Np-incorporation into uranyl phases: A quantum-mechanical evaluation

Density functional theory calculations are used to compare mechanisms of charge-balanced Np5+-incorporation into boltwoodite [K(UO2)(SiO3OH)(H2O)1.5]. The charge-balancing mechanisms considered include: (i) H+ addition (Np5+ + H+ ↔ U6+), (ii) interlayer substitution (Np5+ + Ca2+/Mg2+ ↔ U6+ + K+), and (iii) intra-layer substitution (Np5+ + P5+ ↔ U6+ + Si4+). The choice of the source (of Np5+, H+, Ca2+, Mg2+, P5+) and sink (of substituted U6+, K+, Si4+) phases, the reference phases, for the cations involved in the incorporation reaction greatly affects the final calculated incorporation energy (ΔErxn). The incorporation energies using oxide (ΔErxn = 183.5 kJ/mol) and silicate (ΔErxn = 67.5 kJ/mol) reference phases are compared for the interlayer substitution mechanism. Since both the source of Np in environmental systems and the cations released are typically aqueous complexes, a newly-developed approach of combining cluster and periodic simulations was employed to model exchange with aqueous complexes. For the H+ addition mechanism, incorporation from oxides reference phases (ΔErxn = 93.6 kJ/mol) is less favorable than from aqueous (ΔErxn = 12.5 kJ/mol) reference species. Estimates of the solid-solution behavior of Np5+/P5+- and U6+/Si4+-boltwoodite and Np5+/Ca2+- and U6+/K+-boltwoodite solid solutions are used to predict the limit of Np-incorporation into boltwoodite. For the interlayer and intra-layer substitution mechanisms, the substitution energies are 67.5 and −2.9 kJ/mol, respectively, resulting in a maximum amount of incorporation at 300 °C of 172 ppm and 768 ppm, respectively.