Sorption and Desorption of Cd 2+ and Zn 2+ Ions in Apatite-Aqueous Systems MERIKE PELD,* ,† KAIA TO ˜ NSUAADU, ‡ AND VILLEM BENDER § Departm ents of Chem istry, Chem ical 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 Cd 2+ and Zn 2+ 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 Cd 2+ and Zn 2+ , respectively. The solution composition (pH, and presence of Cl - and NO 3 - ions) had no essential impact on sorption. Desorption of bound cations depended both on the sorption level and solution composition. The amount of desorbed Cd 2+ and Zn 2+ increased proportionally to the amount of sorbed cations. However, apatites having higher sorption capacity release relatively less sorbed cations. Desorption increases with increasing Ca 2+ concentration in the solution, reaching 8-20% of sorbed Cd 2+ in 0.002 M, 10- 35% in 0.01 M, and 33-45% in 0.05 M Ca(NO 3 ) 2 solution. Compared to nitrate solutions, the presence of Cl - ions in the solution promotes the release of bound cations. Desorption of Zn 2+ is slightly higher than that of Cd 2+ . The desorption mechanism was assumed to include both ion- exchange and adsorption of Ca 2+ ions on apatite surface. Introduction Calcium phosphates with apatite structure [Ca 10(PO4)6(OH)2] possess an ability to bind metal ions from solutions and are, therefore, considered to be perspective materials for the immobilization oftoxicmetalsfrom polluted soils,sediments, and waters, allowing rehabitation of soils and revegetation of highly polluted industrial sites. The addition of hydroxy- apatite has been shown to lower the mobility of Cd, Zn, Pb, Cu,Co,Mn,Ni,andU(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 ofapatites as metal immobilizers. The high reversibility of sorption permits regeneration and 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 oflow 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). Dis- solution 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, ion- exchange 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 di- minishes its sorption capacity, while an increase in mag- nesium substitution enhances metal ion sorption (16, 17). The main factoraffectingapatite sorption propertieshasbeen 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 subse- quent desorption of metal ions may depend also on the solution composition, contact time, temperature, solid - liquid ratio, etc. (18-20). The substituted apatites are less studied than pure calcium-hydroxyapatites. However, as substitution is com- mon in naturalapatites, the impact ofdifferent substituents on apatite metal binding properties is of great interest. Interpretation ofthe results ofstudies using naturalapatites iscomplicated,astheycontain different anionicand cationic substituents as well as other accompanying minerals, all affectingmetalsorption and desorption behavior.Therefore in thisresearch carbonate-and fluoride-containingsynthetic apatites with different specific surface area were used. The aim ofthis study was (1) to examine the factors affecting the sorption of Zn 2+ and Cd 2+ from aqueous solutions on synthetic apatites with different chemicalcompositions 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 Cd 2+ and Zn 2+ 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 (CO3 2- in PO4 3- 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 PO4 3- and CO3 2- equals six, and there can be vacancies at Ca 2+ ion and monovalent anion positions.The quantityofOH - groups was calculated basing on electroneutrality principle. To explain the high cation-to-anion ratio in synthesized materi- *Correspondingauthorphone: +372 6202859; fax: +372 6202801; e-mail: mpeld@staff.ttu.ee. † Department of Chemistry. ‡ Department of Chemical Engineering. § Department of Physics. Environ. Sci. Technol. 2004, 38, 5626-5631 5626 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 21, 2004 10.1021/es049831l CCC: $27.50  2004 American Chemical Society Published on Web 09/28/2004