The Elovich isotherm equation: Back to the roots and new developments

This work seeks to debunk the claim that the Elovich isotherm is vastly inferior to the Langmuir or Freundlich isotherm in the correlation of adsorption equilibrium data. This mistaken finding, reported in many published articles, is a result of comparing linearized versions of the three isotherms, which are based on different sets of transformed data. When isotherm discrimination is performed on the basis of untransformed data (the conventional adsorbed phase concentration versus solution phase concentration plot), the Elovich isotherm is shown to be highly and consistently competitive against the Langmuir or Freundlich isotherm. Predictions of untransformed data can be obtained from the three isotherms by using parameter estimates generated by either linear or nonlinear regression. To promote a wider application of the Elovich isotherm in adsorption research, this work shows how a modified form of the Elovich isotherm can be used to evaluate the energy distribution of heterogeneous surfaces.