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期刊界 All Journals 搜尽天下杂志传播学术成果专业期刊搜索期刊信息化学术搜索

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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

Authors:	Ji?í Faimon Martin Blecha

Institution:	(1) Department of Geological Sciences, Faculty of Sciences, Masaryk University, Kotlářská 2, 611 37 Brno, Czech Republic;(2) Present address: Research Institute for Soil and Water Conservation, Žabovřeská 250, 256 27 Praha 5 – Zbraslav, Czech Republic

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) × 10⁻¹² and k _growth ∼ (2.58 ± 0.39) × 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) × 10⁻¹¹ and k _growth ∼ (6.08 ± 2.37) × 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 ${}^{298}K_{\rm SS}^{(I=0)} = 10^{-2.754\pm 0.017}$ and $^{298}K_{\rm DBP}^{(I=0)} = 10^{-2.728\pm 0.003},$ standard Gibbs free energy of silica gel formation was calculated as $\Updelta{G}_{\rm f}^0(298\,{\rm K}) \sim -\hbox{850,463} \pm 98\hbox{ J mol}^{-1}$ and −850,318 ± 20 J mol⁻¹, respectively. Activation energies for silica gel dissolution and growth were determined as $E_{\rm A}^{\rm dissol} \sim 62.0\pm 3.2\,\hbox{kJ mol}^{-1}$ and $E_{\rm A}^{\rm growth} \sim 48.8\pm 4.4\,\hbox{ kJ mol}^{-1},$ respectively. An universal value for growth of any silica polymorph, $E_{\rm A}^{\rm growth} \sim 37.4 \pm 9.4\,\hbox{kJ mol}^{-1},$ is not consistent with the value for silica gel growth, which questions the hypothesis about one unique activated complex controlling the silica polymorph growth.

Keywords:	Dissolution Growth Dynamics Error Experiment Flux Model Reversibility Silica gel
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