DISSOLUTION KINETICS OF SILICA GLASS AS A FUNCTION OF PH BETWEEN 40 AND 85-DEGREES-C

Anhydrous vitreous silica glass was reacted in solutions buffered from pH 1.1 to pH 10.9 between 40 and 85-degrees-C for up to 1500 h. The initial release of silicon to solution is proportional to the square root of time, consistent with a surface diffusion process. At later times, the release becomes linear with time, indicating surface reaction control. The reproducibility of square root of time release rates with previously used glass can be explained within the context of current models of vitreous silica structure. At 65-degrees-C, above pH 6.5 the log diffusional rate constants increase with pH such that delta log k/deltapH = 0.4, while below pH 6.5 the diffusional rate constants are nearly pH-independent. The surface reaction release rate constants show a similar pH dependence with delta log k/deltapH = 0.5 above pH 7, while below pH 7 the rate constants are nearly pH-independent. The activation energy of surface reaction is 22.7 +/- 3.7 kcal mol-1 at pH 4 for vitreous silica. This value is consistent, but slightly higher than that of Rimstidt and Barnes [Geochim. Cosmochim. Acta 44 (1980) 16831 for amorphous silica.