X. XIAO et al.: NOVEL MAGNETIC ALGINATE/HYDROXYAPATITE COMPOSITE WITH HIGH-EFFICIENCY ...
61–68
NOVEL MAGNETIC ALGINATE/HYDROXYAPATITE COMPOSITE
WITH HIGH-EFFICIENCY CADMIUM-ADSORPTION
PERFORMANCE
NOV KOMPOZIT NA OSNOVI MAGNETNEGA ALGINATA IN
HIDROKSIAPATITA Z VELIKO SPOSOBNOSTJO ADSORPCIJE KADMIJA
Xiao Xiao
2,3
, Jiabei Zhou
1,*
, Dali Zhou
4
, Liang Li
2,3
, Yingjiang Wu
2,3
,
Xiang Zhou
2,3
, Lang Du
2,3
, Honggen Chen
2,3
1
School of Chemical Engineering, Sichuan University, Chengdu 610064, China
2
Sichuan Institute of Product Quality Supervision, Inspection and Testing, Chengdu 610100, China
3
Chengdu Product Quality Inspection Research Institute Co., Ltd, Chengdu 610100, China
4
School of Materials Science and Engineering, Sichuan University, Chengdu 610064, China
Prejem rokopisa – received: 2023-06-05; sprejem za objavo – accepted for publication: 2023-12-05
doi:10.17222/mit.2023.927
Heavy-metal pollution (such as Cd(II)) is regarded as a serious environmental problem, posing a great threat to human beings.
In this research, a novel water-dispersible magnetic alginate/hydroxyapatite composite with high-efficiency Cd(II) adsorption
performance was successfully synthesized by a facile wet-chemical method. The magnetic separation experiment and magnetic
property analysis indicate that a magnetic alginate/hydroxyapatite composite can be effectively separated under a magnetic
field. The zeta-potential result and dispersity experiment indicate that the lowest zeta-potential is –39.4 mV at pH = 5, and the
obtained sample dispersed well in a Cd(II) solution after 120 min. The maximum adsorption capacity of a sample on Cd(II) is
135.3 mg g
–1
at pH = 5, and the adsorption of Cd(II) reached equilibrium in 10 min. The adsorption data could be fitted well us-
ing the Langmuir model, and the adsorption kinetic follows a pseudo-second-order kinetic model.
Keywords: magnetic alginate/hydroxyapatite composite, Cd(II), water-dispersible adsorbent, magnetic separation
Onesna`enje s te`kimi kovinami, kot je naprimer dvovalentni kadmij Cd(II)), se smatra za enega od najbolj resnih okoljskih
problemov, ki ogro`a zdravje ljudi. V tem ~lanku avtorji opisujejo raziskavo novega kompozita na osnovi magnetnega alginata
in hidroksiapatita z zelo veliko adsorbcijsko sposobnostjo za Cd(II), ki se dobro dispergira v vodi. Avtorji so ga uspe{no
sintetizirali z enostavno kemijsko metodo. Preizkus magnetnega lo~evanja (separacije) in analiza magnetnih lastnosti sta
pokazala, da lahko izdelani kompozit u~inkovito separiramo s pomo~jo magnetnega polja. Rezultati zeta potenciala in
eksperimenti disperzije so pokazali, da se dose`e najni`ji zeta potencial pri –39,4 mV in pH = 5. Pridobljeni vzorec se je dobro
dispergiral v raztopini po 120 min. Maksimalna adsorpcijska kapaciteta vzorca na Cd(II) je 135,3 mg·g
–1
p r ip H=5i n
adsorpcijsko Cd(II) ravnote`je se vzpostavi po 10 min. Pridobljeni eksperimentalni podatki adsorpcije se lahko dobro opi{ejo z
Langmuirjevim modelom in kinetika adsorpcije dvovalentnega kadmija sledi psevdo kineti~nemu modelu drugega reda.
