SIMULTANEOUS SORPTION OF Cd AND Zn IONS ON SYNTHETIC. MERIKE PELD, KAIA TONSUAADU and MIHKEL VEIDERMA

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1 SIMULTANEOUS SORPTION OF Cd AND Zn IONS ON SYNTHETIC APATITES FROM AQUEOUS SOLUTIONS MERIKE PELD, KAIA TONSUAADU and MIHKEL VEIDERMA Tallinn Technical University Institute of Basic and Applied Chemistry Ehitajate tee 5, Tallinn, Estonia Abstract The heavy metal sorption capacity of synthetic apatites with different levels of fluorine and carbonate substitutions (0-2.7% F, % CO2) has been studied. Compared with hydroxyapatite with a very low carbonate content ( `0.1% CO2), the sorption capacity of cadmium per 100 mg apatite diminishes from 55.1 to 14.1 ƒêmol and the sorption capacity of zinc from 41.5 to 13.2 ƒêmol, with an increase in the flourine and carbonate content. In single cation solutions the sorption capacities of Zn2+ and Cd2+ ions are nearly equal while in the binary solutions the sorption of Cd2+ becomes overwhelming and the sorption of Zn2+ is suppressed. The total amount of cations sorbed from binary solutions equals roughly the amount of zinc or cadmium sorbed separately from single solutions. INTRODUCTION The removal ability of hydroxyapatite (HAp) for heavy metal ions from solutions has been thoroughly studied during the past two decades and the use of apatites (Ap) as removal agents for toxic heavy metals from waste waters has been suggested".the results of previous studies have shown that the sorption of heavy metal ions depends essentially on the structural characteristics of Ap 1,2,7. In addition, the removal of one specific metal ion seems to be affected by the presence of other heavy metal ions3,4,8. It was found that HAp has a considerable removal selectivity for heavy metal ions from solutions 5,8, what might be used for the separation of these ions. In this study the selectivity of the removal of Cd2+ and Zn2+ ions from aqueous solutions by synthetic Ap was investigated. The role of fluorine and carbonate substitutions in Ap structure on its sorption capacity was also examined. Phosphorus Research Bulletin Vol. 10 (1999), 347

2 EXPERIMENTAL PROCEDURE The synthetic Ap were obtained by precipitation 7'9. The synthesized materials were B-type carbonate apatites (CAp) with specific surface area g/m2. The chemical composition of apatites is given in Table 1. TABLE 1. Chemical composition and specific surface area of apatite samples The solutions containing either only one cation or both Cd2+ and Zn2+ ions were prepared from nitrate salts. The ph of the solutions was set at 6, with the addition of nitric acid. The concentration of metal ions was in the range mmol/l. The sorption experiments were performed using batch method. 100 mg of Ap was shaken with 50 ml of solution at room temperature (20 Ž) during 24 hours. The suspensions were centrifuged and the concentrations of Cd, Zn, Ca, P, and F in the solutions were measured. Quantitative analyses of Ca, Cd and Zn ions were performed by use of atomic absorption spectrometry (Carl Zeiss Jena AAS 1N), phosphate was determined spectrophotometrycally as phosphomolybdate complex and fluorine potentiometrically with Radelkis ionselective electrode. The structural changes in the apatite samples before and after the uptake of heavy metal ions were investigated by powder X-ray diffractometry (DRON-4) and IR-spectroscopy (Carl Zeiss Jena IR75). The specific surface area of samples was measured by BET method using Sorptometer EMS-53. Phosphorus Research Bulletin Vol. 10 (1999), 348

3 RESULTS AND DISCUSSION The sorption capacity for Cd 2+ and Zn 2+ is the highest in the case of HAp with a very low CO3 2- substitution level (0.1% CO2), reaching 55.1 p.mol of Cd and 41.5 p mol of Zn per 100 mg apatite. The increase in carbonate content causes diminishing of the specific area and a-decrease in the sorption capacity (Figure 1). Sorption of Cd and Zn in single solutions Sorption of Cd and Zn in binary solution FIGURE 1. Dependence of sorption capacity on carbonate content in Ap. In FHCAp the small fluorine substitution causes a growth in the uptake of Zn2+ ions up to 29 ƒêmol/100mg. For the Ap with higher F content than 1.5% the sorption of Phosphorus Research Bulletin Vol. 10 (1999), 349

