The values of R
2
for the Langmuir adsorption isotherm models for studied
materials do not exceed 0.92 (Table 2). The Qm values calculated by the Langmuir
isotherm model much differ from the experimental data. This can be explained by the
fact that the Langmuir model describes the adsorption on a homogeneous surface and
implies monolayer coverage. The data from Table 2 indicate that the adsorption
processes fit well with the Freundlich model which considers physical and chemical
sorption on heterogeneous surface.
Table 2 Parameters of adsorption isotherm models for Fe
3+
Sample
Langmuir
Freundlich
K
L
(mg/L)
Q
m
(mg/g)
R
2
K
F
(L/g)
n
R
2
Walnut shells
Apricot seed shells
7.9·10
-2
3.5·10
-2
61.1
42.2
0.9088
0.9288
6.43
3.78
2.2
1.72
0.9888
0.9904
The application of plant wastes as sorbents can reduce the content of iron ions in
water twice. The obtained results indicate that the studied materials have a high
sorption capacity for metal cations and are comparable to other lignocellulosic
materials, for example, sorption capacity of orange peel [17] and hazelnut husk [18]
are 13.3 and 13.6 mg/g, respectively.
A study of the kinetics of the Fe
3+
sorption indicated that the maximum sorption
rate of ions occurred within the first 30 minutes of contact (Fig. 4). Further decreases
in the concentration of ions in solution occurred slowly, and full sorption equilibrium
was reached within 250 minutes.
Figure 4. Fe
3+
sorption onto apricot seed shells as a function of time
The kinetic models of Fe
3+
ions sorption onto crushed apricot seed shells are
shown in Fig. 5. The calculated values of the coefficients of the pseudo-first-order
model, pseudo-second-order model and intraparticle diffusion model are given in Table
3.
It can be seen that the pseudo-first-order model, which describes the process of
monosorbtion on a homogeneous surface, doesn’t fit correctly adsorption processes of
Fe
3+
ions sorption and the values of sorption capacities calculated from the plot (Fig. 5
a) do not coincide with the experimental values.
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