Solubility of tetravalent actinides in alkaline CaCl2 solutions and formation of Ca4[An(OH)8]4+ complexes: A study of Np(IV) and Pu(IV) under reducing conditions and the systematic trend in the An(IV) series

The solubility of Np(IV) and Pu(IV) hydrous oxides was studied at 22 ± 2°C under reducing conditions in alkaline CaCl2 solutions. Redox conditions were adjusted either with 2 mM Na2S2O4 or additions of iron powder. In 2.0 and 4.5 M CaCl2 and pHc = 11 - 12, the neptunium and plutonium concentrations increase with a slope of +4 (log [An] vs. pHc) as expected for the formation of the complex Ca4[An(OH)8]4+ recently identified for Th(IV). At CaCl2 concentrations = 1.0 M this effect is negligible for both Np(IV) and Pu(IV). The conditional equilibrium constants log*Ks,(4,1,8) for the reaction An(OH)4(am) + 4 H2O + 4 Ca2+ ¿ Ca4[An(OH)8]4+ + 4 H+ are evaluated with the SIT and Pitzer model using the parameters derived from analogous data for Th(IV) in 0.5 - 4.5 M CaCl2. The log*K°s,(4,1,8) values and the complex formation constants log ß°(4,1,8) follow systematic trends in the series Th(IV), Np(IV) and Pu(IV) which allows the estimation of the corresponding data for U(IV).

FELLHAUER David;  NECK V.;  ALTMAIER M.;  LUETZENKIRCHEN J.;  FANGHAENEL Thomas; 

2011-01-17

OLDENBOURG VERLAG

JRC58132

0033-8230,   

https://publications.jrc.ec.europa.eu/repository/handle/JRC58132,   

10.1524/ract.2010.1751,