MIXING CELL MODELS FOR NONLINEAR NONEQUILIBRIUM SINGLE SPECIES ADSORPTION AND TRANSPORT

The oft-used ''standard'' mixing cell model describing reactive solute transport is extended to cater for nonequilibrium reactions. For trac er transport, the standard model is second-order accurate in the space and time discretization used. However, an error analysis of the stand ard model reveals that its accuracy is degraded when reactions are inc luded. An ''improved'' model which maintains second-order accuracy is developed. The improved mixing cell model is demonstrated to be more a ccurate than the standard model. The solution obtained from the improv ed model is found to agree very well with the results of a numerical C rank-Nicolson solution for various isotherms. The improved mixing cell predictions are also shown to agree very well with the exact solution for the two-site adsorption model, where a linear isotherm is conside red. Application of the improved model was illustrated by simulating t he experimental data from a laboratory Ca-K exchange experiment in whi ch a solution containing Ca was passed through a column filled with K- saturated soil. The model was used in conjunction with the experimenta lly determined nonlinear adsorption isotherm to predict the experiment al breakthrough data. By changing the value of kinetic coefficient, it was demonstrated that the predictions varied markedly. However, the s imulations suggest that the equilibrium model is more appropriate than the assumption of nonequilibrium.