Modified SBA-15 mesoporous silica for heavy metal ions remediation

Modified SBA-15 mesoporous silica for heavy metal ions remediation

Chemosphere 73 (2008) 1499–1504 Contents lists available at ScienceDirect Chemosphere journal homepage: Modified...

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Chemosphere 73 (2008) 1499–1504

Contents lists available at ScienceDirect

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Modified SBA-15 mesoporous silica for heavy metal ions remediation Mihaela Mureseanu a, Aurora Reiss a, Ioan Stefanescu b, Elena David b, Viorica Parvulescu c, Gilbert Renard d, Vasile Hulea d,* a

Faculty of Chemistry, University of Craiova, 165 Calea Bucuresti, 200144 Craiova, Romania National Institute of Cryogenics and Isotope Separation, 4 Uzinei Street, 240050 Rm.Valcea, Romania c Institute of Physical Chemistry, 202 Spl. Independentei, 060021 Bucharest, Romania d Institut Charles Gerhardt, UMR 5253, CNRS-UM2-ENSCM-UM1, Matériaux Avancés pour la Catalyse et la Santé, 8 rue de l’Ecole Normale, 34 296 Montpellier Cedex 5, France b

a r t i c l e

i n f o

Article history: Received 16 January 2008 Received in revised form 7 July 2008 Accepted 16 July 2008 Available online 28 August 2008 Keywords: Salicylaldehyde Schiff bases Copper Nickel Cobalt Zinc

a b s t r a c t N-Propylsalicylaldimino-functionalized SBA-15 mesoporous silica was prepared, characterized and used as an adsorbent for heavy metal ions. The organic–inorganic hybrid material was obtained using successive grafting procedures of SBA-15 silica with 3-aminopropyl-triethoxysilane and salicylaldehyde, respectively. For comparison an amorphous silica gel was functionalized using the same procedure. The structure and physicochemical properties of the materials were characterized by means of elemental analysis, X-ray diffraction (XRD), nitrogen adsorption–desorption, thermogravimetric analysis and FTIR spectroscopy. The organic functional groups were successfully grafted on the SBA-15 surfaces and the ordering of the support was not affected by the chemical modification. The behavior of the grafted solids for the adsorption of heavy metals ions from aqueous solutions was investigated. The hybrid materials showed high adsorption capacity and high selectivity for copper ions. Other ions, such as nickel, zinc, and cobalt were adsorbed by the modified SBA-15 material. The adsorbent can be regenerated by acid treatment without altering its properties. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction The increasing level of heavy metals in water represents a serious risk to human health and ecological systems. The most abundant harmful metals in the liquid effluents are Cr, Ni, Zn, Cu, and Cd. They are considered persistent, bioaccumulative and toxic substances. Adsorption is widely used in the removal of heavy metals from wastewater. Various adsorptive compounds, such as activated charcoal, zeolites and clays are capable of capturing metal ions from dilute aqueous solutions. Porous silica functionalized with various chelating agents is increasingly utilized as an adsorbent because of its high selectivity for metal ions adsorption (Zaporozhets et al., 1999; Soliman et al., 2001; Yamini et al., 2002; Abou-El-Sherbini et al., 2002). The discovery of hexagonally ordered mesoporous silicas (Kresge et al., 1992) has stimulated a renewed interest in adsorbents and catalysts design because of their unique large surface area, well-defined pore size and pore shape. The addition of organic groups by grafting of organosiloxane precursors onto the surface of the pores result in functional mesoporous hybrid materials (Lee and Yi, 2001a; Antoschuk and Jaroniec, 2002; Bibby and Mercier, 2002; Hossain and Mercier, 2002; Walkarius et al., 2002; Yoshitake et al., 2002; Yee Ho et al., 2003; Zhang et al., 2007; Yang et al., 2008). These organic–inor* Corresponding author. Tel.: +33 4 67 16 34 64; fax: +33 4 67 16 34 70. E-mail address: [email protected] (V. Hulea). 0045-6535/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2008.07.039

