. The model is based on several basic assumptions: (i) the sorption takes place at specific homogeneous sites within the adsorbent; (ii) once a dye molecule occupies a site; (iii) the adsorbent has a finite capacity for the adsorbate (at equilibrium); (iv) all sites are identical and energetically equivalent [65]. The nonlinear equation of Langmuir isotherm model (Eq. (7))