Hydrothermal growth kinetics of Np(IV) oxide

Toulouse, France All previous knowledge leading to this estimate is of NpO2(c) is indirect, based on thermodynamic cycles. The phase itself has heretofore not been observed as a precipitate from aqueous solution. Recent attempts (e.g., Nitsche et al., 1993; Efurd et al., 1996) to establish solubility controls on Np in oxidizing groundwaters (including J-13 groundwater) starting from high concentrations (i.e., supersaturation) have shown the formation of one or both of two Np(V) phases, NaNpO2CO3:3.5H2O(c) (with variable stoichiometry) and Np2O5(c). These are both highly soluble, yielding Np concentrations on the order of 1 x 10 -4 to 1 x 10 -3 molal, in accord with existing thermodynamic data for these phases. No evidence was found of the formation of NpO2(c). Under reducing conditions, experiments (Rai et al., 1987 and references cited therein) have shown the formation of Np(IV) polymer, which may be viewed as a hydrated form of NpO2. It is orders of magnitude less soluble than the Np(V) phases, but still orders of magnitude more soluble than NpO2(c). No undersaturation experiments with NpO2(c) are known to have been performed. However, both NpO2(c) and Np(IV) polymer are known to be difficult to dissolve. We have hypothesized that NpO2(c) is simply a phase that is slow to form at low temperature. In this regard, it would be analogous to such minerals as quartz, dolomite, and hematite. It is well known that it is difficult to impossible to demonstrate the formation of such minerals in low temperature experiments on feasible time scales. However, it is possible to demonstrate their formation and measure the kinetics of the process by conducting experiments at elevated temperatures, generally in the range of 150-300 o C (e.g., Rimstidt and Barnes, 1980; Sibley et al., 1984). By developing kinetic models, it should be possible to estimate the appropriate time scales on which such solids might impose solubility constraints on natural groundwaters. To test our hypothesis, we reacted solutions of 1 x 10 -4 molal NpO2 + (balanced with Cl-) at 200 o C in closed Parr bombs (with air present). Some runs were conducted in teflon-lined reactors, others in titanium reactors with passivated surfaces. The pH was not buffered, and EQ3/6 calculations indicated that the initial solutions were supersaturated with respect to NpO2(c) and undersaturated with respect to other possible Np solids. In the initial runs, the starting 25 o C pH was 6-7. Periodically the reactors were cooled, opened, and the Np concentration and pH were measured. The Np concentration slowly dropped (Fig. 1), and the pH dropped to ~4.2. A very fine brown precipitate was observed. XRD analysis shows an eight-line match with NpO2(c), validating our hypothesis. Other experiments have been conducted with higher initial pH, and in which the original pH has been periodically restored by the addition of small amounts of base. These show a more rapid drop in dissolved Np (Fig. 1), indicating that the rate of precipitation is pH dependent. The precipitates are being characterized by XRD, EXAFS, and SEM. Additional experiments at 150, 250, and 300 o C are currently planned to allow the development of a kinetic model that can be extrapolated to lower temperature. The use of a mixed flow reactor is also planned in future work. Figure 1. Neptunium molality as a function of time in 200 o C experiments. Top curve (squares): initial (25 o C) pH 6.5, no attempt to restore the original pH. Middle curve (circles): initial pH of 8, pH readjusted. Bottom curve (diamonds): initial pH of 10, pH readjusted.