Adsorption of orthophosphoric, pyrophosphoric, and tripolyphosphoric acids from aqueous solutions by calcined gibbsite

Article abstract of DOI:10.1248/cpb.c14-00224

In this research, gibbsite (GB) samples calcined at 200-1000°C (GB200-GB1000) were produced. These GBs were used to adsorb orthophosphoric, pyrophosphoric, and tripolyphosphoric acids from aqueous solutions. The properties (amounts of hydroxyl groups, specific surface areas, mean pore diameters, and solution pHs) of the GBs were investigated, and their adsorption capacities for phosphoric acids evaluated. The amount of hydroxyl groups (0.46 mmol/g) and specific surface area (295.3 m²/g) of GB400 were greater than those of the other GBs. The mechanism of phosphoric acid adsorption on the GBs was related to the amount of hydroxyl groups and specific surface area. The optimal pH for phosphoric acid adsorption by GBs was 2.0-3.0. Equilibrium adsorption was reached within 24 h. The adsorption processes followed a pseudo-second-order kinetic model (correlation coefficient, 0.998-0.999). The adsorption capacity increased with increasing temperature. The adsorption isotherm data fitted the Langmuir (correlation coefficient: 0.921-0.992) and Freundlich (correlation coefficient: 0.948-0.997) equations well. Our results will be useful when developing methods for the adsorption of phosphoric acids from aqueous solutions.

