*2.3. Effect of pH, Temperature, and Contact Time on the Adsorption of Hg2+*

First, in order to evaluate the effect of pH, FA48 (0.01 g) was added to 50 mL of the Hg2+ solution at 10, 30, 50 mg/L. The pH of the solution was adjusted to 2, 5, 7, 9, 11 using either nitric acid or sodium hydroxide solutions. The suspension was shaken at 100 rpm at 25 ◦C for 24 h, and filtered using a 0.45 µm membrane filter. Second, in order to evaluate the temperature effect, FA48 (0.01 g) was added to a 50 mL Hg2+ solution at 10, 20, 30, 40, 50 mg/L, and the suspension was shaken at 100 rpm at 7, 25, 45 ◦C for 24 h. The 7 ◦C of solution was prepared as follows. The sample solution was set at 5 ◦C in a water bath shaker personal-11 (TAITEC Co., Nagoya, Japan) in the low-temperature room at 6 ◦C. Finally, to evaluate the effect of contact time, FA48 (0.01 g) was added to 50 mg/L Hg2+ solution (50 mL). The suspension was shaken at 100 rpm and 25 ◦C for 0.5, 1, 3, 6, 12, 18, 21, 24, 30, 42, 48 h. The amount of Hg2+ adsorbed was calculated as described in Section 2.2. In addition, to evaluate the Hg2+ adsorption mechanism, the concentration of potassium ions released from FA48 in the adsorption isotherm experiment was measured using an iCAP-7600 Duo (Thermo Fisher Scientific Inc., Osaka, Japan).
