consider that you have two stages of adsorption. the first part of the curve is normal and then starts another stage. it might be related to change in adsorption mechanism or adsorption sites.

The isotherm of dry materials moisturizing has this shape. Were the conditions for process conducting the same, ie the time of reaching the state of equilibrium and the intensity of mixing? Has enough adsorbent been used? At low concentrations, the whole mass of the adsorbate could be adsorbed. Regards,

To start off I would like to point out that your isotherm is of Type II and not of Type III as you state. With your isotherm you can calculate a surface area with the BET equation. Also I think that you need to run a standard nitrogen adsorption experiment on your biomass derived carbon just to see what sought of adsorption capacity it has. Just because it is carbon does not mean that it has been activated or that it contains micropores. Finally, I do not know what the size of Congo Red is, so bear in mind that if it is a large molecule then only the External Surface Area, that is the area not including the micropores, would be available for adsorption.

dear Rigoberto Barrios , If you mask the last three points of the isotherm, the curve might be concave to x axis. This is in agreement with M. Yates suggestion that the isotherm is type ii. To ensure such a conclusion, you should measure additional points at the low concentration range (as Miroslaw Grzesic previously noted). You could also consider that the behavior at the end of the isotherm (last three points) could be due to the formation of colloids in the solute and not a direct result of adsorption.