Journal of Colloid and Interface Science 236, 252–259 (2001) doi:10.1006/jcis.2000.7411, available online at http://www.idealibrary.com on Sorption Mechanisms of Eu 3+ on CSH Phases of Hydrated Cements Ingmar Pointeau, ∗, § Bernard Piriou,† Michel Fedoroff, 1, ‡ Marie-Genevieve Barthes,‡ Nicolas Marmier, ∗ and Francine Fromage ∗ ∗ Groupement de Recherche en Chimie Inorganique, Universit´ e de Reims, B.P. 1039, 51687 REIMS Cedex, France; †Laboratoire de Structure, Propri´ et´ es et Mod´ elisation du Solide, Ecole Centrale Paris, Grande Voie desVignes, 92295 Chatenay-Malabry, France; ‡Centre d’Etude de Chimie M´ etallurgique, Centre National de la Recherche Scientifique, 15 rue Georges Urbain, 94407 Vitry-sur-Seine, France; and §Agence Nationale pour la Gestion des D´ echets Radioactifs, 1,7 rue Jean-Monnet, 92298 Chˆ atenay-Malabry Cedex, France Received May 30, 2000; accepted December 29, 2000 The sorption mechanisms of Eu 3+ on calcium silicate hydrate (CSH) phases of hydrated cement were investigated as a tool for the prediction of the behavior of trivalent radionuclides with aged/degraded cements in radioactive waste repositories. Fourtech- niques were used: site-selective and time-resolved luminescence spectroscopy, XPS, high-resolution SEM coupled with EDX, and XRD. Results showed that europium is not precipitated in the solu- tion despite its low solubility limit. It is strongly retained on CSH, resulting in a more than 99.8%sorption rate. Two main sorption sites were characterized by luminescence spectroscopy. One site, with a long lifetime, can be interpreted as Eu included in the framework of CSH. Another one, with a shorter lifetime, can be interpreted as a site with a hydrated environment that is high but is less than that of europium hydroxide. It corresponds to superficial complexation or precipitation. C  2001 Academic Press Key Words: sorption; precipitation; europium(III); radioactive wastes;CSH;cement;site-selective and time-resolved luminescence spectroscopy; XPS. INTRODUCTION Concrete is one of the materials which are intended to be used as engineered barriers in high-level radioactive waste reposito- ries. It is important to know the role of cements in the transport of radionuclides in pore water. The present study is devoted to the behavior of Eu(III), taken as a model for trivalent rare earths and actinides. The peculiar behavior of such elements in the pres- ence of concrete is related to their low solubility, due to the high pH values of cement waters. Prior to modeling the migration of these elements in concrete barriers, it is necessary to identify the main retention mechanisms, specifying the role of precipitation and sorption processes. Calcium–silicate–hydrate (CSH) phases, x CaO · SiO 2 · y H 2 O with 0.7 < (x = Ca/Si) < 1.7, are the major (50–70% in mass) hydrate phases in cement paste. The Ca/Si ratio is an indicator of the state of degradation of cements, since the ratio decreases 1 To whom correspondence should be addressed. E-mail: fedoroff@glvt- cnrs.fr. as lixiviation is going on. CSH phases have a nanocrystalline structure and possess hydraulic binding properties. Their high specific surface area inclines them to have sorption properties. CSH phases are thus chosen as a model in the study of the retention of radionuclides in cement materials. Although there are many data on the sorption properties of cements, few publications are devoted to the sorption of lan- thanides and actinides on CSH phases. Ewart et al. (1) report distribution coefficient values of 10 4 L kg −1 for Am and Pu. For the study of sorption processes, these authors used lanthanum at concentrations higher than the solubility limit. Using essentially transmission electron microscopy with CSH precipitated in the presence of lanthanum, these authors suggest that lanthanum is retained both by precipitation of hydroxide and by substitu- tion to calcium. In their study of the retention of Am, Baston et al. (2) quantified the data according to a surface complexation model, in which the main surface sites of concrete are >SiO − and >SiOCaOH. Americium hydroxide species such as Am(OH) 3 and Am(OH) + 2 , which dominate the speciation of americium in concrete water, are assumed to be the sorbed species. Thus the sorption mechanisms of these elements remain uncertain. Incorporation of Cr(III), which is trivalent but belongs to an- other group of elements, was studied during hydration of alite (3CaO · SiO 2 ) leading to the formation of CSH (3, 4). It seems to occur through the formation of complexes including both Ca and Cr(III). Replacement of Si by Cr(III) seems possible in (3) but was not suggested in (4). Precipitation as Cr(OH) 3 was not observed in (4). To better define the retention mechanisms of heavy elements like trivalent lanthanides and, to a certain extent, trivalent ac- tinides like americium (III), and to try to discriminate between precipitation and sorption phenomena, we have chosen Eu 3+ , due to its selective properties in luminescence techniques. This tool was previously applied to the study of the sorption of Eu 3+ on calcite (5) and was used to characterize three main sorption sites. For CSH phases, to simulate long-term behavior, we have chosen a value of the Ca/Si ratio corresponding to aged cements and, at the same time, equal to the ratio of 0.83 in tobermorite, which can be taken as a reference, since it is a crystallized phase (6, 7). For comparison and to test extreme terms of the CSH 252 0021-9797/01 $35.00 Copyright C  2001 by Academic Press All rights of reproduction in any form reserved.