**4. Conclusions**

In this study, the aqueous batch experiments with the amendment of AC, SAC, and FeS sorbents were first carried out to comprehend the Hg removal efficiency in Hg-contaminated sediments influenced by salinity and DOM. The microcosms were then set up to examine the performance of these capping sorbents on Hg-contaminated sediment remediation. The experimental results showed that FeS on Hg removal was not significantly affected by salinity levels and maintained with high removal efficiency. The Hg removal efficiency of AC and SAC increased as salinity increased. In contrast, the Hg removal efficiency of sorbents decreased with the addition of DOM at different salinity levels because DOM competed with sorbents and may occupy the adsorption site, thus inhibited the Hg uptake by sorbents. The microcosm experiments showed that the THg immobilization abilities of three capping sorbents greatly varied as compared to that of control unit. The MeHg concentration of overlying water in the freshwater microcosm with no cap was higher than that in the estuary system. Therefore, Hg compounds in the freshwater system may be more bioavailable to microorganisms in methylated phase as compared to those in the estuary system. To summarize, the capping materials including AC, SAC, and FeS effectively decreased the concentration of overlying water MeHg in the freshwater system of microcosms. Because the production of MeHg in estuary system was low, the efficiency of materials on MeHg sorption was insignificant.

We suggest that future studies should be focused on scale-up design using large microcosms. Notably, because FeS showed the best Hg removal efficiency, resistance to salinity, and only slightly affected by DOM in aqueous adsorption experiments and AC showed as the best MeHg adsorption material, a "mixing cap" using both FeS and AC should be examined and the optimal mixing ratio should be obtained. A mixing cap of FeS and AC may also help preventing the leaching out of FeS from the cap layer, which was observed in our microcosm study.

It is also worth noting that for the vertical up-flow system, the accumulated Hg in capped layer may eventually breakthrough, which could cause sudden concentration shock that leads to instant risk of exposure. Long-term microcosm operation is critical and should be further conducted to obtain design parameters on subsequent pilot tests or full-scale application.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4441/12/7/1991/s1, Table S1. Freshwater and seawater ion species concentration, Table S2. Hg(II) speciation at various salinity levels, Table S3. The partitioning coefficients for Hg adsorptions at various salinity levels, Table S4. A one-way ANOVA or one-way ANOVA on ranks based on normality test, followed by a post hoc test (*p* < 0.05) used to determine the significance differences among various sorbents, Figure S1. Photos of the microcosms on (a) day 25 as the capping materials were initially applied and (b) on day 65, Figure S2. An-Shun site sediment texture, Figure S3. The temperature of microcosms (the symbol F refers to freshwater system and the symbol E refers to estuary system), Figure S4. The dissolved oxygen for the microcosms (the symbol F refers to freshwater system and the symbol E refers to estuary system), Figure S5. The pH value variation of microcosms (the symbol F refers to freshwater system and the symbol E refers to estuary system), Figure S6. The electricity conductivity of overlying water in (a) freshwater system and (b) estuary system, Figure S7. The oxidation reduction potential of overlying water in (a) freshwater system and (b) estuary system, Figure S8. DOM variation of microcosms for (a) freshwater system; (b) estuary system, Figure S9. The total Fe variation of microcosms in (a) freshwater system and (b) estuary system.

**Author Contributions:** Conceptualization, B.-L.C., Y.T., T.C.C., and H.-C.H.; methodology, B.-L.C., C.-J.H., Y.T., Y.-L.W., and H.-C.H.; formal analysis, B.-L.C. and C.C.; resources, H.-C.H.; writing—original draft preparation, B.-L.C. and C.-J.H.; writing—review and editing, Y.-L.W. and H.-C.H.; supervision, T.-C.C. and H.-C.H.; funding acquisition, H.-C.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research and the APC were funded by the Environmental Protection Administration, Taiwan under Grant no. 08BT547001 and theMinistry of Science and Technology of Taiwan, Taiwan under Grant no.MOST 105-2221-E-002-008-MY3.

**Acknowledgments:** We greatly appreciate the financial and technical supports from the Environmental Protection Administration, Taiwan and the Ministry of Science and Technology of Taiwan, Taiwan. The opinions expressed in this paper are not necessarily those of the sponsors.

**Conflicts of Interest:** The authors declare no conflict of interest.
