Silver (1971) presented an interesting approach,to evaluating the speciation of Pu in natural waters. The stability of Pu0, at environmental pH's was recognized and discussed. The major difficulty in appreciating Silver's approach, however, lies in his use of “alpha facrors" to account for changes in ion activities due to complexation. For example, millimolar Pu(IV) concentrations are predicted in certain circumstances where complexation is active. 12 9 yor % How realistic are such concentrations is a question which remains to be answered, An example of an Eh-pH approach is discussed at this point to both illustrate the basic concept and to demonstrate the type of complexities which do not appear in an analysis of Pu alone. The oxidation of Pu(III) to Pu(IV) at environmental pH's might be written as follows: 3+ + 2 O= Pu(OH), (s) + 4 H +e + Eh = E° + n - a The Nernst } where E°, the formal potential in yolts, and 0.059 are constants: n is the change in electrons in the reaction; and ( ) represents ion activity. The activity of a pure solid is taken as unity, thus equation 2? reduces to *p0> (g) = - log of the O» pressure, , AQUA Pu on yA = ae Z pe N SP UO.OH).? 2 2 20 4 & $3 2 “ i % © oe <-> y 28 r <3 “Fe Y 4 “@ 2 + 4 “9 Pug, 3 a <r, ? gy 4a e “tO 32 F (Pucon),) (Ht)! (Pu~ Pu0,CO,” Q) where aqueous Pu? is in equilibrium with solid phase Pu(OH),. equation of reaction (1) is: 0.059 log 16 : g re %, Rai and Serne (1977) took a slightly different approach in evaluating Pu speciation. In addition to hydrolytic species, Rai and Serne considered other inorganic ligands (F, HPO, , SOQ, , Cl , CO3 } in their paper. Their analysis was aimed at predicting soil solution activities of soluble Pu species and solid phase compounds under varying redox and pH regimes. It wag concluded that under reducing conditions (p0, = 80 atm.)* and pH 8, Pu? and its complexes dominate. In oxidizing edSironments, Pu02C0,0H fs the predominant solubl2 species at pH 8,0. This approach 1s illustrated in Figure 2, which was taken from their analysis of the activity of various Pu ions in solutions in equilibrium with PuOg(s). The assumed conditions were p02 = 16 atm.; pCO, = 3.52 atm.; pCl = pS$0,2 = 2.5; pF = 3.5; and pHyp0, = 5.0. As Figure 2 illustrates, their analysis was a comprehensive attempt to integrate the chemistry of Pu with the characteristics of natural solutions. The dominant solid phase under those conditions, according to their analysis, was Pul>(s). My . Pug ols? -log A;, moles/t Andelman and Rozgzel (1970) examined the stability relationships of Pu in aqueous solutions. Pentavalent Pu apparently was not considered because of its tendency to dieproportionate; however, Kraus (1949), Silver (1971), and Polzer (1971) recognized that this reaction was pH and concentration dependent. Consequently, Pu? was determined to be the dominant species 1f complexation by CO3 was not considered, but masked by PuCO3* if carbonate was included. Schell and Watters (1975), in their review, used the solubility diagram derived by Andelman and Rozzell (1970) to demonstrate the complexity of Pu species in natural waters. It was pointed out by Andelman and Rozzell, however, that the effect of redox potential was not considered for that diagram. 4 NX 3a % \5 aN 2> > ; - | (2) 3 5 7 9 H 11 . a : P . . : : : Fig. 2. The activity of various Pu ions in the soil selutien in equilibrium with Pud. (5) uncer oxidizing conditicns (Rai and Serne,1977).