Interaction of Freshly Precipitated Silica Gel with Aqueous Silicic Acid Solutions under Ambient and Near Neutral pH-conditions: A Detailed Analysis of Linear Rate Law

Abstract

Interaction of freshly precipitated silica gel with aqueous solutions was studied at laboratory batch experiments under ambient and near neutral pH-conditions. The overall process showed excellent reversibility: gel growth could be considered as an opposite process to dissolution and a linear rate law could be applied to experimental data. Depending on the used rate law form, the resulting rate constants were sensitive to errors in parameters/variables such as gel surface area, equilibrium constants, Si-fluxes, and reaction quotients. The application of an Integrated Exponential Model appeared to be the best approach for dissolution data evaluation. It yielded the rate constants k dissol * (4.50 ± 0.68) 9 10 -12 and k growth * (2.58 ± 0.39) 9 10 -9 mol m -2 s -1 for zero ionic strength. In contrast, a Differential Model gave best results for growth data modeling. It yielded the rate constants k dissol * (1.14 ± 0.44) 9 10 -11 and k growth * (6.08 ± 2.37) 9 10 -9 mol m -2 s -1 for higher ionic strength (I * 0.04 to 0.11 mol L -1 ). The found silica gel solubility at zero ionic strength was somewhat lower than the generally accepted value. Based on the 298 K ðI¼0Þ SS ¼ 10 À2:754AE0:017 and 298 K ðI¼0Þ DBP ¼ 10 À2:728AE0:003 ; standard Gibbs free energy of silica gel formation was calculated as DG 0 f ð298 KÞ$ À 850,463 AE 98 J mol À1 and -850,318 ± 20 J mol -1 , respectively. Activation energies for silica gel dissolution and growth were determined as E dissol A $62:0 AE 3:2 kJ mol À1 and E growth A $48:8 AE 4:4 kJ mol À1 ; respectively. An universal value for growth of any silica polymorph, E growth A $37:4 AE 9:4 kJ mol À1 ; is not consistent with the value for silica gel growth, which questions the hypothesis about one unique activated complex controlling the silica polymorph growth.

Structure of Silica Polymers and Reaction Mechanism for Formation of Silica-Rich Precipitated Phases in Direct Aqueous Carbon Mineralization

“…This simulation methodology then predicted the formation of crystalline silica polymorphs chalcedony, followed by cristobalite, then silica gel (SiO 2 ) (a precursor to a SiO 2 ), and then finally a SiO 2 , all in conjunction with the formation of magnesite. Silica phases constrained to amorphous solids (amorphous silica and silica gel) are considered appropriate because of the delayed formation of crystalline phases such as chalcedony and quartz. , …”

“…Silica phases constrained to amorphous solids (amorphous silica and silica gel) are considered appropriate because of the delayed formation of crystalline phases such as chalcedony and quartz. 50,51 3.5. Characterization of HAL7 and the Carbonation Product To Identify Precipitated Phases during Direct Aqueous Carbonation.…”