Interaction of U(VI), Np(VI), and Pu(VI) ions with unsaturated heteropolytungstates of the 17th and 11th series in aqueous solutions

The stability constants of the complexes of U(VI), Np(VI), and Pu(VI) with the heteropolyanions (HPAs) P2W17O61 10-, SiW11O398-, and PW 11O397- in solutions with pH from -0.3 (2 M H+) to 5-5.5 in the presence of Na or K salts (up to 2 M) and without them were measured. All the complexes have exclusively the 1: 1 composition; their stability constants βM(VI) in neutral solutions at a low ionic strength are close to 108 1 mol-1. In 0.1-2.0 M acid solutions, log βM(VI) for the complexes with P 2W17O6110- is within 1.4-3.9. The slope of the pH dependence of log βM(VI) does not exceed 1.75; this fact suggests that no more than two protons are displaced from HPA upon complexation in acid solutions. In the presence of 1-2 M sodium salts, the βM(VI) values reach a maximum at pH ∼3 and drastically decrease with a further increase in pH. Actinides(VI) interact with HPAs appreciably more weakly than do actinides(III), which is apparently due to the fact that the denticity of HPAs in the complexes with An(VI), apparently, does not exceed 2. © 2007 Pleiades Publishing, Inc.