Schiff base anchored with silver nanoparticles as effective adsorbent for removal of cadmium(II) heavy metal from industrial wastewater

Article abstract of DOI:10.14233/ajchem.2020.22694

The current study deals with the examination of the capacity of Schiff base anchored with silver nanoparticles for removal of cadmium(II) ions from industrial wastewater. Schiff base was synthesized using refluxing of salicylaldehyde and 4-aminoantipyrine

Full text of DOI:10.14233/ajchem.2020.22694

Asian Journal of Chemistry; Vol. 32, No. 8 (2020), 1941-1946
A
SIAN
J
OURNAL OF HEMISTRY
C
https://doi.org/10.14233/ajchem.2020.22694
Schiff Base Anchored with Silver Nanoparticles as Effective Adsorbent for
Removal of Cadmium(II) Heavy Metal from Industrial Wastewater
*,
PREETI SHARMA and VEDULA UMA
Department of Chemistry, Faculty of Science, S.P.C. Government College, Ajmer-305001, India
*Corresponding author: E-mail: drvuma@gmail.com
Received: 23 February 2020;
Accepted: 30 April 2020;
Published online: 27 July 2020;
AJC-19971
The current study deals with the examination of the capacity of Schiff base anchored with silver nanoparticles for removal of cadmium(II)
ions from industrial wastewater. Schiff base was synthesized using refluxing of salicylaldehyde and 4-aminoantipyrine in alcoholic
medium. The characterization of Schiff base were studied by elemental analysis, FTIR, NMR, UV-visible and mass spectral studies. The
silver nanoparticles were synthesized using the chemical reduction method and characterized. Then, silver nanoparticles anchored to the
Schiff base by suitable method and again characterized. Peanut shells were used as solid phase for removal of Cd(II) ions. The effects of
several parameters to optimize the adsorption of Cd(II) ions on solid phase, including pH, contact time, initial metal ion concentration and
adsorbent weight were investigated. The maximum removal efficiency of Cd(II) ions on solid phase using Ag nano@Schiff base was
achieved under experimental conditions of pH 6 (% removal = 81%), contact time of 15 min (% removal = 93%), initial metal ion
concentration of 0.5 ppm (% removal = 95%) and adsorbent weight of 3 mg (% removal = 89%). The results showed that extraction of
Cd2+ on AgNPs@Schiff base follows Freundlich adsorption isotherm.
Keywords: Heavy metal, Schiff base, Adsorption, Silver nanoparticles.
a nitrogen analogue of an aldehyde or ketone in which the
carbonyl group has been replaced by an imine or azomethine
group. Schiff base ligands are easily synthesized and form
complexes with metal ions. Over the last few years, many reports
have come to their biological applications, including anti-
bacterial [8], antiviral activity [9], anticancer [10], antimalarial
[11], antifungal [12] and anti-inflammatory [13]. One of the
Schiff base ligand characteristics is that its ability in water
treatment process is currently under study [14,15]. However,
these materials have a relatively low adsorption efficiency [16].
Hence, it is necessary to find more effective adsorbents, which
could be highly selective, cost-effective and environment friendly
[17-19].
Several studies have been reportedly recently that these
requirements can be achieved by using nanomaterials [20-23].
It was also reported that the nanomaterials such as graphene,
zinc oxide, titanium oxide, magnesium oxide, ferric oxide,
manganese oxide and carbon nanotubes (CNTs) played a vital
role in the wastewater treatment processes [24-32]. Among
inorganic nanoparticles, silver nanoparticles (AgNPs) have
INTRODUCTION
Today, one of the major ecological concerns on a world
scale is heavy metal pollution from both anthropogenic and
natural sources and increasing day by day [1]. However, the
majority of this heavy metal pollution is related to anthropo-
genic activities such as smelting, mining, industrial wastes,
untreated urban sewage sludge discharge and many other human
activities [2]. Such high levels of toxic heavy metals have an
adverse effect on the mankind [3]. Many procedures are used
to eliminate several toxic heavy metals from wastewaters before
discharging it into the mother nature which includes membrane
filtration, chemical precipitation, electrochemical process, ion
exchange, super critical fluid extraction, membrane bioreactors,
advanced oxidation process and adsorption [4]. Now a days,
for heavy metal extraction, adsorption technique has evolved
as an effective and economic method [5-7].
Schiff base is synthesized when any primary amine reacts
with an aldehyde or ketone under specific condition. Struct-
urally, a Schiff base (also known as immine or azomethine) is
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1942 Sharma et al.
Asian J. Chem.
attracted many researchers' attention due to their physical,
chemical and biological prop-erties as compared to their bulk
form [33,34]. The aim of this study is to evaluate the maximum
capacity of a silver nano complex of Schiff base synthesized
