MODELING CA/K EXCHANGE KINETICS ON MONTMORILLONITE AND VERMICULITE

Abstract

A small percolation chamber was constructed that allows the investigation of cation exchange kinetics on clay minerals under temperature controlled conditions. A mathematical model for the kinetics of cation exchange was developed that accounts for either linear or Gapon-type exchange isotherms on external (quick) and interlayer (slow) sorption sites, respectively. With montmorillonite, as indicated by the course of the potassium percolation curve, almost the whole of the cation exchange capacity could be attributed to slow sorption sites. Potassium percolation curves could best be modeled assuming that no quick sorption took place. Only the mass exchange rate coefficient was estimated. A linear exchange isotherm was assumed, and the cation exchange capacity was set to the value obtained from batch exchange isotherms. At longer times, however, the simulation deviated considerably from the experiment. We concluded that the presence of easily expandable interlayers added effects to the cation exchange that are not considered by the model. With vermiculite, potassium percolation curves needed three parameters to be estimated: mass exchange rate coefficient, partitioning factor, and Gapon coefficient for quick sorption sites. Cation exchange capacity was set to the value obtained in batch experiments. A linear exchange isotherm was assumed for interlayer sorption sites. According to the model about 64% of the cation exchange capacity (CEC) could be attributed to slow sorption sites. Modeled and observed results are in good agreement. A comparison of parameter estimation results revealed that a Gapon coefficient for slow sorption sites is redundant, whereas the assumption of a linear exchange isotherm for quick sorption sites always leads to erroneous results. A faster mass transfer from slow sorption sites into solution has been observed with increasing temperature.