The evaluation of the heat of adsorption can provide useful infor-mation related to the nature of adsorption process. The heat of
adsorption determined at constant amounts of adsorbed sorbate is
known as isosteric heat of adsorption (H) and it can be calculated
using the Clausius–Clapeyron equation (8) [22]
H − R
d(ln Ce)
d(1/T)
(8)
where R is the molar gas constant (8.314 J mol−1
K
−1
) and Ce
(mol L
−1
) is the MB concentration at the equilibrium. The H can
be evaluated from the slope of the ln Ceversus 1/T (Fig. 5).
The calculated enthalpy change was −12.1 kJ mol−1
indicating
that the adsorption process was exothermic in nature. The same
behavior was found by Khan et al. for the adsorption of MB on sheep
wool [
, pH 6 and
an adsorbent dosage of 1 g L
−1
.
Figs. 2d and 3d show the influence of temperature on the adsorp-tion capacity and removal efficiency, respectively. In particular,
both adsorption capacity and removal efficiency slightly decreased
with increasing the temperature; this suggests that the interactions
between the cationic dye and active chemical groups of keratin
were lower at higher temperatures