Iler The Chemistry Of Silica
Glendora Starr
N2 - The use of modern spectroscopic techniques in the study of the formation of aq. silica gels is described. The oligomerization process of monomeric silicic acid was studied by silicon-29 NMR spectroscopy; at high pH values cyclic trimeric silicate species were favored compared to the linear structure. Aggregation of primary silica particles of mol. size (
AB - The use of modern spectroscopic techniques in the study of the formation of aq. silica gels is described. The oligomerization process of monomeric silicic acid was studied by silicon-29 NMR spectroscopy; at high pH values cyclic trimeric silicate species were favored compared to the linear structure. Aggregation of primary silica particles of mol. size (
BT - The colloid chemistry of silica : developed from a symposium sponsored by the Division of Colloid and Surface Chemistry, at the 200th national meeting of the American Chemical Society, Washington, DC, August 26 - 31, 1990
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Organic compounds in natural waters are thought to interact at the silicate surface and accelerate mineral weathering processes. Natural and contaminating organic acids produce protons which contribute to proton promoted mineral weathering. Organic acid anions can also complex with metal ions in solution, lowering metal activity and increasing the apparent solubility of the mineral. The major impact of organic acids, however, may be that metal-organic complexes can form at the solid-solution interface, weakening cation-oxygen bonds, thus catalyzing the dissolution reaction.
Chemical weathering by organic acids may be an important process in oil field formation waters and soils where there are high concentrations of complexing organic compounds. Organic ligand concentrations in the bulk groundwater solution are lower, but they still might be responsible for a significant amount of mineral weathering. In groundwaters contaminated by organic compounds, however, the rate of weathering processes will be greatly accelerated by the presence of organic ligands produced by metabolizing microbes.
The overall dissolution reaction involves the multi-step process of initial rapid exchange of cations for protons at the mineral surface, followed by a slow, rate determining hydrolysis and subsequent detachment of silica and alumina species from the remaining framework. The extent of the reaction and identity of the process depend on the environmental conditions and the organic species present.
Natural and contaminating organic compounds are reported to play an important role in geochemical processes. Organic ligands are suggested to chelate and mobilize heavy metals, sorb to mineral surfaces, and enhance the dissolution and the precipitation of inorganic minerals in aqueous systems (e.g., Bennett, 1991). Wide variety of organic compounds in natural and waste waters can act as complexing agents for metal ions (Smith and Martell, 1974-1982). The presence of significant concentrations of organic acid anions has been revealed from analyses of subsurface oilfield water (e.g., Surdam et al., 1984; Surdam and MacGowan, 1987) and are thought to enhance the formation of secondary porosity.
Dissolved organic compounds are suspected of interacting with a wide variety of inorganic solutes in natural waters. Dissolved organic substances can mobilize and transport relatively insoluble metals in natural waters ( e.g., Stumm and Morgan, 1981). Many investigators have found that organic acids enhance the dissolution of aluminosilicate minerals or quartz (Huang and Keller, 1970; Tan, 1980; Bennett et al., 1988; Bennett, 1991; Welch and Ullman, 1993). Grandstaff (1986) found that some organic acids accelerate olivine dissolution and that the relative reactivity of organic compounds is related to the ability of the organic ligand to form complexes with metals. Dissolved organic compounds produced by biodegrading crude oil were shown to be actively etching quartz and aluminosilicate minerals (Bennett and Siegel, 1987; Hiebert and Bennett, 1992). Huang and Longo (1992) found evidence of enhanced development of secondary porosity in feldspar dissolution in organic-bearing aqueous solutions at reservoir temperatures (95C/1 bar and 100C/88 bar).
Previous researchers have examined, to varying degrees, the kinetics of silicate dissolution, the dissolution of silicates in the organic/inorganic electrolyte solutions, and the mechanisms of silicate dissolution. While the reaction kinetics of aluminosilicate hydrolysis in inorganic aqueous solutions has been examined in detail (e.g., Berner, 1978; Aagaard and Helgesson, 1982; Helgesson et al. 1984; Chou and Wollast, 1985; Rimstidt and Dove, 1986; Anbeek, 1992; Wollast and Chou, 1992), mechanisms of dissolution at the molecular level are often difficult to distinguish due to difficulties in isolating the competing reactions and processes in the system.
