Freundlich And Langmuir: An Analysis Of Absorption Isotherm Models

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Adsorption isotherm models : To study the proper model, the isotherm data were analyzed and were fitted them into two models, Freundlich and Langmuir, because they are the most suitable and employed models. Equilibrium uptake of CV and MB was investigated with sorbent weight of 0.5 g of fennel seeds in contact with 100 mL of solutions and at pH 4 for CV and 6 for MB. The flasks were shaken at 200 rpm and the equilibrium concentration of the remaining CV and MB was determined by spectrometer. The analysis of the isotherm data is important to make models, actually represent the results .
Langmuir Isotherm

Lamgmuir model is suitable for studying the monolayer adsorption, because the process is take place at homogeneous sites. Constant
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The Freundlich model can be expressed as [8]:

.................................................................................................... (6)

where qe mass of adsorbate is adsorbed per unit mass of adsorbent (mg/g), Ce is the equilibrium concentration of adsorbate (mg/l), Kf indicates adsorption capacity and n an intensity factor of the adsorption process , which varies with the heterogeneity of the adsorbent. The adsorption is better favorability when 1/n is to be grater. The fractional values of 1/n ranged between 0 and 1. The constants Kf and 1/n were calculated from the intercept and slope of the plot of ln qe versus ln Ce. Fig.10 shows the linear plot of Freundlich isotherm for adsorption of CV and MB onto fennel seeds. The calculated parameters are shown in table 2. The Freundlich isotherm model was found best fitted with experimental data as the value of R2 equal to 0.992 and 0.993 for CV and MB, table 2. Kf is a Freundlich constant that shows the adsorption capacity on heterogeneous sites level. The adsorption intensity given by the Freundlich coefficient (1/n) is smaller than unit indicating the favorable adsorption . These results indicate the Freundlich equation represents a better fit than Langmuir for CV and MB. The Langmuir equation is shown a homogeneous adsorption, while Freundlich equations is demonstrated a heterogeneous adsorption. The correlation
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the value of R2 is the indicator for the successful of the process. Whereas, R2 values are close to 1, high value, the best adsorption process[20,21]. The Pseudo first order is given in equation:. ......................................................................................(7)
Where
qe CV or MB amount that adsorbed onto unit weight of adsorbent at equilibrium (mg/gm). qt CV or MB amount that adsorbed onto unit weight of adsorbent at any time t (mg/g)
K1 is the rate constant for pseudo-first-order model (L/min).
The value of qe and K1 were found from slope and intercept of linear relation between log (qe- qt) with t at the given concentrations. The straight line between log (qe- qt) and the time emphasize the applicability of the equation (7) , fig. (11).
The pseudo second-order is given in equation: ........................................................................................................(8)
Where:
qe CV or MB amount that adsorbed onto unit weight of adsorbent at equilibrium (mg/gm). qt CV or MB amount that adsorbed onto unit weight of adsorbent at any time t

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