Natural Resources, 2012, 3, 71-74
http://dx.doi.org/10.4236/nr.2012.32011 Published Online June 2012 (http://www.SciRP.org/journal/nr) 71
Preparation of Calcium Phosphate with Oyster Shells
Hiroaki Onoda, Hironari Nakanishi
Department of Informatics and Environmental Sciences, Kyoto Prefectural University, Kyoto, Japan.
Received April 16th, 2012; revised May 10th, 2012; accepted May 18th, 2012
ABSTRACT
Oyster shells have received attention for use as a calcium resource. For this study, calcium phosphate was prepared
from phosphoric acid and oyster shells. The influences of the concentration of phosphoric acid and pH in the prepara-
tion conditions were studied from the yields of calcium phosphate and unreacted carbonate, and the Ca/P ratios in pre-
cipitates. The yield of calcium phosphate and carbonate was low in the preparation condition with 0.1 mol/l of phos-
phoric acid. The obtained precipitates were the mixture of calcium hydrogen phosphate dihydrate and the unreacted
calcium carbonate. The reactivity of the oyster shells with phosphoric acid was discussed from the yields and Ca/P ra-
tios in precipitates.
Keywords: Calcium Phosphate; Oyster Shells; Calcium Carbonate; Yield; Powder Properties
1. Introduction
Recently, interest in the recycling of waste materials is
increasing. From this viewpoint, seashells have been used
as a raw material for various applications [1,2]. Generally,
edible parts of shellfish are few. Therefore, large amounts
of waste material are produced from shellfish processing.
Because seashells are easy to gather, they are suitable for
use as a raw material. To date, seashells that are light
with little pigment, for ex ample scallops, hav e been used
as a calcium resource [3,4]. Nevertheless, it is difficult to
use seashells that are dark and which have much pig-
ment.
Calcium phosphates are important materials for many
applications, such as ion exchangers, adsorbents, and so
on [5,6]. Calcium phosphates were prepared from sea
urchin shells and artificial phosphorus waste fluid in
earlier studies [7,8]. The obtained precipitates’ main
component was CaHPO4·2H2O. The precipitates con-
taining organic coloring materials were obtained in the
process with the sea-urchin shell heated at lower tem-
peratures as a pretreatment. Sea urchin shells have overly
strong coloring materials. In contrast, corbicula and
oyster shells were expected to produce calcium phos-
phate with fewer coloring materials [9]. In previous work,
corbicula shells were used to prepare calcium phosphate
[10]. The corbicula shells with and without heating reacted
with phosphoric acid to calcium hydrogen phosphate
dihydrate. Oyster is one of the popular seafoods in all o ver
the world. Therefore, the oyster shells are easy to gather,
these oyster shells have a possibility to use as one of cal-
cium resource.
In this work, we tried to use this oyster shell as a cal-
cium resource to prepare calcium phosphate. The concen-
tration of phosphoric acid and pH value were varied to
clarify the formation mechanism of calcium phosphate.
The chemical composition and powder properties of pre-
cipitates were investigated. The obtained phosphate ma-
terials were also assessed for the comparison with cal-
cium phosphate prepared from phosphoric acid and
commercial calcium carbonate.
2. Experimental
Oyster shells were washed and milled to a powder condi-
tion. Half were heated at 400˚C for 1 hour. The shell
powders without heating and heated at 400℃ were mixed
with 0.1, 0.05, and 0.01 mol/l of phosphoric acid in the
ratio of Ca/P = 1, 2, and 10, respectively. At this time,
the weight of oyster shells was calculated as that of cal-
cium carbonate. These solutions were adjusted with am-
monia water to pH 5, 7, and 9. The precipitates in these
solutions were filtered off and dried. For comparison,
commercial calcium carbonate was reacted with phos-
phoric acid in the same conditions as those used for oys-
ter shells.
A part of the precipitates was dissolved in hydrochloric
acid solution. The ratios of phosphorus and calcium in the
precipitates were also calculated from ICP results of these
solutions, using SPS1500VR, Seiko Instruments Inc. XRD
patterns of materials were recorded on a Rigaku Denki
RINT 2000 X-Ray diffractometer using monochromated
CuKα radiation.

