Ion-Exchange Properties of Microporous Tungstates

Hydrothermally prepared microporous, hexagonal tungsten bronze (HTB) phases display promising distribution coefficients ( K d ) for both Cs + (2−100 ppm) and Sr 2+ (0.5−60 ppm) cations in acidic (1.0 M HNO 3 ) radioactive waste (radwaste) simulants. The selectivity for Cs + and Sr 2+ increases significantly on isomorphous substitution of molybdenum into the tungstate framework and is optimal for material of composition Na 0.2 Mo 0.03 W 0.97 O 3 · Z H 2 O. This composition results from attempts to prepare materials doped at a 10 atom % level viz. Na 0.2 Mo 0.1 W 0.9 O 3 · Z H 2 O (Mo 0.1 −HTB). Both the parent HTB and Mo 0.1 −HTB phases reach maximum uptake of Cs + and Sr 2+ after 30−60 min contact. The cation exchange capacity (CEC) of the parent HTB phase for Cs + and Sr 2+ 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 Mo 0.1 −HTB sorbent displays good tolerance of the alkali metal cations, Na +, K +, and Ca 2+, with respect to the sorption of Cs + from acidic solutions. In contrast, the sorption of Sr 2+ from similar solutions is reduced to a much greater extent by the presence of these cations.