Silicates adsorption

Solid phase adsorption Silicic acid-H2S04-5% butanol in chloroform Titration-manual Milk, cream Harper et al. (1956)... 

Brunauer and co-workers [129, 130] found values of of 1310, 1180, and 386 ergs/cm for CaO, Ca(OH)2 and tobermorite (a calcium silicate hydrate). Jura and Garland [131] reported a value of 1040 ergs/cm for magnesium oxide. Patterson and coworkers [132] used fractionated sodium chloride particles prepared by a volatilization method to find that the surface contribution to the low-temperature heat capacity varied approximately in proportion to the area determined by gas adsorption. Questions of equilibrium arise in these and adsorption studies on finely divided surfaces as discussed in Section X-3. 

Table XI-1 (from Ref. 166) lists the potential-determining ion and its concentration giving zero charge on the mineral. There is a large family of minerals for which hydrogen (or hydroxide) ion is potential determining—oxides, silicates, phosphates, carbonates, and so on. For these, adsorption of surfactant ions is highly pH-dependent. An example is shown in Fig. XI-14. This type of behavior has important applications in flotation and is discussed further in Section XIII-4.

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). 

The crystalline mineral silicates have been well characterized and their diversity of stmcture thoroughly presented (2). The stmctures of siHcate glasses and solutions can be investigated through potentiometric and dye adsorption studies, chemical derivatization and gas chromatography, and laser Raman, infrared (ftir), and Si Fourier transform nuclear magnetic resonance ( Si ft-nmr) spectroscopy. References 3—6 contain reviews of the general chemical and physical properties of siHcate materials. 

Silicates. For many years, siUcates have been used to inhibit aqueous corrosion, particularly in potable water systems. Probably due to the complexity of siUcate chemistry, their mechanism of inhibition has not yet been firmly estabUshed. They are nonoxidizing and require oxygen to inhibit corrosion, so they are not passivators in the classical sense. Yet they do not form visible precipitates on the metal surface. They appear to inhibit by an adsorption mechanism. It is thought that siUca and iron corrosion products interact. However, recent work indicates that this interaction may not be necessary. SiUcates are slow-acting inhibitors in some cases, 2 or 3 weeks may be required to estabUsh protection fully. It is beheved that the polysiUcate ions or coUoidal siUca are the active species and these are formed slowly from monosilicic acid, which is the predorninant species in water at the pH levels maintained in cooling systems. 

Filter aids should have low bulk density to minimize settling and aid good distribution on a filter-medium surface that may not be horizontal. They should also be porous and capable of forming a porous cake to minimize flow resistance, and they must be chemically inert to the filtrate. These characteristics are all found in the two most popular commercial filter aids diatomaceous silica (also called diatomite, or diatomaceous earth), which is an almost pure silica prepared from deposits of diatom skeletons and expanded perhte, particles of puffed lava that are principally aluminum alkali siheate. Cellulosic fibers (ground wood pulp) are sometimes used when siliceous materials cannot be used but are much more compressible. The use of other less effective aids (e.g., carbon and gypsum) may be justified in special cases. Sometimes a combination or carbon and diatomaceous silica permits adsorption in addition to filter-aid performance. Various other materials, such as salt, fine sand, starch, and precipitated calcium carbonate, are employed in specific industries where they represent either waste material or inexpensive alternatives to conventional filter aids. 

The most important minerals of the lanthanide elements are monazite (phosphates of La, Ce, Pr, Nd and Sm, as well as thorium oxide) plus cerite and gadolinite (silicates of these elements). Separation is difficult because of the chemical similarity of the lanthanides. Fractional crystallization, complex formation, and selective adsorption and elution using an ion exchange resin (chromatography) are the most successful methods. 

Zeolites are naturally occurring hydrous aluminum-sodium silicates in porous granule form. They are capable of exchanging their sodium base for calcium or magnesium and of expelling these alkaline earth metals for sodium by treatment with salt. Thus, they are a type of ion-exchange media. (Some zeolites act as molecular sieves by adsorption of water and polar compounds.)... 

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... 

With its oxygen functionality, graphite oxide has chemical properties more akin to those of layered disulfides or sheet silicates than to those of graphite (Gi, T1,A2). Many studies have been of an extremely applied nature the possibility of fluorination (LI, N1), redox potentials in the presence of hydrogen peroxide (V2), the apparent density (L2), the adsorption isotherms with nitrogen (L3), and the diffusion of Cs in graphite oxide (R2). 

FIGURE 2.18 Adsorption, followed by drag and separation mechanism for exfoliation of silicate layers. 

In the same spirit DFT studies on peroxo-complexes in titanosilicalite-1 catalyst were performed [3]. This topic was selected since Ti-containing porous silicates exhibited excellent catalytic activities in the oxidation of various organic compounds in the presence of hydrogen peroxide under mild conditions. Catalytic reactions include epoxidation of alkenes, oxidation of alkanes, alcohols, amines, hydroxylation of aromatics, and ammoximation of ketones. The studies comprised detailed analysis of the activated adsorption of hydrogen peroxide with... 

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. 

Adsorption (including both physical adsorption and ion exchange) by clay minerals and silicates... 

The influence of calcium on the adsorption of high molecular weight EOR polymers such as flexible polyacrylamides and semi-rigid xanthans on siliceous minerals and kaolinite has been studied in the presence of different sodium concentrations. Three mechanisms explain the increase in polyacrylamide adsorption upon addition of calcium (i) reduction in electrostatic repulsion by charge screening,... 

This study aims at determining the effects of calcium on the adsorption of polyacrylamides and xanthans on siliceous minerals and kaolinite. 

Adsorption on Siliceous Minerals. The adsorption of polyacrylamides on siliceous minerals in the presence of monovalent ions has been discussed previously (9, 10). While PAM adsorption is unaffected by monovalent ions since it is not governed by electrostatic factors, HPAM adsorption is increased due to reduction in electrostatic repulsion by charge screening. 

Adsorption on Siliceous Minerals. All adsorption studies of xanthan (XCPS) in the presence of calcium are conducted at pH 6.5 to avoid precipitation which has been reported at pH>7 for xanthan solutions containing calcium (25). 

For polyacrylamides, as a function of polymer ionicity, the presence of calcium induces a maximum in intrinsic viscosity and a minimum in adsorption density on siliceous minerals. This holds important practical implications in EOR since an optimal polymer ionicity can be selected according to field conditions. 

Layer silicate minerals have a high selectivity of trace transition and heavy metals and greater irreversibility of their adsorption. Some chemisorbing sites such as -SiOH or AlOH groups may be at clay edges and form hydroxyl polymers at the mineral surface. Another possible reason for the high selectivity may be hydrolysis of the metal and strong adsorption of the hydrolysis ion species. 

Both organic and inorganic ligands such as Cl and dissolved organic carbon (fulvie acid and carboxylic acids) decrease metal adsorption. In the arid soils with higher pH, folic acids increase the solubility of metals such as Cu and Zn. The interaction between the transition of heavy metals and silicate surfaces was reviewed by McBride (1991). 

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