Kaolinite surface complex

The surface characteristics of kaolinite was discussed in Chapter 3.4 and in Fig. 3.9. While the siloxane layer may - to a limited extent - participate in ion exchange reactions. The functional OH-groups at the gibbsite and edge surfaces are able to surface complex heavy metal ions. (Schindler et al., 1987). 

Assuming a correlation between surface complexation and aqueous hydrolysis exists, the trend in strengths of surfaces complexes for An in different oxidation states onto a given mineral would be in the order An4+ > AnC>2+ > An3+ > AnOj. Several authors have provided evidence for linear relations between the first hydrolysis constant of metals and the intrinsic constant associated to the formation of surface species of metals as S-OMamorphous silica (Schindler Stumm 1987), hydrous ferric oxides (Dzombak Morel 1990), aluminum (hydr-)oxides and kaolinite (Del Nero et al. 1997, 1999a). 

Gay minerals and zeolites are interesting with respect to possibilities for geometric influences. Activation can be produced, as in enzyme catalysis, by constraining the reactive molecule, via surface complexation, in a configuration in which it is destabilized relative to that of the free molecule, yet still accessible to other reactants. A possible example is hydrazine complexed with kaolinite. The conformation of hydrazine is flattened relative to that of the free molecule (See Giff Johnston s paper in Part m of this volume). It has been shown that hydrazine is readily air-oxidized by kaolinite (Coyne, submitted for publication). 

Carroll-Webb S. A. and Walther J. V. (1988) A surface complex model for the pH-dependence of corundum and kaolinite dissolution rates. Geochim. Cosmochim. Acta 52, 2609-2623. 

Surface complexation models have been successfully used to describe the pH dependent adsorption of As(III) and As(V) by several minerals including ferrihydrite, goethite, amorphous As(OH)3, gibbsite, kaolinite,... 

Effects of Humic Substances on Mineral Dissolution. Although humic substances appear to adsorb to oxide surfaces at least in part through surface complex formation, they have only slight effects on the dissolution of oxide and silicate minerals in laboratory studies. Both inhibition (at pH 4) and acceleration (at pH 3) of aluminum oxide dissolution have been observed 61), but in neither case was the effect dramatic (Table I). Kaolinite dissolution at pH 4.2 was also observed to be only minimally affected by humic substances (62). 

NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY Nuclear magnetic resonance (NMR) spectroscopy can be applied to aqueous samples and can distinguish between inner- and outer-sphere ion surface complexes. The adsorption behavior of the cations Cs+ and Na" " was studied on the surfaces of silica, boehmite, kaolinite, and illite (Kim and Kirkpatrick, 1997). Cesium was adsorbed both as inner-and outer-sphere surface complexes and in the diffuse layer, while Na was adsorbed only as outer-sphere surface complexes and in the diffuse layer. The adsorbed Na ions were fully hydrated, while the Cs ions had direct contact with the surface oxygen atoms. 

Strontium adsorption onto soil minerals is an important retardation mechanism for Sr " ". Chen et al. (1998) investigated the adsorption of Sr " " onto kaolinite, illite, hectorite, and montmorillonite over a range of ionic strengths and from two different electrolyte solutions, NaNO3 and CaCb- In all cases, the EXAFS spectra suggested Sr adsorbed to clay minerals as an outer-sphere mononuclear complex. Sahai et al. (2000) also found that on amorphous silica, goethite, and kaolinite substrates, Sr"+ adsorbed as a hydrated surface complex above pH 8.6. On the other hand, Collins et al. (1998) concluded from EXAFS spectra that Sr " " adsorbed as an inner-sphere complex on goethite. 

O Day, P. A., Parks, G. A., and Brown, G. E. (1994). Molecular structure and binding sites of cobalt(II) surface complexes on kaolinite from x-ray absor ption spectroscopy. Clays Clay Miner. 42, 337-355. 

Peacock, C.L. and Sherman, D.M., Surface complexation model for multisite adsorption of copper(n) onto kaolinite, Geochim. Cosmochim. Acta, 69, 3733, 2005. 

The dissolution is controlled by the detachment of Al. Since the dissolution of silica is not promoted in presence of oxalate and salicylate (Bennett et al., 1988 Wieland, 1988), we may conclude that Si centers do not form stable surface complexes with these ligands. Hence, the siloxane layer of kaolinite and muscovite is not reactive with respect to dissolution reactions. Therefore, the detachment of both Al and Si is a consequence of the formation of surface complexes with Al sites. 

O Day, P.A., G.E. Brown, and G.A. Parks. 1994a. X-ray absorption spectroscopy of Cobalt(II) multi-nuclear surface complexes and surface precipitates on kaolinite. J. Colloid Interf. Sci. 165 269-289. 

Morton JD, Semrau JD, Hayes KF (2001) An X-ray absorption spectroscopy study of the structure and reversibility of copper adsorbed to montmorillonite clay. Geochim Cosmocliim Acta 65 2709-2722 Muller B, Sigg L (1992) Adsorption of lead(II) on the goethite surface Voltammetric evaluation of surface complexation parameters. J Coll Interf Sci 148 517-532 Neder RB, Burghammer M, Grasl T, Schulz H, Bram A, Fiedler S (1999) Refinement of the kaolinite structure from single-crystal synchrotron data. Clays Clay Miner 47 487-494 Needleman HL (1983) Low level lead exposure and neuropsychological performance In Lead versus health - Sources and effects of low level lead exposure. Rutter M, Russell JR (eds) Wiley, Chichester, p 229-248... 

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