A green approach of preparation of fine active alumina with high specific surface area from sodium aluminate solution

Article abstract of DOI:10.1039/c8ra09853k

Fine active alumina (FAA) with a high specific surface area (SSA) is used in catalysis, adsorbents and other applications. This study presents a novel method of preparing high surface area FAA via a phase evolution from gibbsite through ammonium aluminum

Full text of DOI:10.1039/c8ra09853k

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A green approach of preparation of fine active
alumina with high specific surface area from
sodium aluminate solution
Cite this: RSC Adv., 2019, 9, 5628
Guoyu Wu,
Guihua Liu, * Xiaobin Li, Zhihong Peng, Qiusheng Zhou
and Tiangui Qi
Fine active alumina (FAA) with a high specific surface area (SSA) is used in catalysis, adsorbents and other
applications. This study presents a novel method of preparing high surface area FAA via a phase
evolution from gibbsite through ammonium aluminum carbonate hydroxide (AACH) to FAA.
4 2 3
Thermodynamic calculations showed that increasing the pH and (NH ) CO concentration both
promoted the transformation of gibbsite to AACH. Fine gibbsite precipitated from a sodium aluminate
solution could thus be efficiently changed to AACH and subsequently to FAA. Minimal particle
aggregation was achieved from gibbsite to AACH to FAA owing to the filling of capillaries by NH
CO , the formation of boehmite and interfacial hydrophobicity. Furthermore, capillary pressures of 1.25–
6.56 MPa during the AACH roasting process prevented the collapse of mesopores. The high capillary
pressure, numerous open mesopores, and inhibition of aggregation produced FAA with an extremely
3
and
2
4
Received 30th November 2018
Accepted 31st January 2019
2
ꢀ1
ꢁ
high SSA. The SSA of FAA was as high as 1088.72 m
g
following the roasting of AACH at 300 C for
DOI: 10.1039/c8ra09853k
180 min. This FAA was demonstrated to remove phosphate from wastewater with an adsorption capacity
ꢀ
1
rsc.li/rsc-advances
of 300.28 mg g .
1
3,14
addition of organic solvents,
adding a surfactant or
1
Introduction
15
16
chelator, modifying the particle surfaces or coating the
17
2 3
g-Al O
is widely used in catalysts, catalyst supports, wastewater particle surfaces. Various approaches, including azeotropic
18 19 20
purication, gas separation, automobile exhaust emission distillation, freeze-drying, supercritical drying and surface
1
21
reduction systems and ue gas cleaning. As many of those modication, are subsequently applied during drying or
applications depend on a large specic surface area (SSA), roasting (calcining) of the precursor. Additionally, the pore
increasing the SSA of active alumina is an important research structure (pore size, pore volume) can be tuned so as to increase
5,6
22–25
challenge.
Because the SSA of ne alumina is oen less than 250 m
the SSA, such as surfactants,
templates or pore-expanding agents.
, the SSA is typically increased by preparing nano-alumina as increase the SSA of nano-alumina to the range of 200 to 700 m
templates,
coupling
2
26
27–29
Those techniques
ꢀ
1
2
g
ꢀ1
well as by regulating the pore structure of the material. Various
g . Even so, such methods are expensive and tend to generate
methods have been developed for the preparation of nano- large amounts of high salinity waste water. Meanwhile, the ne
2
alumina, including chemical vapor deposition, mechanical active alumina (FAA) can be prepared in an economically
3
4
5,6
7
grinding, chemical pyrolysis, wet sol–gel, microemulsion,
friendly, economical manner using a sodium aluminate solu-
8
–10
11
precipitation
the wet method has been extensively investigated owing to its particles, although the low SSA of this material (<300 m g
low cost, simplicity and ready industrial application. However, limits its applications. Therefore, a green, economical methods
the unavoidable aggregation of nano-particles during precipi- of synthesizing FAA have a high SSA (>500 m g ) is required.
tation from solution as well as during drying or roasting can
Ammonium aluminum carbonate hydroxide (AACH) is oen
and hydrothermal synthesis. Among these, tion, which leads to less aggregation compared to nano-
30
2
ꢀ1
)
2
ꢀ1
greatly increase the particle size. Hence, various additional used as a precursor for FAA production. AACH, in turn, is
processes to inhibit aggregation have also been reported. In typically prepared by adding ammonium bicarbonate to an
31
those approaches, an Al(OH) or AlOOH precursor is rst care- aluminum sulphate solution. This process generates a large
3
4 2 4
fully prepared, such as by diluting a more concentrated solu- amount of wastewater containing (NH ) SO . If AACH could be
12
tion, modifying the solvent properties of a solution via the generated from gibbsite rather than amorphous aluminum
hydroxide, an inexpensive sodium aluminate solution could be
used to prepare FAA via a precipitation process that does not
School of Metallurgy and Environment, Central South University, Changsha 410083,
generate wastewater. Trimm et al. demonstrated that AACH can
Hunan, China. E-mail: liugh303@csu.edu.cn
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be synthesized from ne amorphous aluminum hydroxide by gibbsite is then precipitated from the supersaturated sodium
pumping CO into a dilute sodium aluminate solution. This aluminate solution, in which caustic soda is recycled. In addi-
2
2
ꢀ1
material was found to have a low SSA (from 200 to 300 m g
aer the roasting process.
