Surface Acidity and As(V) Complexation of Iron Oxyhydroxides: Insights from First-Principles Molecular Dynamics Simulations

Iron hydroxides are ubiquitous in soils and aquifers and have been adopted as adsorbents for As­(V) removal. However, the complexation mechanisms of As­(V) have not been well understood due to the lack of information on the reactive sites and acidities of iron hydroxides. In this work, we first calculated the acidity constants (pK_as) of surface groups on lepidocrocite (010), (001), and (100) surfaces by using the first-principles molecular dynamics (FPMD)-based vertical energy gap method. Then, the desorption free energies of As­(V) on goethite (110) and lepidocrocite (001) surfaces were calculated by using constrained FPMD simulations. The point of zero charges and reactive sites of individual surfaces were obtained based on the calculated pK_as. The structures, thermodynamics, and pH dependence for As­(V) complexation were derived by integrating the pK_as and desorption free energies. The pK_a data sets obtained are fundamental parameters that control the charging and adsorption behavior of iron oxyhydroxides and will be very useful in investigating the adsorption processes on these minerals. The pH-dependent complexation mechanisms of As­(V) derived in this study would be helpful for the development of effective adsorbent materials and the prediction of the long-term behavior of As­(V) in natural environments.