J. Chem. SOC., Faraday Trans. I , 1982, 78, 3605-3611 Synthetic Hydroxyapatites as Inorganic Cation Exchangers Part 2 B Y TAKASHI SUZUKI,* TOSHIAKI HATSUSHIKA AND MICHIHIRO MIYAKE Department of Applied Chemistry, Yamanashi University, Takeda-4, Kofu-shi 400, Japan Received 19th March, 1982 Ihe removdl of c d k i u h as Pb2+ Mn2+, Co2+ and Cu2+ in aqueous solution by four synthetic hydroxyapatites (S- 1, S-2, S-3, S-4) has been investigated using both batch and column methods. The removal is due not only to an adsorption effect but also to an ion-exchange reaction between the cations in solution and the Ca2+ ions of the apatites. The order of the ions according to the amount exchanged was as follows: Pb2+ > Cu2+ > Mn2+ 1: Co2+. Pb2+ ions were readily removed by the apatites and the maximum value for the exchange of Pb2+ ions was 230 mg per g of S-4 apatite.The apatites, particularly S-4, would seem to be possible agents for the removal of toxic Pb2+ ions. The selectivity of the apatites for the cations can be explained by considering the radii and the electronegativities of the ions. It is well known that hydroxyapatite [Ca,,(PO,),(OH),] is the major inorganic constituent of biological hard tissues1 such as bones and teeth and consequently its surface characteristics have been investigated from various standpoints. Macromolecules2 such as polypeptides and proteins may be adsorbed, in the usual sense of physical adsorption or chemisorption, on its surface, while some ions may ex- change with the ions of the hydroxyapatite lattice. Some of these exchange reactions, i.e.anion-exchange reactions3 between the hydroxy ions of the lattice and anions in solution, have been extensively studied. However, little attention has been given to cation-exchange r e a c t i o n ~ ~ - ~ between calcium ions of the lattice and cations in solution. In a previous paper,' we noted that some synthetic hydroxyapatites could be used as inorganic cation exchangers for metallic cations. The purpose of the present paper is to examine the cationic characteristics in more detail and to clarify the selectivity of the apatites for various cations. EXPERIMENTAL MATERIALS Four hydroxyapatite samples were prepared as follows. Sample 4 (S-4) was prepared by precipitation from hot water, i.e. by dropwise titration of phosphoric acid solution into a boiling calcium hydroxide solution, according to the method of Brown et a1.8~9 Sample 3 (S-3) was prepared by a similar precipitation method as described by Moreno et 111.l~ Sample 1 (S-1) and TABLE I.-SPECIFIC SURFACE AREAS AND Ca/P MOLAR RATIOS OF FOUR SYNTHETIC HY DROXY APATITES apatite s- 1 s-2 s-3 s-4 ~ ~~~~~ Ca/P molar ratio 1.60 1.56 1.66 1.66 surface area/m2 g-l 30.0 11.4 30.7 36.0 36053606 HYDROXYAPATITE CATION EXCHANGERS sample 2 (S-2) were commercially obtained materials (S-1 from Wako Pharmacy Co and S-2 from Nihon Chemical Co).S-1 and S-2 were synthesized for chromatographic use by a precipitation method. The specific surface areas of the samples were determined by the B.E.T. method using nitrogen adsorption at liquid-nitrogen temperatures, assuming the cross-section area of a nitrogen molecule to be 16.2 A2.The specific surface areas and Ca/P molar ratios of the four samples are summarized in table 1 . All chemicals were analytical reagent grade commercial materials and used without further purification. BATCH METHOD 1 g of each sample was mixed and stirred with 400 cm3 of 100-1000 ppm aqueous solutions containing the cations Pb2+, Mn2+, Co2+ and Cu2+ in the form of nitrates at 20 "C. After 2 h, the supernatant solutions were analysed by atomic absorption spectroscopy and EDTA titration method. Thus, the amount of Ca2+ released into solution and the amount of cation absorbed by the samples were determined. COLUMN METHOD In order to study the removal of the cations by the apatites in detail, the samples (1, 3 and 5 g) were weighed into a glass column (length, 300 mm; diameter, 10 mm) and 400 cm3 of the 100-1000 ppm solutions was passed through the column at a flow rate of 3 cm3 min-' at 20 "C.After washing with 100 cm3 of distilled water, the eluents were analysed. In addition, structural changes in the samples before and after uptake were investigated using powder X-ray diffraction. RESULTS AND DISCUSSION Table 2 summarizes the results for the removal of Pb2+ ions from 100-1000 ppm aqueous solutions (400 cm3) by S-4 (1 g) apatite at 20 OC using the batch method. Note that the molar ratio Pb2+/Ca2+ is almost equal to 1.0. This means that the uptake of Pb2+ is an ion-exchange reaction between Pb2+ ions in solution and Ca2+ ions in the apatite, as in the Cd2+-Ca2+ system discussed previ~usly.