Fabrication and covalent modification of highly chelated hybrid material based on silica-bipyridine framework for efficient adsorption of heavy metals: isotherms, kinetics and thermodynamics studies

Abstract

Adsorbent materials are essential in clean-up processes. Research of efficient materials is a well-established technology. In this work, a novel and excellent host for heavy metals was synthesized by covalent immobilization of bipyridine tripodal receptor onto silica particles. The new engineered surface was well analyzed and evaluated by BET, BJH, EA, FT-IR, SEM, TGA and solid-state ¹³C NMR. The adsorption properties were investigated using Pb(II), Cd(II), Zn(II) and Cu(II) metals by varying all relevant parameters such as pH, contact time, concentration, thermodynamic parameters, kinetics, Langmuir and Freundlich isotherms, etc. The hybrid material has been found to exhibit high distribution coefficients for heavy metals. Adsorption kinetics follows a pseudo-second-order model, as a rapid process as evidenced by equilibrium achieved within 20 min. The resulting adsorption isotherms of the material were better represented by the Langmuir model than the Freundlich model. The thermodynamic parameters (ΔH°, ΔS° and ΔG°) revealed that the adsorption was endothermic and spontaneous. In addition, the proposed material demonstrates a high degree of reusability over a number of cycles, thus enhancing its potential for application in heavy metals recycling. All metal ions were determined by atomic absorption measurements.

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Introduction

Water pollution caused by heavy metal ions has become a major issue globally due to their persistence and harmful effects upon aquatic life, human health^1,2 and the environment.^3–5 It has been reported that metals such as lead (Pb), cadmium (Cd), zinc (Zn) and copper (Cu) are among the most hazardous and are included on the US Environmental Protection Agency's list of priority pollutants. Therefore, their removal from natural and waste water has been drawing more and more attention.

Several techniques have been developed for the removal of these toxic metals from aqueous solution, such as ion exchange,⁶ reverse osmosis,⁷ adsorption,^8,9 precipitation,¹⁰ and filtration.¹¹ Compared with other techniques, adsorption is considered to be one of the most promising and widely applied methods owing to its low cost, high efficiency, operational simplicity, and reversibility.¹² Many kinds of adsorbents have been used for the removal of heavy metals from aqueous solution, such as activated carbon,¹³ clays,¹⁴ chelating resin,¹⁵ cellulose,¹⁶ zeolites,¹⁷ and silica gel.¹⁸ Nevertheless, it is of importance to further develop cheaper and available adsorbents with good performance.

To this end, silica-based organic–inorganic hybrids have rapidly emerged as a versatile and attractive class of adsorbents with advanced properties that are often difficult to achieve either from totally inorganic or from totally organic materials. These hybrids are robust solids displaying: (i) high specific surface area, (ii) three-dimensional structure giving rise to highly and regular porous framework, (iii) good mechanical and hydrothermal stabilities, (iv) exceptionally good accessibility to active centers, (v) fast mass transport rates inside the porous structure, and (vi) quite easy manufacture under mild conditions.^19–22 Indeed, in recent years, these organic–inorganic hybrids have become powerful adsorbent materials for removal of organic^23,24 and inorganic^25–29 contaminants from waters and wastewaters. Furthermore, the silica network exhibits good mechanical properties and stability in a wide range of pH. Also, it can be easily derivatized to a variety of organic functionalities by chemical and covalent coupling,^30–36 and it is much less expensive than the corresponding sepharoses.³⁷

Some organic–inorganic hybrid materials containing pyridine derivatives have already been reported, showing high bond stability for transition metals.^38–40 In this context, the ability of pyridine and its derivatives to act as ligands with sp² hybrid nitrogen donors has been the research subject of many preconcentration and coordination chemists. This is evident from the large number of articles on this topic.^41–43

We therefore propose the use of a newly designed pyridine ligand with the aim of developing a selective recovery system for heavy metals. Indeed, we successfully fabricated a new highly chelated hybrid material based on silica-supported N,N-bipyridine tripodal ligand. The active donor atoms (of grafted ligand) are so oriented to act as a convergent chelating bidentate donor due to the possible coordination to the same metal center, forming thus a more stable five-membered ring.³⁸ The synthesized adsorbent showed high affinity, high adsorption capacity and less equilibration time for efficient removal of heavy metals. Parameters that can affect the sorption efficiency of the metal ions Pb(II), Cd(II), Zn(II) and Cu(II) were studied in aqueous solutions using atomic absorption.

