One-pot Synthesis of Mesoporous La(OH)3 Adsorbent and its ...

One-pot Synthesis of Mesoporous La(OH) 3
Adsorbent and its Application for Phosphate Removal
Wei-Ya Huang 1,2,3 , Xuelei Wang 2 , Dan Li 1* , Goen Ho 1 , Yuan-Ming Zhang 2*
1 Environmental Engineering, School of Engineering and Information Technology, Murdoch University,
Murdoch, Western Australia, 6150, Australia;
2 Department of Chemistry, Jinan University, Guangzhou, 510632, China;
3 Department of Materials Science and Engineering, Taizhou University, Linhai, 317000, China;
* Corresponding authors, D. Li: Tel.: +61 08 9360 2569, Email: L.Li@murdoch.edu.au; Y-M Zhang: Tel.:
+86 20 8522 1264, Email: Tzhangym@gmail.com
Abstract— In this study, we reported the one-pot synthesis of
mesoporous La(OH) 3 adsorbent (Meso-La(OH) 3 ) and its
application in phosphate removal for the first time. The
synthesized La(OH) 3 sample exhibited an irregular particle
shape and possessed mesopores. In the phosphate adsorption
test, the experiment equilibrium data were fitted better by
using the Langmuir model than the Freundlich model,
suggesting that the adsorption feature be monolayer. Our
results showed that Meso-La(OH) 3 had a maximum phosphate
adsorption capacity of 57.65 mg P/g.
Keyword-La(OH) 3 ; mesoporous; phosphate; adsorption
I. INTRODUCTION
To date, water eutrophication has become a severe global
environmental problem, which leads to overgrowth of algae,
depletion of dissolved oxygen, and in turn deterioration of
water quality [1]. One of the major factors causing water
eutrophication is the discharge of excessive nutrients,
especially phosphate, into water systems. Until now, intensive
effort has been devoted toward various strategies to remove
phosphate from water, including biological activated sludge
process, physical adsorption, and chemical precipitation [2].
Among those methods, physical adsorption has been
commonly studied in the removal of phosphate due to the good
removal efficiency and removal rate [2-4]. In the adsorption
method, it is necessary to develop highly efficient, low-cost
and easily available adsorbents. So far, a large number of
adsorbents have been studied to remove phosphate anions from
aqueous solution [5-9], such as goethite, vesuvianite, layered
double hydroxides, etc.
In recent years, the utilization of materials with
mesoporosity as adsorbents has become an increasing area of
research. In particular, after introducing the M41S family of
ordered mesoporous silica [10], their unique structural
properties, such as high surface area and uniform pore size,
have attracted great interest. However, despite the mesoporous
silicas, e.g. MCM-41, MCM-48 and SBA-15, possess many
attractive features as adsorbents, their adsorption capacities for
phosphate are reported to be extremely low [11, 12], since their
surface Si-OH can hardly capture phosphate anions from
aqueous solution. Metal doping method is suggested as an
effective way to improve the adsorption capacities of
mesoporous silicas working as phosphate adsorbents. For
instance, Zhang et al. fabricated a series of lanthanum-doped
mesoporous MCM-41 as adsorbents to remove phosphate [13].
It was found that the phosphate adsorption capacities increased
when greater amount of La was incorporated. The sample
La 25 M41 prepared with the Si/La molar ratio of 25 possessed a
high Langmuir adsorption capacity, which was 22.0 mg P/g
[13]. Yang et al. reported that the La-doped SBA-15 adsorbent,
La 100 SBA-15, which was synthesized with the
La(NO 3 ) 3·6H 2 O/SBA-15 mass ratio of 1, had a maximum
adsorption capacity of 45.63 mg P/g [14]. As compared to
calcium, aluminum or iron, lanthanum-modified adsorbents
show several promising advantages in phosphate removal, such
as superior adsorption capacity, wide operating pH range, and
high removal rate in low phosphate concentration [15, 16].
Although the metal doping of silica-based materials has been
studied in phosphate removal; some demerits are recognized,
such as possibly reduced pore sizes of mesoporous materials,
time-consuming and costly fabrication.
In this paper, the objective is to prepare mesoporous
La(OH) 3 via a one-pot synthetic method and investigate its use
for phosphate removal, which to the best of our knowledge is
reported for the first time. Herein, it is expected that our
adsorbent will possess a superior adsorption capacity, which
will be a highly efficient adsorbent for removing phosphate. In
this paper, the fabrication, characterization and phosphate
adsorption performances of mesoporous La(OH) 3 are detailed.
II.
EXPERIMENTALS
A. Synthesis of La(OH) 3
In a typical synthesis, 1.0 g La(NO 3 ) 3·6H 2 O was dissolved
in 1 mL deionized water; after which, 1 mL propionic acid and
30 mL glycol were added with stirring to form a uniform

solution. The resulting mixture was sealed in an autoclave and
heated at 180 °C for 200 min. The product was then
centrifuged, washed with water and ethanol, and dried at 80 °C
in air. The as-synthesized sample is denoted as Meso-La(OH) 3.
