Crystal Chemistry of Colloids Obtained by Hydrolysis of Fe(III) in the Presence of SiO 4 Ligands

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Crystal Chemistry of Colloids Obtained by Hydrolysis of Fe(III) in the Presence of SiO4 Ligands. Emmanuel Doelsch1, Armand Masion1, Jérôme Rose1, William E.E. Stone1, Jean Yves Bottero1 and Paul M. Bertsch2 1CEREGE, Europole Méditerranéen de l'Arbois, BP 80, 13545 Aix-en-Provence Cedex 04, France. 2AACES-SREL, University of Georgia, PO Drawer E, Aiken, SC 29802, USA. ABSTRACT The crystal chemistry of a number of Fe-Si systems (Si/Fe 0-4, pH 3-10) was investigated by combining local scale spectroscopic methods (EXAFS, FTIR and NMR) and at the semi local scale (SAXS). The Fe clusters within the precipitates have two growth regimes depending on the Si/Fe ratio: the growth is three and two dimensional for Si/Fe ” and Si/Fe •UHVSHFWLYHO\7KHSUHVHQFHRI Fe-O-Si bonds within the precipitated phases has been demonstrated. Their formation and relative proportion was found to be very dependent on the pH and Si concentration The size of silica domains within the precipitates was shown to increase with increasing Si/Fe and/or decreasing pH. The high fractal dimension (Df) of the aggregates is attributed to the presence of the SiO4 ligands, but the evolution of Df linearly depends on the polymerization state of iron. INTRODUCTION Iron and silicon are two of the most abundant elements of the earth's crust. Fe-Si associations are of great relevance for materials science as well as environmental issues. Numerous studies concerning Fe-Si systems have focussed on the effects of Si on the crystallization of Fe phases [1-4]. It has been shown that, even at low Si levels, the crystallization of Fe was delayed or even inhibited. Most of the available literature on Fe-Si phases investigates systems that underwent some degree of "ripening" (i.e. heating and/or aging). Very few authors investigated the nucleation mechanisms of Fe-Si colloids on freshly prepared samples. Thus surprisingly little is known about the crystal chemistry of Fe-Si systems. The present study aims at investigating the local (i.e. up to 5 Å) and semi-local (up to 400 Å) structure of freshly prepared Fe-Si colloids or precipitates in order to elucidate the mechanisms that might influence the subsequent evolution of these systems upon heating and/or aging. MATERIALS AND METHODS Appropriate amounts of tetra ethylorthosilicate (TEOS) were added to a FeCl3 stock solution so as to obtain a final Fe concentration of 0.2 M and Si/Fe molar ratios of 0, 0.25, 0.5, 1, 2 and 4. These systems were base hydrolyzed (NaOH) to reach pH values of 3, 5, 7 and 10. When required for further analysis, the suspensions were centrifuged at 40,000 rpm (2 hours) and freeze-dried. X-ray diffraction (XRD) analyses were performed using a Philips PW 3710 diffractometer with Co Kα radiation at 40 kV and 40 mA. Fe K-edge extended X-ray absorption fine structure (EXAFS) measurements required the use of the synchrotron beamlines D42 (LURE, Orsay, France) and X23A2 (NSLS, Upton (NY), USA). Data reduction and modeling of the EXAFS data were accomplished as described previously [5, 6]. GG3.36.1