Thermochemical and kinetic investigations of amorphous silica dissolution in NaOH solutions

Dissolution of amorphous silica in 4, 8 and 11 mass% of sodium hydroxide solutions was followed by microcalorimetry at temperature between 353.15 and 403.15 K. Decreasing the Na2O/SiO2 molar ratio leads to the successive formation of the following dissolved entities Si4O11Na6, Si2O5Na2, Si3O7Na2 and Si5O11Na2. Their formation enthalpies were determined at 393.15 K as −5640.8, −2492.2, −3401.8 and −5214.7 kJ mol−1, respectively. A kinetic investigation showed that dissolution cannot be described by one-step controlled process. Isoconversional model allows determining a complex variation in the activation energy of dissolution in the range 6–21 kJ mol−1.