Sorption and Desorption of Cd2+ and Zn2+ Ions in Apatite

Sorption and Desorption of Cd and Zn Ions in Apatite-Aqueous Systems ..... of superparamagnetic nanoparticles on the physicochemical properties of nan...
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Environ. Sci. Technol. 2004, 38, 5626-5631

Sorption and Desorption of Cd2+ and Zn2+ Ions in Apatite-Aqueous Systems M E R I K E P E L D , * ,† KAIA TO ˜ NSUAADU,‡ AND VILLEM BENDER§ Departments of Chemistry, Chemical Engineering, and Physics, Tallinn University of Technology, 5 Ehitajate Road, 19086 Tallinn, Estonia

As a low-soluble phosphate mineral capable of binding various metal ions, apatite can be used to immobilize toxic metals in soils and waters. In the present research the factors affecting sorption and desorption of Cd2+ and Zn2+ ions on/from apatites are investigated. Batch experiments were carried out using synthetic hydroxy-, fluoride-, and carbonate-substituted apatites having various specific surface area (SSA). Apatite sorption capacity was found to depend mainly on its SSA, ranging from 16 to 78 and from 11 to 79 mmol per 100 g of apatite for Cd2+ and Zn2+, respectively. The solution composition (pH, and presence of Cl- and NO3- ions) had no essential impact on sorption. Desorption of bound cations depended both on the sorption level and solution composition. The amount of desorbed Cd2+ and Zn2+ increased proportionally to the amount of sorbed cations. However, apatites having higher sorption capacity release relatively less sorbed cations. Desorption increases with increasing Ca2+ concentration in the solution, reaching 8-20% of sorbed Cd2+ in 0.002 M, 1035% in 0.01 M, and 33-45% in 0.05 M Ca(NO3)2 solution. Compared to nitrate solutions, the presence of Cl- ions in the solution promotes the release of bound cations. Desorption of Zn2+ is slightly higher than that of Cd2+. The desorption mechanism was assumed to include both ionexchange and adsorption of Ca2+ ions on apatite surface.

Introduction Calcium phosphates with apatite structure [Ca10(PO4)6(OH)2] possess an ability to bind metal ions from solutions and are, therefore, considered to be perspective materials for the immobilization of toxic metals from polluted soils, sediments, and waters, allowing rehabitation of soils and revegetation of highly polluted industrial sites. The addition of hydroxyapatite has been shown to lower the mobility of Cd, Zn, Pb, Cu, Co, Mn, Ni, and U (1-12) and decrease the exchangeable contents of Cd, Pb, Ni, U, Al, Ba, Co, Mn, Cu, and Zn in sediments and soils (4, 11, 13, 14), inhibiting their uptake by plants. The reversibility of metal sorption is essential for the potential application of apatites as metal immobilizers. The high reversibility of sorption permits regeneration and * Corresponding author phone: +372 6202859; fax: +372 6202801; e-mail: [email protected]. † Department of Chemistry. ‡ Department of Chemical Engineering. § Department of Physics. 5626

