An experimental study of sepiolite precipitation and dissolution rates and mechanisms at 60 C

<p>Clay mineral precipitation in geothermal systems can be detrimental for geothermal energy harvesting and subsurface CO<sub>2</sub> storage efforts. One of the clays that precipitate in such systems is sepiolite [1]. Sepiolite precipitation of can lead to a loss in host rock permeability and a decrease in the aqueous Mg concentrations, thereby hindering Mg-carbonate formation and thus limiting CO<sub>2</sub> mineral storage. Conversely, sepiolite dissolution can provide Mg and thus enhance CO<sub>2</sub>­ mineral storage. Water rock interactions in such systems occur often close to equilibrium and temporal and regional changes in the saturation state of sepiolite can affect both the dissolution and precipitation rate of this mineral. Hence, to improve quantitative models on CO<sub>2</sub> mineral storage and hydrothermal energy harvesting scenarios and to gain an increased understanding in clay mineral dissolution/precipitation mechanisms in general we have measured sepiolite dissolution and precipitation rates as a function of its saturation state.</p><p>A series of mixed flow precipitation and dissolution experiments were performed at 60 °C with varying flow rates and saturation indices ranging from -8 to 18. All experiments were performed in the presence of pure, crystalline sepiolite seeds. The reactors were placed in a shaker water bath to ensure that they were well mixed and at constant temperature. Sepiolite precipitation rates were calculated from the difference in Mg and Si concentration between the inlet and outlet solution.  The solid phase was recovered from a mixed flow experiment that ran for over 3 months. The resulting solid contained approximately 40 w.t% newly precipitated material. The precipitation of crystalline sepiolite was confirmed from the stoichiometric Si/Mg depletion in solution, from Energy-dispersive X-ray spectroscopy and from X-ray diffraction spectra of the solid phase. Precipitation/dissolution rates varied between 10<sup>-16.3</sup>0 and 10<sup>-18.77</sup> mol/cm<sup>2</sup>/s depending on the affinity of the precipitation reaction. The results show that the precipitation and dissolution rates of sepiolite depend linearly on the affinity of the precipitation/dissolution reaction, with dissolution and precipitation rates increasing at higher reaction affinities. The linearity of sepiolite precipitation and dissolution further suggests that dissolution is mechanistically the reverse of precipitation and that both processes are consistent with transition state theory. The relatively high precipitation rates at increased Mg/Si concentrations imply that, under basic conditions, sepiolite precipitation could be detrimental to the permeability of the host rock and the availability of Mg during CO<sub>2</sub> sequestration.</p><p> </p><p>[1] Oelkers, E. H., et al. "Using stable Mg isotope signatures to assess the fate of magnesium during the in-situ mineralisation of CO<sub>2</sub> and H<sub>2</sub>S at the CarbFix site in SW-Iceland." GCA 245 (2019): 542-555.</p>