FTIR spectroscopic characterization of thermally treated Cu2+, Cd2+, and Li+ montmorillonites

The aim of this study was to investigate the effect of ion size and charge of exchangeable cations on the extent of their fixation upon heating. Series of materials were prepared by heating of Cu2+-, Cd2+-, and Li+-saturated montmorillonites to various temperatures. The effect of these treatments on the cation exchange capacity (CEC) and structure was investigated chemically and by FTIR spectroscopy. CEC values showed that the layer charge of the mineral decreases with increasing temperature of heating and that the amount of fixed cations, in meg g(-1), is in the order Li+ > Cu2+ >Cd2+ at all temperatures. Heating to 300 degrees C of Li-exchanged Cu-JP200 sample, containing both Li+ and Cu2+ exchangeable cations, leads to preferential fixation of Lif. Analysis of the stretching OH region revealed that most of Li+ ions migrate upon heating into the octahedral vacancies, thus creating local trioctahedral AlMgLiOH groupings. Cu2+ ions are trapped in the hexagonal cavities of the tetrahedral sheet. Cd2+ ions are too large to enter deep enough into the hexagonal cavities to get fixed. While both Cu2+ and Cd2+ ions are of the same charge, the ionic radius of Cd2+ is too large to permit its penetration close to the OH groups. Cu2+ and Li+ ions are of similar size and therefore expected to enter vacant octahedral sites. However, IR spectroscopy indicates that only Li+ ions migrate into the previously vacant octahedra. It appears that both the size and the charge of interlayer cations affect their final position after fixation upon heating. (C) 1999 Elsevier Science B.V. All rights reserved.