Critical Review and Discussion of the Nonlinear Form of Radke–Prausnitz Model in Adsorption Solid–Liquid Phases

The Radke–Prausnitz model, a typical three-parameter model belonging to the Langmuir–Freundlich-type equation, is increasingly being used to correlate the equilibrium data of various adsorption processes. However, very few published works provide the correct information on (1) the nonlinear form of this model, (2) the units of its parameters, and (3) its original citation. This work provides a critical analysis of the Radke–Prausnitz model with special emphasis on promoting its proper application in adsorption research. The correct expression and citation of the models used in adsorption studies are provided. The correct units of its parameters aRP, bRP, and β are L/kg, (mol/kg)/(mol/L)β, and unitless when the solution phase concentration Ce is given in mol/L and the adsorbed phase concentration qe is defined in mol/kg. The Radke–Prausnitz model can be reduced to the Henry, Freundlich, Langmuir, or Redlich–Peterson model in some special cases. The Radke—Prausnitz model is valid when the exponent β is in the range of 0≤β≤1. A new adsorption equilibrium constant of this model is introduced. The feasibility of using the new equilibrium constant to calculate the thermodynamic parameters (ΔG°, ΔH°, and ΔS°) of adsorption processes is discussed herein.