The activated carbon surface was saturated from phenol . So it was not be able to absorb more phenol (plateau of the phenol removal curve).How much is the error associated to the values of the curves?. 

For low initial concentration of adsorbate (100–200 mg/l), modified Langmuir

model represents the data very well with the average relative

error (ARE) of 8.34, 8.31 for phenol and resorcinol respectively. Whereas, for high initial concentration of adsorbate (400–1000 mg/l), extended Freundlich model represents the data very well with ARE of 7.0, 6.46 for phenol and resorcinol respectively.