where Ce (mg/L) is the equilibrium dye concentration, qe (mg/ g) is the amount of CR adsorbed on the adsorbent at equilibrium, the parameters qm (mg/g) and b (L/mg) are the Langmuir constants that relate qe for a complete monolayer and energy of adsorption, respectively, and KF (mg/g) and 1/n indicate adsorption capacity and adsorption intensity, respectively.
The necessary features of the Langmuir isotherm can be articulated in terms of a dimensionless parameter called the separation parameter (RL), which is defined as follows:
equation(6)
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where Co is the initial dye concentration (mg/L). The RL value indicates the shape and favourability of the adsorption isotherm. RL values between 0 and 1 indicate a favourable adsorption system.
Various parameters calculated from the Langmuir and Freundlich isotherm plots are given in Table 2. The value of R2 was found to be higher for the Langmuir isotherm (0.999) compared to the Freundlich isotherm (0.970). In addition, the values of q calculated from the Langmuir isotherm are quite close to the experimental values indicating that Langmuir isotherm is followed for adsorption of CR on CCDA ( Fig. 10a). The value RL lies between 0 and 1, indicating a favourable adsorption process (Fig. 10b). The best fit of Langmuir model indicated monolayer adsorption of CR onto the surface of CCDA with finite number of identical sites (single bondHCdouble bond; length as m-dashO) which are homogeneously distributed over the adsorbent (CCDA) surface. These results indicated that the adsorption of CR by CCDA is a chemical process which takes place by monolayer formation between CR and CCDA via imine linkages between aldehyde groups of CCDA and primary amine groups of CR. Similarly, there is also some contribution to the whole adsorption process from the interaction of dye molecules with the functional groups on the cellulose chains. Freundlich isotherm is presented in Fig. 10c.