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recurrent utilization of sorbent material, which makes such apatites suitable for water purification. The low reversibility favors the use of apatites as soil additives that immobilize toxic metals into a stable form of low solubility, thus reducing the metals bioavailability for plants, or as barrier materials that restrict metals dispersal and migration into deeper soil layers and eventually groundwater. Depending on metal ion and apatite characteristics, sorption can proceed via different mechanisms. Sorbed metal ions can be bound at the surface (adsorption), or introduced into the apatite strucure, followed by the filling of cationic vacancies in a nonstoichiometric apatite (absorption), or exchanged with apatite lattice cations (ion-exchange). Dissolution of calcium apatite and the formation of new metal phosphate phases is also possible (dissolution-precipitation method) (1, 2, 4-6, 8, 10, 15). Among these processes which can transform toxic metals into less soluble forms, ionexchange and the reprecipitation of a partly substituted apatite phase are the most desirable, as the metal is incorporated into the apatite structure as a high stability reaction product. The ability of apatite to bind metal ions depends on its structure and chemical composition. For example, a partial carbonate and fluorine substitution in hydroxyapatite diminishes its sorption capacity, while an increase in magnesium substitution enhances metal ion sorption (16, 17). The main factor affecting apatite sorption properties has been found to be its specific surface area (17). The stability of sorption products may be affected by the same apatite characteristics. The extent of both sorption and the subsequent desorption of metal ions may depend also on the solution composition, contact time, temperature, solidliquid ratio, etc. (18-20). The substituted apatites are less studied than pure calcium-hydroxyapatites. However, as substitution is common in natural apatites, the impact of different substituents on apatite metal binding properties is of great interest. Interpretation of the results of studies using natural apatites is complicated, as they contain different anionic and cationic substituents as well as other accompanying minerals, all affecting metal sorption and desorption behavior. Therefore in this research carbonate-and fluoride-containing synthetic apatites with different specific surface area were used. The aim of this study was (1) to examine the factors affecting the sorption of Zn2+ and Cd2+ from aqueous solutions on synthetic apatites with different chemical compositions and specific surface areas, (2) to evaluate the stability of sorbed metal ions in apatite, and (3) to examine the impact of the solution composition on desorption process.

Experimental Section Materials. Synthetic Ca-apatites were used as materials for sorption of Cd2+ and Zn2+ ions from aqueous solutions. Apatites were prepared by precipitation from aqueous solutions at pH 9-10 (21). Three Ca-Cd-apatite samples were precipitated in order to obtain the material for comparison with Ca-apatites having sorbed cadmium. IR spectra showed that synthesized apatites were carbonate apatites of B-type (CO32- in PO43- position) (see Figure 1 in Supporting Information). They differed in fluorine and carbonate contents and in SSA (Table 1). Their chemical formulas were calculated assuming that the sum of PO43and CO32- equals six, and there can be vacancies at Ca2+ ion and monovalent anion positions. The quantity of OH- groups was calculated basing on electroneutrality principle. To explain the high cation-to-anion ratio in synthesized materi10.1021/es049831l CCC: $27.50

 2004 American Chemical Society Published on Web 09/28/2004

TABLE 1. Chemical Composition and Specific Surface Areas of Apatitesc sample

specific surface area, m2/g

chemical formula

Ca-Hydroxyapatites

A-1-4 A-1-3 HA-1a HA-2a HA-3a,b

Ca9.66Ca0.34(PO4)5.96(CO3)0.04(OH)1.36OH0.64‚1.24H2O Ca9.88Ca0.12(PO4)5.94(CO3)0.06(OH)1.81OH0.19.1.20H2O Ca9.93Ca0.07(PO4)5.85(CO3)0.15(OH)2‚2.05H2O + 0.30Ca(OH)2 Ca9.89Ca0.11(PO4)5.77(CO3)0.23(OH)2‚2.80H2O + 0.15Ca(OH)2 Ca9.92Ca0.08(PO4)5.84(CO3)0.16(OH)2‚2.59H2O + 0.30Ca(OH)2

D-3-2 D-3-1 D-2-3b D-2-4 A-3-1 A-2-1 D-2-2a,b D-2-1a

Ca9.63Ca0.37(PO4)5.40(CO3)0.60(OH)1.13F0.74OH0.13‚1.67H2O Ca9.57Ca0.43(PO4)5.26(CO3)0.74(OH)1.17F0.71OH0.12‚1.65H2O Ca9.36Ca0.64(PO4)5.60(CO3)0.40(OH)0.83F0.28OH0.89‚1.39H2O Ca9.20Ca0.80(PO4)5.23(CO3)0.77(OH)0.88F0.29OH0.83‚1.68H2O Ca9.92Ca0.08(PO4)5.83(CO3)0.17(OH)1.23F0.77‚1.32H2O Ca9.88Ca0.12(PO4)5.76(CO3)0.24(OH)1.56F0.44‚1.70H2O Ca9.53Ca0.47(PO4)5.63(CO3)0.37(OH)1.08F0.35OH0.57‚1.45H2O Ca9.41Ca0.59(PO4)5.05(CO3)0.95(OH)1.50F0.27OH0.23‚1.97H2O

