Precipitation Pathways of Amorphous Silica

Amorphous silica is one of the most common phases to precipitate from geothermal fluids. It precipitates by self-assembly of monomeric silica (H4SiO4) via heterogeneous and homogeneous nucleation and subsequent growth of nuclei by addition of dissolved silica. The mechanism and the factors controlling the individual steps of silica precipitation have been studied in numerous laboratory experiments over the last decades and are, despite their complexity, well understood. However, due to the higher complexity of natural geothermal fluids (e.g high fluid flow rates, microbial activity or complex fluid chemistries), these findings cannot be directly applied to the study of silica scaling inside geothermal power plants and silica sinters around hot springs. In the first part of this thesis we present the results from the first ever time-resolved study of silica precipitation inside in-use geothermal pipelines. Silica scales formed primarily via heterogeneous nucleation on steel surfaces, resulting in a silica layer rapidly covering these surfaces. This pathway of silica deposition was controlled by surface roughness, total silica concentration and temperature and allowed the deposition of up to 1 g of silica per day and m2. Homogeneous nucleation also occurred and lead to the formation of silica microspheres which were deposited preferentially into depressions and along edges or were aggregated to fan- and ridge-shaped structures growing towards the flow, depending on the fluid flow regime. While the 3D structures could result in more turbulent flow, decreasing the flow rate, the formation of the silica layer could potentially even be beneficial for the operation of a geothermal power plant as it passivates the surface against corrosion. For the second part of this thesis, we studied the interaction of a silica solution with a protein (=lysozyme), during which hybrid composites were formed. By investigating these final products in detail, we determined that, depending on the timing of the silica-lysozyme interactions (during or after silica polymerisation) and the ratio of silica-to-protein, the resulting composites showed different structures and surface properties. This is of interest for biomineralisation as it elucidates how biomolecules interact with dissolved silica and how microorganisms can control this process.