Speaker
Description
Selective separation of rare-earth elements (REEs) is a central challenge in sustainable REE supply and recycling and can be useful in the radiochemical applications of these elements, for example in radiopharmaceutical chemistry and waste treatment. A hydrothermal route to synthesize tetravalent metal oxides (ZrO2, TiO2, SnO2) and their sulfated analogues was tested and their performance in separating La, Eu, and Lu in aqueous solutions was evaluated. Sulfation was achieved for zirconia and titania but not for tin oxide, as confirmed by FTIR and TGA, with high sulfate loading on ZrO2 (~24 wt%) and low loading on TiO2 (~1.5 wt%), while SnO2 remained essentially unchanged. Sulfation altered not only crystallinity, surface area and pore structure but also brought changes to sorption behavior.
Batch sorption experiments revealed that sulfated zirconia exhibits earlier onset and higher uptake than pristine ZrO2, with modest loss of selectivity. For titania, sulfation shifted preference from Eu to Lu, despite the low sulfate loading, indicating strong surface-chemical control over selectivity. Tin oxide showed identical behavior with or without sulfate treatment, consistent with unsuccessful sulfation.
The sorption of the lanthanides was manipulated using aqueous complexants. EDTA suppressed Lu uptake across all oxides while increasing La and Eu sorption on Zr-based materials at low pH, reversing the initial selectivity and markedly enhancing separations. The highest separation factor, SF(La/Lu)=95 (and SF(Eu/Lu)=32), was obtained with SnO2 in EDTA at pH ~4.7, where Lu sorption was suppressed heavily. In contrast, addition of EDTA suppressed the sorption of all lanthanides on titanium-based materials. Citrate generally increased uptake at low pH without the severe suppression seen with EDTA. On sulfated zirconia citrate inverted selectivity to favor light REEs (La>Eu>Lu), yielding moderate SFs (e.g., La/Lu≈12).
Overall, hydrothermal sulfation was shown to alter the crystal phase, pore size and surface area along with ion-exchange behavior of tetravalent metal oxides. Coupled with complexing agents, it enables pH-programmable and ligand-directed control of lanthanide selectivity, highlighting inorganic sulfated oxides, especially ZrO2 and TiO2 and EDTA assisted SnO2 as promising platforms for solid-phase separations of REEs.