Distribution coefficients, trivalent

From sorption experiments with the rare earths and trivalent actinides, it was found that the distribution coefficients for kaolin and attapulgite were quite different, the latter having much higher values ( 1). Therefore, the radiation damage study... 

Figure 6. Theoretical distribution coefficients for trivalent ion sorption on sorbent containing 99.2% clay and 0.8% oxide. Arbitrary constants (see text).

Similarly, the distribution coefficients for trivalent cations in octahedral sites are predicted to show the order ... 

We found that absorption of Am3+ is greatly affected by aluminum nitrate. Trivalent Am can be satisfactorily absorbed from 0.1M HN03, but it can not be absorbed from 0.1M HN03 and 0.5M A1(N03)3. In 0.1M HN03 and 0.01M A1(N03)3 solution, the volume distribution coefficient of Am3+ is five times less than in 0.1M HN03. Therefore, if there is a great quantity of Al3+ in the feed solution, it should be removed. 

The berkelium (IV) extraction coefficients have been determined by stripping solvents previously loaded with tetravalent cerium and berkelium in the presence of sodium bismuthate. Sodium bismuthate has been found to be an efficient oxidizing agent for trivalent cerium. Because of its small solubility it does not affect the distribution coefficients of tetravalent cerium. These two properties have been demonstrated by comparing the distribution coefficients of cerium (IV) measured by spectrophotometry with those of cerium oxidized by sodium bismuthate and measured by beta counting of the cerium isotope tracer. The data are summarized in Table I and indicate no real difference in the distribution coefficients of cerium obtained by these two methods when using trilaurylmethylammonium salts-carbon tetrachloride as solvent. 

In IN to 2N nitric solutions, berkelium was not extracted either in TLMA nitrate or TBP, whereas cerium was extracted. Furthermore, the berkelium (IV) already extracted from 6N HNO3 into TBP was completely back-extracted by IN HNO3, while cerium was not stripped as much. A similar experiment made after adding sodium bismuthate proved that aqueous cerium was entirely at the four valence state, while berkelium showed a low distribution coefficient (smaller than O.OI) corresponding mainly to the trivalent state. This result was confirmed by a... 

In the above equilibrium, extractant dependency studies have indicated that n = 1 for Am" and Bk " and = 2 for Cf ". These stoichiometries have been observed for extractions into chloroform, and the self-adduct formation with trivalent actinides has been possibly cormected with tetrads where Cm is one of the minima. Extractions into xylene, however, leads to the formation of selfadducts with all four actinides due to better distribution coefficients in xylene over chloroform. The formation of self-adducts is due to ligand concentration, ionization constant of the ligand, basicity of the bound ligand, solvent identity, and oxidation state of the metal ion. 

Figure 4. Distribution coefficients of trivalent TPE in the neutral organophos-phorus compound-chloroform-nitric acid systems. The Roman numerals correspond to numbers of reagents in the text.

To decrease hydrolysis and to increase selectivity we have employed complex-forming agents such as DTPA and tartaric acid ( 1 ) The results show that Am extraction in the NaOH-tartaric acid solution is approximately the same as that of Eu. At low concentrations of the alkali the distribution coefficients are small as the concentration increases, the extraction of Am and Eu sharply increases to reach a maximum at 2 M NaOH ( 99%) and to stay constant up to 10 M. The R = f/NaOH/ dependence by Am and Eu extraction in the form of tartrates by aliquat-336 was different, namely the extraction was high at low alkali concentrations and poor at NaOH concentration greater than 2 M. It may be assumed that DOP forms strong chelates with trivalent actinides and lantha-... 

The synergistic mixture (HDEHP + TOPO) was used in Experiment 3 (runs 3a-3b). The DF Cm/Am values obtained were very similar (7 and 7.5) and not higher than previously, while DF Am/Cm are respectively 1,300 and 6,100 with two and three scrub stages. The aqueous raffinate was found to contain hexavalent and trivalent americium (the apparent distribution coefficient in the final extraction stage was 0.2). 

