Probing atomic-scale processes at the ferrihydrite-water interface with reactive molecular dynamics

Interfacial processes involving metal (oxyhydr)oxide phases often control the mobility and bioavailability of nutrient and contaminant elements in soils, sediments, and water. Consequently, these processes influence ecosystem health and functioning and have shaped the biological and environmental co-evolution of Earth over geologic time. Here we employ reactive molecular dynamics simulations, supported by synchrotron X-ray spectroscopy, to derive accurate surface complexation model constants for ferrihydrite-water systems containing aqueous \ce{MoO_4^{2-}}. This is achieved by employing a force-field that is adept at capturing the realistic dynamics of surface restructuring, surface charge equilibration, and the evolution of the interfacial water hydrogen bond network in response to adsorption processes or proton transfer events. We inform surface complexation models by exploring the free energy landscape of \ce{MoO_4^{2-}} adsorption at the ferrihydrite-water interface at different concentrations. We find how hydration and adsorption induce changes in surface charge, prompting the surface to restructure into more disordered ferrihydrite phases. We observe how the dynamics of adsorbed complexes are concentration-dependent and how surface restructuration, along with adsorption, influence the interfacial hydrogen bond network. By incorporating reactivity, reactive molecular dynamics opens new avenues to study mineral-water interfaces, enriching and refining surface complexation models beyond their foundational assumptions.