USING NUCLEATION THEORY TO UNDERSTAND THE DISSOLUTION KINETICS OF VITREOUS AND BIOGENIC SILICA

Realization that silicon is a biological element has renewed interest in the demineralization kinetics of vitreous and biogenic silicas. In some settings, these materials constitute a significant and reactive pool of silicon in the global biogeochemical system. Silica reactivity is complex owing to rate-enhancing effects (up to 100 fold) of the major solutes in natural waters (Ca, Mg, Na, K) and the variable levels of dissolved silica that present a broad range of undersaturations, i.e. chemical driving force to dissolution. We study this interplay to arrive at a model that quantifies dissolution and growth within a mechanism-based model that is based in surface energy across the continuum of chemical driving force.

Experimental measurements of vitreous silica dissolution kinetics were conducted at intermediate to high undersaturations (C/Ce = 0.05-0.4) at 150°C and circumneutral pH. Rates measured in solutions containing CaCl2 have a strong exponential dependence upon degree of undersaturation, just as recently reported for quartz (Dove et al., 2005, PNAS). However, the crystal-based nucleation models that explain quartz behavior cannot be applied to amorphous materials. We hypothesize the solutes enhance amorphous silica dissolution rates through analogous processes that increase Si detachment rate and/or the population of Si—O bonds susceptible to dissolution.

By defining a statistical volume of the reacting unit, we find that classical polynuclear theory explains the dependence of dissolution on chemical potential and quantifies the surface energy of amorphous silica in the absence and presence of solutes. The polynuclear model also 1) predicts kinetic behavior reported for biogenic and colloidal silicas and 2) predicts both the growth and dissolution rates of colloidal silica. These findings reiterate the idea that growth and dissolution can be understood as symmetric, reversed processes, just as determined recently for quartz and silicates.