SBA-16 Cage-Like Porous Material Modified with APTES as an Adsorbent for Pb2+ Ions Removal from Aqueous Solution

Tridimensional cubic mesoporous silica, SBA-16, functionalized with aminopropyl groups, were employed as adsorbents for Pb2+ ion removal from aqueous solution. The adsorption capacity was investigated for the effect of pH, contact time, temperature, and concentration of 3-aminopropyltriethoxysilane (APTES) employed for adsorbent functionalization. The textural properties and morphology of the adsorbents were evaluated by N2 physisorption, small-angle X-ray diffraction (XRD), diffuse reflectance spectroscopy (UV-vis), and transmission electron microscopy (TEM). The functionalization of the SBA-16 was evaluated by elemental analysis (N), thermogravimetric analysis (TG), Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS). Batch adsorption studies show that the total Pb2+ ions removal was archived on adsorbent having an optimized amount of aminopropyl groups (2N-SBA-16). The maximum of Pb2+ ions removal occurred at optimized adsorption conditions: pH = 5–6, contact time 40 min, and at a low initial lead concentration in solution (200 mg L−1). Under the same adsorption conditions, the amino-functionalized SBA-16 with cubic 3D unit cell structure exhibited higher adsorption capability than its SBA-15 counterpart with uniform mesoporous channels.

In order to improve the efficiency of the adsorption process, we investigated the adsorption capacity of SBA-16 substrate, scarcely employed for heavy ions removal [31][32][33][34][35][36]. The SBA-16 substrate exhibits an interesting three-dimensional cubic-like (Im3m) structure with interconnected micro-and mesopores, large surface area, and adequate pore diameter for the accommodation of the guest molecules [38]. Recently, the potential application of SBA-16 material synthesized from rice husk ash as an adsorbent for acetone vapors was investigated by Zeng and Bai [32]. It was found that SBA-16 exhibited superior adsorption capacity than ZSM-5 and MCM-41. This was ascribed to their higher specific surface area, which provides more adsorption sites for adsorption of acetone vapors, and the presence of micropores [32]. Lesaint et al. [35] used SBA-15-and SBA-16 mesoporous silica functionalized with mercaptopropyl groups as adsorbents for Hg 2+ ions removal from aqueous solution. For both types of adsorbents, the evaluation of the diffusion rates of Hg 2+ species in the mesoporous solids showed faster binding rates for the adsorbents functionalized by post-synthesis grafting than for their counterparts prepared by the one-step co-condensation route [35]. β-diketone functionalized SBA-15 and SBA-16 mesoporous materials were successfully used for the removal of copper from aqueous solution by Ouargli et al. [27]. Their SBA-16-based adsorbent exhibited a larger copper extraction capacity than its SBA-15-based counterpart.
This work aims to study the adsorption capacity of SBA-16 functionalized with variable amounts of amine groups. Considering that lead is one of the most toxic pollutants released into aquatic systems from many industrial processes [12], the adsorption capacity of amine-functionalized SBA-16 adsorbents was investigated for the removal of Pb 2+ ions from aqueous solution. In order to optimize adsorption conditions, the adsorption experiments were conducted at varying adsorbent and ligand ratio (TEOS/APTES ratio), contact time, pH, initial Pb 2+ concentration, and temperature. The physicochemical characteristics of the amine-modified SBA-16 adsorbents before and after Pb 2+ adsorption were investigated by elemental analysis, thermal analysis, N 2 adsorption-desorption isotherms, Fourier-transform infrared spectroscopy, DRS UV-vis, and XPS techniques. The influence of the adsorbent morphology on the adsorbent capacity is discussed.

Preparation of the Pure SBA-16
The SBA-16 substrate was synthesized according to the method described previously [22,23]. In the synthesis, 8 g of the triblock Pluronic F127 copolymer (EO 106 PO 70 EO 106 , Sigma-Aldrich, 99%) was dissolved in a solution of 60 mL of deionized water and 240 mL of 2M HCl. After 1 h of stirring, 26 mL of tetraethyl orthosilicate (TEOS, Aldrich, 98%) was added to the water-copolymer solution. This mixture was further stirred for 24 h at a constant temperature of 35 • C. The obtained suspension was transferred into a tightly closed polypropylene bottle and kept at 80 • C for 24 h without stirring. The precipitated solid was filtered and washed thoroughly with deionized water. After drying in air at 110 • C, the solid was calcined at 500 • C for 6 h to remove the organic template.

Functionalization of the SBA-16 with APTES
The SBA-16 mesoporous silica functionalized with 3-aminopropyltriethoxysilane (APTES, Aldrich, 97%) was prepared according to the procedure described in an earlier contribution [1]. Briefly, 1.0 g of dry SBA-16 was introduced into APTES-ethanol solution containing different amounts of APTES while continuously stirring. The molar ratio of TEOS to APTES was 1:0.2, 1:0.3 and 1:0.4, hereinafter called 2.6N/SBA-16, 3.8N/SBA-16, and 5.1N/SBA-16. Then, the liquid suspension was stirred at room temperature in an inert atmosphere for 30 min, whereas deionized water was slowly added to conduct the hydrolysis process of the organic functional group alkoxide from APTES. Then, the solid was dried overnight at room temperature followed by heating at 110 • C for 24 h. The SBA-16 mesoporous materials modified with amine groups will be referred to hereafter as xN/SBA-16, where x corresponds to the N content in the amine-functionalized adsorbents.

