3.3.1. Physicochemical Properties of Sediments Used in Microcosm Experiments

The physicochemical properties of the An-Shun site sediment are summarized in Table 2. The pH value of the sediment was about 7.5, slightly alkaline due to the estuary water system of that region. The slightly alkaline condition may favor to immobilize and decrease the toxicity of most metals as they may precipitate in the alkaline environment. The OC content in the An-Shun site sediment was about 0.8 wt%, which was relatively low when compared to other sediments. The organic matter in the sediment may potentially affect the interaction between pollutants and sediment. The sand, silt and clay contents of the sediment were 71.4, 14.3, and 14.3 wt%, respectively. The An-Shun site sediment could thus be classified as a sandy loam according to the soil texture classification of the United States Department of Agriculture (USDA) (Figure S2). In general, the texture of sediment is tightly correlated with OC content and CEC. The fine-textured sediment may contain high OC and CEC due to its large surface areas, whereas the OC and CEC of sandy-textured sediment was relatively low. The results of physicochemical properties analysis of An-Shun site sediment were consistent with the fact that a sandy loam showed low OC content and low CEC.

**Table 2.** The physicochemical properties of An-Shun site sediment.


#### 3.3.2. Environmental Factor Variation during Microcosms Operation

The vertical up-flow microcosms with a closed circulation system are set up as shown in Figure 1 and Figure S1. Two kinds of waters were applied in the microcosms, including freshwater and estuary water. The microcosms were operated for 65 days. The operation time from days 1 to 25 was the process of sediment acclimation. After that, the capping materials were delivered to the microcosms and the water was collected periodically from the outflow as the overlying water sample. The appearance of the microcosms, which was capped with the AC, SAC, or no cap were seen to be clear, while the column capped with FeS was found to gradually appear in brown inside the microcosms. It has been speculated that the addition of FeS may promote a chemical reaction to form iron hydroxide or iron oxide, which was found attached on the wall of the column and the buffer tank.

The recorded temperature in the microcosms is shown in Figure S3. The range of temperature was between 20 and 27 ◦C during the test period between winter and spring. The DO results are shown in Figure S4. The DO value at the beginning of the experiment was approximately 4 mg/L, started to decrease and remained stable at 3 ± 0.5 mg/L as the operation time extended. This result could be explained by the microbial activity in the microcosms. The microbes would consume DO to carry out respiration process, thus its concentration decreased at the beginning. Owning to the operational defects, the oxygen outside the microcosm fluxed in and caused the increment of DO. The pH value of overlying water is shown in Figure S5. It was between 7.6 and 7.9 in all microcosms.

The results of EC in the freshwater system and estuary system are shown in Figure S6. In the freshwater system, the EC of overlying waters of microcosms was around 2300−3270 μS/cm. The EC values decreased as the operation time increased because the microcosms would be refilled with the artificial freshwater and get diluted after sampling periodically (Figure S6a). Moreover, the An-Shun site sediment was estuary sediment, thus it contained a portion of salt and contributed to the background levels of salinity. In the estuary water system of microcosms, the results of EC are shown in Figure S6b. The EC ranged from 37,000 to 38,000 μS/cm.

The ORP results of the overlying water from freshwater system and estuary system are shown in Figure S7. In the freshwater system, the ORP of overlying water of microcosms was around 40–160 mV. As for the estuary system, the ORP of overlying water of microcosms was about −22–145 mV.

The results of DOM in the overlying water are shown in Figure S8. The DOM content in the overlying water was low, mainly related to the low DOM content in An-Shun site sediment. The An-Shun site sediment contained less OC content according to the previous results shown; consequently, less DOM would be released from the sediment to the overlying water.

The total Fe in overlying water is shown in Figure S9. In the freshwater system, the microcosm capped with FeS would contribute significant dissolved Fe to the overlying water while the concentration of Fe in the microcosm with no capped was also high. The Fe content of those treatment were higher if compared to microcosms with AC and SAC. The similar trend was also observed in the estuary system.

