Arsenazo(I), even in the presence of a large excess of UO " 2- Solution spectrophotometry (Figs. 4 and 5) can be utilized if the plutonium oxidation state is known or controlled. The spectrophotometric method is very sensitive if a colored complex such as Arsenazo(III) is used. Analytically usehil absorption maxima and molar absorption coefficients ( s) are given in Table 10. Laser photoacoustic spectroscopy has been developed for both elemental analysis and speciation (oxidation state) at concentrations of lO " — 10 M (118). Chemical extraction can also be used to enhance this technique. 

Much was also learned at the Metallurgical Laboratory about the solution chemistry of plutonium during these first few years of investigation. This included elucidation of the ionic species present in aqueous solutions of different acids and determination... 

Investigations of the chemical properties of plutonium have continued in many laboratories throughout the world as it has become available. This has led to the situation where the chemistry of this relative newcomer is as well understood as is that of most of the well-studied elements. The four oxidation states of plutonium—III, IV, V, and VI—lead to a chemistry which is as complex as that of any other element. It is unique among the elements in that these four oxidation states can all exist simultaneously in aqueous solution at appreciable concentration. As a metal, also, its properties are unique. Metallic plutonium has six allotropic forms, in the temperature range from room temperature to its melting point (640 C), and some of these have properties not found in any other known metal. 

The investigation of plutonium chemistry in aqueous solutions provides unique challenges due in large part to the fact that plutonium exhibits an unusually broad range of oxidation states -from 3 to 7-and in many systems several of these oxidation states can coexist in equilibrium. Following the normal pattern for polyvalent cations, lower oxidation states of plutonium are stabilized by more acidic conditions while higher oxidation states become more stable as the basicity increases. 

Reduction Reactions of Uranium, Neptunium, Plutonium and Americium in Aqueous Solutions" National Technical Information Service, U. S. Department of Commerce Springfield, Virginia, 1975. (b) Buxton, G. V. and... 

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). 

All the oxidation states III, IV, V and VI can exist in aqueous solutions in the Eh-pH-range of environmental interest (37). Strong complexes are formed between plutonium and oxygen containing ligands (02-, OH-, CO32-, etc.) as well as F but not with S2-. 

Lemire, R.J. Tremaine, P. "Uranium and Plutonium Equilibria in Aqueous Solutions to 200°C" AECL-6655 Whiteshell Nucl. Res. Est. Pinawa, 1980. 

It has already been stated that plutonium can exist in aqueous solution in four oxidation states and an examination of Table 3 shows that the potentials of the couples Pu3+/Pu4+ and PuOj/PuO are less negative than the Pu /PuOJcouple. As a result of these lower couples it is possible for a simultaneous oxidation and reduction of Pu4+ and PuOj- This process of simultaneous oxidation and reduction is termed disproportionation and the process is represented thus ... 

As mentioned in the Introduction, the actinyl ions are not stable under all chemical conditions. Plutonium can coexist in solution in several oxidation states, the stability of which often depends strongly on acidity (26). As a result, great care must be taken to obtain pure solutions of PuOl(27). On the other hand, the neptunyl ion NpO is the most stable form of neptunium in aqueous solution. It is noteworthy that the exchange between the oxygen atoms of PuO and H20 is very slow (ti/2 > 10 h) (25), whereas it is quite fast (h/2 2.2 s) in the case of NpO. ... 

Heat capacity data for ions in aqueous solution over the temperature range 25-200°C. Such data for ionic species of uranium, plutonium, other actinides and various fission products such as cesium, strontium, iodine, technetium, and others are of foremost interest. 

Plutonium-239 exists in aqueous solutions in five possible oxidation states, Pu(III), Pu(IV), Pu(V), Pu(VI), and as recently reported, Pu(VII) (38). The latter apparently occurs only in very basic aqueous solutions. Pu(II) oxidizes readily in aqueous solution. 

Pentavalent plutonium, Pu( V), is believed to exist only as the oxyion Pu02+ in aqueous solutions. Generally Pu(V) quickly disproportionates to form Pu(VI) and Pu(IV) in solutions of moderate acidity (40). 

Hexavalent plutonium, Pu(VI), is relatively stable in aqueous solution and exists as the oxyion plutonyl, Pu022+ (40). The chemical properties of Pu(VI) compounds are somewhat similar to those of U(VI). 

T. W. Newton, The Kinetics of the Oxidation Reduction Reactions of Cranium, Neptunium, Plutonium, and Americium in Aqueous Solution, TID-26506, U.S. Energy, Research, and Development Administration (ERDA) Technical Information Center, Washington, D.C., 1975. 

The plutonium ions in aqueous solution possess characteristic colors bluc-lavcndcr for Pu3+. yellow-brown to green for Pu4+, and pink-orange for Pu02 +. 

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