*3.1. Synthesis of Adsorbents 1 and 2*

The aim of this work was to improve the sorption efficiency of silica for removal of toxic heavy metals from polluted water. For this purpose, two mesoporous silica materials (adsorbents) were prepared. Adsorbent **1** was silica functionalized with Schiff base-propyl-thiol and adsorbent **2** was silica functionalized with propyl-thiol. The Schiff base was synthesized from 2-amino-4,5-dimethoxybenzoic acid and 4-hydroxybenzaldehyde via a condensation reaction in ethanol using acetic acid as a catalyst (Scheme 1). The Schiff base purity was confirmed by FTIR (Figure 1), 1H and 13C NMR spectroscopy (See Supporting Information; Supplementary Materials, Figures S1 and S2). The synthetic route of adsorbent **1** is shown in Scheme 2, the organic precursor 3-(triethoxysilyl) propane-1-thiol was added to a suspension containing Schiff base 2-(4-hydroxybenzylideneamino)-4,5-dimethoxybenzoic acid), ethanol, water and ammonia as a catalyst to obtain Schiff base-thiol silica gel (adsorbent **1**) through the sol-gel reaction. The silica functionalized with propyl-thiol (adsorbent **2**) was prepared by co-condensation (direct synthesis) of the organic precursor 3-(triethoxysilyl) propane-1-thiol in the presence of ammonia as a catalyst (Scheme 3).

**Figure 1.** FTIR spectra of (a) free Schiff base and (b) adsorbent **1**.

The two adsorbents (**1** and **2**) were isolated as yellowish and white powders, respectively. The hybrid materials were washed several times with water, ethanol, and dried under vacuum before characterization. The synthesized adsorbents were completely characterized by FTIR, XRD, SEM, energy-dispersive X-ray spectroscopy (EDX), TGA, and BET analyses.

### *3.2. Characterization*

### 3.2.1. The FT-IR Spectra of Free Schiff Base and the Schiff Base Functionalized Silica (Adsorbent **1**)

The FT-IR spectra of free Schiff base and the Schiff base functionalized silica (adsorbent **1)** are shown in Figure 1. Figure 1a displays the characteristic peaks of free Schiff base. The broad band at 3442 cm−<sup>1</sup> was assigned to the vibration of the phenolic group υ(OH). In addition, the absorptions at 1601 and 1521 cm−<sup>1</sup> in the free Schiff base were due to carbonyl υ(C=O) and azomethine υ(C=N), respectively. These vibrations appeared at lower frequencies due to strong intramolecular hydrogen bonds in the solid state of the free Schiff base. The weak peaks observed at 2921 cm−<sup>1</sup> belonged to the stretching modes of the aliphatic -C-H bond. Figure 1b, shows the FT-IR spectrum of adsorbent **1.** The characteristic vibration of the phenolic group υ(OH) at 3442 cm−<sup>1</sup> in free Schiff base (Figure 1a) was decreased indicating successful functionalization of Schiff base on the silica surface. However, adsorbent **1** displayed broad bands in the region of 3250 to 3600 cm−1, which were assigned to the silanol stretching modes ν(Si-OH). The intensive absorption peaks observed at 1122–1032 cm−<sup>1</sup> are due to asymmetric ν(Si–O-Si) and the band at 804 cm−<sup>1</sup> is due to symmetric stretching vibrations of Si-O-Si [33]. The weak peaks observed at 2930 cm−<sup>1</sup> belong to the stretching modes of the aliphatic -C-H bond. The appearance of bands around 1665–1402 cm−<sup>1</sup> were caused by C=O, C=N, and C=C vibrations, confirming the anchoring of the organic molecule (Schiff base) onto the silica surface [34]. Moreover, the characterization features of adsorbent **1** compared with the free Schiff base were the shift of the carbonyl group from 1601 cm−<sup>1</sup> (in free Schiff base) to 1665 cm−<sup>1</sup> (in adsorbent **1**) and the disappearance of the thiol S-H peak at 2556 cm−<sup>1</sup> compared to adsorbent **2**.

