Solvent extraction study of Np(IV) sulfate complexation in Na+-Np4+-OH--SO42--HSO4--ClO4- and Na+-Np4+-OH--SO42--HSO4--Cl- systems

Solvent extraction was used to study the complexation of Np(TV) with sulfat e in NaClO4 and NaCl solutions ranging in concentration from 0.1 M to 5.0 M . A reducing agent (NH2OH.HCl) was used during extractions to maintain the tetravalent state of neptunium. Independent tests confirmed that the oxidat ion state of neptunium did not change during extractions. The distribution ratio of Np(IV) between the organic and aqueous phases was found to decreas e as the concentration of sodium sulfate increased. Fitter's specific ion-i nteraction approach was used to model the solvent extraction results and ca lculate the thermodynamic stability constants of Np(IV)-sulfate complexes. At the sulfate concentrations used in these experiments, NpSO42+ and Np(SO4 )(2)(0) were found to be the dominant aqueous species. Based on the data ob tained over a wide range of concentrations of NaCl4 and NaCl the thermodyna mic equilibrium constants were found to be 1.05 X 10(9) for the formation o f NpSO42+ (Np4+ + SO42- reversible arrow NpSO42+) and 5.62X10(11) for the f ormation of Np(SO4)(2)(Np4+ + 2SO(4)(2-) reversible arrow Np(SO4)(2)).