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Sistema de Información Científica
Red de Revistas Científicas de América Latina y el Caribe, España y Portugal
Rev. Int. Contam. Ambient. 26 (2) 129-134, 2010
SYNTHESIS AND CHARACTERIZATION OF CALCIUM PHOSPHATE AND ITS RELATION TO
CR(VI) ADSORPTION PROPERTIES
Francisco GRANADOS-CORREA*, Juan BONIFACIO-MARTÍNEZ and Juan SERRANO-GÓMEZ
Depto. de Química, Instituto Nacional de Investigaciones Nucleares, A. P. 18-1027. Col. Escandón. Delegación
Miguel Hidalgo. C. P. 11901. México, D. F., México. *Correo electrónico: francisco.granados@inin.gob.mx.
(Recibido junio 2008, aceptado septiembre 2009)
Key words: calcium phosphate, synthesis, characterization, Cr(VI) adsorption, surface area, adsorption
experiments
ABSTRACT
Calcium phosphate with hydroxyapatite structure was synthesized and its ability to
adsorb Cr(VI) from aqueous solution is presented. XRD, BET, IR, TGA, SEM and EDS
techniques were used to characterize the obtained material. A pure phase was obtained
through a simple synthesis process. The speci±c surface area of the synthesized pow-
der was found to be 64.5 m
2
g
-
1
. The X-ray diffraction pattern shows that the calcium
phosphate formed was nanocrystalline with an average grain size of approximately 75
nm. A fast adsorption was observed and in less than 24 h it was found that 2.41
x
10
-
4
meq g
-
1
of Cr(VI) ions were adsorbed on calcium phosphate. Desorption experiments
showed that Cr(VI) adsorption decreased to 2.23
x
10
-
4
meq g
-
1
of calcium phosphate.
This behavior is a consequence of partial dehydration of the synthesized material. Cal-
cium phosphate can be effectively used for removing Cr(VI) from aqueous solutions
in treatment processes of metal wastes.
Palabras clave: fosfato de calcio, síntesis, caracterización, adsorción de Cr(VI), área super±cial, experimentos
de adsorción
RESUMEN
Se sintetizó fosfato de calcio, con la estructura de la hidroxiapatita, y se presenta su
habilidad para adsorber Cr(VI) en solución acuosa. Para caracterizar el material obtenido
se usaron técnicas de DRX, BET, IR, ATG, MEB y EDS. Se obtuvo una fase pura por un
proceso simple de síntesis. El área especí±ca del polvo sintetizado fue de 64.5 m
2
g
-
1
. El
patrón de difracción de rayos-X muestra que el fosfato de calcio formado fue nanocris-
talino con un promedio de grano alrededor de 75 nm. Se observó una rápida adsorción en
menos de 24 h y se encontró que 2.41
x
10
-
4
meq g
-
1
de iones Cr(VI) fueron adsorbidos
sobre el fosfato de calcio. Experimentos de desadsorción mostraron que la adsorción
de Cr(VI) disminuyó a 2.23
x
10
-
4
meq g
-
1
de fosfato de calcio. Este comportamiento
es una consecuencia de la deshidratación parcial del material sintetizado. El fosfato de
calcio puede ser usado efectivamente para la remoción de Cr(VI) de soluciones acuosas
en los procesos de tratamiento de desechos de metales.
F. Granados-Correa
et al.
130
INTRODUCTION
During the last decade, obtaining porous materi-
als for applications in many ±elds of technology has
shown great development. Calcium phosphate with
apatite structure (Ca
10
(PO
4
)
6
(OH)
2
)
is a sparingly
soluble mineral with strong af±nity for adsorption
of radio nuclides and heavy metals. It has been
proposed for use as a back±ll material for geo-
logic repositories for nuclear waste (Qureshi and
Varshney1991), and as an adsorbent in engineered
barrier for environmental restoration (Ewing 2002).
Additionally, it shows promise for other potential
applications such as acidic or basic catalysts, chro-
matographic adsorbent and as biomaterial (Matsuda
et al.
2005, Thakur
et al.
2005).
