Simple approaches to modeling pH-dependent and multicomponent sorption of ions by variable-charge minerals

The mobility of ions in reactive media is controlled by sorption reactions, which are affected by various chemical variables. Empirical models, such as the Freundlich isotherm, are widely used to describe the partitioning of ions between solution and sorbed phases; however, their parameters are only valid for a narrow range of experimental conditions. Presented here are several Freundlich-type isotherms capable of describing pH-dependent sorption of cations and anions, as well as two novel isotherms for describing pH-dependent and multicomponent sorption. Previously published data representing single and multicomponent ion sorption across a wide range of experimental conditions were extracted and used to evaluate the proposed sorption isotherms. The isotherms successfully described single-component ion sorption as a function of pH, ionic strength, and solution–solid ratio, as well as competitive sorption between ions of the same charge or synergistic sorption between a cation and an anion. Importantly, the data were well described using far fewer parameters than corresponding surface complexation models (SCMs). As such, these isotherms offer a convenient alternative to SCMs for describing sorption across a wide range of chemical conditions. This may be especially beneficial for modeling ion behavior in heterogeneous matrices such as soils or in cases where knowledge of molecular-scale sorption mechanisms is either unavailable or unnecessary.