B. Characterization of materials
X-ray powder diffraction (XRD) patter was recorded in the
2θ range of 10–70 ° with a scan speed of 2 °/min by using a
diffractometer (Siemens D500 series) with Cu Kα radiation (30
mA, 40 kV). Surface morphology of the sample was examined
by scanning electron microscopy (SEM, Zeiss Neon 40EsB
FIBSEM). Transmission electron microscopy (TEM,
JEM2010) was used to characterize the sample at an
accelerating voltage of 200 kV. The sample was firstly
dispersed in ethanol via sonication and then was collected with
a carbon copper grid for TEM characterization. Nitrogen
adsorption-desorption isotherm was measured at 77 K using
ASAP 2010 (Micromeritics Inc., USA). Prior to analysis, the
sample was degassed at 120 °C for 12 h under vacuum. The
specific surface area, S BET , was determined from the linear part
of the BET plot (P/P 0 = 0.05-0.20). The pore size, d, was
calculated from the desorption branch of isotherm by using
Barrett-Joyner-Hallenda (BJH). The total pore volume, V total ,
was evaluated from the adsorbed nitrogen amount at a relative
pressure of 0.98.
C. Phosphate adsorption experiments
A series of batch tests were conducted to investigate the
phosphate adsorption performance of the Meso-La(OH) 3
adsorbent. In the equilibrium experiment, 0.025 g of the
adsorbent was added into 50 mL of phosphate solution with
various initial concentrations in a polypropylene bottle, which
was prepared by dissolving anhydrous K 2 HPO 4 (analytical
grade, Sinopharm Chemical Reagent Co., Ltd) in deionized
water. After shaken for 24 h at 25 °C, the solution was
removed by filtering through a syringe nylon-membrane filter
(pore size 0.45 μm; Shanghai Xinya Purification Devices
Factory). The concentraiton of phosphate in filtrate was
analyzed by Autoanalyzer 3 (Bran and Luebbe Inc., Germany).
The amount of phosphate adsorbed on the sample at the
equilibrium (q e ) was calculated by,
q =
e
( 0
(1)
C − Ce)
× V
m
where C 0 and C e are the initial and equilibrium phosphate
concentrations in solution (mg/L), respectively; V is the
volume of solution (L) and m is the mass of adsorbent (g).
The equilibrium data were fitted to the well-known
Langmuir and Freundlich isotherm models [17, 18], as shown
in (2) and (3), respectively:
Langmuir model: C
q
e
1
=
q K
e
+
0
L
C
q
e
0
(2)
Freundlich model:
1
log qe
= log KF
+ logC
(3)
e
n
where C e is the concentration of phosphate solution at
equilibrium (mg/L); q e is the corresponding adsorption capacity
(mg/g); q 0 (mg/g) and K L (L/mg) are the constants in Langmuir
isotherm model which are related to adsorption capacity and
energy or net enthalpy of adsorption, respectively; K F (mg/g)
and n are the constants in Freundlich isotherm model, which
measure the adsorption capacity and intensity, respectively.
III.
RESULTS AND DISCUSSIONS
The formation process of Meso-La(OH) 3 is speculated to be
a two-stage growth model based on the literature [19]. Ethylene
glycol and ester are suggested to work as structure-directing
agents to modify particle surface; thus affecting the nucleation,
aggregation and growth of particles, and subsequently forming
mesoporous structures. As shown in Scheme 1, firstly, the
La 3+ ions hydrolyze to form nano-sized La(OH) 3 crystalline
precursors in the solvothermal synthesis. Ethylene glycol is
absorbed on the nano-sized La(OH) 3 surface. Meanwhile,
propanoic acid reacts with ethylene glycol via an esterification
process, thus enhancing the particle surface modification.
Finally, the pre-formed small particles assemble and grow into
larger secondary particles. After removing the structuredirecting
agents, the mesoporous La(OH) 3 is obtained.
Scheme 1. The formation of Meso-La(OH) 3 [19].
Figure 1. XRD pattern of Meso-La(OH) 3.
Fig.1 shows X-ray diffraction pattern of Meso-La(OH) 3 .
The low intensity of peaks is probably due to the low degree of
powder crystallization. As seen, several peaks are observed in
the XRD pattern, corresponding to the reflections of La(OH) 3
(JCPDS card 36-1481), which indicate that the as-synthesized
sample is La(OH) 3 .

Figure 2. SEM (a) and TEM images (b and c) of Meso-La(OH) 3.
