Fluoride adsorption on goethite in relation to different types of surface sites

Metal (hydr)oxides have different types of surface groups. Fluoride ions have been used as a probe to assess the number of surface sites. We have studied the F− adsorption on goethite by measuring the F− and H interaction and F− adsorption isotherms. Fluoride ions exchange against singly coordinated surface hydroxyls at low F− concentrations. At higher concentrations also the doubly coordinated OH groups are involved. The replacement of a surface OH− by F− suggests that all F charge (−1) is located at the surface in contrast to oxyanions which have a charge distribution in the interface due to the binding structure in which the anion only partially coordinates with the surface. Analysis of our F− data with the CD-MUSIC approach shows that the formation of the fluoride surface complex is accompanied by a redistribution of charge. This is supposed to be due to a net switch in the H bonding as a result of the change of the type of surface complex from donating (FeOH, FeOH2) to proton accepting (FeF). The modeled redistribution of charge is approximately equivalent with the change of a donating H bond into an accepting H bond. At high F− concentrations precipitation of F−, as for instance FeF3(s), may occur. The rate of formation is catalyzed by the presence of high electrolyte concentrations.