Thermo-XRD-analysis of montmorillonite treated with protonated Congo-red. Curve fitting

The adsorption of protonated Congo red (CR) by montmorillonite was investigated by thermo-XRD-analysis. Montmorillonite was loaded at pH 1 with increasing amounts of CR up to 75 mmol per 100 g clay. Diffractograms of samples treated at 420 °C showed broad peaks and were curve-fitted to determine the different basal spacings, which composed the XRD peaks. The broad peak of one sample may contain up to four components. Component A with a maximum at 0.95–1.01 nm, characterizes the presence of tactoids with collapsed interlayers. Component B at 1.01–1.07 characterizes the presence of tactoids with a non-complete collapse due to the presence of hydroxyl-cations in the interlayers. Component C at 1.12–1.31 nm characterizing tactoids with intercalated planar monolayers of charcoal. In the presence of high amounts of adsorbed organic matter components A and B converge whereas component C splits into two sub-components C1 and C2, with maxima at ≈ 1.12 and ≈ 1.26 nm, respectively. The former spacing represents intercalated charcoal monolayers with carbon atoms keying into the hexagonal holes of the oxygen plane. Component D at > 1.60 nm characterizes the presence of tactoids with expanded interlayers containing intercalated charcoal of multi-layer carbon. The presence of components C and D in the curve-fitted diffractograms suggests that before the thermal treatment the adsorbed protonated CR was located inside the interlayer space of the smectite. Depending on the amount of adsorbed CR, three types of diffractograms were identified. With small adsorption components A, B and C were detected. With higher adsorption components A, B, C and D were detected and with very high adsorption, components A, C1, C2 and D were obtained.