Characterization Techniques
Quantitative determination of the nitrogen content was measured by elemental microanalysis on a elementar Analysensysteme GmbH-vario EL III Element Analyzer (Langenselbold, Germany). The textural properties of the adsorbents were evaluated using a Micromeritics TriStar 3000 apparatus (Micromeritics, Norcross, GA, USA) from the nitrogen adsorption-desorption isotherms recorded at −196 • C. Before the measurement, the samples were degassed at 150 • C for 24 h under a vacuum (10 −4 mbar). Their specific total surface area was calculated using the Brunauer-Emmett-Teller (BET) method [24]. The pore size distribution was calculated from the adsorption branch of the N 2 isotherm using the BJH method. The total pore volume (V total ) was calculated from the amount of nitrogen adsorbed at a relative pressure of 0.99 [25].
X-ray diffraction (XRD) patterns of the powder samples were carried out using the Cu Kα radiation with a wavelength of 1.5406 Å in the range: 0.5-3 (low-angle) on a PANalytical diffractometer (Almelo, The Netherlands). The unit cell parameter (a o ) was calculated using Equation (1): where d 110 is the position of the (110) diffraction line (from low-angle XRD). The pore-wall thickness (w t ), which corresponds to the distance between the centers of adjacent mesopores, was estimated using the Equation (2) [26]: where d p is the mean pore diameter. Fourier transform-IR (FT-IR) spectra of the framework vibration (400-1800 cm−1 range) were recorded on a JASCO FT/IR-6300 spectrophotometer (JASCO, Easton, WA, USA) using the potassium bromide pellet method. The stability of the aminopropyl groups was determined by thermogravimetric analysis (Model TGA 2950, TA Inc, New Castle, USA) by weight loss of the samples during their heating in an atmosphere of nitrogen (a heating rate of 5 • C·min −1 ). The materials after lead adsorption were analyzed by UV-vis diffuse reflectance spectra at room temperature on a CARY 5000 UV-Vis-NIR VARIAN instrument (Varian, Santa Clara, CA, USA). X-ray photoelectron spectroscopic studies of the adsorbents were recorded on a VG Escalab 200R spectrometer (Vacuum Generators, Crowborough, UK) equipped with a hemispherical electron analyzer (Vacuum Generators, Crowborough, UK), using an MgKα (hν = 1253.6 eV, 1 eV = 1.603 × 10 −19 J) X-ray source. The details of the XPS measurements are reported elsewhere [1].

Adsorption Experiments
The Pb 2+ adsorption on the SBA-16 and xN/SBA-16 adsorbents was carried out in a batch reactor (Facultad de Ingeniería, Universidad Autónoma de Querétaro, Querétaro, Mexico) using deionized water in all experiments. The various parameters investigated for Pb 2+ adsorption were: contact time (20-120 min, with ranges of 20 min), reaction temperature (30,35, and 40 • C), and pH of solution (the pH value was adjusted using HNO 3 or NaOH 0.1 M). Lead solutions were obtained from lead nitrate (Baker, 99.92%). In a typical run, 0.1 g of adsorbent and 20 mL of 200 mg L −1 of lead solution were placed in a shaker (Thermo Scientific, Waltham, MA, USA) at 150 rpm for 1 h (pH = 5) at room temperature. After this period of stirring, the suspension was centrifuged at 2500 rpm for 5 min, and finally, the adsorbents were recovered from through filtration. Inductively coupled plasma atomic emission spectroscopy (ICP-AES) was employed to determine the initial and final lead concentrations with a Perkin Elmer Optima 3300 DV spectrometer (Perkin Elmer, Waltham, MA, USA) calibrated with 0-10 mg L −1 stock solutions. The emission line used was in accordance with the Environmental Protection Agency (EPA) standard method [27]. For each equilibrium point, the amount of adsorbed Pb 2+ was determined by the difference between initial and final metal concentrations in the solution. Replica experiments indicated that associated error was within ± 3%. The percentage of adsorbed Pb 2+ was calculated according to Equation (3): where C i and C f are the initial and final Pb(II) ions concentration in the solution, respectively. The adsorption capacity of Pb 2+ per unit weight of the modified adsorbent at time t, (Q t ; mg g −1 ), was calculated from the mass balance in Equation (4): where C i (mg L −1 ) and C f (mg L −1 ) are the initial and final concentrations of Pb 2+ at time t, respectively, V is the volume of the aqueous solution, and m is the mass of adsorbent.