)
tion, CO and NH , decomposed from AACH, are transformed
2 3
32,33
However, there have been few into (NH ) CO solution, and AACH is then formed in
4
2
3
reports of AACH synthesis using gibbsite. Furthermore, FAA (NH
with a high SSA of over 500 m g is expected to be obtained no wastewater and gas will be discharged during the prepara-
4
)
2
CO
3
solution. CO
2
and NH
3
are also recycled. Therefore,
2
ꢀ1
from a sodium aluminate solution without wastewater.
In the present study, an inexpensive sodium aluminate
tion of FAA from sodium aluminate solution.
2.2.2 Removal of phosphate from wastewater. The phos-
solution was used to prepare FAA by promoting a sequential phate solution by dissolving KH
2 4
PO into deionized water was
phase evolution from gibbsite to AACH to FAA. The feasibility of used to simulate wastewater containing phosphorus. Aer-
ꢀ
1
changing gibbsite to AACH in an ammonium carbonate solu- wards, 100 mL as-prepared KH PO solution and 2 g L FAA
2
4
ꢁ
tion was initially examined based on thermodynamic calcula- were mixed together at 25 C, removal of phosphate from
tions. The phase evolution process, along with the resultant wastewater was then studied by static shaking table test.
particle size distribution (PSD), particle morphology and pore
structure, were subsequently characterized by X-ray diffraction
(XRD), PSD analysis, scanning electron microscopy (SEM),
2.3 Characterization and methods
transmission electron microscopy (TEM) and N adsorption–
2
2
.3.1 Characterization. Gibbsite, AACH and FAA was iden-
desorption isotherms. The mechanism associated with the
formation of FAA with a high SSA was also studied in detail. In
addition, the FAA resulting from this process was employed to
remove phosphate from wastewater. The results demonstrate
a novel green process for the preparation of FAA with a high SSA
and also improve our understanding of the mechanism by
which particle aggregation is inhibited.
tied by X-ray diffraction (XRD, Rigaku TTR-III) with Cu Ka
radiation (l ¼ 0.154056 nm), the applied tube voltage and
electric current were 40 kV and 250 mA, respectively. Scan range
varied from 10 to 75 , and the scanning rate of 10 min . PSD
was determined using a Mastersizer-2000 (Malvern UK) when
sample was dispersed in deionized water by ultrasonic.
Morphologies were observed on SEM (JSM-6360LV, JEOL, Japan)
and TEM (FEI Tecnai G2 F20, USA). The thermal gravimetric
analysis and differential thermal analysis (TG-DTA) measure-
ments were conducted on Thermal Analysis SDT-2960 (USA)
with a heating rate of 5, 10 and 20 C min and up to 800 C in
an air atmosphere. Aer powder was pressed into a thin tablet
with 0.2 g sample under 20 MPa, the surface properties of
alumina, boehmite and AACH were measured by CL200B
Contact Angle instrument (Shanghai junlun information tech-
nology Co., LTD., China) using the drop method with glycerin,
diiodomethane and deionized water, surface tension and free
ꢁ
ꢁ
ꢁ
ꢀ1
2
Experiment
2
.1 Experimental materials
Sodium aluminate solution with a
soda Na O to alumina Al in sodium aluminate solution) was
prepared with aluminum hydroxide and sodium hydroxide
Xilong Chemical Co., Ltd.). H O , (NH ) CO , NH cH O and
ꢁ
ꢀ1
ꢁ
k
1.45 (molar ratio of caustic
2
2 3
O
(
2
2
4 2
3
3
2
CH CH OH (Sinopharm Chemical Reagent Beijing Co., Ltd,
3
2
China), as well as reagents used in analysis were of analytical
purity.
energy were then calculated based on the Owen rule. N
2
adsorption–desorption isotherms and pore structure were ob-
tained on an ASAP 2020 (Micromeritics Instruments, USA)
2
.2 Experimental
.2.1 Preparation of AACH from gibbsite precipitated from
sodium aluminate solution. 15 mL of sodium aluminate solu-
nitrogen adsorption apparatus. All samples were degassed at
2
ꢁ
1
50 C prior to Brunauer–Emmett–Teller (BET) measurements.
The BET specic surface area was determined by a multipoint
BET method using the adsorption data in the relative pressure
ꢀ1
ꢀ1
tion (CNa O ¼ 2.26 mol L , CAl O ¼ 1.57 mol L ) and 400 mL of
2
2
3
2 2
deionized water were added into a round ask, 25 mL of H O
P/P
0
range of 0.05–0.30.
ꢀ1
(10 wt%) was then added at speed of 5 mL min to generate
2
2.3.2 Methods. The concentration of caustic soda (Na O)
a gibbsite seed in vigorous agitation, followed by precipitation
of the gibbsite at 50 C for 60 min. The ne gibbsite was ltered
and washed with boiling deionized water. Aerwards, the ne
gibbsite was placed into a ask containing 100 mL of (NH ) CO
2 3
and alumina (Al O ) in sodium aluminate solution were
ꢁ
measured by titration. Phosphate concentration was deter-
34
mined by the molybdate blue spectrophotometric method.
