Goethite surface hydroxyls

Fig. 10.2 Surface hydroxyl configuration on the goethite 001, 101, 100 and 210 faces. Distances of O and Fe ions to the projection plane are indicated next to the corresponding row of ions. Rows of singly, doubly, and triply coordinated O ions are indicated as S, D, and T, respectively. Solid line rectangles represent the two-dimensional (surface) unit cell. Dotted-line rectangles show contiguous singly coordinated hydroxyls (Barron and Torrent, 1996, with permission).

Barron,V. Torrent, J. (1996) Surface hydroxyl configuration of different crystal faces of hematite and goethite. J. Colloid Interface Sci. 177 407-411... 

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 ... 

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. 

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... 

Zhang, J.S., Stanforth, R., and Pehkonen, S.O., Proton-arsenic adsorption ratios and zeta potential measurements Implication for protonation of hydroxyls on the goethite surface, 7. Colloid Interf. Sci., 315, 13, 2007. 

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. 

It should be noted that minerals, and oxide minerals in particular, have different types of OH groups, depending on the coordination of the O atoms, as revealed by spectroscopic studies. Goethite (a-FeOOH) has four types of surface hydroxyls whose reactivities are a function of the coordination environment of the O in the FeOH group (Sposito 1984 Sparks 2002). The FeOH groups are A-, B-, or C-type sites, depending on whether the O is coordinated with 1,3, or 2 adjacent Fe(III) ions. The fourth type of site is a Lewis acid-type site, which results from chemisorption of a water molecule on a bare Fe(III) ion. Only A-type sites are basic (can bind H+), and, on the contrary, A-type and Lewis acid sites can release a proton. The B- and C-type sites are considered unreactive. Thus, A-type sites can be either a proton acceptor or a proton donor (i.e., they are amphoteric). Other spectroscopic studies have shown that boehmite (y-AlOOH) and lepidocrocite (y-FeOOH) have two types of OH, presumably associated with different crystal faces (Lewis and Farmer 1986). 

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