Solution amorphous silica

Bessho M, Wajima T, Ida T, Nishiyama T (2011) Experimental study on prevention of acid mine drainage by silica coating of pyrite waste rocks with amorphous silica solution. Environ Earth Sci 64 311-318... 

Hydrated amorphous silica dissolves more rapidly than does the anhydrous amorphous silica. The solubility in neutral dilute aqueous salt solutions is only slighdy less than in pure water. The presence of dissolved salts increases the rate of dissolution in neutral solution. Trace amounts of impurities, especially aluminum or iron (24,25), cause a decrease in solubility. Acid cleaning of impure silica to remove metal ions increases its solubility. The dissolution of amorphous silica is significantly accelerated by hydroxyl ion at high pH values and by hydrofluoric acid at low pH values (1). Dissolution follows first-order kinetic behavior and is dependent on the equilibria shown in equations 2 and 3. Below a pH value of 9, the solubility of amorphous silica is independent of pH. Above pH 9, the solubility of amorphous silica increases because of increased ionization of monosilicic acid. 

The acidic conditions of standard SBA-15 synthesis [35] cause the precipitation of metal nanoparticles without silica encapsulation, or the formation of amorphous silica due to the presence of the polymer used for nanoparticle synthesis. Therefore, the SBA-15 framework was synthesized under neutral condition using sodium fluoride as a hydrolysis catalyst and tetramethylorthosilicate (TMOS) as the silica precursor. Pt particles with different sizes were dispersed in the aqueous template polymer solution sodium fluoride and TMOS were added to the reaction mixture. The slurry aged at 313 K for a day, followed by an additional day at 373 K. Pt(X)/SBA-15-NE (X = 1.7, 2.9, 3.6, and 7.1nm) catalysts were obtained by ex-situ calcination (see Section 3.2). TEM images of the ordered... 

Scheme 2. Encapsulation of size- and shape-controlled Pt nanoparticles under neutral hydrothermal synthesis conditions of SBA-15. Silica templating block copolymers and silica precursors were added to PVP-protected Pt nanoparticle solutions and subjected to the standard SBA-15 silica synthesis conditions. Neutral, rather than acidic pH conditions were employed to prevent particle aggregation and amorphous silica formation [16j. (Reprinted from Ref. [16], 2006, with permission from American Chemical Society.)...

It is thought that the precipitation of amorphous silica is caused by conductive cooling from the hydrothermal solution which flows laterally in the chimney (Herzig et al., 1988). 

In these chimneys, coprecipitation of barite and amorphous silica is taking place from the solution characterized by lower temperatures and lower flow rate than the black smoker. 

Solubilities of quartz and amorphous silica in aqueous solutions increase with increasing of temperature (Holland and Malinin, 1979). Solubility of barite depends on salinity and temperature (Blount, 1977). The solubility of barite in hydrothermal solution having more than 1 molal NaCl concentration increases with increasing temperature, while a solubility maximum exists in the solution with NaCl concentration less than ca. 0.2 molal (Blount, 1977). 

Quartz or amorphous silica tends to precipitate from the solution having relatively high temperature and low flow rate and under high A/M condition. 

In the X-ray powder diffraction patterns of the composites, the disappearance of the broad band centered at 22 °20, typical of amorphous silica, indicates that the zeolitisation of the mineral fraction of the parent composite was complete. In no diffraction pattern any sign of crystallised chitosan could be found. The two methods in which the silica-polymer beads were extracted from the aluminate solution after impregnation (methods A and C) allowed the formation of the expected zeolite X, with traces of gismondine in the case of the method C. The method B, in which excess aluminate solution was present during the hydrothermal treatment, resulted in the formation of zeolite A. 

Two principal factors drive reaction in the evaporating fluid. First, the loss of solvent concentrates the species in solution, causing the saturation states of many minerals to increase. The precipitation of amorphous silica, for example,... 

The amorphous silica matrixes are porous network structures that allow other species to penetrate [44]. Thus, the doped dye molecules have the ability to react with targets. However, the reaction kinetics is significantly different than the molecules in a bulk solution. In the synthesis of DDSNs, commonly used silicon alkoxides including TEOS and TMOS have tetrahedron structures, which allow compact polycondensation. As a result, the developed silica nanomatrix can be very dense. The small pore sizes provide limited and narrow pathways for other species to diffuse into the silica matrix. 

Carbon tetrachloride slowly reacts with hydrogen sulfide in aqueous solution yielding carbon dioxide via the intermediate carbon disulfide. However, in the presence of two micaceous minerals (biotite and vermiculite) and amorphous silica, the rate transformation increases. At 25 °C and a hydrogen sulfide concentration of 0.001 M, the half-lives of carbon tetrachloride were calculated to be 2,600, 160, and 50 d for the silica, vermiculite, and biotite studies, respectively. In all three studies, the major transformation pathway is the formation of carbon disulfide. This compound is... 

