Cerium distribution coefficients

Therefore, the preliminary investigation described herein examined several aspects of the behavior of the equilibrium distribution coefficients for the sorption of rubidium, cesium, strontium, barium, silver, cadmium, cerium, promethium, europium, and gadolinium from aqueous sodium chloride solutions. These solutions initially contained one and only one of the nuclides of interest. For the nuclides selected, values of Kp were then... 

Therefore, based on available literature, the following sorption results were expected (l) as a result of the smectite minerals, the sorption capacity of the red clay would be primarily due to ion exchange associated with the smectites and would be on the order of 0.8 to I.5 mi Hi equivalents per gram (2) also as a result of the smectite minerals, the distribution coefficients for nuclides such as cesium, strontium, barium, and cerium would be between 10 and 100 ml/gm for solution-phase concentrations on the order of 10"3 mg-atom/ml (3) as a result of the hydrous oxides, the distribution coefficients for nuclides such as strontium, barium, and some transition metals would be on the order of 10 ml/gm or greater for solution-phase concentrations on the order of 10 7 mg-atom/ml and less (U) also as a result of the hydrous oxides, the solution-phase pH would strongly influence the distribution coefficients for most nuclides except the alkali metals (5) as a result of both smectites and hydrous oxides being present, the sorption equilibrium data would probably reflect the influence of multiple sorption mechanisms. As discussed below, the experimental results were indeed similar to those which were expected. 

For the nuclides studied (rubidium, cesium, strontium, bariun silver, cadmium, cerium, promethium, europium, and gadolinium) the distribution coefficients generally vary from about 10 ml/gm at solution-phase concentrations on the order of 10 mg-atom/ml to 10 and greater at concentrations on the order of 10 and less. These results are encouraging with regard to the sediment being able to provide a barrier to migration of nuclides away from a waste form and also appear to be reasonably consistent with related data for similar oceanic sediments and related clay minerals found within the continental United States. 

Yttrium and the lanthanide FPs, mainly cerium, praseodymium and promethium, will be present in the dissolver solution as trications, which are poorly extracted by TBP/OK. Distribution coefficient measurements show293 that the trisolvates are extracted according to equation (157). 

The different terms of Equation 1 were obtained as follows— /ce, formal potential of the Ce(IV)-Ce(III) couple in the medium, was taken from publications [Ce(IV)]a and [Ce(IV)]o have been measured by direct absorption spectrophotometry [Ce(III)] was calculated by difference between total cerium, titrated by potentiometry, and tetravalent cerium [Bk(IV)]a was calculated from the solvent beta counting, allowing for the measured distribution coefficient of Bk(IV) [Bk(III)] was determined by subtracting the [Bk(IV)]a value from the aqueous counting in all cases [Ce(III)]o and [Bk(III)]o were found to be negligible. 

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. 

Procedure. Aqueous phases were prepared from samples of cerium (IV), cerium (III), berkelium, and acid and diluted by distilled water to the proper concentrations. Samples of cerium were chosen in order to obtain dijSerent cerium (IV)/cerium (III) ratios. The solutions were allowed to stand for six hours to reach the oxidation equilibrium. A 2 cc. sample of the solvent was added to the same volume of aqueous solution and mixed for 15 minutes. After separation by a centrifuge, samples of both phases were taken for the beta counting of berkelium and the spectrophotometric determination of cerium (IV). In addition, one aliquot of the loaded solvent was taken for determining the distribution coefficient of berkelium (IV). 

On the other hand very little berkelium is extracted by tributyl phosphate from IN to 2N HNO3 as shown in Table IV. The determination of the formal oxidation potential of the Bk(IV)-Bk(III) couple in this medium is not reliable. Assuming that all the berkelium extracted is tetravalent and that the distribution coefficient of berkelium (IV) is the same as that of cerium (IV), calculation shows that the difference between the formal oxidation potential of the two couples should be greater than +0.08 volts. 

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... 

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. 

Table 6.18 Distribution coefficients for uranium, thorium, and cerium between organic amines and aqueous sulfate solution"...

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