Results and discussion

Linker synthesis

The synthetic procedure for the new organic–inorganic hybrid material is summarized in Scheme 1. The first stage of the preparation was the synthesis of the ligand (L₂). The reaction was carried out from ethyl picolinate which was converted in the presence of lithium aluminum hydride to the hydroxyl product L₁. This product was then chlorinated using thionyl chloride to give the desired product (L₂). The structure of compounds was determined on the basis of the corresponding analytical and spectroscopic data.

Scheme 1 Synthesis route of modified chelating material.

The second stage involves reacting activated silica with 3-aminopropyltrimethoxysilane in toluene to form amino groups attached to the silica surface. These NH₂ groups on the silica surface were then reacted with ligand L₂ under gentle conditions to form the new chelating adsorbent SiNPr₂ (Scheme 1). The surface of the adsorbent was characterized on the basis of the corresponding analytical and spectroscopic data.

Characterization of the materials

Elemental analysis. The surface immobilization of silica particles was confirmed by the presence of carbon and nitrogen in the modified materials, which were primarily absent in native silica. The elemental analysis in each synthetic step is shown in Table 1; the C and N content appearing in SiNH₂ indicated the successful aminopropylation reaction. In the final step, the increase of C and N content in SiNPr₂ indicated that the ligand L₂ was perfectly attached to SiNH₂.

Table 1 Elemental analysis

Sample	%C	%N
SiNH2	4.46 ± 0.06	1.66 ± 0.02
SiNPr2	6.21 ± 0.04	1.82 ± 0.05

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FT-IR characterization. The modified silica was also confirmed by FT-IR analysis, as shown in Fig. 1. A characteristic feature of the 3-amonopropylsilica (SiNH₂) spectrum (b) when compared with that of native silica (a) was the appearance of a ν(NH₂) band around 1560 cm⁻¹ and a ν(C–H) weak band at 2941 cm⁻¹ corresponding to the carbon chain of the pendant group attached to the inorganic silica matrix. The spectrum of the final material (SiNPr₂) (c) reveals the disappearance of the absorption band at 1560 cm⁻¹ due to the reaction of the primary amine (–NH₂) and the appearance of new characteristic bands around 1552 cm⁻¹ and 1451 cm⁻¹ resulting from C=N and C=C vibrations respectively. These results showed that the methylpyridine units had been grafted onto the surface of silica gel after modification.

Fig. 1 FT-IR spectra of native silica (SiG), SiNH₂ and SiNPr₂.

Scanning electron microscopy (SEM). Micrographs were obtained of the native silica and chemically modified materials (SiNH₂ and SiNPr₂) in order to detect differences in their surfaces (Fig. 2). The surface of the native silica is composed of a macro particle structure. After modification, the surface was changed. It was evident that the loaded functional groups were distributed on the whole surface that made the surface of the product SiNPr₂ become rough.

Fig. 2 SEM images of native silica (A), SiNH₂ (B) and SiNPr₂ (C).

Thermogravimetric analysis (TGA). The TGA curves reflect the thermal stability and the degradation process of the materials. Curves of the native silica, SiNH₂ and SiNPr₂ have been established in the temperature range of 25 °C to 800 °C, and the results are shown in Fig. 3. The profile confirms the high thermal stability for the prepared material. Indeed, the native silica presents a first mass loss stage of 3.15% in the interval of 25 °C to 110 °C assigned to physically adsorbed water, and a second loss of 5.85% from 110 °C to 800 °C assigned to condensation of the free silanol groups which causes siloxane bond formation (Si–O–Si).^44,45 Again two distinct mass loss steps were detected for the SiNH₂ sample. The first one was a small mass loss of 1.56% in the room temperature to 100 °C range attributed to the remaining silanol hydration water. The second mass loss of 9.77% was observed between 208 and 800 °C, which corresponds to the organic matter immobilized on the surface. The final SiNPr₂ material presented two distinct mass loss stages. Following the preceding interpretation, the first mass loss of 2.01% in the 25–109 °C range is assigned to adsorbed water, and the other mass loss of 10.6% between 242 and 800 °C is attributed to the decomposition of the pyridine fraction immobilized on the surface of silica, together with the condensation of the remaining silanol groups. The pronounced increase in mass loss reflects the higher amount of the anchored organic groups.

Fig. 3 Thermogravimetric profiles of native silica (a), SiNH₂ (b) and SiNPr₂ (c).

