Thermodynamics of Interactions of Th, U, Np, and Am with Polyoxotungstates
D.E. Wall*, G.R. Choppin
Department of Chemistry, The Florida State University, Tallahassee, FL, 32306, USA
* Present Address: Sandia National Laboratories Carlsbad Programs Group
4100 National Parks Highway - Carlsbad, NM 88220 USA
The thermodynamics of complex formation of Th4+, UO22+, NpO2+ and Am3+ with the polyoxotungstates, [NaP5W30O110]14-, [As2W21O69]6-, [P2W18O62]6- and [P2W17O61]10- were examined in 0.1 molar NaCl solution at 25 ° C. The respective stability constants of Th(IV), U(VI), Np(V) and Am(III) with [NaP5W30O110]14- are log b101 = 6.10 + 0.05, 3.80 + 0.06, 2.98 + 0.04, 5.85 + 0.05; and with [As2W21O69]6-are 9.9 + 0.1, 6.32 + 0.04, 4.39 + 0.03, 10.1 + 0.1. The log b101 of NpO2+ with [P2W18O62]6- is < 2 , and with [P2W17O61]10- is 4.81 + 0.03. When the primary binding sites have tetragonal geometry the order of the stability constants is given by: Th4+ > UO22+ > Am3+ > NpO2+, whereas the stability constants follow the order: Th4+ > Am3+ > UO22+ > NpO2+ when trigonal or pentagonal binding sites are predominant, due to steric effects. The magnitude of dehydration of the metal ion correlate to entropy of complexation, which depends on the geometry of the binding site.