Adsorption capacity q (eqn (1)) and removal rate of phosphate h
4
2
3
ꢀ
1
t
(1.54 mol L ) solution and 100 mL of deionized water. AACH
(%) (eqn (2)) were calculated according to the following eqn (1)
ꢁ
was produced at 50 C for 24 h when pH was maintained at
approximately 10.0 with addition ammonia (30 wt%). AACH was
then obtained by centrifugation, washed with deionized water
and (2), respectively.
ðC
0
ꢀ C
t
ÞV
q
t
¼
(1)
(2)
ꢁ
and alcohol, and dried at 60 C for 24 h. Finally, FAA was ob-
m
tained by roasting AACH at different temperature for 180 min.
The green economical process in preparing FAA is listed in
Fig. 1.
C
0
ꢀ C
t
hð%Þ ¼
ꢂ 100%
C
0
0 t
In Fig. 1, the spent liquor (spent sodium aluminate solution) where C and C are the initial and t-moment concentration of
ꢀ1
t
is used to dissolve aluminum hydroxide or bauxite, and the ne phosphate in solution (mg L ), q represents the t-moment
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Fig. 1 The green economical approach to prepare FAA with high specific surface area from sodium aluminate solution (containing reaction
equation).
ꢀ
1
adsorption capacity (mg g ), m stands for adsorbent mass (g), was nearly equal to its concentration. Based on the equilibrium
and V is solution volume (L). reactions (8)–(12) and mass balance (13), Fig. 2 presents the
dependence of [N] on pH at 298.15 K.
2 3
Results in Fig. 2 indicated that NH Al(OH) CO could be
T
4
3
Results and discussion
formed at a pH range from 9.92 to 10.28. Furthermore, raising
pH and (NH ) CO concentration widened the region of NH -
3.1 Transformation from gibbsite to AACH
4
2
3
4
Gibbsite can be economically precipitated from the supersatu- Al(OH) CO and narrowed the region of AlOOH. Meanwhile,
2 3
rated sodium aluminate solution relative to preparation of increasing total (NH
4
)
2
CO
3
concentration benetted Al(OH)
Al(OH) CO and limited the
Al(OH) CO could be
in concentrated alkaline
3
or
ꢀ
amorphous Al(OH) by neutralization or hydrolysis. Thus, the Al(OH)
4
transformation into NH
4
2
3
3
possibility of gibbsite changing into AACH was rstly examined formation of AlOOH. However, NH
4
2
3
ꢀ
through thermodynamic calculation, and the transformation changed into AlOOH or Al(OH)
was then veried by experiments.
solution.
.1.1 Reaction behavior of gibbsite in ammonium
3.1.2 Phase evolution from gibbsite to AACH to FAA. Fig. 3
carbonate solution. Since ammonium carbonate solution is shows the XRD patterns obtained on going from gibbsite to
4
3
2
ꢀ
ꢀ
ꢀ
ꢀ
+
weakly alkaline, CO3 , HCO , OH , and NH may exist in the AACH to FAA. The well characteristic peaks were assigned to the
3
4
2
ꢀ
ꢀ
+
solution. Meanwhile, CO3 , HCO3 , OH , or NH ions may gibbsite (PDF no. 74-1775) prepared from sodium aluminate
4
react with aluminum ions. Table 1 lists data of the Gibbs free solution by seeded precipitation (Fig. 3(a), pattern 1). A new
ꢁ
energy for the above substances. Table 2 presents the possible peak at a 2q value of 15.31 in Fig. 3(a) (pattern 2) that was
reaction equations and corresponding reaction constants at assigned to AACH (PDF no. 52-1138) was found aer the gibb-
2
98.15 K.
site changed to AACH for 60 min. Many peaks of AACH were
then observed aer increase in duration (Fig. 3(a), pattern 3 and
+
Ammonium exists in the form of NH
3
, NH
4
and NH
4
-
Al(OH)
2
CO
3
in the system of gibbsite (Al(OH)
3
) changing into 4). There results prove that gibbsite can be changed into AACH
CO solution, which is in well agreement with that in
AACH (NH Al(OH) CO ). The total concentration of ammonium in (NH
)
2
4
2
3
4
3
in the solution, denoted as [N] , was expressed in the following Fig. 2.
T
eqn (13) on the basis of the mass balance and reaction formula.
