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

Abstract

Clay mineral growth can directly affect the chemistry and permeability of many natural systems, including soils, marine sediments, Earth surface waters, geothermal powerplants and carbon dioxide storage sites. Notably, the sluggish precipitation of clay minerals has been hypothesised to hinder Earth surface weathering rates. To date, there are limited data on the rate of clay mineral growth and on the effect of the aqueous solution saturation states on these rates. In this study we quantify the growth and dissolution rates of sepiolite, a 2:1 layered Mg-phyllosilicate (Mg4Si6O15(OH)2 * 6H2O) as a function of aqueous solution saturation state in a series of mixed flow experiments. Results of the both the dissolution and growth experiments are consistent with r=-10-14.801-expΔGrσRT, where r refers to the surface area normalized growth rate of sepiolite in units of mol/cm2/s, ΔGr denotes Gibbs Free energy of the sepiolite dissolution reaction, which is <0 for undersaturated solutions, 0 at equilibrium and >0 for supersaturated solutions, σ refers to Temkin’s stoichiometric number, R stands for the gas constant and T symbolizes absolute temperature. This rate equation suggests that sepiolite dissolution and growth are consistent with transition state theory and the concept of micro-reversibility. The relative decrease in aqueous Mg2+ and Si concentrations in the outlet aqueous solutions of the experiments, and the X-Ray diffraction patterns of the precipitates collected from the experiments, confirm the growth of crystalline sepiolite. The results of longer-term experiments suggest that sepiolite growth rates decrease over time. Such a decrease in the growth rate has been associated with poisoning or destruction of the sepiolite reactive surface sites. Calculations of sepiolite growth coupled to either forsterite or enstatite dissolution, based on laboratory measured rates suggest that the dissolution of the primary mineral is the rate-limiting step during the weathering of most natural systems. These results, however, contradict the saturation states of natural fluids such as observed in Icelandic waters, where clays are strongly supersaturated, whilst primary phases are relatively close to equilibrium. This discrepancy may be due to the consumption of reactive sites over time such that clay mineral precipitation rates in most natural systems are controlled by the relative slow nucleation rates of new reactive sites on the clay mineral surface.