Chemical Analysis and Low-Angle X-ray Diffractions
The nitrogen content of the amine-functionalized adsorbates is listed in Table 1. As expected, the nitrogen content increases with an increase of TEOS/APTES ratio. The SBA-16 substrate exhibits interesting morphology consisting of the three-dimensional channel systems corresponding to Im3m space group symmetry and uniform cage-like mesopores with a cubic symmetry [22]. Its 3D structure consisting of ordered and interconnected spherical mesopores makes it easily accessible for Pb(II) ions. The reaction of silanol groups of calcined SBA-16 substrate with ethoxysilanes resulted in the anchorage of aminopropyl functional groups.
The possible structure changes after SBA-16 grafting with -NH 2 groups was investigated by powder low-angle XRD technique ( for characteristic cubic Im3m spatial groups) suggesting that functionalization of SBA-16 substrate leads to a structure with minor pore arrangement and a major presence of nitrogen molecules [28]. However, a transmission electron micrograph (TEM) image of the representative 3.8N/SBA-16 sample showed a well-ordered Im3m cubic array of mesopores ( Figure 2). Therefore, it can be inferred that the mesoporous structure of SBA-16 adsorbent did not suffer significant modification after its functionalization with amine groups. For 5.1N/SBA-16, the low intensity of its peak at reflection [110] is linked with a high wall thickness/pore size ratio ( Table 1) that originated from a large number of micropores located near the wall surface the pore walls [29].
Materials 2020, 13, x FOR PEER REVIEW 5 of 15 functionalization with amine groups. For 5.1N/SBA-16, the low intensity of its peak at reflection [110] is linked with a high wall thickness/pore size ratio ( Table 1) that originated from a large number of micropores located near the wall surface the pore walls [29].  To determine both structure and symmetry of the synthesized samples, the cubic unit cell parameters and the wall thickness were calculated using XRD data and equations (1) and (2), respectively ( Table 2). The cubic unit cell parameter (a0) of all samples was found to be in the range 13.91-14.19 nm, confirming their Im3m structure [30]. Both 2.6N/SBA-16 and 3.8N/SBA-16 samples show smaller cubic unit cell parameters than the bare SBA-16 sample indicating the possible location of amine groups within the porous structure of the SBA-16 substrate. The higher thermal and hydrothermal stability with respect to the bare SBA-16 substrate could be inferred [31]. Noticeably, the functionalization of SBA-16 material with an optimized amount of amine groups did not change the wall thickness (3.8N/SBA-16). On the contrary, an increase of the pore wall thickness observed for the 5.1N/SBA-16 is probably due to the location of the amine group within the pores in addition to the external surface, in good agreement with a large decrease of the specific surface area calculated using Brunauer-Emmett-Teller (BET) equation. Table 2. Unit cell parameter (ao), and pore wall thickness (wt) of the bare SBA-16 and amine-functionalized adsorbents.
wt/dp Ratio SBA- 16 14.13 8.84 2.6  a As determined from N 2 adsorption-desorption isotherms at 77 K; S BET : specific surface area calculated using Brunauer-Emmett-Teller (BET) equation; V total : adsorption total pore volume; dp: average pore diameter calculated from the isotherm adsorption branch.; b Adsorption efficiency expressed as Pb/N atomic ratio: adsorption conditions were: 0.1 of adsorbent, 20 mL of 200 mg L −1 of Pb 2+ aqueous solution, pH = 5, contact time 60 min, T = 30 • C.
To determine both structure and symmetry of the synthesized samples, the cubic unit cell parameters and the wall thickness were calculated using XRD data and Equations (1) and (2), respectively ( Table 2). The cubic unit cell parameter (a 0 ) of all samples was found to be in the range 13.91-14.19 nm, confirming their Im3m structure [30]. Both 2.6N/SBA-16 and 3.8N/SBA-16 samples show smaller cubic unit cell parameters than the bare SBA-16 sample indicating the possible location of amine groups within the porous structure of the SBA-16 substrate. The higher thermal and hydrothermal stability with respect to the bare SBA-16 substrate could be inferred [31]. Noticeably, the functionalization of SBA-16 material with an optimized amount of amine groups did not change the wall thickness (3.8N/SBA-16). On the contrary, an increase of the pore wall thickness observed for the 5.1N/SBA-16 is probably due to the location of the amine group within the pores in addition to the external surface, in good agreement with a large decrease of the specific surface area calculated using Brunauer-Emmett-Teller (BET) equation.

Textural Properties
Further insight into the textural properties of the samples was evaluated by N 2 adsorption-desorption isotherms. To illustrate this, Figure 3a shows the N 2 isotherms of bare SBA-16, and amino-functionalized materials. Irreversible type IV isotherms with type an H2 hysteresis loop was observed in all amino-functionalized and bare SBA-16 material. Type H2 hysteresis loop in the range from 0.4 to 0.6 P/P 0 is distinctive from materials with pores with diameters between 3-6 nm [32,33], a structural porosity that can be detected because of the characteristic networking pore model from the SBA-16 material. The hysteresis loop observed at high relative pressures corresponds to the textural porosity due to the voids formed between particles during the analysis. These results are typical of materials with cubic pores and pore network connectivity like SBA-16 and reveal that the mesoporous nature of the material is preserved even though the grafting has occurred, as shown in samples 2.6N/SBA-16, 3.8N/SBA-16 and 5.1N/SBA-16. At approximately P/P0 ≈ 0.4, an explicit change is present, and it is related to capillary condensation that fills the mesopores. When the substrate is functionalized with (-NH 2 ), there is an increase of the volume of adsorbed nitrogen, and the inflection point of the step shifted only slightly from relative pressure of 0.4 to 0.41. The minimum value of adsorbed nitrogen suggests the modification in the pores with amine groups, while the slight relative pressure shift of the step is indicative of mesopore sizes, a key requirement to be an efficient adsorbent.