CaCdHA1 CaCdFA1 CaCdFA2

Ca8.05Cd1.93Ca0.02(PO4)5.96(CO3)0.04(OH)2‚1.61H2O + 1.43Ca(OH)2 Ca8.01Cd1.99(PO4)6(OH)0.84F1.16‚*H2O + 1.69Ca(OH)2 Ca7.93Cd2.07(PO4)6(OH)0.81F1.19‚1.64 H2O + 1.83Ca(OH)2

16.1 30.9 36.3 85.4 86.1

Ca-Fluorhydroxyapatites

9.6 15.8 20.4 28.4 31.0 32.8 55.0 113.8

CaCd-Apatites

a

Synthesized with rapid mixing.

b

Heated to boiling temperature after mixing.

c Ca, OH

31.2 4.57 31.2

- vacancies at Ca2+ and OH- positions in apatite structure.

FIGURE 1. The dependence of apatite (D-2-1) solubility and cadmium and zinc sorption on solution pH. als, the excess of calcium is presented as Ca(OH)2. The high pH of synthesis allows the formation of Ca(OH)2 (22); however, neither XRD nor IR spectra showed the presence of Ca(OH)2 as a separate phase. Sorption. Sorption experiments were performed in batch at room temperature. 100 mg of apatite was introduced into 50 mL of Cd(NO3)2, CdCl2, or Zn(NO3)2 aqueous solution (0.002 M, pH 6) and shaken (160 rpm) for 24 h, which was considered to be the necessary time to attain equilibrium (see Figure 2 in Supporting Information). For the case of Cd2+ sorption, some experiments were carried out with binary solutions containing both Cd2+ and Ca2+ (concentration for either 0.001 M). To study the impact of the solution pH, sorption experiments were carried out at pH ranging from 2 to 7. The pH of solutions was adjusted by adding nitric acid or ammonia aqueous solutions. The apatite solubilities were estimated at the same pH values. The suspensions were centrifuged and pH values and Ca, Cd, Zn, P, and F concentrations in the solution determined. The solid product was washed with distilled water and air-dried at 105 °C. The crystal structures of apatites before and after the uptake of Cd2+ and Zn2+ were investigated by XRD and IR spectroscopy. The chemical composition of solid phases was calculated based on the results of solution analysis. Desorption. To estimate the desorption of bound metal ions from apatites, calcium salt solutions were used as surrogates for soil porewaters, and they are supposed to

FIGURE 2. The dependence of cadmium and zinc sorption on specific surface area of apatites at pH 6 in 0.002 M Cd(NO3)2 and Zn(NO3)2 solutions. exchange Ca2+ with bound and exchangeable metal ions in the solid apatite phase (14). In pure water bound cation desorption from sorption products was negligible. Desorption experiments were carried out analogously to sorption experiments, using Ca(NO3)2 aqueous solution with calcium concentrations of 0.002 M, 0.01 M, and 0.05 M at pH 6. The 0.002 M and 0.01 M CaCl2 solutions were used to evaluate the impact of Cl- on cadmium desorption. The influence of pH on Cd2+ desorption was investigated in a series of experiments in the pH range from 4 to 7 in 0.01 M Ca(NO3)2. Desorption experiments were also performed with precipitated Ca-Cd-apatites whose Cd to Ca molar ratio was 0.18 ÷ 0.19 (Table 1) in order to compare the Cd2+ release from apatite depending on the way of its introduction. Analytical Methods. The crystal structure of apatites was investigated by powder X-ray diffractometry (DRON-4, Cu KR radiation at 40 kV, 20 mA, step size 0.04°) and IR spectroscopy (INTERSPECTRUM 2000, range 400-4000 cm-1, KBr pellets with sample to KBr mass ratio 1:300). SSA measurements were performed by the BET-method (adsorptive gas N2, carrier gas He, heating temperature 150 °C) using sorptometers EMS-53 and KELVIN 1040/1042 (Costech VOL. 38, NO. 21, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 2. Sorption of Cd2+ and Zn2+ Ions on Apatites at pH 6 in 0.002 M Cd(NO3)2 and Zn(NO3)2 Solutions sorption of Cd2+ ions sample

