AKION-EXCHANGE ABSORPTION OF QUADRIVALENT ACTINIDE NITRATES
Oct., 1960
1375
SPECIE3 INVOLVED I N THE ANION-EXCHANGE ABSORPTIOS OF QUADRIVALE?\;T ACTISIDE SITKATES' BY J. L. RYAN Hanford Laboratories Operation, General Electric Company, Richland, Washington Received January $6, 1960
The anion-ecchange absorption of the quadrivalent actinides is compared with spectrophotometric and solubility data The plutonium, neptunium and uranium species absorbing on anion exchangers from nitric acid and from calciuni nitrate are PU(L1;03)6-, Wp(NO3),3' and U(N03) =. The data indicate that iii the case of thorium, Th(NOa)a= is absorbed. The presence of distinct maxima in the anion exchange distributions of the quadrivalent actinides in nitric is attributed to the formation of HM(N03)e- and H2M(N03)6a t high acidity. The marked similarity betreen anion exchange of metal complexes and the solubility of quaternary amine salts of these complexes is pointed out.
Anion-exc hange resins have long been known to show very h:gh selectivities for certain of the metal complex ions. The distribution coefficients of many of the complexes requiring high ligand concentration for thleir formation are known to go through maxima als the concentration of the ligand acid is increased 2 - 4 It is also known that in many systems the absorption of the metal complexes by the exchanger is much stronger when a metal salt of the ligand ion is used instead of the ligand a ~ i d . ~ - 6The anion-exchange absorption of a metal complex by resin in the ligand anion form can be represented by the reactions
*
bI+* -- nX -y _J MX,
blXn-"U+m
+ (-) ny -- m
blX,-"s+m
&-y
+ (~TJ- m)H+
+m
(1)
Hnv-mMX-n (3)
where the subscript (r) denotes resin phase and others are aqueous phase. Reactions 1 and 3 may occur stepwise, arid X-Y might be any complexing anion including acid anions. Also an anion which is formed in an intermediate step of reaction 3 (complex acid anion) might absorb. Reaction 3 above has been either diimiissed with only brief consideration or neglect,ed entirely by most previous workers.6-* Recent p a p e r ~ ~ ~published ~O while t'his work was lbeing completed have proposed that formation of t,he free acid of the complex anion (reaction 3) is a fa.ctor affecting anion-exchange behavior. .bt,tempts have been made to explain higher anion-exchange distributions in ligand salts (1) T h i s paper is based on work performed under Contract No. AT(45-1)-1350 for t h e U. S. d t o m i c Energy Commission. Presented before t h e North\rest Regional Meeting of the Arnerican Chemical Society, Richland, Washington, June. 1960. (2) K. A. Kreus and F. Keison, "Proc. Intern. Conf. Peaceful Uses .Itomic Energy, Cenel-a, 19;3." Vol. V l I , pp. 113, 131 (1951il. 1 3 ) .I. L. R y a n a n d E. .I. Wheelwright. Ind. Eng. Ciiem.. 51, 60 (19.58). > J. Danon, J . A m . Chrm. S o e . , 78, 5953 (1956). ) I liy solubility studies and to correlate it t o anionexchange behavior, the insoluble compounds must be salts of the complex anion instead of addition compounds of crystallization. An example is the precipitation of a tan chloride of h'i(I1) from hydrochloric acid and lithium chloride by Cs+ M hereas the complex absorbed on anion-exchange resin from concentrated lithium chloride is brilliant blue, and no anion-exchange absorption is observed in hydrochloric acid. In order t o identify or conipare the species absorbed in anion exchangers and the species present in precipitates spectrophotometric studies of the resin, of inert, high dielectric solvent solutions of the precipitates, and of mulls of the precipitates may be useful as discbussed in this paper. Icraus and MichelsonZGdetermined the solubility (25) T hlorller, 'Inorrani? Cllrniistrv '' J o l \\ ~ ~Irl ~ u . 1%ins Trir N r ~ rYorb. N. Y , 1952 ;I. ,343
Ort. , 1960
ANION-EXCHANGE ABSORPTIONOF QUADRIVALENT ACTINIDENITRATES
of tetramethylammonium and benzyltrimethylammonium chloroaurate in hydrochloric acid and lithium chloride and found that the solubilities of these were reminiscent of the anion-exchange behavior of gold in these media. Since their measurements are in an absence of a constant excess of the quaternary amine cation and both the cation and AuC14- concentration vary with chloride concentration in their solubility measurements the slopes of the solubility curves 2's. chloride are less pronounced than the anion-exchange slopes. AuC14is a stable compleu and as such shows no maximum in anion-ex&inge distribution coefficient us. hydrochloric acid voncentration. The distribution coefficient D) for Au(II1) in hydrochloric acid decreases with increasing hydrochloric acid and increases slowly with increasing lithium chloride. 