title: EXAFS study of Nd(III) uptake by amorphous calcium silicate hydrates (C-S-H).
abstract: Calcium silicate hydrate (C-S-H) phases control the immobilization of many metal cations in cementitious materials. In this study Nd binding to amorphous C-S-H phases with different Ca/Si (C/S) mol ratios (0.56, 0.87 and 1.54) and Nd loadings (7 and 35mumol/g), and which had been aged up to 270 days, has been investigated using extended X-ray absorption fine structure (EXAFS) spectroscopy. The structural parameters derived from EXAFS were compared with those predicted from bond-valence calculations. The study reveals that Nd may form several species in contact with C-S-H phases. The EXAFS parameters determined in samples after one day of reaction indicate the formation of inner-sphere surface complexes. The Nd-Ca and Nd-Si bond-distances tend to increase with time at both Nd loadings. Changes in the coordination numbers N(Si) and N(Ca) were found to be dependent on the (C/S) ratio. At the lowest C/S ratio the number of neighboring Si atoms tends to increase with time while the number of neighboring Ca atoms tends to increase with time at highest C/S ratio. No clear trend was observed for the medium C/S ratio. Nd incorporation into the structures of C-S-H phases is assumed to be the dominant immobilization process based on comparison with bond-distances predicted from structural considerations. After prolonged reaction times (45 days) Nd is expected to be predominantly incorporated into the Ca sheets of the C-S-H structure while small portions of Nd might also be taken up by the interlayer. The study suggests that, in the long term, amorphous C-S-H phases are capable of taking up Nd via exchange processes with Ca(2+) in the Ca sheets and the interlayer.
authors.affiliation: Laboratory for Waste Management, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland. Peter.Mandaliev@psi.ch
authors.forename: P
authors.initials: P
authors.lastname: Mandaliev
authors.forename: R
authors.initials: R
authors.lastname: Dähn
authors.forename: J
authors.initials: J
authors.lastname: Tits
authors.forename: B
authors.initials: B
authors.lastname: Wehrli
authors.forename: E
authors.initials: E
authors.lastname: Wieland