Quantum-mechanical determination of the incorporation of pentavalent plutonium into carbonate and sulfate minerals

Abstract

Pentavalent plutonium is the most abundant form of soluble, mobile, and oxidized Pu in natural systems. The incorporation of Pu into environmentally abundant mineral hosts can strongly influence the transport and concentration of contaminants in both aqueous environments and the subsurface. This study determines the incorporation energy (ΔE, ΔH, and ΔG) of (PuO2)+(HSO4)− and (PuO2)+(HCO3)− from solid Pu2O5 and aqueous (PuO2)+ as sources of Pu(V), as well as the adsorption with subsequent surface incorporation, in a step-by-step approach onto/into different sulfate and carbonate minerals using ab initio computational methods.Solid source and sink reaction energies are calculated for carbonate and sulfate mineral hosts as(MCO3)4 + ½(Pu2O5) + (H2CO3) ⇌ ½(H2O) + (MCO3)3(PuO2)(HCO3) + (MCO3) for carbonate structures and(MSO4)4 + ½(Pu2O5) + (H2SO4) ⇌ ½(H2O) + (MSO4)3(PuO2)(HSO4) + (MSO4) for sulfate structures, where “M” denotes the metal cation being replaced by PuO2+ (M = Ba, Sr, Pb, Ca).For Pu(V) from solid sources, sulfate group host minerals with the largest cations result in the most favorable incorporation energies; for aqueous sources and sinks, this effect is more than compensated by smaller host cations gaining more hydration energy when released. Thus, incorporation favorability using aqueous sources and sinks vs. solids are nearly reversed, with the smallest cationic radii corresponding to the lowest incorporation energy.While previous studies have used a similar methodology for calculating the thermodynamics of incorporation into bulk minerals, what is new in this study is that the method was extended to observe the rate-controlling steps from a species in solution (e.g., PuO2+ and HSO4− or HCO3−), to their co-adsorbed state on the mineral surface, followed by their co-incorporation (by replacing divalent cations and anions from the surface of the host mineral), and finally being incorporated into the bulk, mimicking the stability of co-precipitated or overgrown plutonyl defects. As the plutonyl ion approaches the surface, a small activation barrier has to be overcome (∼0.2 eV), followed by adsorption which is exothermic (∼–2.6 eV) with potential subsequent surface incorporation (endothermic ∼1.2 eV). Combining these steps results in a surface incorporation of plutonyl that is energetically downhill (∼–1.2 eV), which can sequester plutonyl without the addition of any external energy. Keeping the substituted host ions above the incorporation site is more energetically favorable than releasing them, generating a metastable plutonyl overgrowth site where continued growth is more favorable than re-releasing the plutonyl.These findings, with this more process-oriented and environmentally-relevant approach of surface interactions, considering mineral bulk and surfaces, hydration, and enthalpy, entropy, and Gibbs free energy of individual reaction, facilitate the evaluation of incorporation reactions and narrows the list of minerals favorable for the incorporation of actinides to complement (particularly resource-limited) experimental study. In addition, it may serve as a model for future semi-kinetic studies where a full computational treatment of the kinetics is prohibitively computationally expensive.