Uranium (VI) Solubility in Carbonate-Free ERDA-6 Brine

Abstract

When present, uranium is usually an element of importance in a nuclear waste repository. In the Waste Isolation Pilot Plant (WIPP), uranium is present in significant quantities, with about 647 metric tons to be placed in the repository [1]. Therefore, the chemistry of uranium, and especially its solubility, needs to be determined under WIPP-relevant conditions.Long-term experiments were performed to measure the solubility of uranium (VI) in carbonate-free ERDA-6 brine, a simulated WIPP brine, at pC H+ values between 8 and 12.5. These data, obtained from the over-saturation approach, were the first WIPP repository-relevant data for the VI actinide oxidation state. The solubility trends observed pointed towards low uranium solubility in WIPP brine and a lack of amphotericity. At the expected pC H+ in the WIPP (~ 9.5), measured uranium solubility approached 10 -7 M. The objective of these experiments was to establish a baseline solubility to further investigate the effects of carbonate complexation on uranium solubility in WIPP brines, during the ongoing research program in actinide solubility under WIPP-relevant conditions.

Uranium(VI) solubility in carbonate-free WIPP brine

“…The correction factors K were (1.23 ± 0.01) for GWB and (0.94 ± 0.02) for ERDA-6 brine respectively [16]. region investigated [20,21]. However, for NaCl brines, because of the lack of buffer capacity, drifts in the pC H + values were observed, and they were not adjusted back to the desired values.…”

Influence of Carbonate on Uranium Solubility in the WIPP

“…. Similarly to the carbonate-free experiments [2], carbonate was first removed from the brines using a two-step process: acidification of the brines, which ultimately converted carbonate into dissolved carbon dioxide gas, and a slow pump-down process to smoothly remove gas from the brine. From this point, all brine solutions were kept in polypropylene bottles, tightly capped, in an anoxic carbon dioxide-free glovebox, and at a temperature of (25 ± 4) ºC.…”

“…The data obtained when carbonate was present correspond to 17 samplings performed throughout 994 days of experiments. The data in carbonate-free brines are presented elsewhere [2,9], but they are reproduced in Figure 1 for comparison purposes. In all of the experimental solutions containing carbonate, a yellow precipitate could be observed, and the uranium concentrations measured in solution reached a steady state over time.…”

“…Consequently, in WIPP PA, both IV and VI oxidation states of uranium are considered. Uranium is assumed to be uranium (VI) in 50% of the PA vectors and uranium (IV) in 50% of the PA vectors [2]. The solubility of uranium (VI) was conservatively set at 10 -3 M in WIPP PA for all expected WIPP conditions, including the potential and likely effects of carbonate complexation [1].…”

“…The solubility of uranium (VI) in the WIPP is expected to be defined by the combined contribution of hydrolysis and carbonate complexation. Experimental data published in previous papers [2,5,6] were obtained under carbonate-free conditions, so they provided a baseline for carbonate effects on uranium (VI) solubility. In this paper, some results of long-term experiments carried out in the presence of a significant amount of carbonate (up to millimole levels) are given.…”