ganic hybrid materials have been reported to exhibit improved sorption properties toward heavy metal ions, superior to those achieved with silica gel functionalized with the same ligand (Liu et al., 2000; Walkarius and Delacote, 2003). Unfortunately, the most extensively investigated mesoporous material MCM-41 silica shows low mechanical and hydrothermal stability. It has been shown that the low hydrothermal stability of MCM-41 material is due to the hydrolysis of its thin (1–2 nm thickness) pore walls (Landau et al., 1999; De Clercq et al., 2003). In 1998, Zhao et al. (1998a) developed SBA-15 mesostructured silica, which consists of parallel cylindrical pores with axes arranged in a hexagonal unit cell. SBA-15 usually has wider pores than MCM-41 (SBA-15 pores range from 5 to 30 nm), and higher pore volumes. Moreover, in comparison with other mesostructured silica materials, SBA-15 exhibits thicker pore walls (between 3.1 and 6.4 nm) which provide high hydrothermal stability (Zhao et al., 1998b; Khodakov et al., 2005), being suitable for use in aqueous media. SBA-15 surface modifications with organic species for adsorption applications have been presented. Functional groups, such as thiol (Kang et al., 2004; Perez-Quintanilla et al., 2006), imidazole (Li et al., 2007), amino (Liu et al., 2000; Wang et al., 2005), polyol (Wang et al., 2006), iminodiacetic (Gao et al., 2007) were incorporated into the inorganic SBA-15 network. The thiol-functionalized SBA-15 silicas showed exceptional selectivity for adsorbing Hg2+ (Liu et al., 2000; Perez-Quintanilla et al., 2006) and noble metals (Kang et al., 2004) from waste


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streams. The imidazole-derivatized SBA-15 exhibited high adsorption capacity for Cr (VI) (Li et al., 2007). Amino-functionalized SBA15 showed a high affinity for different metal ions, such as Cu2+, Zn2+, Cr3+ and Ni2+ (Wang et al., 2005). Wang et al. (2006) reported that SBA-15 with polyol functional groups show a very good boron adsorption capacity. Very recently, Gao et al. (2007) prepared iminodiacetic acid-modified SBA-15 hybrid material with excellent ability to remove Cd2+ from aqueous solutions. Schiff base-type groups, easily prepared by condensation between aldehydes and amines, are known as very efficient chelating agent for many different metals (Abdel-Latif et al., 2007). Recently it was shown that the immobilisation of Schiff base ligands onto solid supports, such as organoclays (Lagadic, 2006) and MCM-41 silica (Sutra and Brunel, 1996; De Clercq et al., 2003; Singh et al., 2005), led to the formation of hybrid organic–inorganic materials which are able to chemisorb the transition metal ions. Keeping in view the interesting properties of both SBA-15 mesoporous material and Schiff’s base, the present study had two aims. Firstly, we have synthesized and characterized a new hybrid material, i.e., SBA-15 mesoporous silica modified with N-propylsalicylaldimine. Secondly, we have studied the applicability of this material for removal of heavy metal ions from aqueous solutions. The results are compared with those obtained with amorphous silica gel functionalized by the same procedure.

2. Experimental 2.1. Preparation of functionalized SBA-15 silica SBA-15 material was synthesized as described in Zhao et al. (1998a). The organic–inorganic hybrid materials were obtained by a two-step post-grafting procedure with 3-aminopropyl-triethoxysilane (APTES) and salicylaldehyde (SA), respectively. Firstly, the calcined SBA-15 was silanized with APTES according to the previously described procedure (Brunel et al., 1995; Martin et al., 2001). One gram of SBA-15 silica, freshly activated overnight at 453 K under vacuum, and 1 mL of APTES (99% Aldrich) was added to 50 mL of dry toluene. After stirring the solution (reflux, 2 h), the released ethanol was distilled off and the mixture was kept under reflux for 90 min. The NH2-functionalized mesoporous silica (referred as NH2–SBA-15) was filtered and washed with toluene, ethanol and then diethyl ether. It was then submitted to a continuous extraction run overnight in a Soxhlet apparatus using diethyl ether/dichloromethane (v/v, 1/1) at 373 K and dried overnight at 403 K. In the second step, the NH2–SBA-15 solid (1 g), activated under vacuum at 403 K for 1 h, was dispersed in dry toluene (50 mL) before adding 1 mL of SA. The mixture was refluxed at 398 K overnight and the obtained yellow solid (denoted as SA–SBA-15) was collected by filtration and washed successively with toluene, ethanol and diethyl ether. The SA excess was removed by Soxhlet extraction using diethyl ether/dichloromethane (v/v, 1/1). The obtained SA–SBA-15 was dried overnight at 313 K. Silica Gel 60 (from Merck, 0.015–0.040 mm particle size, average pore size of 6 nm, specific surface area of 550 m2 g1) was used as a reference material. Its functionalisation with APTES and SA was performed using the same procedure as described above for SBA-15. The two obtained hybrid materials will be referred as NH2–SG and SA–SG, respectively. 2.2. Characterization Small-angle XRD data were acquired on a Bruker diffractometer using Cu Ka radiation. N2 adsorption–desorption isotherms were measured at 77 K with a Micromeritics ASAP 2010 instrument. The samples were degassed at 323 K for 12 h prior to the adsorp-

