3.2. Adsorption isotherm
Fig. 4 shows the adsorption isotherms of Pb2+ on FeAA at
288, 308 and 318 K. Equilibrium data can be analyzed using
commonly known adsorption isotherms, which provide the basis
for the design of adsorption systems. The most widely used
isotherm equation for modelling of the adsorption data is the
Langmuir equation, which is valid for monolayer sorption onto
a surface with a finite number of identical sites and is given by
Eq. (2).
qe = KLqmCe
1 + KLCe
, (2)
where KL is the adsorption equilibrium constant including the
affinity of binding sites (mM−1), qm the maximum adsorption
capacity (mmol g−1) and qe is the amount of sorbed Pb2+ at equilibrium
(mmol g−1). It represents a practical limiting adsorption
capacity when the surface is fully covered with Pb2+. qm and
KL can be determined from the linear plot of Ce/qe versus Ce
[27,28].
The Freundlich model is an empirical equation based on
sorption on a heterogenous surface. It is given as:
qe = KFC1/n
e , (3)
where KF and n are the Freundlich constants related to the
adsorption capacity and adsorption intensity, respectively. The
Freundlich equation can be linearized by taking logarithms and
constants can be determined [28].
Linear plots of Ce/qe versus Ce and ln qe versus lnCe are
shown in Fig. 5a and b. For each isotherm in Fig. 5a, the values
of qm and KL were calculated from experimental data through