Use of Fe3+ ion probe to study the stability of urea-intercalated kaolinite by electron paramagnetic resonance

The effect of mechanical and chemical activation in processes of urea intercalation in the interlayer spacing of kaolinite and the effect of varying the temperature of the intercalation product between 100 and 200 °C were studied using Fe3+ ions as a probe in electron paramagnetic resonance (EPR) spectroscopy. Other techniques were also used to characterize the samples. Monitoring the heating of urea-intercalated kaolinite, FTIR, and XRD revealed that the product obtained was stable up to a temperature of 150–160 °C. The EPR data indicated that the intercalation process promoted an approximation and increase of the magnetic interactions among the Fe3+ ions. The DRUV–vis analysis of the product before heating showed an absorption band at 680 nm that was absent in the raw kaolinite. This band was attributed to the transition A16→T24(G4) in the adjacent Fe3+ ions, intensified by magnetic coupling among these ions. We suggest that intercalated urea forms hydrogen bonds between the carbonyl's oxygen and the hydroxyls bound to the Fe3+ ions of the kaolinite structure. This would cause the approximation of the Fe3+ ions, maximizing magnetic couplings and intensifying concentrated centers of Fe3+, as was visible by EPR spectroscopy.

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