Adsorption of Ag+ ions using hydroxyapatite powder and recovery silver by electrodeposition
Cite this paper: Vietnam J. Chem., 2021, 59(2), 179-186
Article
DOI: 10.1002/vjch.202000148
Adsorption of Ag+ ions using hydroxyapatite powder and recovery
silver by electrodeposition
Pham Thi Nam1, Dinh Thi Mai Thanh2,3, Nguyen Thu Phuong1, Nguyen Thi Thu Trang1,
Cao Thi Hong1, Vo Thi Kieu Anh1, Tran Dai Lam1,3, Nguyen Thi Thom1*
1Institute for Tropical Technology, Vietnam Academy of Science and Technology,
18 Hoang Quoc Viet, Cau Giay, Hanoi 10000, Viet Nam
2University of Science and Technology of Hanoi, Vietnam Academy of Science and Technology,
18 Hoang Quoc Viet, Cau Giay, Hanoi 10000, Viet Nam
3Graduate University of Science and Technology, Vietnam Academy of Science and Technology,
18 Hoang Quoc Viet, Cau Giay, Hanoi 10000, Viet Nam
Submitted August 31, 2020; Accepted November 9, 2020
Abstract
Nowadays, waste of electrical and electronic apparatuses generated in huge amount surround the earth and has
become a global environmental issue. Electronic waste contains large amounts of metal ions, such as Au, Ag, Cu, Pd,
Pb and Cd etc., resulting in a threat to the environment, ecosystems and human health. Therefore, removal of metal ions
and recovery of precious metals are extremely necessary. Hydroxyapatite material was reported that they can remove
heavy metal ions in water with high efficiency. In this work, Ag+ ions in water were adsorbed using hydroxyapatite
o
(HAp) powder and recovery silver by electrodeposition. The adsorption efficiency of silver was about 61 % at 50 C
after 60 minutes of contact time. The Ag+ adsorption process using HAp powder followed Langmuir adsorption
isotherms with the maximum monolayer adsorption capacity of 18.7 mg/g. 60 % of silver can recovery by
electrodeposition after 4 hours at the apply current of 10 mA at 50 °C.
Keywords. Ag+ ion, Adsorption, hydroxyapatite (HAp), recovery of silver, electrodeposition.
1. INTRODUCTION
silver compounds such as silver nitrate or silver
oxide may cause breathing problems, lung and throat
Among industries, the electronic industry is the irritation and stomach pain.[8] Nowadays, a large
world’s largest and fastest growing manufacturing amount of electronic waste has been discharged into
industry.[1,2] Today, electrical and electronic waste the environment without proper treatment. It carries
are the type of waste that is most interested in the the risk of polluting heavy metals into the ground
current waste stream because they are the fastest and water. Therefore, the treatment of electronic
growing waste stream and grow 3 times faster than waste is necessary.
other types of waste (about 4 percent growth a
In addition, electronic waste also contains a big
year).[3] The amount of electrical and electronic amount of many precious metals such as Au, Ag, Pd,
waste are created about 40 million tons each year. etc. Recovery of precious metals prevents the
Electronic waste contains a lot of heavy metals, pollution as well as prodigality. In Vietnam, some
chemical compounds that easily penetrate soil and materials were synthesized to remove heavy metal
water, threatening the environment and human ions such as: coffee husk, MnFe2O4/GNPs
health.[4-7] This seriously affects human health such composite and chitosan/graphene oxide/magnetite
as cancers, respiratory tract, cardiovascular and nanostructured (CS/Fe3O4/GO) composite.[9-11] The
neurological.[4-7] Since the early part of 19th century, adsorption capacity for Ni(II) of coffee husk is 21.14
physicians have known that silver compounds can mg/g, reported by Do Thuy Tien et al.[9] The
cause some areas of the skin and other body tissues. CS/Fe3O4/GO can remove 60 % of Fe(III) with
Skin contact with silver compounds has been caused adsorption capacity of 6.5 mg/g.[11] Nguyen
mild allergic reactions, such as rash, swelling, and suggested that MnFe2O4/GNPs composite removed
inflammation. The inhalation with high amount of Pb2+ with high adsorption capacity of 322.6 mg/g.[10]
179 Wiley Online Library © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH
Vietnam Journal of Chemistry
Nguyen Thi Thom et al.
