Uptake of Se(IV/VI) oxyanions by hardened cement paste and cement minerals: An X-ray absorption spectroscopy study

The uptake of selenate ((SeO42-)-O-VI) or selenite ((SeO32-)-O-IV) by hardened cement paste (HCP) and important constituents of the cement matrix such as calcium silicate hydrate (C-S-H), portlandite (CH), ettringite (AFt) and monosulfate (AFm) was investigated using X-ray absorption spectroscopy (XAS). The XAS measurements were conducted on samples with Se loadings ranging between 1200 and 8800 ppm. X-ray absorption near edge structure (XANES) spectroscopy shows that redox reactions do not influence uptake processes in the cementitious systems. The EXAFS (extended X-ray absorption fine structure) spectra of Se(lV) and Se(VI) bound to CH, AFt, AFm and are similar to those of SeO42- and SeO32- in solution, indicating a solution-like coordination environinent upon uptake by the cement minerals. Similarly, the spectra of Se(IV)- and Se(VI)-treated HCP samples reveal the absence of backscattering atoms at short distances. These results suggest that the coordination sphere of the SeO42- and SeO32- entities is maintained upon immobilization by HCP and cement minerals and non-specific interactions dominate at the given Se loadings. (c) 2005 Elsevier Ltd. All rights reserved.