Colloidal silica is one of the most undesirable deposits in industrial water treatment. It is rare, compared to other more common mineral scales, such as CaCO3, but its formation and deposition onto critical equipment surfaces can be problematic. Control approaches usually include: (a) removal of "soluble" and colloidal silica before entering the water system, and (b) use of chemical scale inhibitors/dispersants. Nevertheless, silica scale inhibition is not easy because of the chemical nature of colloidal silica. Silica is a random, three-dimensional polymer that forms by propagation of Si-O bonds, which form by polycondensation of monosilicic acid. The amorphous nature of silica precludes use of the "traditional" scale inhibition approaches (usually phosphonate additives and polyacrylate-based polymers) that are principally applied for mineral scales. Silica formation is further complicated by the presence of metal hydroxides (commonly iron, aluminum, and magnesium hydroxides). The scope of the present study is to explore the effects of the presence of such hydroxides in a process system on silica formation.
The polycondensation of silicate to form colloidal silica is a well-known process. Silica formation takes place through an SN2-like mechanism that involves an attack of a mono-deprotonated silicic acid molecule on a fully protonated one. Thus, monomeric silicate species produce silicate dimers, and oligomers, and eventually form colloidal silica particles.1 Nevertheless, this straightforward silica chemistry can be profoundly affected by the presence of certain metal cations, such as calcium,2 magnesium,3 aluminum,4 and iron.5 When such cations are present in a process water they enhance the rate of polymerization of silicate ions and induce the formation of metal silicate precipitates.
The effects of soluble Mg2+ and Al3+ species on the autocondensation of silicate have been recently studied by our group.6,7 Notably, these effects are strongly pH-dependent, and are maximized at the pH values of ∼10 (for Mg2+) and ∼ 7 (for Al3+). These are the pH regimes that also induce in situ formation of Mg(OH)2 and Al(OH)3. Hence, the involvement of Mg(OH)2 or Al(OH)3 in the polymerization of silicate cannot be ruled out.