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 ${\text{NH}}_4^{+}$\$\end{document} cations. The resulting materials (bi-ionic Cu-Na and Cu-NH4 samples, respectively, as well as homo-ionic forms with Cu2+, Na+ and ${\text{NH}}_4^{+}$\$\end{document}) 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 ${\text{NH}}_4^{+}$\$\end{document} on the Cu2+ fixation could be found for the montmorillonite studied.