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

A. Discussion of selected references 691[83HEL]The paper describes an X-ray diffraction study of compounds of the formM 2 NpO 2 SO 4·xH 2 O(s) and M 2 NpO 4 (s) where M = Cs or Rb and x = 10, 4, and 0.5.The most highly hydrated forms were precipitated from room temperature solutionsand the dehydrated forms prepared on heating. Other compounds described are:K 2 NpO 2 (SO 4 ) 2·H 2 O(s), Na 2 NpO 2 (SO 4 ) 2·H 2 SO 4·7H 2 O(s), Na 2 NpO 2 (SO 4 ) 2 (s),(NpO 2 ) 2 SO 4 (s), (H 3 O) 2 Na 2 NpO 2 (SO 4 ) 3·2H 2 O(s), Na 2 NpO 2 (SO 4 ) 2·H 2 SO 4 (s),NH 4 HNpO 2 (SO 4 ) 2·3H 2 SO 4 (s), (NH 4 ) 2 NpO 2 (SO 4 ) 2·3H 2 SO 4·4H 2 O(s), NpO 2 SO 4·H 2 SO 4·6H 2 O(s), and NpO 2 SO 4·H 2 SO 4·2.5H 2 O(s). Bulk analyses andwater content were provided for some of these solids and X-ray diffractionpeak heights for most. Comparison was made to analogous U compounds. There areno thermodynamic data useful for this review.[83KIM/LIE]The results on complexation of Pu(IV) in carbonate-bicarbonate solutions were obtainedfrom a series of solubility studies of 238 PuO 2 (s) at a constant ionic strength of1.0 at room temperature (probably 20 ◦ C). The solubilities are given as a function of pHthat varied from 2 to 10 (the higher pH values were fixed by the ratio of CO 2−3 /HCO− 3 )and were measured after equilibration periods of 4 to 12 months. Spectrophotometricevidence supports the contention that Pu(IV) was the predominant oxidation state inthe initial solutions, but the oxidation state later in the experiment was not confirmed.The stability of the initial solid phase ( 238 PuO 2 (s) prepared from the amorphous hydroxidecalcined at 700 ◦ C for 6 hours) was not verified during the experiment andthe authors postulated that an alternative phase, Pu(OH) 2 CO 3 (s), might have been thesolubility limiting phase in these experiments. No verification of the identity of thephase was made. The authors were aware that radiation effects on the solutions couldgive rise to ambiguities in the interpretation of the solubility data and they consideredthe calculated values to be “preliminary”. Polymers, which may have formed in thecourse of some of the experiments, especially at low total CO 2−3concentrations, werenot considered by the investigators. The data describing Pu(IV) hydrolysis was takenfrom Baes and Mesmer [76BAE/MES]. The data were interpreted as resulting from asequence of carbonate complexes up to the pentacarbonato complex, and included onemixed hydroxycarbonate, PuOHCO + 3. The stability of plutonium carbonate was takenas given in the work of Moskvin and Gelman [58MOS/GEL] and, as a result, all ofthe remaining calculated values were biased by this ill-chosen value (see comments inthis appendix for [58MOS/GEL]). Unfortunately, no attempt was made to perform asensitivity analysis which would have permitted some of these candidate complexes tobe eliminated. In the absence of the original data, we cannot perform these calculationsourselves. We therefore give zero weight to the calculated values of stability constantsfor these Pu(IV) carbonate complexes. However, the observed rapid increase in plutoniumsolubility in high carbonate solutions is strong evidence that the limiting speciesis at least a tri-carbonato species.