In particular, the use of calcium phosphate as an
adsorbent is due to its large speci±c area, high thermal
and chemical stability, high ionic exchange capacity
and its stability towards ionizing radiation. Therefore,
its characteristics resulting from the preparation
method exert an important in²uence on its behavior
as inorganic exchanger to remove contaminants
present in water. In many researches, various types
of phosphates, such as acid metal phosphates of
zirconium, aluminum, titanium and tin, have been
synthesized for use as ionic exchangers (Qureshi
and Varshney 1991, El-Said
et al.
2001, Da-Rocha
et al.
2002).
Various methods to synthesize calcium phosphate
have been reported (Yoshida 1996, Ayres
et al.
1998,
Andronescu
et al.
2002), where calcium is supplied
as aqueous solutions of CaCl
2
, Ca(NO
3
)
2
, CaCO
3
or
Ca(CH
3
COO)
2
and phosphates are supplied as aque-
ous solutions of (NH
2
)HPO
4
, NH
4
H
2
PO
4
, KHPO
4
,
N
2
HPO
4
, o NaH
2
PO
4
.
In this work, calcium phosphate was synthezised
by using the continuous precipitation method, and
its structural and surface characteristics were deter-
mined to test the adsorption of Cr(VI) ions present
in aqueous solution.
MATERIAL AND METHODS
Materials
Calcium phosphate was synthesized at room tem-
perature by the continuous precipitation method as
reported by Gómez-Morales
et al.
(2001). To obtain
10 g of calcium phosphate (hydroxyapatite) 23.51 g
calcium nitrate tetrahydrate Ca(NO
3
)
2
•4H
2
O A.C.S.
reagent (Sigma-Aldrich, 99 wt% purity) and 6.87 g
monobasic ammonium dihydrogen-phosphate (NH
4
)
H
2
PO
4
A.C.S. reagent (Sigma-Aldrich, 98 wt % purity)
were used. The reagents were mixed at a Ca/P molar
ratio of 1.67. K
2
CrO
4
(Baker) was used for the adsorp-
tion experiments.
Synthesis by continuous precipitation
Calcium phosphate was prepared with 23.51 g of
Ca(NO
3
)•4H
2
O, dissolved in 545 mL distilled water
with an initial pH of 5. Later, this solution was mixed
with 62 mL of concentrated NH
4
OH; the ±nal pH of
the solution obtained was 12. Finally, distilled water
was poured into the beaker to attain a total volume
of 890 mL. The same purity criteria were used to
prepare aqueous solution of monobasic ammonium
phosphate: 6.9 g of NH
4
H
2
PO
4
were dissolved in 833
mL of distilled water with a pH value of 5. Then, 37
mL of concentrated NH
4
OH were added to reach
a pH value of 12. Finally, the obtained solution
was diluted with distilled water up to a volume of
1500 mL. To obtain calcium phosphate, ammonium
phosphate solution was slowly poured into calcium
nitrate solution under vigorous stirring for 24 h. The
suspension was left to settle for another 24 h, to ±nally
draw off the supernatant. The precipitate was washed
four times with distilled water under stirring for 18 h
and then the solid and liquid phases were separated
by centrifuging. The calcium phosphate obtained was
dried at 120 ºC for 2.5 hours and calcined at 1050 ºC
for 2.5 hours more. This thermal treatment made
calcium phosphate more crystalline and destroyed
and volatilized the nitrate and ammonium traces
trapped in the solid.
Characterization
Specific surface of calcium phosphate was
obtained by nitrogen adsorption through the BET
method with a surface area analyzer Micromeritics
Gemini 2360. The solid sample was heated for 2 h at
200 ºC. Infrared measurements were performed us-
ing a Nicolet 550 spectrophotometer and the sample
was mixed with KBr in the conventional way. A
Siemens D500 diffractometer coupled to a copper
anode tube was used to obtain the X-ray diffraction
patterns and identify the crystalline compounds.
The K
a
wavelength was selected with a diffracted
beam monochromator. The X-ray tube was operated
at 35 kV and 20 mA. All diffraction patterns were
obtained in scanning mode with a 0.02
o
(2
q
) step
size. Crystalline calcium phosphate (hydroxyapatite)
was identi±ed conventionally with the JCPDS ±les.
