Synthesis and characterization of 1:1 layered uranyl silicate mineral phases

Sodium boltwoodite (Na[(UO2)(SiO3OH)] · x H2O), uranophane (Ca[(UO2)(SiO3OH)]2 · y H2O), and sklodowskite (Mg[(UO2)(SiO3OH)]2 · z H2O) were synthesized in our laboratory using an optimized procedure described herein. Mineral identities were confirmed using powder X-ray diffraction. We also characterized our uranophane using Raman spectroscopy. The surface area of each material we produced was determined by two methods. We used the standard BET approach (gas phase absorption, N2), and we also estimated surface areas in solution using sorption of methylene blue. These two techniques provided markedly different results, possibly due to the collapse of the mineral structure under the vacuum necessary for the BET analysis. In aqueous suspensions, the surface areas are 83 ± 10, 72 ± 7, and 47 ± 4 m2/g, for sodium boltwoodite, sklodowskite, and uranophane, respectively, using the methylene blue sorption method. The amount of water associated to each mineral was determined by thermogravimetric analysis. Results showed that minerals that had been dried in a dessicator contained less structural water than samples that had been allowed to set in aqueous suspension. The numbers of water for dry sodium boltwoodite, uranophane, and sklodowskite were found to be 1.5 (± 0.1), 2.8 (± 0.6), and 6.0 (± 0.6), respectively. The zeta potentials in suspension of 0.1 M NaClO4 were similar for all three minerals, decreasing from − 20 mV to − 45 mV from pH 5 to 12, and remaining stable over a 10 day time period. The combination of a large surface area and negative zeta potential implies that these solids will behave much like clay minerals, serving as important sinks for other dissolved radioactive cations present in radioactive waste repositories.