Adsorption of arsenic(V) onto single sheet iron oxide: X-ray absorption fine structure and surface complexation

Adsorption onto two-dimensional nanosheet materials offers new possibilities for fast and efficient removal of contaminants from waters. Here, the adsorption of As(V) to a new type of iron oxides - single sheet iron oxide (SSI) - has been studied as a function of time, loading and pH. Adsorption of As(V) onto SSI was very fast compared to other iron oxides, with 80% of total As(V) adsorbed within 10 min. Examination by extended X-ray absorption fine structure analysis showed that As(V) forms a bidentate inner-sphere surface complex with SSI. Arsenic(V) adsorption isotherms and adsorption envelopes were well described using a 1-pK Basic Stern surface complexation model involving protonated (Fe₂O₂AsO₂H^-) and unprotonated (Fe₂O₂AsO₂^-2) inner-sphere surface complexes. The surface complexation constants for As(V) binding to SSI is similar to constants found for goethite and ferrihydrite. Simulated adsorption isotherms for intermediate As(V) concentrations also demonstrate that SSI is performing equally well as goethite and ferrihydrite based on surface area normalized adsorption capacities. The binding affinities at micromolar to submicromolar As(V) solution concentrations are similar for SSI and ferrihydrite. SSI has interesting potential as a stable, high-affinity sorbent for use in applications where efficient and fast removal is required.