Silicate minerals, adsorption

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. 

Bloesch, P.M. Bell, L.C. Hughes, J.D. (1987) Adsorption and desorption of boron by goethite. Aust. J. Soil Res. 25 377-390 Blomiley, E.R. Seebauer, E.G. (1999) New approach to manipulating and characterising powdered photo adsorbents. NO on Cl treated Ee20j. Langmuir 15 5970-5976 Bloom, P.R. Nater, E.A. (1991) Kinetics of dissolution of oxide and primary silicate minerals. In Sparks, D.L. Suarez, D.L. (eds.) Rates of soil chemical processes. Soil Sci. 

Koretsky, E. M., Sverjensky, D. A., and Sahai, N. (1998). A model of surface site types on oxide and silicate minerals based on crystal chemistry— implications for site types and densities, multisite adsorption, surface infrared-spectroscopy, and dissolution kinetics. Amer. J. Sci. 298, 349-438. 

Although many soil scientists had considered the possible mechanisms which soils employ for the retention [fixation] of phosphorus, it remained for Haseman et al. (1950) to demonstrate that phosphorus could — and in experimental situations did — replace the silicon of micas and clay minerals in order to form crystalline hydrous aluminium phosphates of sodium, ammonium and potassium. Prior to experimentation by this group, associated with the laboratories of the Tennessee Valley Authority (TVA), most authors attributed the retention of phosphorus by soils to combination with calcium to produce fairly insoluble minerals to adsorptive, exchangeable combination with silicate minerals and to formation of phosphates of iron... 

Most of the recent adsorption literature has emphasized the importance of the acid-base properties of oxide surfaces when explaining or estimating their sorption behavior. However, Sveijensky (1993) has shown that log values for the adsorption of a specific cation by multiple mineral sorbents are a simple linear function of l/e, where e is the dielectric constant of each mineral. He has used this approach to estimate CC and TL model K"" values for the adsorption of up to 18 cations on 7 oxide and silicate mineral surfaces. 

Koretsky et al. calculated site densities for particular low index faces of six oxides and six silicate minerals and average site densities for cleavage and growth faces, and showed that these methods lead to very different results. Full documentation of depth, length of broken bond and Brown bond strengths of broken bonds for particular types of sites is presented. The ranges of TVs calculated by Koretsky et al. [7] and in their literature data collection (tritium exchange, acid-base titrations, NMR, adsorption and desorption of water at various conditions, and saturation experiments with different adsorbates in solution) for six oxides are listed in Table 5.1. 

X-RAY ABSORPTION SPECTROSCOPY X-ray absorption spectroscopy (XAS) includes x-ray absorption near-edge (XANES) and extended x-ray absorption fine structure (EXAFS) spectroscopy. An advantage of XAS spectroscopy is that adsorption experiments can be carried out in aqueous systems (Fendorf et al., 1994). X-ray absorption spectroscopy has been used to examine the sorption of both cations and anions to oxide and silicate minerals found in soils, with an emphasis on ions that are potential contaminants in the environment. 

The partial orders with respect to [OH ] observed for most silicate mineral dissolution reactions can be explained by the surface complexation model (Blum and Lasaga, 1988 Brady and Walther, 1989). Brady -and Walther (1989) showed that slope plots of log R vs. pH for quartz and other silicates at 25 °C is not inconsistent with a value of 0.3. Plots of the log of absorbed OH vs. pH also have slopes of about 0.3, suggesting a first-order dependence on negative charge sites created by OH adsorption. Because of the similarity of quartz with other silicates and difference with the dependence of aluminum oxides and hydroxide dissolution on solution [OH ], Brady and Walther (1989) concluded that at pH >8 the precursor site for development of the activated complex in the dissolution of silicates is Si. This conclusion is supported by the evidence that the rates (mol cm s ) at pH 8 are inversely correlated with the site potential for Si (Smyth, 1989). Thus it seems that at basic pH values, silicate dissolution is dependent on the rate of detachment of H3SiO4 from negative charge sites. 

Huang, P, and Fuerstenau, D.W., The effect of hydrophobicity of silicate minerals on the adsorption of dodecylammonium acetate. Mater. Res. Soc. Symp. Proc., 432, 351, 1997,... 

Thus, B(0H)3 is a Lewis add rather than a Bronsted acid (see Chapter 1). Because boron adsorbs most effectively in the pH 8 to 9 range on A1 and Fe oxides and silicate minerals, its availability is generally low in coarse-textured, acid-leached soils and in calcareous soils. Deficiency in add soils is the result of boron depletion by leaching, while deficiency in calcareous soils is caused by strong adsorption and predpitation as relatively insoluble Ca borate salts. In contrast, B toxicity is most commonly found in alkaline soils of arid regions these soils often contain high levels of Na which forms quite soluble borate salts. A lack of rainfall allows soluble borate to accumulate to phytotoxic levels. 

This list may be lengthened when the effects of adsorption are considered, because a molecule bound on a surface may be activated for photodegradation even if the reaction is not favorable for the same molecule dissolved in solution. For example, paraquat apparently photodecomposes more rapidly when adsorbed on layer silicate minerals than when in solution (Helling et al., 1971). This phenomenon may be related to the fact that adsorption on clay shifts the UV absorption band of paraquat to longer wavelength (256 - 275 nm) and closer to the atmospheric window for UV light (see Figure 10.20). In other words, adsorption on clay increases the probability that paraquat will absorb UV radiation and thereby decompose. 

Adsorption and Surface Chemical Grafting. As with silica and many other silicate minerals, the surface of asbestos fibers exhibit a significant chemical reactivity. In particular, the highly polar surface of chrysotile fibers promotes adsorption (physi- or chemisorption) of various types of organic or inorganic substances (22). Moreover, specific chemical reactions can be performed with the surface functional groups (OH groups from bmcite or exposed silica). 

Most of the colloidal properties of SOM are due to humus. Humus is highly colloidal and is x-ray amorphous rather than crystalline. The surface area and adsorptive capacities of humus per unit mass are greater than those of the layer silicate minerals. The specific surface of well-developed humus may be as high as 900 x 103 m2 kg-1 its exchange capacity ranges from 1500 to 3000 mmol(+) kg-1. 

The second model requires catalysis by adsorbed protons and is applied to silicate minerals with covalent, polymerized structures, like quartz. In this model, protons react quickly with oxide bonds at the surface, accelerate cleavage, and return to solution. As bonds are progressively cleaved, a monomer or small oligomer is released from the surface. The weakness of this model is the enormous difficulty in simultaneously determining rates of dissolution and proton adsorption densities on complicated multi-oxide mineral structures. Protons taken up by leaching alkaline-metal cations from the mineral must be separated from those involved in protonation of bonds in order to assign a value to the rate order. 

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 ... 

The above examples show the general features of ion adsorption on phyUosili-cates, which can be applied also to other silicate minerals the involvement of edge groups, mainly aluminol, in the coordination with metal cations, and in the development of positive charges favoring anion binding. The quantitative treatment of these phenomena will be presented in Part HI. 

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