Environ. Sci. Technol. 2002, 36, 1630-1635
Cadmium Uptake by Hydroxyapatite Synthesized in Different Conditions and Submitted to Thermal Treatment NILCE C. C. DA ROCHA Departamento de Quı´mica Analı´tica, Instituto de Quı´mica, Universidade Federal do Rio de Janeiro, Cidade Universita´ria, Centro de Tecnologia, Bloco A/517, RJ, Brasil REINALDO C. DE CAMPOS Departamento de Quı´mica, Pontifı´cia Universidade Cato´lica do Rio de Janeiro, Rua Marqueˆs de Sa˜o Vicente, 225, RJ, Brasil ALEXANDRE M. ROSSI,* ELIZABETH L. MOREIRA, AND ADEMARLAUDO DO F. BARBOSA Centro Brasileiro de Pesquisas Fı´sicas, Rua Dr Xavier Sigaud, 150, 22290-180, RJ, Brasil GUSTAVO T. MOURE Centro de Pesquisas e Desenvolvimento Leopoldo A. M. de Mello, Cidade Universita´ria, Quadra 7, RJ, Brasil
This paper intends to evaluate the uptake of cadmium ions from aqueous solution by 21 hydroxyapatite samples which have been synthesized in different conditions. It has been determined that the variation on the hydroxyapatite sorption capacity is neither related to sample solubility nor to hydroxyapatite Ca/P molar ratio. Cd2+ sorption is controlled by sample BET surface area, which shows a direct dependence on the hydroxyapatite crystallite dimensions. The hydroxyapatite pore distribution presented modes at 1000 and 60 000 Å, corresponding to intracrystallite voids and voids between the agglomerate of these crystallites, respectively. Pores belonging to the former mode immobilize the major part of Cd2+. The influence of sample thermal treatment on Cd2+ sorption efficiency has been studied using hydroxyapatite samples calcined at temperatures ranging from 500 to 1140 °C. Similarly to nonthermally treat samples, the Cd2+ sorption on calcined hydroxyapatite could be described by Langmuir isotherms. The results showed that the maximum sorption capacity decreased from 0.631 mmol g-1 for the noncalcined sample to 0.150 mmol g-1 for the one calcined at 900 °C. This drop in the sorption capacity could also be explained by a reduction in its specific surface area, which is induced by the increase of the crystal size.
Introduction Hydroxyapatite (HA, Ca10(PO4)6(OH)2) can accept a series of cationic and anionic substitutions (1-2) in its structure. In consequence, it has been proposed for the removal of heavy metals from contaminated soils and wastewater (3-5). Thus, * Corresponding author phone: (55) 21-897370; fax: (55) 215867540; e-mail:
[email protected]. 1630
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 36, NO. 7, 2002
the understanding of the effect of these substitutions on the HA structure (6, 7) and properties (dissolution (8), crystallinity (9), surface activity (9), and thermal stability (10)) has motivated many fundamental works in the last years. In particular, the sorption of heavy metals such as Cd and Pb by HA has been extensively studied because of their high toxicity. However, controversy related to metal sorption mechanisms still remains in the literature (11-15). Such controversy is certainly related to the insufficient understanding of the physicochemical properties of the HA surface. On the other hand, HA shows strong variability in crystallinity, morphology, texture, stability, and solubility when synthesized in different ways and conditions. In the case of cadmium, previous works suggested the occurrence of several sorption mechanisms such as ion exchange (11, 12, 15), surface complexation (13), diffusion through the solid (15), and coprecipitation (13). However, it is not simple to determine the importance of each mechanism along the sorption kinetic once the amount of Ca2+ and PO43- measured in the solution is partially due to the HA dissolution. Moreover, HA solubility depends on the cadmium initially present in the solution as well as in the Cd2+ concentration on the HA surface. Another limitation is the difficulty to detect the formation of cadmium complexes or modifications on the HA structure by X-ray diffraction because of the similar ionic radii of Ca and Cd. According to Jeanjean et al. (14), who studied four different HA, the cationic vacancies have no influence on the maximum Cd2+ uptake but induce dissolution-precipitation processes that can increase the amount of immobilized Cd2+. These authors reported that the maximum Cd2+ uptake by HA was about 0.8 mol of Cd/mol of HA (0.8 mmol of Cd/g of HA) for experiments carried out at pH ) 5. Values of 0.21, 0.59, and 0.432 mmol of Cd/g of