Textural Properties
Further insight into the textural properties of the samples was evaluated by N2 adsorption-desorption isotherms. To illustrate this, Figure 3a shows the N2 isotherms of bare SBA-16, and amino-functionalized materials. Irreversible type IV isotherms with type an H2 hysteresis loop was observed in all amino-functionalized and bare SBA-16 material. Type H2 hysteresis loop in the range from 0.4 to 0.6 P/P0 is distinctive from materials with pores with diameters between 3-6 nm [32,33], a structural porosity that can be detected because of the characteristic networking pore model from the SBA-16 material. The hysteresis loop observed at high relative pressures corresponds to the textural porosity due to the voids formed between particles during the analysis. These results are typical of materials with cubic pores and pore network connectivity like SBA-16 and reveal that the mesoporous nature of the material is preserved even though the grafting has occurred, as shown in samples 2.6N/SBA-16, 3.8N/SBA-16 and 5.1N/SBA-16. At approximately P/P0 ≈ 0.4, an explicit change is present, and it is related to capillary condensation that fills the mesopores. When the substrate is functionalized with (-NH2), there is an increase of the volume of adsorbed nitrogen, and the inflection point of the step shifted only slightly from relative pressure of 0.4 to 0.41. The minimum value of adsorbed nitrogen suggests the modification in the pores with amine groups, while the slight relative pressure shift of the step is indicative of mesopore sizes, a key requirement to be an efficient adsorbent.   Table 1 compiles the values of some textural parameters (BET area, mesopore volume, and pore diameter) for the bare SBA-16 sample and their NH2-modified counterparts. As expected, the BET area and the mesoporous volume strongly decreased after modification according to the sequence SBA-16 (650 m 2 /g) >> 2.6N/SBA-16 (507 m 2 /g) > 3.8N/SBA-16 (500 m 2 /g) > 5.1N/SBA-16 (494 m 2 /g), suggesting that the grafted species appear concentrated not only on the external surface but also within the mesopore network of the SBA-16 substrate. Interestingly, regardless of APTES concentration, all amine-modified samples exhibit very similar specific surface area (in the range 494-507 m 2 /g). It is known that the SBA-16 possesses micropores located within the walls of primary mesopores forming a three-dimensional channel system with a connection between  Table 1 compiles the values of some textural parameters (BET area, mesopore volume, and pore diameter) for the bare SBA-16 sample and their NH 2 -modified counterparts. As expected, the BET area and the mesoporous volume strongly decreased after modification according to the sequence SBA-16 (650 m 2 /g) >> 2.6N/SBA-16 (507 m 2 /g) > 3.8N/SBA-16 (500 m 2 /g) > 5.1N/SBA-16 (494 m 2 /g), suggesting that the grafted species appear concentrated not only on the external surface but also within the mesopore network of the SBA-16 substrate. Interestingly, regardless of APTES concentration, all amine-modified samples exhibit very similar specific surface area (in the range 494-507 m 2 /g). It is known that the SBA-16 possesses micropores located within the walls of primary mesopores forming a three-dimensional channel system with a connection between the mesopores [34]. The micropores are formed during the synthesis of SBA-16 due to the penetration of more hydrophilic EO chains of the tri-block copolymer in the silica wall [35,36]. Thus, the large loss of total pore volume after SBA-16 modification with APTES should also be explained in terms of blocking micropores by large ligand groups.

FTIR of Framework Vibration
Direct probe on the functionalization of SBA-16 surface by -NH 2 groups were obtained from the FTIR spectra of framework vibrations of xN/SBA-16 samples. In Figure 4  of more hydrophilic EO chains of the tri-block copolymer in the silica wall [35,36]. Thus, the large loss of total pore volume after SBA-16 modification with APTES should also be explained in terms of blocking micropores by large ligand groups.

FTIR of Framework Vibration
Direct probe on the functionalization of SBA-16 surface by -NH2 groups were obtained from the FTIR spectra of framework vibrations of xN/SBA-16 samples. In Figure 4    More information should be obtained by analyzing a very broad band in the wavenumber range 3000-4000 cm −1 . For the xN/SBA-16 samples, the N−H stretching vibration is expected to occur at about 3300 cm −1 [39]. Unfortunately, in the same region, all the samples exhibit bands due to silanol groups (ca. 3750 cm −1 ) and adsorbed molecular or hydrogen-bonded molecules (ca. 3630 cm −1 ) [40,41]. As seen in Figure 4, for the amine-modified samples, the band in this region is much broader than for the bare SBA-16. The widening of this band should be originated by the symmetric stretching vibration of N−H groups in the terminal amine groups cross-linked with the -SiOH groups [41]. Figure 5 displays the weight loss of the samples during thermal treatment of the adsorbents in an atmosphere of nitrogen, as determined by thermogravimetry. The thermogravimetric profiles indicate a significant weight loss, which occurs just at very low temperatures. The total weight loss More information should be obtained by analyzing a very broad band in the wavenumber range 3000-4000 cm −1 . For the xN/SBA-16 samples, the N−H stretching vibration is expected to occur at about 3300 cm −1 [39]. Unfortunately, in the same region, all the samples exhibit bands due to silanol groups (ca. 3750 cm −1 ) and adsorbed molecular or hydrogen-bonded molecules (ca. 3630 cm −1 ) [40,41]. As seen in Figure 4, for the amine-modified samples, the band in this region is much broader than for the bare SBA-16. The widening of this band should be originated by the symmetric stretching vibration of N−H groups in the terminal amine groups cross-linked with the -SiOH groups [41]. Figure 5 displays the weight loss of the samples during thermal treatment of the adsorbents in an atmosphere of nitrogen, as determined by thermogravimetry. The thermogravimetric profiles indicate a significant weight loss, which occurs just at very low temperatures. The total weight loss is approximately 10.5wt.% from room temperature to 600 • C. It shows a first inflection point at 45 • C corresponding to dehydration of the SBA-16 material. A major weight loss, at about 250 • C, can be attributed to surface dihydroxylation or structural rearrangement of the aminopropyl groups [42]. Considering that pure SBA-16 material did not exhibit the weight loss above 300 • C, the weight loss observed in the temperature range 300-600 • C is due to the decomposition of aminopropyl groups of the xN/SBA-16 solids [42]. The thermogravimetric analysis corroborates the successful modification of the SBA-16 with APTES. In good agreement with the literature [42], the aminopropyl groups start to decompose at a temperature of 27 • C, higher than the boiling point of APTES liquid, indicating the chemical bonding of 3-aminopropylsilane with -OH groups of the SBA-16 material. is approximately 10.5wt.% from room temperature to 600 °C. It shows a first inflection point at 45 °C corresponding to dehydration of the SBA-16 material. A major weight loss, at about 250 °C, can be attributed to surface dihydroxylation or structural rearrangement of the aminopropyl groups [42].