Full text of DOI:10.1248/cpb.c14-00224

August 2014
Regular Article
Chem. Pharm. Bull. 62(8) 799–805 (2014)
799
Adsorption of Orthophosphoric, Pyrophosphoric, and Tripolyphosphoric
Acids from Aqueous Solutions by Calcined Gibbsite
a
a
,a,b
Fumihiko Ogata, Ayaka Ueda, and Naohito Kawasaki*
a
b
Faculty of Pharmacy, Kinki University; and Antiaging Center, Kinki University; 3–4–1 Kowakae, Higashi-Osaka,
Osaka 577–8502, Japan.
Received March 11, 2014; accepted May 16, 2014
In this research, gibbsite (GB) samples calcined at 200–1000°C (GB200–GB1000) were produced. These
GBs were used to adsorb orthophosphoric, pyrophosphoric, and tripolyphosphoric acids from aqueous
solutions. The properties (amounts of hydroxyl groups, specific surface areas, mean pore diameters, and
solution pHs) of the GBs were investigated, and their adsorption capacities for phosphoric acids evaluated.
2
The amount of hydroxyl groups (0.46mmol/g) and specific surface area (295.3m /g) of GB400 were greater
than those of the other GBs. The mechanism of phosphoric acid adsorption on the GBs was related to the
amount of hydroxyl groups and specific surface area. The optimal pH for phosphoric acid adsorption by
GBs was 2.0–3.0. Equilibrium adsorption was reached within 24h. The adsorption processes followed a
pseudo-second-order kinetic model (correlation coefficient, 0.998–0.999). The adsorption capacity increased
with increasing temperature. The adsorption isotherm data fitted the Langmuir (correlation coefficient:
0
.921–0.992) and Freundlich (correlation coefficient: 0.948–0.997) equations well. Our results will be useful
when developing methods for the adsorption of phosphoric acids from aqueous solutions.
Key words orthophosphoric acid; pyrophosphoric acid; tripolyphosphoric acid; adsorption
Phosphorus is a necessary nutrient for organisms in most inexpensive and widely used in many fields. Moreover, GB
ecosystems, and it is often present in water bodies at low can be used for anion exchange in aqueous solutions. Alu-
concentrations as phosphates, including organic phosphates, minum oxide can be used in the recovery of orthophosphoric
12)
inorganic phosphates, and polyphosphates (particulate phos- acid. Several studies have been carried out on the adsorption
phorus). Excessive discharge of phosphates into rivers and of orthophosphoric acid or condensed phosphates on ferri-
1,2)
lakes is the major cause of eutrophication.
hydrite, Fe(OH) , titania, and titanium dioxide. However, the
3
Condensed phosphates, produced by the condensation of adsorption of condensed phosphates on aluminum hydroxide
13–19)
orthophosphates, are compounds with –P–O–P– bonds. At has seldom been reported.
Until now, the use of GB for
least 750000t of condensed phosphates were used in 1996, the adsorption of condensed phosphates (pyrophosphoric and
mainly in detergents and cleaning agents, and eventually tripolyphosphoric acids) in aqueous solutions has not been
3)
discharged into wastewater treatment plants. It was found reported. The object of this study is to investigate the adsorp-
in a previous study that condensed phosphates account for tion behavior of condensed phosphates on GB. The effects of
approximately 15–75% of the total phosphates in raw sew- temperature, pH, and concentration on the adsorption kinetics
4)
age and 5–40% of those in secondary effluents. Condensed of condensed phosphates were investigated.
phosphates, especially ammonium polyphosphates, are widely
used as fertilizers to provide plants with phosphorus. During Materials and Methods
the past 30 years, most studies of phosphorus have focused
Materials GB (H-42: amorphous aluminum hydrox-
exclusively on orthophosphates, which are the most important ide) was purchased from Showa Denko, Japan. It consisted
5
)
phosphorus species in ecological systems. The important role of moisture (0.23%), Al(OH) (99.6%), Fe O (0.01%), SiO
3
2
3
2
of condensed phosphates in the phosphorus cycle has been (0.01%), Na O (0.03%), and ω-Na O (0.05%). The bulk den-
2
2
3
neglected, mainly because they are thought to be biologi- sity and moisture adsorption capacity were 0.2–0.5g/cm and
cally unavailable without hydrolysis, and some are difficult to 0.90%, respectively. Calcined GB was prepared by treating
31
measure. However, the development of P-NMR spectroscopy virgin GB (20g) in a muffle furnace in the temperature range
has made researchers recognize the importance of condensed 200–1000°C for 2h (the calcined samples are referred to as
phosphates, especially pyrophosphates and tripolyphos- GB200–GB1000). Potassium dihydrogenphosphate, sodium di-
6
–8)
phates.
There is therefore an increasing demand for the phosphate, and sodium tripolyphosphate were purchased from
removal of orthophosphoric, pyrophosphoric, and tripolyphos- Wako Pure Chemical Industries, Ltd.
phoric acids from aqueous solutions. Various techniques such
The specific surface areas and mean pore diameters of the
as ion exchange, adsorption, chemical precipitation, and bio- GBs were measured using a NOVA4200e specific surface
9)
logical methods have been used for phosphate removal. Ad- analyzer (Yuasa Ionic, Japan). The amount of hydroxyl groups
2
0)
sorption methods are most effective because of their low cost, was measured by fluoride ion adsorption. In this procedure,
high uptake capacities, greater selectivities, faster regeneration the adsorbent (0.125g) was added to 0.01mol/L NaF solu-
10,11)
kinetics, low sludge production, and ease of operation.
tion (50mL) at pH 4.6; the pH was adjusted using 0.2mol/L
The aluminum compound gibbsite (GB) is a recyclable acetic acid solution and 0.2mol/L acetate buffer solution. The
material. GB can be easily obtained from bauxite, and it is solution was shaken at 25°C for 24h at 100rpm and then fil-
tered using a 0.45-µm membrane filter. The concentration of
The authors declare no conflict of interest.
fluoride ions in the filtered solution was measured, and the
*
To whom correspondence should be addressed. e-mail: kawasaki@phar.kindai.ac.jp
© 2014 The Pharmaceutical Society of Japan