mmol/ 100 g Ap

A-1-4 A-1-3 HA-1 HA-2 HA-3

19.4 31.1 30.9 58.4 58.2

D-3-2 D-3-1 D-2-3 D-2-4 A-3-1 A-2-1 D-2-2 D-2-1

15.8 19.1 26.0 32.0 27.5 29.0 42.0 78.0

µmol/ m2

sorption of Zn2+ ions mmol/ 100 g Ap

µmol/ m2

Qs

Ca-Hydroxyapatites 12.0 1.30 13.6 10.1 1.18 23.1 8.6 1.05 19.9 7.3 1.01 50.5 7.0 1.02 50.3

8.4 7.5 5.5 5.9 5.8

0.86 0.86 0.62 0.78 0.81

Ca-Fluorhydroxyapatites 16.5 1.32 10.6 12.1 1.11 15.7 12.7 1.18 28.7 11.3 0.97 28.0 8.9 1.02 30.4 8.8 1.04 27.7 7.6 1.03 35.2 6.9 0.81 78.6

11.0 9.9 14.1 9.9 9.8 8.5 6.4 6.9

0.91 0.85 1.34 1.00 0.95 1.03 0.92 0.89

Qs

International). The concentrations of Ca2+, Zn2+, and Cd2+ in solutions were measured by atomic absorption spectrometry (Carl Zeiss Jena AAS 1N instrument), that of PO43by spectrophotometry (SPEKOL 11, Carl Zeiss Jena) as phosphomolybdate yellow complex and F- using ionselective electrode (Fluoride COMB.ISE/BNC). Water content was determined by thermal analysis using a Setaram LabSys 2000 instrument at a heating rate of 10 °C/min in an air flow. The pH was measured with CyberScan pH/Ion 510 electrode connected to a Bench pH/Ion/mV Meter.

Results and Discussion Sorption. The solubility of apatites at pH above 4 was less than 1% and increased with its further decrease, reaching 4-7% at pH 3 and 80-100% at pH 2. In the solutions containing Cd2+ and Zn2+, the release of Ca2+ was higher than in dilute HNO3 solutions (Figure 1). Due to high apatite solubility, at low pH values the amount of cations sorbed on apatite was diminutive (Figure 1). At pH values above 3, the sorption became substantial; the sorption capacity was approached at pH 4. Thus, the impact of pH on sorption is important and must be considered at pH values lower than 4. The experiments with Cd(NO3)2 and CdCl2 solutions revealed that sorption was insensitive to anionic composition. The presence of Ca2+ in the sorption solution had no effect on the binding of Cd2+, either. Zn2+ sorption was slightly lower than that of Cd2+ (Table 2). Similarly to Cd2+, the sorption of Zn2+ depended mainly on the apatite SSA. Sorption ranged from 16 to 78 mmol of Cd2+ and from 11 to 79 mmol of Zn2+ per 100 g of apatite (Figure 2). As the amount of bound cations per surface unit for different apatites was different (Table 2), the impact of other apatite characteristics, like the kind and amount of substituents, should also be taken into consideration. However, our previous studies have shown that the effect of fluoride and carbonate substitutions on the Cd2+ sorption capacity of apatite was relatively small as compared to the impact of SSA (17). Discerning the predominant sorption mechanism was based on the results of IR spectroscopy and XRD analyses as well as on the analysis of molar ratios Qs of cations bound by apatite to Ca2+ released from apatite. When Qs ) 1, the quantities of the bound and released cations are equal that indicates the ion-exchange of cations between the apatite and the solution. However, if the secondary phase formed as a result of dissolution-precipitation also has an apatitic 5628

