Adsorbents silicates

Although 2-line ferrihydrite has been used for dissolution studies, 6-line ferrihydrite has, to date, not been investigated. Fischer (1976) compared the dissolution behaviour of three 2-line ferrihydrites in 0.2 M oxalate and found the slowest dissolution rate for a slowly precipitated sample and faster dissolution for rapidly precipitated samples (hydrolysed by fast addition of NH3 or by bacterial oxidation of Fe citrate). Adsorbed silicate reduced the dissolution rate in oxalate probably by blocking surface Fe sites (Schwertmann Thalmann, 1976). 

Schwertmann, U. Eechter, H. (1982) The point of zero charge of natural and synthetic ferrihydrite and its relation to adsorbed silicate. Clay Min. 17 471-476... 

The second assumption is that hydrolysis occurs only on sites not occupied by adsorbed silicic acid ... 

In the table, it can be seen that the adsorption characteristics remain virtually unchanged when the thickness of the layer of modifying cations is increased by a factor of almost two. Hence, since the adsorbed molecules are separated quite considerably from the surface of the original layer silicate, we conclude that it is interaction with the modifier which contributes mainly to the thermodynamic characteristics of adsorption, while details of the adsorbate - silicate interaction potential play only a marginal role. 

At pH 11.0, if the silica sol is treated as a normal 30-nm-diameter silica (without adsorbed silicate layers), DLVO calculation generates a coagulation condition (Vi-max = 0) at just under 2-nm surface separation. The electrostatic repulsion is inside this separation and cannot overcome the van der Waals attraction. 

Physical Properties. Physical properties of importance include particle size, density, volume fraction of intraparticle and extraparticle voids when packed into adsorbent beds, strength, attrition resistance, and dustiness. These properties can be varied intentionally to tailor adsorbents to specific apphcations (See Adsorption liquid separation Aluminum compounds, aluminum oxide (alumna) Carbon, activated carbon Ion exchange Molecular sieves and Silicon compounds, synthetic inorganic silicates). 

Traditional adsorbents such as sihca [7631 -86-9] Si02 activated alumina [1318-23-6] AI2O2 and activated carbon [7440-44-0], C, exhibit large surface areas and micropore volumes. The surface chemical properties of these adsorbents make them potentially useful for separations by molecular class. However, the micropore size distribution is fairly broad for these materials (45). This characteristic makes them unsuitable for use in separations in which steric hindrance can potentially be exploited (see Aluminum compounds, aluminum oxide (ALUMINA) Silicon compounds, synthetic inorganic silicates). 

Bentonite is a rock rich in montmorillonite that has usually resulted from the alteration of volcanic dust (ash) of the intermediate (latitic) siliceous types. In general, reUcts of partially unaltered feldspar, quartz, or volcanic glass shards offer evidence of the parent rock. Most adsorbent clays, bleaching clays, and many clay catalysts are smectites, although some are palygorskite [1337-76 ]. 

The mesoporous ordered silicas of different type represent the new generation of materials with unique properties. The discovery of these materials became basis for creation of new catalysts, adsorbents, sensors and supporter for other molecules. The most important way of the modifying physical and chemical properties of mesopurous silicas consist in organic components incorporation on the silica surface as part of the silicate walls or their insertion within channels of the mesopores. This ensured that interest in synthesis and study of functionalized mesoporous materials shai ply grew. In spite of it, these materials are studied insufficiently. 

For exposure of reasons of observable discrepancy of results of the analysis simulated experiment with application synthetic reference samples of aerosols [1]. The models have demonstrated absence of significant systematic errors in results XRF. While results AAA and FMA depend on sort of chemical combination of an elements, method of an ashing of a material and mass of silicic acid remaining after an ashing of samples. The investigations performed have shown that silicic acid adsorbs up to 40 % (rel.) ions of metals. The coefficient of a variation V, describing effect of the indicated factors on results of the analysis, varies %) for Mn and Fe from 5 up to 20, for Cu - from 10 up to 40, for Pb - from 10 up to 70, for Co the ambassador of a dry ashing of samples - exceeds 50. At definition Cr by a method AAA the value V reaches 70 %, if element presences an atmosphere in the form of Cr O. At photometric definition Cr (VI) the value V is equal 40%, when the element is present at aerosols in the form of chromates of heavy metals. 