#### 3.3.3. Sequestration of Aqueous THg and MeHg by Thin Layer Capping

The effectiveness of capping materials to immobilize Hg is shown in Figure 7 and Table S4. For the freshwater system, the THg immobilization abilities of three capping materials greatly varied as compared to that of the no-capped control unit. The THg reduction efficiencies of the AC, SAC, and FeS, which were calculated based on the initial THg concentration of the microcosm started and the THg concentration in a given sampling date, fluctuated between −22−82%, −30−78% and −87−62%, respectively, in comparison to the control unit. As for the estuary system, THg reduction efficiencies of the AC, SAC and FeS reached −33−49%, 28−60% and −24−44%, respectively, in comparison to that of the control unit. Therefore, the fluctuation and uncertainty for Hg removal of capping sorbents were observed in both freshwater and estuary systems. According to our previous studies [17,48], the recommended amendment dosage of sorbents for Hg-sediment remediation was approximately 1−5%. The dosage of sorbents being used in this study was 3%, which was supposedly to show significant effect on Hg removal, but the results seemed not be similar to our

previous studies. The horizontal flow microcosm set up was mostly used to stimulate the emission of Hg from the sediment according to the previous studied [48]. However, a different design, vertical up-flow microcosm was adapted in this study, in which the water moved in an up-flow direction and then recirculated back into the columns in a closed system. Therefore, a dynamic equilibrium among the sediment, capping material, and overlying water achieved in the closed circulation system. The Hg in the sediments will be released to the water continuously until an equilibrium was reached. The same process happened in the interface between water and capping material. These processes will lead to the depletion of Hg on the sediments because it was diffused and transferred to the capping materials. Owning to these process, the reduction of Hg in the overlying water of the microcosm was less evident, which was also demonstrated by the ANOVA analysis (Table S4); instant and marked releases of Hg from the caps were occasionally found. The dosage of sorbents applied in this microcosms may need to be reconsidered as it might not be capable to sorb Hg efficiently due to the characteristic of this microcosms system.

**Figure 7.** THg concentration in overlying water of (**a**) freshwater system and (**b**) estuary system.

The Hg concentrations in sediment, sorbents and overlying water after 65 days in the microcosms are shown in Table 3. The Hg concentrations in the freshwater sediments at the end of experiment for AC, SAC, FeS and control unit were 65.35 ± 0.65, 68.87 ± 0.46, 50.38 ± 1.03 and 68.77 ± 2.03 mg/kg, respectively. In the estuary system, the Hg concentrations of sediment for AC, SAC, FeS and control unit were 56.16 ± 2.87, 57.55 ± 0.39, 54.80 ± 2.25 and 69.20 ± 4.02 mg/kg, respectively. The initial Hg content in sediment of the An-Shun site was 76.00 ± 2.59 mg/kg, thus, there were varying losses of Hg from sediment with different capping amendment in both freshwater and estuary systems. The sediment can be a reservoir of Hg, which would provide Hg to the water, capping materials, and the microcosm's auxiliary equipment until an equilibrium reached. Hence, the decrease of Hg in the sediment of the study is reasonable. In the freshwater system, the loss of Hg in microcosms capped with AC and SAC was similar to that in the control unit while the loss of Hg in FeS capping was slightly higher than that in the control unit, AC, and SAC capping. As for the estuary system, the Hg loss from sediment with capping materials was decreased significantly as compared to that in the control unit. On the other hand, the contents of Hg in the sorbents of AC, SAC, and FeS were 4.84 ± 0.81, 6.86 ± 3.52, and 37.66 ± 8.34 mg/kg, respectively, in freshwater system. In the estuary system, the contents of Hg in the sorbents of AC, SAC, and FeS were 3.88 ± 1.82, 2.89 ± 1.49, and 43.83 ± 13.04 mg/kg, respectively. The Hg content in FeS was significantly greater as compared to those in AC and SAC in both freshwater and estuary systems. Notably, the Hg sorption capacity of sorbents in the batch experiments was relatively high when compared to those for the sorbents applied in the microcosms. Although the Hg concentration of overlying water, approximately 200 ng/L was relatively low as compared to the batch experiment test, the mechanism of Hg sorption might

