Passivation of a montmorillonite by the silica created in acid activation

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The evolution of a Spanish montmorillonite with acid treatment has been studied. With a high concentration of acid, 8 mol dm–3 HCl, there is a break in the normal tendency of the observed behaviour. The silica created at the beginning of the reaction protects the clay structure and impedes subsequent attack. This passivation of the material is confirmed by the results obtained from chemical analysis, IR spectroscopy, X-ray diffraction (XRD) and adsorption isotherm experiments. With 5 mol dm–3 acid treatment the specific surface area reaches a maximum value five times that of the starting material, and in the sample treated with 8 mol dm–3 acid it is still 3–4 times the original value.