**3.5.2.3 Equilibrium isotherm models**

The adsorption isotherm is fundamental in describing the specific relation between the concentration of adsorbate and the adsorption capacity of an adsorbent, and it is important for the design of adsorption system. In this study, two important isotherms are selected, that is, Langmuir and Freundlich models, to describe the experimental results of MB dye adsorption on POHP are summarized in Table 10.

The Langmuir isotherm assumes that the adsorption occurs at specific homogeneous sites on the adsorbent and is the most commonly used model for monolayer adsorption process, as represented by Fig. 21 and the following equation:

$$\mathbf{C\_e} / \,\mathrm{q\_e} = 1/\mathrm{bqm} + \mathrm{C\_e}/\mathrm{gm} \tag{13}$$

Where the constant b is related to the energy of adsorption (Lmg−1), and qm is the Langmuir monomolecular adsorption capacity (mg mg−1).

Fig. 20. Adsorption isotherms of MB onto POHP

The adsorption isotherm of MB dye onto POHP is represented in Fig 20. The adsorption capacity of MB dye increases with the increase of dye concentration. This may be attributed to the extent of a driving force of concentration gradients with the increase of dye concentration. Then tends to level off, this is due to the saturation of the sorption site on the adsorbent.

The adsorption isotherm is fundamental in describing the specific relation between the concentration of adsorbate and the adsorption capacity of an adsorbent, and it is important for the design of adsorption system. In this study, two important isotherms are selected, that is, Langmuir and Freundlich models, to describe the experimental results of MB dye

The Langmuir isotherm assumes that the adsorption occurs at specific homogeneous sites on the adsorbent and is the most commonly used model for monolayer adsorption process,

 Ce/ qe = 1/bqm + Ce/ gm (13) Where the constant b is related to the energy of adsorption (Lmg−1), and qm is the Langmuir

Fig. 19. Adsorption kinetics of MB dye onto POHP.

adsorption on POHP are summarized in Table 10.

as represented by Fig. 21 and the following equation:

monomolecular adsorption capacity (mg mg−1).

**3.5.2.2 Adsorption isotherm** 

**3.5.2.3 Equilibrium isotherm models** 

The essential characteristics of the Langmuir isotherm can be expressed in terms of a dimensionless constant separation factor RL represented by the equation .

$$\mathbf{R\_{L}} = \mathbf{1/(1+bC\_{o})} \tag{14}$$

where Co (mg L−1) is the initial dye concentration. If the value of RL lies between 0 and 1, the adsorption is favorable.

The Freundlich isotherm is an empirical equation assuming that the adsorption process takes place on heterogeneous surfaces and adsorption capacity is related to the concentration of dye at equilibrium is described by Fig. (21) and the following equation:

$$\log \mathbf{q}\_{\mathbf{e}} = \log \mathbf{K}\_{\mathbf{f}} + 1/\mathbf{n} \log \mathbf{C}\_{\mathbf{e}} \tag{15}$$

where Kf (mg mg−1) is roughly an indicator of the adsorption capacity and 1/n is the adsorption intensity.


Table 10. Langmiur and freundlich isotherm constants of the adsorption of MB dye on POHP.

Electropolymerization of Some Ortho-Substituted Phenol Derivatives on Pt-Electrode from

OHP Ortho Hydroxyphenol POCP Poly orthochlorophenol POHP Poly orthohydroxy phenol MB Methelene blue dye

WE Working electrode

SB Salt bridge CE Counter Electrode

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It is shown that the applicability of the above adsorption isotherms was compared by judging the correlation coefficients. The results indicate that the Langmuir isotherm fits quite well with the experimental data (r2= 0.99), whereas, the low correlation coefficients (r2 > 0.97) show the poor agreements of the Freundlich isotherm with the experimental data. The value of n for Freundlich isotherm is greater than 1, mean while the values of RL lie between 0 and 1, indicating that MB dye is favorably adsorbed by POHP.

Fig. 21. Langmiur and Frendlich isotherms for adsorption of MB dye on POHP.
