Actinides hydroxides

Hydrolysis reactions are common to all actinide ions in nearneutral solutions, and take place either in parallel with or predominantly over other complexation reactions. In connection with the migration studies of actinide ions in natural waters, attention recently has been focused on hydrolysis reactions of actinides since these reactions are important in determining the solubility of the actinide hydroxide or oxide. Although numerous studies have been made (1-4) to determine stability constants of various hydrolysis products, much of the necessary data are still lacking. The acquisition of these data and further improvement or verification of the existing data is desirable. 

Four types of colloids were considered in the WIPP program intrinsic actinide colloids, mineral colloids, microbes, and humic acid colloids (US DOE, 1996). Intrinsic actinide colloids, consisting of polymerized hydrated actinide hydroxides, are not stable in the neutral to moderately basic pH conditions expected in the WIPP, and were assumed not to contribute to the total actinide concentrations in solution. Mineral colloids are destabilized and tend to flocculate in the high-ionic-strength WIPP brines (Kelly et al., 1999). In the performance assessment calculations for the WIPP, a highly conservative value of 2.6 X 10 mol actinide per liter, for each actinide, was assumed to be bound to mineral colloids and to contribute to the mobile fraction. Actinides sorbed onto microbes and humic acids were estimated to contribute significantly to the concentration of mobile actinides in WIPP brines as discussed above (Section 9.06.3.2.2). 

Hydroxides. Pure and mixed metal actinide hydroxides have been studied for their potential utility in nuclear fuel processing. At the other end of the nuclear cycle, the hydroxides are important in spent fuel aging and dissolution, and environmental contamination. Tetravalent actinides hydrolyze readily, with Th more resistant and Pu more likely to undergo hydrolysis than and Np. All of these ions hydrolyze in a stepwise marmer to yield monomeric products of formula An(OH) with = 1,2,3 and 4, in addition to a number of polymeric species. The most prevalent and well characterized are the mono- and tetra-hydroxides, An(OH) and An(OH)4. Characterization of isolated bis and tri-hydroxides is frustrated by the propensity of hydroxide to bridge actinide centers to yield polymers. For example, for thorium, other hydroxides include the dimers. 

Recent work on the hydrolysis constants of actinide ions and the solubility products of the actinide hydroxides has been reviewed. The data in the literature cover a wide range of ionic strengths and are usually obtained at ambient temperature or 25° C. A number of solubility stucUes have been reported, approached boA from undersaturation and oversaturation, but in most cases the solid materials that result from these studies are not well-characterized. In solutions with high concentrations of salts, radiolysis effects play a major role in tiie determination of the oxidation states... 

Rard s (1985) assessment of europium thermodynamics included AG = 150.7 5.7 kJ mol(K = 3.9 x 10 ) for the similar solubility equilibrium of Eu(OH)j. From this AG and other thermodynamic properties he calculated AfG [Eu(OH)3(s)] = - 1198.9 7.8k Jmol- and Af/f [EU(OH)3(s)] = - 1336.5 8.3kJmol . Clearly, the calorimetric and equilibrium measurements for these hydroxides need to be reconciled. For actinide hydroxides approach-to-equilibrium measurements have been made the scatter among measurements and estimates indicates that equilibrium may not have been reached (Morss 1992a). One calorimetric measurement of an actinide hydroxide enthalpy of formation has been made (table 4, Morss and Williams 1994) from which K p[Am(OH)3] = 7 x 10 has been calculated. This C p is significantly smaller than that of structurally similar Nd(OH)j. 

W (weeks) Labile salts and complexes, e.g., nitrates, chlorides The complex/salt tends to dissociate at the deposition site. This may lead to precipitation of the actinide, in the absence of strongly complexing ligands, to give amorphous actinide hydroxides. This will be leached slowly from the deposition site with half-times in the region of 10 to 100 days. 

Mesmer [57] have exhaustively surveyed the literature and have evaluated heterogeneous equilibria for lanthanide and actinide hydroxides and hydrated oxides. The thermodynamic activity products (recommended by this author) to calculate free energies and ), in basic solution are listed in Table 17.3. In general, the values selected by Baes and Mesmer have been adopted however, in some cases more recent results have been selected or averaged [53,60]. These more recent results yield a monotonic relation between Ln(OH)j unit-cell volumes and K. For tetravalent ions, no hydroxides have been characterized and we assume that the dioxides are in equilibrium with hydrated dioxides, if any exist. Hence K values refer to reactions such as the following (see [361]) ... 

The reduction potentials for the actinide elements ate shown in Figure 5 (12—14,17,20). These ate formal potentials, defined as the measured potentials corrected to unit concentration of the substances entering into the reactions they ate based on the hydrogen-ion-hydrogen couple taken as zero volts no corrections ate made for activity coefficients. The measured potentials were estabhshed by cell, equihbrium, and heat of reaction determinations. The potentials for acid solution were generally measured in 1 Af perchloric acid and for alkaline solution in 1 Af sodium hydroxide. Estimated values ate given in parentheses. 