Fig. 2 shows SEM and TEM images of Meso-La(OH) 3 . In
Fig. 2a and b, the sample exhibits an irregular shape with a
wide range of particle size distribution. From the TEM image
(Fig. 2c), there are clear voids with a diameter of 5-10 nm
inside the particle, revealing the mesostructure of Meso-
La(OH) 3 .
Figure 4. Langmuir and Freundlich adsorption isotherms of Meso-
La(OH) 3.
TABLE II. LANGMUIR AND FREUNDLICH ISOTHERMS PARAMETERS IN
THE PHOSPHATE ADSORPTION BY USING MESO-LA(OH) 3.
Sample
Meso-
Langmuir
Freundlich
q 0 (mg/g) K L (L/mg) R 2 n K F(mg/g) R 2
57.65 2.41 0.964 4.98 27.98 0.802
La(OH) 3
TABLE III. COMPARISON OF PHOSPHATE ADSORPTION CAPACITIES
BETWEEN MESO-LA(OH) 3 AND OTHER LANTHANUM-MODIFIED ADSORBENTS
SELECTED FROM LITERATURE.
Adsorbents
Temperature
(°C)
Adsorption capacity
(mg P/g)
Ref.
Figure 3. N 2 adsorption-desorption isotherm and the corresponding BJH
pore size distribution plot (the inset) of Meso-La(OH) 3.
TABLE I. TEXTUAL STRUCTURE CHARACTERISTICS OF MESO-LA(OH) 3.
Sample S BET (m 2 /g) d (nm) V total (cm 3 /g)
Meso-La(OH) 3 65.6 8.74 0.153
Fig. 3 shows N 2 adsorption–desorption isotherm of Meso-
La(OH) 3 and its corresponding BJH pore size distribution plot
(the inset in Fig. 3). Table I summarizes the textual structure
characteristics, including BET surface area (S BET ), pore
diameter (d), and total pore volume (V total ). The N 2 adsorption–
desorption isotherm exhibits type IV isotherm model according
to the IUPAC classification standard, implying the
mesostructure in the sample [20]. The inset in Fig. 3 shows a
wide pore size distribution in Meso-La(OH) 3 , proving the
presence of mesopores in the particles.
PhoslockTM 23 10.2 [21]
La-treated bark fiber 25 10.9 [16]
La-loaded orange
waste
30 13.9 [22]
La-modified MCM-41 25 17.7 [23]
La-doped SBA-15 25 45.6 [14]
Meso-La(OH) 3 25 57.7 Our work
Fig. 4 shows the Langmuir and Freundlich adsorption
isotherms of Meso-La(OH) 3 . In Fig. 4 and Table II, the
experimental equilibrium data are fitted better by using the
Langmuir model (R 2 = 0.964) than the Freundlich model (R 2 =
0.802), suggesting the adsorption feature onto Meso-La(OH) 3
be monolayer. To further evaluate the applicability of our
fabricated adsorbent, Table III presents the comparison of
adsorption capacity of Meso-La(OH) 3 , which was calculated by
the Langmuir model, with other lanthanum-modified
adsorbents used for phosphate removal in literature. The
maximum adsorption capacity of Meso-La(OH) 3 estimated by
the Langmuir model is 57.7 mg P/g. It clearly shows that,
despite there is a slight difference in test temperatures, our
prepared Meso-La(OH) 3 exhibits a superior phosphate
adsorption capacity, when compared to those reported
adsorbents. Especially, its phosphate adsorption capacity is
significantly greater than those of lanthanum-modified
mesoporous silicas, e.g. MCM-41 and SBA-15, respectively.

CONCLUSION
We have successfully prepared the mesoporous La(OH) 3
(Meso-La(OH) 3 ) via a simple one-pot synthetic method and
utilized it to remove phosphate for the first time. The assynthesized
Meso-La(OH) 3 exhibited an irregular shape and
possessed mesopores, which were confirmed by using TEM,
SEM and nitrogen adsorption-desorption isotherm. Moreover,
our adsorbent had a better phosphate adsorption capacity, as
compared with those reported in literature. Therefore, our study
offers a simple and scalable approach to fabricate an adsorbent
with a superior phosphate adsorption performance. Future work
will study its performance and feasibility for practical
phosphate removal in wastewater, which especially contains a
wide range of contaminants, such as organic substances and
competitive ions, in detail. On the other hand, the optimization
and adjustment on the porous structure of our adsorbent is
expected to further enhance its phosphate adsorption capacity.
ACKNOWLEDGEMENTS
This work was supported by Natural Science Foundation of
Guangdong (No. S2011040001667), the Fundamental Research
Funds for the Central Universities (No. 21611310), the
National High Technology Research and Development
Program of China (863 Program) (No. 2009AA064401), and
the Key Laboratory of Mineralogy and Metallogeny
Cooperation Foundation (No. KLMM20110204). Ms Wei-Ya
Huang’ study was supported by the program of China
Scholarships Council (No.201206780010).
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