¹³C-NMR characterization. Important features related to immobilization of pendant groups on the inorganic structure of the hybrid formed can be obtained through ¹³C NMR spectroscopy in the solid state⁴⁵ (Fig. 4). Indeed, the spectrum of SiNH₂ presented three well-formed peaks, at 9.02, 24.79 and 42.62 ppm, attributed to the propyl carbon. The signal at 50.62 ppm was assigned to the carbon of unreacted methoxy. The spectrum of the SiNPr₂ material showed peaks in the range of 122–156 ppm assigned to carbon atoms of dimethylpyridine units.

Fig. 4 ¹³C NMR spectra of SiNH₂ and SiNPr₂.

Surface properties. The surface areas of the materials were determined through the BET method and gave the results as S_BET of native silica > SiNH₂ > SiNPr₂. The decrease in surface area (Fig. 5) after incorporation of organic groups can be easily interpreted by the presence of pendant organic groups which partially block the adsorption of nitrogen molecules on the surface and consequently cause a decrease in the surface area. This hindrance is more pronounced for the largest organic moieties, originating from the bipyridine tripodal units. Moreover, the isotherm curves are of type IV according to the classification of IUPAC, and show for partial pressures P/P₀ > 0.4 a pronounced hysteresis. In addition, the hysteresis loops are of type H₂ which indicates that there is a uniform pore diameter distribution.

Fig. 5 Nitrogen adsorption–desorption isotherm plots of SiNH₂ and SiNPr₂.

Table 2 summarizes the physical parameters for the native silica, SiNH₂ and SiNPr₂ that were calculated from nitrogen adsorption–desorption isotherms. It was observed that the surface area and total pore volume decrease sequentially. The results can be attributed to the introduction of organic functional groups into the mesoporous channels which may reduce the pore size and increase the density of the material.^46–48

Table 2 Physical properties of silica derivatives

Silica derivative	Specific surface area SBET (m^2 g^−1)	Pore volume (cm^3 g^−1)
Native silica	305.21 ± 0.79	0.770 ± 0.002
SiNH2	283.08 ± 0.77	0.690 ± 0.002
SiNPr2	272.09 ± 0.99	0.680 ± 0.011

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Chemical stability. Chemical stability of the newly synthesized material SiNPr₂ was examined in various acidic and buffer solutions (pH 1–7). No destruction of the material structure was noticed even after 24 h of contact as determined by elemental analysis. The high stability exhibited by the attached organofunctional moiety is presumably due to the pendant group. It has been shown that when the length of the hydrocarbon bridge was more than two methylene groups, the rupture of Si–C bond did not occur in a mineral acid medium, due to the length of the chain; longer chains no longer have a functional handle that can undergo beta-elimination of the Si cation.^49,50

Heavy metal adsorption

Effect of pH. pH is one of the main variables affecting the sorption process, the speciation of the metal ions, the surface charge of the sorbent and the degree of ionization of the adsorbate during the reaction.⁵¹ Indeed, the effect of solution pH on the removal of Pb(II), Zn(II), Cu(II) and Cd(II) by SiNPr₂ was determined within the pH range of 1–7 and the results are given in Fig. 6. The removal of metals increased with an increase in solution pH. The maximum removal of Pb(II), Zn(II), and Cu(II) was observed at pH 6–7 while for Cd(II), it was in the range 5–7.

Fig. 6 Effect of pH on the adsorption of metal ions on SiNPr₂, at optimum concentration, t = 60 min and temperature of 25 °C. The coefficients of variation were lower than 5% for the data presented.

At lower pH values, the retention of metal ions by the functionalized silica SiNPr₂ is not significant since the donor atoms of the ligand must be almost entirely in their protonated form and a low interaction or a strong electrical repulsion force occurred between the donor atoms and the metal ions. With increasing pH, the protonation becomes weaker, and the interactions of donor atoms with metal ions become stronger, which enhances the chelation and adsorption of metal ions. Above pH 7, a variation of metal species in solution occurred because of the hydrolysis of metal ions, leading to the precipitation of metal hydroxide M(OH)₂; this makes it difficult to distinguish between hydrolyzed or adsorbed M(II).

This is consistent with the point of zero charge (PZC) corresponding to the pH of the medium at which the material surface charge is zero. Indeed, the pH_PZC value of native silica was 2.3, determined by using a simple previously described method.⁵² Surface coverage of SiNPr₂ leads to an increase of the PZC to give pH_PZC of 6.3 due to the basicity of the ligand used. This pH_PZC corresponds to the total deprotonation of the ligand, and therefore to a maximum sorption of metal ions.

