**1. Introduction**

Excessive heavy metal concentrations in drinking water pose significant threats to human health and the environment. For example, lead (Pb) is a very commonly used metal with a relatively high atomic mass, atomic number, and density of 11.34 g/cm3. Pb is widely used in battery production, paints, and fertilizers. However, exposure to high Pb levels can cause major problems in the biosynthesis of hemoglobin (Hb), which is important for oxygen transport in humans and other vertebrates. In addition, Pb can cause serious damage to the liver, kidneys, and nervous system. Elimination of highly toxic heavy metal ions, such as Pb, mercury (Hg), chromium (Cr), copper (Cu), cobalt (Co), and zinc (Zn), from water is necessary and recently has been the subject of extensive research. Polluted water can cause serious environmental problems and can cause harm to both humans and other animal life [1–4].

A number of techniques are used for treatment of wastewater discharge. Some of the primary methods include precipitation, flocculation, flotation, and chelation reactions. Other methods include ion-exchange columns, solvent extraction, reverse osmosis, and electrolytic plating on an anode. Nonetheless, these methods suffer from being costly and inappropriate for removing a lot of

contaminants; for instance, large amounts of chemicals are needed for lime precipitation, and its electrolytic recovery suffers from corrosion [5–13]. Recently, low-cost adsorbents derived from functionalized inorganic–organic hybrid materials have been extensively used as adsorbents for removal of heavy metal from wastewater because of their notable properties, such as strong binding affinities and high adsorption capacities towards heavy metal ions; in addition, they have large surface areas and good chemical, thermal, and mechanical stabilities [14–24].

Among the different types of adsorbents is mesoporous silica material, which is widely used and a well-known adsorbent for removal of toxic heavy metals. This material consists of a porous structure, and there are two types: (1) hexagonal (MCM-41), which is a silica-based mesoporous material with size between 50 and 120 nm, with different morphologies and hollow and core-shell spheres and (2) three-dimensional cube (MCM-48), which has four different structures (worm-like, helical, radial, and lamellar). MCM-41 has been extensively used in different applications, such as catalysis, adsorption, and separation processes, and as drug carriers [25]. Recently, modifications of MCM-41 with organic compounds in order to enhance the properties of MCM-41 and to improve its adsorption capacities towards toxic heavy metals has attracted more interest [26–29]. For example, Liu and co-workers [30] prepared novel zwitterion hybrid polymer adsorbents for adsorption of Pb(II) and Cu(II) ions from aqueous solution. Pavan et al. [31] prepared a stable mesoporous aniline/silica sorbent material for adsorption of Co, Zn, and cadmium chlorides (CoCl2, ZnCl2, and CdCl2, respectively) from aqueous and ethanol solutions.

On the other hand, the stable azomethine group (-C=N-R, in which R is either an alkyl or aryl), also known as an imine or Schiff base, has been shown to play an important role in stabilizing metal complexes because it can act as a good binding site for many transition metals and therefore can form stable coordination complexes. Recent studies have demonstrated that mesoporous silica materials (MCM-41) can be immobilized with organic groups such as thiols, anilines, and Schiff bases and then used successfully for removal of toxic heavy metals from contaminated water. For example, Radi et al. [32] anchored a Schiff-base on silica gel for adsorption of Cd(II), Cu(II), and Zn(II) from aqueous solutions. The author concluded that the adsorbent can be regenerated and used several times without loss of its activity. However, the disadvantage of their adsorbent was the maximum sorption of Cu(II), Zn(II), and Cd(II) occurred at a pH of >8. Therefore, at this pH, hydrolysis of these metal ions may occur, and this hydrolysis makes it difficult to distinguish between hydrolyzed and adsorbed metals.

The goal of this present study was to prepare new modified mesoporous silica materials as adsorbents with large surface areas, regular pores, and high adsorption capacities for removal of toxic heavy metal ions from contaminated water. For this purpose, we have fabricated two hybrid materials, namely, Schiff base-modified and propyl-thiol-modified silicas (adsorbents **1** and **2**, respectively) by a direct co-condensation method using (3-mercaptopropyl) trimethoxysilane as the silica source. The removal of heavy metal ions such as Pb(II) from aqueous solutions by adsorbents **1** and **2** was examined by considering the effect of different factors, such as pH and contact time, between the absorbent and surrounding solution.

### **2. Materials and Methods (Experimental Section)**

### *2.1. Materials*

2-amino-4,5-dimethoxybenzoic acid, 4-hydroxybenzaldehyde, (3-mercaptopropyl) trimethoxysilane, and the 1000 mg/L standard solution with Pb(II) were purchased from Sigma Aldrich and used without modifications. HCl 0.5 M and NaOH 0.5 M were used to adjust the required pH. All reagents and solvents (analytical grade) were used without any further purification. All solutions were prepared with fresh deionized water obtained from a Milli-Q water system.

### *2.2. Instrumentation*

The residual Pb (II) concentration was determined by inductively coupled plasma mass spectrometry (ICP-MS; Agilent 7500). A digital pH meter was used to determine the pH of water samples. Fourier-transform infrared (FTIR) spectra were obtained with a Perkin Elmer System 2000. Scanning electron microscopy (SEM) images were obtained on an FEI-Quanta 200. The 1H and 13C nuclear magnetic resonance (NMR) spectra of the Schiff base were obtained with a CP MAX CXP 300 MHz. A specific area of modified silica was determined by using the Brunauer–Emmett–Teller (BET) equation. Nitrogen adsorption–desorption was obtained by means of a Thermoquest Sorpsomatic 1990 analyzer after the material had been purged in a stream of dry nitrogen. Thermal gravimetric analysis (TGA) was conducted on a SDT Q 600 analyzer. Powder X-ray diffraction (XRD) patterns for determining phase purity and structure of the end product were performed on an XRD P-6000-Shimadzu X-ray diffractometer (40 kV/20 mA) using a conventional θ–2θ reflection geometry and Cu K α radiation (λ = 1.5406 A◦).

### *2.3. Preparation of the Schi*ff *Base 2-(4-Hydroxybenzylideneamino)-4,5-Dimethoxybenzoic Acid*

2-amino-4,5-dimethoxybenzoic acid (1 mmol) and 4-hydroxybenzaldehyde (1 mmol) were mixed and heated to reflux for 4 h in ethanol (40 mL). The orange precipitate of the Schiff base was filtered, washed with cooled ethanol, and dried. The yield was 80% (Scheme 1). Analytical conditions were set for several different methods: (1) FTIR (KBr, cm<sup>−</sup>1): 1521 (νC=N Schiff base), 1601 (νC=O), 1452–1354 (νC=C), 3442 phenolic hydroxide ν (OH); (2) 1HNMR (DMSO, ppm, 400 MHz): 3.62 (s 3H), 3.72 (s, 3H), 6.32 (s H), 6.91–0.6.93 (d, 2H), 712 (s, H), 7.75–7.77 (d, 2H) and 9.78 (s, H); and (3) 13C NMR (dimethyl sulphoxide, DMSO, ppm, 400 MHz): 55.64, 55.38, 99.49, 101.05, 113.59, 116.31(2C), 128.84, 132.61(2C), 139.55, 148.71, 154.94, 163.80, 169.62, and 191.50.

**Scheme 1.** Schiff base synthesis.
