Co Speciation in Hardened Cement Paste: A Macro- and Micro-Spectroscopic Investigation

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• 作者：M. Vespa ; R. D&auml ; hn ; D. Grolimund ; E. Wieland ; A. M. Scheidegger
• 刊名：Environmental Science & Technology
• 出版年：2007
• 出版时间：March 15, 2007
• 年：2007
• 卷：41
• 期：6
• 页码：1902 - 1908
• 全文大小：494K
• 年卷期：v.41,no.6(March 15, 2007)
• ISSN：1520-5851

文摘

Cement-based materials play an important role in multi-barrier concepts developed worldwide for the safe disposalof hazardous and radioactive wastes. Cement is used tocondition and stabilize the waste materials and to constructthe engineered barrier systems (container, backfill andliner materials) of repositories for radioactive waste. In thisstudy, Co uptake by hardened cement paste (HCP) hasbeen investigated with the aim of improving our understandingof the immobilization process of heavy metals in cementon the molecular level. X-ray-absorption spectroscopy (XAS)on powder material (bulk-XAS) was used to determinethe local environment of Co in cement systems. Bulk-XASinvestigations were complemented with micro-beaminvestigations to probe the inherent microscale heterogeneityof cement by using mages/entities/mgr.gif">-X-ray-fluorescence (mages/entities/mgr.gif">-XRF) andmages/entities/mgr.gif">-XAS. mages/entities/mgr.gif">-XRF was used to gain information on the spatialheterogeneity of the Co distribution, whereas mages/entities/mgr.gif">-XASwas employed to determine the speciation of Co on themicroscale. The Co-doped HCP samples hydrated for time-scales from 1 hour up to 1 year were prepared undernormal atmosphere, to simulate similar conditions as forwaste packages. To investigate the role of oxygen, furthersamples were prepared in the absence of oxygen. Thestudy showed that, for the samples prepared in air, Co(II)is oxidized to Co(III) after 1 hour of hydration time.Moreover, the relative amount of Co(III) increases withincreasing hydration time. The study further revealed thatCo(II) is predominately present as a Co-hydroxide-likephase and/or Co-phyllosilicates, whereas Co(III) tends tobe incorporated into a CoOOH-like phase and/or Co-phyllomanganates. In contrast to samples prepared in air,XAS experiments with samples prepared in the absenceof oxygen revealed solely the presence of Co(II). This findingindicates that oxygen plays an important role for Cooxidation in cement. Furthermore, the study suggests thatCo(III) species or Co(III)-containing phases should betaken into account for an overall assessment of the Corelease from Co-containing cement-stabilized waste underoxidizing conditions.