The studies used HAp to adsorbed heavy metal ions
in water which were reported for few years ago.[12-15]
The adsorbent of HAp showed a good removal
ability of heavy metal ions. In our reports,
hydroxyapatite (HAp, Ca10(PO4)6(OH)2) powder can
remove some ions such as Pb2+, Cd2+ and Cu2+ with
the efficiency of about 86 % corresponding to the
adsorption capacity of 281 mg/g.[16] However, the
researches for using of HAp to adsorb Ag+ ions in
water are not reported. The aim of this work is to
study the mechanism of Ag+ adsorption using
hydroxyapatite powder and silver deposition.
Herein, HAp powder was used to adsorb Ag+ ions in
water and recovery of silver by electrodeposition.
(3)
(4)
where: t is the contact time (min); Qt is the
adsorption capacity following the time (mg/g); Qe is
the adsorption capacity at the equilibrium (mg/g); K1
is adsorption constant following the pseudo-first-
order law (min-1) and K2 is the equilibrium constant
(g/mg.min).
From the data of the effect of initial Ag+
concentration, we studied the isothermal adsorption
model following Langmuir and Freundlich
adsorption isotherms ((5) and (6) equations,
respectively).
2. MATERIALS AND METHODS
(5)
The chemical precipitation was used to synthesize of
hydroxyapatite powder from Ca(NO3)2 (M = 100.09
g/mol, 99.0 % of pure), (NH4)2HPO4 (M = 132.05
g/mol, 99.0 % of pure) and NH4OH (M = 35.05
g/mol, 28 %). These chemicals were purchased from
lnQe = lnKF + 1/n lnCe
(6)
where: Qe is the equilibrium adsorption capacity; Qm
is maximum single layer adsorption capacity per unit
mass of adsorbent; Ce is the equilibrium concentration
of Ag+; KL and KF are Langmuir and Freundlich
adsorption constant and n is experimental constant.
The effect of pH solution, temperature and
adsorbent mass on Ag+ adsorption capacity was
investigated. 0.1 g HAp was used to remove 50 mL
Ag+ 50 g/L for 60 min at different pH values from 2
to 8. The treatment temperature was adjusted at 20,
VWR chemicals,
Belgium. The
obtained
hydroxyapatite powder has cylinder shape with size
of 18 × 29 nm and the SBET = 75 m2/g.[17] Sulfuric
acid (M = 98.08 g/mol, 95-97 %) and silver nitrate
(M = 169.87 g/mol, 99.0 % of pure) are pure
chemical of Merck. The adsorption of Ag+ ions was
conducted with a 50 mL of AgNO3 solution at
various initial concentrations from 10 to 100 mg/L at
different contact time of 5, 10, 20, 30, 40, 50, 60, 70
and 80 minutes. The adsorbent amount of HAp was
0.1 g. The concentration of Ag+ ions after adsorption
process was determined by atomic absorption
spectrophotometry (AAS). The capacity (Q) and
efficiency (H) of Ag+ adsorption process were
calculated by the equations (1) and (2):
o
30, 40, 50, 60 and 70 C using a thermostatic with
water bath. The mass of HAp changed from 0.05 to
0.15 g.
The Fourier transform infrared spectroscopy
(FTIR) is used to identify of functional groups of
HAp before and after Ag+ adsorption process. The
FTIR spectra were recorded by an IS10 (NEXUS)
using KBr pellet technique at room temperature over
the frequency range from 400 to 4000 cm-1 with a 32
scans and 4 cm-1 resolution. The phase component of
HAp before and after adsorption process was
analysed by X-ray diffraction (XRD) (Siemens
D5000 Diffractometer, CuKα radiation (λ = 1.54056
Å) with a step angle of 0.030°, scanning rate of
0.04285 °/s and 2-theta range of 20-70°). The
surface morphology of HAp before and after
adsorption of Ag+ ions was analyzed by scanning
electron microscopy (SEM S4800, Hitachi). The
element component of AgHAp was determined using
energy dispersive X-ray analysis (Jeol 6490 JED
2300).
