Gibbsite surface charge

Values of Sverjensky Sahai (1996) for gibbsite used to describe surface charge experiments on alumina gel (Fig. 4). Values optimized from surface charge experiments on silica gel (Fig. 3). 

Procedure. The solid used is a Merck alumina gel. The product is predominantly amorphous but its X-ray diffraction pattern exhibits small peaks characteristics of gibbsite. Its surface charge determined by counter-ions adsorption is reported in Fig. 4. Batch experiments were performed using 5 g/L solid contacted with ltr4 m uranyl carbonated solutions at pH 7... 

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 Surface charge of some oxyhydroxides from pH 2 to 10 measured in different electrolyte solutions shown in parentheses. Ferrihydrite [Fe(OH)3 nH20] (0.001 M NaN03) from Hsi (1981) gibbsite [Al(OH)3] and silica gel [Si02 nH20] (1.0 M CsCl) based on Greenland and Mott (1978) goethite [o -FeOOH] (0.005 M CsCl) based on Greenland and Mott (1979) (see also Hsi [1981]) bimessite [cr-Mn02] (0.(X)1 M NaNOs) from Catts (1982).

So far the model is still quite general and the same for silica and gibbsite, the difference between the two surfaces appears upon introduction of the charge density. For silica the surface charge density equals ... 

For gibbsite type surfaces the characteristic protonation reaction is given by Eq. (43) and the surface charge by Eq. (62). In the presence of a simple 1-1 electrolyte of s.a. ions that weakly adsorb at the Stern plane Eqs. (62) and (55) to (59) can be combined to give... 

The surface charge vs. pH curves of other well studied metal hydroxides can all be described fairly well with the gibbsite one-pKn model [31, 34, 52-54]. The more complex reality that these surfaces are multifunctional does not affect the surface charge of rutile and hematite. Results for rutile are plotted in Fig. 9 for a good fit s.a. of the K+ and NOj ions had to be incorporated. Besides the <7s(pH) curves also, C, the capacitance (dus/dpH) is shown. Around the pzc the capacitance provides a good test for the model description [54]. Experimental capacitance values above pH=10 are unreliable. 

However, the nature, crystallinity (Kinniburg and Jackson, 1976, 1981 McKenzie, 1980), crystal size, and surface charge of metal oxides and mixed metal oxides (e.g., Fe-Al oxides Violante et al., 2003) also play an important role in the sorption selectivity of trace elements in cationic form. McBride (1982) compared the sorption behavior of different Al precipitation products of different crystallinity. The Cu sorption capacity followed tlie order noncrystalline Al-hydroxide > poorly crystalline boehmite > gibbsite. Iron and Mn oxides are... 

Jodin, M.C., Gaboriaud, E, and Humbert, B., Limitations of potentiometric studies to determine the surface charge of gibbsite y-AKOHij particles, J. Colloid Interf. Sci., 581, 2005. 

Figure 3.16. Surface charge of goethite (FeOOH) in solutions of 1.0, 0.1, and 0.01 M NaCl, estimated from the difference in proton and hydroxyl uptake (H" — OH ) by the oxide during acid-base titration. PZC, point of zero charge, (Adapted from F. J. Hingston, 1970. Specific adsorption of anions on goethite and gibbsite. Ph.D, dissertation. University of Western Australia, Perth.)...

Koopal reports [20] that surface charge (aj vs. pH, and its derivative with respect to pH dn /dp K) vs, pH can often be described on the basis of pseudo, homogeneity. In the case of transition aluminas (and of gibbsite) this may, at least partially, be caused by the fact that these oxides are remarkably soluble, which will lead to more homogeneous surfaces, as the most active centers, having the highest energies, will dissolve first. 

In summary, the model proposed on the basis of acid-base characteristics of kaolinite platelets explains the pH-dependent charge primarily to the protonation of the hydroxyl groups at the basal gibbsite and the edge surface. We will later illustrate how this charge characteristics (surface protonation) influences the reactivity (dissolution characteristics) of kaolinite. 

Simple Models. The surface chemical properties of clay minerals may often be interpreted in terms of the surface chemistry of the structural components, that is, sheets of tetrahedral silica, octahedral aluminum oxide (gibbsite) or magnesium hydroxide (brucite). In the discrete site model, the cation exchange framework, held together by lattice or interlayer attraction forces, exposes fixed charges as anionic sites. 

Figure 2. Charging behaviour of silica type (panel a) and gibbsite type (panel b) surfaces.

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