by suitable method to remove cadmium(II) ions from industrial
wastewaters. The continuous adsorption process has been used
to evaluate the maximum adsorption capacity of a silver nano
complex of Schiff base.
Extraction of cadmium(II): A silver nanocomplex of
Schiff base was attached with solid phase. About 2 mg of solid
phase was added to a silver nanocomplex of Schiff base dissolved
in 10 mL of ethanol. The obtained precipitate was dried in
oven at 60 ºC for 5 h. [35]. Then, the metal ions were extracted
from industrial wastewater by continuous adsorption method.
A glass column is used in column method, where activated
carbon was put between two layers of glass wool, the first at
the bottom and second at the top to retain it. Then, water sample
containing silver nanocomplex of Schiff base passed through
the column at the flow rate 2.0 mL/ min.
EXPERIMENTAL
4-Aminoantipyryine, salicylaldehyde and sodium boro-
hydride were purchased from CDH Chemicals, India. Methanol,
ethanol, sodium citrate dihydrate and silver nitrate were purc-
hased from Sigma-Aldrich, USA.All the solvents were distilled
dried and purified by standard methods.
The percentage removal of metal ion was calculated as
follows:
Co − Ce
Co
Removal (%) =
×100
(1)
Synthesis of Schiff base: A solution of salicylaldehyde
in ethanol mixed with a solution of 4-aminoantipyrine in
ethanol and stirred for 10 min and then refluxed for 2-3 h. The
yellow coloured precipitate was filtered and recrystallized from
alcohol to remove the impurities (Scheme-I). The purity of
the ligand was confirmed by TLC technique. Colour: yellow;
yield: 81.72%. m.w.: 307.36; m.p.: 184 ºC. Elemental analysis
found %: C, 70.27; H, 5.53; O, 10.41 and N, 13.66.
Synthesis of silver nanoparticles: Established volume
of trisodium citrate dihydrate solution mixed with a fixed volume
of silver nitrate solution and this reaction mixture was added
dropwise to ice cooled sodium borohydride solution. The reac-
tion mixture was stirred vigorously on magnetic stirrer for 10
min.After 10 min, the stirring was stopped and a yellow colloidal
solution of silver nanoparticles was obtained.
Synthesis of Ag nano@Schiff base: Synthesis of silver
nanocomplex of Schiff base was done by binding the synthe-
sized Schiff base on the synthesized colloidal silver nano-
particles solution. To the silver nanoparticles colloidal solution,
salicylaldehyde solution and 4-aminoantipyrine solutions were
added and stirred for 10-15 min obtaining a bright light green
coloured precipitate. Yield: 82.39%. m.w.: 415.22; m.p.: 176
ºC. Elemental analysis found %: C, 52.02; H, 4.09; O, 7.70;
N, 10.11 and Ag, 25.97.
Synthesis of solid phase: Peanut shells were used as a
solid phase in the adsorption process. Peanut shells were first
washed with 0.5% HCl to remove all dirt, dried in oven for
overnight, grounded by a mill, sieved and carbonized for 2 h.
The carbonized material was soaked in NaOH for overnight,
then dehydrated in an oven for overnight and activated for 2 h.
The resulting activated carbon was washed with distilled water,
filtered and finally dried in an oven.
where, Co is the initial concentration of metal ion and Ce is
the concentration of metal ion at equilibrium.
RESULTS AND DISCUSSION
UV-visible spectral studies: In Fig. 1a, four bands are
observed in DMSO medium. Bands A and B appeared at 220
and 250 nm, respectively due to π-π* transition in aromatic
ring. Band C appeared at 350 nm due to n-π* transition in
between (CH=N-) bond, while band D appeared at 400 nm
wavelength due to intramolecular H-bonding between the
hydroxyl group of salicylaldehyde and azomethine nitrogen.
In silver nanoparticles formation, it was observed that colour
of silver nitrate solution turned colourless to yellow, which
indicates the formation of colloidal silver nanoparticles due
to plasmon absorbance and showed surface plasmon resonance
(SPR) peak at 415 nm (Fig. 1b). Previous studies [36] suggested
that an SPR peak located between 410 and 450 nm has been
observed forAgNPs and might be attributed to spherical nano-
particles.
In the UV-visible spectra ofAg nano@Schiff base (Fig. 1c),
an absorption band is observed at 460 nm thus, the spectrum
shows shifting of absorption band at higher wavelength which
confirms the formation of Ag nano@Schiff base. The other
absorption bands at 220, 250 and 350 nm were similar as in
synthesized Schiff base and silver nanoparticles.
FTIR spectral studies: In the FTIR spectrum of Schiff
base (Fig. 2a), a strong band appeared at 1589 cm-1 is due to
ν(C=N). The bands appeared at 3055 and 1281 cm-1, respec-
tively, due to the C-H aromatic stretching and -OH bend vibration,
respectively. In the IR spectrum ofAg nano@Schiff base (Fig.
2b), ν(C=N) peak is observed at 1589 cm-1. The peaks at 3750,
N
OH
OH
N
N
Ethanol
Reflux at 2-3 h
+
N
C
H
O
C
H
N
O
H2N
O
2-Hydroxybenzaldehyde
4-Amino-1,5-dimethyl-2-phenylpyrazol-3-one
4-[(2-hydroxybenzylidene)]-4-amino-1,5-dimethyl-2-phenylpyrazol-3-one
Scheme-I: Synthesis of Schiff base