Solid Compounds. The tripositive actinide ions resemble tripositive lanthanide ions in their precipitation reactions (13,14,17,20,22). Tetrapositive actinide ions are similar in this respect to Ce . Thus the duorides and oxalates are insoluble in acid solution, and the nitrates, sulfates, perchlorates, and sulfides are all soluble. The tetrapositive actinide ions form insoluble iodates and various substituted arsenates even in rather strongly acid solution. The MO2 actinide ions can be precipitated as the potassium salt from strong carbonate solutions. In solutions containing a high concentration of sodium and acetate ions, the actinide ions form the insoluble crystalline salt NaM02(02CCH2)3. The hydroxides of all four ionic types are insoluble ... 

Hydroxides. Thorium (TV) is generally less resistant to hydrolysis than similarly sized lanthanides, and more resistant to hydrolysis than tetravalent ions of other early actinides, eg, U, Np, and Pu. Many of the thorium(IV) hydrolysis studies indicate stepwise hydrolysis to yield monomeric products of formula Th(OH) , where n is integral between 1 and 4, in addition to a number of polymeric species (40—43). More recent potentiometric titration studies indicate that only two of the monomeric species, Th(OH) " and thorium hydroxide [13825-36-0], Th(OH)4, are important in dilute (<10 M Th) solutions (43). However, in a Th02 [1314-20-1] solubiUty study, the best fit to the experimental data required inclusion of the species. Th(OH) 2 (44). In more concentrated (>10 Af) solutions, polynuclear species have been shown to exist. Eor example, a more recent model includes the dimers Th2(OH) " 2 the tetramers Th4(OH) " g and Th4(OH) 2 two hexamers, Th2(OH) " 4 and Th2(OH) " 2 (43). 

Dioxides are known for all the actinides as far as Cf. They have the fee fluorite structure (p. 118) in which each metal atom has CN = 8 the most common preparative method is ignition of the appropriate oxalate or hydroxide in air. Exceptions are Cm02 and Cf02, which require O2 rather than air, and Pa02 and UO2, which are obtained by reduction of higher oxides. 

Hydrothermal hydrolysis of metal ions is useful in producing crystalline phases which contain metals in a state of partial hydrolysis, i.e., a state intermediate between that of the hydrated metal ion and that of the hydrous hydroxide. Such reactions have been used to produce numerous crystalline phases of actinides (1-4), Group IV metal ions (5-14) and lanthanides (15-21). 

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. 

Hydrous Oxides and Hydroxides in the Lanthanide and Actinide Series, Final Report June 1, 1969-May 31, 1972. U.S. AEC Report 0R0-3955-3, Oak Ridge Operation Office, Oak Ridge, TN, 1972. 

Waste Treatment. Figure 2 outlines the current waste recovery and treatment processes, and proposed changes. Acid waste streams are sent through nitric acid and secondary plutonium recovery processes before being neutralized with potassium hydroxide and filtered. This stream and basic and laundry waste streams are sent to waste treatment. During waste treatment, the actinides in the aqueous waste are removed by three stages of hydroxide-iron carrier-flocculant precipitation. The filtrate solution is then evaporated to a solid with a spray dryer and the solids are cemented and sent to retrievable storage. 

Ferrite is introduced into the aqueous media by two techniques. With the in situ method, ferrite is formed within the actinide-containing solution by addition of Fe(II), Fe(III), and sodium hydroxide. With the preformed ferrite method, ferrite solids are prepared separately and added to the actinide solution. 

Holm et al. [74] used a spectrometry for the determination of 237neptunium in seawater. The actinides are preconcentrated from a large seawater sample by hydroxide precipitation. The neptunium was isolated by ion exchange, fluoride precipitation, and extraction with TTA. 238Neptunium or 235neptunium was used to determine the radiochemical yield. 

Results and Discussion. Changes in concentrations of the actinides in solution in the blank containers are shown in Figure 9. A large fraction of the Pu, Am and Cm was removed from solution while only a small amount of the U and none of the Np was lost from solution. Since the starting concentration of Pu exceeds the solubility for the hydroxide (or hydrated oxide), the Pu probably precipitated as colloidal-size particles (2) the behavior of Am and Cm cannot be explained by a similar mechanism. However, there is recent experimental evidence that indicates the solubility products for Am and Cm carbonates may be of the order 10 ( ). If this is the case, there was sufficient... 

Hydroxides and oxides. Protactinium(V), neptunium(V) and plutonium(V) hydroxides precipitate from alkaline aqueous solutions of the actinides(V) the last two appear to be of the form Mv02(0H)-xH20. 

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