Goethite surface hydroxyl groups

Robl, R. (1958) t)ber ferromagnetische Eisenox-yde Angew. Chem. 12 367-371 Rochester, C.H. Topham, S.A. (1979) Infrared study of surface hydroxyl groups on goethite. J. Chem. Soc. Faraday Trans. I. 75 591-602 Rochester, C.H. Topham, S.A. (1979a) Infrared study of surface hydroxyl groups on hematite. J. Chem. Soc. Faraday Trans. I. 75 1073-1088... 

Previous workers concluded that cadmium ions adsorb onto the goethite surface by the formation of bidentate surface complexes.11,12 However, the cadmium adsorption data and our calculations (see Figure 3) shows that cadmium adsorbs on goethite mainly on one surface hydroxyl group, forming monodentate complexes, according the following reaction ... 

Rochester, C. H., and Topham, S. A. (1979a). Infrared study of surface hydroxyl groups on goethite. J. Chem. Soc. Faraday Trans. I 75, 591—602. 

It has been found that adsorption/desorption of anions such as Cl and CIO4 on soil constituents is very rapid. In fact, reequilibrium is too rapid to be observed using p-jump relaxation. Fortunately, the electric-field pulse technique can be used for such systems. This method was employed by Sasaki et al. (1983) to study CI and CIO4 adsorption on goethite. Two relaxations on the order of microseconds were observed in acidified aqueous suspensions of a-FeOOH with either NaCl or NaClO4. The fast relaxation was dependent on the applied electric field intensity and was attributed to a physical diffusion phenomenon. The slower relaxation was independent of the applied electric field intensity and was interpreted in terms of the association/dis-sociation reaction of counter ions with protonated surface hydroxyl groups as ion pairs... 

Figure 1.9. Surface hydroxyl groups on goethite singly (A-type), triply (B-type), and doubly (C-type) coordinated to Fe(III), along with Lewis acid site hydroxyls. The drawing on the right shows an inner-sphere surface complex with HP04 at the A-type hydroxyl group. The dashed lines indicate hydrogen bonds.

Sun and Doner (31) examined the OH stretching modes of deuterated goethite in the presence of arsenate using FTIR. Based on transmission and ATR modes in dry systems, the authors concluded that arsenate oxyanions replaced singly coordinated surface hydroxyl groups to form bidentate binuclear complexes. Reactions presented by Sun and Doner (31) indicate HAs04 as the surface species, bound to metal centers via two O bonds. 

Oxides/Oxyhydroxides. For natural solids that are oxides or oxyhydroxides (e.g., quartz, Si02 goethite, a-FeOOH gibbsite, Al(OH)3), their water-wet surface is covered by hydroxyl groups (recall Fig. 11.2). These hydroxyl moieties can undergo proton-exchange reactions with the aqueous solution much like dissolved acids ... 

Sun and Doner (1996) used deuterated goethite to examine the affect of As(III/V) adsorption on surface hydroxyl (OH) groups. Deuterium exchanges for H in surface OH groups, forming OD groups on the surface. The peaks corresponding to OD vibrations are shifted relative to OH bands. 

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