Structural Characterization of Reduced-Charge Montmorillonites. Evidence Based on FTIR Spectroscopy, Thermal Behavior, and Layer-Charge Systematics

Abstract

AbstractIn the present study, the gradual layer-charge reduction of two Li-saturated smectites, SAz-1 from Arizona, USA, and FEO-G from Troodos, Cyprus, with octahedral charge of 0.54 electrons per half unit cell (e/huc) and 0.39 e/huc, respectively, was monitored by X-ray diffraction of K-saturated, ethylene glycol-solvated samples, by thermogravimetry-differential thermogravimetry, and by mid- and near-Fourier transform infrared spectroscopy after heating at 80–300ºC. With increasing heating temperature, the layer charge and cation exchange capacity (CEC) of both smectites decreased gradually due to Li fixation. At temperatures >200ºC, ~25% residual CEC was observed, suggesting incomplete Li fixation due to kinetic constraints. Dehydration of the original Li-smectites occurred in two steps, one peaking at ~100ºC and another at 175–180ºC. The latter decreased upon progressive Li fixation and vanished from smectites treated above ~125ºC. Dehydroxylation occurred at 635–640ºC in both smectites and was not affected by Li fixation. The second derivative analysis of the infrared spectra showed that Li fixation was manifested in both smectites by the growth of two new sharp OH-stretching fundamentals at ~3640 and 3670 cm−1 and their overtones at ~7115 and 7170 cm−1. The new bands constitute pairs of fixed energy and relative intensity which grow simultaneously at the expense of the broad OH-stretching and overtone features of the original smectites. Based on this result, Li fixation is suggested to be accompanied by the simultaneous formation of two distinct trioctahedral-like structural OH species, which is compatible with Li+ occupying trans-octahedral vacancies in both smectites.