SREL Reprint #2555

X-ray absorption spectroscopy study of immobilization processes for heavy metals in calcium silicate hydrates: 1. Case of lead

Jérôme Rose1, Isabelle Moulin1, Jean-Louis Hazemann2, Armand Masion1, Paul M. Bertsch3,
Jean-Yves Bottero1, Franqis Mosnier4, and Claude Haehnel5

1CEREGE Equipe physico-chimie des interfaces, UMR 6635, CNRS/Université Aix-Marseille III,
Europole Méditerranéen de l'Arbois, BP80, 13545 Aix en Provence Cedex 4, France
2Laboratoire de Cristallographie CNRS, 38042 Grenoble Cedex 9, France
3SREL, University of Georgia, PO Drawer E, Aiken, South Carolina 29802
4CIRSEE, 38 Avenue du Président Wilson, 78230 Le Pecq, France
5ATILH, 7 place de la Défense, 92974 Paris la Défense Cedex, France

Abstract: Trace amounts of heavy metals can be detected in cement. The major source is due to heavy metals naturally present in some of the raw materials used for clinkers manufacture. One way to predict the long-term behavior of these metals is to know which types of links occur between heavy metals and major elements (Ca, Si, Al, Fe). Since tricalcium silicate (C3S) is the major phase of cement-base material, a XAS study at the PB-LIII edge has been carried out to elucidate the binding mechanisms of Pb by calcium silicate. At low concentration, under the precipitation of the oxide and oxy-hydroxide, a strong retardation of the hydration of the C3S has been observed for C3S doped with Pb. EXAFS results confirmed by XANES at the Pb-LIII edge indicate that Pb is chemically adsorbed at the surface of the C3S. This Pb layer acts as a diffusional barrier to water and leads to the retardation of C3S hydration. When C3S is previously hydrated without Pb, lead is strongly retained by the hydrated C3S (called CSH for calcium silicate hydrate). A previous 29Si NMR1 study has demonstrated that Pb is chemically fixed to the CSH structure through a Pb-O-Si bond. XAS experiments have confirmed that Pb is incorporated within the matrix of CSH and directly linked at the end of the silicate chains through Pb-O-Si bonds. The Pb-Si distance is ≈3.75Å. All EXAFS results have been confirmed by ab initio calculation including multiple scattering.

SREL Reprint #2555

Rose, J., I. Moulin, J. L. Hazemann, A. Masion, P. M. Bertsch, J. Y. Bottero, F. Mosnier, and C. Haehnel. 2000. X-ray absorption spectroscopy study of immobilization processes for heavy metals in calcium silicate hydrates: 1. Case of lead. Langmuir 16:9900-9906.

This information was provided by the University of Georgia's Savannah River Ecology Laboratory (srel.uga.edu).