Sorption mechanisms of zinc to calcium silicate hydrate:: Sorption and microscopic investigations

Knowledge of the binding mechanisms of heavy metals to cement minerals is essential for the prediction of the longterm leachability of secondary building materials and cement-stabilized wastes containing heavy metals. In this study, the sorption of Zn(II) to calcium silicate hydrate (C-S-H(I)) in pre-equilibrated aqueous suspensions has been investigated as a function of time (up to 87 d), pH (11.7, 12.48, and 12.78) and Zn(II) concentration (4.8-4800 muM). Electron probe microanalysis (EPMA) was performed in order to determine where Zn(II) was bound. At high Zn(II) concentrations (> 1000 muM), the precipitation of beta (2)-Zn-(OH)(2) (< pH 12) and calcium zincate (Zn(2)Ca(OH)(6). 2H(2)O, > pH 12) was observed. Surface precipitation could not be discerned. At lower concentrations, it was found that the sorption process was initially very rapid with over 50% sorbed within 30 min but that the sorption continued more slowly to at least 87 d. The data could be interpreted in terms of the Freundlich isotherm up to a Si:Zn(II)(sorbed) atomic ratio of approximately 6:1. Zinc was observed by EPMA to incorporate into the C-S-H(I) particles but did not appear to substitute for Ca or Si. The incorporation of Zn(II) in the interlayer of C-S-H(I) or sorption to internal surfaces of crystalline appear to be the most probable mechanisms for the observed Zn(II) sorption to C-S-H(I).