Fig. 3(b) shows the phase evolution from gibbsite through
AACH and boehmite to FAA. Boehmite (PDF no. 21-1307) was
(13) observed during the roasting of AACH at 150 C, and active
+
ꢁ
[N]
T
¼ [NH
3
] + [NH
4
] + [NH
4
Al(OH)
2
CO
3
]
alumina g-Al
2
O
3
(PDF no. 04-0880) was produced during
As solubility of aluminum hydroxide is minimal in the
ꢁ
roasting at 500 C for 180 min. The g-Al O evidently had a low
degree of crystallization and contained defect sites and high
reactive sites, as demonstrated by the broad diffraction peaks in
2
3
ꢀ
weakly alkaline (NH ) CO solution, little Al(OH) exists in the
4
2
3
4
solution. Meanwhile, no corresponding activity coefficient in
the (NH CO solution was reported. Therefore, the ion activity
4
)
2
3
Table 1 Gibbs free energy of aluminum ions, carbonate ions, or compounds at 298.15 K (ref. 35–38)
2ꢀ
4ꢀ
3
4
+
3ꢀ
32ꢀ
Substances
AlO
ꢀ827.48
Al(OH)
ꢀ1305.7
Al(OH)
ꢀ1154.01
AlOOH
ꢀ1831.51
NH
ꢀ79.5
HCO
ꢀ586.77
CO
ꢀ527.98
q
ꢀ1
f
D G /(kJ mol
)
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a
Table 2 Reaction equation and their equilibrium constant at 298.15 K (ref. 39–41)
Serial number
Ions reaction equation
lg K
2ꢀ
+
2
3
(
(
(
(
(
(
(
(
(
(
(
(
1)
2)
3)
4)
5)
6)
7)
8)
AlO + H + H
O ¼ Al(OH)
15.66
14.97
15.19
0.99
10.33
9.24
14.00
16.38
27.03
4.34
14.99
0.155
ꢀ
ꢀ
+
Al(OH)
Al(OH)
4
+ H ¼ Al(OH)
3
2
+ H O
+
4
+ H ¼ AlOOH + 2H
2
O
+
+
3ꢀ
+ HCO
(NH
4
)
2
CO
3
+ H ¼ 2NH
4
ꢀ
+
2ꢀ
3
HCO
NH
3
¼ H + CO
+
ꢀ
3
$H
2
O ¼ NH
4
+ OH
+
ꢀ
ꢀ
ꢀ
H + OH ¼ H
2
O
+
3ꢀ
+
Al(OH)
Al(OH)
Al(OH)
Al(OH)
4
4
3
3
+ NH
+ NH
4
+ HCO + H ¼ NH
4
Al(OH)
Al(OH)
Al(OH) CO + H
CO
2
CO
3
+ 2H
+ 2H
2
O
2
O
+
2ꢀ
+
9)
4
+
+ CO
3
+ 2H ¼ NH
4
2
CO
3
4
+ HCO ¼ NH
3ꢀ
4
2
3
2
O
10)
11)
12)
+ NH
+ NH
Al(OH)
+
2ꢀ
+
4
+ CO
3
+ H ¼ NH
4
Al(OH)
2
3
+ H
2
O
+
ꢀ
NH
4
2
CO
3
¼ AlOOH + NH
4 3
+ HCO
a
q
lg K for eqn (8)–(12) are obtained according to the following equation DG298 ¼ ꢀRT ln K.
The gibbsite particles in Fig. 5(a) were approximately 0.5 mm in
size, which was consistent with the results in Fig. 4. Aer
ultrasonication was applied to disperse the gibbsite in anhy-
drous alcohol (90 W power for 240 min), gibbsite particles
approximately 30 to 60 nm in size were found to be aggregated
(Fig. 5(b)). Fig. 5(c) showed uniform, ne AACH bers with
lengths of approximately 1.00–1.50 mm and diameters of close
to 0.05 mm. The fact that the well crystallized AACH was readily

ltered compared with nano-gibbsite led to inexpensive sepa-
ration from solid to liquid in slurry. Roasting of the AACH at
ꢁ
5
00 C for 180 min gave FAA particles with ber-like shapes
(Fig. 5(d)) and a similar morphology to the AACH. Many FAA
primary particles with sizes less than 30 nm were also observed,
with numerous nanopores having diameters of over 2 nm
scattered throughout the particles.
Thus, particle aggregation during the transformation from
gibbsite through AACH to FAA was effectively inhibited. This
4 2 3
Fig. 2 Phase diagram of aluminum compound in (NH ) CO system at process allows the economical, environmentally friendly prep-
ꢀ
1
2
98.15 K. 1#-pH ¼ 9.92, [N]
T
¼ 1.0 mol L , 2#-pH ¼ 10.04, [N]
T
¼
aration of FAA from a sodium aluminate solution, based on
recycling of the spent sodium aluminate solution and the
ꢀ1
ꢀ1
1
.5 mol L , 3#-pH ¼ 10.15, [N]
T
¼ 2.0 mol L , 4#-pH ¼ 10.22, [N]
T
¼
ꢀ1
ꢀ1
2.5 mol L , 5#-pH ¼ 10.28, [N]
T
¼ 3.0 mol L
.
decomposition gases NH
roasting step.
3 2
, and CO produced during the
42
Fig. 3(b), pattern 3. Those data demonstrate that g-Al
2
O
3
is
successfully produced by phase evolution from gibbsite to
AACH and FAA.
3.2 Pore structure of FAA
3.1.3 PSDs of gibbsite, AACH and FAA. The particle size of The pore structure (pore size, pore volume, and open or closed
the precursor has a signicant effect on the SSA of the FAA, and pore) of the FAA, which is related to its SSA, is affected by the
Fig. 4 presents the PSD curves for the gibbsite, AACH and FAA roasting process. Thus, the effects of both the roasting
prepared in this work. It was evident that the PSD plot of the temperature and heating rate were investigated, with the aim of
FAA resembled those of the gibbsite and AACH. The mean minimizing the particle aggregation.
particle sizes (d(50)) of gibbsite, AACH and FAA were 0.586,
3.2.1 Effect of roasting temperature on mass loss. Because
.554 and 0.417 mm, respectively, conrming that each was the H O, CO and NH are generated during the thermal decom-
0
2
2
3

ne powder. Although extensive particle aggregation typically position of AACH, the gas volume produced per unit time,
occurs during the powder drying and roasting (or calcining) which in turn is related to the heating rate, will determine the
processes, such aggregation was effectively inhibited in the FAA pore structure. Fig. 6 provides the thermal gravimetric and
present work by the phase evolution process to synthesize the differential thermal analysis (TG-DTA) curves obtained from
FAA.
