**Boon-Lek Ch'ng 1, Che-Jung Hsu 1, Yu Ting 1, Ying-Lin Wang 1, Chi Chen 1, Tien-Chin Chang <sup>2</sup> and Hsing-Cheng Hsi 1,\***


Received: 7 May 2020; Accepted: 12 July 2020; Published: 14 July 2020

**Abstract:** This study aimed to investigate the Hg removal efficiency of iron sulfide (FeS), sulfurized activated carbon (SAC), and raw activated carbon (AC) sorbents influenced by salinity and dissolved organic matter (DOM), and the effectiveness of these sorbents as thin layer caps on Hg-contaminated sediment remediation via microcosm experiments to decrease the risk of release. In the batch adsorption experiments, FeS showed the greatest Hg2<sup>+</sup> removal efficiencies, followed by SAC and AC. The effect of salinity levels on FeS was insignificant. In contrast, the Hg2<sup>+</sup> removal efficiency of AC and SAC increased as increasing the salinity levels. The presence of DOM tended to decrease Hg removal efficiency of sorbents. Microcosm studies also showed that FeS had the greatest Hg sorption in both freshwater and estuary water; furthermore, the methylmercury (MeHg) removal ability of sorbents was greater in the freshwater than that in the estuary water. Notably, for the microcosms without capping, the overlying water MeHg in the estuary microcosm (0.14−1.01 ng/L) was far lesser than that in the freshwater microcosms (2.26−11.35 ng/L). Therefore, Hg compounds in the freshwater may be more bioavailable to microorganisms in methylated phase as compared to those in the estuary water. Overall, FeS showed the best Hg removal efficiency, resistance to salinity, and only slightly affected by DOM in aqueous adsorption experiments. Additionally, in the microcosms, AC showed as the best MeHg adsorber that help inhibiting the release of MeHg into overlying and decreasing the risk to the aqueous system.

**Keywords:** mercury; methylmercury; salinity; sediment; remediation