27 If the solubility data of ref. 26 are recalculated as a (solubility squared) it becomes proportional to the AuC14- activity a t constant amine concentration and agrees much more closely with the anionexchange data for An(II1) in ref. 27. It appears that with Au(II1) increasing hydrochloric acid promotes formation of chloroauric acid whereas increasing lithium chloride promotes increasing D and decreasing quaternary amine salt solubility through the effect of the highly hydrated lithium ion on the activity coefficient of water and thus on the chloroaurate ion (salting out effect). No maximum is observed in hydrochloric acid because formation of the ALuC14- is complete a t extremely lorn chloride concentration. It is possible using generalized reactions 1, 2 and 3 to predict probable anion exchange distributions LIS. ligand concentration data for ligand acid and ligand salt solutions. Consider first a very stabli: coniplex anion (high complexation constant) which LS strongly absorbed by the anion resin (Le., AuCl4-, PtCls=). If the ligand acid is stronger than the complex acid (likely the case in HC1, HXO?, HBr, etc.) the distribution coefficient will decrease with increased ligand arid concentration due to reaction 3. In ligand salt solution a t low concentration, the distribution coefficient will decrease due to common ion effect in reaction 2 . At higher ligand salt concentration where resin invasion it; significant the distribution coefficient will probably increase with such salts as those of Li, Ca, A1 and other ions which are highly hydrated and chaiige water activity markedly. (2G) IC. A. Kraus and D. C Miehelson, in Oak Ridge National Laboratory Chemistry Division Annual Progress Repoi t , ORNL2786 Period endins Jiiiic 20 1957, p. 104. ( 2 7 ) K. \ ICraiiq, 1) < h I i ( 11cIs~inand F. Nrlson ./. A m . Chem. soc.. 81, 3204 I 9 X ) .
1385
With salts of NH4+and similar cations less increase or actual decrease in distribution coefficient will be observed with increased ligand salt. Next consider complex anions of lower stability but strongly held by the resin (Le., PU(NO~)G-, ZnC14-). Here if the complex acid is a relatively weak acid compared to the ligand acid a maximum in the distribution coefficient os. ligand acid will result because of the combined effects of reactions 1 and 3. The position of the maximum will depend on the relative contributions of reactions 1 and 3. It must be pointed out that this maximum may be above the attainable ligand acid concentration if the complex formation constant is quite low or if the complex acid is moderately strong (probably the case with Zr, Hf, Pa, Nb, etc., in HCI). In ligand salts, complexes in this category will show markedly increasing distribution coefficients as the ligand salt is increased up to the point where conlplexation is complete, after which slower increase will probably be observed in salts such as those of Li, Ca, Al, etc., and probably decrease in salts of S H 4 + , etc. Third, consider complexes of high stability but more weakly absorbed by the resin (example T h sulfate5). Here the highest distribution will occur at low ligand concentrations and n-ill decrease with both increasing ligand acid and ligand salt through common ion effect in reaction 2 , but the decrease will be much more rapid in acid than in salt solution if the complex acid is weak. In the case of complexes of both low stability and low resin affinity, absorption will be slight a t any ligand concentration. A further factor is introduced when the ligand acid is a polybasic acid such as sulfuric acid. Here increasing acid promotes bisulfate formation which may compete with the complex anion in the resin without contributing to its formation. Although the preceding division of complex anions into four general categories was made, it must be realized that such division does not in reality exist but instead only a continuous gradation of properties occurs. The relative stability of the complex, dissociation constaiit of the ligand acid, dissociation constant of the complex acid, affinity of the resin for the complex, and activity coefficient effects (salting effects) of the cations of ligand salts used must all be considered. Acknowledgment.-The author expresses his appreciation to R. L. Braun who carried out the neptunium and thorium ion-exchange and solubility measurements and who measured many of the neptunium absorption spectra. The author also thanks the personnel of the Analyticd Lahoratories Operation who carried out most of the :Lnalyses.
