**1. Introduction**

Mercury (Hg) has been known as a global contaminant due to the characteristics of long-range transportin the atmosphere, persistencein the environment, bioaccumulationin the food chain, and adverse effects in human health and ecosystem [1]. The increased accumulation of Hg compounds in sediment may cause the high possibility of Hg being transformed to methylmercury (MeHg) by organisms, which is a neurotoxic compound occurred under anoxic conditions [2–5]. The bioaccumulation and biomagnification of MeHg via food chain transfer may pose a high risk to human through fish consumption [6,7]. Therefore, the strategies for sediment remediation are needed to decrease the Hg contaminant release and the possibility of direct or indirect contact with benthic organisms and water surface.

In-situ capping is a feasible approach to remediate contaminated sediment. The main approach is to allow the sediment left in place but decreasing the chance of further contamination from resuspension of contaminants by the capping layer. This technique could decrease the need for handing sediment and decrease the potential of exposure and consequential spills of sediment. The cost is relatively low as compared to traditional dredging and excavation. Thin layer capping may refer as active capping, which involves a chemically reactive material placing in the subaqueous to sequestrate the emission of contaminants from sediment and decrease the bioavailability, mobility, and toxicity of contaminants [8]. Owing to the physical and chemical sorption properties of reactive materials, the amount of sorbents needed and the thickness of the capping layer to achieve the considerable results are lesser as compared to traditional capping [8]. Besides, remediation technology not only decreases the cost, but also minimizes the exposure of benthic organisms to contaminated sediment and decreases the ecological risk associated with contaminated sediments [9].

The active materials may carry a series of reactions to remove contaminants, adsorption, absorption, and precipitation of contaminants and shift them from the aqueous phase to a solid. It works by increasing the contaminated-solid partition coefficient and extend the isolation time before contaminant breaking through the capping layer. A wide variety of active materials are applied and preferred according to the specific conditions of the remediation site. Carbonaceous materials such as activated carbon (AC), biochars, and surface-modified black carbon could effectively decrease organic contaminants and immobilize Hg. AC has several functional groups such as carboxyl, lactone, and phenolic groups. With its high specific surface area, AC has the potential to be an option for remediation of organic pollutant and Hg-contaminated sites [10–12].

Sulfurized activated carbon (SAC) is generally formed by heating the carbon in the presence of elemental sulfur [13,14] or sulfurous gases [15,16]. SAC provides sulfur-containing functional groups, which show high affinity for Hg compounds to form mercuric sulfide at the surface. Hence, SAC has been verified to further enhance the adsorption capacity of Hg as compared to untreated AC in aqueous adsorption tests [17].

Iron sulfide (FeS) minerals have been widely applied for Hg immobilization on account of the high affinity to Hg ions [18–22]. The mercuric sulfide (HgS) is a stable compound and hardly soluble with a low solubility product constant (Ksp) of 2 <sup>×</sup> 10−<sup>54</sup> for red cinnabar [23] and 4 <sup>×</sup> 10−<sup>54</sup> for black metacinnabar [24,25]. Additionally, FeS can effectively immobilize other divalent metals such as Cd2+, Co2<sup>+</sup>, Zn2<sup>+</sup>, and Ni2<sup>+</sup> through adsorption and coprecipitation [26,27].

The objective of this research attempts to decrease the release of both ionic Hg (Hg2+) and MeHg from sediment to surface water and minimize the negative impacts of Hg contamination on the ecological environment with in-situ thin-layer capping practice, which has been shown to have potentials to decrease Hg contamination in sediment. However, previous application of in-situ capping was primarily focused on systems like contaminated river and lake sediment, there is few studies correlated with wetland and estuary, which are complex systems and effected by tidal flow. Besides, the knowledge of capping material's stability is still limited and easily affected by environmental factors. Therefore, it is necessary to investigate the influence of these environmental factors, such as salinity and dissolved organic material (DOM), on the Hg sorption effectiveness of active capping materials. AC, SAC, and FeS were examined as the capping materials because of their potentially suitable physicochemical properties for Hg sorption. In this study, both aqueous batch sorption experiments and lab-scale vertical up-flow microcosms were conducted to comprehend the impact of environmental factors on the stability of materials and applicability to Hg-contaminated sediment.

#### **2. Materials and Methods**

The test sediment was collected from a Hg-contaminated seawater pond in China Petrochemical Development Corporation, An-Shun, Tainan city, Taiwan, designated as An-Shun site. The sediment within 0-15 cm depth was collected using a stainless crab bucket. For the sediment pretreatment procedures, the sediment was air dried in a hood, and the branches and benthic biotas were removed. The sediment was grounded and sieved through a 20-mesh screen to obtain homogenized sediment. After pretreatment, the sediment was stored at room temperature and covered with a black plastic bag.

Three kinds of sorbents were tested in the experiments. Commercial coconut-shell AC was obtained from Li Jing Viscarb Co. Ltd., Taiwan and sieved to obtain a size range from 18 to 30 mesh. Sieved AC was dried in an oven at 105 ◦C for 24 h. To obtain SAC, the commercial AC was pretreated with elemental sulfur following the protocol described in Hsi et al. [28]. The prepared SAC had the size range and pretreatment conditions the same as the AC and has been examined in our previous study [29]. FeS was purchased from Sigma-Aldrich.
