Simple approaches to modeling pH-dependent and multicomponent sorption of Ions by variable-charged minerals

Authors: Padilla, Joshua - Josh

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/10/2023
Publication Date: 5/3/2023
Citation: Padilla, J.T. 2023. Simple approaches to modeling pH-dependent and multicomponent sorption of Ions by variable-charged minerals. Soil Science Society of America Journal. https://doi.org/10.1002/saj2.20555.
DOI: https://doi.org/10.1002/saj2.20555

Interpretive Summary: Accurately modeling the behavior of a chemical in soil or geologic media requires accounting for essential variables such as pH and the presence of other chemicals. Models such as the Freundlich isotherm are widely used for their simplicity, but they cannot account for changes in environmental variables. Instead, surface complexation models (SCMs) can account for changes in environmental variables, but require many parameters. This paper proposes several modifications to the Freundlich isotherm so that reactions between a chemical and solid material is modeled across a wide range of experimental conditions. Previously published data were extracted and used to evaluate the modified Freundlich isotherms. These modifications provided similar or superior descriptions of experimental data compared to SCMs requiring fewer parameters than SCMs. The simplicity of these isotherms makes them an attractive alternative to more complex SCMs.

Technical Abstract: 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 models 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 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 heterogenous matrices such as soils, or in cases where knowledge of molecular scale sorption mechanisms is either unavailable or unnecessary.