related to specific activity or oxidation state differences. It is particularly important with Pu and possibly Np that the oxidation states present be defined as chemical form differences may influence biological or environmental interpretations (Dahlman et al., 1976). The most stable oxidation states of the transuranium elements are probably Np(V), Pu(Iv), Am(I1II), Cm(ITIL). While Np, Pu, and Am can exhibit at least four oxidation states in aqueous solutions (III, IV, V, Vi), only Np and Pu are likely to exist in more than one oxidation state environmentally, although Am(IV) might be stable at high pH's and in oxidizing environments (Pourbaix, 1966). -|.80 -1.40 r Pud + e -1.00 Pud,, (OH), Three general topics will be addressed in this paper. They encompass the potential oxidation states of Pu in the environment as predicted by thermodynamic procedures; what oxidation states of Pu have been evaluated in natural or laboratory solutions which have environmental relevance; and the effect that oxidation state has on plant availability of the actinide elements. -.20 Eh-pH DIAGRAMS AND MINERAL STABILITY CALCULATIONS Stability field diagrams relating the species of Pu which dominate solid and solution phases under varing Eh-pH conditions have been constructed by Kraus (1949), Pourbaix (1966), Andelman and Rozzell (1970), and Polzer (1971), using thermodynamic constants for the reactions of interest. An evaluation of the phase relationships between Pu species will not be attempted here. Rather, those studies wiil be briefly reviewed as will a particular example. The reader ig referred to Garrels and Christ (1965) or Pourbaix (1966) for more information on the construction of these types of diagrams. 0.6 Kraus (1949) presented an early assessment of the effect of pH on the stability of the various Pu spectes. The stability of Pu(V) at weakly acidic to neutral pH's, and the instability of Pu(III) to oxidation by Oo were among the major points made. While this early paper was orfented towards Pu chemistry at macroaconcentrations, it is of interest historically. Polzer (1971) presented an extensive examination of the stability of Pu oxidation states under environmental pH's. Both solid and aqueous phase speciation was considered. Figure 1 is taken from that analysis. Because of different sign conventions, the Eh values in the figure are opposite in sign from the usual presentation of environmental-type Eh-pH diagrams (Garrels and Christ, 1966). It will be noted that in the pH range of 4 to 9, and the Eh range of, +0.6 to -0.3 volts (-0.6 to +0.3 volts in the figure), soluble Pu> and Pu, species dominate. Because of its low solubiijty, Pu(IV¥) was not considered. The line between the domains of Pu?’ and Pu02 represents the point of equal concentrations of the two species. Polzer (1971) pointed out that this analysis (Figure 1) considered only uncomplexed species (except with OH }. 2.2 Pu (metal) oO 1L 2 4 6 pH 8 10 \2 in Fig. 1. Stability relations of dissolved plutonium species (Polzer, water at 25°C and one atmosphere total pressure 1971). 450 14