Adsorption of Ag⁺ ions using hydroxyapatite powder and recovery silver by electrodepositio

Cite this paper: Vietnam J. Chem., 2021, 59(2), 179-186 Article 
DOI: 10.1002/vjch.202000148 
179 Wiley Online Library © 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH 
Adsorption of Ag+ ions using hydroxyapatite powder and recovery 
silver by electrodeposition 
Pham Thi Nam
1
, Dinh Thi Mai Thanh
2,3
, Nguyen Thu Phuong
1
, Nguyen Thi Thu Trang
1
, 
Cao Thi Hong
1
, Vo Thi Kieu Anh
1
, Tran Dai Lam
1,3
, Nguyen Thi Thom
1*
1
Institute for Tropical Technology, Vietnam Academy of Science and Technology, 
18 Hoang Quoc Viet, Cau Giay, Hanoi 10000, Viet Nam 
2
University of Science and Technology of Hanoi, Vietnam Academy of Science and Technology, 
18 Hoang Quoc Viet, Cau Giay, Hanoi 10000, Viet Nam 
3
Graduate 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 
(HAp) powder and recovery silver by electrodeposition. The adsorption efficiency of silver was about 61 % at 50 
o
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 
Among industries, the electronic industry is the 
world’s largest and fastest growing manufacturing 
industry.
[1,2]
 Today, electrical and electronic waste 
are the type of waste that is most interested in the 
current waste stream because they are the fastest 
growing waste stream and grow 3 times faster than 
other types of waste (about 4 percent growth a 
year).
[3]
 The amount of electrical and electronic 
waste are created about 40 million tons each year. 
Electronic waste contains a lot of heavy metals, 
chemical compounds that easily penetrate soil and 
water, threatening the environment and human 
health.
[4-7]
 This seriously affects human health such 
as cancers, respiratory tract, cardiovascular and 
neurological.
[4-7]
 Since the early part of 19
th
 century, 
physicians have known that silver compounds can 
cause some areas of the skin and other body tissues. 
Skin contact with silver compounds has been caused 
mild allergic reactions, such as rash, swelling, and 
inflammation. The inhalation with high amount of 
silver compounds such as silver nitrate or silver 
oxide may cause breathing problems, lung and throat 
irritation and stomach pain.
[8]
 Nowadays, a large 
amount of electronic waste has been discharged into 
the environment without proper treatment. It carries 
the risk of polluting heavy metals into the ground 
and water. Therefore, the treatment of electronic 
waste is necessary. 
In addition, electronic waste also contains a big 
amount of many precious metals such as Au, Ag, Pd, 
etc. Recovery of precious metals prevents the 
pollution as well as prodigality. In Vietnam, some 
materials were synthesized to remove heavy metal 
ions such as: coffee husk, MnFe2O4/GNPs 
composite and chitosan/graphene oxide/magnetite 
nanostructured (CS/Fe3O4/GO) composite.
[9-11]
 The 
adsorption capacity for Ni(II) of coffee husk is 21.14 
mg/g, reported by Do Thuy Tien et al.
[9]
 The 
CS/Fe3O4/GO can remove 60 % of Fe(III) with 
adsorption capacity of 6.5 mg/g.
[11]
 Nguyen 
suggested that MnFe2O4/GNPs composite removed 
Pb
2+
 with high adsorption capacity of 322.6 mg/g.
[10]
Vietnam Journal of Chemistry Nguyen Thi Thom et al. 
© 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 180 
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 Pb
2+
, Cd
2+
 and Cu
2+
 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. 
2. MATERIALS AND METHODS 
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 
VWR chemicals, Belgium. The obtained 
hydroxyapatite powder has cylinder shape with size 
of 18 × 29 nm and the SBET = 75 m
2
/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  ...  1.00 1.25 1.50 1.75
0.7
0.8
0.9
1.0
1.1
1.2
L
o
g
 Q
Log C
e
y 
= 
0.
33
33
x 
+ 
0.
56
85
R
2 =
 0
.9
66
0
Figure 6: The Ag
+
 adsorption isotherm follows the 
Freundlich isothermal model using HAp powder 
0 10 20 30 40 50 60
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
C
e/
Q
 (
g
/L
)
C
e
 (mg/L)
y =
 0.