Thermogravimetry (TG)
Considering that pure SBA-16 material did not exhibit the weight loss above 300 °C, the weight loss observed in the temperature range 300-600 °C is due to the decomposition of aminopropyl groups of the xN/SBA-16 solids [42]. The thermogravimetric analysis corroborates the successful modification of the SBA-16 with APTES. In good agreement with the literature [42], the aminopropyl groups start to decompose at a temperature of 27 °C, higher than the boiling point of APTES liquid, indicating the chemical bonding of 3-aminopropylsilane with -OH groups of the SBA-16 material.

DRS UV-vis
Next to the lead adsorption with initial Pb 2+ concentrations of 200 and 400 ppm, UV-Vis spectra were recorded to exhibit the changes on the 3.8N/SBA-16 material. Figure 6 shows only a little larger lead absorption occurring at higher Pb 2+ ion concentration in solution (400 mg L −1 ). The scheme of Pb 2+ ions complexation with the amino group of 3.8N/SBA-16 is shown in the inlet of Figure 6. It is assumed that van der Waals electrostatic interaction is taken place at the amino groups in the surface of the adsorbent with the Pb 2+ ions, explaining its mechanism of adsorption [1]. The absorption spectra of all adsorbents display two bands: an intense band appears at 210 nm, and a less intense band is observed at ca. 310 nm [43]. These transitions contain both ligand-to-metal charge transfer (N 2p → Pb 6sp) and intraatomic (Pb 6s 2 → Pb 6sp) character (for Pb in Oh: a*1g 2 → a*1g 1 t*1u 1 ) [44]. Both absorption bands can be used to gain qualitative information about the affinity of Pb 2+ for the amine groups of the xN/SBA-16 adsorbents.

DRS UV-Vis
Next to the lead adsorption with initial Pb 2+ concentrations of 200 and 400 ppm, UV-Vis spectra were recorded to exhibit the changes on the 3.8N/SBA-16 material. Figure 6 shows only a little larger lead absorption occurring at higher Pb 2+ ion concentration in solution (400 mg L −1 ). The scheme of Pb 2+ ions complexation with the amino group of 3.8N/SBA-16 is shown in the inlet of Figure 6. It is assumed that van der Waals electrostatic interaction is taken place at the amino groups in the surface of the adsorbent with the Pb 2+ ions, explaining its mechanism of adsorption [1]. The absorption spectra of all adsorbents display two bands: an intense band appears at 210 nm, and a less intense band is observed at ca. 310 nm [43]. These transitions contain both ligand-to-metal charge transfer (N 2p → Pb 6sp) and intraatomic (Pb 6s 2 → Pb 6sp) character (for Pb in O h : a* 1g 2 → a* 1g 1 t* 1u 1 ) [44].

X-ray Photoelectron Spectroscopy
To clarify the type of Pb species formed on the surface of the amine-functionalized SBA-16 after Pb 2+ ion adsorption, the most optimized 3.8N/SBA-16 sample was also studied by the XPS technique. Table 3 lists the binding energies of Si 2p, O 1s, N 1s and Pb 4f7/2 core electrons of 3.8N/SBA-16 sample, whereas Figure 7a,b shows its N 1s and Pb 4f7/2 core-level spectra, respectively. For comparison purposes, some XPS data of the SBA-15-based counterpart [1] are included in Table 3. In good agreement with our previous study [1], the 3.8N/SBA-16 sample shows the O 1s peak at about 532.9 eV which is characteristic of oxygen in Si-O-Si bonds together with two peaks (Pb4f7/2 and Pb4f5/2) derived from spin-orbit splitting (Figure 7b). The binding energy of the Pb 4f7/2 core level appeared at 139.0 eV and 139.2 eV. According to the literature [45], the Pb 4f binding energies are stronger than the energies corresponding to the orthorhombic PbO compound (137.4 eV), and it is similar to those reported for Pb(NO3)2 (138.6 eV). There is no evidence that implies precipitation of Pb occurs as hydroxides or carbonates throughout the adsorption process with the XPS results. Finally, -NH2 bonds are attributed to the binding energy at 400.1 eV, while positively charged -NH3 + groups are identified at binding energies of 402.0 eV [46]. Thus, both -NH2 and protonated NH3 + species seem to be present previous to and afterward Pb 2+ adsorption. -NH2 groups are predominant in the material, and these can be coordinated with Pb 2+ ions through the pair of free electrons. Noticeably, the nitrogen atoms exposed in the surface for the 3.8N/SBA-16 was much lower than for its SBA-15-based counterpart prepared with the same TEOS/APTES molar ratio of 3.

