Goethite surface

Direct in situ X ray (from synchroton radiation) adsorption measurements (EXAFS) (Hayes et al., 1987, Brown et al., 1989) permit the determination of adsorbed species to neighboring ions and to central ions on oxide surfaces in the presence of water. Such investigations showed, for example, that selenite is inner-spherically and selenate is outer-spherically bound to the central Fe(lll) ions of a goethite surface. It was also shown by this technique that Pb(II) is inner-spherically bound to 5-AI2O3 (Chisholm-Brause et al., 1989). 

Figure 6. CIR-FTIR spectra of SAL and iron SAL complex in solution and SAL on goethite. (upper) 0.1 M SAL in 1 M KC1 at pH 5.5 (middle) SAL on goethite in D.O with goethite surface groups subtracted at pD 4.5, 0.01 M KC1 and 100 g/L solid concentration (lower) aqueous iron SAL complex at pH 1.6.

He, J Ma, W Song, W Zhao, J Qian, X Zhang, S Yu JC. Photoreaction of aromatic compounds at a-FeOOH/H20 interface in the presence of H2O2 evidence for organic-goethite surface complex formation. Water Research, 2005 39, 119-128. 

Lengweiler et al. (1961) found that the solubility of goethite, like that of ferrihydrite, increased as the pH rose above 12. For ferrihydrite, equilibrium between the solid and Fe(OH)4 was reached quite rapidly, whereas for goethite, equilibrium was not reached even after 40 days (25 °C). A value of 1.40 + 0.1 for of goethite (surface area ca. 100 m g" ) was only reached after 3 years (Fig. 9.5) (Bigham et al., 1996). As expected on thermodynamic grounds, the solubility of goethite was 10 to 10 times less than that of ferrihydrite. 

Fig. 10.12 The effect of increasing amounts of goethite (surface area 51 m g ) on the electrophoretic mobility of kaolinite at various pH. The figures on the curves indicate the amount of goethite added (mg g ) (Venema and Glasauer, unpubl.).

The acidity of organic ligands is enhanced by coordination with the oxide surface, i.e. the surface promotes deprotonation of the functional groups (COOH or OH). Such ligands, therefore, adsorb on the surface at a pH 2-3 units lower than that at which complexation with Fe in solution would occur (Kummert and Stumm, 1980). An example of this is the deprotonation of the alcoholic OH group of tartaric acid upon adsorption on the goethite surface (Cornell and Schindler, 1980). The appropriate reaction for the acid in water is. 

Adsorption of ferricyanide (a contaminant of sites of former gas plants) on goethite decreases with rising pH and is dependent on ionic strength it is fully reversible and is thought to involve both inner and outer sphere complexes. Adsorption of ferrocyanide on goethite involves initial formation of inner sphere complexes followed by precipitation of a Berlin Blue-like phase on the goethite surface (Rennert and Mansfeldt, 2001). 

Goethite (surface are 50 m g" ) has been reported to catalyse the hydrolysis of carboxylate and phosphorothioate esters (Torrents and Stone, 1994). The authors suggest that the sulphur of the thioester binds to the surface Fe of the goethite and thereby reduces the electron density at the P atom which in turn facilitates the nucleophilic attack by OH and promotes hydrolysis. 

Felmy, A.R. Rustad, J.R. (1998) Molecular statics calculations of proton binding to goethite surfaces Thermodynamic modeling of the surface charging and protonation of goethite in aqueous solution. Geochim. Cosmochim. Acta 62 25—31... 

Effect of heating on microcrystalline synthetic goethite. Clays Clay Min. 34 17-24 Koch, F. Cohen, J.B. (1969) The defect structure of Fei.xO. Acta Cryst. B 25 275-287 Koch, G. Mailer, P.J. (1987) Adsorption of nitrogen and water vapor onto goethite surfaces. Thermochim. Acta 114 139-144... 

N. Carroll, S.A. (2000) X-ray absorption spectroscopy of strontium coordination. II Sorption and precipitation at kaolinite, amorphous silica and goethite surfaces. J. Coll. 

Using Eq. 11 -34 (Box 11.1) and the goethite surface site density, its pA i and pK values, and the ionic strength of the solution, find the intensity of surface charging of the goethite at the three pH values of interest (note that [Pg.446]

Next, consider the factors determining benzoate bonding to the goethite surface. From Evanko and Dzombak (1999), you have Kjnt = 107,89 for the adsorption reaction ... 

Excess Surface Charge. The reaction at the goethite surface producing charged sites by adsorption of H+ and OH" as potential determining ions can be represented as follows,... 

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 ... 

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