Modeling of aqueous phase adsorption: Is it time to bid adieu to the Harkins-Jura isotherm?

The Harkins-Jura isotherm equation was originally developed to interpret sigmoid or type II isotherms exhibited by gas-solid adsorption systems. Surprisingly, in the field of water contaminant adsorption, it has been exclusively used to describe hyperbolic or type I isotherms since the mid-2000s. Given the availability of several isotherm models capable of handling type I data, we must ask ourselves: Is there a need for the Harkins-Jura isotherm? To gain acceptance into the pool of curve-fitting models for type I data, the Harkins-Jura isotherm must demonstrate superiority over existing, well-established models. When pitted against the grand masters of such models, i.e., the Langmuir and Freundlich equations, the Harkins-Jura isotherm was found to be distinctly inferior in the modeling of type I adsorption data with and without an apparent plateau. Worse still, the Harkins-Jura isotherm was also found to perform poorly in the correlation of sigmoid or type II data, which was its original intended purpose. Given its very limited modeling power, we argue that it is time to bid adieu to the mediocre Harkins-Jura isotherm.(c) 2022 Elsevier B.V. All rights reserved.