X-ray Photoelectron Spectroscopy
To clarify the type of Pb species formed on the surface of the amine-functionalized SBA-16 after Pb 2+ ion adsorption, the most optimized 3.8N/SBA-16 sample was also studied by the XPS technique. Table 3 lists the binding energies of Si 2p, O 1s, N 1s and Pb 4f 7/2 core electrons of 3.8N/SBA-16 sample, whereas Figure 7a,b shows its N 1s and Pb 4f 7/2 core-level spectra, respectively. For comparison purposes, some XPS data of the SBA-15-based counterpart [1] are included in Table 3. In good agreement with our previous study [1], the 3.8N/SBA-16 sample shows the O 1s peak at about 532.9 eV which is characteristic of oxygen in Si-O-Si bonds together with two peaks (Pb4f 7/2 and Pb4f 5/2 ) derived from spin-orbit splitting (Figure 7b). The binding energy of the Pb 4f 7/2 core level appeared at 139.0 eV and 139.2 eV. According to the literature [45], the Pb 4f binding energies are stronger than the energies corresponding to the orthorhombic PbO compound (137.4 eV), and it is similar to those reported for Pb(NO 3 ) 2 (138.6 eV). There is no evidence that implies precipitation of Pb occurs as hydroxides or carbonates throughout the adsorption process with the XPS results. Finally, -NH 2 bonds are attributed to the binding energy at 400.1 eV, while positively charged -NH 3 + groups are identified at binding energies of 402.0 eV [46]. Thus, both -NH 2 and protonated NH 3 + species seem to be present previous to and afterward Pb 2+ adsorption. -NH 2 groups are predominant in the material, and these can be coordinated with Pb 2+ ions through the pair of free electrons. Noticeably, the nitrogen atoms exposed in the surface for the 3.8N/SBA-16 was much lower than for its SBA-15-based counterpart prepared with the same TEOS/APTES molar ratio of 3.    Figure 8 shows the percentage of Pb 2+ ions adsorption onto xN/SBA-16 adsorbents functionalized with variable amounts of aminopropyl groups and pure silica SBA-16. The adsorption was performed at pH = 5.0, temperature 30 °C, and contact time of 60 min. As seen in this Figure, the adsorption of Pb 2+ did not occur on the pure silica SBA-16. On the contrary, all amine-functionalized SBA-16 materials were good adsorbents for Pb 2+ removal. The adsorption of Pb 2+ increased with an increase of the TEOS/APTES molar ratio up to 3.3. At this point, the maximum percentage of the removal of the Pb 2+ ions from the solution was about 99% (3.8N/SBA-16). At higher -NH2 concentration, a drop in the Pb 2+ removal percentage occurs (5.1N/SBA-16). Thus, in good agreement with the previous study on the SBA-15-based adsorbents [1], the best results were obtained using a TEOS/APTES molar ratio 3.3. This is probably because a large amount of APTES molecules with aminopropyl chain shielding close to silanol groups limits the entrance of Pb 2+ ions into the inner porous structure of the SBA-16 substrate [47]. However, considering the nitrogen content determined by elemental microanalysis (Table 1) Table 3. Binding energies (eV) of core-electrons and surface atomic ratios of the most optimized 3.8N/SBA-16 adsorbent (from XPS) before and after Pb 2+ ion adsorption.

XPS Data
Before  Figure 8 shows the percentage of Pb 2+ ions adsorption onto xN/SBA-16 adsorbents functionalized with variable amounts of aminopropyl groups and pure silica SBA-16. The adsorption was performed at pH = 5.0, temperature 30 • C, and contact time of 60 min. As seen in this Figure, the adsorption of Pb 2+ did not occur on the pure silica SBA-16. On the contrary, all amine-functionalized SBA-16 materials were good adsorbents for Pb 2+ removal. The adsorption of Pb 2+ increased with an increase of the TEOS/APTES molar ratio up to 3.3. At this point, the maximum percentage of the removal of the Pb 2+ ions from the solution was about 99% (3.8N/SBA-16). At higher -NH 2 concentration, a drop in the Pb 2+ removal percentage occurs (5.1N/SBA-16). Thus, in good agreement with the previous study on the SBA-15-based adsorbents [1], the best results were obtained using a TEOS/APTES molar ratio 3.3. This is probably because a large amount of APTES molecules with aminopropyl chain shielding close to silanol groups limits the entrance of Pb 2+ ions into the inner porous structure of the SBA-16 substrate [47]. However, considering the nitrogen content determined by elemental microanalysis (Table 1), the Pb/N atomic ratio follows the trend: 2.6N/SBA-16 > 3.8N/SBA-16 > 5.1N/SBA-16. Taking into account that the same trend follows their specific surface area, one might conclude that both functionalization and textural properties of the adsorbent are important factors influencing the adsorption efficiency. For the most optimized adsorbent 3.8N/SBA-16, a series of experiments were conducted to determine the optimum contact time, pH, adsorption temperature, and the initial lead concentration in the solution. The influence of temperature on the Pb 2+ ions adsorption onto the 3.8N/SBA-16 sample is shown in Figure 9a. The adsorption was performed in the temperature range of 30-40 °C. As seen in this figure, the percentage of the Pb 2+ removal in the temperature range studied was very high: 99.5% at 30 °C and 100% at 40 °C, suggesting that the adsorption process is endothermic. Because of the easy mobility of the molecules at 30 °C, this temperature was selected for further experiments.