Figure 8.22 shows the effect of silica in the system (aqueous solution saturated with respect to amorphous silica) under reducing alkaline conditions, the metasilicate FeSi03 forms at the expense of magnetite through the equilibrium... 

It has been previously reported [21, 22] that metal colloids are formed by radiochemical reactions in water/alcohol solutions, in which the reduction of metal salts takes place by solvated electrons and free radicals produced under UV or y-ray irradiation. Ichikawa et al. have applied this photoreduction method to the surface-mediated reaction of metallic ions and succeeded in synthesizing metal/aUoy nanowires in the constrained cavities of mesoporous supports such as FSM-16 and MCM-41 [18-20, 23-25]. The adsorbed water and alcohol work not only as solvents in the nanoscale silica void space but also as a source of reducing species for metallic ions to metals under UV-vis and y-ray [11, 18, 19] irradiation. The results indicate the dense formation of Pt nanowires inside the charmels of mesoporous supports, such as FSM-16, which act as the templates. In fact, no any Pt wire is observed on the external surface of FSM-16 or amorphous silica surface. Short wires, 10 nm long, are also observed as a minor species in the samples in the initial stage of UV and y-ray irradiation. 

Fig. 3. Plot of logio normalized ion-exchange rate at amorphous silica saturation vs. the amount of excess alkalis (Na, K), denoted by the molar ratio XAlk/(Al + IVB + FeT). All boron is treated as four-fold coordinated (IVB) and total iron (FeT) is regarded as ferric. The ion-exchange rate subtracts out the contribution of alkalis to solution from matrix dissolution. As the amount of excess alkali increases, the ion-exchange rate increases. This increase in rate reflects the increasing amount of alkalis in non-bridging oxygen (NBO) configurations. Error bars represent 2-

In 1968, Stober et al. (18) reported that, under basic conditions, the hydrolytic reaction of tetraethoxysilane (TEOS) in alcoholic solutions can be controlled to produce monodisperse spherical particles of amorphous silica. Details of this silicon alkoxide sol-gel process, based on homogeneous alcoholic solutions, are presented in Chapter 2.1. The first attempt to extend the alkoxide sol-gel process to microemul-sion systems was reported by Yanagi et al. in 1986 (19). Since then, additional contributions have appeared (20-53), as summarized in Table 2.2.1. In the microe-mulsion-mediated sol-gel process, the microheterogeneous nature (i.e., the polar-nonpolar character) of the microemulsion fluid phase permits the simultaneous solubilization of the relatively hydrophobic alkoxide precursor and the reactant water molecules. The alkoxide molecules encounter water molecules in the polar domains of the microemulsions, and, as illustrated schematically in Figure 2.2.1, the resulting hydrolysis and condensation reactions can lead to the formation of nanosize silica particles. 

The solution in equilibrium with amorphous silica at ordinary temperatures contains monomeric monosilicic acid, Si(OH)4. The acid is dibasic, dissociating in two steps (36) ... 

Solutions of monosilicic acid may also be obtained by careful hydrolysis of tetrahalo-, tetraalkoxy-, or tetraacyloxysilanes by electrolysis or acidification of alkali silicate solutions or by ion exchange (qv). By operating under carefully controlled conditions at low temperature and pH, solutions may be obtained that remain supersaturated with respect to amorphous silica for hours at temperatures near 0°C. Eventually, however, polymerization reactions involving the formation of siloxane linkages occur, leading ultimately to the formation of colloidal particles and further aggregation or gel... 

Properties. Colloidal silica is a stable aqueous dispersion or sol of discrete amorphous silica particles having diameters of 1 to 100 nm. Silica sols do not gel or setde out of solution for at least several years of storage. Aqueous sols that contain up to 50% silica have been developed (30,31). Particle sizes of approximately 130 nm in diameter are possible (32), but slowly setde out of solution. 

In the absence of a suitable solid phase for deposition and in supersaturated solutions of pH values from 7 to 10, monosilicic acid polymerizes to form discrete particles. Electrostatic repulsion of the particles prevents aggregation if the concentration of electrolyte is below ca 0.2 N. The particle size that can be attained is dependent on the temperature. Particle size increases significantly with increasing temperature. For example, particles of 4—8 nm in diameter are obtained at 50—100°C, whereas particles of up to 150 nm in diameter are formed at 350°C in an autoclave. However, the size of the particles obtained in an autoclave is limited by the conversion of amorphous silica to quartz at high temperatures. Particle size influences the stability of the sol because particles <7 nm in diameter tend to grow spontaneously in storage, which may affect the sol properties. However, sols can be stabilized by the addition of sufficient alkali (1,33). 

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