*2.4. Preparation of Schi*ff *Base-Thiol Silica (Adsorbent 1)*

The Schiff base 2-(4-hydroxybenzylideneamino)-4,5-dimethoxybenzoic acid) (3) (first reaction) was added to a solution of deionized water/EtOH (25.0 mL, 50% *v*/*v*). The resulting suspension was stirred for 30 min. To this suspension, 3-(triethoxysilyl) propane-1-thiol (two equivalents) in deionized water/EtOH (25.0 mL, 50% *v*/*v*) was added. A few drops of base were added as catalyst. The mixture was stirred for 72 h at 60◦C. The resulting yellowish solid (adsorbent **1**) was filtered and washed several times with distilled water and ethanol. The solid was then dried under vacuum overnight at 105 ◦C before characterization (Scheme 2).

The propyl-thiol functionalized silica material (adsorbent **2**) was prepared by the sol-gel method (direct synthesis) after stirring (3-mercaptopropyl) trimethoxysilane in an ethanol/water mixture (50% *v*/*v*) using base as catalyst (Scheme 3). The mixture was stirred for 72 h at 60 ◦C. The resulting white solid was filtered and washed several times with distilled water and ethanol. The solid was then dried under vacuum overnight at 105 ◦C (Scheme 3).

**Scheme 2.** The synthesis of adsorbent 1.

**Scheme 3.** Preparation of adsorbent **2**.

### *2.5. Sorption Experiment*

### 2.5.1. pH Optimization

The effects of pH on the adsorption of Pb(II) by adsorbents **1** and **2** was investigated at room temperature. Adsorption checks were carried out by adding 10 mg of adsorbent **1** or **2** to each flask in a series of 100 mL Erlenmeyer flasks. To each flask, 50 mL of Pb(II) solution (25.52 mg/L) was added. Hydrochloride acid (HCl; 0.5 M) and sodium hydroxide (NaOH; 0.5 M) were used to adjust the initial pH of the solutions between 2 and 7. A pH meter was used to measure the pH of the solutions. Each suspension was stirred at room temperature for 2 h at 25 ◦C over various pH ranges (2.0, 4.0, 6.0, and 7.0). The solution was then filtered, and the amount of Pb(II) ions in the filtrate was analyzed for residual Pb(II) ions using inductively coupled plasma mass spectrometry (ICP-MS) in triplicate.

### 2.5.2. Effect of Contact Time for Pb(II)

The 1000 mg/L standard solution of lead Pb(II) ions was diluted with deionized water to obtain 50 mL of 40 mg/L. Adsorption checks were carried out by adding 20 mg of each adsorbents **1** and **2** to three Erlenmeyer flasks (100 mL). To each flask, 50 mL of Pb(II) solution (of 40 mgL−1) was added. The pH of solutions was adjusted to the optimum values (6.5 ± 0.2). The mixture was stirred constantly at 300 rpm in order to allow contact at different contact times. Five milliliter portions from each solution were filtered at measured time intervals (0 to 340 min), and the residual Pb(II) ion concentrations were measured using ICP-MS in triplicate. Results expressed as ppm for adsorbents **1** and **2**, respectively. The sorption capacities at equilibrium (mg g<sup>−</sup>1) and the percentage adsorption (% ads) of adsorbents **1** and **2** were calculated using the following Equations (1) or (2) [4]:

$$
\eta c = \left[\mathbb{C}\_0 - \mathbb{C}\_c\right] / m \times V \tag{1}
$$

$$\text{V@}\_{\circ}\text{ads} = [\text{C}\_{0} - \text{C}\_{\epsilon}] \text{/C}\_{0} \times 100 \tag{2}$$

where *qe* is the equilibrium uptake capacity (mg g<sup>−</sup>1), *C*<sup>0</sup> is the initial concentration of Pb(II) (mg L<sup>−</sup>1), *Ce* is the equilibrium concentration of Pb(II) expressed in mg L<sup>−</sup>1, V is the volume of aqueous solutions (L), and m is the weight of the adsorbent **1** or **2** (g).

### **3. Results and Discussion**
