3.2.2. Sorption isotherms
In order to determine the sorption capacity of MNHAP adsorbents
towards examined metal ions, sorption studies over a large
initial concentration range from 10−4 to 10−2 mol L−1 were carried
out. The maximum adsorption capacities of the MNHAP adsorbent
for heavy metals were evaluated using the adsorption isotherms.
The Langmuir adsorption isotherm was employed to describe the
adsorption behavior in the present study and the results are shown
in Fig. 5. The linear form of Langmuir isotherm equation is given by
Eq. (2):
1
qe
= 1
qmbCe
+ 1
qm
(2)
where qe and Ce are the equilibrium concentrations of metal ions in
the adsorbed (mol g−1) and liquid phases (mol L−1), respectively, qm
is the maximum adsorption capacity (mol g−1), and b is Langmuir
constant which is related to the energy of adsorption (L mol−1). The
constants b and qm can be determined from the slope and intercept
of the linear plot 1/qe versus 1/Ce. The results showed that
the values of correlation coefficient for the adsorption of Cd2+ and
Zn2+ ontoMNHAPadsorbents were 0.9999 and 0.9950 respectively,
which demonstrated the good fitting of experimental data by this
model. The Langmuir constant for Cd2+ and Zn2+ was 2.112×103
and 1.957×103 Lmol−1 respectively, which illustrated that the
MNHAP had better adsorption affinity for Cd2+ than for Zn2+ since
the Langmuir constant was proportional to the binding energy. The
qm values for the adsorption of Cd2+ and Zn2+ by the MNHAP were
1.964 and 2.151mmolg−1, respectively. Comparing with the maximum
adsorption capacity of HAP reported by previous studies, Xu