Evidence for a new composition of Am(IV) complexes with tungstophosphate (alpha(2)-P2W17O6110-) and tungstosilicate (alpha-SiW11O398-) ligands in nitric acid medium

Lacunary heteropolyanions (LHPA) such as alpha(2)-P2W17O6110- or alpha-SiW1 1O398- are known to be strong complexing agents for tetravalent actinides. This property stabilizes Am(IV) in acidic media by formation of 1:2 complex es (Am-IV(P2W17O61)(2)(16-)) [1]; otherwise Am(IV) disproportionates very q uickly and thus cannot be isolated. Previous studies report the same stoich iometry of Am(IV) complexes with tungstophosphate (1:2) and visible absorpt ion spectra have been recorded for this species [1-2]. In the present work, evidence is given for a new composition of Am(IV) comp lexes with the relevant LHPA: stoichiometry 1.1 (Am(IV)LHPA). The visible-n ear IR spectra of such complexes have been recorded; they are rather simila r to 1:2 complexes spectra, but a small band at 560 nm is specific to 1:1 s pecies, whereas another small signal at 789 nm is specific to 1:2 complexes . Molar absorptivity are about twofold bigger for 1:2 than for 1:1 complexe s for wavelengths from 500 to 700 nm. An efficient way to prepare 1:1 Am(TV) complexes is to perform electrochemi cal oxidation of Am(III) in a slightly acidic solution (similar to 0.1 M [H NO3]) with an anode potential of less than 2 V/SHE. Such potentials are req uired to avoid direct oxidation of Am(III) to Am(VI) which is possible with out LHPA and higher potentials. An [LHPA(total)]/[Am(total)] ratio near or of less than one is required to favor the formation of 1:1 rather than 1.2 complexes. Oxidation mediators such as Ag(II) cannot be used, as they would cause oxidation of Am(IV) to Am(VI) under these conditions. Stability constants of Am(III) and Am(IV) with the relevant LHPA have been evaluated for a 1 M nitric acid medium. The methods used are based on absor ption spectroscopy and potentiometric measurement. In this work, Am(IV) stability under alpha radiolysis was also studied. The rate of autoradiolytic reduction of Am(IV) was shown to be independent of the composition of Am(IV) complexes. A mechanism for the destruction of Am(IV)-LHPA complexes by Th(IV) is propo sed. Destruction occurs in two stages. The first is instantaneous, consisti ng in the destruction of 1:2 Am(IV) complexes by Th(TV) to form 1:1 Am(IV) complexes. The second stage is rather slow tone hour to completion), and in volves the destruction of 1:1 Am(IV) complexes by Th(IV). The released "fre e" Am(IV) disproportionates into Am(III), (V) and (VI).