~ Moreover, the removal ratio of Pb2+ ions from 100-200 ppm solutions by S-4 is loo%, and the amount of Pb2+ ions removed from the 1000 ppm solution is ca.230 mg, even though the removal ratio is 57%. This is interesting because the maximum removal of Cd2+ ions, found to be one of the most easily removed ions,l' is only 80 mg under the same conditions. In other words, Pb2+ ions are readily removed by S-4, which has potential as an agent for the removal of Pb2+ ions. Table 3 shows the removal ratios of Pb2+ ions from 400 cm3 of 100 ppm solutions by 1 g of various synthetic hydroxyapatites (S-1 to S-4) at 20 "C using the batch method. As shown in the table, the removal ratios are different for the different TABLE 2.-REMOVAL OF Pb2+ IONS FROM 100-1000 ppm SOLUTIONS BY s-4 APATITE AT 20 "c USING THE BATCH METHOD concentration (PPm) 100 200 500 1000 Pb2+/mg per 400 cm3 39.9 77.9 171.9 231.1 ( x lop4 mol) (1.93) (3.76) (8.26) (1 1.2) ( x mol) (1.92) (3.75) (8.16) (11.1) Pb2+ : Ca2+ 1: 1 1 : l 1 : 0.99 1 : 0.99 molar ratio Ca2+/mg per 400 cm3 7.7 15.0 32.7 44.4 removal ratio (%) 100 100 86 57T.S U Z U K I , T. HATSUSHIKA A N D M. MIYAKE 3607 TABLE 3.-REMOVAL OF Pb2+ IONS FROM 100 ppm SOLUTION BY VARIOUS APATITES AT 20 " c USING THE BATCH METHOD sample s- 1 s-2 s-3 s-4 Pb2+/mg per 400 cm3 34.2 17.5 39.9 39.9 ( x mol) (1.65) (0.84) (1.93) (1.93) ( x mol) (1.65) (0.82) (1.92) (1.94) molar ratio of Pb2+ (%) Ca2+/mg per 400 cm3 6.6 3.3 7.7 7.8 Pb2+ : Ca2+ 1 : l 1 : 0.98 1 : l I : 1 removal ratio 89 45 100 100 apatites; e.g.the removal ratio of S-2 is only 45%. S-2 was found to be a mixture of Ca,,(PO,),(OH), and Ca,(PO,),. H20 by powder X-ray diffraction and to have a small surface area of 11.4 m2 g-l (cf. table 1). These seem to be the cause of the small removal ratio. However, granular S-2 seems to possess an advantage over the other apatites in that it can be used for chromatography without applying pressure. Fig. 1 shows the results for the removal of Pb2+ ions from 100-1000 ppm solutions (400 cm3) by 1 g of the four samples using the column method. It is clear that the removal of Pb2+ ions is similar to that observed using the batch method and that the order of the removal ratios for the samples at each original concentration is as follows : s-4 > s-3 > s-1 > s-2.Furthermore, no structural changes were detected by powder X-ray diffraction. Thus these samples, especially S-4, could be used as agents for the removal of Pb2+ ions. Indeed, it was found that only 3 g of S-4 is required to remove 400 mg of Pb2+ ions using the column method, as shown in fig. 2. Table 4 shows the uptake phenomena of Mn2+ ions by S-4 under the same conditions as in table 2. From table 4 it was calculated that the molar ratio '0°1 I 8 Kl 0 X 0 t t X 0 A 0 I X I I I I 0 LO 80 200 LOO original amount of Pb*+/mg per 400 cm3 FIG. 1.-Removal of Pb2+ ions at 20 O C using the column method. 0, S-4; A, S-3; x , S-2; 0, S-I.3608 HYDROXYAPATITE CATION EXCHANGERS i A t 0 200 LOO amount of Pb2+ in original solution/mg per 400 cm3 FIG.2.-Removal ratios of Pb2+ ions on 3 g of 0, S-4 and A, S-3 apatites using the column method. TABLE 4.-REMOVAL OF Mn2+ IONS FROM 100-1000 ppm SOLUTIONS BY s-4 APATITE AT 20 OC USING THE BATCH METHOD original Mn2+ concentration (ppm) 100 200 500 1000 Mn2+/mg per 400 cm3 10.2 13.8 20.2 24.2 ( x mol) (1.86) (2.51) (3.68) (4.40) Ca2+/mg per 400 cm3 7.7 10.1 15.2 17.7 ( x mol) (1.93) (2.53) (3.79) (4.42) Mn2+: Ca2+ 1:1.04 1:1.01 1:1.03 1:1.01 molar ratio removal ratio 25 17 10 6 of Mn2+ (%) Mn2+/Ca2+ is almost equal to 1.0, as in the case of Pb2+/Ca2+, but that the removal ratios of Mn2+ ions are much smaller than those of Pb2+ ions (cf. the fifth line of table 4). The trend of smaller amounts of Mn2+ being exchanged was also found for S-1, S-2 and S-3.Fig. 3 shows the removal ratios of Mn2+ ions by 5 g S-4 and S-3, which have high removal ratios for the cations, from 100-1000 ppm solutions (400 cm3) using the column method. Note that a large amount of S-4 ( 5 g) is needed to remove completely the Mn2+ ions in the solution with a low concentration of 100 ppm (40 mg per 400 cm3). These results suggest that the synthesized hydroxyapatites have the same selectivity for metallic cations as that found previously.'T. SUZUKI, T. HATSUSHIKA A N D M. MIYAKE 3609 E h ' O 0 I $ f 2 0 A 0 A 0 A 0 A t 0' I I I I 40 80 2 00 L 00 original amount of Mnz+/mg per 400 cm3 FIG. 3.