2.4.1. Microcosm Design

The microcosm shown in Figure 2 was designed by modifying the system used in Ting et al. [29]. Multi-columns containing contaminated sediment with mixed caps on the top were set to stimulate the release of metal compounds and examine the efficiencies of capping materials. Water entered the system from the bottom of the column, vertically moved upwards to fill the column, and then discharged to the outside of the system.

**Figure 2.** The lab-scale microcosm system.

#### 2.4.2. Microcosm Operation

Six freshwater columns were set up to investigate the efficiencies and stabilities of different mixed caps. Dried incubated sediment was added into the columns and each column contained 200 g of sediment. Five columns were capped by the mixed adsorbents with five different ratios, which were determined according to the results of batch experiments. The sixth column was not capped as the controlled unit. To start up the system, each column was filled with water and allowed to settle for 24 h. Next, the microcosm was activated to start inflow and this day was counted as the operation day 1. The flow rate was maintained to be 0.2 L day−<sup>1</sup> constantly by using a peristaltic pump (Gamma ST100SV2, Shakopee, MN, USA).

Notably, in order to investigate the stabilization of different mixed caps, the pH of artificial water was adjusted to 3 ± 0.1 by adding HNO3 during day 101–114 to simulate the extreme condition with acid influent.