Adsorption Experiments
The effect of contact time on the adsorption capacity of the 3.8N/SBA-16 sample is shown in Figure 9b. The initial concentration of Pb 2+ ions in aqueous solution was 200 mg L −1 . As seen in Figure  9b, the initial rate of Pb 2+ adsorption on the 3.8N/SBA-16 was fast, and equilibrium was reached after 40 min with total Pb 2+ removal. Thus, the contact time of 60 min was chosen to perform further adsorption experiments. The comparison of the percentage of Pb 2+ removal after a contact time of 120 min indicated a larger adsorption capacity of the 3.8N/SBA-16 sample with respect to its SBA-15-based counterpart [1] (99.8% vs. 91.5%). The fast adsorption of Pb 2+ ions during the first 40 min strongly suggests the uniform distribution of-NH2 groups on the surface of SBA-16 adsorbent.
The effect of variation of pH of solution on the capacity of Pb 2+ adsorption on the most optimized adsorbent is shown in Figure 9c. In good agreement with a study on SBA-15-based adsorbent [1], the highest adsorption of Pb 2+ was achieved at pH > 3. An increase of pH from 3 to 5 led to an increase of Pb 2+ adsorption from 25.0 to 99.6wt.%. This result indicates that pH = 5 is the most appropriate to increase the extent of adsorption. This behavior may be likely related to the competitive adsorption of Pb 2+ , and H3O + ions on the NH2-modified SBA-16 surface. At low pH, the number of H3O + exceeds that of the Pb 2+ ions being the surface of the adsorbent covered mainly by H3O + ions leading to a lower extent of Pb 2+ adsorption. When pH increases, more and more H3O + ions leave the adsorbent's surface, making the sites available to the Pb 2+ adsorption [38].
Finally, the effect of initial lead concentration was evaluated (Figure 9d). Initial Pb 2+ concentration was adjusted in the ranges of 100-600 mg L −1 . As seen in Figure 9d, at a moderate initial lead concentration of 105 and 208 ppm, the adsorption of Pb 2+ is high (95.1% and 99.5%, respectively). As expected, an increase in lead concentration from 200 to 600 ppm led to a drastic decrease in adsorption capacity. Assuming that the mechanism of lead adsorption on the siliceous adsorbents modified with -NH2 groups is probably through van der Waals electrostatic interaction of Pb 2+ ions with surface -NH2 groups [1], at a higher initial lead concentration (> 300 ppm), more Pb 2+ was left in solution because lead in solution is much higher than the amount required to saturate the binding site. For the most optimized adsorbent 3.8N/SBA-16, a series of experiments were conducted to determine the optimum contact time, pH, adsorption temperature, and the initial lead concentration in the solution. The influence of temperature on the Pb 2+ ions adsorption onto the 3.8N/SBA-16 sample is shown in Figure 9a. The adsorption was performed in the temperature range of 30-40 • C. As seen in this figure, the percentage of the Pb 2+ removal in the temperature range studied was very high: 99.5% at 30 • C and 100% at 40 • C, suggesting that the adsorption process is endothermic. Because of the easy mobility of the molecules at 30 • C, this temperature was selected for further experiments.
The effect of contact time on the adsorption capacity of the 3.8N/SBA-16 sample is shown in Figure 9b. The initial concentration of Pb 2+ ions in aqueous solution was 200 mg L −1 . As seen in Figure 9b, the initial rate of Pb 2+ adsorption on the 3.8N/SBA-16 was fast, and equilibrium was reached after 40 min with total Pb 2+ removal. Thus, the contact time of 60 min was chosen to perform further adsorption experiments. The comparison of the percentage of Pb 2+ removal after a contact time of 120 min indicated a larger adsorption capacity of the 3.8N/SBA-16 sample with respect to its SBA-15-based counterpart [1] (99.8% vs. 91.5%). The fast adsorption of Pb 2+ ions during the first 40 min strongly suggests the uniform distribution of-NH 2 groups on the surface of SBA-16 adsorbent.
The effect of variation of pH of solution on the capacity of Pb 2+ adsorption on the most optimized adsorbent is shown in Figure 9c. In good agreement with a study on SBA-15-based adsorbent [1], the highest adsorption of Pb 2+ was achieved at pH > 3. An increase of pH from 3 to 5 led to an increase of Pb 2+ adsorption from 25.0 to 99.6wt.%. This result indicates that pH = 5 is the most appropriate to increase the extent of adsorption. This behavior may be likely related to the competitive adsorption of Pb 2+ , and H 3 O + ions on the NH 2 -modified SBA-16 surface. At low pH, the number of H 3 O + exceeds that of the Pb 2+ ions being the surface of the adsorbent covered mainly by H 3 O + ions leading to a lower extent of Pb 2+ adsorption. When pH increases, more and more H 3 O + ions leave the adsorbent's surface, making the sites available to the Pb 2+ adsorption [38].