Effect of adsorption kinetics. The kinetics of the adsorption process was investigated to study the effect of the initial concentration of metal ions on q_e with respect to time, and the time required to achieve equilibrium between aqueous and solid phase.⁵³ The kinetics study was carried out using optimum pH, optimum concentration and temperature of 25 °C. Two simple kinetic models, namely the pseudo-first-order model (eqn (1)) and the pseudo-second-order model (eqn (2)),⁵⁴ are the most often used to analyze the rate of sorption:

ln(q_e − q_t) = ln q_e – k₁t (1)

where q_e and q_t are the amounts of metal ions adsorbed on the adsorbent (mg g⁻¹) at equilibrium and at time t, respectively. k₁ and k₂ (min⁻¹) are the rate constants of the pseudo-first-order and the pseudo-second-order adsorption respectively.

The effect of contact time on the adsorption of metals by the SiNPr₂ material is shown in Fig. 7. Indeed, the kinetic curve showed that the adsorption was rapid and the plateau was reached after about 20 min of contact. The rapid adsorption can be explained by the availability and the orientation of the three nitrogens (active donor atoms), as convergent chelating sites, in the coordination of metal ions forming thus a more stable five-membered ring.³⁸

Fig. 7 Effect of shaking time on the adsorption capacity of Pb(II), Zn(II), Cu(II) and Cd(II) at optimum pH, optimum concentration and temperature of 25 °C. The coefficients of variation were lower than 5% for the data presented.

We also note that the adsorption affinity depends on several factors such as the nature, charge and size of metal ions, and the affinity of donor atoms towards each metal. It is therefore not surprising to have different affinities and adsorption of the same adsorbent with respect to different metals.

On the other hand, the rate of metal ion adsorption is one of the important characteristics that define the efficiency of sorption. It was evaluated by fitting the experimental data to the linear form of pseudo-first-order and pseudo-second-order kinetics using eqn (1) and (2) respectively. The fitted kinetic parameters are summarized in Table 3. The results show that the pseudo-second-order model has a better correlation coefficient R². Moreover, the calculated values of q_e by this model are also much closer to the experimental data than those of the pseudo-first-order kinetic model. These results suggested that the chemical process was likely to be the rate-limiting step of the adsorption mechanism.⁵⁵ Indeed, it is well known that the pseudo-second-order equation may be applied for chemisorption processes with a high degree of correlation in several literature cases where a pseudo-first-order rate mechanism has been arbitrarily assumed.

Table 3 Kinetics of heavy metal removal onto SiNPr₂

Parameter	Metal
	Pb(II)	Zn(II)	Cu(II)	Cd(II)
qe(exp) (mg g^−1)	99.68	82.68	64.84	42.18
Pseudo-first-order
qe (mg g^−1)	30.63	15.95	09.55	30.16
k1 (min^−1)	0.118	0.108	0.108	0.251
R^2	0.991	0.923	0.978	0.921
Pseudo-second-order
qe (mg g^−1)	100.00	83.33	64.93	42.91
k2 (g mg^−1 min^−1)	15.87 × 10^−3	28.80 × 10^−3	51.56 × 10^−3	38.24 × 10^−3
R^2	0.995	0.997	0.998	0.995

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Adsorption isotherms. In order to determine the relationship between the amount of metal ions adsorbed and the concentration of metal ions remaining in the aqueous phase, adsorption isotherm studies were performed under equilibrium conditions. These adsorption data for each metal ion were fitted to both the Langmuir and Freundlich isotherm equations. The Langmuir isotherm describes the monolayer sorption of metal ions on the surface of the sorbent, while the Freundlich isotherm model describes both multilayer sorption and sorption on heterogeneous surfaces. The linearized forms of the Langmuir isotherm (eqn (3))^56,57 and the Freundlich isotherm (eqn (4))⁵⁸ are expressed as follows:where q_e is the amount of solute sorbed on the surface of the sorbent (mg g⁻¹), C_e is the equilibrium ion concentration in the solution (mg L⁻¹), q is the saturated adsorption capacity (mg g⁻¹) and n is the Freundlich constant. K_L (L g⁻¹) and K_F (mg g⁻¹) are the Langmuir and the Freundlich adsorption constants respectively.

The adsorption behavior of heavy metal ions onto the adsorbent SiNPr₂ at different initial heavy metal ion concentration was investigated and the results shown in Fig. 8. The isotherms showed a sharp initial slope indicating that the material acts as a high-efficiency adsorbent at low metal concentration. In addition, when aqueous Pb(II), Zn(II), Cu(II) and Cd(II) concentration increased, a constant saturation value was reached.