be different at the low Hg concentration. The recovery of Hg in the microcosms was approximately 67−90% in both freshwater and estuary systems. The low recovery in some cases can be explained by the phenomenon of bacteria respiration, which could produce a large portion of batting biomass and colloids that cause Hg to attach onto. Moreover, the Hg might be adhered to the surface of the microcosm's auxiliary equipment, such as column surface, buffer tank, pipelines, and others that may cause the decreasing recovery of Hg (Figure S1). Therefore, although FeS showed great Hg sorption capacity during the batch and caused Hg partitioning from sediment to the cap during microcosm test, Hg may be released from cap again, through redissolution or release of small-scale FeS particles containing Hg, leading to not only the low recovery but also the risk of unexpected release of Hg for sediment with vertical up-flow.


**Table 3.** Recovery calculation based on the Hg content in sediment, sorbents and overlying water after 65 days in the microcosms.

<sup>1</sup> The Hg content in sediment for the day 65. <sup>2</sup> Recovery was obtained by the sum of Hg content in sediment, sorbents and overlying water divided by the total Hg content in initial sediment (i.e., 76.0 ± 2.59 mg/kg and 22.8 mg) then multiplying by 100.

The results of MeHg content in the overlying water of freshwater system and estuary system are shown in Figure 8. In the freshwater system, the amounts of MeHg in the overlying water from microcosms capped with AC, SAC, and FeS were significantly decreased when compared to that of the control unit; the MeHg concentrations of AC, SAC, FeS, and control unit were 0.04−0.70, 0.14−0.94, 0.13−1.64, and 2.26−11.35 ng/L, respectively. In contrast, the MeHg concentrations of AC, SAC, FeS, and control unit were 0.10−0.14, 0.15−0.87, 0.04−2.77, and 0.14−1.01 ng/L in the estuary system, respectively. Therefore, the production of MeHg under the treatment of AC and SAC capping was decreased tremendously, while the reduction efficiency of MeHg in the microcosms capped with FeS was not significant.

**Figure 8.** MeHg concentration in overlying water of (**a**) freshwater system and (**b**) estuary system.

The production of MeHg of the control unit in the freshwater system was significant as compared with that in the estuary system. A previous study showed that the salinity level of the environment was negatively correlated with MeHg production [49]. In the high salinity condition, Hg has tendency to form complex with chlorine salt and sulfurous compounds, reducing the trends of Hg methylation [50]. The researchers suggested that a high-salinity condition may inhibit Hg methylation because sulfate with high concentration would reduce to toxic sulfides by microorganisms, and poisons the Hg methylating bacteria, reducing the formation of MeHg as well. In the freshwater system, the MeHg reduction efficiency in microcosms capped with AC was the best, SAC followed, and FeS the least, suggesting that the SAC and FeS provided a portion of the iron and sulfur elements to dissolve into water and enhanced the methylation ability of sulfate-reducing bacterial and Fe(III)-reducing bacteria. As a result, the MeHg reduction efficiency in microcosms capped with SAC and FeS was poorer as compared to that with AC.

The MeHg contents in the sediment are shown in Table 4. The formation of MeHg in the control unit of freshwater system was higher than that in the estuary system. The formation of MeHg capped with AC and FeS was low when compared to that in the control unit in freshwater system, except for the microcosm capped with SAC, which was slightly higher. As a results, the MeHg released to overlying water in freshwater system was inhibited by both limiting the MeHg formation and the effectiveness of caps. In the estuary system, the MeHg contents in both capped with sorbents and control unit were similar and smaller than those in freshwater system.


**Table 4.** The MeHg content in sediment on day 65.