AACH decomposition at various heating rates.
3
.1.4 Morphological structures of gibbsite, AACH and FAA.
In Fig. 6, similar TG-DTA curves were acquired at heating
ꢁ
ꢀ1
To further clarify the particle size variations among the gibbsite, rates of 5, 10 and 20 C min . However, increasing the heating
AACH and FAA, SEM and TEM images are presented in Fig. 5. rate shied the onset of thermal decomposition from 195.27 to
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Fig. 3 XRD patterns of (a) AACH synthesized from gibbsite over different time spans and (b) gibbsite, AACH, boehmite and FAA. Gibbsite was
ꢀ
1
ꢀ1
ꢁ
precipitated from a sodium aluminate solution (CNa O ¼ 2.26 mol L , CAl O ¼ 1.57 mol L ) at 50 C for 65 min, while AACH was produced in
2
2
3
ꢀ
1
ꢁ
ammonium carbonate solution (C(NH ) CO ¼ 1.54 mol L ) held at 50 C for 180 min. Boehmite was obtained after the AACH was roasted at
4
2
3
ꢁ
ꢁ
1
50 C for 180 min, and FAA was synthesized by roasting the AACH at 500 C for 180 min.
ꢁ
ꢀ1
a mass losses of 56.03 wt% occurred at 20 C min comparison
to 56.83 wt% in the form of boehmite from AACH. Moreover,
ꢁ
a 1.55–1.85 wt% mass loss was observed above 500 C, owing to
43
the formation of g-Al O . From those results, the AACH
2
3
appears to have decomposed according to the following series
of reactions.
D
NH
4
AlðOHÞ CO
3
¼ AlOOHðboehmiteÞ þ NH
3
2
[ þ CO [
2
2
þH O[
D
2
AlOOH ¼ Al
2
O
3
ðamorphousÞ þ H
2
O[
D
Al
2
O
3
ðamorphousÞ ¼ g-Al O
3
2
AlOOH was interestingly found during the transformation of
AACH into FAA. Its surface property and water mass loss
4
4,45
differed from those of Al(OH)
3
(2Al(OH)
3
¼ Al
2 3 2
O + 3H O),
Fig. 4 PSD curves of gibbsite, AACH and FAA. Gibbsite was precipi-
formation of boehmite may mitigate the hard aggregation of
particles in roasting process.
ꢀ
1
tated from a sodium aluminate solution (CNa O ¼ 2.26 mol L , CAl O
¼
2
2
3
ꢀ1
ꢁ
1
.57 mol L ) at 50 C for 65 min, while AACH was produced in
ꢀ1
ꢁ
3.2.2 Effect of roasting temperature on contact angle and
surface energy. Contact angle measurements are oen used to
evaluate surface properties such as hydrophilicity, hydropho-
bicity and surface free energy. Thus, the extent of particle
ammonium carbonate solution (C(NH ) CO ¼ 1.54 mol L ) at 50 C for
4
2
3
ꢁ
1
80 min. The FAA was obtained after AACH roasted at 500 C for
80 min.
1
ꢁ
2
01.47 to 219.92 C, while reducing the extent of mass loss. aggregation on going from AACH to AlOOH to FAA was dis-
There results suggest that a greater volume of gas will be cussed by determining the contact angles of AACH, AlOOH and
released per unit time when applying a more rapid heating FAA with similar PSDs. The solid surface energy of each material
46
during the roasting of AACH. A strong endothermic peak was also estimated using Owen's method. Table 3 displays the
ꢁ
appears at 200 C in each curve, attributed to the formation of resulting data.
boehmite as an intermediate (in Fig. 3(b)). Increasing the
The results in Table 3 indicated that the contact angles
heating rate may accelerate the formation of boehmite because tended to vary signicantly when glycerol, diiodomethane and
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Fig. 5 SEM and TEM images of (a and b) gibbsite, (c) AACH and (d) FAA. Gibbsite was precipitated from a sodium aluminate solution (CNa O
¼
2
.26 mol L , CAl O ¼ 1.57 mol L ) at 50 C for 65 min, while AACH was produced in ammonium carbonate solution (C(NH ) CO ¼ 1.54 mol Lꢀ1) at
ꢀ
1
ꢀ1
ꢁ
2
5
2
3
4
2
3
ꢁ
ꢁ
0 C for 180 min. The FAA was obtained after AACH roasted at 500 C for 180 min.
water were acted as the liquid phase. Elevating the temperature minimal increase in particle size on going from AACH to FAA
gradually decreased the contact angle with glycerol, while the (Fig. 4). This nding reveals that the interface energy of the
contact angles of AACH and boehmite remained nearly constant powder is dependent on particle size.
