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. Serie s 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 chemical ly and by FTIR spectroscopy. CEC values showed that the layer charge of the mineral decreases with increasing temperature of heating and that the amou nt 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, con taining both Li+ and Cu2+ exchangeable cations, leads to preferential fixat ion of Lif. Analysis of the stretching OH region revealed that most of Liions migrate upon heating into the octahedral vacancies, thus creating loca l trioctahedral AlMgLiOH groupings. Cu2+ ions are trapped in the hexagonal cavities of the tetrahedral sheet. Cd2+ ions are too large to enter deep en ough into the hexagonal cavities to get fixed. While both Cu2+ and Cd2+ ion s are of the same charge, the ionic radius of Cd2+ is too large to permit i ts penetration close to the OH groups. Cu2+ and Li+ ions are of similar siz e and therefore expected to enter vacant octahedral sites. However, IR spec troscopy indicates that only Li+ ions migrate into the previously vacant oc tahedra. 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.