Environ. Sci. Technol. 2001, 35, 3669-3675
Sorption of Antimony onto Hydroxyapatite ANA G. LEYVA Comisio´n Nacional de Energı´a Ato´mica, Unidad de Actividad Fı´sica, Centro Ato´mico Constituyentes, Avda. Gral Paz 1499, 1650-San Martı´n, Argentina JULIETA MARRERO Comisio´n Nacional de Energı´a Ato´mica, Unidad de Actividad Geologı´a, Centro Ato´mico Ezeiza, Avda. del Libertador 8250, 1429-Buenos Aires, Argentina PATRICIA SMICHOWSKI AND DANIEL CICERONE* Comisio´n Nacional de Energı´a Ato´mica, Unidad de Actividad Quı´mica, Centro Ato´mico Constituyentes, Avda. de los Constituyentes 1499, 1650-San Martı´n, Argentina
We prepared synthetic hydroxyapatite [HAP; Ca5(PO4)3-x(CO3)x(OH)1+x (x ) 0.3)] and then investigated this material’s ability to remove trivalent antimony [Sb(III)] from water. The HAP was characterized by X-ray diffraction analysis, scanning electron microscopy, X-ray energy dispersive spectroscopy, X-ray photoelectron spectroscopy, and infrared spectroscopy. The sorption of Sb(III) to HAP was measured over an Sb(III) concentration range of 0.05-50 mg L-1, at constant ionic strength (I ) 0.01 mol dm-3) in equilibrated aqueous suspensions (34 g dm-3) for 5 < pH < 8 in vessels that were closed to the atmosphere. Under these conditions, we found that HAP particles were enriched in Ca after incongruent dissolution of the solid. More than 95% of the Sb(III) in solution adsorbed to the solid-phase HAP in 0.25 µm. These results provide direct evidence of the efficiency of the filtration technique we used. Samples (0.010-0.34 g) of the HAP were placed in 15-mL Nalgene polycarbonate centrifuge tubes and equilibrated in 0.01 M KCl (Merck) for various periods of time (ranging from hours to 30 d) at pH values ranging from 5 to 11. The HAP suspensions were not in contact with air at any time: contact with air was avoided by using adequate seals, lab equipment, and procedures. Different Sb(III) concentrations, ranging from 0.05 to 100 mg L-1, were established in the equilibrated suspensions by addition of known quantities of Sb(III) stock solution. Supernatant pH values were measured with a pH meter equipped with a combination glass electrode (Hanna) both before and after the adsorption experiments. The pH meter and probe system was standardized with buffers of pH 4, 7, and 10 (Hanna). Sb(III)-amended samples were filtrated after they were shaken overnight. The Ca and P contents of the filtrates were analyzed by inductively coupled plasma-atomic emission spectrometry (ICP-AES) using a sequential Perkin-Elmer ICP 400 instrument (Norwalk, CT). 3670
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TABLE 1. HAP Equilibrium Data reactions
log K
ref
+ H2PO4 a H3PO4 H+ + HPO42- a H2PO4H+ + PO43- a HPO42Ca2+ + H2PO4- a CaH2PO4+ Ca2+ + HPO42- a CaHPO4 Ca2+ + PO43- a CaPO4Ca5(PO4)3OH a 5Ca2+ + 3PO43- + OH-
2.21 7.18 12.18 1.50 2.83 6.54 59
34 35 36 37 37 37 30
H+
-
Coupled hydride generation-inductively coupled plasmaatomic emission spectrometry (HG-ICP-AES) was used for the determination of Sb at trace levels. The detection limit of Sb by HG-ICP-AES was 0.3 µg L-1. No Sb signal was observed for HG-ICP-AES analysis of blank solutions. Results presented in this study are average values based on three replicate measurements. The electrophoretic mobility of the batch suspensions was examined, at different pH conditions, using a Rank Brothers Mark II apparatus (London, U.K.) equipped with a cylindrical cell (2 mm diameter) thermalized at 20 °C. Several techniques, including XRD, FTIR, SEM, and EDAX, were used to study the solid phase after Sb adsorption. Thermodynamic Description of the Sb(III)-HAP-H2O System. Chemical equilibrium modeling was accomplished using MINEQL (version 3.21, Environmental Research Software, Hallowell, ME). Calculations over the range of the pH conditions we investigated were made with HAP considered as an infinite solid, assuming congruent dissolution of HAP in a system closed to the atmosphere with total carbonate concentration ranging from 1 × 10-4 to 1 × 10-6 mol dm-3. The thermodynamic data used in these calculations are summarized in Table 1.
