*2.2. Amount of Hg2+ Adsorbed Using FA Series*

Approximately 0.01 g of each pretreated adsorbent, namely CFA, FA1, FA3, FA6, FA12, FA24, and FA48, was mixed with 50 mL of 50 mg/L Hg2+ solution. Subsequently, the reaction mixture was shaken at 100 rpm and 25 ◦C for 24 h. The resulting sample was filtered through a 0.45 µm membrane filter. The concentration of Hg2+ was measured using an inductively coupled plasma optical emission spectrometer (iCAP-7600 Duo, Thermo Fisher Scientific Inc., Osaka, Japan). The quantity of Hg2+ adsorbed was calculated using the levels before and after adsorption in Equation (1).

$$q = \frac{(\mathbb{C}\_0 - \mathbb{C}\_\varepsilon)V}{W} \tag{1}$$

where *q* is the quantity adsorbed (mg/g); *C*<sup>0</sup> is the initial concentration (mg/L); *C<sup>e</sup>* is the equilibrium concentration (mg/L); *V* is the solvent volume (L); and *W* is the weight of the adsorbent (g).