Fig. 8 Effect of concentration on metal ion adsorption onto SiNPr₂ (adsorption dose = 10 mg, V = 10 mL, temperature = 25 °C and pH = 6) for Pb(II), Zn(II), Cu(II) and Cd(II). The coefficients of variation were lower than 5% for the data presented.

Parameters of the Langmuir and Freundlich models were calculated by plotting C_e/q_e versus C_e and ln q_e versus ln C_e, respectively. The Langmuir and Freundlich isotherm parameters for adsorption of Pb(II), Zn(II), Cu(II) and Cd(II) are given in Table 4. Comparison of the R² values shows that the Langmuir isotherm fitted quite well with the experimental data (R² > 0.99), indicating a uniform solid surface on the sorbent, and a regular monolayer sorption.

Table 4 Adsorption isotherm parameters for the removal of heavy metals onto SiNPr₂

Metal	Langmuir isotherm model			Freundlich isotherm model
	q (mg g^−1)	KL (L mg^−1)	R^2	KF (mg g^−1)	n	R^2
Pb(II)	111.11	0.061	0.991	8.490	1.88	0.942
Zn(II)	91.743	0.072	0.991	7.42	1.808	0.939
Cu(II)	68.027	0.221	0.997	12.54	2.529	0.823
Cd(II)	45.045	0.139	0.994	8.985	2.794	0.824

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Thermodynamic studies. In order to evaluate the effect of temperature on the sorption, the experimental values were fitted to the plots of the distribution coefficient value K_d as a function of temperature. The thermodynamics of metal ion sorption onto SiNPr₂ from aqueous solution were studied between 25 and 45 °C. The thermodynamic parameters such as change in Gibbs free energy (ΔG°), enthalpy (ΔH°) and entropy (ΔS°) were determined using the following equations:⁵⁹

ΔG⁰ = ΔH⁰ − TΔS⁰ (7)

where C₀ (mg L⁻¹) is the initial concentration of metal solution, C_e (mg L⁻¹) is the equilibrium concentration, V (mL) is the volume of solution, m (g) is the dosage of sorbents, R is the universal gas constant (8.314 J mol⁻¹ K) and T (K) is the absolute temperature.

The thermodynamics of Pb(II), Zn(II), Cu(II) and Cd(II) sorption onto SiNPr₂ from aqueous solution was studied between 25 and 45 °C using optimum concentration of all metal ions (Fig. 9). The ΔH°, ΔS° and ΔG° values were obtained from the slope and intercept of ln k_d versus T⁻¹ using eqn (5), (6) and (7) respectively.

Fig. 9 Effect of temperature on the sorption of metal ions onto SiNPr₂ (shaking time = 60 min, pH = 6, V = 10 mL, m = 10 mg of SiNPr₂ at optimum concentration).

Table 5 shows that the values of ΔG° are negative at all studied temperatures, indicating the spontaneous nature of the adsorption process. The positive values of enthalpy ΔH° show that the adsorption is endothermic. The positive value of ΔS° reveals the increase in randomness at solid solution interface during the adsorption of all four ions onto SiNPr₂.

Table 5 Adsorption models used in this work and their parameters

Metal	ΔH° (kJ mol^−1)	ΔS° (J K^−1 mol^−1)	T (±1 °C)	ΔG° (kJ mol^−1)
Pb(II)	4.33	14.77	25	−0.08
			35	−0.22
			45	−0.37
Zn(II)	11.46	38.62	25	−0.08
			35	−0.47
			45	−0.85
Cu(II)	17.46	61.46	25	−0.92
			35	−1.53
			45	−2.15
Cd(II)	9.92	34.53	25	−0.40
			35	−1.74
			45	−0.78

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Selectivity of SiNPr₂. Competitive adsorption experiments with Pb(II) for SiNPr₂ were carried out from the mixed Pb(II)–Zn(II)–Cu(II)–Cd(II) quaternary system using an aqueous solution containing optimum concentration of each metal ion, through the batch method. Fig. 10 shows the adsorption capacity of metal ions in the quaternary system. It can be seen that SiNPr₂ has excellent adsorption and high affinity especially for Pb(II). However, the extraction seems to decrease compared to the value obtained in the absence of foreign metal ions. Yet, the remarkable selectivity toward Pb(II) favors the material to be used as a promising potential adsorbent, for the removal of Pb(II) from aqueous solutions containing competing ions.

