Mulders, JJPA; Oelkers, EH; (2020) An experimental study of sepiolite dissolution rates and mechanisms at 25 °C. Geochimica et Cosmochimica Acta , 270 pp. 296-312. 10.1016/j.gca.2019.11.026.

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Abstract

The steady-state dissolution rates of sepiolite were measured as a function of pH from mixed-flow reactor experiments at 25 °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 °C, based on Si release rates at far from equilibrium conditions, r+ in mol/cm2/s at pH ≥ 4.44 can be described by. r+=10-15.291(a2H/aMg2+)1/8 , where ai 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 2H+, 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 SiO2 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 SiO2 dissolution rates in citrate bearing aqueous fluids, preventing the formation of substantial SiO2-rich surface layers. To further examine the extent of non-stoichiometric 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 SiO2. 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 SiO2 was retained in an amorphous solid phase, demonstrating the independence of the Mg compared to Si release to the aqueous solution.

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