*3.2. Adsorption E*ffi*ciency*

The aqueous metal removal efficiency by the adsorbents was calculated with the following equation:

$$\text{R} = \frac{\text{C}\_0 - \text{C}\_t}{\text{C}\_0} \times 100\% \text{.} \tag{1}$$

where R (%) is the removal efficiency of adsorbents, C0 (mg L−1) is the initial metal concentration detected in blank solution, and Ct (mg L<sup>−</sup>1) is the concentration of remaining sorbate at any time.

Figure 3 and Table 3 show the removal efficiencies of five materials towards five metals at various initial concentrations, which illustrated roughly that OSP and HAP had the best affinities to Cr, Zn, Cu, and Ni, and CB took the third place. As for Cu, HAP performed the largest removal efficiency than the others with a reduction of 98.39%. For Ni, OSP presented a significant removal rate of 76.47% at a low initial concentration and, contrarily, poor efficiency at high initial metal concentrations. For the adsorption of Hg, inversely, FeS showed the best removal efficiency, which was up to 100%. CB was the second excellent one with a removal rate of 86.4%.

**Figure 3.** Removal efficiencies of five materials towards (**a**) Ni, (**b**) Cr, (**c**) Cu, (**d**) Zn, and (**e**) Hg at various initial concentrations.


**Table 3.** Removal efficiencies (in percentage) of five adsorbents for five metals (*n* = 3).

#### *3.3. Microcosm*

#### 3.3.1. Mixed Caps Design

Five columns covered by different mixed materials and one controlled column were set as shown in Table 4 for conducting the microcosm experiments. The design of capping mixing ratio was based on the integrating consideration of removal efficiencies, cost of preparation, and utilization of recycling resources.

**Table 4.** Six columns covered by different mixed materials.


Based on the aforementioned results from batch experiments, HAP and OSP were selected as the major materials to adsorb Ni, Cr, Cu, and Zn; FeS and CB were selected as the major materials to immobilize total Hg (THg) and MeHg. Although the adsorption effect of kaolinite on these five metals was not significant, it has been shifted that kaolinite may probably be more capable of stabilizing the caps than other clays [30] and was therefore added for the purpose of resisting flow disturbance. Column 1 was designed as the ideally best column with appropriate amounts of each material. Column 2 replaced FeS completely with CB and Column 3 did the opposite for comparing FeS with CB. As far as costs and complexity of preparation were concerned, Column 4 tried to reduce the proportion of HAP to grope for a cheaper mixed amendment with equal ability. The cap ratio of Column 5 was almost the same as Column 1, except for the absence of 10 wt% kaolinite, which was designed to investigate the necessity of existence of kaolinite. The last column was composed of only sediment without caps as the controlled group.

#### 3.3.2. Results of Sediment Incubation

The concentrations of the five metals before and after 60 days of incubation and the concentrations of the five metals in the supernatant after incubation are shown in Table 5. Concentrations of metals in the sediment after incubation had almost met the experimental requirements. It is worth mentioning that the concentrations of Ni and Zn of supernatant were much higher than those of the other three metals.


**Table 5.** Concentrations of the five metals in sediment before and after 60 days of incubation and the concentrations of the five metals in the supernatant after incubation.
