Charge-Neutral Adsorption Modeling.

The adsorption of cations and anions by minerals can be modeled with strict charge-neutral ion exchange models. Two examples will be presented to illustrate this. The first example is the retention of multi-competitive ion exchange reactions of phthalate, chloride, hydrogen, and sodium ions on goethite as a function of pH. The model presented accurately predicts the competitive adsorption behavior of all these ions simultaneously. The second example is the retention of various cations in the nanopore channels of zeolite (ZSM-5) as a function of pH. Here, the retention of sodium, potassium, and cesium are accurately predicted either as single ions or as mixed competing multi-ion mixtures. The effect of strongly adsorbed cations on the pK of weakly adsorbed cations is believed to be due to a shift in solubility properties of the mixed-ion systems. The software IExFit was used to optimize the model parameters.