Goethite surface species

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). 

Infrared spectroscopy showed that SO2 adsorbed on goethite to form sulphito (OSO2) species and on lepidocrocite to form sulphate and sulphite complexes (Ka-neko and Matsumoto, 1989). XPS measurement suggested that H2S adsorbed on iron oxide to give both Sg and SH3 as surface species, whereas only Sg formed if the surface had been pre-exposed to water vapour (Prasad et ah, 1993). 

Therefore, from a knowledge of the intrinsic acidity constants, Ns and the value of kj, the distribution of surface species, using data from Lumsdon and Evans (1994). 

P.-C. Zhang and D. L. Sparks, Kinetics and mechanisms of molybdate adsorption/desorption at the goethite/water interface using pressure-jump relaxation, Soil Sci. Soc. Am. J. 53 1028 (1989). The rate coefficients inEq. 4.42 need not be modified by the model surface-species activity coefficients adopted in this paper. 

Predictions of Titratable Charge and Surface Species Distribu-tions of Goethite in Seawater... 

The complexation constants of the individual major seawater ions with otFeOOH determined in single salt solutions can be used to predict the titratable charge and surface species distribution of goethite in seawater. This prediction can then be compared with the experimentally determined charge of goethite in a mixed seawater type electrolyte. 

Distribution of Surface Species on Goethite in Seawater as a Function of pH... 

The simplified mass and proton balance model determined what the surface species distribution of goethite would be in a mixed, seawater type electrolyte. This surface species distribution was used to calculate a surface charge for goethite. 

Van Geen A., Robertson A. P., and Leckie J. O. (1994) Complexation of carbonate species at the goethite surface implications for adsorption of metal ions in natural waters. Geochim. Cosmochem. Acta 58, 2073 -2086. 

Figure 9.13. Surface complex formation with ligands (anions) as a function of pH. (a) Binding of anions from dilute solutions (5 x 10 M) to hydrous ferric oxide [TOTFe= 10 M]. Based on data from Dzombak and Morel (1990). I = 0.1. (b) Binding of phosphate, silicate, and fluoride on goethite (a-FeOOH) the species shown are surface species. (6 g FeOOH per liter, Pj = 10 M, Si/ = 8 x 10 M.) The curves are calculated with the help of experimentally determined equilibrium constants (Sigg and Stumm, 1981).

Figure 5. Evidence for surface species (right) in the normalized, background-subtracted, k3-weighted EXAFS spectra (left) of selenate and selenite ions reacted with goethite. The EXAFS spectra of selenate or selenite ions in aqueous solution are shown as dashed curves, with vertical arrows denoting contributions from Fe(III) in the selenite spectrum. Reactive surface OH groups on goethite are...

Figure 10.25 Distribution plot of percent surface species on goethite at 25°C in 0.1 M NaNOj solutions for ZU(VI) = 10" M computed with the TL model. Cg = 1 g/L, = 45 m /g (Hsi 1981 Hsi and Langmuir 1985).

Figure 13 Adsorption of Pu(V) by goethite as a function of pH in 0.1 M NaN03 solutions with EPu(V) = 1 X 10" M, and a sorbent/Hquid ratio of 0.55 g/L. The experimental data (open circles) are from Sanchez et al. (1985). The various curves are model fits to the experimental data assuming that adsorption is as one of three possible mononuclear surface species. The best fit of the data with DL, CC, and TL models is obtained assuming that Fe0H2-Pu020H" is the adsorbed species. From Turner (1995).

The examples shown is Section D indicate that the shape of calculated uptake curves (slope, ionic strength effect) can be to some degree adjusted by the choice of the model of specific adsorption (electrostatic position of the specifically adsorbed species and the number of protons released per one adsorbed cation or coadsorbed with one adsorbed anion) on the one hand, and by the choice of the model of primary surface charging on the other. Indeed, in some systems, models with one surface species involving only the surface site(s) and the specifically adsorbed ion successfully explain the experimental results. For example, Rietra et al. [103] interpreted uptake, proton stoichiometry and electrokinetic data for sulfate sorption on goethite in terms of one surface species, Monodentate character of this species is supported by the spectroscopic data and by the best-fit charge distribution (/si0,18, vide infra). 

Common inorganic ligands can enhance or inhibit the sorption of metal ions from aqueous solutions onto mineral surfaces depending upon whether or not they result in the formation of ternary surface complexes or stable solution species (Fig. 5). For example, the common ligand sulfate enhances the sorption of Pb(II) on goethite surfaces over the... 

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