First-Principles Insights into the Selective Separation of MoS₄^2– and WO₄^2–: Crucial Role of Hydration Structures

The selective separation of MoS₄^2- and WO₄^2- using quaternary ammonium salt through solvent extraction or ion exchange methods has been well-established in the metallurgical industry. However, the conventional electrostatic adsorption theory falls short in explaining the separation mechanism. Through first-principles density functional theory (DFT) calculations and newly self-developed deep potential molecular dynamics (DPMD) simulation method, our work first reveals that the disparity in hydration structures of MoS₄^2- and WO₄^2- plays a crucial role in their selective separation. It is proposed that MoS₄^2- and WO₄^2- anions undergo hydration to form [MoS₄(H₂O)<i>_n</i>]^2- and [WO₄(H₂O)<i>_n</i>]^2-, respectively, facilitated by hydrogen bond (H-bond) interactions. Emphasis is placed on the discrepancy between MoS₄^2- and WO₄^2- in hydration structures by the hydration energy, Hirshfeld charge, evaluation of weak interactions, hydration radius, hydration coordination number, and H-bonds distribution. MoS₄^2- presents a larger first hydration radius and a lower first hydration coordination number due to weaker interactions with H₂O, while WO₄^2- is subjected to enhanced hydration shielding, resulting in MoS₄^2- anions being more susceptible to be selectively separated by a quaternary ammonium salt. This insight paves the way for the selective separation of MoS₄^2- and WO₄^2-, further bridging the gap between theory and industry applications.