8
00
Vol. 62, No. 8
amount of fluoride ions adsorbed on the adsorbent was calcu- was calculated using Eq. 1.
lated. The solution pH was measured using a digital pH meter
(
Mettler-Toledo, Japan) after adding the adsorbent (1g) to dis- Results and Discussion
tilled water (50mL), allowing the mixture to stand for 2min at
Properties of GBs Table 1 shows the properties of the
25°C, and filtering it through a 0.45µm membrane filter. The GBs. It can be seen that the pH of the GB increased with in-
amount of phosphorus on the GB surface was measured using creasing calcination temperature (the pH of virgin GB is 6.8).
an electron probe microanalyzer (JXA-8530F, JEOL, Japan); The number of hydroxyl groups (0.46mmol/g) and specific
2
the conditions were accelerating voltage 45.0kV and beam surface area (295.3m /g) of GB400 were greater than those
diameter 5µm.
of the other GBs. The mean pore diameter of GB400 (13.0Å)
Saturated Amounts of Phosphoric Acids Adsorbed on was smaller than those of the other GBs. The structure of
GBs The adsorbent (1.0g) was added to phosphoric acid gibbsite for calcined gibbsite at 200°C is the same as that
solutions (100mg/L; 100mL). The suspension was shaken at of virgin gibbsite, but when the calcination temperature was
2
5°C for 48h at 100rpm, and then filtered through a 0.45µm higher than 300°C, the structure of gibbsite was destroyed,
membrane filter. The concentration of orthophosphoric acid and when the calcination temperature was 400°C, boehmite
was measured using a DR/890 colorimeter (HACH, U.S.A.). structure (AlOOH) appeared and amorphous aluminum oxide
The concentrations of condensed phosphoric acids (pyro- was produced. It must be pointed out that when the calcination
phosphoric and tripolyphosphoric acids) were determined as temperature is higher than 1000°C, gibbsite became aluminum
follows. The sample solution was added to 5.25mol/L sulfuric oxide with crystal structure which is a very stable substance.
acid (2mL), and the mixture was heated for 30min. The solu- These results indicate that the changes in the GB properties
tion was then cooled to room temperature, and then 5.0mol/L (pH, amount of hydroxyl groups, specific surface area, and
sodium hydroxide (2mL) was added. The total volume of the mean pore diameter) depend upon the calcination tempera-
21–23)
sample solution was 25mL. The concentrations of pyrophos- tures.
phoric and tripolyphosphoric acids were measured using a
DR/890 colorimeter.
Amounts of Phosphoric Acids on GBs The amounts
of phosphoric acids adsorbed on the GBs are shown in Fig.
The amount of phosphoric acid adsorbed on the GB was 1. GB400 adsorbed the largest amounts of phosphoric acids.
calculated from the difference between the concentration be- The amounts adsorbed increased in the order orthophosphoric
fore and after adsorption (Eq. 1):
acid>pyrophosphoric acid>tripolyphosphoric acid.
The relationship between the amount of phosphoric acids
adsorbed and adsorbent properties was evaluated by plotting
X =(C − C
0
e
)V/M
(1)
where X is the amount of phosphoric acid adsorbed (mg/g), the amount adsorbed against the specific surface area and
C is the concentration before adsorption (mg/L), C is the
0
e
concentration after adsorption (mg/L), V is the solvent volume
(
L), and M is the mass of the adsorbent (g).
Adsorption Isotherms of Phosphoric Acids in Solutions
with Different pHs GB400 (0.05g) was added to phosphoric
acid solutions (10–50mg/L; 50mL) under acidic (pH 2.0–3.0),
neutral (pH 6.5–7.5), and basic (pH 11.0–12.0) conditions,
which was adjusted by hydrochloric acid and sodium hydrox-
ide solution. The sample solution was shaken at 25°C for 48h
at 100rpm. The amount of phosphoric acid on GB400 was
calculated using Eq. (1).
Effect of Contact Time on Adsorption of Phosphoric
Acids on GB400 GB400 (0.05g) was added to phosphoric
acid solutions (50mg/L; 50mL). The sample solution was
shaken at 25°C for 1–48h at 100rpm. The adsorbed amount
was calculated using Eq. (1).
Adsorption Isotherms of Phosphoric Acids on GB400 at
Different Temperatures GB400 (0.05g) was added to phos-
phoric acid solutions (10–50mg/L; 50mL) under acidic condi-
tions (pH 2.0–3.0). The sample solution was shaken at 5–45°C
Fig. 1. Amounts of Phosphoric Acids Adsorbed by GBs
Initial concentration: 100mg/L, Sample volume: 100mL, Adsorbent: 1.0g, Tem-
for 48h at 100rpm. The amount of phosphoric acid on GB400 perature: 25°C, Contact time: 48h, 100rpm.
Table 1. Properties of the GBs
Amount of hydroxyl group
Specific surface area
Mean pore diameter
Samples
pH
2
(mmol/g)
(m /g)
(Å)
GB200
GB400
GB600
GB800
GB1000
8.6
9.2
0.09
0.46
0.43
0.38
0.31
4.6
295.3
176.1
128.1
36.8
68.6
13.0
10.4
10.5
10.7
38.0
48.2
140.9

