*3.3. Adsorption Study*

3.3.1. Effects of pH on the Adsorption of Pb(II) by Adsorbent **1**

The effects of the pH value on the adsorption of Pb(II) ions by adsorbents **1** and **2** are shown in Figure 14. The results show an increase in percent adsorption (% ads) for sorption of Pb(II) ions by both adsorbents with increasing pH reaching a maximum of 91.30% and 73.54% at pH 7.0 for adsorbents **1** and **2** respectively.

**Figure 14.** Effects of pH on the adsorption of Pb(II) (mean ± RSD) by adsorbent **1** and adsorbent **2**. (Experimental conditions: Co = 25.52 mg/L, dosage = 0.01 g per 50 mL, shaking time 2 h, mixing rate = 300 rpm; T = 25 ◦C). Each pH measurement was done in triplicate.

From the plot in Figure 14, it can be observed that the Pb(II) ion uptake sharply increased from pH 2.0 to 7.0. At low pH values, the interaction of Pb(II) with adsorbents **1** and **2** decreases because the surfaces of the adsorbents were fully covered by hydronium (H3O<sup>+</sup>) ions, resulting in minimum adsorption. As pH increased, the interaction of Pb(II) with adsorbents **1** and **2** increased, and the adsorption also increased. Hence, the optimum pH for the maximum sorption of Pb(II) was pH = 7.0. Above a pH of 7.0, we observed precipitation of Pb(II) ions in the form of Pb(OH)2 in a control sample (without adsorbents**)**.

3.3.2. Effect of Contact Time and Kinetics for the Adsorption of Pb(II) by Adsorbents **1** and **2**

The kinetics of Pb(II) removal were determined in order to understand the adsorption behavior of adsorbents **1** and **2**. Figure 15 illustrates the Pb(II) ions adsorption on adsorbents **1** and **2** as a function of contact time (0–340 min). As can be seen from Figure 15, the rate of adsorption of Pb(II) ions was fairly rapid in the initial stages (in the first 50 min). After 60 min, the rate of adsorption slowed down as time progressed and reached a constant value around 60 min (equilibrium time). The initial rapid adsorption rate before 50 min could be explained by the fact that at the beginning of the adsorption process, the sites for adsorption were available and open for Pb(II) ions leading to a higher adsorption rate. As time progressed to greater than 60 min, the adsorption of Pb(II) ions slowed down because all binding sites were occupied by Pb(II) ions.

**Figure 15.** Kinetic adsorption of Pb(II) on adsorbent **1** (a) and adsorbent **2** (b) as a function of contact time (pH 6.5 ± 0.2, initial metal concentration = 20 mg/L, dosage = 50 mg/50 mL, mixing rate = 50 rpm, T = 25 ◦C).

3.3.3. Adsorption Kinetics for the Adsorption of Pb(II) by Adsorbents **1** and **2**

The dynamics of the adsorption by the adsorbents **1** and **2** were evaluated using the Lagergren's pseudo-first-order and the McKay and Ho's pseudo-second-order models as defined in Equations (3) and (4), respectively [41,42]:

$$\text{Log } (q\_\varepsilon - q\_t) = -K\_1 \text{ t/2.303 } + \text{Log } (q\_\varepsilon) \tag{3}$$

$$t/q\_t = 1/K\_2 q \varepsilon^2 + t/q\_\varepsilon \tag{4}$$

where q*<sup>e</sup>* is the adsorption capacity (mg g<sup>−</sup>1) at equilibrium, q*<sup>t</sup>* is the adsorption capacity (mg g<sup>−</sup>1) at any time t, k1 is the rate constants of first-order sorption (min−1), and k2 (g mg−<sup>1</sup> min−1) is the rate constant for the pseudo-second-order adsorption.

Figure 16 shows the linear plot of *t*/*qt* versus *t* for the Lagergren's pseudo-second-order model for the adsorption of Pb(II) for adsorbents **1** and **2**, respectively. The equilibrium rate constants of the pseudo-second-order model (k2) were 1.23 <sup>×</sup> 10−<sup>4</sup> and 1.03 <sup>×</sup> 10−<sup>3</sup> g mg−<sup>1</sup> min−<sup>1</sup> for Pb(II) for adsorbents **1** and **2,** respectively (Table 4). The pseudo-second-order model (Equation (4)) fit the experimental data very well with a correlation coefficient (R2) of 0.99, which was close to unity as shown in Table 4 and Figure 16. On the other hand, our calculations demonstrated that the resulting experimental data were not fitted to the pseudo-first-order model. Therefore, the adsorption kinetics of Pb(II) ions on the three adsorbents showed that the Pb(II) adsorption followed a second-order reaction. Moreover, the experimental values of qe (Exp.) for both adsorbents were very close to the calculated values qe (Cal.) measured from the pseudo-second-order equation.

