Abstract
A new approach to acid activation of raw Ca-bentonite was explored. The method consisted in dehydration of clay by thermal pretreatment at 200 °C, followed by immediate impregnation with H2SO4 solution. The acid concentration was 1.5 × or 2.0 × cation exchange capacity (CEC) of clay. The volume of the liquid was adjusted so as to leave the material in the apparently dry state. Structural evolution of the activated solids after 1, 2, 3, and 4 weeks of storage was monitored with X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), 27Al magic angle spinning nuclear magnetic resonance (MAS NMR), and chemical analysis. In the macroscopically dry solids, the rehydrated interlayer Ca2+ underwent rapid exchange with H3O+ and formed extra-framework gypsum. Acid attack on montmorillonite structure resulted in continuous removal of layer forming Mg, Al, and Fe cations, with Mg2+ being eliminated most efficiently. No significant damage to the montmorillonite lattice was observed. Al was extracted both from the tetrahedral and the octahedral sheets. Under less acidic conditions, the monohydrated H-montmorillonite changed upon storage to bi-hydrated form, as a result of clay auto-transformation. Higher concentrations of acid in the pore network of clay stabilized the H-form of montmorillonite. The data indicate that compositional transformation of acid impregnated bentonite extended beyond the one month of aging investigated in the present work.
Highlights
The present study aims at providing insight into the evolution of the clay structure upon the applied acid treatment
Diffractograms illustrate the evolution of the X-ray diffraction (XRD) patterns with the time of storage
The non-treated bentonite shows the d001 value equal 14.9 Å, as expected for the Ca-form of montmorillonite, in which the Ca2+ cations are encased in the double layer of water molecules
Summary
Transformation of clay minerals upon contact with an acidic environment may occur in nature, as a result of common geological weathering processes, or as an undesired side effect of human activities, e.g., mining (due to the drainage of acidic waste waters into the ecosystem) [1,2]. Activation Procedure Mt + 6 M HCl, acid/clay ratio 200 mL/g, 95 ◦C, 1–24 h, washed with water and dried at 60 ◦C. Bent + H2SO4 (0–70 wt% of the mixture), acid/clay ratio 20 mL/g, 97 ◦C, 6 h, washed and dried 4 h at 105 ◦C. Bent + H2SO4 or HCl (1, 5, or 10 M), acid/clay ratio 100 mL/g, 80 ◦C, 1.5–96 h, washed with water and freeze-dried. Bent + 2 M HCl, acid/clay ratio 7 mL/g, microwave heated to 100 ◦C for 1–20 min, washed with water and freeze-dried. Bent + HCl (0.05–0.5 M), acid/clay ratio 0.1 mL/g, 60–100 ◦C, washed with water, dried 12 h at 55 ◦C. Activation Procedure Mt + 3.2 M HNO3, acid/clay ratio 49 mL/g, 104 ◦C, 4–24 h, washed with water, dried and calcined at 450–1150 ◦C for 4 h. The present study aims at providing insight into the evolution of the clay structure upon the applied acid treatment
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