Klju~ne besede: kompozit na osnovi magnetnega alginata in hidroksiapatita, dvovalentni kadmij, adsorbent, ki se dispergira
v vodi, magnetna separacija
1 INTRODUCTION
Heavy-metal pollution (such as Cd(II)) is regarded as
a serious environmental problem, posing a great threat to
human beings. The rapid development of industry is con-
sidered as the main source of the introduction of
heavy-metal ions into soil and ground water.
1
Cd(II)
from electroplating, metallurgy and mining industrial
sewage is the main heavy-metal pollutant in water and
soil ecosystems.
2
Cd(II) entering the human body could
cause an endocrine disorder and a high carcinogenicity,
as well as bone fracture and renal dysfunction.
3
There-
fore, it is very necessary to remediate Cd(II)-polluted
soil and water.
Classic accepted techniques used for the removal of
Cd(II) from wastewater include adsorption, in-situ solidi-
fication chemical precipitation, membrane filtration, and
ion-exchange.
4–7
Among the above methods, adsorption
was regarded as one of the most promising processes due
to its high efficiency, low cost and simplicity of applica-
tion. For the adsorption technology, adsorbent plays a
key role. Traditional adsorbents that were used to remove
Cd(II) from wastewater included clay, activated carbon
and natural zeolite.
8–11
Rao et al. studied the adsorption
of heavy-metal ions in wastewater by kapok-tree-
shell-activated carbon, and the results showed that the
adsorption capacity of kapok-activated carbon for Cu(II)
and Cd(II) reached 20.8 and 19.5 mg/g, respectively.
12
However, these materials had defects, suffering from low
efficiency or difficult separation from the treated solu-
tion.
13,14
HAP and HAP-related materials have been increas-
ingly used to adsorb Cd(II) due to their high adsorption
capacity, low water solubility, high chemical stability
and low cost.
15–17
Although the HAP has high capacity of
Cd(II) adsorption, it is difficult to separate from solution.
Materiali in tehnologije / Materials and technology 58 (2024) 1, 61–68 61
UDK 622.778:620.168 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 58(1)61(2024)
*Corresponding author's e-mail:
zjb@scu.edu.cn (Jiabei Zhou)

For such an adsorbent, it is necessary to apply an effi-
cient method of purification that involves materials that
can be recycled and does not generate secondary waste.
18
The traditional separation technologies such as filtra-
tion, sedimentation and centrifugation are unsuitable or
inefficient methods to recover HAP from a liquid phase.
To overcome the above problems, magnetic adsorbents
are considered owing to the efficient and fast separation
from the liquid phase by a magnetic field.
19
Recently, a
new water-dispersible magnetic "fluid" adsorbent (sus-
pensions with adsorbents behave like fluid) has been re-
ported with the advantages of low surface potential, fast
adsorption rate and easy separation.
20
For this, the
HAP-related water-dispersible magnetic adsorbent needs
to be rationally designed.
In this work, a novel water-dispersible magnetic
alginate/hydroxyapatite composite (magnetic alginate/
hydroxyapatite composite, M-ALG/HAP) was synthe-
sized as a high-efficiency and easily separable Cd(II) ad-
sorbent. M-ALG/HAP as Cd(II) adsorbent had magneti-
cally separable properties, a high specific surface, a low
zeta-potential and was well dispersed in an aqueous solu-
tion. The obtained M-ALG/HAP demonstrated a high-ef-
ficiency adsorption performance. The adsorption capac-
ity (135.3 mg g
–1
) of Cd(II) was higher than that for the
traditional adsorbent. and the equilibrium time of adsorp-
tion was within 10 min.
2 EXPERIMENTAL PART
2.1 Materials
All the chemicals used in this study were of analyti-
cal reagent grade. Sodium alginate, calcium chloride
(CaCl
2
·2H
2
0), sodium dihydrogen phosphate (NaH
2
PO
4
)
sodium hydroxide (NaOH), iron(II) chloride tetrahydrate
(FeCl
2
·4H
2
O), iron(III) chloride hexahydrate
(FeCl
3
·6H
2
O), Cd(II) chloride (CdCl
2
·2.5H
2
O) and hy-
drochloric acid (HCl) were from Chengdu Kelong
Chemical Co. Ltd. (Sichuan, China).