**Figure 16.** Lagergren pseudo-second-order kinetics adsorption for Pb(II) onto adsorbents **1** and **2** (Experimental conditions: pH 6.5 ± 0.2, initial metal concentration = 20 mg/L, dosage = 50 mg/50 mL, mixing rate = 50 rpm, T = 25 ◦C).

**Table 4.** Coefficients of sorption kinetics (Lagergren's pseudo-second-order model) for Pb (II) removal by adsorbents **<sup>1</sup>** and **<sup>2</sup>** and their correlation coefficient (R2). (Experimental conditions: pH 6.5 <sup>±</sup> 0.2, initial metal concentration = 20 mg/L, 50 mg/50 mL, mixing rate = 50 rpm, T = 25 ◦C).


Furthermore, the effect of temperature on the adsorption behavior for Pb(II) ions was also investigated. The effect of temperature on Pb(II) ions adsorption by adsorbents **1** and **2** is shown in Figure 17. The removal of Pb(II) ions by both adsorbents increased slightly with increasing solution temperature from 25 to 50 ◦C. This result suggested that the adsorption mechanism associated with Pb(II) ions on both adsorbents involved a temperature-dependent process to some extent. The adsorbent's weak sensitivity to temperature is essential to practical applications, thereby enabling both synthesized adsorbents to be potentially applied to the practical treatment of Pb(II) ions at room temperature.

**Figure 17.** Effect of the temperature on Pb (II) ion adsorbtion (mean ± RSD) on adsorbent **1** and adsorbent **2**. (pH 6.5 ± 0.2, initial metal concentration = 20 mg/L Pb(II), dosage = 50 mg/50 mL, at different temperatures for 60 min). Each measurement has been done in triplicate.

### 3.3.4. Adsorption Isotherms

The adsorption isotherms were used to determine the affinity of adsorbents **1** and **2** to Pb(II) ions. The amount of Pb(II) ion per gram of adsorbent (*qe*) was defined as shown in Equation (1) above. The sorption of Pb(II) into adsorbents **1** and **2** was described by Langmuir [43,44] and Freundlich [45] models:

• The Langmuir Isotherm Model [43]

$$\mathbf{q}\_{\rm e} = \mathbf{q}\_{\rm m} \,\mathbf{K}\_{\rm L} \,\mathbf{C}\_{\rm e} / 1 + \mathbf{K}\_{\rm L} \,\mathbf{C}\_{\rm e} \tag{5}$$

where *qe* is the is the lead concentration adsorbed per specific amount of adsorbent (mg g<sup>−</sup>1), *Ce* is the equilibrium concentration of Pb(II) expressed in mg L<sup>−</sup>1, and qm is the maximum amount of lead ions required to form a monolayer (mg g<sup>−</sup>1). The Langmuir equation can be rearranged to a linear form (Equation (6)) for the convenience of plotting and determination of the Langmuir constant (KL, the first coefficient related to the energy of adsorption) as below. The values of qm and KL can be determined from the linear plot of 1/Ce versus 1/*q*e:

$$1/\text{q}\_{\text{e}} = 1/\text{q}\_{\text{Im}} + 1/\text{K}\_{\text{L}} \text{ qm} \times 1/\text{C}\_{\text{e}} \tag{6}$$

The dimensionless equilibrium parameter or separation factor (R*L*) could be expressed as in the following equation [44]:

$$R\_L = 1/1 + K\_L C\_0 \tag{7}$$

where C0 is the initial concentration of the lead ion, and K*<sup>L</sup>* is the Langmuir constant. The value of R*<sup>L</sup>* is between 0 and 1 for favourable adsorption, while R*<sup>L</sup>* > 1 represents unfavorable adsorption, and R*<sup>L</sup>* = 1 represents linear adsorption. The adsorption process is irreversible if R*<sup>L</sup>* = 0.

