Investigation of the pH dependence of the kinetics of quartz dissolution at 25 °C

Abstract

The dissolution kinetics of two samples of commercial ground Fontainebleau sand consisting of > 99.6% quartz are studied at 25 °C in constant ionic strength conditions and in the pH range 4.5–10.5. The strong pH dependence of the reaction rate is attributed to changes in the surface speciation of quartz. The ionization of the surface silanol groups is investigated using potentiometric methods and the results are compared with literature data using a simple surface speciation model that accounts for the effects of the electrical double layer on the hydrogen ion activity. The surface speciation model produces titration curves in good agreement with the experimental data and offers an interpretation of the relationship between the density of ionized groups, [SiO–(s)], and pH.The rate constant, k+, for the dissolution reaction: SiO2(solid)+ 2H2O → Si(OH)4(aq) is determined from 25 experiments as: log k+=– 14.547 ± 0.162 + 0.361 ± 0.022 pH at 25 °C, ionic strength of 0.010 mol dm–3 and pH in the range 4.5–10.5. The constant, k+, is in units of mol m–2 s–1. The results are compared with the available literature data at 25 °C. Although there is general agreement between the experimental data and the surface speciation model concerning the pH dependence of the dissolution rates, the variation between the absolute values of the rate constants from different quartz samples is large. The pH dependence observed in this work and that of Brady and Walther, is consistent with a pK for the dissociation of surface silanol groups and surface density of terminal silanol groups of ca. 7.5 and 11 nm–2, respectively. A slope of ca. 0.33 obtained from the regression of pH and log k+ is predicted from the surface model. Similar gradients have been observed in other studies using quartz and also for a range of other silicate minerals for solution pH 7.