Selective dissolution of americium by ferricyanide ions in basic aqueous solutions

Americium exhibits a soluble form in aqueous alkaline media under oxidizing conditions which is not the case for the other Transplutonium Elements (TPE). This property can be exploited for High Level Liquid Waste (HLLW) treatment to extract Am, one of the main radionuclides responsible for the long term radiotoxicity of nuclear waste. The Soluble Am compound can be obtained by adding a concentrated basic solution of ferricyanide ions (Fe(CN)63-) to a trivalent americium hydroxide precipitate. The method allows complete and rapid extraction of americium via its soluble form in alkaline solutions. Under these conditions, other TPE and lanthanides remain in the solid state as precipitates of highly insoluble trivalent hydroxides. In the case of dissolution involving large amounts of americium, the formation of the soluble americium species is followed by the appearance of a reddish precipitate in the basic solution. Dissolution of the reddish solid in NaOH or NaOH/Fe(CN)63- media demonstrated the existence of a media dependent solubility of the precipitate, and therefore the existence of at least two forms of soluble Am. Spectroscopic studies (UV-visible, EXAFS-XANES) of this reddish solid led to the determination of an Am oxidation state (pentavalent americium) and its possible formula (Na2AmVO2(OH)3.nH2O). Electrochemical studies show that the only possible oxidation reaction of trivalent americium in the working media yields the pentavalent form, and that the hexavalent state is unattainable. Stoichiometric and spectroscopic studies show that not all the ferricyanide ions required for complete dissolution of Am remain in the free Fe(CN)63- form. This observation supports the view that this dissolution of Am(III) solid compound is much more complex than a simple oxidation by the ferricyanide ions. The existence of a molecular interaction between Am(V)O2+ and ferricyanide ions is highly probable. This work demonstrates that the selective dissolution of americium by a basic ferricyanide solution could set the stage for a process whereby Am can be separated from other transplutonium elements and from lanthanides. The dissolved americium is highly pure but its low solubility poses a possible limitation to process development. From the more fundamental standpoint, the broad outlines of a mechanism accounting for americium dissolution by ferricyanide ions in basic media were established from experimental findings. (authors)