Comparative Sorption Properties of Metal(III) Phosphates S. Mustafa, 1 A. Naeem, S. Murtaza, N. Rehana, and H. Y. Samad National Center of Excellence in Physical Chemistry, University of Peshawar, Peshawar, 25120 Pakistan Received September 23, 1998; accepted July 15, 1999 Sorption behavior of metal(III) phosphates toward Zn 2 was investigated using different concentrations, pHs, and tempera- tures. Ion exchange between protons from the surface and metal cations from solutions was found to be responsible for metal sorption by metal(III) phosphates. Dissociation constants of metal(III) phosphates and binding constants of the Zn 2 with metal(III) phosphates were determined using Henderson–Hassel- bach and modified Langmuirequations, respectively. The sorption process in all the three metal phosphates was found to be endo- thermic in nature while the dissociation process was observed to be endothermic in AlPO 4 and FePO 4 and exothermic in CrPO 4 . © 1999 Academic Press Key Words: ion exchange; sorption; potentiometric titration; AlPO 4 ; CrPO 4 ; FePO 4 . INTRODUCTION Phosphates, Fe, and Al are important constituents of soils (1– 4). Phosphate, mainly derived from the mineral apatite and fertilizers, is associated with Ca, Fe, Al, clay mineral, and organic matter in soils. This association ranges from simple adsorption/ion exchange to precipitation reaction (5, 6). The interaction of phosphates with metal ions like Ca, Fe, and Al may lead to the formation of relatively insoluble phosphates, which may then act as further scavengers for the metal cations like Pb 2+ , Zn 2+ , and Cd 2+ , thus, significantly reducing their concentrations in aqueous solution systems (6). While sorption properties of the calcium phosphates like hydroxyapatite [Ca 5 (PO 4 ) 3 OH] are known quite well (6, 7), the mechanism of the process is still a point of conjecture. Moreover, extensive literature (2, 8 –10) is available about the exchange of metal ions on inorganic/organic ion exchange materials including phosphates of zirconium and titanium. In regard to the sorption properties of metal(III) phosphates like aluminum, chromium, and iron, very little is reported in the literature (11, 12). The metal(III) phosphates with their ion exchange properties and mechanism of exchange were observed to resemble the zirco- nium phosphates. The metal(III) phosphates are insoluble solid materials which exist in various amorphous, crystalline, and intermediate states of crystallinity (13, 14), possess pH-depen- dent surface charge properties, and as such, may be considered as important sink for trace metal ions in the environment, similar to the corresponding hydrous oxides of iron and alu- minum (2, 15). Our earlier papers (11, 12, 16, 17) indicate that AlPO 4 , CrPO 4 , and FePO 4 exhibit remarkable affinity to transition metal ions. The present study, being an extension of this previous work, describes the comparative sorption studies of Zn 2+ on AlPO 4 , CrPO 4 , and FePO 4 . In addition, the main aim of this work is to elucidate on the binding mechanism of divalent metal cations like Zn 2+ with metal(III) phosphates. EXPERIMENTAL All the reagents used were of analytical grade. Preparation and Characterization of Metal(III) Phosphates Metal(III) phosphates were precipitated by mixing a 1 M solution of trisodium phosphate with a 1 M solution of metal(III) nitrate, according to the reaction M NO 3  3 + Na 3 PO 4 3 MPO 4 + 3NaNO 3 , [1] where M represents iron, aluminum, and chromium. The suspensions were equilibrated for 1 h at 40°C with continuous stirring. Thick yellow precipitates formed when the final pH at the completion of the precipitation was equal to 4. The resulting precipitates after settling down were dialyzed for 10 days with daily renewal of the doubly distilled water. The suspensions were filtered and the precipitates were washed until they were free of nitrate and chloride ions. Then, they were dried at 110°C for 48 h, ground to a fine powder, passed through a 60-mesh sieve, and stored properly in the sealed reagent bottles. Characterization of Metal(III) Phosphates The samples thus prepared were examined by infrared (IR) spectroscopy, X-ray diffraction (XRD), surface area, thermal, and electron microprobe analysis. X-ray patterns of the powder samples were determined using a JEOL X-ray diffractometer, model JDX-7E with Mn-filtered CuK radiation. IR spectra of metal(III) phosphates mixed with KBr crystals were recorded with a Pye Unicam model Sp 1 To whom correspondence should be addressed. Journal of Colloid and Interface Science 220, 63–74 (1999) Article ID jcis.1999.6429, available online at http://www.idealibrary.com on 63 0021-9797/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.