Solubility of tetravalent actinides in alkaline CaCl_2 solutions and formation of Ca_4[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

摘要

The solubility of Np(IV) and Pu(IV) hydrous oxides was studied at 22 ± 2 °C under reducing conditions in alkaline CaCl_2 solutions. Redox conditions were adjusted either with 2 mM Na_2S_2O_4 or additions of iron powder. In 2.0 and 4.5 M CaCl_2 and pH. = 11-12, the neptunium and plutonium concentrations increase with a slope of +4 (log[An] vs. pH_c) as expected for the formation of the complex Ca_4[An(OH)_8]~(4+)recently identified for Th(IV). At CaCl_2 concentrations < 1.0 M this effect is negligible for both Np(IV) and Pu(IV). The conditional equilibrium constants log ~*K_s (4, 1.8) for the reaction An (OH)_4( am) + 4H_2O + 4Ca~(2+) - Ca_4[An(OH)_8]~(4+) + 4H+ are evaluated with the SIT and Pitzer model using the param-eters derived from analogous data for Th(IV) in 0.5-4.5 M CaCl_2. The log ~*K_s(4,1,8) values and the complex formation constants log β(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).