Before synthesizing M-ALG/HAP, the 2-M NaOH,
0.6-M NaH
2
PO
4
and 1-M CaCl
2
solution were prepared
for standby. The M-ALG/HAP were synthesized via a
wet chemical method. 0.64 g sodium alginate was added
to 180 mL of ultrapure water, and stirred for 6 h to ob-
tain a light-yellow sodium alginate solution. Sequen-
tially, 1-M CaCl
2
(8 mL) and 0.6-M NaH
2
PO
4
(8 mL)
were added dropwise into the sodium alginate solution,
then the pH of the solution was adjusted to 13 with 2-M
NaOH solution, aged for 20 h to a white suspension. Un-
der nitrogen atmosphere, the FeCl
2
and FeCl
3
mixed so-
lution was drop-wise added to the suspension, stirred for
1.5 h with 75 °C, aged 1 day, centrifuged, washed with
water three times, and freeze-dried to obtain the
M-ALG/HAP sample. For comparison, the pure HAP
was synthesized by the coprecipitation method.
2.2 Characterization
The chemical structure and phase composition of
M-ALG/HAP were analyzed by FT-IR (Nicolet 6700),
XPS (EscaLab 250Xi) and XRD (Shimadzu XRD-6100).
The zeta potentials of the M-ALG/HAP in Cd(II) solu-
tion were measured by Zetasizer Nano (Malvern Instru-
ment). The morphology of the sample was characterized
by SEM (FEI Inspect F50). The BET-specific surface
area and pore size distribution were measured by an Au-
tomated Specific surface area and Porosity Analyzer
(ASAP 2020). The magnetic properties of the
M-ALG/HAP were measured by a vibrating-sample
magnetometer (VSM, Lakeshore 735).
2.3 Cd(II) adsorption test
Considering the ranging of the initial Cd(II) concen-
tration and adsorption time to the adsorption capacity of
the M-ALG/HAP, adsorption experiments were carried
out using the batch-equilibration technique.
40 mg of M-ALG/HAP sample was added to 100 mL
of different concentrations of Cd(II) solution ranging
from 25 to 500 mg L
–1
. The kinetics adsorption experi-
ments were performed at an initial concentration
100 mg L
–1
, 25 °C, pH = 5.0. The Cd(II) concentration of
the solution was tested by ICP-AES (IRIS Adv, USA).
The amount of Cd(II) adsorbed on the M-ALG/HAP was
calculated from the balance relationship.
3 RESULTS AND DISCUSSION
Figure 1 shows the XRD pattern of synthetic
M-ALG/HAP. From Figure 1, the diffraction peaks of
M-ALG/HAP can be well-indexed as hexagonal HAP
(PDF 73-0294), indicating that the sample has hexagonal
HAP.
21
Meanwhile, M-ALG/HAP has an obvious diffrac-
tion peak at about 2 = 35°, which corresponds to the
characteristic diffraction peak of the (311) crystal plane
of Fe
3
O
4
(PDF No.70-0417), indicating that the sample
has a spinel structure Fe
3
O
4
.
22
The above XRD results
demonstrate that HAP and Fe
3
O
4
exist in M-ALG/HAP.
X. XIAO et al.: NOVEL MAGNETIC ALGINATE/HYDROXYAPATITE COMPOSITE WITH HIGH-EFFICIENCY ...
62 Materiali in tehnologije / Materials and technology 58 (2024) 1, 61–68
Figure 1: XRD pattern of synthetic M-ALG/HAP

For the analysis the interaction between the HAP and
alginate in M-ALG/HAP composite the FTIR spectra of
HAP and the prepared M-ALG/HAP were recorded (Fig-
ure 2). The adsorption bands of the HAP and
M-ALG/HAP at 3450 cm
–1
and 1635 cm
–1
were attrib-
uted to the stretching and bending vibration of the
hydroxyl (-OH) groups.
