Nucleation and Growth Mechanisms of Iron Oxyhydroxides in the Presence of PO 4 Ions. 3. Speciation of Fe by Small Angle X-ray Scattering Armand Masion,* ,† Je ´ro ˆme Rose, † Jean-Yves Bottero, † Denise Tchoubar, ‡ and Pierre Elmerich § Laboratoire des Ge ´ osciences de l’Environnement URA 132 CNRS, CEREGE, Europole Me ´ diterrane ´ en de l’Arbois, BP 80, 13545 Aix en Provence Cedex 04, France, Centre de Recherche sur la Matie ` re Divise ´ e, Laboratoire de Cristallographie UMR 812, Universite ´ d’Orle ´ ans, BP 6706, 45067 Orle ´ ans Cedex 2, France, and Elf-Atochem, Centre d’Application de Levallois, 95 rue Danton, 92300 Levallois-Perret, France Received November 18, 1996. In Final Form: April 14, 1997 X The speciation of Fe(III) within the aggregates formed in partially hydrolyzed Fe-PO4 solutions was determined by small angle X-ray scattering. The simulation of the scattering curves by models consisting of hard spheres representing the various Fe species shows that the presence of PO 4 hinders the polymerization of Fe(III). At a hydrolysis ratio R ([OH]/[Fe]) of 1.0, 60% of Fe are Fe monomers and 40% are edge sharing dimers. At higher R, the aggregates consist essentially of edge-sharing Fe dimers (>84%). The Fe speciation derived from the modeling of the scattering curves confirmed and refined the results obtained on the same samples by extended X-ray absorption at the Fe k-edge spectroscopy in a previous study. Introduction The speciation of metal ions, and especially Al(III) and Fe(III), in nonhomogeneous solid phases (aggregates) is of great interest in natural systems as well as in industrial processes. The speciation of Al during its hydrolysis in the absence of ligands different from OH has been extensively studied over the past 2 decades mainly by nuclear magnetic resonance (NMR) and infrared (IR) spectroscopies. 1-5 The existence of the tridecameric Al 13 polycation in solution and its proportion under diverse synthesis conditions have been clearly determined. 1,3,6-8 The presence of organic ligands during Al hydrolysis resulted in strong hindrance of the formation of soluble Al 13 due to the complexation of the precursors of the tridecamer. 9,10 The modeling of small angle X-ray scat- tering (SAXS) curves yielded a quantitative speciation of Al contained in the aggregates formed at higher pH. 11-13 The solid phase consisted essentially of uncondensed Al monomers, 12 the initially formed Al 13 being depolymerized by the organic ligands. 13 The speciation of Fe(III) in the early stages of the hydrolysis is less understood than that for Al. The use of extended X-ray absorption fine structure (EXAFS) spectroscopy was necessary to provide precise data on the nucleation and polymerization steps: 14,15 for FeCl 3 solu- tions and with increasing pH, Fe octahedra form edge- sharing dimers, trimers by adding a double corner sharing monomer onto the dimer, and finally the Fe 24 polycation which has the local structure of -FeOOH. An unstable Fe 13 polymer having a structure analogous to Al 13 was described on the basis of IR results, 16 but no evidence of this species was found in the EXAFS studies. SAXS studies of partially hydrolyzed ferric salts showed that, whatever the salt used, the size of the aggregates was approximately 10 nm and the structure evolved from linear to branched with increasing hydrolysis ratio R ) [OH]/ [Fe]. 