3.3.4. Metal Immobilization

The concentrations of five metals and MeHg are shown in Figure 5 and Tables S5–S10 (in Supplementary Material). The results of one-way ANOVA and LSD test (*p* < 0.05) are shown in Tables 6 and 7. In consistence with the expectation, Column 1 showed the most considerable comprehensive consequences of inhibiting the release of metals to the overlying water. It was unexpected that even without 10 wt% of kaolinite, Column 5 achieved almost the same results as Column 1, which is also proved by no significant (*p* > 0.05) correlation in ANOVA (Table 6) between the two columns for all metals, indicating that kaolinite did not show greater stabilization ability for mixed caps as compared to the other test materials.

**Figure 5.** The remaining proportion (%) of (**a**) Ni, (**b**) Cr, (**c**) Cu, (**d**) Zn, (**e**) Hg, and (**f**) the concentration of MeHg in six columns. The remaining proportion in aqueous phase was calculated through dividing the concentration of each contaminant by the concentration on the first day.


**Table 6.** Metal concentrations in six columns before adding acid (day 1–100).

Different letters for metal concentrations in six columns indicate a significant difference at *p* < 0.05. For example, the concentrations of Ni in Column 1 with letter b are significantly different from the concentrations of Ni in Column 6 with letter a.

When comparing Column 1 (50% OSP + 35% HAP) with Column 4 (80% OSP + 5% HAP), it is demonstrated by Figure 5 that the releases of Ni, Cr, Cu, and Zn were inhibited well in both columns. However, Column 4 with less proportion of HAP could not achieve an equal effect to Column 1 for Ni, Cr and MeHg immobilization (Figure 5), and a significant correlation was observed by ANOVA (*p* < 0.05) (Table 6) in MeHg concentrations of the two columns (Table 6).


**Table 7.** Metal concentrations in six columns after adding acid (day 101–135)

Different letters for metal concentrations in six columns indicate a significant difference at *p* < 0.05. For example, the concentrations of Ni in Column 1 with letter a and b are significantly different from the concentrations of Ni in Column 3 with letter c.

To compare FeS with CB, 5% FeS in Column 3 made it more effective in immobilizing Ni than 5% CB in Column 2. On the other hand, Column 3 did worse than Column 2 for lowering the concentration of MeHg in the long term, which was demonstrated doubly by Figure 5 and statistical analysis (Table 6).

The stabilization of different mixed caps under extreme condition with acid influent during day 101–114 is also shown in Figure 5. It was observed that the concentrations of Hg and MeHg in every column increased during the days with acid influent (Figure 5f), especially for the controlled column without capping (Table 7). Also, Column 6 released the most amount of Zn during day 101–114 (Figure 5d). The experimental results also suggested that columns with caps were less affected by acid influent than the controlled one, which could be supported by strong significant correlations observed by ANOVA between Column 6 and the other columns.
