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

Mareschal L., Ranger J., Turpault M.P.. 2009. Geochimica et Cosmochimica Acta, 73 (2) : p. 307-319.

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 diluteHCLat pH 2.7, at 50 _Cfor 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 inCECand a progressive displacement of d(001) 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 vermiculitem_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. (Résumé d'auteur)

Mots-clés : vermiculite; espagne

Thématique : Chimie et physique du sol

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