The thermogravimetric analysis (TGA) were
carried out with a TA Instrument TGA-51 under a
²owing nitrogen atmosphere in a heating condition
CALCIUM PHOSPHATE AND ITS RELATION TO CR(VI) ADSORPTION PROPERTIES
131
of 10 ºK min
-
1
; 15.0 mg samples and high purity N
2
gas were used.
Topography and morphology of the synthesized
material were determined using a scanning electron
microscopy (SEM; JOEL JSM-5900 LV) with an
accelerating voltage of 30 keV and a maximum
magniFcation of 1000X. Semi-quantitative analysis
of the selected micro area of calcium phosphate (hy-
droxyapatite) samples were carried out by energy dis-
persive X-ray analysis (EDS) technique using Oxford
microprobe coupled to the electronic microscope.
Chromium adsorption
Batch experiments were carried out at room
temperature mixing in closed vials, 0.1 g of calcium
phosphate and 10 mL of 1.0
x
10
-
4
M Cr(VI) solution
at pH 5.5. Samples were stirred for 10 s and shaken
for 48 h. The liquids were separated from the solids
by centrifuging (5 min at 3000 rpm). Chromium con-
centrations in each aliquot of 5 mL solutions was de-
termined by using a Shimadzu ultraviolet-visible 265
spectrophotometer analyzer at
l
=540 nm using the
1,5 diphenylcarbazide method. EfFciency of Cr(VI)
adsorption on calcium phosphate was determined in
milliequivalents of ion adsorbed per gram of solid.
The Cr(VI) adsorption by calcium phosphate could be
explained by physical adsorption and as this process
is reversible, physical adsorption was estimated by
desorption experiments. The best Cr(VI) exchanged
sample was suspended in deionized water and chro-
mium content in the aqueous phase was measured as
a function of time, until an equilibrium was reached
between the sample and the aqueous phase.
RESULTS AND DISCUSSION
The speciFc area of the synthesized material was
found to be 64.5 m
2
g
-
1
; a value which coincides
with data reported by Yoshida (1996) for calcium
phosphate type hydroxyapatite. The infrared spec-
trum of calcium phosphate is shown in
fgure 1
. The
bands at
n
1
= 962.6 and
n
2
= 1040 cm
-
1
are assigned
to the fundamental frequencies of the PO
4
3
-
group.
A low intensity band can be observed at 957 cm
-
1
which is assigned to the vibration of the P-OH
group. The broad band at 3192 cm
-
1
results from
an overlapping of hydrogen vibrations: stretching
vibrations of structural OH
-
and physically adsorbed
water. The low intensity band at 1630 cm
-
1
is as-
signed to bending vibrations of strongly adsorbed
water. The band observed at 1432 cm
-
1
is very prob-
ably due to the presence of a very small amount of
calcium carbonate. To lower frequencies, two bands
assigned to the PO
4
3
-
ion can be found: the intense
band at 1032 cm
-
1
(
n
3
) and the low intensity band
at 567 cm
-
1
(
n
4
). Thus, the IR spectrum in Fgure
1 indicates the high purity of the analyzed calcium
phosphate (hydroxyapatite).
Figure 2
shows the XRD pattern of the obtained
powders: the examination of the Fgure indicates that
the synthesized material was only a crystalline single
phase. The indexation calculations reveal that this
synthesized material has a hexagonal hydroxyapatite
structure with unit cell parameters calculated from the
XRD patterns of Ca
10
(PO
4
)
6
(OH)
2
(a=b=9.385 Å and
c=6.870 Å) obtained from the procedure described by
Suryanarayana
et al.
(1998), in strong agreement with
literature data (Da-Rocha
et al.
2002). According to
Sherrer equation, the calcium phosphate formed was
all nanocrystalline with an average grain size of about
75 nm. The most intense and sharp lines are observed
50
45
40
35
30
% Transmittance
25
20
3432.556
2362.128
2337.427
1633.375
1034.630
603.815
565.
704
15
10
5
4000
3500
2500
1500
500
3000
2000
Wavenumber (cm
-
1
)
1000
Fig. 1.
IR spectrum of calcium phosphate
Intensity (u.a.)