Results and Discussion The apatite powder had a specific surface area of 61 m2 g-1 and a particle size of ∼0.45 µm. The average equivalent spherical diameter of the aggregates was 35 µm. The X-ray diffraction pattern of the precipitate (Figure 1) was assigned to HAP (ICDD 9-432) (20). The broadening of the X-ray diffraction line was attributed to the small crystal size and/ or the enrichment of the HAP with carbonate. From the shift of the (300) reflection (d ≈ 2.72 Å), about 10% of the CO32concentration was assignable to shifts observed by LeGeros et al. (21). FTIR analysis revealed the presence of carbonate on the surface of the HAP. Figure 2 shows the transmittance
FIGURE 2. FTIR spectra of HAP, with and without Sb(III), from 2200 to 600 cm-1. Inset: a complete spectrum (4000-600 cm-1 region) shows the hydroxyl peak at 3574 cm-1. infrared spectrum of synthetic HAP in the 4000-650 cm-1 region before and after interaction with 50 µg L-1 Sb(III). A narrow band located near 965 cm-1 (962 cm-1 in Figure 2) represents the ν1 mode of PO43- ions in apatite. The main signal of phosphate appears in the triply degenerate ν3 domain (1000-1100 cm-1). The adsorption band at 3570 cm-1 confirmed the presence of OH- groups. The ν2 peak of CO32is located at 875 cm-1; this absorption results from out-ofplane stretching. The ν3 mode, near 1400 cm-1, is the strongest IR peak for carbonate. This peak is actually composed of two bands (1454 and 1421 cm-1, in Figure 2) (22, 23). The shape of the ν3 signal and the absence of the C-O absorption bands at 710 cm-1 indicate that no calcite was associated with the HAP. Carbonate ions can substitute for either OH- or PO43ions in the apatite structure (type A CO32- or type B CO32-) (24, 25). The lack of a band at 1540 cm-1 shows that carbonate is located in type B sites, substituting for phosphate [14551420-878 cm-1 (23) assuming a shift in ν2 peak to 875 cm-1]. The presence of the band at 650-1630 cm-1 reveals the presence of adsorbed water (26). The ratio of Ca to P on the precipitate’s surface (using data obtained by EDX analysis) showed 6% enrichment in Ca. We interpreted this as an independent line of evidence for the substitution of phosphate by carbonate. For semiquantitative analysis with EDX, the sample was spread on one side of double-adhesive tape and coated with silver to improve its conductivity. Thermodynamic Considerations. The calculated and measured values of total dissolved phosphorus (PT) and total calcium (CaT) are shown for HAP at pH values between 5 and 10 in Figure 3. The differences between the calculated and the measured values of CaT were consistently small for CaT values that varied over nearly 3 orders of magnitude (0.010.00001 M). This pattern was not evident for comparisons of calculated vs measured levels of phosphorus: for this element, PT was always >0.001 M. Based on measured values for CaT and PT, the ratio of CaT to PT was much lower than 1.66 for pH values greater than 6.6. This finding demonstrated incongruent dissolution of HAP with Ca enrichment of the suspended particles. A reaction that could reasonably account for the observed responses of P and Ca is a 10% replacement of phosphate by carbonate in the synthetic HAP samples we used. Carbonate substitution has a destabilizing effect on the apatite structure, which results in an increase in solubility (27). Our experimental systems
FIGURE 3. Solubility of HAP in water at different pH conditions at 20 ( 0.1 °C. Solid and dashed lines represent total dissolved calcium (CaT) and phosphate (PT), respectively, calculated using MINEQL, assuming congruent dissolution of HAP and 0.01 ( 0.001 mol KCl dm-3 ionic strength. The open squares and the solid circles show measured values of PT and CaT, respectively, for equilibrated HAP samples. were closed to the atmosphere, so the total carbonate concentration would depend on the amount of dissolved solid (it would range from 3 × 10-4 to 3 × 10-6 mol dm-3). EDX analysis of HAP that had been treated with Sb under various pH conditions showed that (i) HAP particles were enriched in Ca and (ii) the ratio of Ca to P increased as pH increased. The latter trend was confirmed by XPS analysis: the ratio of Ca to P was 1.9 at pH 7 and was 2.2 at pH values >7. The ratio of Ca to P for solid-phase HAP was also calculated using data for concentrations of Ca and P remaining in solution after equilibration. This method yielded a Ca to P ratio of 1.87, implying a 3% enrichment of the HAP with Ca. These results are in agreement with the data we obtained using EDX and XPS. Our results indicate that sorption kinetic and equilibrium studies of metals with aqueous suspensions of HAP should use samples that are preequilibrated for at least 7 d, especially when experimental units have low ratios of surface area to solution volume. Well-equilibrated systems also should improve the comparability of the results of experiments to values reported in the literature. Adsorption of Sb onto HAP. Our experiments used Sb concentrations far away from the saturation level of Sb(OH)3. VOL. 35, NO. 18, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Adsorption isotherm of Sb(III) on HAP. Solid squares show values obtained from our experiments. The solid line corresponds to a Langmuir description of the adsorption process. Γmax ) 6.7 ( 0.1 × 10-8 mol m-2 (1.4 ( 0.2 × 10-4 mol dm-3 g-1) and Kads ) 1.5 ( 0.2 × 103 dm3 mol-1. A linear representation of the adsorption data using this model is shown in the inset. (See text for more details.) Under these conditions, no changes in the concentration of Sb(III) were detectable in “blank” experiments in which Sb was put into contact with the filtered aqueous phase of HAP suspensions. Nearly complete removal of Sb from solution was achieved in e0.5 h under the following experimental conditions: 5 < pH < 8, when Sb initial concentrations of the equilibrated HAP suspensions were between 0.05 and 50 mg/L. These results compare favorably with rates of uptake of Pb, Cd, and U by HAP (18, 28) for other studies that used high surfacearea to solution-volume ratios. After treatment, under the experimental conditions described above, Sb concentrations were
