the prepared nanoparticles was linear with an R
2
of a range
0.94–0.97
demonstrating
that
adsorbate-adsorbate
and
adsorbate-nano-adsorbent interactions both control the dye
removal process. Additionally, the parameters of the Temkin
model, summarized in
Table 3
show higher values of the heat
of sorption (b
t
= 127.54–132.7 J/mol) suggesting chemical
adsorption for methylene blue sorption. The same trends were
also observed within the work of
Ngulube et al. (2018
) when
developing calcined magnesite as adsorbents for cationic and
anionic dyes.
The favorability of the adsorption of methylene blue using
the prepared nanoadsorbents could be assessed through the
exponent ‘‘n
” determined from Freundlich parameters and it
has observed that n ranges from 1.16 to 1.52. It was reported
that when the value of n lies between 1 and 10 it represents
beneficial adsorption (
Aljeboree et al., 2017
).
The enthalpy (
DH°) and the entropy (DS°) values were
determined from plotting Ln (K
L
) as a function of the inverse
of the temperature (1/T) (
Fig. 7
e). The positive value of the
enthalpy (
DH° = 19.07 kJ/mol) confirms that the interaction
between the prepared nano-adsorbent and methylene blue is
endothermic. This result is also supported by the increase
of the capacity removal with temperatures values, as described
above. The positive value of the entropy change (
DS° = 16.91
J/mol) displays the increased disorder and randomness at the
solid-solution interface of methylene blue with the nano-
adsorbents that brings about some structural changes in the
dye as adsorbate and the nanoparticles as adsorbent
(
Elmorsi, 2011
). Indeed, it was reported that this trend could
reflect the affinity of the prepared nano-adsorbent towards
dye molecules (
Barkat et al., 2009
). The positive values of
the free energy (
DG° = 13.52–14.08 kJ/mol) means that the
sorption of methylene blue is non spontaneous.
4. Conclusion
Herein, nano-adsorbents were successfully prepared for the
first time using Cynomorium coccineum extract, in which
the biological extract acts as a capping as well as a reducing
agent. Evidence of the formation of the metal nanoparticles
was
confirmed
structurally
and
morphologically
using
FT-IR, SEM, XRD, EDX, and TGA. FT-IR has shown
the presence of the functional groups characteristics of the
studied biological extract. The purity of the particles was
confirmed through EDX, showing the presence of elemental
copper oxide which was surrounded with some elements of
the plant. The crystalline nature of the particles was found
to be affected by the temperature of drying. The average
crystallite size was calculated to be about 14.2 nm. The
results registered within the TGA analysis was in agreement
within those reported using EDX. The prepared nanoparti-
cles exhibited outstanding performance in the adsorption of
cationic dyes. The bio-sorption capacity achieved 64 mg/g
at room temperature which is a level compared to some
other adsorbents previously published in the literature. The
values of B and b
t
indicated endothermic adsorption and
strong dye-nanoadsorbent interaction. The mean free energy
(E = 100–129.1
kJ/mol),
calculated
from
Dubinin-
Radushkevich
suggested
a
chemi-sorption
phenomenon.
Considering the environmentally friendly plant support and
the good sorption performances, the prepared nanoparticles
could be used for the purification of contaminated waters
and explored in other environmental applications. Further
works will be extended for the development of new metallic
nanoparticles using other biological extracts and it will be
interesting to check their performances in photocatalysis
and textile printing.
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