Theequilibrium data for the adsorption at constant temperature are commonly knownas adsorption isotherms. In the present work results obtained afterexperimental study were analyzed using Langmuir (1918) and Freundlich (1906) isotherms.
Langmuir adsoption isotherm model isvalid for monolayer adsorption. The adsorbent surface consists of finite numberof identical active sites and all the adsorption sites have equal affinity forthe adsorbate molecules. A linearized form of Langmuir model is represented bythe equation (5):Ce/qe = 1/(KLqm) + Ce/qm (5)Where, Ce is the metal ionconcentration in (mg/L) at equilibrium, qe is the metal adsorptioncapacity of the adsorbent in (mg/g) at equilibrium; qm and KLare the Langmuir constants that are obtained by plotting a graph between Ce/qe and Ce as shown infig. 6 (a). The intercept and the slope will give the values of qm andKL.The Freundlich isotherm is used todescribe the adsorption equilibrium between adsorbent and adsorbate in aqueoussystems. It is applicable on heterogeneous surfaces and gives the idea ofmultilayer adsorption.
Linearized form of Freundlich adsorption isotherm isrepresented by the following equation (6).ln(qe) = ln(KF) + (1/n) lnCe (6)Where, qe is the monolayeradsorption capacity of the adsorbent at equilibrium (mg/g), KF is the Freundlich constant and 1/n is the characteristic constant of thesystem that indicates adsorption capacity. From the plot lnqe vs lnCeas shown in fig. 6 (b) ln KF and 1/n can be calculated as interceptand slope, respectively.
Langmuirand Freundlich adsorption isotherms for Cu (II) from aqueous solution arepresented in above fig. 10. It indicates that the experimental data fitted wellto all the isotherm models. By comparing the correlation coefficients (Table 3),it was observed that Langmuir isotherm is more favorable for adsorption of Cu(II) by activated Parthenium which isbased on monolayer sorption on to the surface restraining finite number ofidentical sorption sites.