Modeling Speciation and Solubility in Aqueous Systems Containing U(IV, VI), Np(IV, V, VI), Pu(III, IV, V, VI), Am(III), and Cm(III) Peiming Wang 1 • Andrzej Anderko 1 • Jerzy J. Kosinski 1 • Ronald D. Springer 1 • Malgorzata M. Lencka 1 Received: 18 October 2015 / Accepted: 18 October 2016 Ó Springer Science+Business Media New York 2017 Abstract A comprehensive thermodynamic model, referred to as the Mixed-Solvent Electrolyte model, has been applied to calculate phase equilibria and chemical speciation in selected aqueous actinide systems. The solution chemistry of U(IV, VI), Np(IV, V, VI), Pu(III, IV, V, VI), Am(III), and Cm(III) has been analyzed to develop the parameters of the model. These parameters include the standard-state thermochemical properties of aqueous and solid actinide species as well as the ion interaction parameters that reflect the solu- tion’s nonideality. The model reproduces the solubility behavior and accurately predicts the formation of competing solid phases as a function of pH (from 0 to 14 and higher), temperature (up to 573 K), partial pressure of CO 2 (up to p CO 2 = 1 bar), and concen- trations of acids (to 127 molkg -1 ), bases (to 18 molkg -1 ), carbonates (to 6 molkg -1 ) and other ionic components (i.e., Na ? , Ca 2? , Mg 2? , OH - , Cl - , ClO  4 , and NO  3 ). Redox effects on solubility and speciation have been incorporated into the model, as exemplified by the reductive and oxidative dissolution of Np(VI) and Pu(IV) solids, respectively. Thus, the model can be used to elucidate the phase and chemical equilibria for radionuclides in natural aquatic systems or in nuclear waste repository environments as a function of environmental conditions. Additionally, the model has been applied to systems relevant to nuclear fuel processing, in which nitric acid and nitrate salts of plutonium and uranium are present at high concentrations. The model reproduces speciation and solubility in the U(VI) ? HNO 3 ? H 2 O and Pu(IV, VI) ? HNO 3 ? H 2 O systems up to very high nitric acid concentrations (x HNO3  0:70). Furthermore, the similarities and differences in the solubility behavior of the actinides have been analyzed in terms of aqueous speciation. Keywords Actinides  Aqueous solutions  Phase equilibria  Solubility  Speciation  Transuranium elements Electronic supplementary material The online version of this article (doi:10.1007/s10953-017-0587-x) contains supplementary material, which is available to authorized users. & Peiming Wang pwang@olisystems.com 1 OLI Systems, Inc., 240 Cedar Knolls Road, Suite 301, Cedar Knolls, NJ 07927, USA 123 J Solution Chem DOI 10.1007/s10953-017-0587-x