10
20
30
2θ degrees
Calcium phosphate
(hydroxyapatite structure)
40
50
60
Fig. 2.
XRD pattern of calcium phosphate
F. Granados-Correa
et al.
132
in the 2θ angle range between 20 and 60º. These lines
are coincident with the lines of the XRD spectrum
reported in JCPDS 9-0432 ±le, which corresponds to
hydroxyapatite. The XRD pattern in
fgure 2
shows
that water detected by IR spectroscopy in the calcium
phosphate sample does not alter the structure of this
material.
The thermogravimetric analysis (TGA) results
of the synthesized calcium phosphate (hydroxy-
apatite) are shown in
fgure 3.
The TGA curves
display a continuous slow mass loss with a constant
temperature increase. In the derivative curve the
peak observed at about 43 ºC indicates a relatively
slow moisture loss at an almost constant rate. At
about 150 ºC there is a small slope change in the
TGA curve, indicating a different type of mass loss,
which is that of strongly adsorbed water whose loss
rate is rather low and can be seen in the derivative
curve as a non-intense and small peak which ends
at about 325 ºC. The water molecules were lost as
calcium phosphate was increasingly heated. A fast
slope change of the TGA curve is presented over
the 300 ºC with a maximum at about 325 ºC in
the derivative curve, indicating again a mass loss
which can be due to the dehydroxylation of the hy-
droxyapatite sample. A small peak at about 360 ºC
is also observed in the derivative curve and it may
be due to the decomposition of the carbonate ions
detected by IR spectroscopy in the solid simple.
These carbonate ions were formed as a result of
CO
2
absorbed by the calcium phosphate. Finally,
another fast change in the TGA curve can be seen
at about 600 ºC and the mass loss in this case may
be due to phosphate ion decomposition observed as
the two peaks displayed at 642 and 697 ºC. These
results show that the calcium phosphate is stable at
temperatures below 600 ºC.
Figure 4
shows the micrograph of hydroxyapatite
obtained by SEM. The material has a particle size
range of 5-40
m
m, indicating a high dispersion in
particle size. The graphic results of the semiquanti-
tative elemental chemical analysis yielded by EDS
technique shows that the elemental composition of
the ±nal synthesized white powders were found to be
P 27.47 wt%, Ca 19.64 wt% and O 52.89 wt%. These
results reveal high purity of the calcium phosphate
obtained by the continuous precipitation method
since no other chemical element was detected by the
EDS technique when hydroxyapatite was analyzed
in different zones of the sample.
The amount of chromium ions retained on calcium
phosphate is shown in
fgure 5
. At ±rst, a fast Cr(VI)
adsorption on calcium phosphate was observed and
at 24 h of contact time, a maximum adsorption was
found to be 2.41
x
10
-
4
meq Cr(VI) g
-
1
. Cr(VI)
adsorption data as a function of the adsorbate con-
centration were examined in terms of the Freundlich
adsorption model. In
fgure 6
the logarithm of the
amount adsorbed (log
a
e
) of Cr(VI) at equilibrium has
been plotted versus logarithm of Cr(VI) concentra-
tion (log
C
e
) in solution at equilibrium at pH 5.5. The
obtained straight line in
fgure 6
shows that Cr(VI)
adsorption data ±t well with the Freundlich isotherm
in its logarithmic form:
Log q
e
= 1/n log C
e
+ Log K
(1)
K
and
1/n
(0<
1/n
<1) being Freundlich constants
which refer to the adsorption capacity and intensity
of adsorption. The values of these constants can be
0
200
600
800
100
42.91 ºC
0.02060%/ºC
641.59 ºC
0.003761%/ºC
696.75 ºC
0.004592%/ºC
0.025
0.020
0.015
0.010
0.005
0.000
-
0.005
98
96
94
400
Temperature (ºC)
Weight (%)
Deriv. (% / ºC)
Fig. 3.
Thermogravimetric analysis of calcium phosphate
Fig. 4.
