Stoichiometry of a dissolution reaction of a trioctahedral vermiculite at pH 2.7

Transformation of vermiculite to hydroxy-interlayered vermiculites (HIV) significantly modifies the physicochemical properties of the original mineral. HIV is a common phase in acid soils, nevertheless its formation remains poorly understood. The main goal of this paper was to clarify the kinetics and process of interlayer aluminization of pure vermiculite using an experimental design. For this purpose, we monitored the dissolution of Na-saturated vermiculite in dilute HCL at pH 2.7, at 50 °C for 672 h in stirred flow-through reactors. Both reacted samples at different dissolution steps, and the leaching of elements, were investigated. The main result was a rapid change to hydroxy-interlayered vermiculite, with a decrease in CEC and a progressive displacement of d(0 0 1) reflection near 1.4 nm after K saturation, resulting from formation of hydroxy-interlayer material. Vermiculite was found to dissolve non-stoichiometrically for 500 h; after that, the release rate for Si, Mg and Al became stoichiometric with respect to vermiculite chemistry. By contrast, Fe sustained non-stoichiometric release throughout the whole experiment. At the steady state, i.e., after 500 h, a dissolution rate of 8.8 ± 0.1 × 10⁻¹¹ mol vermiculite m⁻² s⁻¹ was found with respect to Si. Both Al and Fe precipitated in the interlayer space, and their amounts calculated at the end of the experiment were 3.74 × 10⁻⁴ mol g⁻¹ of vermiculite for Al and 8.74 × 10⁻⁵ for Fe. The rate of interlayer aluminization increased for 60 h and then regularly decreased. Al-interlayering stopped after 288 h, but Fe still precipitated in the interlayer space.A comparison with the same mineral incubated for three years in acid soils revealed that the reaction was proton-promoted. The same pattern of CEC decrease and interlayer aluminization was observed, but the kinetics were slower due to soil environmental conditions.