Kinetics of silica oligomerization and nanocolloid formation as a function of pH and ionic strength at 25°C

A study of sodium silicate in aqueous solution and sorbed by synthetic magnetite using in situ ATR-FTIR spectroscopy

“…In the modelling the precipitation of amorphous silica under thermodynamic conditions was included. In this context it should be mentioned that the kinetics of the formation of silica precipitate is rather slow especially at high pH, low ionic strength, and low silica concentration, as shown by Icopini et al [36]. The half-life of molybdate-reactive silica was >2700 h at a silica concentration of 4.2 mM at pH 8-9 and silicate concentration of 12.5 mM at pH 11 with ionic strength of 0.01 M. Starting at pH 11, would therefore imply that the formation of amorphous silica precipitate during the period of solution spectra collection or sorption can be ignored.…”

“…The half-life of molybdate-reactive silica was >2700 h at a silica concentration of 4.2 mM at pH 8-9 and silicate concentration of 12.5 mM at pH 11 with ionic strength of 0.01 M. Starting at pH 11, would therefore imply that the formation of amorphous silica precipitate during the period of solution spectra collection or sorption can be ignored. The formation of nanocolloidal silica is strongly limited at high pH although at pH 7 and 10 mM silicate concentration about 10% of the silicic acid could be formed to nanocolloidal silica after 1 h [36], since the solution becomes supersaturated when pH is lowered [13]. Equilibrium protonation/deprotonation parameters and the formation constants are shown in Table 1 along with the chemical formula of each species and the calculated equilibrium distributions are plotted in Fig.…”

Nucleation, Growth, and Aggregation of Mineral Phases: Mechanisms and Kinetic Controls

“…Microscopic observations of field-derived samples indicate that the first silica precipitates are made up of tens of nanometer-sized spheroids. These nuclei are bigger than the critical nucleus estimated for experimental silica precipitation studies (∼3 nm, Icopini et al, 2005;Tobler et al, 2006) and thus it is asserted that the observed particles in the field-derived samples represent already aggregates of smaller nuclei. However, it is asserted that once such initial nuclei have formed this will be followed by aggregation, growth or Ostwald ripening.…”

“…(i.e., Ostwald ripening) followed again by growth to form either large nanoparticles (several hundred nm up to 1 µm) or aggregation (e.g., Iler, 1979Iler, , 1980Perry, 2003;Benning et al, 2004a, b andIcopini et al, 2005; see also Chap. 1 (Eq.…”

“…Yet, due to the infinite resupply of aqueous monomeric silica in the geothermal solution, nucleation will be a continuous process. Many experimental studies -both with purely inorganic or mixed organic inorganic solutions -have shown that once silica nanoparticles formed in solution, their growth, ripening or aggregation behavior is strongly affected by pH, ionic strength, temperature (Iler, 1980;Rothbaum and Wilson, 1977;Makrides et al, 1980;Weres et al, 1981;Icopini et al, 2005, Tobler et al, 2006, surfactant type and concentration (e.g., Boukari et al, 1997;Green et al, 2003). Furthermore, the structure and complexity of the formed aggregates can be described by applying concepts of fractal geometry (Pfeifer and Obert, 1989;Lin et al, 1990) with the condensation of primary monomers into nanoparticles or aggregates producing fractal objects with a fractal dimension D f .…”