Solid adsorbents water

A number of highly porous solids adsorb water preferentially when contacted by wet solvent mixtures and can remove water to very low concentrations. While they can be used on a once-through basis they are capable of being regenerated for many cycles of reuse by heating and such regeneration is economical for long-term operations. 

Fig. XVI-7. Dielectric isotherms of water vapor at 15°C adsorbed on a-FeiOa (solid points indicate desorption). A complete monolayer was present at P/P = 0.1, and by P/P = 0.8 several layers of adsorbed water were present. (From Ref. 110.)...

The results obtained for the adsorption of butane on a ball-milled caldte" are also of interest. When the solid was outgassed at 150°C to remove physically adsorbed water, the butane isotherm was ofType II with c = 26 (Fig. 5.5, curve (ii)) but outgassing at 25°, which would leave at least a monolayer of molecular water on the surface, resulted in a Type 111 isotherm (Fig. 5.5, curve (i)). Though butane is nonpolar its polarizability is... 

Fig. 5.20 Adsorption isotherms for water vapour on x-Fe,Oj at 15°C for various outgassing temperatures. Solid points indicate second isotherm after 25°C evacuation of physically adsorbed water. (Courtesy Zettlemoyer.) Outgassing temperature,<, 25°C , I00°C O, 250°C ...

A high percentage 01 water remains after the sublimation process, present as adsorbed water, water of hydration or dissolved in the diy amorphous solid this is difficult to remove. Usually, shelf-temperature is increased to 25 to 40°C and chamber pressure is lowered as far as possible. This stiU does not result in complete diying, however, which can be achieved only by using even higher temperatures, at which point thermally induced product degradation can occur. 

Trace contaminants are also significant at charged solid surfaces, affecting both the charging process and the surface conductivity. In ambient air atmospheres their effect is often determined by interaction with adsorbed water vapor, whose dominant concentration may be sufficiently large to form a monolayer. Topical antistatic agents for solids typically rely on interaction with adsorbed water and can lose effectiveness at low relative humidity (4-2.1). 

Adsorption, which utilizes the ability of a solid adsorbent to adsorb specific components from a gaseous or a liquid solution onto its surface. Examples of adsorption include the use of granular activated carbon for the removal of ben-zene/toluene/xylene mixtures from underground water, the separation of ketones from aqueous wastes of an oil refinery, aad the recovery of organic solvents from the exhaust gases of polymer manufacturing facilities. Other examples include the use of activated alumina to adsorb fluorides and arsenic from metal-finishing emissions. 

Lateral density fluctuations are mostly confined to the adsorbed water layer. The lateral density distributions are conveniently characterized by scatter plots of oxygen coordinates in the surface plane. Fig. 6 shows such scatter plots of water molecules in the first (left) and second layer (right) near the Hg(l 11) surface. Here, a dot is plotted at the oxygen atom position at intervals of 0.1 ps. In the first layer, the oxygen distribution clearly shows the structure of the substrate lattice. In the second layer, the distribution is almost isotropic. In the first layer, the oxygen motion is predominantly oscillatory rather than diffusive. The self-diffusion coefficient in the adsorbate layer is strongly reduced compared to the second or third layer [127]. The data in Fig. 6 are qualitatively similar to those obtained in the group of Berkowitz and coworkers [62,128-130]. These authors compared the structure near Pt(lOO) and Pt(lll) in detail and also noted that the motion of water in the first layer is oscillatory about equilibrium positions and thus characteristic of a solid phase, while the motion in the second layer has more... 

Transport in solution or aqueous suspension is the major mechanism for metal movement from the land to the oceans and ultimately to burial in ocean sediments. In solution, the hydrated metal ion and inorganic and organic complexes can all account for major portions of the total metal load. Relatively pure metal ores exist in many places, and metals from these ores may enter an aquatic system as a result of weathering. For most metals a more common sequence is for a small amount of the ore to dissolve, for the metal ions to adsorb onto other particulate matter suspended in flowing water, and for the metal to be carried as part of the particulate load of a stream in this fashion. The very insoluble oxides of Fe, Si, and A1 (including clays), and particulate organic matter, are the most important solid adsorbents on which metals are "carried."... 

Water taken up by solid materials is generally classified as water bound by physical forces or water bound by chemical bonds. Physically bound water includes adsorbed water, trapped or liquid-inclusion water, and absorbed water. The physical adsorption of water occurs when water condenses or is held on the surface the surface includes the cracks, crevices, etc. of real materials. Liquid inclusion occurs during the crystallization process when bubbles of water are trapped. 

