Clay minerals, anionic

Effect on Oxide—Water Interfaces. The adsorption (qv) of ions at clay mineral and rock surfaces is an important step in natural and industrial processes. SiUcates are adsorbed on oxides to a far greater extent than would be predicted from their concentrations (66). This adsorption maximum at a given pH value is independent of ionic strength, and maximum adsorption occurs at a pH value near the piC of orthosiUcate. The pH values of maximum adsorption of weak acid anions and the piC values of their conjugate acids are correlated. This indicates that the presence of both the acid and its conjugate base is required for adsorption. The adsorption of sihcate species is far greater at lower pH than simple acid—base equihbria would predict. 

Two classes of clays are known [3] (i) cationic clays (or clay minerals) that have negatively charged alumino-silicate layers balanced by small cations in the interlayer space (e.g. K-10 montmorillonite) and (ii) anionic clays which have positively charged brucite-type metal hydroxide layers balanced by anions and water molecules located interstitially (e.g. hydrotalcite, Mg6Al2(0H)igC034H20. 

Frost RR, Griffin RA (1977) Effect of pH on adsorption of As and selenium from land fill leachate by clay minerals. Soil Sci Soc Am J 41 53—57 Goh K-H, Lym TT (2005) Arsenic fractionation in a fine soil fraction and influence of various anions on its mobility in the sub surface environment. Appl Geochem 20 229-239... 

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. 

Cornell, R. M., and U. Schwertmann (1979), "Influence of Organic Anions on the Crystallization of Ferrihydrite", Clays and Clay Minerals 27,402-410. 

Figure 2. Calculated variation of AG° (Na Cs) (kJ mol ) with the equivalent anionic radius (A) at different interlayer molalities (35). Data of Maes and Cremers (17) are also shown. (Figure supplied by D.D. Eberl). Reproduced with permission from Ref. 35. Copyright 1980, The Clay Minerals Society.

Mineral segregation in industry relies heavily on the selective adsorption of macromolecules onto the surfaces of those minerals that have particular industrial applications. This selectivity is governed mainly by the surface chemistry of the mineral and the type of polymer used as a flocculant. " Effectiveness of flocculation depends upon the charge, concentration and molecular weight of the polymer, and also the pH and salt concentration of the clay suspension. The bonding between the anionic flocculant polyacrylamide (PAM) and clay mineral surfaces has been effectively reviewed recently by Hocking et al and the reader is referred to this should they require an in-depth literature review. For more information on general colloidal chemistry of clay suspensions the reader is referred to the review of Luckham and Rossi." ... 

This paper contributes to the literature by quantifying anionic polymer adsorption onto the clay minerals kaolinite, feldspar, mica and quartz by X-ray photoelectron spectroscopy (XPS). XPS measures the sorbed amount directly rather than by a subtraction technique. This enables an insight into how effective selective flocculation is for obtaining kaolinite from a mineral mixture. Atomic force microscopy (AFM) is also used to image polymer adsorption onto mineral surfaces and the effectiveness of this technique applied to mineral surfaces is discussed here. 

The silicates are divided into five subgroups, which differ in the structures of their silicate anions. Each silicate anion consists of one or more Si04 tetrahedrons which are linked in various ways. This group of substances is discussed more elaborately than the other ones because these minerals are most abundant in the lithosphere and because the clay minerals in clay also belong to the silicates. 

We have already learnt that the building unit of the silicates is the Si04 pyramid. By linking these pyramids in various ways the different kinds of silicate anions are formed. The classification of the silicates is based on these kinds of ions. The silicate anions of clay minerals have a layered structure, the top view of which is again represented in figure 8.5. 

Cation exchange capacity The ability of a solid substance (especially clay minerals) to adsorb cations. The cation exchange capacity of a material represents the total negative charge on the surface of the material and is generally expressed in milliequivalents per 100 g of material (compare with anion exchange capacity). 

Clay minerals and clay colloids are the products of the advanced weathering of primary silicates. They are comprised mainly of silica and alumina, often with appreciable amounts of alkali and alkaline earth metals and iron. Most also have varying amounts of water bound to their surfaces and can take on a variety of different chemical and physical properties depending on the amount of water adsorbed. They have the ability to exchange or bind cations and anions and are capable of complex formation with a wide variety of organic molecules. 

McBride, M. B. (1982b). Organic anion adsorption on aluminum hydroxides Spin probe studies. Clays Clay Miner. 30,438-444. 

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