Fig. 10 Effect of foreign metal ions.

Applicability of SiNPr₂ for extraction of heavy metals from real samples. The applicability of the studied adsorbent was tested by its application for removal of Pb(II) and Zn(II) from water samples. To this end, two water samples were collected, (i) Ghiss River (Al Hociema) and (ii) Touissit-boubekker River (Jerada-Oujda), using polyethylene bottle and filtered through a 0.45 μm nylon filter. As the selected samples do not contain lead, we proceeded by adding an optimal concentration of standard Pb(II) (150 mg g⁻¹) to solutions. This investigation revealed (Table 6) that significant elimination of existing metals can be achieved in less than one hour of contact. However, the extraction seems to decrease compared to the value obtained in aqueous solutions because of possible interference of the organic matter and alkali metals naturally existing in real water.

Table 6 Results of the application of SiNPr₂ for extraction of heavy metals from water samples (with and without adding 150 mg L⁻¹ of Pb²⁺)

Water sample	Metal ion	Cfound ± 0.05 (mg L^−1)	Adsorption capacity (mg g^−1)^b	Adsorption capacity (mg g^−1)^c
a nd: not detectable.b Adsorption capacity without adding lead.c Adsorption capacity with adding 150 mg L^−1 of lead.
Ghiss river (Al Hociema)	Zn(II)	1.15	0.4	—
	Cd(II)	1.45	0.02	—
	Cu(II)	nd^a	—	—
	Pb(II)	nd	—	70.65
Touissit river (Jerada)	Zn(II)	12.05	6.35	—
	Cd(II)	2.25	0.41	—
	Cu(II)	nd	—	—
	Pb(II)	nd	—	72.39

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Regeneration of SiNPr₂. The sample was easily regenerated by soaking it in 6 N HCl for a few minutes (5–10 mL of 6 N HCl per g of support). This new solid extractor has a good stability and can be reused many times without decreasing its extraction percentage. Indeed, after five cycles of adsorbent regeneration (Table 7), no significant change in the percentages of adsorption was observed, which is remarkable. The stability of the organic groups on the solid surface was also confirmed by TGA, with no distinct changes in the sorbent material being observed after five cycles. This suggests that SiNPr₂ has excellent chemical stability as a highly efficient adsorbent for the recovery of heavy metals.

Table 7 Adsorption/regeneration of hybrid material toward: Pb(II), Zn(II), Cu(II) and Cd(II)

Cycle number	Pb(II)	Zn(II)	Cu(II)	Cd(II)
1	99.68	82.68	64.84	42.18
2	99.00	82.12	64.80	42.01
3	99.03	82.08	64.81	42.03
4	98.89	81.87	64.70	42.06
5	99.10	81.80	64.55	42.01

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Comparison with alternative sorbents. Table 8 shows the adsorption of Pb(II) by other sorbents reported in the literature. It is clear that the functionalized silica described in this work presents further improvement and shows better values and higher affinity for the effective adsorption of Pb(II).

Table 8 Comparison of the adsorption capacity (mg g⁻¹) of various sorbents toward: Pb(II), Zn(II), Cu(II) and Cd(II) from the literature

Support: silica gel/ligand	Pb(II)	Zn(II)	Cu(II)	Cd(II)	Ref.
This work	99.68	82.68	64.84	42.18	—
TOES	75.60	71.50	54.70	65.80	28
Nitrothiophene	52.41	35.72	61.52	38.45	29b
C,N-Bipyrazole	2.28	0.56	—	0	29c
Ketoenol furane	18.75	23.36	32.08	52.15	29e
Gallic acid	12.63	—	15.38	6.09	60
Tris (2-aminoethyl)amine	64.61	—	—	36.42	61
3-Aminopropyl	23.70	—	19.20	14.10	62
PMAEEDA	61.90	17.16	19.96	19.34	63
Dithiocarbamate	42.19	26.01	25.00	10.01	64

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Conclusions

Based on the experimental results, the following conclusions are drawn:

(1) A novel engineered material based on a hybrid material (SiNPr₂) with highly chelated bipyridine receptor has been successfully synthesized via a simple heterogeneous procedure and the surface was well characterized.

(2) The adsorption depends on pH in the range from 1 to 7. The maximum adsorption value of 99.68 mg g⁻¹, 82.68 mg g⁻¹, 64.84 mg g⁻¹ and 42.18 mg g⁻¹ for Pb(II), Zn(II), Cu(II) and Cd(II), respectively, was obtained in the pH range 6–7, and can be reached in only 20 min, suggesting rapid external diffusion and surface adsorption.