• The Freundlich Isotherm Model [45]

$$q\_{\mathcal{C}} = K\_F \subset\_{\mathcal{C}}^{1/n} \tag{8}$$

$$\text{Log q}\_{\text{f}^\circ} = \text{Log K}\_{\text{f}^\circ} + (1/\text{n})\,\text{Log C}\_{\text{c}} \tag{9}$$

where, *qe* is the equilibrium uptake capacity (mg g<sup>−</sup>1) and *Ce* is the equilibrium concentration of Pb(II)) expressed in mg L<sup>−</sup>1. kF is the first coefficient related to the energy of adsorption, and n is a coefficient. Both constants kF and n can be calculated from the plot of log *qe* vs. log Ce.

\* The coefficients were calculated from linearized forms of sorption isotherms models Equations (6) and (9).

### 3.3.5. Linear Fitting of the Isotherm Models

Linear fitting of the Langmuir and Freundlich isotherm models is shown in Figures 18 and 19 respectively; the corresponding parameters of sorption isotherm models calculated are illustrated in Table 5. From the results shown in Table 5, the adsorption process of Pb(II) on both adsorbents can be described by the linear form of the Langmuir Isotherm Model, which produced higher R<sup>2</sup> values of 0.959 and 0.983 for Pb(II) onto adsorbents **1** and **2,** respectively, compared to the Freundlich isotherm model, which produced low R<sup>2</sup> values of 0.902 and 0.871 for Pb(II) onto adsorbents **1** and **2** respectively. Therefore, the adsorption isotherms for both adsorbents fitted well with the Langmuir model more than with Freundlich model; hence, the sorption process of Pb(II) ions involved in both adsorbents occurred by chemical complexation (chemisorption). The separation factor R*<sup>L</sup>* (Equation (7)) was found to be between 0 and 1 (Table 5) indicating a favorable adsorption process [44].

**Figure 18.** The Langmuir Isotherm Model adsorption for Pb(II) onto (**A**) adsorbent **1** and (**B**) adsorbent **2**. (Experimental conditions: dosage = 0.015 g (adsorbent **1**) and 0.1 (adsorbent **2**) per 50 mL; T = 25 ± 1 ◦C; contact time = 60 min.; pH = 6.5 ± for Pb(II).

**Figure 19.** The Freundlich Isotherm Model adsorption for Pb(II) onto (**C**) adsorbent **1** and (**D**) adsorbent **2**. (Experimental conditions: dosage = 0.015 g (adsorbent **1**) and 0.1 (adsorbent **2**) per 50 mL; T = 25 ± 1 ◦C; contact time = 60 min.; pH = 6.5 ± for Pb(II).


**Table 5.** Coefficients of two different sorption isotherm models for Pb(II) removal by adsorbents **1** and **2** and their correlation coefficient (R2). (Experimental conditions: dosage = 0.015 g (adsorbent **1**) and 0.1 (adsorbent **2**) per 50 mL; T = 25 ± 1 ◦C; contact time = 60 min.; pH = 6.5 ± 0.2 for Pb(II).

a. The Langmuir Isotherm Model

b. The Freundlich Isotherm Model

3.3.6. Complexation of Pb(II) by Adsorbent **1**

Adsorbent **1** showed adsorption of Pb(II) ions due to the presence of Schiff base functionality (azomethine (C=N), carboxylate oxygen, and active thiol and silanol groups on the surface), whereas adsorbent **2** adsorbs Pb(II) ions due to the presence of active thiol and silanol groups only on the surface. Adsorbent **1** showed better adsorption of Pb(II) ions, which may be attributed to the presence of potentially good coordinating Schiff base sites and active carboxylate oxygen, in addition to thiol and hydroxyl groups. The shift of asymmetric stretching vibration of νas(COO−) of adsorbent **1** at 1665 cm−<sup>1</sup> to a lower wave number 1642 cm−<sup>1</sup> and the disappearance of the thiol S-H peak at 2556 cm−<sup>1</sup> after Pb(II) ion was loaded confirmed the chemical complexation of Pb(II) by adsorbent **1** (Figure 2b). The FTIR results (Figure 2b) support the structures proposed for Pb(II) ion interaction with adsorbent **1** (complex) during the adsorption process [46]. The Pb (II) ions interaction with adsorbent **1** is shown in Figure 20 below.

**Figure 20.** Pb(II) ion adsorbent **1** complex formation.
