# Immobilization of (Aqueous) Cations in Low pH M-S-H Cement
## 4 Discussion
However, the presence of a co-precipitated phase as a hydrotalcite-like phase, not identified by SEM/EDX due to arbitrary selection of locations, in which the amount of Al and Mg would be considerably higher than in the M-S-H precipitates is the most acceptable reason for these differences. The precipitation of the semi-amorphous hydrotalcite was recently observed in addition of M-A-S-H phases [57]. Our experiments demonstrated that there are different mechanisms involved in the incorporation of the secondary metals in M-S-H. These mechanisms are controlled by properties of ion association (i.e., aqueous speciation and surface complexation) and the solubility of the secondary phases. This study shows that the mechanism behind the phenomena of absorption and coprecipitation controls the inclusion of the secondary metal cation into the M-S-H matrix. The combination of these effects explains the element specific incorporation mechanism characteristic to each ion considered in this study. However, a common tendency to delay the nucleation onset of M-S-H has been observed in presence of secondary metal cations. Recent studies which investigated the solid stabilization of Pb using magnesium oxychloride cement and magnesium phosphate cement proposed that in general, metals are immobilized as precipitated amorphous metal hydroxides in those Mg-based cementitious materials [31]. In the obtained precipitates, amorphous metal hydroxides could certainly be present in the form of nanoparticles distributed homogeneously through the M-S-H matrix. In this case, the resolution of the applied experimental techniques (EDX-TEM) would not allow detecting these particles in the M-S-H matrix. It can be speculated that the nanoparticles (ca. 2-5 nm) with higher electron contrast present in the precipitates formed from Pb and Fe bearing solutions could be metal hydroxide phases of both ions trapped in the M-S-H matrix. However, the resolution of the applied analysis cannot evidence the difference in their composition. Another possibility is the incorporation of the metal cations within the semicrystalline structure of M-S-H. Assuming a sepiolite-like M-S-H, the metals are expected to be preferentially incorporated in the so-called exchangeable sites (i.e., surface of particles and interlayer) replacing the Mg ions. However, as has been shown for Al, they also can be incorporated in both SiO\({}_{2}\) and MgO layers [54]. Based on our results, it seems that the mechanism of incorporation could vary significantly for the different metals. The fact that the overall (Mg + metal)/Si corrected ratio (Figure 6, second column) is not far from the Mg/Si ratio in the reference M-S-H for most of the samples could be interpreted as isomorphic substitution of cations for Mg. Nevertheless, this mechanism is postulated under the assumption that no secondary phases are embedded in the M-S-H matrix. Mechanistic understanding of the incorporation mechanism for each element would require detailed analysis of the cation exchange capacity (CEC) and structural characterization such as solid state nuclear magnetic resonance (SS-NMR) and/or pair distribution function analysis (PDF). In this context, the reversibility studies would be pivotal for elucidating the mechanism of immobilization and thus, are in the focus of ongoing studies.