Modeling ion exchange equilibrium: Analysis of exchanger phase non-ideality

The accurate modeling of ion exchange under the scope of mass action law requires activity coefficients for both solution and exchanger phases. Generally, the Pitzer model is adopted for the liquid mixture due to the excellent behavior provided up to high ionic strengths (6 molal). For the sorbent, the Wilson equations are almost always chosen despite other models being available from thermodynamics. In this work the performance of the NRTL, Margules, and UNIQUAC models is compared with that achieved by Wilson, in order to evaluate their applicability in the field of ion exchange equilibrium. The analysis of the models is accomplished with experimental data collected from literature. The methods of Gaines and Thomas, and Ioannidis et al. are used in calculations, as well as the ion association effects in solution. The results point out the similar behavior provided by Wilson, NRTL, UNIQUAC, and Margules: 5.45, 6.52, 6.52, and 6.65%, respectively. Nonetheless, in the region of high concentrations, the Margules equations input an abnormal trend on the global ion exchange model. With respect to the predictive ability of the models, once more they behave similarly, with some better results accomplished with Margules.

► Modeling ion exchange isotherms with activity coefficients in the liquid and sorbent.
► Non-idealities in the sorbent calculated by NRTL, UNIQUAC, Margules and Wilson models.
► All models provide similar representations of ion exchange equilibrium data.
► NRTL, UNIQUAC, Wilson show similar predictive capacity of ion exchange equilibrium.