3.2.5. SEM of Adsorbents **1**

The SEM images of adsorbent **1** are shown in Figure 5. As revealed in Figure 6, adsorbent **1** showed uniform distribution of spherical particles with different sizes. The presence of the organic components (Schiff base and propyl thiol groups) on the overall surface caused the surface of adsorbent **1** to be rough as a result of the microporous structure. Furthermore, the EDX pattern of adsorbent **1** is shown in Figure 6. EDX data in Table 1 confirm the formation of adsorbent **1,** as shown by the Si, S, N, C, and O peaks, thus confirming the presence of organic components (Schiff base and propyl thiol groups) on the silica surface.

**Figure 5.** Scanning electron microscopy (SEM) images of pure adsorbent **1** before adsorption.

**Figure 6.** Energy-dispersive X-ray spectroscopy (EDX) spectrum of adsorbent **1** before adsorption.


**Table 1.** (EDX) data of adsorbent **1** before adsorption.

Figure 7 presents an SEM micrograph of Pb(II) ions loaded onto adsorbent **1**. SEM analysis revealed that adsorbent **1** displayed agglomerated spherical particles with different sizes and granular morphologies. The accompanying EDX spectrum of the silica-functionalized Schiff base shown in Figure 8 confirms the presence of Pb(II) ions on the surface of adsorbent **1**. The EDX data in Table 2 confirmed that adsorbent **1** was very effective in removing Pb(II) ions from aqueous solutions. The EDX results were in good agreement with the results obtained by inductively coupled plasma mass spectrometry (ICP-MS).

**Figure 7.** SEM images of adsorbent **1** after adsorption.



**Figure 8.** (EDX) spectrum of adsorbent **1** after adsorption.

Figure 9 shows the field emission scanning microscope (FESEM) image of adsorbent **2**. As shown in Figure 10, the hybrid is spherical with a rough surface. The SEM image showed a homogeneous material with pores covering the entire structure. The pore size varied between 2 and 8 μm. The presence of the overall surface of the propyl thiol groups caused the surface of adsorbent **2** to be rough with a microporous structure. Moreover, the EDX pattern of adsorbent **2** (Figure 10) confirms the purity of adsorbent **2** and also confirms the presence of propyl thiol groups on the silica surface. The EDX spectrum also presents a minor peak of Au and Cl that resulted from the thin film of gold deposited on the insulating sample in order to make it conductive and, hence, easy to visualize using SEM.

**Figure 9.** Field emission scanning electron microscopy (FESEM) images of adsorbent **2** before adsorption.

**Figure 10.** EDX images of adsorbent **2** (inset is the EDX data of adsorbent **1** before adsorption).

Figure 11 presents an SEM micrograph of Pb(II) ions loaded onto adsorbent **2**. SEM analysis revealed that adsorbent **2** displayed spherical particles with different sizes and rough morphology. The accompanying EDX spectrum of the silica-functionalized Schiff base, shown in Figure 12, confirms the presence of Pb(II) ions on the surface of adsorbent **2** (see Table 3). However, the EDX spectrum confirmed that adsorbent **2** was also effective in removing Pb(II) ions from aqueous solutions. EDX studies have shown that both adsorbents (**1** and **2**) have strong affinities to divalent metal cations (Pb(II)).

**Figure 11.** SEM images of adsorbent **2** after adsorption.

**Figure 12.** EDX spectrum of adsorbent **2** after adsorption.


**Table 3.** EDX data of adsorbent **2** after adsorption.

### 3.2.7. TGA of Adsorbent **1** and **2**

The TGA thermal details of adsorbents **1** and **2** are presented in Figure 13. The thermogram of adsorbent **1** (Figure 13a) shows that there are essentially two regions of weight loss; in the first region, a slight weight lost in the range of 45 to 285 ◦C due to the loss of physiosorbed solvents used in the preparation (2.85%) can be seen. The second region appears in the range of 285 to 475 ◦C (with lost weight equal to 28.1% and with DTGmax at 346 ◦C) due to decomposition of the Schiff base and propyl-thiol groups immobilized on the silica surface [39]. The remaining weight loss in the range of 475 to 885 ◦C (with total weight loss equal to 26.23%) was due to the condensation of the hydroxyl group silanol (Si-O-OH) to yield siloxane bonds (Si-O-Si) [38]. At 885 ◦C, adsorbent **1** around 57.18% was degraded, leaving 42.82% behind as residue.

### 3.2.8. Surface Properties of Adsorbents **1** and **2**

Using nitrogen adsorption isotherms, the surface area, pore volume, and pore size of adsorbents **1** and **2** were measured. It is noted that adsorbent **1** exhibited a BET surface area of 22.0452 m2/g, the pore volume **(**BJH) of adsorbent **1** was 0.120986 cm3g−<sup>1</sup> with pore sizes of 15.7644 nm (BET) and 22.7447 nm (BJH). On the other hand, adsorbent **2**, which was prepared by direct hydrolysis and condensation of 3-(triethoxysilyl) propane-1-thiol, exhibited a BET surface area of 9.3475 m2g−1. The pore volume **(**BJH) of adsorbent **2** was 0.049107 cm3g−<sup>1</sup> with pore sizes of 9.68719 nm (BET) and 25.0027 nm (BJH). The reduced surface area and pore volume of adsorbent **2** were due to the smaller size of the propyl-thiol group. The noticeable increase in the BET surface area of adsorbent **1** was

attributed to the Schiff base and propyl-thiol groups anchored on the silica framework. This indicates that adding more organic moieties to silica framework results in a large surface area, which increases the possibility of extracting metal ions [40].

**Figure 13.** Thermogravimetric (TGA) curve of (a) adsorbent **1** and (b) adsorbent **2**.