### 3.2.2. The FT-IR Spectra of Adsorbent **1** before and after Loading Pb(II) Ions

The FT-IR spectra of adsorbent **1** before and after loading Pb(II) ions are shown in Figure 2. It can be seen that the results for the Pb(II) ions loaded sample were almost same as the pre-adsorption sample, However, some of the bands after lead ions were loaded shifted to a lower value than that of the pre-adsorption sample, confirming the adsorption of the Pb(II) ions on the silica surface. This suggests the involvement of the oxygen atom of the carboxylate anion with the azomethine nitrogen and the Pb(II) ion [35]. The infrared spectrum in Figure 2b revealed the asymmetric stretching vibration of νas(COO−) of adsorbent **1** at 1665 cm−<sup>1</sup> was shifted to a lower wave number (1642 cm−1) after the Pb(II) ion was loaded, indicating that the lead ion coordination took place via the oxygen atom of the carboxylate anion [36].

**Figure 2.** FTIR absorption spectrum of adsorbent **1**, (a) before absorption and (b) after adsorption.

### 3.2.3. The FT-IR Spectra of Adsorbent **2** before and after Loading Pb(II) Ions

Figure 3 shows the FT-IR spectroscopy results of adsorbent **2** samples taken before and after Pb(II) ion adsorption studies. The spectrum of synthesized adsorbent **2** before adsorption studies (Figure 3a) presents a broad band centered at 3495 cm−1, which was assigned to the OH vibrations in the silica framework, while the strong absorption peak located at 1128 cm−<sup>1</sup> was due to asymmetric ν(Si–O-Si), and the band at 804 cm-1 was due to symmetric ν(Si–O) stretching vibrations [33]. The weak peaks observed at 2921 cm−<sup>1</sup> belong to the stretching modes of the aliphatic -C-H bond. The characteristic thiol S-H functional group peak was detected at 2556 cm−<sup>1</sup> confirming the anchoring of the organic molecule (propyl-thiol) onto the silica surface. Compared with the spectrum of adsorbent **2** after Pb(II) ion loading (Figure 3b), it can be seen that the results for the lead ion loaded sample were similar to that before adsorption, except the intense broad peak centered at 3431 cm−<sup>1</sup> was attributed to coordinative water in the coordination sphere of Pb(II) Schiff base complex. The characteristic peaks at 2556 cm−<sup>1</sup> has diminished, suggesting that adsorbent **2** interacted with soft Lewis acid Pb(II) ions through the thiol groups.

### 3.2.4. XRD of Adsorbents **1** and **2**

The XRD patterns of Schiff base-functionalized silica (adsorbent **1**) and propyl-1-thiol functionalized silica (adsorbent **2**) are illustrated in Figure 4. The XRD patterns of both adsorbents **1** and **2** showed weak broad peaks at a low 2θ angle (7.47◦) and broad peaks at a high 2θ angle (20.21◦) for adsorbent **1** and at 23.57◦ for adsorbent **2,** indicating the amorphous nature of the silica in both adsorbents [37]. Furthermore, the shift of the broad peak at a high 2θ angle (23.57◦) for adsorbent

**2** to 20.21◦ for adsorbent **1** suggests structural perturbations resulting from the incorporation of the Schiff base within the silica framework [34]. The XRD patterns of adsorbent **1** show sharp peaks at 37.73, 43.79, 64.29, 77.47, 81.81, and 98.37 of 2θ. These sharp peaks were due to the ligand (Schiff base) immobilized on the surface of silica [38].

**Figure 3.** FTIR absorption spectrum of adsorbent (2), (a) before absorption and (b) after adsorption.

**Figure 4.** The X-ray diffraction (XRD) patterns of (a) adsorbent **1** and (b) adsorbent **2**.
