Removal and Deposition of Silica from Water

Since silica exists in water both in soluble and colloidal forms, the mechanisms of removal and deposition are quite different, and often must be considered separately. 

Since it is not possible here to review the large volume of literature on water treatment, only general principles are considered, along with some examples. 

In considering the phenomenon of precipitation it should be pointed out that two entirely different mechanisms may be involved  

Nucleation. Most discussions of precipitation concentrate on the question of how particles are initially formed. The mechanism has been considered in detail, for example, by Nielsen (240) and Walton (174) in their treatises on precipitation. The formation of amorphous silica particles by polymerization of Si(OH) involves some aspects of the theory of nucleation. This is dealt with in Chapter 3. 

Coagulation. In this case, a precipitate is formed by coagulation of already existing colloidal particles. It does not involve nucleation and is often considered as a separate subject. 

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The first group comprises both dry absorbent (molecular sieves, activoted olumino and silica gel) and continuous absorption processes. The dry obsorbent processes will remove virtually oil of the free ond dissolve water from the condensate streom, however they were rejected on the basis that salt deposits in the absorbed water could contomlnote the porous moteriol during regeneration. 

Another series of catalysts represented in Figure 129 was activated by a similar two-step process, except that the chromium was deposited onto the calcined silica from an aqueous solution. The silica was allowed to soak in aqueous CrCh solution overnight before the water was removed by evaporation, and the sample was finally calcined in air at 500 °C. Polymers were made, and the measured LCB levels are represented by the lower line in Figure 129. Again LCB levels were found to increase with activation temperature, but this method produced the lowest LCB levels of the three series in Figure 129. A moment s reflection makes it clear why this should be so. All these samples were rehydroxylated by the water... 

Kondo et al. reported a novel fabrication method of polymer nanotubes by combination of an alternate layer-by-layer (LbL) assembly of PLLA and PDLA and the siliea template method. Silica nanoparticles with a diameter of 300 nm were alternately immersed in aeetonitrile solutions of PLLA and PDLA at 50 °C. The immersion process was performed for 10 cycles to deposit 10 double layers of PLLA and PDLA. The resulting partieles were then treated with 2.3% aqueous hydrofluoric acid to remove the siliea core. The hollow eapsules have a spherical shape with a diameter of 320 nm and a shell thiekness of approximately 60 nm. Tubular assemblies with an average diameter of 300 nm and lengths of 2-5 pm are obtained by evaporating water at ambient temperature from a water dispersion of the hollow eapsules on a polyethylene terephthalate (PET) substrate. 

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