Biosilicification and silica nanoparticle formation occur in many modem terrestrial environments and they also played an important role in ancient geological settings. This thesis presents results from (i) field studies in Icelandic geothermal waters that aimed at quantifying the parameters that control the growth rate and texture of sinters and the diversity and silicification of associated microbial communities and (ii) lab studies that focussed on the\ud kinetics and mechanisms of silica nanoparticle forination under conditions mimicking natural geothermal environments.\ud \ud The analysis of growth rates and textures of sinters from five geochemically very different Icelandic geothermal areas showed that the inorganic silica precipitation rate was strongly influenced by temperature, pH, ionic strength, and silica concentration. In addition, the presence\ud of thick biofilms seemed to have aided the precipitation process by simply providing "sticky" surfaces. In turn, the structural and textural development of sinters was affected by the precipitation rate and mechanism (subaqueously and/or subaerially) as well as the presence and\ud absence of microbial communities. As a result, porous, subaequeouss inters developed at sites with medium to high sinter growth rates and low microbial activity. Conversely, dense, heterogeneouss inters formed in geothermal waters characterized by low precipitation rates and extensive biofilms. With time these biofilms became fully silicified and well preserved within the sinter edifices. The diversity of microbial communities in hot spring environments appeared to be directly controlled by the physico-chernical conditions of the geothermal waters (i. e., T,pH, salinity and sinter growth rate) and the most dominant phylotypes were related to Aquificae, Deinococci and y-Proteobacteria.\ud \ud The rates and mechanisms of the initial steps of silica polymerisation and silica nanoparticle formation were quantified in-situ and time-resolved using synchrotron-based small angle x-ray scattering (SAXS). The experiments were carried out in near neutral pH solutions with initial\ud Si02 between 640 - 1600 ppm, ionic strength of 0.02 - 0.22 M, and added organics (glucose, glutarnic acid, xanthan gum). The polymerization reactions were induced either by neutralising a high pH solution or by rapid cooling of a supersaturatedh ot silica solution. From the analysis\ud of the time-resolved SAXS data, a kinetic model for the nucleation and growth of silica nanoparticles was derived suggesting a 3-stage process: (1) homogeneous nucleation of critical nuclei (I -2 run; depending on the concentration regimes), (2) 3-dimensional, surface-controlled particle growth following 1st order reaction kinetics and (3) Ostwald ripening and particle aggregation. At the end of this 3-stage process, regardless of the tested silica concentration, ionic strength or added organics, the final particle diameter was about 8nm characterised by open, polymeric (i. e., mass fractal) structures. The kinetics of particle growth were unaffected by the two different methods to induce silica polymerisation (pH-drop vs. T-drop) however, the growth processes proceeded substantially slower if silica polymerisation was induced by fast\ud cooling as opposed to pH-drop. In contrast, the addition of organics did not affect the reaction rates.\ud \ud The nucleation and growth of silica nanoparticles under constant re-supply Of fresh silica solution (i. e., hot springs) was simulated using a flow-through geothermal simulator system. The effect of silica concentration ([Si02D, ionic strength (IS), temperature and organic additives on the size and polydispersity of silica nanoparticles was quantified. VVhile the applied increase\ud in IS did not affect the size (30 - 35 nm) and polydispersity (± 9 nm) observed at 58 C, an increase in [Si02] notably enhanced silica polymerisation and also resulted in slightly smaller particle sizes. The biggest effect was observed with a decrease in temperature (58 to 33 C) or the addition of glucose: in both cases particle growth was restricted to sizes below 20 mn. Conversely, the addition of xanthan gum induced the development of a thin silica-rich film that enhanced silica aggregation
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