3.2.2. Effect of Initial Concentration of Hg2<sup>+</sup>

The batch experiments were tested within the initial concentration of Hg2<sup>+</sup> from 5 to 135 μg/L using a diluted standard Hg2<sup>+</sup> solution. The Hg sorption capacity of sorbents and the initial Hg2<sup>+</sup> concentration is shown in Figure 3. The experimental results showed that the Hg sorption capacity of sorbents increased with the increment of the initial concentration of Hg2+, with linear adsorption behaviors within the test concentration range. The FeS could maintain a high Hg removal efficiency of up to 90% in the range of the initial concentration of Hg2<sup>+</sup> given, which illustrates that FeS is an excellent sorbent for Hg removal followed by SAC and AC.

**Figure 3.** Hg sorption at different Hg2<sup>+</sup> concentrations.

#### 3.2.3. Effect of Salinity

The salinity level of water in the Hg-contaminated site may affect the Hg removal ability of sorbents. Thereby, three artificial synthetic waters, including freshwater, estuary water, and seawater with different salinity levels, were prepared to study the effects of salinity on Hg removal ability of sorbents. The Hg removal efficiency of sorbents for each water system are presented in Figure 4, indicating that FeS had the largest Hg removal efficiency, followed by SAC and then AC. From the lowest salinity (freshwater) to the highest salinity level (seawater), the Hg removal efficiency increased for both AC and SAC. Although the effect of salinity levels on FeS was insignificant, the Hg removal efficiency of FeS was still the highest as compared to AC and SAC. The calculated KD values are listed in Table S3, which shows that KD values increased as the salinity increased. FeS also performed the largest KD values, followed by SAC and then AC.

**Figure 4.** Comparison of various salinity levels affecting the sorbents' Hg removal.

Notably, our results were contrary to those in previous studies, which have shown that a high level of salinity may decrease the removal efficiency of Hg by sorbents [38,39]. The previous studies have demonstrated that an increase in NaCl concentration would decrease the sorption efficiency of AC and kaolin. However in our research, the different salinity levels were prepared by adding various salts to form the artificial waters. Hence, the complexation and species of Hg in this study may be more complicated than the cases with only presence of NaCl. Table S2 displays the simulated speciation of Hg(II) compounds, indicating that the fraction of Hg(OH)2 in the freshwater system (i.e., 95.97%) was significantly higher than that in both systems of estuary water (i.e., 0.094%) and seawater (i.e., 0.014%). Hg(OH)2 has been reported that it could easily decompose to the elemental form as Hg0, Hg<sup>0</sup> is more difficult to be captured from the water solution than other oxidized form because of its extremely low solubility (i.e., 5.6 <sup>×</sup> 10−<sup>5</sup> g/L) [29]. Additionally, Thiem et al. [40] showed that the addition of calcium ion would enhance Hg removal of AC. They speculated that the calcium ion may react with the surface group on AC to form a new adsorption site, leading to an increment of Hg removal capacity in the solution. Besides, according to the KD values, the increase of salinity has a positive effect on the partition behavior of aqueous Hg to the adsorbent; furthermore, FeS has a fabulous capability for converting Hg from the liquid phase to the solid phase.
