Surface area sorbing

The results of the studies on the effect of silica particle size on sorption indicate that, in the range of silica diameter of 1.4 X 10 2 to 3.9 X 10 2 cm, there is very little difference in the final amount of plutonium sorbed, as shown in Table III, the total weight of silica being the same for each size fraction. The only differences lie in the surface density of the sorbed species and the rate at which the equilibrium was approached. As the surface area decreased (i.e., as the silica size increased), there was more plutonium deposited per unit area of surface. Also, the smaller size particles with the higher surface areas sorbed the plutonium at a faster rate. These results are difficult to interpret, except with the model of a limited number of sorbable species, all of which are taken up eventually by the silica. They would not, however, be consistent with the model of a limited number of sorbing sites. 

Fig. 1. Normal soda Bayer hydrate heated for one hour. A, change in surface area B, total water, LOI plus sorbed water after exposure to 44% rh and C,...

Of the various inorganic soil constituents, smectites (montmorillonite clays) have the greatest potential for sorption of pesticides on account of their large surface area and abundance in soils. Weak base pesticides, both protonated and neutral species, have been shown to be sorbed as interlayer complexes. Sorption of atrazine on smectites ranges from 0 to 100% of added atrazine, depending on the surface charge density of the smectite (36). 

The most fundamental manner of demonstrating the relationship between sorbed water vapor and a solid is the water sorption-desorption isotherm. The water sorption-desorption isotherm describes the relationship between the equilibrium amount of water vapor sorbed to a solid (usually expressed as amount per unit mass or per unit surface area of solid) and the thermodynamic quantity, water activity (aw), at constant temperature and pressure. At equilibrium the chemical potential of water sorbed to the solid must equal the chemical potential of water in the vapor phase. Water activity in the vapor phase is related to chemical potential by... 

The sorption of water vapor onto nonhydrating crystalline solids below RHq will depend on the polarity of the surface(s) and will be proportional to surface area. For example, water exhibits little tendency to sorb to nonpolar solids like carbon or polytetrafluorethylene (Teflon) [21], but it sorbs to a greater extent to more polar materials such as alkali halides [34-37] and organic salts like sodium salicylate [37]. Since water is only sorbed to the external surface of these substances, relatively small amounts (i.e., typically less than 1 mg/g) of water are sorbed compared with hydrates and amorphous materials that absorb water into their internal structures. 

Unfortunately, the literature is relatively sparse with examples showing the water uptake profile onto crystalline, nonhydrating substances below RHq. This is most likely due to the difficulty in accurately measuring the small amounts of water that are sorbed. Alkali halides are an exception, however, likely due to their well-characterized particle morphologies [34—37]. Figure 2 shows a water uptake isotherm onto recrystallized sodium chloride [37]. Note that the amount of water sorbed as a function of relative humidity is normalized to the specific surface area of the sample. Since water is sorbed only to the external surface of... 

An important consideration in constructing certain types of geochemical models, especially those applied to environmental problems, is to account for the sorption of aqueous species onto sediment surfaces (e.g., Zhu and Anderson, 2002). Because of their large surface areas and high reactivities (e.g., Davis and Kent, 1990), many components of a sediment - especially clay minerals, zeolites, metal oxides and oxyhydroxides, and organic matter - can sorb considerable masses. 

Since laboratories follow different aging procedures, results of their studies can be difficult to compare. Values reported for surface area and site densities vary over a relatively broad range (Dzombak and Morel, 1990). It is not clear, furthermore, how closely the synthetic material resembles sorbing ferric oxides (e.g., ferrihydrite) encountered in nature. 

As an example of an equilibrium calculation accounting for surface complexation, we consider the sorption of mercury, lead, and sulfate onto hydrous ferric oxide at pH 4 and 8. We use ferric hydroxide [Fe(OH)3] precipitate from the LLNL database to represent in the calculation hydrous ferric oxide (FeOOH /1H2O). Following Dzombak and Morel (1990), we assume a sorbing surface area of 600 m2 g-1 and site densities for the weakly and strongly binding sites, respectively, of 0.2 and 0.005 mol (mol FeOOH)-1. We choose a system containing 1 kg of solvent water (the default) in contact with 1 g of ferric hydroxide. 

A more robust way to write a rate law for a catalytically promoted reaction is to include the concentrations of one or more surface complexes, in place of the surface area As. In this case, the simulation can account not only for the catalyzing surface area, since the mass of a surface complex varies with the area of the sorbing surface, but the effects of pH, competing ions, and so on. 

The concentrations of other metals attenuate when the metals sorb onto the surfaces of precipitating minerals (see Chapter 10). Hydrous ferric oxide, the behavior of which is well studied (Dzombak and Morel, 1990), has a large specific surface area and is capable of sorbing metals from solution in considerable amounts, especially at moderate to high pH HAO may behave similarly. The process by which hfo or HAO form and then adsorb metals from solution, known as coprecipitation, represents an important control on the mobility of heavy metals in acid drainages (e.g., Chapman etal., 1983 Johnson, 1986 Davis etal., 1991 Smith et ai, 1992). 

For moderate reduction the changes are completely reversible but they are progressively less so with more thorough reduction (Stucki et al 1984 Komadel et al 1995 Gates et al 1996). There are concomitant changes in the clay s physical and chemical properties, including its surface area, swelling behaviour, and capacity to sorb cations. 

EQUILIBRIUM SORPTION VALUES. The equilibrium sorption values for the extracts at various pressures of benzene are shown in Table II. The results show that O-methylated extract sorbs the most benzene at the lower pressure and that the O-octylated extract sorbs the least. At the higher pressme, the order is reversed. We believe the data shown in Table II reflect changes in the relative amounts of adsorption and absorption (swelling) with increasing size of the added alkyl groups. This interpretation is based on surface area and solubility measurements described below. 

Thus, the data in Table II can be readily explained if one considers the overall sorption process to consist of both adsorption and absorption. At low pressures, adsorption makes a relatively large contribution to the overall sorption process, and the values reflect the relative surface areas of the extracts. At higher pressures, absorption of benzene becomes relatively more important, and the equilibrium sorption values reflect the solubilities of benzene in the extract. It is interesting to note that the O-octylated extract sorbs more benzene than the O-butylated extract at the higher pressure, in spite of the fact that the O-butylated extract has a higher surface area. We conclude that benzene is more soluble in the O-octylated extract. 

Approximately 40 to 50% of the total amount of phenolics sorbed was retained by the organic matter fraction (27). In surface soil layers, organic matter is frequently intimately associated with the mineral components present, providing a large surface area and reactive sites for surface interaction. Soil acidity has a major influence on phenolic adsorption by the organic carbon fraction, since the degree of dissociation of the phenolic acids is pH-dependent. Whitehead and coworkers (28) observed that the extractability of several phenolic acids was highly dependent upon the extractant pH between pH 6 and 14. The amount extractable continually increased with extractant pH thus the extracted acids could not be readily classified into distinct fractions. 

The use of ion exchange resins and natural or synthetic inorganic exchange materials in the nuclear industry is well documented ( ). In the waste solidification application, the titanates or niobates offer no unique sorption properties. They do, however, provide a relatively high overall sorption capacity for a variety of nuclides in materials which can be converted into a stable ceramic host for the sorbed ions. After the sorption process, the column bed must be consolidated to reduce surface area. The project emphasis was directed toward a stable waste form and a considerable effort was devoted to producing and characterizing a highly dense form with favorable physical, chemical and thermal properties (l ). 

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