Modelling of potassium exchange in a natural, polyionic montmorillonite under hydrothermal conditions

A smectitic material (<20 um) containing 15% illite as random mixed-layer and minor amounts of other minerals (3% quartz, plagioclase and cristobalite) was put in contact with a solution of KCl to develop a kinetic study of smectite to illite transformation. The results presented in this paper are part of an experimental kinetic study of the smectite to illite transformation. Here, only data from very short run times are included, in which the K+ ion exchange for the natural cations was the only process occurring. The run conditions were combinations of the following variables: K concentration in solution, 0.025, 0.05, 0.1, 0.3, 0.5 and 1 M; temperature, 60, 120, 175 and 200°C; time, 1, 5 and 15 days. The solid: solution ratio was 1:5. During these short experimental times, the only process detected was K+ ion exchange and the ion equilibrium between smectite and solution was reached. The natural cations present in the smectite interlayer were Na+, Ca2+, Mg2+ and a minor amount of K+. The study of the K+ ion exchange process showed that it occurred in two successive steps: first, Na+ ions were removed; second, Ca2+ and Mg2+ ions were expulsed, only after the almost complete Na+ ion removal. The two steps were characterized using the Langmuir equation as a tool. The overall cation exchange reaction was modelled using the equation:image in whichXad is the equivalent fraction of the absorbed cation in the smectite andXin its equivalent fraction already present in the natural smectite,An is an experimental parameter different for each ion, andCn is the concentration (eqdm3) of each ion involved in the process. In this way the influence of the adsorbed cations as well as the adsorption of potassium is taken into account. The sign of each summation term is positive for the ion whose adsorption is being measured and negative for the other ones. This equation seems to be able to accurately describe any ion exchange process in a closed system, even polyionic and heterovalent ones.