Evaluation of Titanosilicate Materials for the Sorption of Cesium and Strontium from High Level Wastes

Treatment of high-level waste (HLW) stored in the U.S. Department of Energy (U.S. DOE) sites of Hanford and Savannah River involves the separation of radioactive cesium (137Cs) and strontium (90Sr). Challenges in treating the liquid HLW includes high alkalinity or acidity, and high salt content, many orders of magnitude above Cs and Sr concentrations. Ion exchange is the preferred process for removing these compounds, due to the availability of selective media and large volume-processing capacity by column operations. The crystalline silicotitanate (Na2Ti2O3SiO4·2H2O), i.e. CST, which structure is analog to the mineral sitinakite, is considered the reference material for this application. However, uptake of Cs by sitinakite declines at higher pH, and the thermal stability of the sitinakite structure may have been overestimated. At temperatures above 200 oC, sitinakite suffers drastic dehydration, which is suggested to affect its selectivity for Cs. As thermal stability is an important parameter in this application, the titanosilicate Na2TiSiO5, analog to the mineral natisite, should be considered as a potential sorbent. Despite rare investigations on natisite, a recent study suggested that replacement of part of Ti by other metals might improve natisite selectivity for Cs and Sr. In the present study, sitinakite was synthesized and heat-treated at temperatures ranging from 50 oC to 550 oC. Structural characterization was conducted using X-ray diffraction (XRD) and Fourier Transform Infrared (FTIR) spectroscopy. Batch experiments were performed for the uptake of Cs and Sr at neutral conditions (DI water). Sitinakite doped with tin was also heat treated and assessed for Cs and Sr uptake from solutions with initial pHs of 4, 6 and 10. In a second study, natisite and three metal-substituted variants (Al-, Zr-, and Sn-natisite) were synthesized and assessed for sorption of Cs and Sr. Natisite materials were evaluated along sitinakite, at increasing conditions of acidity, alkalinity, and competition with Na+ and Ca2+. All samples were assessed by ICP-MS for final concentrations of Cs or Sr. Uptake of Cs and Sr decreased with increasing processing temperatures, although the sorption of the first was noticeably more affected. These results matched the data obtained by XRD and FTIR, which indicated dehydration and phase transformation at temperatures consistent with the observed decrease in uptake. These data are supported by previous studies that linked Cs selectivity by sitinakite to its hydration. Uptake of Cs by Sn-sitinakite decreased at increasing alkalinity for all heat-treated samples. Still, samples treated at lower temperatures sorbed more Cs at high pH than Sn-sitinakites exposed to higher temperatures. In contrast, heat-treatment barely affected the removal of Sr by Sn-sitinakite, and higher uptake rates were observed at high pH. In the second study, sorption data revealed higher removal of Cs by sitinakite over natisites in the majority of the conditions. However, uptake of Cs by sitinakite deteriorated significantly at increasing acidity and alkalinity. Sitinakite and natisite materials removed comparable amounts of Sr at low pH or in solutions containing Na+ or Ca2+. Increasing concentrations of Ca2+ dramatically affected the removal of Sr and at 1 M Ca2+, the sorption of Sr was negligible by all sorbents evaluated. In alkaline solutions, Sn-natisite and Zr-natisite removed up to 95% and 90%, of Sr, respectively, largely outperforming sitinakite. An increase in Sr uptake by all natisite materials was observed at 0.1 M NaOH solutions, but sorption plunged at 3 M NaOH. The results obtained in the first study clearly established that exposing sitinakite to elevated temperatures is detrimental to its sorption for Cs and Sr, and the impact is more pronounced for cesium. In contrast, the sorption of Sr by heat-treated Sn-sitinakite did not seem to be compromised at high alkalinity. The results from the second study indicated that metal-substitutions on natisite improved the sorption for Cs and Sr, especially for the latter. The improvement may be associated with the size of the metal ion replacing Ti, in which larger metals would widen the layer aperture. The results obtained by Zr-natisite and Sn-natisite for Sr in alkaline conditions are very promising considering the DOE’s HLW conditions and deserve further investigation