About the possibility to liberate naturally present fixed cations from dioctahedral smectites by hydrothermal treatment

The chemical analysis of pure dioctahedral smectites exchanged by one of the typical CEC-index cations (e.g. Cu-trien) often shows significant Na-, Ca-, and K-contents which, in contrast to Mg, are not expected to occur in the octahedral sheet. They are supposed to occur in the interlayer but are not exchangeable - hence termed “fixed cations”. The aim of the study was to gain new evidence for the presence of naturally present fixed cations and find out if at least a part of these can be liberated by hydration after hydrothermal treatment.

The methodological approach for cation liberation tests included different treatments with water at 60 °C and 150 °C, partly using an ultrasonic bath, and different reaction times prior to addition of the Cu-trien index cation for CEC analysis.

The study proved that at least part of the naturally present fixed cations could be rendered exchangeable (liberated) by the hydrothermal treatment before an index cation is added. The differences were small but significant.

Liberation of fixed cations was thought to increase the CEC but the increased amount of exchangeable cations was accompanied by a decrease of the CEC, which can probably be explained by partial smectite dissolution. This hypothesis was supported by measurements conducted at higher solid to liquid ratio in order to suppress smectite dissolution. The CEC decrease was less pronounced but at the same time less cations were liberated. In conclusion, the naturally present fixed cations are strongly bound and, supposedly, can only be liberated if the smectite structure is affected.

Future work will be devoted to finding alternative methods which are suitable for quantification of naturally present fixed cations in bentonites/smectites.