Metal Oxide - Since metals are less electrophilic than silicon, metal oxide adsorbents show even stronger selectivity for polar molecules than do siliceous materials. The most commonly used metal oxide adsorbent is activated alumina, used primarily for gas drying. Occasionally, metal oxides find applications in specific chemisorption systems. For example, several processes are under development utilizing lime or limestone for removal of sulfur oxides from flue gases. Activated aluminas have surface areas in the range of 200 to 1,000 ftVft Average pore diameters range from about 30 to 80 A. 

The separation capacity of a TLC method can be easily improved by use of a two-dimensional high performance TLC technique (2D HPTLC). Various plant essential oils (menthae, thymi, anisi, lavandulae, etc.) have been analysed by 2D TLC with florisil (magnesium silicate) as the adsorbent, using dichloromethane/ -heptane (4 6) in the first direction and ethyl acetate/n-heptane (1 9) in the second direction (69). 

If an acid solution of a fluorosilicate is rendered faintly alkaline with aqueous sodium hydroxide and then shaken with freshly precipitated cadmium oxide, all the silicic acid is adsorbed by the suspension. The alkali fluoride is then determined in the filtrate. 

A further resolution of the higher oxidation states in aquatic systems occurred in 1978 when scientists at Argonne National Laboratory(20) and Oak Ridge National Laboratory(21) independently established the capability to identify Pu(V) as the oxidized form that exists in natural waters. Both methods are based upon preferential adsorption on finely divided solids. In the Argonne procedure, adapted from a Japanese method for determining Np(V)(22), Pu(IV) and Pu(VI) adsorb onto silicic acid while Pu(V) does not. The Argonne scientists also have shown that the oxidized form of plutonium in natural waters carries on CaC03 when it is formed by... 

If the materials of interest are adsorbed on the surface of the solid impurities by polar forces, for example, due to contamination by substances such as silica, aluminum silicate or metal oxide, then they... 

Thermal reduction at 623 K by means of CO is a common method of producing reduced and catalytically active chromium centers. In this case the induction period in the successive ethylene polymerization is replaced by a very short delay consistent with initial adsorption of ethylene on reduce chromium centers and formation of active precursors. In the CO-reduced catalyst, CO2 in the gas phase is the only product and chromium is found to have an average oxidation number just above 2 [4,7,44,65,66], comprised of mainly Cr(II) and very small amount of Cr(III) species (presumably as Q -Cr203 [66]). Fubini et al. [47] reported that reduction in CO at 623 K of a diluted Cr(VI)/Si02 sample (1 wt. % Cr) yields 98% of the silica-supported chromium in the +2 oxidation state, as determined from oxygen uptake measurements. The remaining 2 wt. % of the metal was proposed to be clustered in a-chromia-like particles. As the oxidation product (CO2) is not adsorbed on the surface and CO is fully desorbed from Cr(II) at 623 K (reduction temperature), the resulting catalyst acquires a model character in fact, the siliceous part of the surface is the same of pure silica treated at the same temperature and the anchored chromium is all in the divalent state. 

Initial °Th and Pa are generally considered to be associated with a detrital component that becomes cemented, or occluded, within the speleothem. This component may be composed of clays, alumino-silicates or Fe-oxyhydroxides (Fig. 3) with strongly adsorbed and Pa. Th and Pa incorporated in speleothems and similar deposits may also have been transported in colloidal phases (Short et al. 1998 Dearlove et al. 1991), attached to organic molecules (Langmuir and Herman 1980 Gaffney et al. 1992) or as carbonate complexes in solution (Dervin and Faucherre 1973a, b Joao et al. 1987). 

Prepared from sodium silicate using the sol-gel procedure. Silica gel is the most widely used general purpose adsorbent for sample cleanup although it may irreversibly bind some strongly basic substances. Generally considered to be slightly acidic in character but this may vary with the method of prq ration. Activated by heating at about 180 C for 8-12 hours. 

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