The fitting of adsorption data to Freundlich indicates heterogeneous adsorption where as Langmuir indicates homogeneous adsorption. What could be inferred if data fits well to both models? Is the adsorbate homogeneously or heterogeneously adsirbed on adsorbent?
Respected Sir Arun Kumar
The asymptotic behavior was observed for some adsorbents only.
When both equations would fit the experimental results the data range is too limited to make the distinction. In many cases of adsorption reported in literature the data range is rather limited. For a good indication of heterogeneiety the chemical potential range (or concentration in the case of dilute solution) of the adsorbate should preferably be at least three orders of magnitude and this is often not the case. Yet, with strongly heterogeneous surfaces it is more difficult to reach saturation adsorption and in many cases this is the reason that the Freundlich equation gives a better result even if the concentration range is limited.
In that case, to my opinion, you should check whether the Langmuir isotherm model follows the endothermic/exothermic nature of adsorption process you observed in your study, or not. I have experienced in my studies as well as in the studies of others where the Langmuir isotherm models fits well with the adsorption data for individual temperatures, but does not follow the endothermic/exothermic nature of the adsorption. You may find a decreasing qm with the increasing temperature, where in actuality, the adsorption was found to be more favorable at the higher temperatures or vice versa. This simply violets the practical observations and we should not consider the Langmuir isotherm rather I should say the common Langmuir isotherm model to be the most suitable isotherm model satisfying our experimental outcomes. Moreover, the common Langmuir isotherm model does not consider the heterogeneity of the adsorbent which would often lead to an erroneous interpretation when applied for an energetically heterogeneous adsorbent.
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