Structural study of Pb(II) and Sb(V) adsorption on the hydroxylated hematite(1102) surface

Abstract

Surface complexation reactions at mineral-water interface play a key role in dictating the mobility and bioavailability of aqueous species in the environment, considering the minerals (like iron-(oxyhydr)oxides), ubiquitous in nature, are usually of high specific surface area and contain a lot of potentially reactive surface sites for binding ions. Thus, the fundamental understanding of surface complexation reactions is essential to better model the environmental fate and transport of toxic aqueous species. In the current study, the hematite (1102) surface was selected as a model substrate for iron-(oxyhydr)oxides commonly found in the environment to investigate the surface complexation reactions of Pb(II) and Sb(V) using the crystal truncation rod (CTR) surface diffraction. The hematite (1102) surface displays two surface terminations, the half layer termination (HLT) and the full layer termination (FLT) depending on sample preparation schemes. Previous CTR studies of clean hydrated hematite (1102) surface indicated that the chemically mechanical polishing procedure favors a HLT surface, while an annealing procedure favors the FLT surface. Our CTR results on the clean annealed hematite (1102) surface provides structural evidence that the substrate annealed in air undergoes a surface termination transformation that occurs through the projection of the near surface Fe atoms from original lattice sites to occupy top vacant Fe sites. The adsorption of Pb(II) on the hydrated hematite(1102) surface was also studied using CTR diffraction. Our findings demonstrate that aqueous Pb(II) adsorbs at two types of bidentate edgesharing sites on the HLT surface, whereas the surface adsorption of Pb(II) occurs only at one type of edge-sharing site on the FLT surface. The site preference could be rationalized through comparing O-Fe-O bond angles for different edge-sharing binding sites. It was found that Pb binding at edge-sharing sites with relatively large O-Fe-O bond angles (>100°) is unfavorable as a result of forming a Pb complex species with extremely long (weak) Pb-O bonds (>2.5Å). On the contrary, an edge-sharing site with relatively small O-Fe-O bond angles would be more favorable for binding Pb(II) species, since it would give rise to a surface complex species with reasonable Pb-O bond lengths (~2.25Å). Differing from the case of Pb(II), the Sb(V) adsorption on the hematite (1102) surface with the HLT occurs only at a tridentate site in a binuclear edge-sharing/corner-sharing binding mode. The bidentate binding configurations are apparently not favorable due to the steric constraints caused by the Sb-bonded hydroxyl groups, which are abnormally close to the surface oxygen groups. The molecular scale structural details presented in this study improve the understanding of the surface adsorption of Pb(II) and Sb(V) on the hematite(1102) surface. Our findings also give evidence that the surface reactivity is largely determined by the surface structure through steric constraints as a function of sorbate type.