Preparation of Calcium Phosphate with Oyster Shells

doi:10.4236/nr.2012.32011

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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.

Email: [email protected]

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

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.

○

○○

○

(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.

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

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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