Permanent charge, edge charges

The presence of hydroxyaluminum- and hydroxyaluminosilicate polymer in interlayered montmorillonite greatly promotes the adsorption of Cd, Zn, and Pb (Saha et al., 2001). The adsorption selectivity sequences of montmorillonite (Pb > Zn > Cd) and interlayered montmorillonite (Pb Zn Cd) resemble the metal selectivity on amorphous Fe and Al hydroxides (Saha et al., 2001). On montmorillonite, the metals are predominantly adsorbed on the permanent charge sites in an easily replaceable state. However, a substantial involvement of the edge OH" groups of montmorillonite in specific adsorption of the metals is also observed, especially at higher pH (Saha et al., 2001). 

There are two types of charges at the surface of mineral particles (1) permanent charge and (2) variable charge. Permanent charge is due to isomorphous substitutions, whereas variable charge is caused by dissociation of mineral-edge hydroxyls. 

Effect of Permanent Charge on the Study of Edge Charges... 

In the application of surface complexation models to clay minerals or to soils dominant in clays, the assumption is often made that metal ion adsorption occurs primarily on the aluminol and silanol groups of clay edges. The effect of the permanent charge sites on the adsorption process may not be considered. This simplification may be inappropriate, particularly for metal and metalloid anions, since repulsive electrostatic forces emanating from clay faces may spill over and affect the adsorption process on clay edges (Secor and Radke, 1985). 

The surface charge characterization of clay minerals, when permanent charges from isomorphic substitutions of ions in a clay crystal lattice are present besides the variable edge charges, is more complicated than that of metal oxides. In this case, the intrinsic surface charge density, Cin, can be defined as the sum of the net permanent structural charge density, oq, and the net proton surface charge density, ffo.H, i-C-, [2,... 

Fig. 3 Experimental points of net proton surface excess amounts from the reversible backward titration cycles of sodium montmoril-lonite at different NaCl concentrations. The different lines represent the results of numerical fitting (FITEQL [28]) using the diffuse-double-layer option of the surface complexation model assuming reactions of and Na" ions with permanently charged ion-exchange sites in parallel with protonation/deprotonation reactions on amphoteric edge sites...

Two further mechanisms are known to trap electronic charge in thin films intermolecular and resonance stabilization. In resonance stabilization, electron attachment to a molecular center produces an anion in a vibrationally excited state that is then de-excited by energy exchange with neighboring molecules. When the initial anion ground state lies below the band edge or lowest conduction level of the dielectric, then the additional electron may become permanently trapped at the molecular site. In this case, a permanent anion is formed (e.g., the case of O2 [220]). Intermolecular stabilization refers... 

Figure 14. Surface protonation and ion exchange equilibria at the kaolinite surfaces. The inset represents the protonation and ion-exchange reactions at the permanent negatively charged surface sites of the siloxane layer (0.1 MNaN03, [Al] = 1.6x 10-4 M, [XO], = 1.46x 10 3 M). The excess proton density, rHV, at the surface hydroxyl group is displayed as a function of pH. Surface protonation is interpreted as a successive protonation of two distinct types of OH groups localized at the gibbsite and edge surfaces. The pHZPC of the edge surface is about 7.5.

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