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FIGURE 3. The dependence of total desorption of Cd2+ ions on apatite sorption capacity in different Ca2+ solutions. structure, as it has been observed for Pb2+ (19, 20, 23), Qs also equals 1 and ion-exchange and dissolution-precipitation mechanisms cannot be distinguished on the Qs basis. When Qs > 1, a quantity of bound metal ions is more than that of released ones, indicating that nonstoichiometric sorption (surface-complexation or filling the cationic vacancies in apatite crystal lattice) dominates. When Qs < 1, dissolution of apatite phase and precipitation of new phosphate phase having lower cation to phosphate molar ratio occurs. These different processes may also occur simultaneously, and this complicates the determining of the sorption mechanism. For the sorption of both Cd2+ and Zn2+, the release of Ca2+ depended on the number of bound cations, while the solubility of phosphate ions was not affected by sorption (Figure 1). Qs values were close to 1 (Table 2), indicating the dominance of the ion-exchange or the dissolution-precipitation mechanism, provided that the new phase had the same cation to phosphate molar ratio as for the initial material, probably being partially Cd- or Zn-substituted apatite. Fluorine release from apatite during the sorption was negligible. The results of IR spectroscopy and XRD analysis also supported the hypothesis of ion-exchange sorption mechanism, for there was no evidence of formation of new phases in the spectra (see Figures 1 and 3 in Supporting Information). The decrease in apatite unit cell parameters as a result of sorption was in accordance with smaller unit cell parameters of synthetic Ca-Cd-apatites compared with Ca-apatites (Table 3). The molar ratios of Cd to Ca and Zn to Ca in apatites, achieved as a result of sorption, reached the values of 0.082 and 0.077, respectively. However, the Cd to Ca molar ratios in sorption products remain smaller than those in precipitated Ca-Cd-apatites (0.18-0.19). The restrictions for sorption may be caused by diffusional impediments, hindering the penetration of metal ions deeper into apatite particles. Desorption. Desorption of bound cations from apatite depended on both the solution composition and apatite characteristics. The effect of pH on desorption was studied in 0.01 M Ca(NO3)2 solution, varying pH values from 4 to 7 and using one fluorapatite (D-2-1) and two hydroxyapatite (HA-3, A-14) samples (Table 4). In this pH range, desorption constituted 14-47% from sorbed Cd2+, depending on apatite characteristics. Desorption slightly increased with pH decrease. For hydroxyapatites, the release of Cd2+ was more affected by the solution pH than for fluorapatite. Similarly to apatite dissolution and sorption, in the pH range of 4-6 the release of Cd2+ caused an increase in pH,

