Silicate minerals, adsorption water

Koretsky et al. calculated site densities for particular low index faces of six oxides and six silicate minerals and average site densities for cleavage and growth faces, and showed that these methods lead to very different results. Full documentation of depth, length of broken bond and Brown bond strengths of broken bonds for particular types of sites is presented. The ranges of TVs calculated by Koretsky et al. [7] and in their literature data collection (tritium exchange, acid-base titrations, NMR, adsorption and desorption of water at various conditions, and saturation experiments with different adsorbates in solution) for six oxides are listed in Table 5.1. 

In view of the problems associated with the expanding 2 1 clays, the smectites and vermiculites, it seemed desirable to use a different clay mineral system, one in which the interactions of surface adsorbed water are more easily studied. An obvious candidate is the hydrated form of halloysite, but studies of this mineral have shown that halloysites also suffer from an equally intractable set of difficulties (JO.). These are principally the poor crystallinity, the necessity to maintain the clay in liquid water in order to prevent loss of the surface adsorbed (intercalated) water, and the highly variable morphology of the crystallites. It seemed to us preferable to start with a chemically pure, well-crystallized, and well-known clay mineral (kaolinite) and to increase the normally small surface area by inserting water molecules between the layers through chemical treatment. Thus, the water would be in contact with both surfaces of every clay layer in the crystallites resulting in an effective surface area for water adsorption of approximately 1000 tor g. The synthetic kaolinite hydrates that resulted from this work are nearly ideal materials for studies of water adsorbed on silicate surfaces. 

Other papers in this series dealt with silicates which, unfortunately, were not chemically uniform. The mineral tobermorite (Ca3Si207, 2 H20) was made from Ca2Si04 under various experimental conditions180). Strangely, the yalues of nAp derived from the adsorption of nitrogen were in some instances only one-third or one-quarter those calculated from the adsorption of water vapor. When only the latter data were counted, then Qp appeared to be a roughly linear function of nAp, and Us was 388 + 30 erg/cm2. The Us for the trihydrate Ca3Si207, 3 H20 was said to be 320 70 erg/cm2. 

The flotation process is applied on a large scale in the concentration of a wide variety of the ores of copper, lead, zinc, cobalt, nickel, tin, molybdenum, antimony, etc., which can be in the form of oxides, silicates, sulfides, or carbonates. It is also used to concentrate the so-called non-metallic minerals that are required in the chemical industry, such as CaF2, BaS04, sulfur, Ca3(P03)2, coal, etc. Flotation relies upon the selective conversion of water-wetted (hydrophilic) solids to non-wetted (hydrophobic) ones. This enables the latter to be separated if they are allowed to contact air bubbles in a flotation froth. If the surface of the solids to be floated does not possess the requisite hydrophobic characteristic, it must be made to acquire the required hydrophobicity by the interaction with, and adsorption of, specific chemical compounds known as collectors. In separations from complex mineral mixtures, additions of various modifying agents may be required, such as depressants, which help to keep selected minerals hydrophilic, or activators, which are used to reinforce the action of the collector. Each of these functions will be discussed in relation to the coordination chemistry involved in the interactions between the mineral surface and the chemical compound. 

Clays are aluminosilicates with a two-dimensional or layered structure including the common sheet 2 1 alumino- and magnesium- silicates (montmorillonite, hectorite, micas, vermiculites) (figure 7.4) and 1 1 minerals (kaolinites, chlorites). These materials swell in water and polar solvents, up to the point where there remains no mutual interaction between the clay sheets. After dehydration below 393 K, the clay can be restored in its original state, however dehydration at higher temperatures causes irreversible collapse of the structure in the sense that the clay platelets are electrostatically bonded by dehydrated cations and exhibit no adsorption. 

Enzymes are immobilized by attachment to or confinement in water-insoluble materials (Fig. 1). Enzymes can be immobilized by adsorption on biologically inert carriers like organic polymers, glass, mineral salts, metal oxides, and different silicates. Since enzymes retain their activity for a longer time in an undissolved form, many reactions catalyzed by enzymes can be carried out in continuous systems. Immobilized enzymes can be used in agitated vessels, fluidized or Fixed bed tower reactors40). 

In this chapter we will first discuss briefly the basic theory behind ab nitio methods. Then a discussion of transition state theory, in light of the ab initio capabilities, will be taken up. This discussion will be followed by an analysis of the molecular mechanisms in water-silicate reactions and the ab initio elucidation of the adsorption and kinetic barriers involved in the bulk chemical reactions occurring at mineral-water interfaces. 

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. Silicates 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 ipKa of orthosilicate. The pH values of maximum adsorption of weak acid anions and the p Ka 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 silicate species is far greater at lower pH than simple acid—base equilibria would predict. 

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