where C
e is the adsorbate concentration at equilibrium (mg/L),
qe is the adsorption capacity at equilibrium (mg/g), b is the
adsorption equilibrium constant (L/mg) related to the apparent
energy of adsorption, qm is the maximum adsorption capacity
of absorbent (mg/g), n indicates the bond energies between
metal ion and the adsorbent, and k F (L/g) is related to bond
strength [2].
The linear plots of Langmuir and Freundlich equations
representing Cu(II) adsorption by the chicken eggshells are
illustrated in Figs. 3 and 4. The adsorption constants of
Langmuir and Freundlich equations and their correlation
coefficients (R2) are presented in Table I. The value of R2 is a
measure of the goodness-of-fit of experimental data on the
isotherm models [2]. Results showed that the adsorption
process by chicken eggshells was better repres ented by the
Freundlich equation compared to the Langmuir isotherm, where
the R2 values were found to be 0.9818 and 0.7360, respectively.
This indicated that the uptake of copper on chicken eggshells
was happened through heterogeneous adsorption.
From Table I, the k F constant of Freundlich isotherm, which
expresses the selective uptake of copper and affinity of
adsorbent was 13.8995 L/g, whereas the 1/n value was found to
be 0.6473. Generally, the value of 1/n that is related to the
distribution of site bonding energies is ranging between 0.2 –
0.7 [11]. If 1/n is higher than 1, it indicates that the surface
areas of adsorbent covered by the metal ions increase, and the
energies of adsorption will decrease [11].