TABLE 5. Desorption of Bound Cd2+ and Zn2+ Ions from Apatites at pH 6

TABLE 3. Apatite Unit Cell Parameters synthetic Ca-apatites

synthetic Ca-Cd-apatite

cell parameters, Å sample

a

c

sample

Cd/Ca molar ratio in apatite

A-1-3 A-3-1

9.428 9.397

6.887 6.888

CaCdHA1 CaCdFA2

0.18 0.19

cell parameters, Å sample

a

c

9.409 9.373

6.843 6.844

c

Cd/Ca molar ratio in apatite

sample

9.423 6.889 HA-1 + 9.425 6.888 HA-2 + Cd2+ 9.419 6.883 HA-3 + Cd2+ Cd2+

HA-1 HA-2 HA-3

a

synthetic Ca-apatites after Cd2+ sorption

synthetic Ca-apatites

0.01 M Ca(NO3)2 desorption of Cd2+

cell parameters, Å

0.033 0.069 0.067

cell parameters, Å

a

c

9.415 6.885 9.405 6.872 9.418 6.870

TABLE 4. Desorption of at Different Solution pH

Ions in 0.01 M Ca(NO3)2 Solution desorbed Cd2+

initial pH

final pH

mmol/100 g

%

Qdes

4 5 6 7

HA3 + 52.9 mmol Cd/100 g Ap 5.91 15.2 6.05 14.6 6.08 13.4 6.27 13.3

29.7 27.7 25.3 25.1

-2.5 -2.4 0.4 1.1

4 5 6 7

D21 + 76.7 mmol Cd/100 g Ap 6.22 11.9 6.2 11.7 6.23 11 6.32 11

15.6 15.2 14.4 14.3

-3.5 -1.0 1.3 1.6

4 5 6 7

A14 + 14.3 mmol Cd/100 g Ap 5.75 6.7 6.01 6.2 6.18 6 6.35 4.8

46.6 43.1 41.6 33.7

-4.9 -3.3 2.4 3.1

whereas at initial pH 7, pH decreased during desorption. Thus, the final pH of the solution was 5.7-6.4 (Table 4). The changes in the solution pH can be explained by the apatite partial dissolution consuming protons (1) as well as by the equilibrium processes on apatite surface resulting in proton consumption (2) or release (3) by surface groups, depending on the solution pH (10, 23):

Ca10(PO4)6(OH)2 + 14H+ / 10Ca2+ + 6H2PO4- + 2H2O (1) ≡Ca-OH + H+ / ≡Ca-OH2+ or ≡P-O- + H+ / ≡P-OH (2) ≡Ca-OH2+ + OH- / ≡Ca-OH + H2O or ≡P-OH +

OH- / ≡P-O- + H2O (3)

At pH above 5, surface complexes ≡Ca-OH and ≡P-Oare predicted to be essential, while at pH below 4 ≡CaOH2+ and ≡P-OH become significant. In the case of sorption and desorption, an additional impact of bound cations via formation of surface groups such as ≡Ca-OCd+, ≡P-OCd+, ≡Ca-OZn+, and ≡P-OZn+ on these processes can be assumed. The molar ratio Qdes of Ca2+ bound from solution to the cations released from apatite decreased with a decrease in pH, and at low pH values it became even negativesinstead of binding Ca2+ from solution, as it should be in the case of

0.01 M Ca(NO3)2 desorption of Zn2+

total, total, total, mmol/ relative, mmol/ relative, mmol/ relative, % % sample 100 g % Qdes 100 g Qdes 100 g Qdes A-1-3 HA-1 HA-2 D-3-2 D-3-1 D-2-3 D-2-4 A-2-1 D-2-2

8.8 7.9 11.5 4.5 5.9 6.3 6.3 6.6 10.2

28.3 25.6 19.7 28.5 30.9 30.0 21.7 22.8 24.3

4.1 6.7 7.3 10.0 6.3 14.2 14.5 4.6 6.2 6.7 2.1 7.3 2.5 7.5 0 8.1 1.9 12.0

0.002 M Ca(NO3)2 desorption of Cd2+

Cd2+

0.01 M CaCl2 desorption of Cd2+

31.2 32.4 24.3 29.1 35.1 34.8 25.9 27.9 28.6

5.8 8.5 5.5 8.3 4.1 12.9 3.9 4.3 3.1 6.7 0.2 7.8 1.7 7.7 0 7.9 0.1 11.4

36.8 41.7 25.5 40.6 42.7 27.9 27.5 28.2 32.6

3.8 5.1 3.2 1.6 4.7 0.4 1.1 0.7 2.5

0.002 M CaCl2 desorption of Cd2+

desorption of Zn2+

total, total, total, mmol/ relative, mmol/ relative, mmol/ relative, % % sample 100 g % Qdes 100 g Qdes 100 g Qdes A-1-3 HA-1 HA-2 D-3-2 D-3-1 a