4 Zn2+ ions begins to diminish again (curve 4 in Figure 2). The changes in sorption of Cd2+ ions with an increase in the fluorine content are negligible. Compared with Cd2+, the sorption of Zn2+ is less affected by the carbonate content and the specific surface area of the Ap - the wide variance in the values of sorption capacities of samples with comparable surface areas is shown in Figure 2. FIGURE 2. Dependence of sorption capacity on specific surface area of Ap in single cation solutions: 1- HAp; %F; %F; %F. The results of sorption experiments are given in Table 2. In order to describe the selectivity of sorption, the removal ratio R is used: Rs - molar ratio of Cd2+ and Zn2+ ions, sorbed separately from single cation solutions; RB - molar ratio of Cd2+ and Zn2+ sorbed simultaneously from binary solution. The cation exchange ratio Q shows the ratio of heavy metal ions sorbed on Ap to Ca2+ ions released during the sorption process. According to these results the sorption of Cd2+ and Zn2+ proceeds with different mechanisms. The sorption capacity for Cd is directly related to the specific surface area and the role of Ap surface in sorption process is obvious. For FHAp the value of Qcd is generally smaller than 1, therefore the sorption proceeds mainly by ion-exchange in the surface layers of Ap crystals. For HAp Qcd>1 indicates the involvement of adsorption processes. In the case of Zn2+, considering the smaller size of Zn2+ ions, diffusion farther into Ap lattice can be assumed 2, and the greater value of Qzn is an indication of absorption processes beside ion-exchange. Phosphorus Research Bulletin Vol. 10 (1999), 350

5 TABLE 2. Results of sorption experiments In single solutions the sorption capacities of Ap for Cd2+ and Zn2+ are nearly equal (Figure 1) and the removal ratio Rs is close to one. In binary solutions the sorption of Cd2+ is considerably higher than that of Zn2+ and RB increases up to value The sorption of Zn2+ is more suppressed in Ap with a low carbonate content and a high specific area. The reason for that may be the partial occupancy of cationic sites in surface layers by Cd2+ ions that impedes the diffusion of Zn2+ ions farther into Ap crystal. However, the total uptake of metal ions remains the same as in single solutions, so the sorption capacity of Ap seems to have a constant value despite of the kind of metal ions sorbed. The value of ion exchange ratio QCd+Zn is close to 1. The molar ratio of heavy metal ions and calcium in Ap after sorption reaches 0.62:10 for Cd and 0.24:10 for Zn. The amounts of P dissolved from Ap were negligible and the solubility of F was not observed at all. The XRD spectra of Ap after sorption experiments were identical with those of initial Ap and no other solid phases beside Ap were detected. The ER. spectroscopy did not reveal any structural changes either. On the bases of the results above we assume that sorption of Cd2+ takes place mainly by ion-exchange in the surface layers of Ap, while for Zn2+, beside ion-exchange, absorption into Ap structure also occurs. SUMMARY The uptake of Cd 2+ and Zn 2+ ions depends on the substitutions in Ap structure as well as on the composition of the solution. Compared with HAp, the sorption capacities Phosphorus Research Bulletin Vol. 10 (1999), 351

6 of Ap with fluorine and carbonate substitutions are considerably smaller. The sorption of Cd 2+ ions is more related to the specific surface area of Ap than the sorption of Zn2+, indicating the existence of different mechanisms of sorption. In binary systems the sorption of Cd2+ exceeds considerably the sorption of Zn2+, while the total uptake of metal ions remains the same as in single solutions. ACKNOWLEDGEMENT The authors would like to thank Mrs.M.Einard for performing the chemical analyses, Mrs.H.Veskimae for BET measurements and Mr.V.Bender for XRD analyses. The study has been supported by Grant No.2119 of the Estonian Scientific Foundation. REFERENCES 1. J.Jeanjean, S.McGrellis, J.C.Rouchaud, M.Feodoroff, A.Rondeau, S.Perocheau, A.Dudis, J.Solid State Chem., 126, 195 (1996). 2. Y.Xu, F.W.Schwartz, S.J.Traina, Environ.Sci.Technol., 28, 1472 (1994). 3. Y.Takeuchi, H.Arai, J.Chem.Eng.Jap., 23, 75 (1990). 4. Q.Y.Ma, S.J.Traina, T.J.Logan, J.A.Ryan, Environ.Sci.Technol., 28, 1219 (1994). 5. T. Suzuki, T.Hatsushika, Y.Hayakawa, J.Chem.Soc. Faraday Trans.1, 77, 1059 (1981). 6. J.Jeanjean, U.Vincent, M.Fedoroff, J.Solid State Chem., 108, 68 (1994). 7. K.Tonsuaadu, M.Peld, M.Veiderma, Toxicological and Environ.Chem., 64, 145 (1997). 8. H. Sawa, A.Takenaka, M.Hasegawa, K.Aoki, Phosphorus Res.Bull., 8, 55 (1998). 9. K.Tonsuaadu, M.Peld, T.Leskela, R.Mannonen, L.Niinisto, M.Veiderma, Thermochim.Acta, 256, 55 (1995). Phosphorus Research Bulletin Vol. 10 (1999), 352

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