Ion-Exchange Properties of Microporous Tungstates

Hydrothermally prepared microporous, hexagonal tungsten bronze (HTB) phases display promising distribution coefficients (Kd) for both Cs+ (2-100 ppm) and Sr2+ (0.5-60 ppm) cations in acidic (1.0 M HNO3) radioactive waste (radwaste) simulants. The selectivity for Cs+ and Sr2+ increases significantly on isomorphous substitution of molybdenum into the tungstate framework and is optimal for material of composition Na0.2Mo0.03W0.97O3·ZH2O. This composition results from attempts to prepare materials doped at a 10 atom % level viz. Na0.2Mo0.1W0.9O3·ZH2O (Mo0.1HTB). Both the parent HTB and Mo0.1-HTB phases reach maximum uptake of Cs+ and Sr2+ after 30-60 min contact. The cation exchange capacity (CEC) of the parent HTB phase for Cs+ and Sr2+ is ca. 0.2 mmol·g-1 (26.6 g·kg-1) and 0.12 mmol·g-1 (10.5 g·kg-1), respectively, with a 100% increase displayed on 0.03 atom % molybdenum incorporation into the tungstate framework. The optimized Mo0.1-HTB sorbent displays good tolerance of the alkali metal cations, Na+, K+, and Ca2+, with respect to the sorption of Cs+ from acidic solutions. In contrast, the sorption of Sr2+ from similar solutions is reduced to a much greater extent by the presence of these cations.