C C 100
i
(1)
o
H
(%)
Co
(2)
C C V
i
o
Q
(mg / g)
m
where, C0 (mg/L) is the initial Ag+ concentration; Ce
(mg/L) is Ag+ concentration at an equilibrium in the
solution after adsorption process; V (L) is the
solution volume (V = 50 mL) and m (g) is the mass
of adsorbent (HAp, m = 0.1 g).
The kinetics of Ag+ adsorption process using
HAp powder were investigated by the effect of
contact time (changed from 5 to 80 minutes)
following Lagergren’s pseudo-first-order law and
McKay and Ho’s pseudo-second-order law. The
equations of the two models are showed in (3) and
(4) equations, respectively.
The recovery of silver was performed in an
electrochemical cell containing 0.5 g of Ag-HAp
which was dispersed into 5 mL of 0.1 M H2SO4
solution with a cell of three electrodes: the working
electrode was a plate of Au (S = 0.0201 cm2), the
Vietnam Journal of Chemistry
Adsorption of Ag+ ions using hydroxyapatite…
counter electrode was Pt plate (S = 0.0201 cm2) and adsorption process reached the adsorption
the reference electrode was Ag/AgCl. Silver was
deposited on the surface of Au plate by applying
current at 2, 4, 6, 8 and 10 mA with the different
time from 30 minutes to 4 hours at 50 °C. The
remaining Ag+ ion concentration in 0.1 M H2SO4
solution was also determined by AAS method.
equilibrium. The adsorption kinetics was
investigated to determine sufficient residence time
on the absorber surface. This is reflected by the
change of Ag+ ion concentration adsorbed during the
batch adsorption studies following the contact time.
The experimental data were analyzed using two
models:[14,16,18] the pseudo-first-order law and the
pseudo-second-order law.
3. RESULTS AND DISCUSSION
3.1. Standard curve
14
12
10
8
55
50
45
40
35
30
The standard curve of Ag+ was constructed with the
change concentration of Ag+ from 0 to 30 mg/L. The
variation of absorbance according to the
concentration of Ag+ was shown in figure 1. The
results showed that the absorbance value increased
with increasing of concentration of Ag+. The
concentration of Ag+ after adsorption process can be
extrapolated from the standard curve.
H
Q
6
0
60
80
20Time4(0min)
2.0
Figure 2: The variation of adsorption efficiency and
capacity as a function of the contact time
g+
A
1.5
C
9
8
2
.05
9
0
The pseudo-first-order law and the pseudo-
second-order law equations were constructed and
shown in figures 3 and 4. The correlation coefficient
(R2) of two models showed that the pseudo- second-
order law described better for the Ag+ adsorption
process. The parameters of the pseudo-second-order
law were calculated and were shown in table 1.
.99
=
1.0
0.5
0.0
0
y
2
=
R
0
5
10 15 20 25 30
CAg+ (mg/L)
Figure 1: The variation of absorbance as a function
0.75
y
of the Ag+ concentration
=
-
0
.
0
1
R
2
1
1
=
x
+
0
.
9
0.50
0.25
0.00
0
.
8
6
1
8
3.2. Effect of contact time
6
4
1
The change of the efficiency and adsorption capacity
of 0.1 g HAp powder in 50 mL of Ag+ solution (50
mg/L, pH0 = 5.9) at 20 °C according to the contact
time was shown in figure 2. The contact time
increased from 5 to 60 minutes, the adsorption
efficiency and capacity increased rapidly and
reached stability after 60 minutes. It is clear that
there is an initial rapid uptake of metal ions, but as
time progresses the uptake around the 60 minutes
mark no further adsorption takes place. The
efficiency reaches about 50 % corresponding to the
adsorption capacity of 12 mg/g after 60 minutes. At
the contact time of 70 and 80 minutes, the
adsorption efficiency and capacity were not
significantly changed. From these results, they are
possible to conclude that after 60 minutes, Ag+
0
10 20 30 40 50
t (min)
Figure 3: The model of the kinetic of Ag+
adsorption process using HAp powder according to
Lagergren's pseudo-first-order law
Table 1: The parameters of Ag+ adsorption process
using HAp powder
The pseudo-second-order law
K2 (g/mg.min)
0.0034
Qe (mg/g)
12.32
R2
0.9938
Vietnam Journal of Chemistry
Nguyen Thi Thom et al.
