An experimental study of sepiolite dissolution rates and mechanisms at 25 °C

Abstract

The steady-state dissolution rates of sepiolite were measured as a function of pH from mixed-flow reactor experiments at 25 o C in both citrate bearing and citrate-free aqueous solutions. Dissolution at pH ≤5.5 and pH ≥9.18 was found to be non-stoichiometric, with relatively more Mg being released at low pH, while Si was preferentially released at high pH. The steady-state dissolution rate of sepiolite at 25 o C, based on Si release rates at far from equilibrium conditions, r + in mol/cm 2 /s at pH ≥4.44 can be described bywhere a i refers to the activity of the subscripted aqueous species. This rate equation is based on a dissolution mechanism by which interstitial Mg is exchanged for 2 H + , followed by the rate limiting release of partially detached Si-tetrahedra. At pH ≤4.44 and in citrate-free aqueous solutions, sepiolite dissolution rates based on Si release are similar to those measured for amorphous SiO 2 despite the fact that the bulk fluid phase was undersaturated with respect to this solid. In contrast, in citrate bearing aqueous solutions, the Si release rate increases with decreasing pH following a trend consistent with the corresponding higher pH values. This difference is attributed to the increase of amorphous SiO 2 dissolution rates in citrate bearing aqueous fluids, preventing the formation of substantial SiO 2 -rich surface layers. To further examine the extent of nonstoichiometric dissolution and its implications on solid phase transformations, batch experiments were performed in citrate bearing aqueous solutions at pH 2.57, 3.40 and 4.31. During these longer term experiments the fluid phase attained saturation with respect to amorphous SiO 2 . From mass balance calculations, Energy-Dispersive X-ray spectroscopy (EDS) and X-Ray powder Diffraction (XRD), it can be inferred that after 688 hours, 98% of the Mg was removed from the solid structure during pH 2.57 batch experiments, while SiO 2 was retained in an amorphous solid phase, demonstrating the independence of the Mg compared to Si release to the aqueous solution.

A rate law for sepiolite growth at ambient temperatures and its implications for early lacustrine diagenesis

“…This analysis yielded a Mg to Si ratio of 0.57, and trace Al (Al to Si = 0.05:1). This Mg to Si ratio is lower than the ideal ratio (0.67) but consistent with the ratio (0.56) reported by Mulders and Oelkers (2020). Deviations from the ideal ratio could, in principle, be caused by the presence of trace (i.e., less than the several percent detection limit of XRD) amounts of quartz (or another silicabearing mineral), but SEM images strongly suggest that the samples are entirely composed of fibrous sepiolite (Fig.…”

“…For example, it is intriguing to note that our measured rates, which span 3 orders of magnitude from 10 -11 to 10 -14 mol/m 2 /s (Fig. 6), overlap the sepiolite dissolution rates determined by Mulders et al (2020) at pH = 9.2 and 9.7 (~10 -13 mol/m 2 /s), although it should also be noted that the substantial saturation state dependence causes our rates drop to ~10 -16 mol/m 2 /s and -15 mol/m 2 /s as equilibrium is approached (Fig. 9a), and the dependence of sepiolite dissolution rates on the approach to equilibrium is not constrained.…”

An experimental study of sepiolite dissolution and growth rates as function of the aqueous solution saturation state at 60 °C

“…The Valdemore, Spain sepiolite sample used in this study was obtained from the Clay Mineral Society. Preparation of the sample has previously been described in Mulders and Oelkers (2020). The BET surface of the sample was measured using the Brunauer-Emmett Teller (BET) method (Brunauer et al, 1938) and was determined to be 214.0 ±10% m 2 /gm from N 2 adsorption measurements using a DIN-ISO 9277.…”

“…Such processes could lower the efficiency of CO 2 storage and decrease the harvesting potential of geothermal energy. Conversely, clay mineral dissolution can provide the divalent metals necessary for CO 2 sequestration (Harrison et al, 2012;Mulders and Oelkers, 2020;Thom et al, 2013).…”

“…Several studies have reported on the nucleation, growth and dissolution of sepiolite (Arizaleta et al, 2020;Baldermann et al, 2018;Mulders et al, 2018;Mulders and Oelkers, 2020;Tosca and Masterson, 2014;Tutolo and Tosca, 2018;Wollast et al, 1968), a 2:1 layered Mg-phyllosilicate (Mg 4 Si 6 O 15 (OH) 2 *6H 2 O) characterized by its continuous inverted tetrahedral sheets and discontinuous octahedral sheets. Sepiolite is found in volcanic sediments, silicious lake sediments, and it has been inferred to precipitate in the subsurface carbon storage site near the Hellisheiði geothermal powerplant in Iceland (Bowles et al, 1971;Gislason et al, 2010;Perraki and Orfanoudaki, 2008;Stoessell and Hay, 1978;Tutolo and Tosca, 2018).…”