Solubility of tetravalent actinides in alkaline CaCl2solutions 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

Abstract

AbstractThe solubility of Np(IV) and Pu(IV) hydrous oxides was studied at 22±2 °C under reducing conditions in alkaline CaCl2solutions. Redox conditions were adjusted either with 2 mM Na2S2O4or additions of iron powder. In 2.0 and 4.5 M CaCl2and 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 CaCl2concentrations ≤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 reactionAn(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 logβ°(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).