August 2014
801
Fig. 2. Relationship between Amount of Phosphoric Acids Adsorbed and Properties of the GBs
●: Orthophosphoric acid, ○: Pyrophosphoric acid, ▲: Tripolyphosphoric acid.
Fig. 3. Amount of P onto GB Surface
amount of hydroxyl groups of the adsorbent (Fig. 2). The cor-
Adsorption Isotherms of Phosphoric Acids in Solutions
relation coefficients between the amount adsorbed and the with Different pHs Figure 4 shows that the amounts of
specific surface area or the amount of hydroxyl groups ranged phosphoric acids adsorbed on GB400 in acidic solution (pH
from 0.885 to 0.908 or from 0.928 to 0.994, respectively 2.0–3.0) were greater than those in neutral (pH 6.5–7.5) or
suggesting that the specific surface area and the amount of basic (pH 11.0–12.0) solutions. Kawasaki et al. reported that
hydroxyl groups are closely related to phosphoric acid ad- the optimal pH for adsorption of orthophosphoric acid on alu-
21)
sorption. A previous study reported similar trends. We also minum oxide at different temperatures was acidic (pH around
21)
evaluated the amount of phosphorus on the GB400 surface 4.0). Many researchers have reported that the adsorption of
2
4)
(
Fig. 3). It can be seen that amount of phosphorus on GB400 condensed phosphates improved with decreasing pH. The
after adsorption was greater than that before adsorption. This increase in the amount of condensed phosphates adsorbed on
indicates the presence of phosphorus adsorption sites on the aluminum hydroxide at low pH could be attributed to electro-
GB400 surface.
static attraction and acid catalysis, as illustrated by the follow-

8
02
Vol. 62, No. 8
Fig. 4. Adsorption Isotherms of Phosphoric Acids on GB400 in Solutions with Different pHs
Initial concentration: 10–50mg/L, Sample volume: 50mL, Adsorbent: 0.05g, Temperature: 25°C, Contact time: 48h, 100rpm.
2
5,26)
ing equation
:
−
−
−
Al − OH + L  Al − L + OH (L shows phosphate anions)
−
Protonation of the leaving group (OH ) can weaken the
Al–O bond and facilitate OH removal as H O. The adsorp-
−
27)
2
tion of condensed phosphates is therefore favored when the
+
amount of aqua sites (Al–OH ) increases with decreasing
2
2
8)
29)
pH. Xing et al. reported similar results. The phosphate
removal rate by adsorption changes insignificantly in the pH
range 2.0–3.0. With further increases in pH up to 11.0–12.0,
there is a steady decrease. This can be attributed to increas-
−
ing competition for the adsorption sites between OH and
3
0)
phosphoric acid species. Another possible reason is that a
higher pH causes the adsorbent surface to carry more negative
charges, resulting in stronger repulsion of negatively charged
1)
species in solution. These results suggest that the optimal Fig. 5. Effect of Contact Time for the Adsorption of Phosphoric Acids
−
−
on GB400
species for adsorption by GB400 are H PO , H P O , and
2
4
3
2
7
−
31,32)
Initial concentration: 50mg/L, Sample volume: 50mL, Adsorbent: 0.05g, Tem-
perature: 25°C, Contact time: 1–48h, 100rpm, ●: Orthophosphoric acid, □: Pyro-
phosphoric acid, ▲: Tripolyphosphoric acid.
H P O .
4
3
10
Effect of Contact Time on Phosphoric Acid Adsorption
on GB400 Figure 5 shows the adsorption rates of phos-
phoric acids on GB400; equilibrium was reached within 24h.
The amount adsorbed increased in the order orthophosphoric
ln(q
e
− q
t
)=ln q
e
1
− k t
(2)
acid>pyrophosphoric acid>tripolyphosphoric acid. In order to where q and q are the amounts of phosphoric acid adsorbed
e
t
elucidate the kinetic mechanism that controls the adsorption on the adsorbent (mg/g) at equilibrium and at time t (h), re-
process, pseudo-first-order and pseudo-second-order models spectively, and k is the rate constant (1/h).
1
33)
were used to interpret the experimental data. The pseudo-
The pseudo-second-order rate model is given as
first-order rate model is expressed as