07
78
7 x
R
2 = 
0.9
83
42
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. 
1 2 3 4 5 6 7 8 9
4
6
8
10
12
14
16
 Q
pH solution
Q
 (
m
g
/g
)
10
20
30
40
50
60
 H H
 (
%
)
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 
© 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 183 
3.5. Effect of temperature treatment 
In this section, the Ag
+
 treatment temperature was 
adjusted from 20 to 70 
o
C using a thermostatic. The 
results show that the temperature increases from 20 
to 50 
o
C, the adsorption efficiency and capacity 
increase strongly (figure 9). It is clear that the 
temperature promotes movement of ions as well as 
ion exchange reaction. The temperature continues to 
increase, the adsorption efficiency and capacity 
nearly do not change. Therefore, the temperature 
value of 50
o
C is chosen to remove Ag
+
 ions. 
10 20 30 40 50 60 70 80
11
12
13
14
15
16
 Q
Temperature (
o
C)
Q
 (
m
g
/g
)
45
50
55
60
65
 H
H
 (
%
)
Figure 9: The variation of Q and H according to 
temperature 
3.6. Effect of adsorbent mass 
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. 
0.05 0.10 0.15
12
14
16
18
20
22
24
26
 Q
HAp mass (g)
Q
 (
m
g
/g
)
45
50
55
60
65
70
75
80
85
 H
H
 (
%
)
Figure 10: The variation of Q and H as a function of 
HAp mass 
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 
o
C for 60 minutes of the contact time. 
3.7. Characterization of HAp before and after 
treatment 
The characterizations of HAp powder before and 
after adsorption process were analyzed using FT-IR 
and XRD. The functional groups in the HAp 
molecule before and after Ag
+
 adsorption process 
were determined using FTIR spectra (figure 11). It 
can be seen clearly that Ag
+
 adsorption process does 
not change the functional groups in HAp molecule. 
For both of spectra, the characteristic peaks of OHˉ 
and PO4
3-
 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 
PO4
3-
 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]
4000 3500 3000 2500 2000 1500 1000 500
570-605
1040
1105
AgHAp
HAp
Wavenumber (cm
-1
)
T
 (
%
)
PO
4
3-
PO
4
3-
OH
-
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]
20 30 40 50 60 70
JCPDS: 00-009-0432
HAp
2 (degree)
In
te
ns
it
y Ag-HAp
Figure 12: XRD patterns of HAp before and after 
Ag
+ 
adsorption process 
Vietnam Journal of Chemistry Nguyen Thi Thom et al. 
© 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 184 
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). 
Figure 13: SEM images of HAp and AgHAp 
Figure 14: EDX spectrum of AgHAp 
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. 
Figure 15: The cathodic polarization curve of Au 
electrode in 5 mL of H2SO4 solution containing 0.5 g 
of HAp and Ag-HAp 
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: 
Ag
+
 + 1e → Ag (7) 
Ag - 1e → Ag+ (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. 
0 50 100 150 200 250
0
200
400
600
800
1000
1200
Time (min)
A
g
+
R
em
a
n
in
g
 (
m
g
)
 2 mA
 4 mA
 6 mA
 8 mA
 10 mA
Figure 16: The concentration of Ag
+
 ions remains in 
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 3.20 3.52 5.20 12.80 14.80 
45 7.20 8.64 10.24 18.80 22.80 
60 10.40 12.80 15.20 27.04 32.64 
90 12.80 16.40 24.00 34.40 39.20 
120 16.40 20.80 26.40 39.60 42.00 
150 19.20 23.60 30.80 42.00 45.20 
180 21.60 27.36 32.96 44.80 50.40 
210 24.00 30.00 36.00 47.60 55.60 
240 26.00 33.60 40.80 51.60 59.60 
Vietnam Journal of Chemistry Adsorption of Ag
+
 ions using hydroxyapatite 
© 2021 Vietnam Academy of Science and Technology, Hanoi & Wiley-VCH GmbH www.vjc.wiley-vch.de 185 
4. CONCLUSIONS 
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 (R
2
) 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. 
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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 
E-mail: nguyenthomsp@gmail.com; ntthom@itt.vast.vn. 
*This paper is dedicated to the 40
th
 anniversary of Institute for Tropical Technology if accepted for publication.