DFT+U Study of Arsenate Adsorption on FeOOH Surfaces: Evidence for Competing Binding Mechanisms
The Journal of Physical Chemistry C 117(30): 15571-15582
On the basis of periodic density functional theory (DFT) calculations including an on-site Coulomb repulsion term U, we study the adsorption mechanism of arsenate on the goethite (101), akaganeite (100), and lepidocrocite (010) surfaces. Mono- and bidentate binding configurations of arsenate complexes are considered at two distinct iron surface sites—directly at 5-fold coordinated Fe1 and/or 4-fold coordinated Fe2 as well as involving ligand exchange. The results obtained within ab initio thermodynamics shed light on the ongoing controversy on the arsenate adsorption configuration, and we identify monodentate adsorbed arsenate complexes as stable configurations at ambient conditions with a strong preference for protonated arsenate complexes: a monodentate mononuclear complex at Fe1 (dFe1–As = 3.45 Å) at goethite (101) and a monodentate binuclear complex at Fe2 (dFe2–As = 3.29 Å) at akaganeite (100). Repulsive interactions between the complexes limit the loading capacity and promote configurations with maximized distances between the adsorbates. With decreasing oxygen pressures, a mixed adsorption of bidentate binuclear complexes at Fe1 (dFe1–As = 3.26–3.34 Å) and monodentate binuclear arsenate at Fe2 (dFe2–As = 3.31–3.50 Å) and, finally, rows of protonated bidentate complexes at Fe1 with dFe1–As = 3.55–3.59 Å are favored at α-FeOOH(101) and β-FeOOH(100). At lepidocrocite (010) with only Fe2 sites exposed, the surface phase diagram is dominated by alternating protonated monodentate binuclear complexes (dFe2–As = 3.38 Å) and hydroxyl groups. At low oxygen pressures, alternating rows of protonated bidentate mononuclear complexes (dFe2–As = 3.10 Å) and water are present. Hydrogen bond formation to surface hydroxyl groups and water plays a crucial role in the stabilization of these adsorbate configurations and together with steric effects of the oxygen lone pairs leads to tilting of the arsenate complex that significantly reduces the Fe–As distance. Our results show that the Fe–As bond length is mainly determined by the protonation state, arsenate coverage, steric effects, and hydrogen bonding to surface functional groups and to a lesser extent by the adsorption mode. This demonstrates that the Fe–As distance cannot be used as a unique criterion to discriminate between adsorption modes.