with diiodomethane. In contrast, the contact angle initially
3.2.3 Effect of roasting temperature on the pore diameter
increased and then decreased with increasing temperature distribution of FAA. The functionality and SSA of FAA are both
48
3
when using water (Table 3 (q )). These data suggest that the dependent on its pore structure. Because the gas volume
hydrophobicity increases in the order of AACH > boehmite > release varies with the roasting temperature, the pore structure
FAA, thereby preventing particle aggregation. In addition, the will also be expected to vary along with the AACH thermal
solid surface energy remained essentially constant at roasting decomposition temperature. Fig. 7 provides the N adsorption–
2
ꢁ
temperatures ranging from 150 to 500 C but signicantly desorption isotherms and pore size distribution date obtained
ꢁ
increased at 800 C. This nding is in agreement with the at specic roasting temperatures.
ꢁ
ꢀ1
Fig. 6 TG-DTA curves obtained from AACH at heating rates of (a) 5, (b) 10 and (c) 20 C min . Gibbsite was precipitated from a sodium
ꢀ1
ꢀ1
ꢁ
aluminate solution (CNa O ¼ 2.26 mol L , CAl O ¼ 1.57 mol L ) at 50 C for 65 min, while AACH was produced in ammonium carbonate solution
2
2
3
(C
(NH ) CO ¼ 1.54 mol Lꢀ ) at 50 C for 180 min.
1
ꢁ
4
2
3
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a
Table 3 Surface properties of the original aluminum oxide and its hydrate formed by the roasting process
1
#-Al(OH)
3
2#-AACH
3#-boehmite
4#-FAA-R2
5#-FAA-R3
6#-FAA-R4
7#-FAA-R5
8#-FAA-R8
ꢁ
Roasting temperature ( C)
—
—
150
200
300
400
500
800
ꢁ
q
1
q
2
q
3
(glycerol, )
63.78
38.92
28.34
40.26
62.71
36.59
30.82
41.37
50.23
36.95
33.79
44.67
47.55
27.19
53.45
47.81
30.14
23.09
25.14
44.82
27.30
21.62
22.62
44.93
15.45
14.28
20.73
45.50
14.37
13.52
18.10
61.97
ꢁ
(diiodomethane, )
ꢁ
(water, )
ꢀ2
s
g (mJ m )
a
47
ꢁ
0
.2 g sample was pressed into a thin tablet under 20 MPa for 1 min. 1#-Al(OH)
3
, 2#-AACH, 3#-boehmite, roasting AACH at 150 C for 180 min, 4#-
ꢁ
ꢁ
ꢁ
FAA-R2, roasting AACH at 200 C for 180 min, 5#-FAA-R3, roasting AACH at 300 C for 180 min, 6#-FAA-R4, roasting AACH at 400 C for 180 min, 7#-
FAA-R5, roasting AACH at 500 C for 180 min, 8#-FAA-R8, roasting AACH at 800 C for 180 min.
ꢁ
ꢁ
Fig. 7(a) and (b) present the N
isotherms of FAA roasted at various temperatures. Type IV 400 and 500 C, primarily due to the presence of (NH ) CO in
curves were obtained at 200, 300 and 800 C, as well as type water in the capillaries. These samples also produced type H4
H1 hysteresis loop according to the IUPAC classication hysteresis loops consistent with narrow pores.
system, conrming the presence of open mesopores. In
2
adsorption–desorption contrast, type I curves were generated by the AACH roasted at
ꢁ
4
2
3
ꢁ
Fig. 7 The effect of roasting temperature on (a and b) N
2
adsorption–desorption isotherms and (c and d) pore diameter distribution. STP:
ꢀ1
ꢀ1
standard temperature and pressure. Gibbsite was precipitated from a sodium aluminate solution (CNa O ¼ 2.26 mol L , CAl O ¼ 1.57 mol L ) at
2
2
3
ꢁ
ꢀ1
ꢁ
5
0
C for 65 min, while AACH was produced in ammonium carbonate solution (C(NH ) CO ¼ 1.54 mol L ) at 50 C for 180 min. FAA was
4
2
3
synthesized by roasting the AACH roasted at different temperature for 180 min.
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Table 4 Pore characteristics of AACH synthesized via thermal decomposition at various heating rates
2
ꢀ1
3
ꢀ1 a
b
Surface area (BET) (m g
)
Total pore volume (cm g
)
Average pore diameter (nm)
Temperature
( C)
Rapid heating
rate
Slow heating
rate
Rapid heating
rate
Slow heating
rate
ꢁ
c
d
Samples
Rapid heating rate
Slow heating rate
1
2
3
4
5
#-FAA-R2
#-FAA-R3
#-FAA-R4
#-FAA-R5
#-FAA-R8
200
300
400
500
800
982.52
1088.72
813.23
722.59
336.43
771.41
925.61
618.68
552.77
241.24
0.55
0.56
1.32
1.65
1.85
0.28
0.45
0.54
0.55
0.58
2.44
2.80
7.17
102.03
109.59
2.54
2.85
4.43
4.71
5.73
a
b
c
Single point total pore volume of pores at P/P
0
¼ 0.99. Estimated using the BJH desorption branch of the isotherm. Rapid heating rate: sample
ꢁ
ꢁ
ꢀ1
ꢁ
d
was roasted by a rapid heating rate (20 C min ) from 200 C to a given temperature in muffle furnace. Slow heating rate: sample was roasted
according to heating rate of 5 C min from 100 C to in muffle furnace.