Preparation of Calcium Phosphate with Oyster Shells 72
Furthermore, the powder properties of the obtained
precipitates were estimated from the particle shape and
size distribution. Scanning electron microscopy (SEM)
images of samples were observed using JGM-5510LV,
JEOL Ltd. The particle size distributions of these m ate rials
were measured with laser diffraction/scattering particle
size distribution HORIBA L A- 910.
3. Results and Discussion
3.1. Preparation of Calcium Phosphates
The phosphorus-rich solution disturbs the water environ-
ment at many points in the world [11]. Therefore, in this
work, the concentration of phosphoric acid changed to a
lower one to prevent the phosphorus-rich filtered solu tio n.
At low phosphoric acid concentration, not all of the oys-
ter shells were able to react with phosphoric acid. Table 1
presents the yields of precipitates obtained in various con-
ditions. This yield was calculated on the assumption that
the obtained precipitate was a mixture of CaHPO4·2H2O
and CaCO3. The yield of calcium phosphate and carbon-
ate was low in the preparation condition with 0.1 mol/l of
phosphoric acid. On the other hand, the yields in the
conditions with 0.05 and 0.01 mol/l of phosphoric acid
were higher than those with 0.1 mol/l of phosphoric acid,
because of the unreacted calcium carbonate and insolu-
ble part of oyster shells. The yields became lower by the
reaction with phosphoric acid. Oyster shells without heat-
ing indicated high yi elds by usi ng 0.01 m ol/l of phosphori c
acid, because this condition was unsuitable to react with
phosphoric acid. In the whole, oyster shells indicated the
lower yields of calcium phosphate than corbicula shells
reported in previous work [10]. The conditions of cor-
bicula shells with heating and 0.1 mol/l of phosphoric
acid indicated over 90% in the yields. Oyster shells had
the different behavior in the reactivity with phosphoric
acid from corbicula shells.
The ratio of CaHPO4·2H2O and CaCO3 in precipitates
was estimated from Ca/P ratio in the results of ICP
measurements. Table 2 shows Ca/P ratio in precipitates
from ICP results. The theoretical Ca/P ratio was 1, 2, and
10 in the cases with 0.1, 0.05, and 0.01 mol/l of phos-
phoric acid, respectively. The Ca/P ratio in CaHPO4 is 1,
therefore the difference from this ratio was related with
that the precipitates contained calcium carbonate. Sam-
ples prepared with 0.05 and 0.01 mol/l of phosphoric
acid had smaller ratio of Ca/P than theoretical ratio.
Table 1. Yeilds of samples prepared with oyster shells/s%.
No. Ca resource Concentration of H3PO4/mol/l pH Without heating With heating
1 shell 0.1 5 37.9 44.1
2 shell 0.1 7 65.0 59.6
3 shell 0.1 9 72.3 50.5
4 shell 0.05 5 86.7 87.6
5 shell 0.05 7 76.8 85.6
6 shell 0.05 9 76.9 76.9
7 shell 0.01 5 90.1 78.8
8 shell 0.01 7 87.8 75.9
9 shell 0.01 9 92.3 74.0
10 reagent 0.1 7 96.8 -
11 reagent 0.05 7 94.0 -
12 reagent 0.01 7 93.0 -
Table 2. Ca/P ratios in precipitates from ICP measurements.
No. Ca resource Concentration of H3PO4/mol/l pH Without heating With heating
1 shell 0.1 5 1.19 1.30
2 shell 0.1 7 0.78 1.37
3 shell 0.1 9 0.82 1.50
4 shell 0.05 5 1.55 0.70
5 shell 0.05 7 0.85 0.89
6 shell 0.05 9 0.97 1.04
7 shell 0.01 5 4.91 3.29
8 shell 0.01 7 2.73 3.73
9 shell 0.01 9 4.91 4.84
10 reagent 0.1 7 1.13 -
11 reagent 0.05 7 2.32 -
12 reagent 0.01 7 11.21 -

Preparation of Calcium Phosphate with Oyster Shells 73
When oyster shells were difficult to react with phosphor-
ric acid, the Ca/P ratio became large from the unreacted
calcium carbonate. Because the calcium ratio was lower
than theoretical ratio, the initial calcium ratio in oyster
shells was low. In the conditions of 0.1 mol/l of phospho-
ric acid, the oyster shells heated at 400˚C indicated the
higher ratio of Ca/P than those without heating. In previ-
ous paper about corbiculla shells, these calcium ratios in
precipitates prepared from corbicula shells indicated the
similar Ca/P ratios with the theoretical ratios [10]. Oyster
shells had the different behavior on these Ca/P ratios
from corbicula shells. Seashells had the various kinds of
the reactivity with phosphoric acid.
