Kaolinite surface charge

Brady, P. V., Cygan, R. T. Nagy, K. L. 1996. Molecular controls on kaolinite surface charge. Journal of Colloid Interface Science, 183, 356-364. 

The effects of organic molecules and phosphate on the adsorption of acid phosphatase on various minerals, and kaolinite in particular, have been investigated by Huang et al. [97]. The Langmuir affinity constant for AcP adsorption by kaolinite follows the series tartrate (K — 97.8) > phosphate (K= 48.6) > oxalate (K — 35.6) > acetate (K= 13.4). At low concentration, acetate even promoted the adsorption of acid phosphatase. It was considered that competitive interactions between anionic adsorbates can occur directly through competition for surface sites and indirectly through effects of anion adsorption on the surface charge and protonation. 

Type. The interlayer spaces of 2 1 silicates may be blocked by poorly ordered sheets of A1 hydroxy polymers, such as [Al(OH)2 5° +] (n > 6). Such A1 interlayers neutralize a considerable part of the surface charge and restrict swelling, and effectively convert 2 1 clays into materials similar to kaolinite. 

SDS and kaolinite (i.e., at pH 4.6 both the kaolinite surface and SDS have net negative charges). The initial enhancement of sorption that occurs with increasing NaCl at low SDS concentrations most likely results from a screening effect between SDS and kaolinite that allows SDS molecules to first sorb enhancement at higher SDS concentrations most likely results from decreasing repulsions between sorbed SDS headgroups when hydrophobic forces become more important. For the nonionic Tween 80 surfactant, isotherms for 0 and 0.1 M NaCl show that differences in sorption are minor for these conditions, consistent with results from Brownawell et al. (1997). 

Kretzschmar, R., Hesterberg, D., and Sticher, H. (1997). Effects of adsorbed humic acid on surface charge and flocculation of kaolinite. Soil Sci. Soc. Am. I. 61,101-108. 

Clay colloids provide a good example of the kinds of structures that can be formed upon flocculation. The association of plate-like clay particles is complicated by the fact that minerals such as montmorillonite, illite, and kaolinite can exhibit different surface charges at different particle surfaces. 

Chorover, J. and Sposito, G., Surface charge characteristics of kaolinitic tropical soils, Geochim. Cosmochim. Acta, 59, 875, 1995. 

Finally, clays such as the smectites almost invariably have a net negative structural charge because of isomorphous substitution of cations of lower charge than would be present in a balanced structure. In kaolinite, the amphoteric nature of the hydrated aluminum and silica surface contributes more to surface charge than does substitution. As a result of either substitution or surface dissociation, a region of counter ions (exchangeable and... 

Successive surface protonation at the gibbsite plate and the edge surface can account for the pH-dependent surface charge of kaolinite (Wieland and Stumm, 1992). 

Figure 10.5 shows the strong surface charge variation with pH of several common oxyhydroxides of Al, Fe(IIi), Mn(IV), and Si. As shown in Fig. 10.2, the surface charge of kaolinite also exhibits a marked pH dependence, behaving like that of the oxides and hydroxides in Fig. 10.5. 

Simple ion exchange describes the competitive adsorption onto clays of most metal cations present in solution at concentrations from about 10 to 10 mol/kg. It has most often been used.to describe the sorption of alkaline earth and alkali metal cations onto clays. In the case of minerals having pH-dependent surface charge (e.g., kaolinite, metal oxyhydroxides) simple ion exchange or the power-exchange function (see below) may also fit the adsorption data mea.sured in systems at constant pH. 

The cation exchange capacity of clays results from lattice imperfections or defects, isomorphous substitutions, and/or broken bonds on clay particle surfaces. Explain how the CEC s of kaolinite, the smectites, and illite, and their variation with pH, reflect these sources of their surface charge. 

When the amount of modifier on the montmorillonite surface increases, a regular decrease in differential heats of adsorption is observed, with the simultaneous increase of the specific retention volumes per unit area of external surface and Henry s constants for the hydrocarbons considered (Tables 7, 8). At the same time, an increase in the amount of long-chain cationic surfactants on the kaolinite surface leads to the decrease in both differential heat of adsorption and Henry s constant for benzene (Table 8), not surprisingly, because of a more complete covering of the kaolinite surface hy the presorbed modifying layer and a decrease in the charge of the polar NHj groups of the modifier [41]. 

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