Vol. 32, No. 8 (2020)
Schiff Base Anchored with Silver Nanoparticles as Effective Adsorbent for Removal of Cadmium(II) 1943
1.4
0.9
1.2
400
460
(a)
(b)
(c)
0.8
1.2
1.0
0.8
0.6
0.4
0.2
0
1.0
0.8
0.6
0.4
0.2
0
415
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
250
240
250
350
330
350
0
100
200
300
400
500
0
100
200
300
400
500
0
100
200
300
400
500
600
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
Fig. 1. UV-vis spectra of Schiff base (a); silver nanoparticle (b); and Ag nano@Schiff base (c)
110
105
100
95
90
85
80
75
70
(a)
(b)
95
90
4000
3500
3000
2500
2000
1500
1000
4500
4000
3500
3000
2500
2000
1500
1000
Wavenumber (cm–1)
Wavenumber (cm–1)
Fig. 2. FTIR spectra of Schiff base (a); and Ag nano@Schiff base (b)
3300 and 3050 cm-1 are observed due to the -OH, -CH aromatic
and -CH aliphatic, respectively. An appearance of new strong
absorption band is also observed at 687 cm-1 which indicates
the formation of Ag nano@Schiff base.
at 6.8-7.5 ppm, singlet for =C-CH3 at 2.4 ppm and singlet for
N-CH3 at 3.1 ppm (Fig. 4).
SEM studies: Fig. 5a shows that the synthesized silver
nanoparticles have particle size 30-40 nm confirms the nano-
structure of silver nanoparticles, while Fig. 5b, indicates the
floral structure of Ag nano@Schiff base, which is suitable for
adsorption studies.
Effect of pH: From Fig. 6, it is observed that the rate of
removal of Cd(II) increases rapidly, from 29% to 81% as pH
increased from 1 to 6 and then become constant when increases
the pH value from 7 to 8.At the lower pH value (< 6), cadmium
is presented in form of Cd2+ ions. Therefore, the lower adsor-
ption percentage of Cd2+ on silver nanocomplex of Schiff base
at lower pH values partly depends on the electrostatic repulsion
occurring between H+ and Cd2+ on the silver nanocomplex of
Mass spectral studies: The mass spectrum of Schiff base
(Fig. 3a) showed a peak at m/z 308.02 due to M++1. This is
equivalent to the molecular weight of the proposed structure.
In the mass spectrum of Ag nano@Schiff base (Fig. 3b), a
peak observed at m/z 403.07 is due to coating of silver nano-
particles on Schiff base. Molecular weight of Ag nano@Schiff
base was almost 415.22 because of addition of silver. Thus,
the molecular ion peak was shifted from 307.36 to 415.22, which
indicate the coating of silver nanoparticle on Schiff base.
NMR studies:The 1H NMR spectrum of Schiff base shows
singlet for azomethine proton at 9.8 ppm, multiplet for C6H5
308.0259
308.0253
100
100
(a)
(b)
%
%
0
0
60 100 140 180 220 260 300 340 380 420 460 500 540 580
60 100 140 180 220 260 300 340 380 420 460 500 540 580
m/z
m/z
Fig. 3. Mass spectra of Schiff base (a); and Ag nano@Schiff base (b)