ꢁ
ꢀ1
In Fig. 7(c) and (d), two peaks of pore diameter size were
found. Numerous mesopores and few macropores in FAA would
promote its functioning as an adsorbent because there pores
4
9–51
can provide efficient transportation pathways to molecules.
ꢁ
Increasing the temperature from 200 to 800 C enlarged the
pore diameters, while smaller pores (2–30 nm) were corre-
spondingly disappeared. There results suggest that smaller
pores are preferentially destroyed during thermal decomposi-
tion of the AACH, and that the appearance of large pores can be
inhibit particle aggregation due to great capillary pressure.
3.2.4 Variations in capillary pressure at different roasting
temperatures. To obtain additional information regarding
particle aggregation during the roasting process, the capillary
pressures (static pressure) were calculated according to the
capillary pressure equation P
Tables 3 and 4, as well as in Fig. 6 and 7. Capillary pressures
dynamic pressure) were also calculated using the van der Waals
c
¼ 2s cos q/r, using the data in
(
equation of state, based on the data in Fig. 6(c). Fig. 8 plots the
capillary pressures as functions of the roasting temperature.
There data showed that the capillary pressure was reduced
from 46.56 to 2.65 MPa when the roasting temperature was
a
b
Fig. 8 Static (1#) and dynamic (2#) capillary pressure in pore
ꢁ
a
increased from 200 to 400 C, because large amounts of NH
3
structures as functions of roasting temperatures. Calculated
according to the equation, P
pressure (Pa), s is the interfacial tension of the interface (mN m ), q is
the contact angle ( ), r is the pore diameter size (m). Calculated
52
2
and CO lled the mesopores aer decomposition of the AACH.
c
¼ 2s cos q/r. Where P
c
is the capillary
ꢀ
1
Accordingly, the high capillary pressure prevented mesopore
from collapse, leading to minimal particle aggregation and
numerous open mesopores. Although the heating rate of the
thermal analysis instrument (SDT-2960) was limited, additional
increases in the heating rate would be expected to further
increase the amount of gas released per unit time. Conse-
quently, the pressure in the capillary (as shown in curve 1 in
Fig. 8) was increased (relative to that in curve 2 in Fig. 8). Thus,
a rapid heating rate, favoring the inhibition of particle aggre-
gation, was adopted in the following experiments.
ꢁ
b
2
53,54
according to the equation, (P + a/V
gas pressure (Pa), a and b are species-dependent critical parameters,
is a gas of the molar volume (L), R is the gas constant (R ¼ 8.314 J
0
)(V
0
ꢀ b) ¼ RT.
Where P is the
V
(
0
ꢀ1
ꢁ
mol K) ), T is the real temperature ( C). V
0
was calculated according
to mass loss in Fig. 6(c).
NH
3 2
and CO over an extremely short time compared to
ꢁ
ꢀ1
decomposition at a heating rate of 5 C min in Fig. 6. A rapid
heating rate was therefore employed to obtain FAA with a super-
high SSA. Table 4 summarizes the pore structures (surface area,
pore volume and pore diameter) in AACH obtained when
various roasting temperature and heating rates were adopted.
Elevating the temperature (in Table 4) increased the average
pore size and pore volume of the FAA, while reducing the BET
surface area. A rapid heating rate also signicantly increased
the FAA surface area and total pore volume relative to those
In addition, the NH gas generated in this process produced
3
a weakly alkaline solution in the capillary that was conducive to
the formation of AlOOH (Fig. 1) instead of Al(OH) . The data in
3
Table 3 also showed that boehmite was more hydrophobic than
ꢁ
gibbsite (Al(OH)
2
3
) (In Table 3 qAlOOH ¼ 33.79 > qAl(OH)3
¼
ꢁ
8.34 ). This nding suggests that the formation of boehmite
contributes to the inhibition of aggregation during the AACH
roasting process.
ꢁ
ꢀ1
obtained using a low heating rate of 5 C min . Surprisingly,
the maximum alumina BET surface area was 1088.72 m g at
3.2.5 Effect of heating rate on pore structure. Particle
2
ꢀ1
aggregation was further inhibited by the generation of more
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Fig. 9 (a) Adsorption capacity and removal rate as functions of phosphate concentration and (b) fitting results by Langmuir and Freundlich
isothermal adsorption models. For the adsorption of phosphate, 0.2 g FAA (FAA-R5, rapid heating rate) and 100 mL phosphate solution were
ꢁ
added into flask at pH of 5.0. Removal of phosphate from solution was carried out at 25 C with agitation of 150 rpm and contact time 10 h.
