Charge density minerals

Extra radial flexibility has been proved necessary in order to model the valence charge density of metal atoms, in minerals [6,11], and coordination complexes [5], and similar evidence of the inability of single-exponential deformation functions to account for all the information present in the observations have also been found in studies of organic [12, 13] and inorganic [14] molecular crystals. 

Clay minerals or phyllosilicates are lamellar natural and synthetic materials with high surface area, cation exchange and swelling properties, exfoliation ability, variable surface charge density and hydrophobic/hydrophilic character [85], They are good host structures for intercalation or adsorption of organic molecules and macromolecules, particularly proteins. On the basis of the natural adsorption of proteins by clay minerals and various clay complexes that occurs in soils, many authors have investigated the use of clay and clay-derived materials as matrices for the immobilization of enzymes, either for environmental chemistry purpose or in the chemical and material industries. 

The results of this study clearly show the complex dependence of the flocculation process on polymer dosage and charge density. It is seen that the form of dependence varies markedly among the responses monitored. In addition to the factors studied here, it can also be expected to depend upon several other physicochemical conditions of the system, including the type of mixing. The final state of flocculation achieved by a mineral/polymer system will depend upon many interactions in the system as determined by various chemical and hydrodynamic properties of the particles, polymer, dissolved organics and the fluids. 

Focusing, on the Na-Cs pair, the AG is less pronounced with decreasing charge density and tends to vanish at zero charge density, corresponding to a tendency of equal differences in surface and solution terms in eq. (1). This situation is possible if the hydration status of the adsorbed cations tends to equal that of solution cations. It follows therefore that the action of forces that tend to dehydrate the interlamellar cations such as the increase in charge density of the mineral or the Increase in electrolyte concentration (32), enhance the selectivity of the least hydrated cation. 

The inductive effect of the carbon chain in the clay phase amounts to (only) 5 to 7 % of the effect in the gas phase. Ammonium cations in the interlamellar region of clay minerals are therefore less hydrated than in equilibrium solution. The free energy of alkylammonium exchange increases with charge density from Laponite (42) < Red Hill montmorillonite (40) < Camp Berteau montmorillonite (41) in line with the smaller interlamellar hydration status of the adsorbed cation at higher charge density. 

Exchange in zeolites of alkali, alkaline earth, transition metal ions and small organic ammonium ions, has been reviewed (111), and in general, the exchange is characterized by small AG values comparable to those found in clay minerals. Althoufft identical selectivity orders for alkali and alkaline earth metal ions are obtained, as in montmorillonite, the opposite variation of AG with charge density is found. 

Structural surface charge density, defined as the number of Coulombs per square meter, as a result of isomorphic substitutions in soil minerals. 

The type of clay present in a soil influences triazine sorption (Brown and White, 1969). Furthermore, variations in surface properties among different samples of the same clay type greatly influence sorption. For instance, sorption of atrazine on 13 clay samples, of which smectite was the dominant mineral, ranged from 0% to 100% of added atrazine (Figure 21.7), and was inversely correlated to the surface charge density of the smectites (Laird et al., 1992). Such data illustrate the complexity of sorption processes and the reason why simple predictive models relying on % OC, % clay, or surface area normalizations may fail to predict accurately the sorption of triazine by a particular soil. 

Fig. 3. The structure of the EDL at the mineral-water-electrolyte interface. 1-Layer of charging ions 2j-inner and 2,-outer Helmholtz layer (Grahame and Stem plane, resp.) 3-diffuse layer and 4-slipping or shear plane [after Ref. 16]. V o-phase potential and -Stern s poten-tial.a - H20 dipols, b - hydrated counterions, c - negatively charged ions, d - thickness of the G-S layer o - charge density...

Ions causing electrical charge of a mineral surface are called potential-determining ions (PDI). The surface charge density of a mineral os according to the mechanism a) can be calculated from the change of pH value in an aqueous suspension of the solid by using Eq. (1) ... 

Akhtar and Lai128) infer the adsorption mechanism of a collector on a mineral surface from the mutual position of IPpH and PZC which they determine by electrophoresis. Their deduction is based on experiments performed with hematite in solutions of Na oleate (NaOl) and dodecylamine hydrochloride (DDA-HC1). The surface of hematite is assumed to consist of MeOHj, MeOH and MeO-. Chemisorption of the collector is discussed according to Table 3 which is based on Eq. 72 for charge density at the inner Helmholtz plane ... 

The net permanent structural surface charge density, denoted gq and measured in coulombs per square meter (C/m2), is created by isomorphic substitutions in minerals [4]. These substitutions in clay minerals produce significant surface charge only in the 2 1 layer types. In these minerals, Co < 0 invariably because of structural cation substitutions. The relation between gq and the layer charge jc is [3]... 

The intrinsic surface charge density reflects particle charge developed from either isomorphic substitutions or adsorption involving H+ or OH-. A widely used technique for measuring intrinsic surface charge density is the Schofield method. In this method [3], clay mineral particles are reacted with an electrolyte solution (e.g., NaCl) at a given pH value and ionic strength the specific surface excess of the cation and the anion adsorbed from the electrolyte is determined and the value of is calculated with the equation... 

The Poisson-Boltzman (P-B) equation commonly serves as the basis from which electrostatic interactions between suspended clay particles in solution are described ([23], see Sec.II. A. 2). In aqueous environments, both inner and outer-sphere complexes may form, and these complexes along with the intrinsic surface charge density are included in the net particle surface charge density (crp, 4). When clay mineral particles are suspended in water, a diffuse double layer (DDL) of ion charge is structured with an associated volumetric charge density (p ) if av 0. Given that the entire system must remain electrically neutral, ap then must equal — f p dx. In its simplest form, the DDL may be described, with the help of the P-B equation, by the traditional Gouy-Chapman [23-27] model, which describes the inner potential variation as a function of distance from the particle surface [23]. 

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