1944 Sharma et al.
Asian J. Chem.
Fig. 5. SEM image of silver nanoparticle (a); and Ag nano@Schiff base (b)
100
98
96
94
92
90
88
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
0
1
2
3
4
5
4
3
2
1
Initial concentration of Cd2+
Fig. 7. Effect of initial concentration on removal (%) of Cd2+ by silver
nanocomplex of Schiff base
0
11
10
9
8
7
6
5
4
3
2
1
0
Effect of contact time: Fig. 8 shows the effects of contact
time on the percentage removal of Cd2+ ions by silver nano-
complex from a water sample at the optimized conditions. The
maximum adsorption achieved after 15 min of adsorption at
pH 6 and then reached at equilibrium. Thus, 15 min is consi-
dered as the optimum contact time for Cd2+ adsorption on silver
nano complex of Schiff base.
Effect of adsorbent weight: Fig. 9 shows the effects of
different amount of adsorbent weight on percentage removal
of Cd2+ ions by a silver nanocomplex of Schiff base from a
water sample at constant initial metal ion concentration, pH,
Fig. 4. 1H NMR spectra of Schiff base
100
80
60
40
20
0
95
90
85
80
75
70
65
60
0
2
4
6
8
10
pH
Fig. 6. Effect of pH on removal (%) of Cd2+ by silver nanocomplex of
Schiff base
Schiff base. The removal rate of Cd2+ ion is maximum at pH
value 6 due to the precipitation of cadmium as Cd(OH)2. Thus,
pH 6 is the most optimum value for this chelation.
Effect of Cd2+ concentration: Fig. 7 shows the effect of
initial metal ion concentration on percentage removal of Cd2+
ions by a silver nanocomplex of the Schiff base from a water
sample. It is observed that increasing the initial concentrations
of Cd2+ decreases the removal capacity. Maximum removal of
Cd2+ takes place at concentration of 0.5 ppm of Cd2+ metal ion
in water sample. The reason is attributed due to the saturation
of all binding sites with metal ions and establishment of equili-
brium between adsorbate and adsorbent [37,38].
0
5
10
15
20
25
Contact time (min)
Fig. 8. Effect of contact time on removal (%) of Cd2+ by silver nanocomplex
of Schiff base

Vol. 32, No. 8 (2020)
Schiff Base Anchored with Silver Nanoparticles as Effective Adsorbent for Removal of Cadmium(II) 1945
100
95
90
85
80
75
70
65
60
55
50
Co − Ce
m×V
qe =
(2)
Here, Co and Ce are the initial and final concentration of Cd(II)
in the wastewater sample (mg L-1), qe is the amount of adsorbed
Cd(II), V is the volume (L) of wastewater sample and m is the
composite mass (g).
In this study, Langmuir and Freundlich models demons-
trated for Cd(II) on AgNPs@Schiff base are shown in Figs.
10a-b.
The general linearized form of Langmuir is as follows:






