The Flory-Huggins Isotherm and Water Contaminant Adsorption: Debunking Some Modeling Fallacies

The Flory-Huggins (FH) isotherm first appeared in the literature of water contaminant adsorption in the mid-2000s. It has come into the limelight and received considerable attention in recent years, in large part due to regular coverage in review articles. Here, we point out that a linear form of the FH isotherm is incompatible with the original nonlinear version. We show conclusively that the original FH isotherm was destroyed beyond recognition by the linearization method. As a result, the linearized FH isotherm suffers from the following serious defects: (1) Its parameter estimates are incorrect and nonsensical. (2) It is not possible to plot standard isotherm curves in the form of adsorbed phase concentration versus liquid phase concentration. These anomalies, or "red flags", that arise in the use of the linearized FH isotherm in data correlation are reported for the first time. To avoid publishing meaningless research, it is advisable to use the original form of the FH isotherm in data fitting. To go beyond routine application (e.g., simple data correlation and thermodynamic calculation), this work describes a novel application that uses the FH isotherm to evaluate the energy distribution of heterogeneous adsorbents.