Surface charge 349 silicates

Clay may promote hydrolysis of the metal at low pFI, but also inhibit hydrolysis at high pH (McBride, 1991). At a higher pH, clay prevents complete hydrolysis of the metal due to the affinity of the charged polymeric metal ions for the silicate surface. This keeps the metal from becoming a separate hydroxide phase. 

In characterizing layered silicate, including layered titanate (HTO), the surface charge density is particularly important because it determines the interlayer structure of the intercalants as well as the cation exchange capacity (CEC). Lagaly proposed a method of calculation consisting of total elemental analysis and the dimensions of the unit cell [15] ... 

Surface Charge on Carbonates, Silicates,Sulfides and Phosphates... 

Bronsted acid sites) or metal atoms with unsatisfied coordination (Lewis acid sites) react with water to form surface charge (13). Isomorphic substitution in the interlayer region of layered silicates results in a negative surface charge. In each case chemical "exchange" of ions between phases results in the formation of surface charge and the development of an electrical potential. 

Subsequent work showed that a modification of the synthesis procedure produced a 10A hydrate which> if dried carefully, would maintain the interlayer water in the absence of excess water (27). This material is optimal for adsorbed water studies for a number of reasons the parent clay is a well-crystallized kaolinite with a negligible layer charge, there are few if any interlayer cations, there is no interference from pore water since the amount is minimal, and the interlayer water molecules lie between uniform layers of known structure. Thus, the hydrate provides a useful model for studying the effects of a silicate surface on interlayer water. 

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. 

Of particular interest for chemical transport into a predominantly smectite medium is the shrink-swell property of the clay material. The swelling properties of smectites are explained by two concepts. The first one, developed by Sposito (1973), shows that smectite swelling is caused by the hydration and mobility of the cations, which in turn balance the negative charge of the layer silicates. The second concept, presented by Low (1981), emphasizes the direct interaction of water molecules with the silicate surface. Both viewpoints fit the common observation that smectite swells in a high-hydration environment and at low electrolyte concentrations and shrinks when water is lost and salt is added to the bulk solution. 

Clay minerals are characterized by a high surface charge and a very small particle size. A detailed presentation of two types of layered silicate clay (kaoUnite and smectite) is given in Chapter 1. 

Pederson, L. R., McGrail, B. P., McVay, G. L., Petersen-Villalobos, D. A. Settles, N. S. 1993. Kinetics of alkali silicate and aluminosilicate glass reactions in alkali chloride solutions Influence of surface charge. Physics and Chemistry of Glasses, 34, 140-148. 

CDA was tested with KBr single crystals and alkali silicate soft glass, a microscope slide. The KBr crystals behaved as expected for a dielectric containing local dipoles which reorient in an externally applied field. The soft glass showed a monotonous (E-l) vs. T behavior with a decreasing positive surface charge. 

The mechanism a) is characteristic for simple and complex metal oxides and a number of silicates whereas the mechanism b) is typical for sparingly soluble salts such as CaF2, BaS04, CaC03. These mechanisms represent two extreme cases of a surface charge formation since in most minerals both mechanisms proceed simultaneously in proportions depending on the chemical composition and crystalline structure31). 

The results show that the sorption of cyanide on soils and sediments is fast it reaches equilibrium within 10 minutes. The sorbed quantity, however, is low. From 10 4-10 3 mol/dm3 cyanide solutions, it is about 10 7 mol/g. This means an approximately 10-3 dm3/g distribution ratio for cyanide ion. This value is typical for the anion sorption of soils, and it is explained by the interfacial properties of soil components. The main mineral components of soils (primary silicates, clay minerals, oxides) have negative surface charges at pH applied (about 8.5), inhibiting the... 

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