Ce
qe
Ce
qm
1
=
+
(3)
qmKL
0
1
2
3
4
5
Adsorbent weight (mg)
Fig. 9. Effect of adsorbent weight (mg) on the removal (%) of Cd2+ by
The general linearized form of Freundlich isotherm is as
follows:
silver nanocomplex of Schiff base
1
logqe = logKF + logCe
and contact time.The percentage of adsorbed cadmium increased
as the weight of adsorbent was increased from 1 to 3 mg and
then, followed by a decrease of percentage removal of Cd2+ as
adsorbent weight increased.
Adsorption isotherm: The relation of equilibrium data
is essential to apply Langmuir [39] and Freundlich [40] isotherms
in order to analysis the experimental data of theAgNPs@schiff
base adsorbent for Cd (II) ions. For the evaluation of adsorption
equilibrium curve of Cd(II), added 3 mg of AgNPs@schiff
base (adsorbent) in 50 mL wastewater sample with various
concentrations of Cd(II) at pH 6.
n
where, Ce is the final concentration of Cd(II) in the wastewater
sample (mg L-1), qe is the equilibrium adsorption capacity (mg
mg-1), and KL and KF are the equilibrium constant (L mg-1)
and qm is the maximum amount of Cd(II) per unit mass of
adsorbent. Obtained parameters from the graph are given in
Table-1.
From the above outcomes the synthesized adsorbent silver
nanocomplex of Schiff base equilibrium capacity approved.
The maximum adsorption capacity obtained in this work is
compared to other adsorbents indicates the better efficiency
of the synthetic adsorbent (Table-2).
The equilibrium adsorption capacity (qe) is calculated by
following equation:
1.8
0.007
(a)
(b)
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
0.006
0.005
0.004
0.003
y = 0.7051x + 2.0172
R2 = 0.9962
y = 0.0119x + 0.0033
R2 = 0.9165
0.002
0.001
0
0
0.05
0.10
0.15
Ce (mg L–1)
0.20
0.25
0.30
-2.0
-1.5
-1.0
-0.5
0
0.5
log Ce
Fig. 10. Langmuir (a) and Freundlich (b) adsorption isotherms for Cd2+ on silver nanocomplex of Schiff base
TABLE-1
LANGMUIR, FREUNDLICH PARAMETERS AND CORRELATION COEFFICIENT OF Pb(II) ADSORPTION ON AgNPs@SCHIFF BASE
Langmuir: Ce/qe = (1/qm KL) 1/Ce + 1/qm
Freundlich: log qe = log KF + 1/n log Ce
qm (mg g-1)
KL (L g–1)
R2
KF (L g–1)
104.7
R2
1/n
84.38
3.58
0.9165
0.9962
0.705
TABLE-2
ADSORPTION CAPACITIES OF DIFFERENT ADSORBENTS TOWARDS Cd(II) METAL ION
Adsorbents
Chromium doped NiO nanoparticles
Zero valent silver nanoparticle
qmax (mg g–1)
0.1119
0.845
Ref.
[[411]]
[422]]
Cd(II) Imprinted silica supported hybrid sorbent with an anchored Schiff base
Sol-gel derived MgO-SiO2
31.4
94.05
[433]]
[444]]
AgNPs@Schiff base
84.38
This study

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Conclusion
In this work, a Schiff base anchored silver nanocomplex
attached with solid phase (activated carbon) has been used as
novel adsorbent for the removal of cadmium ions effectively.
The results showed that the maximum removal capacity of
Cd2+ ions byAg nano@Schiff base is obtained under the optim-
ized conditions of pH 6 (% removal = 81%), initial metal ion
concentration of 0.5 ppm (% removal = 95%), contact time of
15 min (% removal = 93%) and adsorbent weight of 3 mg (%
removal = 89%). The removal of cadmium ions from industrial
wastewater is achieved due to the high surface area and low
particle size, which provides maximum adsorption active sites.
In addition, it was seen that Freundlich isotherm has a better
value of correlation coefficient (R2) of 0.9962 with the experi-
mental findings rather than with Langmuir isotherm. The adsor-
ption mechanism of AgNPs@Schiff base is defined by both
chelating and ion exchange.
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ACKNOWLEDGEMENTS
The authors are sincerely thankful to the Research Center
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