# Optimal Mixture Designs for Heavy Metal Encapsulation in Municipal Solid Waste Incineration Fly Ash
## 3 Results and Discussion
### Encapsulation and Hydrate Nature  
Several mixtures show excellent encapsulation behavior. While mixtures containing LFS generally show greater encapsulation than other mixtures, they perform very poorly in encapsulating Sb. The calcium aluminates, such as mayenite, in LFS enhance heavy metal encapsulation due to the formation of large amounts of LDH phases.  
The major hydrates formed in the mixtures that play a role in encapsulation are [PERSON]'s salt, ettringite, and C-S-H. [PERSON]'s salt shows significant ionic substitutions at its aluminum and sulfate sites. The aluminum site can accommodate different trivalent and tetravalent cations in varying proportions. Monovalent and divalent anions and cations may also be incorporated in  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Mixture.**} & \multicolumn{2}{c}{Cr} & \multicolumn{2}{c}{Cu} & \multicolumn{2}{c}{Zn} & \multicolumn{2}{c}{Pb} & \multicolumn{2}{c}{Sb} & \multicolumn{2}{c}{Cd} & \multicolumn{2}{c}{Hg} \\ \cline{2-13}  & C & P & C & P & C & P & C & P & C & P & C & P & C & P \\ \hline
**MSWIFA + Water** & \(\mathbf{x}\) & - & & - & - & - & - & - & - & - & \(\mathbf{x}\) & - \\ \hline
**C70B0** & & & & & & & & & & & & & \\ \hline
**C56B14** & & & & & & & & & & & & & \\ \hline
**C42B28** & * & & & & & & & & & & & \\ \hline
**C28B42** & & & & & & & & & & & & \\ \hline
**B70M20** & & & & & & & & & & & & \\ \hline
**B50M40** & & & & & & & & & & & & \\ \hline
**B30M60** & & & & & & & & & & & & \\ \hline
**L0B60** & & & & & & & & & & & & \\ \hline
**L20B40** & & & & & & & & & & & & \\ \hline
**L40B20** & & & & & & & & & & & & \\ \hline
**L60B0** & & & & & & & & & & & & \\ \hline
**G14B16** & & & & & & & & & & & & \\ \hline
**G6B24** & & & & & & & & & & & & \\ \hline
**G2828** & & & & & & & & & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 4: The encapsulation performance for each heavy metal for all groups. The notations are as follows—C: leachate concentration; P: encapsulation percentage; \(\forall\): within EPA drinking water limit; \(\mathbf{x}\): exceeds EPA drinking water limit; \(\mathbf{\ast}\): encapsulation percentage varies significantly over time (generally increases); -: not applicable.  
these phases [2; 8; 10; 14; 17; 22; 34; 35; 41; 45; 49]. The uptake of divalent metal cations in these phases occurs through the precipitation of the metal hydroxides on the surfaces, intercalation by structural incorporation, and adsorption by the surface hydroxyl groups [17; 49]. Ettringite can act as a host to numerous ions, including Ca\({}^{2+}\), Al\({}^{3+}\), SO\({}_{4}\)\({}^{2-}\) and OH\({}^{\prime}\), through chemical substitution in both the column and channel sections of its structure [9]. C-S-H can also host a number of heavy metals in both ionic and salt form. Immobilization mechanisms of C-S-H include adsorption, physical wrapping, and ionic substitution. As the amount of Al in mixtures increases, the amount of Al in the C-S-H can also increase, which may lead to encapsulation of greater amounts of heavy metals [45].  
Table 5 shows the main hydrates and encapsulation mechanisms of representative mixtures.  
Among the mixtures tested, mixtures C56B14, C42B28, C28B42 and B70M20 never exceed EPA drinking water limits for any heavy metal and show adequate early compressive strength (above 20 MPa at 3 days). While mixtures with LFS generally show high encapsulation, they show poor performance with Sb (EPA drinking water limit is exceeded). In addition, several mixtures with LFS show poor early strength (below 10 MPa at 3 days). Therefore, if one were to use zero cement mixtures for MSWIFA encapsulation, the mixture B70M20 would be ideal, but the mixtures with LFS would not.