This adsorbed amount of dye molecules per unit mass of
adsorbent increased with the increase in dye concentration
(
Fig. 6
d). In fact, in the case of the low initial concentrations,
very intense dye adsorption was observed. However, with the
increase in dye concentrations, the sorption amounts increased
slowly after which the equilibrium was achieved. This indicates
the fact that the adsorption sites were saturated at high dye
concentration because at the adsorbent surface there is a lim-
ited number of binding sites. At equilibrium, the adsorption
capacity of methylene blue achieves 64 mg/g. Compared to
other common adsorbents studied in the literature (
Table 2
),
this registered amount of Methylene blue removal is interesting
and thus the prepared copper nanoparticles from Cynomorium
coccineum
extract could be seen as a good adsorbent. As an
example, this value registered for copper oxide nanoparticles
is about four times higher than the sorption capacity registered
within multi-wall carbon nanotubes (15.9 mg/g) (
Ji-Lai et al.,
2009
) and hydroxyapatite nanoparticles (14.7 mg/g) (
Wei
et al., 2005
) used as adsorbents of Methylene Blue. It is about
three times higher than zeolites prepared from kaolins col-
lected from different sources (21.4 mg/g) (
El-Mekkawi et al.,
2016
).
Experimental results show that as the temperature was
increased from 22 to 55
°C, the adsorbed quantity of dye
slightly increased (
Fig. 5
d). At 55
°C, q
t
is about 73 mg g
1
and it is 64 mg g
1
, after equilibrium at 22
°C. The improved
dye removal with an increase in temperature may be attributed
to the kinetic effects due to the enhanced diffusion of the dye
molecules or it can be attributed to new adsorption sites being
‘‘activated
” (
Zhou et al., 2014
) on the prepared nano-
adsorbents at high temperature. This physicochemical behav-
ior can be attributed to the possibility of enlargement of the
pore sizes of the sorbent particles at high temperature. The ele-
vated temperature can, moreover, break the internal bonds
near the edge of the active sites thus increasing the sorption
capacities (
Weng et al., 2009
). According to
Gu¨zel et al.
(2015
), these trends may be clarified by the fact that the
increase in temperature adds strength to the adsorbate mole-
cule scatter rate across the external limit layer and the internal
pores of the adsorbent particles as a result of reduced solu-
tion’s viscosity.
3.3. Kinetic study
The adsorption rate was checked using four kinetic models:
pseudo-first order, pseudo-second-order, Elovich, and Intra-
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