Finally, the effect of initial lead concentration was evaluated (Figure 9d). Initial Pb 2+ concentration was adjusted in the ranges of 100-600 mg L −1 . As seen in Figure 9d, at a moderate initial lead concentration of 105 and 208 ppm, the adsorption of Pb 2+ is high (95.1% and 99.5%, respectively). As expected, an increase in lead concentration from 200 to 600 ppm led to a drastic decrease in adsorption capacity. Assuming that the mechanism of lead adsorption on the siliceous adsorbents modified with -NH 2 groups is probably through van der Waals electrostatic interaction of Pb 2+ ions with surface -NH 2 groups [1], at a higher initial lead concentration (>300 ppm), more Pb 2+ was left in solution because lead in solution is much higher than the amount required to saturate the binding site. Summarizing, the SBA-16 functionalized with an optimized amount of aminopropyl groups demonstrated to be more efficient adsorbent than its SBA-15-based counterpart (Figure 8). The highest sorption capacity was achieved with the 3.3 TEOS/APTES molar ratio adsorbent due to its large specific surface area and unlimited accessibility for Pb 2+ ions to -NH2 groups. The SBA-16-based adsorbents were stable until the temperature of 250 °C. The optimum pH value for removal of Pb 2+ ions from aqueous solution was found to be in the range 5 to 6. The equilibrium for lead adsorption was reached at about 40 min.
The comparison of the adsorption capacity of the 3.8N/SBA-16 studied in this work with that reported previously for the SBA-15-based counterpart [1] strongly suggests a larger adsorption capacity of the former with respect to the latter (39.8 and 36.4 mg of Pb 2+ per g of adsorbent, respectively). The abilities of both adsorbents appeared to be closely related to their pore structure, pore density and the amount of the grafted amine groups. Indeed, because of the much larger amount of organoalkoxysilane precursor, the SBA-15 functionalized with (3-aminopropyl) trimethoxysilane exhibited a larger adsorption capacity [16] than SBA-15 functionalized with (3-aminopropyl) triethoxysilane (APTES) [1] (90 mg g −1 vs. 39.8 mg g −1 ). This could be explained in terms of the cage-like structure and two-times smaller pore diameter of SBA-16 with respect to SBA-15 counterpart. In such a case, additional Pb 2+ ion trapping might well occur within the inner pore structure of SBA-16. Moreover, the easier accessibility of Pb 2+ ions to amine groups as well as the steric difficulty for their leaving out from the cubic-structure smaller pores of SBA-16 adsorbent might explain the enhancement of its sorption capacity. Summarizing, the SBA-16 functionalized with an optimized amount of aminopropyl groups demonstrated to be more efficient adsorbent than its SBA-15-based counterpart (Figure 8). The highest sorption capacity was achieved with the 3.3 TEOS/APTES molar ratio adsorbent due to its large specific surface area and unlimited accessibility for Pb 2+ ions to -NH 2 groups. The SBA-16-based adsorbents were stable until the temperature of 250 • C. The optimum pH value for removal of Pb 2+ ions from aqueous solution was found to be in the range 5 to 6. The equilibrium for lead adsorption was reached at about 40 min.
The comparison of the adsorption capacity of the 3.8N/SBA-16 studied in this work with that reported previously for the SBA-15-based counterpart [1] strongly suggests a larger adsorption capacity of the former with respect to the latter (39.8 and 36.4 mg of Pb 2+ per g of adsorbent, respectively). The abilities of both adsorbents appeared to be closely related to their pore structure, pore density and the amount of the grafted amine groups. Indeed, because of the much larger amount of organoalkoxysilane precursor, the SBA-15 functionalized with (3-aminopropyl) trimethoxysilane exhibited a larger adsorption capacity [16] than SBA-15 functionalized with (3-aminopropyl) triethoxysilane (APTES) [1] (90 mg g −1 vs. 39.8 mg g −1 ). This could be explained in terms of the cage-like structure and two-times smaller pore diameter of SBA-16 with respect to SBA-15 counterpart. In such a case, additional Pb 2+ ion trapping might well occur within the inner pore structure of SBA-16. Moreover, the easier accessibility of Pb 2+ ions to amine groups as well as the steric difficulty for their leaving out from the cubic-structure smaller pores of SBA-16 adsorbent might explain the enhancement of its sorption capacity.

Conclusions
This work demonstrated that ordered SBA-16 mesoporous silica material grafted with an optimized amount of amine-functional groups is extremely effective in the removal of lead ions from aqueous solutions.
The highest sorption capacity was achieved with the 3.3 TEOS/APTES molar ratio adsorbent. The cage-like structure of amine-modified SBA-16 demonstrated to be more effective for Pb 2+ elimination from the aqueous solution than the two-dimensional channel system of amine-modified SBA-15. The amine-free SBA-16 adsorbent did not show Pb 2+ ions adsorption at all. In the case of the SBA-16-based adsorbents, the easier accessibility of Pb 2+ ions to amine groups as well as the steric difficulty for their leaving out from the cubic-structure smaller pores might explain the enhancement of its sorption capacity. Although this research was developed to study the adsorption capacity of the material to remove Pb (II) ions on ideal aqueous solutions, it is intended to prove its efficiency on real samples in future work, so the interference of more ions and organic materials could be assessed.