(3) The adsorption kinetics can be fitted to the pseudo-second-order model, and shows homogeneous characteristics. Comparison with different isotherm models indicated that the Langmuir model gave the best fit to the experimental data.

(4) The increase in adsorption capacity at increased temperature indicates that the adsorption of heavy metals onto the hybrid sorbent is endothermic in nature.

(5) The functionalized material displayed an excellent adsorption capacity for Pb(II) in competitive mode and in natural real water samples.

(6) The hybrid material (SiNPr₂) showed higher performance for heavy metal removal compared to literature reports.

(7) The adsorbent can be regenerated several times without loss of its adsorption capacity.

These results suggest that this novel material has potential for the removal of heavy metals from aqueous solution, thus opening important perspectives.

Experimental

Materials and methods

All solvents and other chemicals (Aldrich, purity > 99.5%) were of analytical grade and used without further purification. Silica (E. Merck) with particle size in the range of 70–230 mesh, median pore diameter 60 Å, was activated before use by heating it at 160 °C during 24 h. The silylating agent 3-aminopropyltrimethoxtsilane (Janssen Chimica) was used without purification. All metal ions were determined by atomic absorption measurements performed with a Spectra Varian A.A. 400 spectrophotometer, equipped with air–acetylene flame. The wavelength used for monitoring Pb, Cd, Cu and Zn was 283.3, 228.8, 324.8 and 213.9 nm, respectively. Metal ion detection was in the range 1–12 ppm, 0.1–0.6 ppm, 1–4 ppm and 0.1–0.6 ppm for Pb, Cd, Cu and Zn respectively. The calibration-curve method was used to elucidate the results of measurements. The pH value was measured by a pH 2006, J. P. Selecta SA. Elemental analyses were performed by Microanalysis Centre Service (CNRS). FT-IR spectra were obtained with a PerkinElmer System 2000. SEM images were obtained with an FEI-Quanta 200. The mass loss determinations were performed in 90 : 10 oxygen/nitrogen atmospheres with a PerkinElmer Diamond TG/DTA, at a heating rate of 10 °C min⁻¹. The solid-state ¹³C NMR spectrum was obtained with a CP MAX CXP 300 MHz. The specific area of modified silica was determined by using the BET equation. The nitrogen adsorption–desorption isotherms were obtained by means of a Thermoquest Sorpsomatic 1990 analyzer, after the material had been purged in a stream of dry nitrogen.

Syntheses

Synthesis of pyridine-2-ylmethanol. To a solution of LiAlH₄ (5.66 g; 0.14 mol) in 90 mL of THF was slowly added ethyl picolinate (6 g; 39.69 mmol) at 0 °C. The mixture was stirred under reflux for 4 h. After cooling, water (5.66 mL), 15% aqueous sodium hydroxide (5.66 mL) and then water (17 mL) were added successively to the mixture at 0 °C. The solid material was filtered and the residue was washed with hot THF. The filtrate and THF washing were concentrated under reduced pressure. The residue was passed through a short silica column (CH₂Cl₂/MeOH, 9/1) to give an 86% yield of L₁ as a brown liquid. R_f = 50% (CH₂Cl₂/MeOH, 9/1; silica). IR (KBr, cm⁻¹): ν(OH) = 3411; ν(C=N) = 1553; ν(C=C) = 1452. ¹H NMR (DMSO-d₆): δ 4.53 (s, 2H, −CH₂); 5.38 (s, 1H, OH); 7.21 (m, 1H, Py-H_β); 7.44 (d, 1H, Py-H_δ); 7.76 (t, 1H, Py-H_γ); 8.44 (d, 1H, Py-H_α). ¹³C NMR (DMSO-d₆): δ 64.52 (1C, CH₂); 120.60 (1C, Py-C_β); 122.35 (1C, Py-C_δ); 137.02 (1C, Py-C_δ); 148.89 (1C, Py-C_α); 162.31 (1C, Py-C_ε). MS: m/z, 110.05 (M + H)⁺.

