2.2.2. Sample Preparation


#### 2.2.3. Adsorption Process

The samples were put into the reciprocating water bath shaker and shaken at 130 rpm at 30 ◦C for 24 h to achieve an adsorption equilibrium.

In order to determine the suitable dosage of sorbents in the following experiments, the dosages 5, 10, 15, 20, 25, 30, 40 and 50 mg were tested with the Hg concentration of 74 μg/L in 50 mL solution. To determine the adsorption isotherms of sorbents, 4.3, 25, 56, and 135 μg/L Hg2<sup>+</sup> were tested with 20 mg of sorbents in 50 mL solution.

In order to study the influence of salinity on Hg removal, the Hg2<sup>+</sup> solutions (Hg concentration = 197.1 ± 10 μg/L) with three different salinity levels of artificial waters were tested in this study, including freshwater (pH = 7.6 ± 0.1), estuary water (pH = 8.2 ± 0.1), and seawater (pH = 8.3 ± 0.1). The compositions of freshwater and seawater are listed in Table S1. The freshwater was prepared according to Lewis et al. [34]; the seawater was prepared according to Kester et al. [35]. The estuary water was prepared by mixing fresh water and seawater at a 1:1 volume ratio. The concentrations of ion species listed in Table S1 were confirmed by ion chromatography (Metrohm 792 Basic IC, Herisau, Switzerland). Hg(II) species at various salinity levels were simulated by using MINTEQ 3.1 and listed in Table S2.

To evaluate the effect of DOM on sorbents' Hg removal efficiency, the DOM solution was prepared by humic acid (HA, Sigma Aldrich, Saint Louis, MO, USA). HA of 10 mg was dissolved in ultrapure water and pre-adjusted to pH 7 by 0.1 M NaOH. The solution was stirred for 1 h to promote fast dissolution, and then filtered through a 0.2 μm mixed cellulose ester filter (DISMIC-25AS, Toyo Roshi Kaisha, Tokyo, Japan). The filtrate was collected and stored at 4 ◦C refrigerator before using. The actual concentration of DOM was verified by a Total OC analyzer (OI Analytical Aurora 1030W, College Station, TX, USA). The DOM was controlled at a concentration of approximately 2.5 mg-C/L and the Hg concentration is 196.2 ± 5 μg/L.

2.2.4. Preservation of Sample and Analysis


$$\mathcal{R} = \frac{\mathcal{C}\_0 - \mathcal{C}\_t}{\mathcal{C}\_0} \times 100\% \,\tag{1}$$

where R (%) is the Hg removal efficiency of sorbents, C0 (ng/L) is the initial Hg concentrations detected in blank solution, and Ct (ng/L) is the concentration of remaining Hg at any time.

Equation (2) was used to determine the partitioning coefficient (KD) for Hg adsorption by the adsorbent:

$$\mathbf{K}\_{\rm D} = \frac{\mathbf{q}\_{\rm e}}{\mathbf{C}\_{\rm e}} \tag{2}$$

where qe is the equilibrium Hg adsorption capacity (mg/g) and Ce is the equilibrium Hg concentration (μM). The calculated values of KD were listed in Table S3.