August 2014
803
2
GB400 increased with increasing temperature. These results
suggest that the adsorption of phosphoric acids on GB400 is
t/q
t
=1/
(
k
2
q
e
)
+t/q
e
(3)
where k is the pseudo-second-order rate constant (g/mg/h). related to chemisorption. The two most widely used adsorp-
2
Plotting t/q versus t results in a straight line, which enables tion isotherms are the Langmuir and Freundlich isotherms. In
t
the calculation of the values of q and the rate constant, k .
this work, the experimental data were analyzed in terms of
e
2
Table 2 lists the rate constants obtained from the models. these isotherms. The Langmuir isotherm model evaluates the
The correlation coefficient for the pseudo-second-order ad- adsorption system based on monolayer adsorption on the ac-
sorption model was the highest (>0.998), and the adsorption tive sites of the adsorbent. The Langmuir model is expressed
36)
capacities calculated using the model were close to those by
determined experimentally. The pseudo-second-order adsorp-
tion model should therefore be used to describe the rates of
C
e
/q
e
=1/W
s
a+C
e
/W
s
(4)
adsorption of phosphoric acids on GB400. The correlation where C is the equilibrium concentration (mg/L), q is the
e
e
coefficient greater than 0.998 supports the assumption behind amount of phosphoric acid adsorbed on GB400 at equilibrium
3
4,35)
the model that adsorption occurs via chemisorption.
(mg/g), and W and a are Langmuir constants related to the
s
Adsorption Isotherms of Phosphoric Acids on GB400 monolayer adsorption capacity and adsorption energy, respec-
Figure 6 shows the adsorption isotherms of phosphoric acids tively.
on GB400. The amounts of phosphoric acids adsorbed on
The Freundlich isotherm assumes heterogeneous adsorp-
Table 2. Fitting Results of Kinetic Data Using Pseudo-First-Order and Pseudo-Second-Order Model
Pseudo-first-order model
Pseudo-second-order model
qe, exp
Samples
2
(mg/g)
k
h
1
q
e, exp
k
q
e, cal
r
r
−
1
(
)
(mg/g)
(g/mg/h)
(mg/g)
Orthophosphoric acid
Pyrophosphoric acid
Tripolyphosphoric acid
19.58
28.79
26.05
0.067
0.087
0.255
5.54
8.08
6.98
0.908
0.960
0.936
0.018
0.009
0.012
20.83
30.27
37.18
0.998
0.999
0.999
Fig. 6. Adsorption Isotherms of Phosphoric Acids on GB400 at Different Temperatures
Initial concentration: 10–50mg/L, Sample volume: 50mL, Adsorbent: 0.05g, Temperature: 5–45°C, Contact time: 48h, 100rpm.

8
04
Vol. 62, No. 8
Table 3. Freundlich and Langmuir Constants for Phosphoric Acids Adsorption onto GB400
Freundlich constants
Langmuir constants
Samples
r
r
Ws
mg/g)
a
log k
0.81
1/n
0.26
(
(L/mg)
Orthophosphoric acid
5°C
5°C
5°C
5°C
5°C
5°C
5°C
5°C
5°C
0.950
0.979
0.997
0.992
0.977
0.965
0.948
0.961
0.965
18.75
24.30
29.98
26.61
31.63
40.54
24.91
30.02
37.33
0.36
3.15
2.43
2.98
2.32
1.61
3.07
2.31
1.66
0.921
0.964
0.972
0.978
0.988
0.992
0.972
0.992
0.990
2
4
1.01
1.14
1.13
1.18
1.24
1.11
1.16
1.20
0.20
0.22
0.18
0.20
0.25
0.18
0.20
0.25
Pyrophosphoric acid
2
4
Tripolyphosphoric acid
2
4
tion, based on delivery to the adsorption sites or the diverse
nature of the adsorbed phosphoric acids. The Freundlich ad-
sorption equation is
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(
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) Kotabe H., Gypsum & Lime, 210, 307–316 (1987).
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=log k +1/n log C
e
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where k (mg/g) is the Freundlich capacity constant and n is
the Freundlich intensity constant. The relevant Freundlich and
Langmuir constants are shown in Table 3. The correlation co-
efficient of the Langmuir constant is 0.921–0.992, and the cor-
relation coefficient of the Freundlich constant is 0.948–0.997.
7
)
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1
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38)
if 1/n>2, adsorption is considered to be difficult. Our results
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5
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1
2
6.05mg-P/g) were greater than those using red mud granular
1
below 2.0mg-P/g) or aluminum hydroxide (pyrophosphoric
acid and tripolyphosphoric acid: below 12.0mg-P/g).
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1
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Conclusion
In this study, we showed that GB calcined at 400°C
(GB400) had the greatest adsorption capacity for orthophos-
2
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2
2
2
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2
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12–318 (2007).
tion of phosphoric acids was reached within 24h. These data
were well described by the pseudo-second-order model. The
amounts of phosphoric acids adsorbed increased with increas-
ing temperature. The adsorption isotherm data for GB400 fit-
ted both the Freundlich and Langmuir equations. These find-
ings will be useful for developing methods for the adsorption
of phosphoric acids.
3
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