*3.5. Effect of Contact Time on the Adsorption of Hg2+*

To investigate the effect of contact time on the removal of Hg2+ from aqueous media using FA48, the duration was varied from 0.5 to 48 h (Figure 5). Rapid adsorption was observed within 0.5 h from the start of the adsorption process, following which the rate of adsorption of Hg2+ fluctuated with increase in adsorption time. Finally, adsorption equilibrium was achieved at approximately 3 h under our experimental conditions. In this study, the adsorption might be mainly attributed to two factors: the interaction between Hg2+ and active adsorption sites, such as specific surface area and pore volume (mentioned in Section 3.2), and ion exchange with K<sup>+</sup> in the interlayer of FA48 (mentioned in Section 3.3).

**Figure 5.** Effect of contact time on the adsorption of Hg2+ onto FA48. Initial concentration: 50 mg/L, sample volume: 50 mL, adsorbent: 0.01 g, temperature: 25 ◦C, contact time: 0.5, 1, 3, 6, 12, 21, 24, 30, 42, and 48 h, agitation speed: 100 rpm.

In addition, to evaluate the kinetic adsorption mechanism of Hg2+ using FA48, pseudofirst-order and pseudo-second-order models were selected to interpret the kinetics data using Equations (5) and (6) [36,38].

$$\ln(q\_{e,\text{exp}} - q\_t) = \ln q\_{e,\text{cal}} - k\_1 t \tag{5}$$

$$\frac{t}{q\_t} = \frac{t}{q\_{\varepsilon, \text{cal}}^2} + \frac{1}{k\_2 \times q\_{\varepsilon, \text{cal}}^2} \tag{6}$$

where *qe,exp* and *q<sup>t</sup>* are the quantities of Hg2+ adsorbed at equilibrium and at time *t* (mg/g), respectively, *qe,cal* is the quantity of Hg2+ adsorbed in the calculation (mg/g), *k*<sup>1</sup> (1/h) and *k*<sup>2</sup> (g/mg/h) are the rate constants of the pseudo-first-order and pseudo-second-order models, respectively. The calculated results are shown in Table 5.

**Adsorbents** *qe,exp* **Pseudo-First-Order Model Pseudo-Second-Order Model** *k***1 (1/h)** *qe,cal* **(mg/g)** *<sup>r</sup> k***2 (g/mg/h)** *qe,cal* **(mg/g)** *<sup>r</sup>* FA48 12.42 0.02 2.34 0.515 0.085 11.7 0.996

**Table 5.** Kinetic parameters for the adsorption of Hg2+ using FA48.

From Table 5, it is evident that the correlation coefficient (*r*) in the pseudo-second-order model (0.996) was significantly higher than the pseudo-first-order model (0.515), indicating that the pseudo-second-order model is more suitable for describing the adsorption kinetics of Hg2+ in this study. Additionally, the value of *qe,exp* was closest to the value of *qe,cal* of the pseudo-second-order model than that of the pseudo-first-order model. In addition, it is strongly suggested that the adsorption of Hg2+ onto FA48 is because of chemisorption, as assumed by this model [45,46].

In addition, the Elovich model (Equation (7)) was also used to describe adsorption kinetic in this study. This model describes activated adsorption, and predicts an energetically heterogeneous solid surface of adsorbent which means adsorption kinetics is not affected by interaction between the adsorbent particles [26].

$$q\_t = 1/\beta \ln \left( \alpha \beta \right) + 1/\beta \ln t \tag{7}$$

where *q<sup>t</sup>* is the quantity of Hg2+ adsorbed at time *t* (mg/g), *α* is the initial adsorption rate (mg/g/h), *β* is the related to the extent of surface coverage and activation energy for chemisorption (g/mg).

From the result, the value of *<sup>α</sup>*, *<sup>β</sup>*, and *<sup>r</sup>* (correlation coefficient) was 8.4 <sup>×</sup> <sup>10</sup><sup>3</sup> mg/g/h, 1.1 g/mg, and 0.888, respectively. The Elovich equation is suitable to describe adsorption behavior of Hg2+ using FA48 that relates to the nature of chemical sorption under our experimental conditions [47].
