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) n ]2– and [WO4(H2O) n ]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.