7
1.2
1.1
1.0
0.9
0.8
0.7
5
5
8
6
1
4
6
5
.
.5
0
5
4
3
2
1
0
0
+
9
x
0
+
9
x
8
4
3
3
7
3
9
0
0
.
3
6
.
0
.3
6
=
2
0
.9
=
y
=
0
2
y
=
R
R
0
20
40
t (min)
60
80
0.50 0.75 1.00 1.25 1.50 1.75
Log Ce
Figure 6: The Ag+ adsorption isotherm follows the
Freundlich isothermal model using HAp powder
Figure 4: The model of the kinetic of Ag+
adsorption process using HAp powder according to
McKay and Ho's pseudo- second-order law
4.5
4.0
3.3. Effect of initial Ag+ concentration
7
x
8
7
7
0
2
.
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
4
0
3
8
=
9
.
y
0
2
=
The change of the efficiency and adsorption capacity
of 0.1 g HAp powder in 50 mL of Ag+ solution with
different initial concentrations varying from 10 to
100 mg/L at pH0 = 5.9, temperature of 20 °C after 60
minutes of the contact time was shown in figure 5. It
is found that adsorption capacity was the result of
increasing equilibrium metal ion concentrations in
solution. The increased concentrations were able to
increase the numbers of Ag+ ions at the absorber
surface and enhance the probability of adsorption.
The data were analyzed based on two isothermal
adsorption models of Langmuir and Freundlich
(figures 6 and 7). The equilibrium equations are
widely used for modelling equilibrium data obtained
from adsorption systems.
R
0
10 20 30 40 50 60
Ce (mg/L)
Figure 7: The Ag+ adsorption isotherm follows the
Langmuir isothermal model using HAp powder
3.4. Effect of pH solution
The effect of pH solution in the range of 2 to 8 on
Ag+ adsorption ability using HAp powder is
presented in figure 8. The pH solution increases
leading to the increase of adsorption efficiency. It is
clear that at low pH values (pH ~ 2 or 3), the
efficiency of Ag+ removing is low because of
proton-competitive sorption reactions between H+
ions and Ag+ ions. When the pH solution increases,
the competing effect of H+ ions decreases leading to
the efficiency of removal Ag+ increases. In the pH
range of 6 to 8, the Ag+ removal efficiency does not
change. So, pH value of 5.9 (pH0) was the optimum
pH value for the Ag+ removal process.
30
25
20
15
10
5
100
80
H
Q
60
40
20
0
0
20 40 60 80 100
Initial Ag+ concentration (mg/L)
Figure 5: The variation of adsorption efficiency and
capacity as a function of the initial Ag+
concentration
16
14
12
10
8
60
50
40
30
20
10
From the correlation coefficient (R2) of the two
equations, it is shown that the Langmuir isothermal
described better for the Ag+ adsorption process than
the Freundlich isothermal. It can be said that Ag+
adsorption process on the surface of HAp was
monolayer. The value of the maximum adsorption
capacity calculated from the Langmuir isothermal
model was about 18.7 mg/g.
H
Q
6
4
1
2
3
4
5
6
7
8
9
pH solution
Figure 8: The variation of Q and H as a function of
the initial pH solution
Vietnam Journal of Chemistry
Adsorption of Ag+ ions using hydroxyapatite…
3.5. Effect of temperature treatment
3.7. Characterization of HAp before and after
treatment
In this section, the Ag+ treatment temperature was
o
adjusted from 20 to 70 C using a thermostatic. The The characterizations of HAp powder before and
results show that the temperature increases from 20 after adsorption process were analyzed using FT-IR
o
to 50 C, the adsorption efficiency and capacity and XRD. The functional groups in the HAp
increase strongly (figure 9). It is clear that the molecule before and after Ag+ adsorption process
temperature promotes movement of ions as well as were determined using FTIR spectra (figure 11). It
ion exchange reaction. The temperature continues to can be seen clearly that Ag+ adsorption process does
increase, the adsorption efficiency and capacity not change the functional groups in HAp molecule.