4.7 4.2 4.7 2.7 3.5

15.1 13.6 8.0 17.1 18.3

3.2 4.5 4.3 6.1 4.3

5.3 5.1 5.2 3.0 3.8

17.0 16.5 8.9 19.0 19.9

0.8 0.8 0.8 0 0.4

a a a a a

a a a a a

a a a a a

Not determined.

ion-exchange, Ca2+ release from apatite into the solution occurred. Binding/release of Ca2+ did not depend on the level of Cd2+ desorption, but it was essentially affected by the solution pH. No substantial dissolution of apatite at these pH values occurred, and phosphate solubility was below 5 mmol per 100 g of apatite. The impact of Ca2+ on Cd2+ desorption was studied at three Ca2+ concentrations. The contents of bound Cd2+ in apatite samples are presented in Table 2. Cd2+ release increased with increasing Ca2+ concentration (Figure 3). Cd2+ desorption reached 8-20% of sorbed Cd2+ in 0.002 M Ca(NO3)2 solution, 10-35% in 0.01 M Ca(NO3)2 solution, and 33-45% in 0.05 M Ca(NO3)2 solution. Molar ratio Qdes depended on the solution composition and increased with increasing Ca2+ concentration (Table 5), thus indicating that the amount of the bound Ca2+ depends more on its content in the solution than on the amount of Cd2+ released from apatite. In contrast to sorption results, where no differences for chloride and nitrite solutions were observed, desorption of Cd2+ was found to be affected by the anionic composition of the solution. In chloride solutions the release of Cd2+ ions was higher than in nitrate ones (Figure 3). Significantly smaller values of molar ratio Qdes in chloride solutions (Table 5) indicate that depending on the anionic composition of the aqueous medium different processes are involved in the release of bound cations. As assumed by Boisson et al. (14), soluble cadmium chlorocomplexes are formed, and therefore the use of CaCl2 solutions results in greater desorption. A comparison of desorption results for different apatites revealed that desorption of both Cd2+ and Zn2+ depended primarily on the amount of cations bound at sorption. Total desorption (the amount of desorbed cations, mmol per 100 g of apatite) increased with increasing the sorption level (Figures 3 and 4), while the relative sorption (the percentage of the desorbed cations from the previously sorbed cations) diminished accordingly (Figures 4 and 5). The latter result implies that the stability of sorption grows with the apatite sorption capacity. VOL. 38, NO. 21, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. The dependence of total and relative desorption of Zn2+ ions on apatite sorption capacity. FIGURE 6. Desorption of cadmium from apatite samples, obtained on sorption from single Cd-solutions and binary Cd+Ca-solutions.

TABLE 6. Desorption of Cadmium from Precipitated Ca-Cd-Apatites at pH 6 desorption of Cd2+ in Ca(NO3)2 solution

sample CaCdHA1

CaCdFA1

CaCdFA2

FIGURE 5. The dependence of relative desorption of Cd2+ ions on apatite sorption capacity in different Ca2+ solutions. Desorption of Zn2+, both total and relative, was slightly higher than that of Cd2+ (Table 5). This differs from the results obtained by Xu et al. (10) for commercial synthetic hydroxyapatites, which were nonstoichiometric apatites with vacancies in Ca-sites, that could also affect the behavior of sorbed cations. Sorption of Cd2+ prevailing that of Zn2+ (Figure 2) indicates Cd2+ being more suitable for Ca-apatite lattice than Zn2+, which explains easier displacement of Zn2+ from apatite. In contrast to sorption, which is considered to take place mainly via the ion-exchange with Ca2+ of apatite lattice (1, 6, 12, 18), the desorption process seems to be more complicated. The amount of bound Ca2+ and released metal ions differed greatly, and molar ratio Qdes, which should be equal or close to 1 in the case of ion-exchange processes, ranged from nearly zero to 10 and even was negative in the case of low solution pH (Table 4). The values of Qdes depended more on the solution composition than on the release of bound cations. When sorption of Cd2+ was carried out in binary Cd2+ and Ca2+ solutions, the presence of Ca2+ affected neither the amount of bound Cd2+ nor the Qs values (Table 3). On desorption, the amounts of Cd2+ released from these samples were lower than in the case of single solutions used for sorption (Figure 6), and molar ratios Qdes were also different. Qdes of the samples bound Cd2+ from single solution varied in the range of 3.1-10.3, while for the samples bound Cd2+ from binary solutions it was only 0-2.6. Obviously, when 5630