Synthesis of 2-(chloromethyl)pyridine. A solution of 2.5 mL of thionyl chloride in 5 mL of methylene chloride was slowly added to compound L₁ (1.5 g; 13.74 mmol) in 30 mL of methylene chloride at 0 °C. This mixture was stirred for one night at room temperature. The solvent was removed under reduced pressure and the residue was dissolved in 60 mL of ether. The mixture was then neutralized with about 10 mL of saturated sodium bicarbonate solution and the ether solution was dried over anhydrous sodium sulfate. After evaporating the mixture, the residue was filtered through a short alumina column (CH₂Cl₂) to give a 76% yield of L₂ as a red viscous product. R_f = 0.72 (CH₂Cl₂/alumina). IR (KBr, cm⁻¹): ν(C=N) = 1555; ν(C=C) = 1454. ¹H NMR (DMSO-d₆): δ 4.79 (s, 1H, CH₂); 7.43 (t, 1H, Py-H_β); 7.60 (d, 1H, Py-H_δ); 824 (t, 1H, Py-H_γ); 8.37 (d, 1H, Py-H_α). ¹³C NMR (DMSO-d₆): δ 56.08 (1C, CH₂); 124.21 (1C, Py-C_β); 125.60 (1C, Py-C_δ); 145.45 (1C, Py-C_γ); 146.70 (1C, Py-C_α); 149.02 (1C, Py-C_ε). MS: m/z, 129.63 (M + 2)⁺ (32.45%).

Synthesis of 3-aminopropylsilica (SiNH₂). 25 g of activated silica gel was dispersed into dried toluene (150 mL) in a necked flask, refluxed and mechanically stirred under N₂ gas atmosphere for 2 h, and then 3-aminopropyltrimethoxysilane (10 mL) was gradually added into the solution with continuous stirring. The mixture was refluxed for 24 h. The final product was filtered, washed with toluene and ethanol. It was then Soxhlet-extracted with a mixture of ethanol and dichloromethane (1/1) for 12 h, to remove the silylating reagent residue. The immobilized silica gel was dried in vacuum at room temperature. Elemental analyses: %C = 4.46; %N = 1.66.

Synthesis of bipyridine tripodal-substituted silica (SiNPr₂). For the synthesis of SiNPr₂, a mixture of 3-aminopropylsilica (SiNH₂) (4 g), L₂ (6.75 g, 52.91 mmol) and sodium carbonate (8.41 g, 79.36 mmol) was reacted in 150 mL of dry acetonitrile. The reaction was stirred and refluxed under nitrogen for 6 days. Then the substituted silica was filtered off and washed with hot water to dissolve sodium carbonate. The product was transferred to a Soxhlet extraction apparatus for reflux-extraction in acetonitrile, methanol and dichloromethane for 12 h, successively. The product was dried under vacuum at 70  °C for 24 h.

Heavy metal adsorption experiments

In order to test the metal adsorption ability of the synthesized materials, a set of adsorption experiments was carried out by stirring 10 mg of functionalized silica in 10 mL of a single-metal solution at 25 °C. The aqueous systems selected were Pb(II), Zn(II), Cu(II) and Cd(II) with optimum concentration. The pH values were adjusted with dilute hydrochloric acid and sodium hydroxide solution. After shaking for 1 h, adsorbent-solution mixtures were filtered to collect the final solutions. Metal concentrations, both in the initial and final solution, were determined by flame atomic absorption spectrometry (FAAS). The equilibrium adsorption capacity of the adsorbent was calculated by the following equations:⁴⁴

Q_M = (C₀ − C_e) × V/W (8)

Q_W = Q_M × M (9)

where Q_M is the amount of the metal ion on the adsorbent (mmol g⁻¹), Q_W is the amount of the metal ion on the adsorbent (mg g⁻¹), V is the volume of the aqueous solution (L), W is the weight of the adsorbent (g), C₀ is the initial concentration of metal ion (mmol L⁻¹), C_e is the equilibrium metal ion concentration in solution (mmol L⁻¹) and M the atomic weight of the metal (g mol⁻¹).

For each set of data obtained, standard statistical methods were used to determine the mean values and standard deviations. Confidence intervals of 95% were calculated for each set of samples in order to determine the margin of error.

Batch experiments

The effects of pH, time of contact, concentration, kinetics, temperature of the system and competitive extraction of metals were tested and evaluated by the batch method. The adsorption properties of SiNPr₂ for Pb(II), Zn(II), Cu(II) and Cd(II) were investigated. After the extraction, the suspension was separated; the residual metal concentration was determined by FAAS using standard solutions for calibration.

Acknowledgements

The authors extend their appreciation to the PPR2-MESRSFC-CNRST-P10 project (Morocco). Sincere appreciation is also extended to the Deanship of Scientific Research at King Saud University for its supporting this Prolific Research group (PRG-1437-29).

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