Adsorbent mineral oxides

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. 

Od-fumace blacks used by the mbber iadustry contain over 97% elemental carbon. Thermal and acetylene black consist of over 99% carbon. The ultimate analysis of mbber-grade blacks is shown ia Table 2. The elements other than carbon ia furnace black are hydrogen, oxygen, and sulfur, and there are mineral oxides and salts and traces of adsorbed hydrocarbons. The oxygen content is located on the surface of the aggregates as C O complexes. The... 

Clay minerals, oxides, and humic substances are the major natural subsurface adsorbents of contaminants. Under natural conditions, when humic substances are present, humate-mineral complexes are formed with surface properties different from those of their constituents. Natural clays may serve also as a basic material for engineering novel organo-clay products with an increased adsorption capacity, which can be used for various reclamation purposes. 

Before we look at some other surfaces, we should briefly address the H-donor (electron acceptor) properties, HDsurf, of the mineral oxides discussed so far. As can be seen from Fig. 11.5b (data for mineral oxides) and Table 11.1, HDsurf values decrease with increasing RH and become more similar with increasing RH. Furthermore, between 30 and 90% RH the HDsurf values can also be estimated by linear interpolation. However, in contrast to the vdW parameter, at 90% RH this value is smaller than that of the bulk water surface. This may have to do with the orientation of the water molecules caused by the nearby solid surface, but an unambiguous explanation is still missing. Between 90 and 100% RH, when the thickness of the adsorbed water layer rapidly grows, one can anticipate that this difference disappears. 

Natural minerals and rocks can be modelled as mixtures of clay-like adsorbents and oxide-like adsorbents. Various isotherms calculated from these assumptions are similar to experimentally observed isotherms for adsorption of Eu(III) on montmorillonite and corundum. Montmorillonite is of course a clay mineral, but it does have oxide-like groups. 

Compared with literature data for adsorption of surfactants from aqueous solutions on oxide surfaces(/4,75), the kinetic data obtained in this work for C18/glass are of similar orders of magnitude to the former systems. The values of 1/k and k.for CIg/Al are greater than those for oxide adsorbent studied, indicating the strong adsorbing ability of metallic aluminum (even oxidized) relative to the mineral oxides. 

Redox reactions are of importance in the dissolution of Fe-bearing minerals. Reductive dissolution of Fe(III)(hydr)oxides can be accomplished with many reductants especially organic and inorganic reductants, such as ascorbate, phenols, dithionite, and HS. Fe(II) in the presence of complex formers can readily dissolve Fe(III)(hydr)oxides. The Fe(II) bound in magnetite and silicate and adsorbed to oxides can reduce O2 (White, 1990 White and Yee, 1985). 

Effects of Humic Substances on Mineral Dissolution. Although humic substances appear to adsorb to oxide surfaces at least in part through surface complex formation, they have only slight effects on the dissolution of oxide and silicate minerals in laboratory studies. Both inhibition (at pH 4) and acceleration (at pH 3) of aluminum oxide dissolution have been observed 61), but in neither case was the effect dramatic (Table I). Kaolinite dissolution at pH 4.2 was also observed to be only minimally affected by humic substances (62). 

Adsorption of cations or anions may be greatly favored or inhibited when they occur as complexes rather than as free (uncomplexed) ions. For example, the hydroxide complexes of uranyl ion (UOl ) are strongly adsorbed by oxide and hydroxide minerals, whereas urany) carbonate complexes are poorly adsorbed by these minerals (Hsi and Langmuir 1985). In fact, carbonate, sulfate, and fluoride complexes of metals are often poorly adsorbed in general, whereas OH and phosphate complexes are usually readily adsorbed, particularly by oxide and hydroxide solids. Metal adsorption is considered at some length in Chap. 10. 

The tendency of cations to be adsorbed by oxide and hydroxide minerals with increasing pH, is usually proportional to their tendency to form hydroxyl complexes in solution. Explain this statement. How would this general rule help you to decide under what different pH conditions equal concentrations of Cr +, Ca % and Na would tend to be adsorbed by the same solid ... 

There are good reasons to believe that the application of the Nemst equation and the DDL model to oxides and other variable-charge mineral surfaces is inappropriate. To begin with, cations and anions may adsorb on oxides by direct coordination to the charged surface group. Even for monovalent cations and anions, a high percentage of the bonds with oxide surfaces are believed to be of the inner-sphere type. 

Special examples of mixture adsorption are competitive adsorption of the different forms of the same substance, such as pH-dependent ionic and undissociated molecular forms, monomers, and associates of the same substance, as well as potential-dependent adsorption of the same compound in two different orientations in the adsorbed layer. Different orientations on the electrode surface—for example, flat and vertical—are characterized with different adsorption constants, lateral interactions, and surface concentrations at saturation. If there are strong attractive interactions between the adsorbed molecules, associates and micellar forms can be formed in the adsorbed layer even when bulk concentrations are below the critical micellar concentration (CMC). These phenomena were observed also at mineral oxide surfaces for isomerically pure anionic surfactants and their mixtures and for mixtures of nonionic and anionic surfactants (Scamehorn et al., 1982a-c). 

Steenberg39 in a comprehensive and informative review of many aspects of surface oxides has contributed much to our understanding of their characteristics. He classifies activated carbons into H and L types. The L carbons are those that preferentially adsorb alkali, and the H types are those that adsorb mineral acids and but little alkali. Many associated properties are described and discussed. 

The emphasis in examining sorption with spectroscopic techniques has been mostly related to reaction with oxide surfaces, primarily Fe and Al. In addition to the simplicity of the systems, these are regarded as the primary adsorbent minerals for oxyanions in natural systems. 

It is also recognized (4) that in the case of ionic adsorbents, adsorption is determined in part by the surface charge of the adsorbent. In general, and OH" are potential - determining in mineral oxide-water systems therefore the surface charge, and the adsorption of sulfonate anions, would depend on the pH of the system (5) Somasundaran and Hanna (5) have found in fact that sulfonate adsorption on kaolinite decreases with increasing pH since the fraction of positive sites on the surface is expected to decrease. 

An experimental study by Wu et al. (1998) on the competitive adsorption resulted in the magnitude order of metal ions (Ag", Ni, Zn, Cu, Cd, Pb , Cr ) adsorbed onto oxide and silicate minerals in near-neutral solution with low ionic strength in mol/nm as follows ... 

Schulz, J.C. Warr, G.G. Adsorbed layer structure of cationic and anionic surfactants on mineral oxide surfaces. Langmuir mm, 18, 3191-3197. 

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