17,18 Differences were found in the subunit size: with ferric nitrate it ranged from 7 to 13.5 Å depending on R, 17 whereas it remained constant at 16 Å with the chloride salt. 18 Recently, the hydrolysis of FeCl 3 in the presence of PO 4 ligands has been investigated by Fe k-edge and P k-edge EXAFS spectroscopy. 19,20 The PO 4 ions displayed a strong affinity toward Fe. They were bound to Fe octahedra from the lowest pH values, 20 thus occupying growth sites and limiting the polymerization of Fe to edge sharing dimers or possibly trimers. 19,20 Double corner * To whom correspondence should be addressed. Phone: (33) 442 97 15 34. Fax: (33) 442 97 15 40. E-mail: masion@cerege.fr. † Laboratoire des Ge ´osciences de l’Environnement URA 132 CNRS. ‡ Centre de Recherche sur la Matie `re Divise ´e, Laboratoire de Cristallographie UMR 812, Universite d’Orle ´ans. § Elf-Atochem, Centre d’Aplication de Levallois. X Abstract published in Advance ACS Abstracts, June 1, 1997. (1) Bottero, J. Y.; Cases, J. M.; Fiessinger, F.; Poirier, J. E. J. Phys. Chem. 1980, 84, 2933-2939. (2) Akitt, J. W.; Farthing, A. J. Chem. Soc., Dalton Trans. 1981, 1606-1608. (3) Bertsch, P. M.; Thomas, G. W.; Barnhisel R. I. Soil Sci. Soc. Am. J. 1986, 50, 825-830. (4) Bottero, J. Y.; Axelos, M.; Tchoubar, D.; Cases, J. M.; Fripiat, J. J.; Fiessinger, F. J. Colloid Interface Sci. 1987, 117, 47-57. (5) Bradley, S. M.; Kydd, R. A.; Howe R. F. J. Colloid Interface Sci. 1993, 159, 405-412. (6) Furrer, G.; Trusch, B.; Mu ¨ ller C. Geochim. Cosmochim. Acta 1992, 56, 3831-3838. (7) Kloprogge, J. T.; Seykens, D.; Jansen, J. B. H., Geus, J. W. J. Non-Cryst. Solids 1992, 142, 94-102. (8) Parker, D. R.; Bertsch, P. M. Environ. Sci. Technol. 1992, 26, 914-921. (9) Thomas, F.; Masion, A.; Bottero, J. Y.; Rouiller, J.; Genevrier, F.; Boudot, D. Environ. Sci. Technol. 1991, 25, 1553-1559. (10) Thomas, F.; Masion, A.; Bottero, J. Y.; Rouiller, J.; Montigny, F.; Genevrier, F. Environ. Sci. Technol. 1993, 27, 2511-2516. (11) Masion, A.; Tchoubar, D.; Bottero, J. Y.; Thomas, F.; Villie ´ras, F. Langmuir 1994, 10, 4344-4348. (12) Masion, A.; Bottero, J. Y.; Thomas, F.; Tchoubar, D. Langmuir 1994, 10, 4349-4352. (13) Masion, A.; Thomas, F.; Tchoubar, D.; Bottero, J. Y.; Tekely, P. Langmuir 1994, 10, 4353-4356. (14) Combes, J. M.; Manceau, A.; Calas, G.; Bottero, J. Y. Geochim. Cosmochim. Acta 1989, 53, 583-594. (15) Bottero, J. Y.; Manceau, A.; Villie ´ ras, F.; Tchoubar, D. Langmuir 1994, 10, 316-319. (16) Bradley, S. M.; Kydd, R. A. J. Chem. Soc., Dalton Trans. 1993, 2407-2413. (17) Bottero, J. Y.; Tchoubar, D.; Arnaud, M.; Quienne, P. Langmuir 1991, 7, 1365-1369. (18) Tchoubar, D.; Bottero, J. Y.; Quienne, P.; Arnaud, M. Langmuir 1991, 7, 398-402. (19) Rose, J.; Manceau, A.; Bottero, J. Y.; Masion, A.; Garcia, F. Langmuir 1996, 12, 6701-6707. (20) Rose, J.; Flanck, A. M.; Masion, A.; Bottero, J. Y.; Elmerich, P. Langmuir 1997, 13, 1827-1834. 3882 Langmuir 1997, 13, 3882-3885 S0743-7463(96)02015-X CCC: $14.00 © 1997 American Chemical Society