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a

desorption of Cd2+ in CaCl2 solution

concn of total, total, Ca2+ in soln, mmol/ relative, mmol/ relative, % mol/L 100 g % Qdes 100 g Qdes 0.002 0.01 0.05 0.002 0.01 0.05 0.002 0.01 0.05

0.16 0.62 1.69 2.17 2.78 3.01 0.53 1.77 6.02

0.10 0.39 1.05 1.31 1.69 1.83 0.31 1.05 3.58

0.6 6.9 51.5 8.0 13.6 32.2 5.5 6.6 16.5

0.21 0.86 a 2.44 3.00 a 0.65 2.37 a

0.13 0.53 a 1.48 1.82 a 0.38 1.41 a

1.9 11.2 a 1.1 8.4 a 0.3 6.4 a

Not determined.

sorption proceeded in the presence of Ca2+, Ca2+ binding during Cd2+ desorption was impeded, although during sorption there occurred equal Ca2+ release in both binary and single solutions. Further investigations are needed to elucidate such an impact of Ca2+ presence during sorption on apatite desorption behavior. In the case of precipitated Cd-Ca-apatites, Cd2+ release increased also with increasing Ca2+ concentration in the solution (Table 6). Compared to the desorption of previously sorbed Cd2+ from Ca-apatites, Cd2+ release from precipitated Cd-Ca-apatites was much lower, reaching in 0.05 M Ca(NO3)2 solution only 3.6 and 1.1% (6 and 1.7 mmol per 100 g of apatite) of the total cadmium content in fluorapatites and hydroxyapatites, respectively. Similarly to Ca-apatites, the desorption in CaCl2 solution was higher than in Ca(NO3)2 solution (Table 6). The easier displacement of Cd2+ from Ca-fluorapatite indicates the restrictions in introducing substituents into fluorapatite structure, which can be explained by a higher regularity of fluorapatite as compared to hydroxyapatite structure. The release of Cd2+ was accompanied by high sorption of Ca2+ from solution, especially in the case of Ca-Cd-hydroxyapatite (Table 6). The IR spectra did not reveal any structural changes caused by the desorption process. Phosphorus and fluorine release into the solution was negligible, indicating that apatite dissolution is not responsible for the desorption process. As Ca2+ behavior during desorption is not related to the number of the released cations, ion-exchange cannot be the only

mechanism for Cd2+ and Zn2+ desorption. High Qdes values indicate a Ca2+ adsorption on apatite surface. Thus, in contrast to sorption, the desorption is more complicated and includes both an ion-exchange mechanism and a Ca2+ adsorption on an apatite surface.

Acknowledgments The authors gratefully acknowledge the help of Marve Einard with the chemical analyses and Helgi Veskima¨e with the specific surface area measurements. The work was supported by Estonian Science Foundation, Grant No. 4299.

Supporting Information Available The IR spectra before and after cadmium sorption (Figure 1), the kinetics of cadmium and zinc sorption on apatite sample A-3-1 (Figure 2), and the XRD before and after cadmium sorption (Figure 3). This material is available free of charge via the Internet at http://pubs.acs.org.

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Received for review February 2, 2004. Revised manuscript received August 16, 2004. Accepted August 19, 2004. ES049831L

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