-Removal ratios of Mn2+ ions on 5 g of 0, S-4 and A, S-3 apatites using the column method. 0 1 I I I I I 100 200 300 LOO 500 original concentration (ppm) FIG.4.-Removal of Co2+ ions from 100-500 ppm solutions by 0, S-1, A, S-3 and 0, S-4 apatites using the batch method.3610 HYDROXYAPATITE CATION EXCHANGERS Fig. 4 shows the results for the removal of Co2+ ions by 1 g of S-4, S-3 and S-1 from 400 cm3 of 100,200 and 500 ppm solutions using the batch method. It was found that the amount of Co2+ ions removed and the order of the apatites (S-4 > S-3 > S-1) are almost the same as in the case of Mn2+ ions. Fig. 5 presents the removal of Cu2+ ions from 400 cm3 of 100-500 ppm solutions by 1 g of S-4, S-3 and S-1 using the batch method. The amounts of Cu2+ ions removed I 1 I are greater than those of Mn2+ and Co2+ ions, but smaller than those of Pb2+ ions. In other words, the order of the removal ratios of the cations is as follows: Pb2+ > Cu2+ > Mn2+ N Co2+.Fig. 6 shows the relationship between the ionic radius12 and ele~tronegativityl~ of various cations, where MI Ca and M2 Ca represent the ranges of radii of two different Ca2+ ions1* in the hydroxyapatite. The following results are obtained from fig. 6. First, the radii of easily removed ions such as Pb2+ and Cd2+ fall within the range of the radius of M2 Ca (0.9-1.3 A) and they have large electronegativity values. Secondly, Mg2+ and Ba2+ ions, which are hardly removed by the apatites,' have radii which fall outside the range and low electronegativity values. Thirdly, Cu2+, Co2+ and Mn2+ ions show intermediate behaviour, i.e. their radii fall outside the range but they have high electronegativity values.From these results, we conclude that if cations have large electronegativity values and radii close to the range 0.9-1.3 A then they are more easily removed by the hydroxyapatites. This work was supported by a Grant in Aid for Research from the Japanese Government Ministry of Education. We thank Mrs Hideko Hatsushika for her secretarial help during the preparation of the manuscript.T. S U Z U K I , T. HATSUSHIKA A N D M. MIYAKE 361 1 2.0 1 0 CdvD 0 MI Ca(VI) - I I 0.6 0.7 0.8 0.9 1.0 1 . 1 1 . 2 1.3 1.4 ionic radius/A FIG. 6.-Relationship between electronegativity and ionic radius. See, for example, C. L. Kibby and W. K. Hall, The Chemistry of Biosurfaces (Marcel Dekker, New York, 1972), vol. 2, p. 663. V. Hlady and H. F. Milhofer, J.Colloid Interface Sci., 1979, 69, 460. See, for example, I. Zipkin, A. S. Posner and E. D. Eanes, Biochim. Biophys. Acta, 1962, 59, 255; R. A. Young, Proc. 2nd Int. Congr. Phosphorus Compounh (IMPHOS, Paris, 1980), p. 73. C. Y. C. Pak and F. C. Bartter, Biochim. Biophys. Acta, 1967, 141, 410. T. Suzuki, T. Hatsushika and Y. Hayakawa, Proc. Zndlnt. Congr. Phosphorus Compounds (IMPHOS, Paris, 1980), p. 165. T. Suzuki, T. Hatsushika and M. Miyake, Proc. 3rd Congr. Inorganic Phosphorus Chemistry (The Chemical Society of Japan, Tokyo, 1981), p. 16. T. Suzuki, T. Hatsushika and Y. Hayakawa, J. Chem. Soc., Faraday Trans. 1, 1981, 77, 1059. Y. Aunimelech, E. C. Moreno and W. E. Brown, J. Res. Nut1 Bur. Stand., Sect. A , 1973, 77, 149. H. Mcdowell, T. M. Gregory and W. E. Brown, J. Res. Natl Bur. Stand., Sect. A, 1977, 81, 273. lo E. C. Moreno, T. M. Gregory and W. E. Brown, J . Res. Natl Bur. Stand., Sect. A , 1968, 72, 773. l 1 T. Suzuki and Y. Hayakawa, Proc. 1st Int. Congr. Phosphorus Compounds (IMPHOS, Paris, 1977), l 2 R. D. Shannon and C. T. Prewitt, Acta Crystallogr., Sect. B, 1969, 25, 925. l 3 L. Pauling, The Nature of the Chemical Bond (Cornell University Press, Ithaca, New York, 3rd edn, l4 M. I. Kay, R. A. Young and A. S. Posner, Nature (London), 1964, 204, 1050. p. 381. 1960). (PAPER 2/475)