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

Abstract

The uptake of selenate (Se VIO 4 2−) or selenite (Se IVO 3 2−) 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(IV) and Se(VI) bound to CH, AFt, AFm and C–S–H are similar to those of SeO 4 2− and SeO 3 2− in solution, indicating a “solution-like” coordination environment 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 SeO 4 2− and SeO 3 2− entities is maintained upon immobilization by HCP and cement minerals and non-specific interactions dominate at the given Se loadings.