Modified competitive Langmuir model for prediction of multispecies PFAS competitive adsorption equilibria on colloidal activated carbon

Competitive adsorption of four perfluoroalkyl substances (PFAS), i.e., perfluorobutyl sulfonate (PFBS), perfluorohexane sulfonate (PFHxS), perfluorooctanoate (PFOA), and perfluorooctane sulfonate (PFOS), on colloidal activated carbon (CAC) was studied and a new predictive model, modified competitive Langmuir model (MCLM), was proposed to predict the competitive adsorption equilibria. The new model is a modification of the competitive Langmuir model (CLM) that gives additional weighting to the molecular weight of the PFAS molecules. A comparative study was done to test the capability of the new model by comparing its predictions to the widely used CLM and ideal adsorbed solution theory (IAST), as well as the experimental data from the batch adsorption experiments on seven different mixtures of the abovementioned PFAS. The models were tested on various combinations of two and three PFAS analytes. The analysis showed better predictions by MCLM over the existing models in most of the cases. Additionally, an error analysis was performed to fit the single-solute isotherms that indicated a better fit of the Langmuir isotherm over Freundlich isotherm. Moreover, the study also showed approximately equal impact of PFOA and PFHxS on PFOS adsorption and a higher impact of PFHxS, compared to PFOA, on PFBS adsorption which hints at equal hydrophobicity but higher adsorption of PFHxS compared to PFOA due to its smaller size and higher pore accessibility.