In some instances, adsorption data fits both to the Langmuir and Freundlich model. What could be the justification to this as langmuir states the monolayer adsorption whereas Freundlich implies heterogeneous adsorption? Thank you. Attach references wherever possible.
Perhaps these experiments did not reach the plateau (ie, the saturation of the adsorbent material). At lower concentrations (remembering that this is very particular to each adsorbate) covering the slope of the curves, both Langmuir and Freundlich isotherms could be well-adjusted to the experimental results. However, monolayer adsorption and on a heterogeneous surface would also be possible. The Sips equation is a combination of both Langmuir and Freundlich isotherms, where n_sips is a constant related to surface heterogeneity.
If to reach this saturation requires an unusual concentration (very high), it would be common for experiments not to reach the plateau.
Emperical model like Langmuir or Freundlich can't explain actual mechanism of adsorption rather you need surface complexation model and spectroscopic measurements. In your case, it is recommended to repeat experiments under optimum conditions.
I also agree with all the above explanations given by various scientists/Profs.
The well explained answere are given. Other than that its very important while dealing with such models whether they are linearized or non-linear. If you have use linearized equations i suggest to try with non-linear. Next step should be use of D-R and Tumkin models.
