THE DISSOLUTION KINETICS OF AMORPHOUS SILICA INTO SODIUM CHLORIDE SOLUTIONS: EFFECTS OF TEMPERATURE AND IONIC STRENGTH

Abstract

The kinetics of amorphous silica, SiO2 (am), dissolution was quantified in deionized water and NaCl solutions. By using two sources of pure SiO2 glass (fused purified quartz and pyrolyzed SiCl4), rates were measured at 40°C to 250°C by applying three types of reactor systems to assess kinetic behavior over the full temperature range. Dissolution rates of the two materials are similar within experimental error. Absolute rates of amorphous silica dissolution in deionized water exhibit an experimental activation energy, Ea,xp, of 81.9 +/- 3.0 and 76.4 +/- 6.6 kJ/mol for the fused quartz and pyrolyzed silica, respectively. These values are similar to estimates for quartz within experimental errors. Absolute dissolution rates of SiO2 (am) in deionized water are ~10x faster compared to quartz. Amorphous silica dissolution rates are significantly enhanced with the introduction of NaCl to near-neutral pH solutions such that 0.05 molal sodium ion enhances rates by 21x compared to deionized water. The new kinetic data are combined with previous measurements of SiO2(am) dissolution rates in 'pure' water to evaluate the temperature dependence of dissolution. The comprehensive data set spans 25°C to 250°C and yields the Arrhenius expression log k+ = 0.82191 - 3892.3/T(K) to give an apparent activation energy for dissolution of 74.5 +/- 1.4 kJ/mol. These findings step toward the larger goal of understanding silica polymorph reactivity in the complex fluid compositions of natural systems.