Computational Insights Into As(V) Removal From Water By The Uio-66 Metal-Organic Framework

Metal-organic frameworks (MOFs) such as UiO-66 have shown great promise for the removal of toxic As(V) species from water, but the precise adsorption mechanism is not yet well understood. In this study, we use density functional theory calculations and cluster models to determine accurate energetics and geometries for the adsorption of As(V) species present under varying pH conditions, namely, arsenic acid and related oxyanions. As a result of this analysis, we identify a new adsorption mode in which two As(V) molecules adsorb in a monodentate fashion on two neighboring Zr open metal sites resulting from capping ligand displacement. This mode is preferred over other adsorption modes previously proposed in the literature at acidic and neutral pH. Multidentate adsorption modes involving adsorption of a single As(V) molecule onto both neighboring Zr sites become more competitive as the pH increases, consistent with the lower adsorption performance observed at basic pH. Additionally, we study the influence of the ligands capping the Zr sites on As(V) adsorption thermodynamics. We show that the hydroxide-water ligand pair is the capping ligand most easily displaced by adsorbing As(V) species, suggesting that defects present in the aqueous environment are the ideal site for As(V) species adsorption. Furthermore, we find that adsorption of phosphate oxyanions can thermodynamically compete with As(V) adsorption, thus potentially explaining the observed decreased As(V) adsorption performance in the presence of phosphates. The molecular-level insights gained in this study are used to draw general design principles for the development of new MOFs that can be employed to remove toxic oxyanions from water.