ꢁ
3
00 C when applying a rapid heating rate. This value is much
From Fig. 9(b), the data were in well agreement with the
5
6,57
greater than values obtained for nano-alumina produced by sol– Freundlich isotherm model,
suggesting that multilayer
gel, neutralization or hydrolysis. Roasting the AACH for and monolayer adsorption occur simultaneously on the FAA
80 min at a rapid heating rate achieved SSA values of 722.59 surface. The maximum theoretical phosphate adsorption
20
1
2
ꢀ1
ꢁ
ꢀ1
and 336.43 m g at 500 and 800 C, respectively, both of which capacity for the FAA was 526.32 mg g according to the q
are also signicantly greater than values for ne alumina and calculated from the Langmuir model. The efficient adsorption
nano-alumina.
is closely correlated with the pore structure, total pore volume plentiful open mesopores. These data also suggest that this
and average pore diameter. FAA could be employed for the efficient purication of
m
21,22
There data demonstrate that the heating rate of phosphate by this material is attributed to its high SSA and
Based on the above, FAA having a d(50) of 0.417 mm with wastewater.
a super-high SSA, a large amount of open mesopores and
minimal aggregation is attributed to high capillary pressure, the
formation of boehmite as an intermediate and the low surface
energy of the particles.
4 Conclusions
(
1) FAA was prepared via a green, economical process involving
phase evolution from gibbsite through AACH to FAA. Increase
in both the pH and (NH CO concentration promoted the
transition of gibbsite to AACH. In this process, ne gibbsite was
The as-prepared FAA, having plentiful open mesopores, repre- precipitated from a saturated sodium aluminate solution, fol-
sents an excellent adsorbent for the purication of wastewater. lowed by AACH preparation from a (NH CO solution and the
Thus, this FAA was used to efficiently remove phosphate from subsequent synthesis of FAA by roasting the AACH.
wastewater. Fig. 9(a) presents the removal rates and adsorption (2) The particle size remained nearly unchanged in the
3
FAA
.3 Efficient removal of phosphate from wastewater with
4
)
2
3
4
)
2
3
capacities for various solutions, while Fig. 9(b) plots the tting transformation from gibbsite to AACH to FAA. However,
of the data based on isothermal Langmuir and Freundlich increasing the roasting temperature increased the average pore
adsorption models.
diameter and pore volume while reducing the SSA. A rapid
Fig. 9(a) conrmed that the adsorption capacity of the FAA heating rate increased the BET surface area and pore volume,
2
ꢀ1
increased rapidly along with the phosphate concentration at such that the FFA SSA reached 1088.72 m g aer heating at
ꢀ
1
ꢁ
concentrations less than 900 mg L and then increased slowly. 300 C for 180 min.
There data suggest that the phosphate uptake was primarily due
(3) The particle aggregation was effectively inhibited by the
to physisorption rather than chemisorption. Increases in the formation of boehmite as an intermediate and by the interface
phosphate concentration at a constant FAA load reduced the hydrophobicity and high capillary pressure during phase
removal rate. The FAA was found to exhibit an absorption evolution from gibbsite to AACH to FAA. High capillary pres-
ꢀ
1
capacity of 300.28 mg g at equilibrium, signicantly greater sure, plentiful open mesopores and inhibition of aggregation all
ꢀ
1
55
than the 49.67 mg g achieved with nano-alumina.
contributed to the super-high SSA of the FAA.
5636 | RSC Adv., 2019, 9, 5628–5638
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(
4) The adsorption capacity of the FAA for phosphate from 18 W. Q. Cai, H. Q. Li and Y. Zhang, Mater. Chem. Phys., 2006,
ꢀ
1
wastewater was as high as 300.28 mg g , which reects the
96(1), 136–139.
high SSA and open mesopores of this material.
19 W. Ling and I. K. Lloyd, J. Am. Ceram. Soc., 2010, 74(11),
2934–2936.
2
2
0 T. M. Sullivan, US Pat., 5238669 A, 1993.
1 L. Bokobza and J. P. Chauvin, Polymer, 2005, 46(12), 4144–
Conflicts of interest
4151.
There are no conicts to declare.
2
2 H. C. Lee, H. J. Kim, H. R. Chang, K. H. Lee, J. S. Lee and
S. H. Chung, Microporous Mesoporous Mater., 2005, 79(1),
61–68.
3 Y. Li, J. Liu and Z. Jia, Mater. Lett., 2006, 60(29), 3586–3590.
4 Z. Zhu, H. Liu, H. Sun and D. Yang, Microporous Mesoporous
Mater., 2009, 123(1), 39–44.
5 J. C. Ray, K. S. You, J. W. Ahn and W. S. Ahn, Microporous
Mesoporous Mater., 2007, 100(1–3), 183–190.
6 J. H. Kim, K. Y. Jung, K. Y. Park and S. B. Cho, Microporous
Mesoporous Mater., 2010, 128(1–3), 85–90.
7 N. Cruise, K. Jansson and K. Holmberg, J. Colloid Interface
Sci., 2001, 241(2), 527–529.
28 M. I. F. Mac ˆe do, C. C. Osawa and C. A. Bertran, J. Sol-Gel Sci.
Technol., 2004, 30(3), 135–140.
29 B. Xu, T. Xiao, Z. Yan, X. Sun, J. Sloan, S. L. Gonz ´a lez-Cort ´e s,
F. Alshahrani and M. L. H. Green, Microporous Mesoporous
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Acknowledgements
2
2
The authors gratefully acknowledge the nancial support from
the National Natural Science Foundation of China (No.
51874366), and project (FA2017029) supported by Science and
2
2
2
Technology Program of Chongzuo, China.
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