Dissolution Mechanism of Serpentine and the Re-Precipitation of Silica Under Constant pH Conditions

The scientific evidence for climate change being the result of increasing levels of carbon dioxide and other greenhouse gases in the atmosphere is overwhelming. There is genuine impetus to reduce the net level of emissions of carbon dioxide and other greenhouse gases into the receiving environment. Perhaps nature has already provided us with the basis of a process to tackle this problem. Mimicking the natural weathering processes, in which calcium or magnesium silicates are transformed into carbonates via reaction with CO₂ gas and/or aqueous CO₂, is the basic premise upon which mineral carbonation technology is based. To make the process commercially viable, the magnesium silicate rocks must initially dissolve in aqueous phase, to release Mg ions. Dissolution of Mg is the rate-determining step in the aqueous carbonation of thermally activated serpentine. Understanding the factors influencing the dissolution of Mg-bearing silicates is especially important, since it has significant implications for the mineral carbonation process.

The presence of Si along with Mg added complexity to the dissolution process. The dissolution of silica-containing rocks in water results in the accumulation of silica in water as suspended particles, in a colloidal or polymeric state, and as silicic acids or silicate ions. The polymeric silica can precipitate, under certain conditions, potentially providing a diffusion barrier that hinders further dissolution of Mg if deposited on the surface of the partially leached particles

In this work, the dissolution of thermally activated serpentine at various pH values and solid to liquid ratios with a particular focus on Si dissolution and re-precipitation was studied. X-ray powder diffraction (XRD), scanning electron microscopy (SEM) along with EDS mapping and ICP-OES have been used to characterize the leached particles and liquid phase(s) during and after dissolution of the thermally activated serpentine in acidic solutions. The “molybdosilicate method” [1] has been employed to study silicon species (i.e. monomeric Si and polymeric Si) in the solution and to determine the concentration of the molybdate-reactive silica.

Dissolution experiments were performed in an open system at room temperature and pressure for 7 h at different pH levels and solid to liquid (S/L) ratios. For each pH level studied, buffer solutions were used to maintain a constant pH during the course of dissolution. Re-precipitation of silica was first studied by measuring the concentration of Si in the liquid phase over long period (7 h). This was followed by SEM imaging of the surface of an inert substrate that was added to the solution. The presence of silica deposits on the surface was confirmed by SEM-EDS. Further experiments have shown and confirmed re-precipitation of silica at certain pH and S/L ratios.

References:

1. Standard methods for the examination of water and wastewater, Part 4000 inorganic nonmetalic constituents. Vol. 2. 1915: American Public Health Association.