X-ray Absorption Spectroscopy Study of Immobilization Processes for Heavy Metals in Calcium Silicate Hydrates: 1. Case of Lead

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 29 Si NMR 1 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 ≈3.75 Å. All EXAFS results have been confirmed by ab initio calculation including multiple scattering.

Stabilization of As-, Pb-, and Cu-contaminated soil using calcined oyster shells and steel slag

“…Therefore, As, Pb, and most likely Cu are expected to be partially encapsulated in pozzolanic products or incorporated into stable phases at low pH. Moreover, Pb is reported to be sequestered into Pb-Si-O compounds at high alkaline pH values (Rose et al 2000).…”

“…Moreover, lead silicates are also possible compounds responsible for Pb immobilization. Rose et al (2000) reported that Pb could be incorporated within the CSH structure through linkages to Si-O chains upon hydration of tricalcium silicate, which is the main compound in Portland cement. Lead silicate (Pb 3 SiO 5 ) has been reported as the compound responsible for Pb immobilization when the fly ash was used on Pb contamination at 3.125 %.…”

Nickel speciation in cement-stabilized/solidified metal treatment filtercakes

“…The application of XAS to complex matrices in relation to environmental fate and behavior of contaminants is still relatively rare, but it has been employed in a limited number of studies of the speciation of metals in cement pastes, including chromium [9], cobalt [10], copper [11], lead [12], neodymium [13], selenium [14], tin [15], uranium [16] and zinc [17], as well as nickel [18][19][20]. Apart from Rinehart et al [9], who examined the oxidation state of chromium in a s/s contaminated soil and showed that this was not affected by S/S, and Hsaio et al [11], who indicated that S/S respeciated CuCl2 and reduced forms of copper in municipal waste incinerator air pollution control residues to Cu(OH)2, other studies have all prepared hydrated cement pastes by mixing pure solutions of soluble metal salts with cement powder.…”

EXAFS study of U(VI) uptake by calcium silicate hydrates

“…Although X-ray absorption fine structure (XAFS) spectroscopy has become an increasingly important tool for investigating immobilization processes in cementitious systems (e.g., [14][15][16][17][18][19][20][21][22][23][24][25][26]), very few studies have used XAFS to develop a molecular-level understanding of the U(VI) uptake by cementitious materials [27,28]. In these studies the coordination environment of U(VI)-doped untreated and hydrothermally treated concrete was probed in the alkaline pH range (pH 9-12).…”