Adsorption of Molybdenum on Goethite Surface: A First-Principles Molecular Dynamics Simulation

Understanding molybdate interactions with iron oxides is essential for characterizing the geochemical behavior of Mo and for elucidating its geochemical cycle in comparison to tungsten (W). However, the molecular-scale structures and thermodynamic stabilities of Mo surface complexes remain poorly resolved. Here, first-principles molecular dynamics (FPMD) simulations are used to investigate the adsorption of molybdate on the goethite (110) surface. Considering molybdate (MoO₄^2-) and its possible protonation states (HMoO₄^- and H₂MoO₄), as well as their different coordination environments, FPMD results show that bidentate corner-sharing complexes remain stable for all species while protonated species spontaneously deprotonate during the simulations. These results indicate that deprotonated states are common under environmentally relevant pH conditions, while protonated species undergo spontaneous deprotonation. Free-energy calculations show that 4-coordinated bidentate complexes are more favored than 5-coordinated bidentate, monodentate, and outer-sphere species. Comparison with our previous study of W adsorption shows that, although Mo and W display comparable desorption free energies, W more readily forms 5-coordinate structures whereas Mo prefers 4-coordinate binding, implying a greater tendency of W for lattice incorporation. These findings provide molecular-level insight into Mo-iron oxide interactions and a mechanistic understanding of Mo and W mobility in natural environments.