*3.2. Adsorption of Hg2+*

Figure 1 shows the quantity of Hg2+ adsorbed by the FA series. The adsorbed Hg2+ was in the order CFA, FA1 (0–0.48 mg/g) < FA3 (2.2 mg/g) < FA6 (3.5 mg/g) < FA12 (4.0 mg/g) < FA24 (7.5 mg/g) < FA48 (11.6 mg/g) in the current experiment conditions. The adsorption capability of Hg2+ using the FA series depended on the duration of the hydrothermal activation treatment using potassium hydroxide solution. Next, we evaluated

the relationship between the adsorption capacity of Hg2+ and the physicochemical properties of the FA series. As a result, the positive correlation coefficient between the quantity of Hg2+ adsorbed and CEC, specific surface area, and pore volume (*d* 5 20 Å) were 0.928, 0.659, and 0.882, respectively. These results indicate that CEC and pore volume strongly affect the adsorption of Hg2+ from aqueous solutions. Additionally, in this study, FA48 was selected to evaluate the adsorption capability for Hg2+ removal from aqueous solutions.

**Figure 1.** Quantity of Hg2+ adsorbed onto FA series. Initial concentration: 50 mg/L, sample volume: 50 mL, adsorbent: 0.01 g, temperature: 25 ◦C, contact time: 24 h, agitation speed: 100 rpm, pH: 3.0.

A comparison of the Hg2+ adsorption capability of FA48 with that of the other adsorbents is listed in Table 2 [24,26,32–35]. FA48 exhibited potential in Hg2+ adsorption from aqueous solutions compared to other reported adsorbents (except for coal gangue and multifunctional mesoporous material).


**Table 2.** Comparison of Hg2+ adsorption capacity of FA48 with other reported adsorbents.

*3.3. Adsorption Isotherms of Hg2+*

Figure 2 shows the adsorption isotherms of Hg2+ using FA48 at different temperatures. The quantity of Hg2+ adsorbed using FA48 did not significantly vary with different

temperatures. Therefore, in this study, the adsorption temperature did not strongly affect the adsorption capability of FA48.

**Figure 2.** Adsorption isotherms of Hg2+ at different temperatures. Initial concentration: 10, 20, 30, 40, and 50 mg/L, sample volume: 50 mL, adsorbent: 0.01 g, temperature: 7, 25, and 45 ◦C, contact time: 24 h, agitation speed: 100 rpm.

Additionally, to investigate the adsorption properties and interactions, the adsorption isotherm data were evaluated using the Freundlich and Langmuir isotherm models. The Freundlich isotherm model was applied to multilayer adsorption, while the Langmuir isotherm model showed monolayer adsorption at specific homogenous sites [24].

The Freundlich isotherm model can be represented as follows [36]:

$$
\log q = \frac{1}{n} \log \mathbb{C} + \log K\_F \tag{2}
$$

where *q* is the quantity of Hg2+ adsorbed (mg/g), *K<sup>F</sup>* and 1/*n* are the Freundlich isotherm constants, *C* is the equilibrium concentration (mg/L). In general, the adsorption reaction in the aqueous phase fits this model. In the Freundlich isotherm model, the isotherm curve depends on the value of *n*. In particular, when the value of 1/*n* is 0.1–0.5, adsorption occurs easily, when 1/*n* is over 2, it is difficult to adsorb [37].

The Langmuir isotherm model can be represented as follows [38]:

$$\frac{1}{q} = \frac{1}{q\_{\text{max}}} + \left(\frac{1}{K\_L q\_{\text{max}}}\right) \left(\frac{1}{\mathcal{C}}\right) \tag{3}$$

where *K<sup>L</sup>* is the Langmuir isotherm constant (L/mg) and *q*max is the maximum quantity adsorbed (mg/g). The Langmuir isotherm model is a theoretical model that can explain monolayer adsorption onto homogenous surfaces. In addition, this model considers adsorption sites.

Table 3 shows the Freundlich and Langmuir model constants for the adsorption of Hg2+ using FA48. The obtained data fitted both models (correlation coefficient of the Freundlich and Langmuir equations were ≥ 0.960 and ≥ 0.904, respectively). The maximum quantity adsorbed at 7 to 45 ◦C was not significantly different in this study, which is supported by the adsorption isotherm data in Figure 2. In addition, the value of 1/*n* was from 0.27 to 0.33 in this study. Therefore, the adsorption of Hg2+ using FA48 from aqueous solutions is more favorable.


**Table 3.** Freundlich model and Langmuir model constants for the adsorption of Hg2+ .

Finally, adsorption properties were evaluated using Sips equation (Equation (4)). The Sips model was derived from the Langmuir and Freundlich equations. This model predicts the heterogeneous adsorption system and overcoming the drawback associated with Freundlich model [39]. The Sips equation was expressed as follows:

$$\frac{1}{q\_{\varepsilon}} = \frac{1}{Q\_{\text{max}} K\_{\text{S}}} \left( \frac{1}{\mathcal{C}\_{\varepsilon}} \right)^{1/n} + \frac{1}{Q\_{\text{max}}} \tag{4}$$

where *K<sup>S</sup>* is the Sips equilibrium constant (L/mg), *Q*max is the maximum quantity adsorbed (mg/g). *n* is the Sips model exponent, which can be employed to describe the system's heterogeneity. If the value of *n* is equal to 1, this equation will become a Langmuir equation. It means a homogeneous adsorption process [40,41].

Table 4 shows the Sips model constants for the adsorption of Hg2+. The value of correlation coefficient of Sips equation was from 0.841 to 0.959 under our experimental conditions. The values of *Q*max at 7–45 ◦C was not significantly changed, which is similar trends to the adsorption isotherm data (Figure 2). In addition, the heterogeneous factor values (*n* = 0.4–1.1) indicate that heterogeneous adsorption process is related to the adsorption mechanism of Hg2+ using FA48.

**Table 4.** Sips model constants for the adsorption of Hg2+ .


Moreover, to evaluate the adsorption mechanism of Hg2+ using FA48, more detailed investigations were conducted in this study (Figure 3). First, the relationship between the quantity of Hg2+ adsorbed and the quantity of K<sup>+</sup> released from FA48 was evaluated in this study. As a result, the correlation coefficient value (*r*) was positive at 0.946, indicating that ion exchange with K<sup>+</sup> in the interlayer of FA48 was one of the mechanisms of Hg2+ adsorption from aqueous media. As mentioned in Section 3.2, the positive correlation coefficient between the quantity of Hg2+ adsorbed and the value of CEC was 0.928. These trends were similar to those reported in previous studies [28,32]. Additionally, the X-ray photoelectron spectroscopy analysis was conducted in this study. The peak intensity of Hg(5p) at 67 eV was newly detected after the adsorption of Hg2+, indicating that Hg2+ was present on the FA48 surface after adsorption, and was not detected before adsorption. Generally, Hg(4f) peaks at 101 and 105 eV were detected after adsorption. However, Si(2p) and Hg(4f) peaks overlapped in this study. Therefore, it was difficult to elucidate and/or detect these peaks in our experiments.

**Figure 3.** Relationship between the quantity of Hg2+ adsorbed and K<sup>+</sup> released (**A**) and the X-ray photoelectron spectroscopy analysis before and after adsorption of Hg2+ (**B**). Initial concentration: 50 mg/L, sample volume: 50 mL, adsorbent: 0.01 g, temperature: 25 ◦C, contact time: 24 h, agitation speed: 100 rpm.