The general dissolution reaction of silicates in inorganic aqueous systems involves multi-step process of initial rapid exchange of cations (K, Na, and/or Ca) for protons at the mineral surface, followed by a slow, rate determining hydrolysis (formation of activated complex) and subsequent detachment of silica and alumina species from the remaining framework (e.g., Aagaard and Helgeson, 1982). A number of studies have demonstrated that mineral hydrolysis occurs via surface complexes formed by the adsorption and desorption of protons and/or ligands from solution (Blum and Lasaga, 1988; Carroll-Webb and Walther, 1988; Brady and Walther, 1989). Destruction of framework bonds are known to occur through decomposition of these surface complexes when they are in the activated state. Therefore, the overall dissolution rate can be expressed as
where S is the surface concentration of the reaction precursor (surface species which is in equilibrium with the activated complex) and k is constant. Also the reaction rate depends on pH and temperature of the reactants. The pH-dependency of hydrolysis is thought to be controlled by the acid-base properties and bonding in the metal-oxygen bond, and the mechanism of hydrolysis (e.g., Casey and Bunker, 1990).
Various conceptual models have been suggested in order to interpret the weathering mechanism of slightly soluble minerals in aqueous solutions. The models are based on experimental observations that the dissolution of most oxides and silicates at low temperature is a surface-controlled process.
It has been suggested that the dissolution kinetics of most slightly soluble oxides and silicates is controlled by the concentration of adsorbed charged species at the mineral surface produced by these reactions, in particular by H+ and OH- (e.g., Chou and Wollast, 1984; Knauss and Wolery, 1986; Carroll-Webb and Walther, 1988; Brady and Walther, 1989, 1990; and many others). Surface protonation-deprotonation and other charged ligand surface complex reactions were considered reaction steps preceeding the rate controlling elementary reaction. Based on transition state theory (Eyring, 1935), these reactions are thought to increase the concentration of activated complexes in a rate determining detachment reaction (Aagaard and Helgeson, 1982; Helgesson et al., 1984; Wieland et al., 1988). This is likely to occur by weakening cation-oxygen bonds at the mineral or oxide surface through bond polarization by the charged surface complexes (Zinder et al., 1986). Therefore, the rate of oxide mineral dissolution directly related to the concentrations of charged surface complexes produced by surface adsorption reactions (Furrer and Stumm, 1986; Brady and Walther, 1989; Xie and Walther, 1992). However, it is not possible to predict the order of the rate relative to the concentration of the surface species, or the order of a given detachment reaction, from surface coordination chemistry (Brady and Walther, 1989).
In simple solutions, variations in hydrolysis rate with pH are controlled by the acid-base properties of bridging oxygens, or terminal hydroxyl oxygens at the mineral surface (Casey and Bunker, 1990). Variation in surface charge concentration with solution pH is characteristic for a given oxide surface, just as the extent of protonation is characteristic of a dissolved oxyacid at a given pH (Casey and Bunker, 1990). The acid base reactions on the mineral surfaces are synonymous with the sorption of hydroxyl or hydrogen ions onto the oxide surface. The sorption reactions saturate the material with a net surface charge, which can be measured in a potentiometric titration or through studies of electrophoretic mobility (e.g., Furrer and Stumm, 1986).
Although the dependency of the surface speciation on pH and ionic strength of the solution, resulting in the increase or decrease in dissolution rates, has been observed by many investigators (e.g., Casey and Bunker, 1990; Xie and Walther, 1992), the ionic strength effect on surface speciation and dissolution rate is not clearly defined. In simple solutions, in theory, increase in ionic strength causes increase in the activity of neutral species like silica. The silica dissolution rates, as a result, should decrease due to the increase in the degree of saturation. However many investigators have reported increase in quartz dissolution rates by increasing the ionic strength (e.g., Dove and Crerar, 1990; Bennett, 1991). Considering that the ionic strength effect varies by the electrolyte species in the solution and the nature of the surface reaction sites, the increased dissolution rates with increasing ionic strength may be attributed to the interaction between surface species and the specific electrolyte species (e.g., Na+ or K+).
Amrhein and Suarez (1988) found that the presence of complex-forming organic ligands resulted in a dissolution rate that increased linearly with decreasing pH. Welch and Ullman (1992) found the rates of plagioclase dissolution in solutions containing organic acids were up to ten times greater than the rates determined in solutions containing inorganic acids at the same acidity.
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