and increased with increasing temperature. Its maximum value
was determined as 0.14 mmol g−1 at 318 K. The increase of sorption
equilibrium constant with temperature showed that there
was a chemical interaction between adsorbent and adsorbate.
Furthermore, entropy and energy factors should be considered
in order to determine what processes will occur
spontaneously in engineering practice. The Gibbs free energy
indicates the degree of spontaneity of the adsorption process
and higher negative value reflects a more energetically favorable
adsorption. The Gibbs free energy change of adsorption is
defined as
G◦ = −RT ln KL, (4)
where R is the universal gas constant (8.314 J mol−1 K−1) and
T is the absolute temperature in Kelvin [29]. The equilibrium
constant may be expressed in terms of standard enthalpy change
of adsorption (H◦) and entropy change of adsorption (S◦) as
a function of temperature. The relationship between the KL and
temperature is given by the van’t Hoff equation:
ln KL = −H◦
RT +
S◦
R , (5)
H◦ and S◦ can be obtained from the slope and intercept of
the plot of lnKL versus 1/T [30].
The equilibrium constants obtained from the Langmuir model
at 288, 308 and 318 K were used to determine the Gibbs free
energy changes. Table 3 shows the Gibbs free energy values for
the adsorption process. The H◦ and S◦ were determined as
25.73 kJ mol−1 and 0.113 J mmol−1 K−1 from Fig. 6, respectively.
A positive standard enthalpy change of 25.73 kJ mol−1
obtained in this study indicates that the adsorption of Pb2+ by
the FeAA adsorbent is endothermic, which fact is evidenced
Fig. 6.