23
The bands at 1100 cm
–1
and
1030 cm
–1
were assigned to the PO4
3–
asymmetric vibra-
tion, and the bands at 605 cm
–1
and 570 cm
–1
were attrib-
uted to the PO4
3–
bending vibration.
24,25
Compared to the
pure HAP, the new adsorption band of M-ALG/HAP at
820 cm
–1
was attributed to the mannuronic acid (in
alginate chemical structure).
26
The above results indicate
that HAP, alginate and Fe
3
O
4
exist in M-ALG/HAP.
Figure 3 shows the XPS spectra of HAP and
M-ALG/HAP. From the survey spectra of M-ALG/HAP,
there are Ca, P, O, Fe and C elements on the surface of
the M-ALG/HAP. The C1s spectra can be divided into
the C-C (284.6 eV), C-O (285.8 eV), C=O (286.6 eV)
and O-C=O (288.7 eV).
27
The peak separation results
showed that there were C-C, C-O, C=O and O-C=O in
M-ALG/HAP, which was attributed to the existence of
these four chemical bonds in the alginate. The above
XPS results further proved the existence of HAP, Fe
3
O
4
and alginate.
Figure 4 presents the different magnification SEM
images of the synthetic M-ALG/HAP. The morphologic
investigation shows that the as-prepared M-ALG/HAP
haad irregular block-like shapes with several hundred
nanometers to several micrometers. It can be seen from
the high-magnification images that the min part of the
block particles is a flocculent structure matrix, which
should be alginate. A large number of nearly spherical
nanoparticles with a particle size of about 10 nm are
coated in and on the surface of the flocculent matrix,
which should be HAP and Fe
3
O
4
. The EDS results of the
M-ALG/HAP are shown in Figure 5. It can be seen from
Figure 5 that there are Ca, P, O, Fe and C elements in
M-ALG/HAP (Cu is from the Copper grid), which fur-
ther confirms the existence of a uniform distribution of
ALG, HAP and Fe
3
O
4
. Using the theoretical analysis of
X. XIAO et al.: NOVEL MAGNETIC ALGINATE/HYDROXYAPATITE COMPOSITE WITH HIGH-EFFICIENCY ...
Materiali in tehnologije / Materials and technology 58 (2024) 1, 61–68 63
Figure 2: FTIR patterns of synthetic M-ALG/HAP
Figure 4: Different magnification SEM images of synthetic
M-ALG/HAP
Figure 3: a) XPS survey spectra of M-ALG/HAP, b) C1s spectra of
M-ALG/HAP

crystal nucleation and growth, the reason for the growth
of the HAP and Fe
3
O
4
into nearly spherical nanoparticles
is that the large number of -OH and -COOH active
groups on the ALG molecular chain provide heteroge-
neous nucleation sites, and the heterogeneous nucleation
is easy to homogenize. A large number of crystal nuclei
are precipitated explosively, the solute concentration de-
creases rapidly, and the grain growth rate is slow, so a
large number of nearly spherical nanocrystalline parti-
cles are formed, and the crystallinity is low.
The VSM result and magnetic separation experiment
process of M-ALG/HAP are shown in Figure 6.Itcan
be seen from Figure 6a that the prepared M-ALG/HAP
has the greatest saturation magnetization (M
s
)o f
10.23 emu g
–1
. Based on this magnetic property of
M-ALG/HAP, a magnetic separation experiment was
carried out, as shown in Figure 6b. It could be observed
that the adsorbent and Cd(II) solution were effectively
separated within 1 min, indicating that M-ALG/HAP
could be rapidly separated from wastewater in a future
practical application.
Figure 7 presents the BJH desorption pore size distri-
bution and N
2
adsorption-desorption isotherm of the pre-
pared M-ALG/HAP. According to the BET result, the
specific surface area and average pore size of
M-ALG/HAP are 111.7 m
2
g
–1
and 9.5 nm, respectively.
The Cd(II) adsorption on M-ALG/HAP benefited from a
high specific surface area. The N
2
adsorption-desorption
isotherm of M-ALG/HAP is type IV with a distinct hys-
teresis loop (P/P
0
> 0.4).
