Thermodynamics of trivalent actinides and neodymium in NaCl, MgCl2, and CaCl2 solutions: Solubility, hydrolysis, and ternary Ca-M(III)-OH complexes

Abstract

Known data on the solubility of Am(OH)3(s) and the hydrolysis of Am(III) and Cm(III), additional information from an extensive solubility study with Nd(OH)3(s) in NaCl, MgCl2, and CaCl2 media of various ionic strengths and spectroscopic (time-resolved laser fluorescence spectroscopy, TRLFS) data for Cm(III) in alkaline CaCl2 solutions are used to evaluate a comprehensive set of standard-state equilibrium constants and ion interaction parameters for the specific ion interaction theory (SIT) and Pitzer equations at 25 °C. The thermodynamic model takes into account the analogous solubility and hydrolysis behavior of trivalent actinides and Nd(III) and covers the entire pH range in dilute to concentrated NaCl, MgCl2, and CaCl2 solutions. In alkali chloride/hydroxide solutions, the formation of the tetrahydroxide complex M(OH)4– requires OH– concentration above 3 mol l–1, whereas in alkaline CaCl2 solutions (at pHc < 12) M(III) complexes with four and six hydroxide ligands are formed. Similar as the recently detected ternary Ca–M(IV)–OH complexes Ca3[Zr(OH)6]4+ and Ca4[Th(OH)8]4+, these complexes are stabilized by the association of Ca2+ ions. The solubility and hydrolysis of Am(III), Cm(III), and Nd(III) in both Ca-free and -containing solutions is consistently described with a model including the ternary Ca–M(III)–OH complexes Ca[M(OH)3]2+, Ca2[M(OH)4]3+, and Ca3[M(OH)6]3+.