nearly do not change. Therefore, the temperature For both of spectra, the characteristic peaks of OHˉ
3-
value of 50oC is chosen to remove Ag+ ions.
and PO4 groups in HAp were observed. A wide
range at 2500 to 3700 cm-1 was characterized for
vibration of OHˉ in water. The vibrations at 1040
and 1105 cm-1 are attributed to the P-O stretching of
PO43- groups. The flexural vibration of the phosphate
group was observed at the wave number of 570 to
605 cm-1. The result is coincident with another
report.[18]
16
15
14
13
12
11
65
60
55
50
45
H
Q
PO43-
10 20 30 40 50 60 70 80
Temperature (oC)
AgHAp
PO43-
OH-
Figure 9: The variation of Q and H according to
HAp
temperature
570-605
1105
1040
3.6. Effect of adsorbent mass
4000 3500 3000 2500 2000 1500 1000 500
The effect of HAp mass from 0.05 to 0.15 g on the
Ag+ adsorption ability is presented in figure 10. The
data show that the amount of Ag+ removed increases
rapidly by increasing of HAp mass from 0.05 to 0.15
g. However, HAp mass increases leading to the
adsorption capacity decreases strongly. Therefore,
the adsorbent mass of 0.1 g is suitable in this study.
Wavenumber (cm-1)
Figure 11: FTIR spectra of HAp before and after
adsorption process
The X-ray diffraction patterns of HAp powder
before and after Ag+ adsorption process were shown
in figure 12. The XRD patterns of HAp and Ag-HAp
samples were similar, which presented the
characteristic peaks for HAp crystal (JCPDS No. 00-
009-0432).[19] This result is in accordance with
previous reports.[20-22]
26
24
22
20
18
16
14
12
85
80
75
70
65
60
55
50
45
H
Q
JCPDS: 00-009-0432
Ag-HAp
0.05
0.10
0.15
HAp mass (g)
HAp
Figure 10: The variation of Q and H as a function of
HAp mass
20 30 40 50 60 70
2 (degree)
From the above data, the suitable condition to
remove 50 mL Ag+ 50 g/L are chosen in this study
including: 0.1 g HAp, pH0 = 5.9, temperature of 50
oC for 60 minutes of the contact time.
Figure 12: XRD patterns of HAp before and after
Ag+ adsorption process
Vietnam Journal of Chemistry
Nguyen Thi Thom et al.
The mechanism of the deposition and dissolution
of Ag on Au electrode can be described as follows:
In H2SO4 solution, HAp and Ag-HAp powders were
dissolved. In the potential range of 0.4 to -0.4 V,
there was the reduction at -0.06 V on the anodic
branch and the oxidation at 0.06 V on the cathodic
branch of Ag:
The SEM images of HAp and AgHAp are
shown in figure 13. The surface morphology of HAp
has cylinder shape. After Ag+ adsorption process,
there is no significant change in particle’s size and
shape. The EDX spectra confirms the present of
silver in HAp after adsorption process (figure 14).
Ag+ + 1e → Ag
Ag - 1e → Ag+
(7)
(8)
Silver was recovered by apply current method
into 0.1 M H2SO4 solution. The different applied
current values were set: 2, 4, 6, 8 and 10 mA with
different time from 30 minutes to 4 hours at a
temperature of 50 °C. The Ag+ concentration
remaining in the solution after recovery process was
shown in figure 16.
Figure 13: SEM images of HAp and AgHAp
1200
1000
800
2 mA
600
4 mA
400
6 mA
8 mA
200
10 mA
0
0
50
100 150 200 250
Figure 14: EDX spectrum of AgHAp
Time (min)
Figure 16: The concentration of Ag+ ions remains in
The cathodic polarization curve of Au electrode
in 5 mL of H2SO4 solution containing 0.5 g of HAp
and Ag-HAp in the potential range of 0.4÷-0.4 V,
with 50 mV/s of scanning rate at a temperature of 50
°C was shown in figure 15. We can see that the
presence of reduction peak of Ag+ at -0.06 V on the
anodic branch and the oxidation peak of Ag at 0.06
V on the cathodic branch of the cathodic
polarization curve of H2SO4 solution containing Ag-
HAp.