18
This is due to the loose pore
structure of ALG, which corresponds to the SEM photos
(Figure 4).
Figure 8 shows the dispersibility of the
M-ALG/HAP in water, and Figure 9 presents the effect
X. XIAO et al.: NOVEL MAGNETIC ALGINATE/HYDROXYAPATITE COMPOSITE WITH HIGH-EFFICIENCY ...
64 Materiali in tehnologije / Materials and technology 58 (2024) 1, 61–68
Figure 5: EDS results of the M-ALG/HAP
Figure 7: a) N
2
adsorption-desorption isotherm of M-ALG/HAP,
b) BJH desorption pore size distribution
Figure 6: a) VSM analysis result of M-ALG/HAP (298 K), b) separa-
tion experiment of M-ALG/HAP from suspension by external magnet
(within 1 min)

of different pH values on the zeta-potential of
M-ALG/HAP. In the range of pH = 3.0 to 8.0, the
zeta-potentials of M-ALG/HAP are all negative, and the
negative zeta-potential can promote the adsorption of
Cd(II) ions by M-ALG/HAP in an aqueous solution. The
zeta-potential of the M-ALG/HAP decreases with the in-
creasing pH from 3.0 to 5.0, the lowest zeta-potential is
–39.4 mv at pH = 5, and then become steady when
pH = 5.0 to 8.0. Furthermore, the zeta-potential of
M-ALG/HAP is lower than that of pure HAP because the
ALG structure contains a large number of -COO- and
-OH functional groups.
12
From Figure 8, the obtained
M-ALG/HAP dispersed well in Cd(II) solution after 120
min benefiting from the low zeta-potential.
3.2 Adsorption isotherms
The adsorption performance and mechanism of
M-ALG/HAP, adsorption isotherms studies over Cd(II)
concentration increasing from 25 mg g
–1
to 500 mg g
–1
were measured (Figure 10). The adsorption experimen-
tal data were fitted with the Freundlich and Langmuir
models, respectively (Figure 10, fitted parameters in Ta-
ble 1). The Langmuir model has a higher related coeffi-
cient (R
2
= 0.926) than the Freundlich model. Therefore,
the Langmuir model is more suitable for describing the
adsorption isotherms of Cd(II) on the M-ALG/HAP, in-
dicating that the adsorption behavior of Cd(II) on
M-ALG/HAP is mainly monolayer adsorption.
28
This is
because the HAP grains in M-ALG/HAP are nearly
spherical particles with small particle size (about 10 nm),
poor crystallinity and many surface defects and active
sites. The maximum theoretical monolayer adsorption
capacity (q
max
) of M-ALG/HAP on Cd(II) is 135.3 mg g
–1
at pH = 5, which was higher than previously reported for
Cd(II) adsorbents.
29,30
Table 1: Fitted parameters of adsorption isotherm.
Freundlich model Langmuir model
qK C
n
ee
=
q
qb C
bC
e
e
e
=
+
max
() 1
R
2
KnR
2
q
max
b
0.837 76.7 0.112 0.926 135.3 0.78
Note: Where the explanatory note of K, n, b and q
max
(mg g
–1
) origi-
nate from reference.
18
3.3 Adsorption kinetics
Figure 11 shows the adsorption kinetics of the Cd(II)
ion on the M-ALG/HAP. As shown in Figure 10, the
equilibrium of Cd(II) adsorption could reached within 10
min, which is faster than on previously reported HAP-re-
lated adsorbents.
16–18
In order to further study the adsorp-
tion mechanism, the adsorption experimental results
X. XIAO et al.: NOVEL MAGNETIC ALGINATE/HYDROXYAPATITE COMPOSITE WITH HIGH-EFFICIENCY ...
Materiali in tehnologije / Materials and technology 58 (2024) 1, 61–68 65
Figure 8: Dispersity of M-ALG/HAP in Cd(II) solution
Figure 10: Isotherm curve Cd(II) adsorption on M-ALG/HAP with
different concentrations
Figure 9: Effect of different pH values on the zeta-potential of
M-ALG/HAP

were fitted by the pseudo-second-order model and
pseudo-first-order model (Figure 11, fitted parameters in
Table 2).
