equilibrium data for the adsorption at constant temperature are commonly known
as adsorption isotherms. In the present work results obtained after
experimental study were analyzed using Langmuir (1918) and Freundlich (1906) isotherms.
Langmuir adsoption isotherm model is
valid for monolayer adsorption. The adsorbent surface consists of finite number
of identical active sites and all the adsorption sites have equal affinity for
the adsorbate molecules. A linearized form of Langmuir model is represented by
the equation (5):
Ce/qe = 1/(KLqm) + Ce/qm (5)
Where, Ce is the metal ion
concentration in (mg/L) at equilibrium, qe is the metal adsorption
capacity of the adsorbent in (mg/g) at equilibrium; qm and KL
are the Langmuir constants that are obtained by plotting a graph between Ce/qe and Ce as shown in
fig. 6 (a). The intercept and the slope will give the values of qm and
The Freundlich isotherm is used to
describe the adsorption equilibrium between adsorbent and adsorbate in aqueous
systems. It is applicable on heterogeneous surfaces and gives the idea of
multilayer adsorption. Linearized form of Freundlich adsorption isotherm is
represented by the following equation (6).
ln(qe) = ln(KF) + (1/n) lnCe (6)
Where, qe is the monolayer
adsorption capacity of the adsorbent at equilibrium (mg/g), KF is the Freundlich constant and 1/n is the characteristic constant of the
system that indicates adsorption capacity. From the plot lnqe vs lnCe
as shown in fig. 6 (b) ln KF and 1/n can be calculated as intercept
and slope, respectively.
and Freundlich adsorption isotherms for Cu (II) from aqueous solution are
presented in above fig. 10. It indicates that the experimental data fitted well
to 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 is
based on monolayer sorption on to the surface restraining finite number of
identical sorption sites.