Figure 1 portrays XRD patterns of samples prepared
in various conditions. Samples prepared with 0.01 mol/l
of phosphoric acid indicated the weak unknown peaks
(Figure 1(c)). Because the large amount of calcium car-
bonate was included in samples prepared with 0.01 mol/l
of phosphoric acid from Table 2, these peaks were re-
lated with calcium carbonate. Other samples prepared
with 0.05 and 0.1 mol/l of phosphoric acid had XRD
peaks of calcium hydrogen phosphate dihydrate (Figures
1(a), (b), (d)). From the yields and the Ca/P ratios, these
samples included a certain degree of calcium carbonate.
However, the peaks of calcium carbonate were not ob-
served in these XRD patterns.
3.2. Powder Properties of Precipitates
Figure 2 presents SEM images of samples prepared in
various conditions. Samples had the mixture of plate-like
and oblong particles in spite of the concentration of phos-
phoric acid, the heatin g co nd itio n of o yst er shells , and pH
in the preparatio n condition [12]. The ratio of the oblong
10 20 30 40 5060
Intensity
2 /deg.
○
○○
○
θ
(a)
(b)
(c)
(d)
Figure 1. XRD patterns of samples obtained at pH 7, (a)
H3PO4 0.1 mol/l, oyster she lls with out heating; (b) 0.05 mol/l,
without heating; (c) 0.01 mol/l, without heating; (d) 0.1
mol/l, with heating; ○: CaHPO4·2H2O.
particles increased in samples prepared with 0.01 mol/l of
phosphoric acid, because of the high Ca/P ratio in precipi-
tates.
Figure 3 shows the particle size distribution of samples
prepared in various conditions. Samples prepared with
0.05 and 0.1 mol/l of phosphoric acid and oyster shells
without heating had the smaller particles than 100 µm
(Figures 3(a) and (b)). These samples indicated the simi-
lar yields and Ca/P ratio (Tables 1 and 2). On the other
hand, sample prepared with 0.01 mol/l of phosphoric
acid and oyster shells without heating had the large parti-
cles than 100 µm (Figure 3(c)). Because this sample had
high Ca/P ratio (Tab le 2), these large particles contained
a certain degree of calcium carbonate. Sample prepared
with 0.1 mol/l of phosphoric acid and oyster shells with
heating had the medium particle size distribution (Figure
3(d)). The particle size distribution of samples was affected
from their yields and the Ca/P ratio in the precipitates.
Figure 2. SEM images of samples, (a) H3PO4 0.1 mol/l, oys-
ter shells without heating; (b) 0.1 mol/l, with heating.
0
5
10
15
20
110100 1000
(a)
(b)
(c)
(d)
Number of particles /%
Particle size /µm
Figure 3. Particle size distributions of samples, (a) H3PO4
0.1 mol/l, oyster shells without heating; (b) 0.05 mol/l, with-
out heating; (c) 0.01 mol/l, without heating; (d) 0.1 mol/l,
with heating.

Preparation of Calcium Phosphate with Oyster Shells 74
4. Conclusion
Calcium phosphates were prepared using oyster shells
and phosphoric acid. The reactivity between oyster shells
and phosphoric acid was estimated from the yield of cal-
cium phosphate and carbonate and the Ca/P ratio in pre-
cipitates. The yields in the co nditions with 0.05 and 0.01
mol/l of phosphoric acid were higher than those with 0.1
mol/l of phosphoric acid, because of the unreacted calcium
carbonate and insoluble part of oyster shells. The obta in ed
samples prepared with 0.1 and 0.05 mol/l of phosphoric
acid indicated XRD peak pattern of calcium hydrogen
phosphate dihydrate. Samples prepared with 0.01 mol/l
of phosphoric acid included a large amount of calcium
carbonate.
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