H2SO4 solution after recovery process
It can be seen clearly that the applied current
increased leading to the deposited amount of Ag on
the surface of Au electrode increased. The recovery
efficiency of Ag was calculated and listed in table 2.
The recovery efficiency of silver reached about 60
% after 4 hours at the apply current of 10 mA.
Table 2: The recovery efficiency of Ag (H %) at
different apply currents for different time.
Time
(min)
H (%)
2 mA 4 mA 6 mA 8 mA
10 mA
30
45
60
90
3.20
7.20
3.52
8.64
5.20
10.24 18.80
12.80
14.80
22.80
32.64
39.20
42.00
45.20
50.40
55.60
59.60
10.40 12.80 15.20 27.04
12.80 16.40 24.00 34.40
120 16.40 20.80 26.40 39.60
150 19.20 23.60 30.80 42.00
180 21.60 27.36 32.96 44.80
210 24.00 30.00 36.00 47.60
240 26.00 33.60 40.80 51.60
Figure 15: The cathodic polarization curve of Au
electrode in 5 mL of H2SO4 solution containing 0.5 g
of HAp and Ag-HAp
Vietnam Journal of Chemistry
4. CONCLUSIONS
Adsorption of Ag+ ions using hydroxyapatite…
results of adsorption of Ni(II) from wastewater using
coffee husk based on activated carbon, Vietnam J.
Sci. Tech., 2018, 56(2C), 126-132.
Green adsorbent of HAp can be removing Ag+ in
water with the efficiency of 61 %. The equilibrium
time of adsorption process was determined after 60
minutes. The results of adsorption isotherms show
that adsorption of Ag+ ions using HAp powder was
mono layer follows Langmuir isothermal model with
the maximum adsorption capacity of 18.7 mg/g. The
experiment data of adsorption kinetics confirms that
Ag+ adsorption process follows the pseudo-second-
order law with the correlation coefficient (R2) of
0.9938.
60 % of silver can be recovered on the surface of
Au electrode by electrodeposition at the applied
current density of 10 mA after 4 hours into the
electrolytic solution of H2SO4. However, in the
electrolyte of H2SO4, hydroxyapatite is dissolved. It
means that the adsorbent of HAp cannot reuse after
desorption process. Therefore, our next work will
study silver recovery into a deep eutectic solvent
(DES) solvent based on choline chloride and urea.
Metal deposition from DES solvent is an area that
has received increasing interest.
10. N. D. Anh. Study on synthesis of MnFe2O4/GNPs
composite and application on heavy metal removal,
Vietnam J. Sci. Tech., 2018, 56(1A), 204-201.
11. L. D. Truong, T. V. Hoang, L. D. Thu, T. N. Quang,
N. T. Minh Hang, N. D. Khoi, T. X. Anh, T. L. Anh.
Synthesis and application of chitosan/graphene
oxide/magnetite nanostructured composite for Fe(III)
removal from aqueous solution, Vietnam J. Sci.
Tech., 2018, 56(2), 158-164.
12. I. Mobasherpour, E. Salahi, M. Pazouki. Comparative
of the removal of Pb2+, Cd2+ and Ni2+ by nano
crystallite hydroxyapatite from aqueous solutions:
Adsorption isotherm study, Arab. J. Chem., 2012, 5,
439-446.
13. G. E. Jai Poinern, S. Brundavanam, S. K. Tripathy,
M. Suar, D. Fawcett. Kinetic and adsorption
behaviour of aqueous cadmium using a 30 nm
hydroxyapatite based powder synthesized via a
combined ultrasound and microwave based
technique, Phys. Chem., 2016, 6(1), 11-22.
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Corresponding author: Nguyen Thi Thom
Institute for Tropical Technology
Vietnam Academy of Science and Technology
18 Hoang Quoc Viet, Cau Giay, Hanoi 10000, Viet Nam
*This paper is dedicated to the 40th anniversary of Institute for Tropical Technology if accepted for publication.
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