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

Metal (hydr)oxides have different types of surface groups. Fluoride ions ha ve been used as a probe to assess the number of surface sites. We have stud ied the F- adsorption on goethite by measuring the F- and H+ interaction an d F- adsorption isotherms. Fluoride ions exchange against singly coordinate d surface hydroxyls at low F- concentrations. At higher concentrations also the doubly coordinated OH groups are involved. The replacement of a surfac e OH- by F- suggests that all F charge (-1) is located at the surface in co ntrast to oxyanions which have a charge distribution in the interface due t o 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 redistrib ution of charge. This is supposed to be due to a net switch in the H bondin g as a result of the change of the type of surface complex from donating (F eOH, FeOH2) to proton accepting (FeF). The modeled redistribution of charge is approximately equivalent with the change of a donating H bond into an a ccepting H bond. At high F- concentrations precipitation of F-, as for inst ance FeF3(s), may occur. The rate of formation is catalyzed by the presence of high electrolyte concentrations. (C) 2000 Academic Press.