Hydrolysis actinides

The techniques used in the work have generally been spectroscopic visible-uv for quantitative determinations of species concentrations and infrared-Raman for structural aspects of the polymer. Although the former has often been used in the study of plutonium systems, there has been considerably less usage made of the latter in the actinide hydrolysis mechanisms. 

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). 

Several oxohalides are also known, mostly of the types An OaXa, An OaX, An OXa and An "OX, but they have been less thoroughly studied than the halides. They are commonly prepared by oxygenation of the halide with O2 or Sb203, or in case of AnOX by hydrolysis (sometimes accidental) of AnX3. As is to be expected, the higher oxidation states are formed more readily by the lighter actinides thus An02X2, apart from the fluoro compounds, are confined to An = U. Conversely the lower oxidation states are favoured by the heavier actinides (from Am onwards). 

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). 

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-... 

Group IVB, actinide, and lanthanide, hydrothermal hydrolysis spectroscopic studies. 58-62... 

Group IVB, actinide and lanthanide hydrothermal hydrolysis (continued)... 

The ionic radii of the commonest oxidation states are presented in Table 2. There is evidence of an actinide contraction of ionic radii as the 5/ orbitals are filled and this echoes the well established lanthanide contraction of ionic radii as the 4/orbitals are filled. Actinides and lanthanides in the same oxidation state have similar ionic radii and these similarities in radii are obviously paralleled by similarities in chemical behaviour in those cases where the ionic radius is relevant, such as the thermodynamic properties observed for halide hydrolysis. 

This behaviour is not restricted to Th(IV) and U(IV) as similar patterns have been found for all tetravalent actinides, An(IV). A more detailed discussion and comparison of An(IV) solubility and hydrolysis is given by Neck Kim (2001). These authors conclude from solubility data... 

Neck, V. Kim, J. I. 2001. Solubility and hydrolysis of tetravalent actinides. Radiochimica Acta, 89, 1-16. 

Actinide ions often form acidic solutions as a result oT hydrolysis ... 

Hydrolysis and Complex Ion Formation. Of the actinide ions, the small, highly charged M4+ ions exhibit the greatest degree of hydrolysis and complex ion formation. 

The redox chemistry of the actinide elements, especially plutonium, is complex (Katz et al., 1980). Disproportionation reactions are especially important for the +4 and +5 oxidation states. Some of the equilibria are kinetically slow and irreversible. All transuranium elements undergo extensive hydrolysis with the +4 cations reacting most readily due to their large charge/radius ratio. Pu (IV) hydrolyzes extensively in acid solution and forms polymers. The polymers are of colloidal dimensions and are a serious problem in nuclear fuel reprocessing. 

Despite the extremely low concentrations of the transuranium elements in water, most of the environmental chemistry of these elements has been focused on their behavior in the aquatic environment. One notes that the neutrality of natural water (pH = 5-9) results in extensive hydrolysis of the highly charged ions except for Pu(V) and a very low solubility. In addition, natural waters contain organics as well as micro- and macroscopic concentrations of various inorganic species such as metals and anions that can compete with, complex, or react with the transuranium species. The final concentrations of the actinide elements in the environment are thus the result of a complex set of competing chemical reactions such as hydrolysis, complexation, redox reactions, and colloid formation. As a consequence, the aqueous environmental chemistry of the transuranium elements is significantly different from their ordinary solution chemistry in the laboratory. 

Also present in many natural waters are humic/fulvic acid, citric acid, and the like. These organics also can complex actinides. In Figure 15.18, we show the relative stability constants for the first complexation reaction of various ligands with actinides of different oxidation states. Clearly, the carbonate and humate ions along with hydrolysis dominate the chemistry. The tetravalent actinide ions will tend toward hydrolysis reactions or carbonate complexation rather than humate/fulvate formation. 

Pu(IV), which forms highly charged polymers, strongly sorbs to soils and sediments. Other actinide III and IV oxidation states also bind by ion exchange to clays. The uptake of these species by solids is in the same sequence as the order of hydrolysis Pu > Am(III) > U(VI) > Np(V). The uptake of these actinides by plants appears to be in the reverse order of hydrolysis Np(V) > U(VI) > Am(III) > Pu(IV), with plants showing little ability to assimilate the immobile hydrolyzed species. The further concentration of these species in the food chain with subsequent deposit in humans appears to be minor. Of the 4 tons of plutonium released to the environment in atmospheric testing of nuclear weapons, the total amount fixed in the world population is less than 1 g [of this amount, most (99.9%) was inhaled rather than ingested]. 

Actinides, particularly the lighter ones, display multiple oxidation states and complex chemical behavior, which makes their chemistry quite fascinating. Some isotopes of these elements, such as 232Th, 233,235,238 and 239Pu, are important for the nuclear industry due to their utility as fissile/fertile materials. Therefore, the separation chemistry of different oxidation states of Th, U, and Pu need to be reviewed with respect to both basic as well as applied aspects. Some fundamental chemical properties of the lighter actinides, including oxidation states, hydrolysis, and complexation characteristics form the basis of their separation. 

The actinide ions in 5+ and 6+ oxidation states are prone to severe hydrolysis as compared to lower oxidation states in view of their high ionic potentials. Consequently, these oxidation states exist as the actinyl ions MOt and MO + even under acidic conditions, which can further hydrolyze under high pH conditions. The oxygen atoms of these ions do not possess any basic property and thus do not interact with protons. The tetravalent ions do not exist as the oxy-cations and can be readily hydrolyzed at low to moderate pH solutions. The degree of hydrolysis for actinide ions decreases in the order M4 > MOT > M3 > MOt, which is similar to their complex formation properties (4). In general, the hydrolysis of the actinides ions can be represented as follows ... 

The Th4+ ion, due to its larger size and lower ionic potential, is quite different from other tetravalent actinide ions, as it does not undergo hydrolysis as readily as U4+ or Pu4+ ions (5). Tetravalent U and Pu ions hydrolyze first in a simple reaction, as given by Equation 2.1, which is followed by a slow irreversible polymerization of hydrolyzed products. 

Modolo, G., Seekamp, S. 2002. Hydrolysis and radiation stability of the alina solvent for actinides(III)/lanthanide(III) separation during the partitioning of minor actinides. Solvent Extr. Ion Exch. 20(2) 195-210. 

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. 

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