Case Study Kaolinite

The weathering of silicates has been investigated extensively in recent decades. It is more difficult to characterize the surface chemistry of crystalline mixed oxides. Furthermore, in many instances the dissolution of a silicate mineral is incipiently incongruent. This initial incongruent dissolution step is often followed by a congruent dissolution controlled surface reaction. The rate dependence of albite and olivine illustrates the typical enhancement of the dissolution rate by surface protonation and surface deprotonation. A zero order dependence on [H+] has often been reported near the pHpzc this is generally interpreted in terms of a hydration reaction of the surface (last term in Eq. 5.16). 

The sequential surface protonation of the kaolinite surface was illustrated in Fig. 3.11. As was explained in Chapter 3.6, the excess proton density can be interpreted as a successive protonation at the edge and of the gibbsite surface. The pHpzc of the edge surface is about 7.5. 

In line with the stoichiometry Al H = 1 1 at the edge surface and Al H = 1 3 at the gibbsite surface and in accordance with Eqs. (5.12b) and (5.17b) the proton promoted rate law indicates a first order reaction 0 = 1) with respect to protons bound to the edge surface and a third order dependence (j = 3) with respect to the mol fraction of protons bound to the gibbsite surface. 

Example 5.1 Change in Surface Protonation as a Consequence of Metal Ion or Ligand Adsorption 

In Fig. 3.5 we illustrated generally that an alkalimetric or acidimetric titration curve of a hydrous oxide dispersion becomes displaced by the adsorption of a metal ion or, - in opposite direction - by the adsorption of an anion (ligand). 

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It can be seen from a number of the case studies reported that additional information on the mechanism of fabrication or functionality of nanoscale features can be found by using a number of these techniques simultaneously or consecutively. Examples of these can be seen in the work presented above from Pap et al. [6], Moghadam et al. [44] and Pisarek et al. [69]. Often one of these techniques is used to back up another. In the work of ClauseU et al., FESEM was used to back up XRD measurements when measuring kaolinite crystal thickness [73], hi another work by Huiqian et al., BET, EESEM, TEM and X-ray scattering were used to characterize tungsten powders. Although BET and SEM could not characterize the particle size of nanometre powders, they provided a method to allow the exclusion of non-nanometre powders [74],... 

Case study II When considering the environmental impact of mining on the Fal Estuary, the most important quarrying activity was the production of china clay (kaolinite), principally from the St. Austell area. Historically, china clay mining resulted in the release of both coarse sand as bedload sediment within rivers and also some fine china clay being released in suspension. The early... 

The tangential electric field drives electric currents around the particle surface. In most electroldnetic theories it is assumed that this current is dominated by the contribution from the diffuse layer ions, for the material inside the shear plane is taken to be immobile and non-conducting. While this assumption seems to be valid for many colloids, including oxide systems, for some systems there is significant electrical conduction in the region between the shear plane and the particle surface. Dielectric dispersion and ESA studies have shown this to be the case in kaolinite [21], bentonite [20], lattices [22,23] and in emulsions [2]. We refer to this phenomenon as stagnant layer conduction. 

In the second study, Kittrick (20) reacted the 0.2 to 5 pm fractions of three montmorillonite clays with low pH (<3.47) solutions for 3 to 4 years. Under these conditions, montmorillonite is unstable with respect to kaolinite. It is uncertain whether the kaolinte formed through precipitation, in which case the nucleation process may have produced numerous small particles, or it formed through the alteration of the pre-existing montmorillonite structure, which could have maintained the existing particle size or even increased it, with growth of the new phase. 

However, the synthesis process most extensively studied by solid-state NMR is that of carbothermal reduction of aluminosilicate minerals such as kaolinite, which are mixed with finely divided carbon and heated in nitrogen at > 1400°C (Neal et al. 1994, MacKenzie et al. 1994a). Under carbothermal conditions the clay decomposes to a mixture of mullite and amorphous silica (MacKenzie et al. 1996b), the latter forming SiC which reacts with the mullite to form P-sialon, in some cases via other sialon phases such as X-sialon (see below). The precise reaction sequence and the nature of the intermediates has been shown by the NMR studies to depend on various factors including the nature of the aluminosilicate starting mineral (MacKenzie er a/. 1994a). 

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). 

MacKenzie (95,96) has developed a method for theTG study of solids in the presence of applied electrical fields. Electric fields of the order of 05 V/m lower the initial decomposition temperature for the dehydroxylation of kaolinite by 60° in some cases. The activation energy for the process is reduced by 3-12 kcal mole-1. Rate constants for the material are increased by electrolysis but this effect falls off at higher temperatures as the normal processes begin to predominate. 

Here the adsorption of the carbamate insecticides and 1-naphthol on kaolinite and bentonite was studied. Adsorption isotherms for Sevin, 1-naphthol, baygon, pyrolan, and dimetilan on bentinite and kaolinite (hydrogen forms) were determined. The adsorption isotherms for the bentonite system are found in Figure 6. These isotherms are similar to those usually encountered in the majority of cases of adsorption of organic solutes from dilute solutions. These types of isotherms are represented according to the classical Freundlich equation ... 

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