HA were also obtained by Christoffersen et al. (8), Xu et al. (13), and Mandjiny et al. (16), respectively. This suggests that the HA sorption capacity can vary according to different conditions of sample preparation. Despite the technological importance of the ceramic hydroxyapatite, almost all of the Cd2+ sorption studies have been performed using noncalcined samples. Hence, fundamental studies still need to be made in order to understand the influence of the thermal treatment on the hydroxyapatite sorption properties. It is well-known that calcination causes strong modifications on the HA bulk and surface structures, which can strongly affect its sorption capacity. For instance, water molecules and carbon dioxide, physically adsorbed inside the HA pores, are lost when exposed to temperatures below 200 °C. In temperatures ranging from 200 to 450 °C, surface water chemically bound to surface Ca2+ and PO43radicals and lattice water occupying OH sites are removed (17). Water adsorption leads to the formation of sites on the HA surface such as HPO42- and H2PO4-. In this temperature interval, sample crystallinity and surface BET area do not change significantly. For higher temperature treatments, the dehydroxylation of OH- groups gradually begins to occur, and BET area decreases strongly. Hydroxyapatite is partially decomposed in tricalcium phosphate (β-TCP), even at temperatures lower than 800 °C, depending on its stoichiometry and impurities content such as carbonate groups. Fast dehydroxylation and particle agglomeration generally occur above 800 °C. This process promotes an increase in the particle mean size and a narrowing of the particle size distribution. Calcination beyond 900 °C induces the loss of the intergranular porosity and the structure densification (18, 19). 10.1021/es0155940 CCC: $22.00
2002 American Chemical Society Published on Web 02/20/2002
TABLE 1. Synthesis Conditions, Ca/P Molar Ratio and Specific Area of the HA Samplesa concentration (mol/L-1)b samples
synthesis temperature (°C)
pH
aging (h)
P
Ca
Ca/P molar ratio
BET area (m2 g-1)
S0 S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16 S17 S18 S19 S20
78.0 ( 0.2 25.0 ( 0.3 25.0 ( 0.7 25.0 ( 0.2 25.0 ( 1.1 90.0 ( 3.0 90.0 ( 1.6 90.0 ( 2.0 90.0 ( 2.0 25.0 ( 0.4 25.0 ( 0.7 25.0 ( 0.5 90.0 ( 2.0 90.0 ( 2.9 90.0 ( 2.0 90.0 ( 1.0 55.0 ( 4.0 55.0 ( 1.0 55.0 ( 1.0 78.0 ( 0.2 80.0 ( 0.2
11.0 7.0 7.0 12.0 12.0 7.0 7.0 12.0 12.0 7.0 12.0 12.0 7.0 7.0 12.0 12.0 9.0 9.0 9.0 11.0 10.5
2 2 20 2 20 2 20 2 20 20 2 20 2 20 2 20 9 9 9 2 2
0.30 0.12 1.20 0.12 1.20 0.12 1.20 0.12 1.20 0.12 1.20 0.12 1.20 0.12 1.20 0.12 0.60 0.60 0.60 0.30 0.30
0.50 0.18 2.00 0.18 2.00 0.18 2.00 0.18 2.00 0.18 2.00 0.18 2.00 0.18 2.00 0.18 1.08 1.08 1.08 0.50 0.50
1.66 ( 0.02 1.53 ( 0.02 1.49 ( 0.02 1.71 ( 0.03 1.60 ( 0.02 1.60 ( 0.02 1.58 ( 0.02 1.75 ( 0.03 1.6 ( 40.01 1.59 ( 0.02 1.61 ( 0.03 1.72 ( 0.02 1.61 ( 0.01 1.63 ( 0.01 1.68 ( 0.02 1.80 ( 0.01 1.67 ( 0.01 1.66 ( 0.02 1.64 ( 0.02 1.64 ( 0.02 1.65 ( 0.1
74 62 52 88 66 39 34 37 49 60 66 99 32 24 63 36 69 65 58 43 30
a
[Ca2+] (mol/L-1) 1.71 × 10-4 2.29 × 10-4 7.11 × 10-5 2.17 × 10-5 7.31 × 10-5 9.08 × 10-5 4.62 × 10-5 6.51 × 10-5 1.70 × 10-4 1.39 × 10-4 9.08 × 10-5 8.18 × 10-5 4.09 × 10-5 6.61 × 10-5 3.37 × 10-5 8.91 × 10-5 8.23 × 10-5 5.00 × 10-5
Dissolved Ca concentration after 72 h in Milli-Q water is also shown. b Solution concentrations before the addition.
Therefore, the purpose of the present work is to evaluate the uptake of Cd2+ ions by hydroxyapatite synthesized in different conditions and submitted to thermal treatments. The dependence of Cd2+ uptake as a function of the HA stoichiometry and specific surface is investigated. Sorption experiments were also carried out with hydroxyapatite samples calcined at temperatures up to 1140 °C, and their sorption capacity are compared to the respective noncalcined ones.
Experimental Procedure Sample Preparation. Hydroxyapatite (HA) powder samples with different crystallinity, stoichiometry, and specific surface area were prepared (20) according to the following reaction:
3(NH4)2HPO4 + 5Ca(NO3)2 + 4NH4OH ) Ca5(PO4)3OH + 10NH4NO3 + 3H2O (1) The (NH4)2HPO4 aqueous solution was added dropwise to the Ca(NO3)2 solution at the precipitation conditions described in Table 1. The precipitate was separated by filtration, repeatedly washed with boiling deionized water, and dried at 100 °C for 24 h. The dried powder was manually ground, and the
