GEOCHEMISTRY OF THE EARTH'S SURFACE AND OF MINERAL FORMATION 275 2rid INTERNATIONAL SYMPOSIUM, July, 2-8, 1990, Aix en Provence, France. IN-SITU X-RAY ABSORPTION STUDY OF THE MECHANISM OF Cr(III) OXIDATION AT THE MN OXIDE/WATER INTERFACE MANCEAU A.* and CHARLET L.** * Laboratoire de MinEralogie-Cristallographie, Tour 16, 4 place Jussieu, 75252 Paris, France. ** Institut ftir Anorganische, Analytische und Physikalische Chemic, Freiestrasse 3, 3000 Bern, Switzerland 1. INTRODUCTION Sorption at oxide/water interfaces plays an important role in the mobility of minor/trace elements in surface waters. For example, recent field studies have shown that Fe and Mn colloids are important scavengers of U 1 and Pb 2, respectively. Despite the relatively low concentration of Mn hydrous oxides as compared to that of Fe, the mixed-valencies of Mn ions render them highly reactive, and redox reactions dramatically change the solubility of sorbed species. For instance, the oxidation of Co(H) to Co(HI) on MnO2 causes its precipitation 3 and explains why Mn and Co are always closely associated in the environment 4-5. One may likely wonder whether hydrous Mn oxides would not act as oxidizing agents of Cr(III) in natural waters? Unlike other transition metals, chromium is solubilized by oxidation to Cr(VI) and precipitated by reduction to Cr(III). Facing the high toxicity 6-7 and solubility of Cr(VI) complexes, the recognition and description at a molecular level of the oxidation of Cr(III) at the Mn oxide/water interface would be of a prime importance for environmental studies. Synchrotron-based X-ray absorption spectroscopy (XAS) is one of the few methods capable of providing, in situ, chemical and structural information on diluted species sorbed on wet surfaces 8. More specifically, the analysis of the absorption edge discontinuity (XANES) at the energy of the sorbing species provides information on its electronic structure 9. Strong spectral changes are observed in going from trivalent to hexavalent chromium 10, which renders XANES spectroscopy particularly valuable to detect the eventual appearence of low amounts of chromate ions. Beyond the XANES regime, comes the EXAFS regime. The EXAFS spectroscopy yields structural information in terms of distance, nature and number of atoms in the two- or three-nearest atomic shells surrounding the sorbed species 11. The application of this spectroscopy to natural samples has already provided first insights into the trapping mechanisms of several trace elements by oxides and silicates12. Concerning experimental investigations of metal complexes sorbed at oxide/water interface, it is then possible to recognize whenever an adsorption is specific or non-specific, and in the former case, to differentiate several classes of sorption behavior, such as true adsorption, a surface precipitation, and a diffusion into the solid (or ad sorption) 8. 2. EXPERIMENTAL AND DATA ANALYSIS A 7mM CrC13 solution was coulometrically titrated at pH 4.0 in order to avoid a contact with hydroxide ions at high concentration. This solution was aged two days before to be used, so that it is expected to have part of its chromium (20%) polymerized 13. The samples were prepared by mixing this solution in equal amounts with a suspension of Mn mineral preadjusted at pH 4.0. The minerals under investigation were : birnessite (Na4MnlnO27.9H20), ramsdellite (MnO2), feitk- nechtite (I3MnOOH) and TMn203. After a two minutes reaction, the pH was recorded and the suspension filtered. The filter pad was analyzed by XAS in the next 20 minutes. Two additional samples were prepared : a Cr-rich feitknechtite (1.2 wt. %) and a birnessite filtered after only 30 sec. reaction time C#BIRI"). A reference chromate sample was also prepared, by mechanical mixing of