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STRUCTURAL CHARACTERIZATION OF (Cu²⁺,Na⁺)- AND (Cu²⁺,NH₄⁺)-EXCHANGED BENTONITES UPON THERMAL TREATMENT

Bauhaus University Weimar, Building Chemistry, Coudraystr. 13C, D-99423 Weimar, Germany

^* E-mail address of corresponding author: torsten.seiffarth{at}bauing.uni-weimar.de

Bentonites are excellent materials for sequestering various metal cations because of the cation fixation ability of the constituent montmorillonite layers, but sometimes, such as in the case of Cu²⁺, the exact location of cation fixation with respect to the clay layers is difficult to determine. Na-montmorillonite was prepared from the <2 µm fraction of the bentonite Calcigel (from Bavaria, Germany) and exchanged by Cu²⁺ and Na⁺ or by Cu²⁺ and cations. The resulting materials (bi-ionic Cu-Na and Cu-NH₄ samples, respectively, as well as homo-ionic forms with Cu²⁺, Na⁺ and NH₄⁺) were heated for 24 h at temperatures of 300 and 450°C and the structural evolution characterized using X-ray diffraction (XRD) analysis, Fourier Transform Infrared (FTIR) spectroscopy, and differential scanning calorimetry (DSC) analysis.

The XRD patterns showed that the Cu sample and the Cu-Na sample have basal spacings of 12.5 Å. Upon heating at 300 and 450°C, the layers collapsed to 9.5 Å. In contrast, the d₀₀₁ value in the NH₄ sample and Cu-NH₄ sample decreased to 10.0 Å and 10.2 Å, respectively, during the heat treatment. The Cu²⁺ ions migrated irreversibly into the montmorillonite structure.

For the NH₄ and the Cu-NH₄ samples, DSC analyses show that NH₃ evolved at between 300 and 400°C though the octahedral sheet was not altered substantially by the H⁺ generated. Infrared spectra show that the bands of the Si-O and OH vibrations of all samples were changed upon heating due to the movement of the dehydrated cations into the hexagonal holes of the tetrahedral sheet. Apparently no Cu²⁺ was trapped in the octahedral sheet. In the case of the Cu-NH₄ form, both Cu²⁺ fixation and de-ammonization occurred during the heat treatment. Other than maintaining the basal spacing, no effect of the presence of NH₄⁺ on the Cu²⁺ fixation could be found for the montmorillonite studied.

Key Words: Acid Activation • Cu Fixation • De-ammonization • DRIFT • FTIR • Hofmann-Klemen Effect • Montmorillonite • NH₄ Clay