*3.4. Effect of Solution pH on the Adsorption of Hg2+*

In aqueous media, pH is one of the most important parameters for heavy metal removal. Thus, the solution pH strongly and directly affects the degree of metal ionization and/or metal binding on the adsorbent surface sites [42]. In this study, the quantity of Hg2+ adsorbed onto FA24 increased with an increase in the solution pH from 2 to 5, and decreased with further increase in pH of 5 to 7 (Figure 4). First, when the solution pH is below 3, Hg2+ is the dominant species; when the solution pH is over 5, Hg(OH)<sup>2</sup> is the dominant species [43,44]. In addition, Hg(OH)<sup>+</sup> exists (1–13% of the total mercury Hg2+) when the solution pH is between 2 and 6. Previous studies have reported that there are several stable Hg2+ products related to either the equilibrium hydrolysis (such as Hg(OH)<sup>3</sup> −, Hg(OH)2, Hg(OH)<sup>+</sup> ) or to the complexation equilibrium with chloride (such as HgCl<sup>4</sup> <sup>2</sup>−, HgCl<sup>3</sup> −, HgCl2, HgCl<sup>+</sup> ). Moreover, mixed species such as Hg(OH)Cl exist in aqueous media [44]. In an acidic solution (pH 2), the FA48 surface is protonated and the electrostatic repulsion between the FA48 surface (positive charge) and Hg2+ species (positive charge) such as Hg(OH)<sup>+</sup> and HgCl<sup>+</sup> easily occurred, resulting in low Hg adsorption. Additionally, the pHpzc value of FA48 was 10.4 in this study, which supports the availability of positive charge on the FA48 surface and the low adsorption of Hg2+ from aqueous media. Next, similar to pH 2, at pH 5, the quantities of Hg2+ species such as Hg(OH)<sup>+</sup> and HgCl<sup>+</sup> decreased. Conversely, HgCl<sup>3</sup> species increased in aqueous media. Therefore, the quantity of Hg2+ adsorbed increased because of the electrostatic interaction between the FA48 surface (positive charge) and Hg2+ species such as HgCl<sup>3</sup> − (negative charge). Finally, when the solution pH was over 7, the quantity of Hg(OH)<sup>2</sup> increased, and the hydroxyl ion (OH−) also increased in the sample solution media [6,43]. Therefore, FA48 showed a low adsorption capability for Hg2+ under alkaline conditions.

**Figure 4.** Effect of pH on the adsorption of Hg2+ onto FA48. Initial concentration: 10, 30, and 50 mg/L, sample volume: 50 mL, adsorbent: 0.01 g, temperature: 25 ◦C, contact time: 24 h, 100 rpm.
