Abstract
The dissolution of minerals in water is typically studied on macroscopic length- and time-scales, by detecting dissolution products in bulk solution and deducing reaction rates from model assumptions. Here, we report a direct, real-time measurement of silica dissolution, by monitoring how dissolution changes the first few interfacial layers of water in contact with silica, using surface-specific spectroscopy. We obtain direct information on the dissolution kinetics of this geochemically relevant mineral. The interfacial concentration of dissolution products saturates at the level of the solubility limit of silica (~millimolar) on the surprisingly short timescale of tens of hours. The observed kinetics reveal that the dissolution rate increases substantially with progressing dissolution, suggesting that dissolution is an auto-catalytic process.
Highlights
The dissolution of minerals in water is typically studied on macroscopic length- and timescales, by detecting dissolution products in bulk solution and deducing reaction rates from model assumptions
We find that equilibration of the silica surface in contact with bulk water occurs on a timescale of several tens of hours, resulting in a steady-state interfacial concentration of ionic dissolution products corresponding to an ionic strength of ~1–10 mM
The local ionic strength at the surface is decreased if the equilibrium concentration of silicic acid exceeds the bulk ionic concentration
Summary
The dissolution of minerals in water is typically studied on macroscopic length- and timescales, by detecting dissolution products in bulk solution and deducing reaction rates from model assumptions. With bulk experiments[2,3,4,5], it has been demonstrated that silica may dissolve up to millimolar concentration of silica species through hydrolysis reactions with interfacial water, over very long (10s–100s of hours) timescales. Very little is known about the dissolution dynamics and how the kinetics affect the structure and composition of the silica/water interface including its physical properties such as the surface charge density, the Debye length, and corresponding surface potential. We find that equilibration of the silica surface in contact with bulk water occurs on a timescale of several tens of hours, resulting in a steady-state interfacial concentration of ionic dissolution products corresponding to an ionic strength of ~1–10 mM.
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