Micrograph and elemental microanalysis X-ray spectrum
of calcium phosphate
CALCIUM PHOSPHATE AND ITS RELATION TO CR(VI) ADSORPTION PROPERTIES
133
estimated by the intercept and the slope (less than 1)
of the straight line, respectively. The values of
K
and
1/n
, computed by using the least squares technique,
were found to be 0.0464 mol g
-
1
and 0.9154 mol g
-
1
respectively, with a correlation factor of 0.9996. The
value of
1/n
(less than one) found in this work conFrm
that the ±reundlich isotherm is valid for the Cr(VI)
adsorption data and this suggest than the adsorbent
surface is heterogeneous in nature with and exponen-
tial distribution of the active centers.
Figure 7
presents the desorption of Cr(VI) from
calcium phosphate; as can be seen, a minimum
amount of desorbed chromium before 3 h in water
was observed. When the calcium phosphate was left
in water for a longer time, 12 h, no more chromium
was left in solution indicating that equilibrium had
2.5
2.0
1.0
meq Cr(VI) x 10
-
4
g
-
1
of calcium phosphate
1.5
0.5
0
10
20
30
40
50
Time (h)
Fig. 5.
Milliequivalents of Cr(VI) per gram of calcium phospha-
te, pH 5.5 and room temperature
5.4
5.2
4.8
4.6
4.4
4.2
3.3
3.4
3.5
3.6
3.7
y = 0.9154 + 1.3328
R
2
= 0.9996
3.8
-log Ce (mol/L)
-log ae (mol/L)
3.9
4.1
4.2
4.3
4.4
4
5
Fig. 6.
±reundlich adsorption isotherm for Cr(VI) ions adsorp-
tion on calcium phosphate
been reached. Then, the chromium ion desorption was
minimum and indicates that physical adsorption in
the surface of calcium phosphate did not considerably
contribute during adsorption.
Figure 7
shows that after
the desorption process, the Fnal adsorption capacity at
equilibrium was 2.23 x 10
-
4
meq Cr(VI) g
-
1
of calcium
phosphate, a value very close to that obtained by the
Cr(VI) adsorption.
The speciFc surface area of a solid material is
related to a number of surface active sites available
to retain the chemical species to be removed from
the contaminated aqueous solution. Therefore, cal-
cium phosphate synthesized in our laboratories has a
good adsorption behavior of Cr(VI) species because
of its speciFc surface area is high: 64.5 m
2
g
-
1
. The
X-ray diffraction pattern showed that the calcium
phosphate formed was nanocrystalline with an aver-
age grain size of 75 nm. This grain size can not be
considered small and to reduce it by fragmentation
to much smaller values can improve the adsorption
behavior of the calcium phosphate because a material
with a higher speciFc area will be obtained. On the
other hand, the high purity of calcium phosphate (as
revealed by the corresponding IR spectrum and EDS
analysis) and the value of the maximum Cr(VI) ad-
sorption found in this work show that a pure material
is required to obtain optimum Cr(VI) removal results.
CONCLUSIONS
Pure and crystallized calcium phosphate pow-
ders with hydroxyapatite structure was synthe-
sized by continuous precipitation method using
Ca(NO
3
)
2
.4H
2
O and NH
4
H
2
PO
4
. This procedure
allows obtaining a material with a speciFc chemical
Fig. 7.
Cr(VI) desorption in water from calcium phosphate
Time (h)
meq Cr(VI) x 10
-
4
g
-
1
calcium phosphate
0.5
0
10
20
30
40
50
1.0
1.5
2.0
2.5
3.0
F. Granados-Correa
et al.
134
composition and controlled powder morphology, both
of which are essential for the application of hydroxy-
apatite as an adsorbent. The maximum sorption of
Cr(VI) was found to be 2.41
x
10
-
4
meq Cr(VI) g
-
1
of calcium phosphate. In desorption experiments the
chromium ion desorption was minimum, and thus a
high selectivity was found for chromium for the ex-
change process in this calcium phosphate. Therefore
the obtained inorganic material presents an optimal
surface and structural properties to be used as an
adsorbent material in the removal of Cr(VI) present
in aqueous wastes.
ACKNOWLEDGEMENTS
The authors thank to the Consejo Nacional de
Ciencia y Tecnología (CONACyT, México) project
52858-II and project ININ-CB-718, for ±nancial
support.
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