The Solubility and Sorption of Uranium (VI) in a Cementitious Repository

Abstract

The solubility behaviour of uranium(VI) has been studied in a number of waters representative of the pore waters to be expected in candidate repository backfill concretes and also in solutions of calcium and sodium hydroxides. The sorption of uranium has been studied on one of these candidates, a Portland cement with a slag filler. The same cement has been hydrothermally treated to simulate the temperature conditions in the repository and the product used for comparative studies.The paper describes the experimental measurements and the results. The aqueous concentration of uranium over the uranates formed in the presence of sodium and calcium hydroxides was constant above pH 7 at 3 × 10−6 M. In the concrete pore waters the solubility of uranium was equal to or lower than this value. The distribution coefficient, Rd, for the sorption of uranium onto the slag cement was 2.5 × 104 ml g−1, and was increased by a factor two after hydrothermal treatment.

Reproducibility of the uptake of U(VI) onto degraded cement pastes and calcium silicate hydrate phases

“…Some experiments were made using subsequent filtration (0.22 µm Millipore  ). The short shaking time (3 days) is compatible with the observation of Brownsword et al [7] who showed through a kinetic study (1, 20 and 33 days) that the measured solubilities did not change significantly with time.…”

“…U(VI) sorption after 14 days, and desorption after 6 days, in suspensions containing CEM I and CEM V degraded cement pastes and C-S-H phases. Literature data are: a -Aged Concrete [10]; b -Fresh concrete: OPC, BFS, fly ash ... [10]; c -Different types of cement: pure or blend of OPC/BFS/PFA [7]; d -Hydrothermaly degraded cements [7]; e -Different types of concrete: OPC/lime/limestone aggregates [12]. dation state of the cement.…”

Thermodynamic Modelling of the Effect of Organic Complexants on Sorption Behaviour

“…The uranium dataset is largely based on that proposed by Lemire [13] except for the uranium(VI) hydrolysis dataset, which has been edited to remove the anionic U0 2 (0H)4~ and (U0 2 ) 3 (0H)7 species. There is some experimental support in the literature for the existence of these species, however recent uranium(VI) solubility studies at Harwell [14] suggest they are formed to a lesser extent than predicted by the Lemire data. The data from reference [14] were used to refine the constant proposed by Lemire for the formation of the U0 2 (OH) 2 species:…”

Determination of uranium(VI) sorbed species in calcium silicate hydrate phases: A laser-induced luminescence spectroscopy and batch sorption study

“…Pointeau et al [17] report sorption distribution ratios (R d values) ranging between 3 Â 10 4 L kg À1 and 1.5 Â 10 5 L kg À1 . In contrast, several studies on the immobilization of uranium by hardened cement paste (HCP) at ambient temperature [17][18][19][20][21][22][23] and at elevated temperature [24] have been undertaken. Sorption was reported to be strong with R d values typically varying between 10 4 L kg À1 and 10 5 L kg À1 .…”