From Table 2 and Figure 11, the R
2
value of the
pseudo-second-order model is higher than those for the
pseudo-first-order kinetic model. Thus, the pseudo-sec-
ond-order model performs a better match between the
experimental and theoretical data. The above results re-
veal that the chemisorption involving valence forces
through the exchange or sharing of electrons between
Cd(II) ions and adsorbent is the rate-limiting step in the
adsorption process of M-ALG/HAP for Cd(II) ion.
31,32
From the above adsorption isotherms and adsorption
kinetics studies, it is shown that M-ALG/HAP has
high-efficiency Cd(II) adsorption performance. This is
because M-ALG/HAP has a small grain size, poor
crystallinity, many surface defects and active sites, high
specific surface area and low zeta-potential.
18,20
The tra-
ditional heavy-metal adsorption process requires a
long-term (over 2 h) constant temperature oscillation
process. Based on the high-efficiency adsorption perfor-
mance, water dispersability and magnetic separation
ability, the M-ALG/HAP further application as an adsor-
bent can greatly reduce the oscillation time and simplify
the adsorbent separation process. Thus, the heavy-metal
adsorption process can be further promoted in the treat-
ment of heavy-metal wastewater.
Table 2: Fitted parameters of adsorption kinetic.
Pseudo-second-order model Pseudo-frist-order model
q
kqt
kqt
t
e
e
=
+
2
2
2
1()
qqe
kt
te
=−
−
() 1
1
k
2
q
e
h
0
R
2
k
1
q
e
R
2
0.31 135.9 428.2 0.996 1.1 132.7 0.990
Note: Where the explanatory note of q
e
(mg g
–1
), q
t
(mg g
–1
), K
1
(min
–1
), k
2
(g mg
–1
min
–1
) originate from reference.
18
4 CONCLUSIONS
Heavy-metal pollution from the rapid development of
industry is regarded as one of the serious environmental
problems, posing a great threat to human beings. In this
work, a novel water-dispersible magnetic alginate/hydro-
xyapatite composite with high-efficiency Cd(II) adsorp-
tion performance was successfully synthesized by a fac-
ile wet chemical method.
The M-ALG/HAP has a high specific surface area of
111.7 m
2
g
–1
, helping to increase the Cd(II) adsorption
capacity. The magnetic separation experiments and VSM
analysis reveal M-ALG/HAP can be rapidly separated
from the suspension by a magnet. The zeta-potential re-
sults and dispersity experiments indicate that the lowest
zeta-potential reached –39.4 mv at pH = 5 and the ob-
tained sample dispersed well in a Cd(II) solution after
120 min.
The maximum adsorption capacity of the sample on
Cd(II) is 135.3 mg g
–1
at pH = 5, and the equilibrium of
Cd(II) adsorption could be reached within 10 min. The
adsorption isotherm could be fitted well by the Langmuir
model, indicating that the adsorption behavior of the
Cd(II) ion on M-ALG/HAP is mainly monolayer adsorp-
tion. The adsorption kinetics follows the pseudo-sec-
ond-order kinetic model, revealing that the chemisorp-
tion involving valence forces through the exchange or
sharing of electrons between Cd(II) ions and that the ad-
sorbent is the rate-limiting step in the adsorption process
of the M-ALG/HAP for Cd(II) ion.
Acknowledgment
The work was financially sponsored by the Sichuan
Province Industry Education Integration Demonstration
Project (Chuancaijiao [2022] No.106).
X. XIAO et al.: NOVEL MAGNETIC ALGINATE/HYDROXYAPATITE COMPOSITE WITH HIGH-EFFICIENCY ...
66 Materiali in tehnologije / Materials and technology 58 (2024) 1, 61–68
Figure 11: Cd(II) adsorption kinetics (data and fitted curve) on M-ALG/HAP. (a) pseudo-first-order model, (b) pseudo-second-order model

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