Figure 2 shows conclusively that DHDECMP can extract trivalent as well as tetravalent and hexavalent actinides from moderately concentrated nitric acid solutions. The valence (III) actinides, Am, Cm, and Cf, have distribution coefficients greater than one above 1.5M HNO3, while valence (IV) and (VI) actinides have values greater than one above 0.5M HNO3. In addition, DHDECMP shows very favorable back extraction or stripping characteristics below 0.3M HNO3. Pu(IV) polymer formation undoubtedly occurred in low-acid measurements and may account for the low distribution coefficients. 

Table 6.18 summarizes distribution coefficients of hexavalent uranium, thorium, and trivalent cerium (representative of rare earths) for four different types of long-chain amines, in sulfate solution with phosphate ion absent. Primary amines have the highest coefficient for thorium and the lowest for uranium, with the converse true of tertiary amines such as were cited for uranium extraction in Chap. 5. Secondary amines extract both metals, with thorium extraction favored by branching distant from the nitrogen. Either primary or secondary amines provide good separation of thorium from cerium. 

Trivalent americium forms relatively unstable complexes with Cl and NOs and more stable complexes with the thiocyanate ion CNS. These americium complexes are more stable than those of the corresponding lanthanide compounds, so that americium can be separated from trivalent lanthanides by anion exchange with concentrated solutions of liQ, liNOs, or NH4CNS. Trivalent americium can be extracted with TBP from a concentrated nitrate solution. It can also be extracted with TBP from a molten LINO3 -KNOs eutectic at 150°C, with much higher distribution coefficients than in extraction from aqueous solutions. Americium is more readily extracted by this process than is trivalent curium [K2]. 

Fig. 22. The regular variation of mineral/melt distribution coefficients for plagioclase with ionic radius, showing the effect of lattice size and valency. Eu enters the Ca " (r = 1.12 A) site more readily than the trivalent lanthanides, the largest (La) of which have preference over the smaller (Yb).

Keder et al. also determined the extraction properties of hexavalent, trivalent and pentavalent actinides as a function of nitric acid concentration, with results shown in Figs. 20 and 21. The maximum distribution coefficients for all these species is very much less than for Pu(IV) and Np(IV), indicating an easy separation of these elements from other actinide elements. For the hexavalent species, the amine concentration dependencies of the distribution coefficients were between first and second power, permitting no unambiguous assignment of the extracted complex. The slope of the Am(III) curve was unity, while that for Pu(III) was approximately 1.5. No explanation of this fact was given. 

Neill and Higgins determined distribution coefficients for Pu(HI) and Pu(IV) for several resins in sulfuric acid solutions. Their results-are shown in Table IV-28. They used Dowex-50 resin to demonstrate a process for recovering Pu from sulfuric acid decladding solutions which contain stainless steel. The Pu is normally trivalent in dilute sulfuric acid solutions, and is adsorbed from 0,5 M acid, scrubbed with 0.5 M sulfuric acid, washed with water to remove sulfate, and eluted with 6 N HNOg. The product Pu solution contained 5% of the original stainless steel materials, primarily iron and chromium. 

Distribution coefficients of Am and other trivalent transplutonium elements from concentrated LiCl solutions are from 150-fold to more than 1000-fold higher than those of trivalent lanthanides [55]. This phenomenon was used by Moore [56] in various analytical applications it was also exploited at ORNL in the development of the Tramex process for plant-scale separation of americium, curium, and other transplutonium elements from fission product lanthanides [7, 57]. 

As a rough approximation (neglecting the activity coefficients), the distribution ratio of a given trivalent metallic cation (DM) can be derived from the logarithm expression of the concentration equilibrium constant Kex ... 

Distributions of this type are traditionally interpreted as the result of particulate scavenging in surface waters followed by remineralization at depth. Type of distribution contrasts strongly with that of Co, the first series transition metal which is located directly above Rh in the periodic table, illustrates that elements from the same group in the periodic table can exhibit very different chemical behavior. The contrasting behavior of Co and Rh is potentially related to the fact the Co has an active redox chemistry in the marine environment whereas Rh is believed to be stable only as Rh(III) complexes. It is unclear why the upper crustal partition coefficient calculated for Rh is so large (Figure 1) by analogy to other trivalent metals a much lower value would be expected. 

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