chemical thermodynamics of neptunium and plutonium - U.S. ...

A. Discussion of selected references 723[86WEI/HEL]The results and discussion essentially duplicate those in [83HEL].[87BEN/HOF]This is a UV-spectroscopic study, with no real data useful for estimating the first hydrolysisconstant of PuO + 2 .[87DEL]Solubility data were presented for the identification of PuO 2 (OH) 4 (CO 3 ) 7−2along witha reported value for the equilibrium constant log 10 K = (1.13 ± 0.20). Data werecollected at room temperature (18- 24 ◦ C) with an equilibration period of 153 days. Thesolutions were 3-5 molal NaOH and 0.25 to 1 molal Na 2 CO 3 . Plutonium was added tothese mixtures as Pu(IV) nitrate, and the concentration of plutonium in the supernatantwas determined by scintillation counting. Observations made on other systems withcomparable high sodium hydroxide concentrations had identified Pu(V) in the alkalinesupernatants. There is an apparent ambiguity in this identification as Pu(VI) was alsoidentified in acidified samples. The author has chosen to write the equilibrium as theaddition of two carbonate ions to PuO 2 (OH) 3−4, the predominant hydrolysis speciesreported to be found in the study. The solid phase apparently changed with time. Thevalue for the association constant and species identification are not considered to bereliable.[87FUK/SIE]This is a solvent extraction study of Pu(III) nitrate complexation performed at (25 ±0.1) ◦ C with aqueous solutions 2 M in NH 4 NO 3 , and using Adogen 464 (trialkyl /C8-C10/ methylammonium nitrate) 20% in xylene as extractant. During the equilibrationHNO 3 was added to the aqueous phase to obtain a pH value of 3.3. The reaction studiedwas [83SOK/SIE]:Pu(NO 3 ) 2−5 (aq) + 2 ANO 3,org Å A 2 Pu(NO 3 ) 5,org + 2NO − 3(aq) (A.64)where ANO 3 is the Adogen nitrate monomer. Then the distribution coefficient is:D =K 5·β◦ ◦ 5·[NO− 3 ]3aq·[ANO 3] 2 org·f Pu 3+ ,aq·f 3 NO − ,aq·f 2 −1ANO 3 ,org·fcpx,org31 + ∑ Ni=1 β i·[NO − 3 ]iaq·f Pu 3+ ,aq·f i −1NO − 3 ,aq·fcpxi,aqThe distribution coefficients for plutonium were compared to those for severallanthanides measured using the same method. The authors assumed that activity coefficientsin the aqueous and organic phases and also complexation in the different aqueousphases were identical, so only the ratio of the products of the constants K ◦ 5 β◦ 5 fromthe above expression remains in the ratio of the distribution coefficients. From this,differences in the standard Gibbs energy of extraction for a couple of metals were