Tetravalent actinide ions

In contrast to the situation observed in the trivalent lanthanide and actinide sulfates, the enthalpies and entropies of complexation for the 1 1 complexes are not constant across this series of tetravalent actinide sulfates. In order to compare these results, the thermodynamic parameters for the reaction between the tetravalent actinide ions and HSOIJ were corrected for the ionization of HSOi as was done above in the discussion of the trivalent complexes. The corrected results are tabulated in Table V. The enthalpies are found to vary from +9.8 to+41.7 kj/m and the entropies from +101 to +213 J/m°K. Both the enthalpy and entropy increase from ll1 "1" to Pu1 with the ThSOfj parameters being similar to those of NpS0 +. Complex stability is derived from a very favorable entropy contribution implying (not surprisingly) that these complexes are inner sphere in nature. 

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. 

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. 

Recently, another class of neutral organophosphorus compounds, namely, N,N-dialkyl carbamoyl methyl phosphonate (CMP) (59) and its phosphine oxide analog (CMPO) have received attention due to their ability to extract even trivalent actinides from acidic solutions along with the hexa- and tetravalent actinide ions. These biden-tate phosphorus-based neutral extractants are reported to be stronger extractants as compared to TOPO (59-62). Pu(IV) and U(VI) are extracted as per the following extraction equilibria ... 

Though TBP and DOSO adducts of Pu(IV) were observed when HTTA was used as the primary extractant, no such adducts were reported with the Pu(IV)-HPMBP system (110, 111). On the other hand, synergism was observed for Pu(IV) extraction with HTTA, HPMBP, and HPBI (with stringent stereochemical requirements) when TOPO was used as the auxiliary ligand (27, 33). Other tetravalent actinide ions such as Th(IV) and Np(IV) have shown similar extraction behavior (29, 30, 34). Some adduct formation constants (I<0 for U(VI) and tetravalent actinide ions are listed in Table 2.4. It is necessary to consider both electronic and steric factors of the ligands to explain the observed trends. 

Tetravalent. The hydrolysis of tetravalent actinide ions can begin to occur in solutions with pH levels < 2. Under dilute conditions, species of the form An(OH) " (n = 1 4) are predicted however, most hydrolysis studies have only been able to identily the first hydrolysis product, An(OH) +. It should be noted that in all of these compounds the remainder of the coordination sphere is made up of bound H2O molecules. The end member of the speciation is the neutral An(OH)4 or An02-2H20. This complex has low solubihty but has been postulated to exist in solutions from solubihty experiments when using the isolated solid as the starting material. Under more concentrated conditions, polymeric materials have been postulated. In modeling the hydrolysis of thorium at concentrations greater than mM, polynuclear species of the form Th2(OH)2 +, Th2(OH)4 +, Th4(OH)g +, Th6(OH)i4 +, and so on, have been included. 

All early actinides from thorium to plutonium possess a stable +4 ion in aqueous solution this is the most stable oxidation state for thorium and generally for plutonium. The high charge on tetravalent actinide ions renders them susceptible to solvation, hydrolysis, and polymerization reactions. The ions are readily hydrolyzed, and therefore act as Bronsted acids in aqueous media, and as potent Lewis acids in much of their coordination chemistry (both aqueous and nonaqu-eous). Ionic radii are in general smaller than that for comparable trivalent metal cations (effective ionic radii = 0.96-1.06 A in eight-coordinate metal complexes), but are still sufficiently large to routinely support high coordination numbers. 

Aqua species. The coordination number of tetravalent actinide ions and U " " has... 

Polyoxometallates. As previously discussed, several classes of polyoxometallates can serve as ligands in the complexation of tetravalent actinide ions. The first of these is the decatungsto-metallates, [An WloOse] , An = Th, niolecular structure of the uranium complex has... 

Using liquid/liquid phase separation by thenoyltrifluoracetone, TTA, in benzene, the authors studied the speciation of Zr in the concentration range 10 to 0.1 M in 2 M perchloric acid solution as well as in 1 M HCIO4/I M LiC104 solutions. TTA is known to selectively extract free tetravalent ions such as Zr or the tetravalent actinide ions. Polymer formation by Zr in the aqueous phase is reflected quantitatively as a decrease in the distribution coefficient. The experiments were conducted very carefully spectroscopic determination of species in the benzene phase, correction for TTA loss of the benzene phase, consideration for complexation of Zr by TTA in the aqueous phase, recrystallisation of starting solids, consideration of impurities in the test, assurance that equilibrium has been reached and discussion of errors related to the variation of proton activity in the aqueous phase due to the extraction reaction. 

Arsenazo 111 partitions quantitatively into the PEG-rich phase from pH 1 to 6 with distribution ratios greater than 100. The extraction of metal ions in the sulfate media with Arsenazo III as an extractant depends markedly on pH. The tetravalent actinide ions Th , Pu, and U " are well extracted at pH 2 in the order Th" " > Pu > and the trivalent... 

The information relating to solvation numbers of tetravalent actinide ions is rather sparse. From NMR peak area, an estimate of the hydration number of Th(IV) in an aqueous-acetone solution of Th(C10 J4 at — 100°C indieated a value of nine (Butler and Symons 1969, Fratiello et al. 1970b) whereas an indirect NMR linewidth method gave a hydration number of ten (Swift and Sayre 1966). An entirely different method for the estimation of hydration numbers from conductivity measurements has been proposed and developed by Gusev (1971,1972,1973). The dependence of conductance on coneentration in acidic solutions of metal salts shows the pattern given in fig. 5. 

The ground-state polarizabilities of the tetravalent actinide ions Th , Pa, and U have been computed at different relativistic levels using the... 

In Table 20.7 are listed radii of trivalent actinide ions (coordination number CN 6) derived from measurements on trichlorides by the method of Bums, Peterson, and Baybarz [288]. Determinations of M-Cl distances have been made for M = U, Pu, Am, Cm, and Cf the other values were estimated by use of unitcell data and curve fitting. All these radii are relative to the trivalent lanthanide radii of Templeton and Dauben [396], who employed data from cubic sesquioxides and assumed atomic positions to establish M-O distances. Also included in Table 20.7 are radii of tetravalent actinide ions obtained from the M-O distances calculated from unit-cell parameters of the dioxides [1] by subtracting 1.38 A for oxygen (the value used [396] for the sesquioxides). For comparison. Shannon s ionic radii, derived from oxides and fluorides, and Peterson s tetravalent radii, derived from dioxides, are shown [537,538]. As... 

The strongly acidic properties of the tetravalent actinide ions have been subject to many investigations. This applies especially to thorium, where the tetravalent oxidation state is of unique importance as the only one existing in solution. The stability of tetravalent thorium, and its easy availability, also allow very thorough studies without the expenditure of an excessive amount of time and effort. The Th" " ion is therefore suitable to use as a model for other tetravalent actinide ions, though it should be remembered that, as the first member of the series, Th" " presents certain specific features. 

Table 21.2 Hydrolysis of tetravalent actinide ions. Constants pfi i listed refer to formation of the first mononuclear complex in solutions of various ionic strengths / in tight and heavy water, at 25°C. Medium (Na,HYZlO, if not otherwise stated.

Moriyama, H., Sasaki, X, Kobayashi, T., and Takagi, I. (2005) Systematics of hydrolysis constants of tetravalent actinide ions. /. Nucl. Sci. TechnoL, 42, 626-635. 

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