**4. Conclusions**

In this study, novel silica functionalized with Schiff-base (adsorbent **1**) and propyl-thiol (adsorbent **2**) were successfully prepared. Direct hydrolysis of 3-(triethoxysilyl) propane-1-thiol gave adsorbent **2**, and mixing 3-(triethoxysilyl) propane-1-thiol with a Schiff base produced a good yield of adsorbent **1**. The new materials were well characterized by FTIR, SEM, XRD, nitrogen adsorption–desorption isotherms, BET surface area, BJH pore sizes, TGA, and 1H and 13C NMR spectra. The newly synthesized adsorbents **1** and **2** displayed good thermal stability and adsorption capacity from aqueous solutions towards Pb(II) ions. The higher removal efficiency for Pb(II) by adsorbent **1** could be attributed to the higher surface area (20.1656 m2/g), which facilitated the interaction of free Pb(II) ions present in solution with the surface of adsorbent **1**. The adsorption isotherms of Pb(II) fitted well with the Langmuir Isotherm Model, indicating that the adsorption mechanism followed Langmuir monolayer adsorption, and the adsorption kinetics of Pb(II) followed the pseudo-second-order kinetic models. In addition, the newly synthesized materials were proven to be cost-effective adsorbents and could be used for the selective extraction of Pb(II) from contaminated water. Overall, our studies suggested that adsorbent **1** could be considered as a good candidate for removing Pb(II) from aqueous solutions.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2227-9717/8/2/246/s1, Figure S1. 1H-nuclear magnetic resonance (NMR) absorption spectrum of the Schiff base. Figure S2. 13C NMR spectra of the Schiff base. Figure S3. Thermogravimetric (TGA) curves of the Schiff base.

**Author Contributions:** Methodology: M.O.A., A.S., and M.M.; Software: M.O.A., A.S.; Writing—Original Draft Preparation: A.S.; Writing—Review & Editing: M.O.A.; Project Administration: M.O.A. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflicts of interest.
