Comprehensive Thermodynamic Model Applicable to Highly Acidic to Basic Conditions for Isosaccharinate Reactions with Ca(II) and Np(IV)

Abstract

Isosaccharinate (ISA−), a degradation product of cellulose codisposed in low-level nuclear wastes, is expected to be one of the dominant complexing ligands for radionuclides, especially tetravalent actinides. This paper presents a comprehensive thermodynamic model for isosaccharinate reactions with Ca(II) and Np(IV). The model is valid for a wide range of pH values (∼2–14), ISA− concentrations (ranging up to 0.1 m), and ionic strengths (ranging up to 6.54 m), and is based on (1) NMR investigations of HISA(aq) (α-D-isosaccharinic acid) and ISL(aq) [dehydration product of HISA(aq)], and the solubility of Ca(ISA)2(c) as a function of pH and concentrations of Ca and ISA−; (2) NpO2(am) solubility in a wide range of pH values (∼2–14) and total ISA concentrations of 0.0016 and 0.008 m and at fixed pH values of approximately 5 and 12 with total ISA concentrations ranging from 0.0001 to 0.1 m; and (3) solvent extraction of Np-ISA solutions, containing fixed NaClO4 concentrations ranging from 0.103 to 6.54 m and at fixed pC H+ values ranging from 1.5 to 1.9, with dibenzoylmethane. Pitzer's ion-interaction approach was used to interpret the data. The different aqueous species required to explain these data included HISA(aq), ISL(aq), Ca(ISA)+, Np(OH)3(ISA)(aq), Np(OH)3(ISA)2 −, Np(OH)4(ISA)−, and Np(OH)4(ISA)2 2−. The values of equilibrium constants for reactions involving these species and determined from these data provided close agreements between the observed and predicted concentrations in all of the systems investigated in this study and those reported previously.