Protein recovery via disruption has also been achieved by adsorbing water from the w/o-ME solution, which causes protein to precipitate out of solution. Methods of water removal include adsorption using silica gel [73,151], molecular sieves [152], or salt crystals [58,163], or formation of clanthrate hydrates [154]. In most of the cases reported, the released protein appeared as a solid phase that, importantly, was virtually surfactant-free. In contrast to the dilution technique, it appears that dehydration more successfully released biomolecules that are hydrophilic rather than hydrophobic. 

Figure 227. Adsorption process of water vapor on solids Solid Adsorbent Liquid Absorbent...

In an open sorption storage system air is transporting water vapor and heat in and out of the packed bed of solid adsorbents (see Figure 235) or a reactor where the air is in contact with a liquid desiccant. In desorption mode a hot air stream enters the packed bed or the reactor, desorbs the water from the adsorbent or the salt solution and exits the bed cooler and saturated. In adsorption mode the previously humidified, cool air enters the desorbed packed bed or the... 

Beichert, P. and Pitts, B. J. F. Knudsen cell studies of the uptake of gaseous HNO3 and other oxides of nitrogen on solid NaCl The role of surface-adsorbed water, J. Phys. Chem., 100,15218-15 228, 1996. 

The next layer of water, held between -30 and —1500 kPa, is available to plants and is therefore called plant available water. The water present between -1500 and -3200 kPa is held in capillaries so tightly that it is not available to plants but can be lost by evaporation. The layer closest to the soil solid is held at more than -3200 kPa and is called hygroscopic water. A soil sample, heated in an oven for 24 hours at 105°C, and then left exposed to the air will adsorb water until a layer of hygroscopic water has been formed, illustrating the strong attraction of soil surfaces for water. 

The rate of adsorption from dilute aqueous solutions by solid adsorbents (zeolites) is a highly significant factor for applications of this process for water quality control. 

The structural state of dendritic macromolecules at air-water (Langmuir mono-layers) and air-solid (adsorbed monolayers, self-assembled films and cast films) interfaces have been reviewed by Tsukruk [17]. Although this work summarizes various characterization techniques for dendritic films by AFM techniques, in this chapter, we will present recent progress on the characterization of the dendritic film surface morphologies. 

In surface precipitation cations (or anions) which adsorb to the surface of a mineral may form at high surface coverage a precipitate of the cation (anion) with the constituent ions of the mineral. Fig. 6.9 shows schematically the surface precipitation of a cation M2+ to hydrous ferric oxide. This model, suggested by Farley et al. (1985), allows for a continuum between surface complex formation and bulk solution precipitation of the sorbing ion, i.e., as the cation is complexed at the surface, a new hydroxide surface is formed. In the model cations at the solid (oxide) water interface are treated as surface species, while those not in contact with the solution phase are treated as solid species forming a solid solution (see Appendix 6.2). The formation of a solid solution implies isomorphic substitution. At low sorbate cation concentrations, surface complexation is the dominant mechanism. As the sorbate concentration increases, the surface complex concentration and the mole fraction of the surface precipitate both increase until the surface sites become saturated. Surface precipitation then becomes the dominant "sorption" (= metal ion incorporation) mechanism. As bulk solution precipitation is approached, the mol fraction of the surface precipitate becomes large. 

Solid-solid contact (inc. solid breakup) Metal to metal Metal to semiconductor Semiconductor to semiconductor Volta potential (equalization of Fermi levels) Electrolytic potential (where adsorbed water films may be present)... 

An adsorbed layer of water molecules at the interface separates hydrated ions from the solid surface. The interfacial electric double layer can be represented by a condenser model comprising three distinct layers a diffuse charge layer in the ionic solution, a compact layer of adsorbed water molecules, and a diffuse charge layer in the solid as shown in Fig. 5-8. The interfacial excess charge on the... 

The diffuse layer of excess electrons and holes in solids is called the space charge layer and the diffuse layer of excess hydrated ions in aqueous solution is simply called the diffuse layer and occasionally called the Gouy layer [Gouy, 1917]. The middle layer of adsorbed water moleciiles, between the diffuse layer on the aqueous solution side and the space charge layer on the soUd side, is called the compact or the inner layer. This compact or inner layer is also called the Helmholtz layer [Helmholtz, 1879] or the Stem layer [Stem, 1924] the plane of the closest approach of hydrated ions to the solid surface is called the outer Helmholtz plane (OHP) [Graham, 1947]. 

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