tion measurements. Specific surface area was calculated by the BET method, the mesopore volume was determined by nitrogen adsorption at the end of capillary condensation, and pore size distribution was determined from the desorption isotherms. FTIR spectra of self-supported wafers previously heated at 423 K under vacuum were performed on a Bruker Vector 22 spectrometer. Atomic absorption spectrophotometry (FAAS) measurements were performed on a Spectra AA-220 Varian spectrophotometer equipped with Varian multi-element hollow cathode lamps and air–acetylene burner. C, H, and N contents were evaluated by combustion on a Fisons EA1108 elemental analysis apparatus. Thermogravimetric analysis was carried out in a Netzsch TG 209C thermobalance. The amount of grafted chains per nm2 (standardized to pure silica weight) was calculated using the method reported by Martin et al. (2001). 2.3. Metal ion adsorption tests Metal nitrates (analytical grade) were dissolved in high purity water in order to prepare initial metal ion solutions with different concentrations. Adsorption tests were carried out at room temperature, in batch conditions, by mixing the adsorbent with aqueous solutions at pH 4.8 (0.1 M phthalate buffer solution), under stirring (600 rpm). After 30 min the adsorbent was filtered and the residual metal concentration in the solution was measured by atomic absorption spectrophotometry. Ion competitive adsorption studies were performed by treating 100 mL of a mixed metal solution containing equimolar amounts (5 mmol L1 of Cu2+, Co2+, Ni2+ and Zn2+ ions) with 100 mg of the adsorbent for 30 min under stirring, at room temperature. For all adsorption tests, deviations between initial and final pH were less then 0.1 pH units.

3. Results and discussion 3.1. Physicochemical properties of SBA-15 and modified SBA-15 materials The organic functional groups were sequentially grafted on the SBA-15 surface by a two-step post-synthesis procedure (Scheme 1). The surface hydroxyl groups were first reacted with the ethoxy groups of aminopropyl triethoxysilane. The amino groups were then reacted with salicylaldehyde molecules to form bidentate Schiff base ligands supported on the SBA-15 surface. Based on the elemental and thermogravimetric analysis, the amount and the density of the functional groups grafted on the SBA-15 surface were measured. Thus, the amounts of grafted aminopropyl and SA groups per nm2 (of pure SBA-15 silica) were 3.1 and 3.0, respectively. As previously reported, the average number of silanol groups (quantified by 29Si NMR spectroscopy) for the SBA-15 material is 3.6 OH nm2 (Palkovits et al., 2006). Based on this density of silanol groups, the calculated yield of the aminopropyl grafting was about 86%. For the SA coupling reaction onto the aminopropyl functionalized silica the yield of coupling was more than 98%, providing the efficiency of the second step of the chemical process described by Scheme 1. From the elemental analysis data it was also established that the C/N and H/N atom ratios in the NH2–SBA-15 sample were 6.7 and 17.9, respectively. This result suggests that the stoichiometry between the silanol group and the silylating agent was 1:1, indicating that most of the aminopropyl silane chains are linked to the pore wall surface only by one Si– O–Si bond (Scheme 1). The C/N molar ratio calculated for the SA– SBA-15 sample was 13.2, indicating a 1:1 stoichiometry between the aminopropyl group and SA. In comparison with SBA-15, a lower loading of functionality was obtained for the amorphous silica gel. Thus, the amount of

M. Mureseanu et al. / Chemosphere 73 (2008) 1499–1504


Scheme 1. Schematic illustration of the SBA-15 functionalization.

aminopropyl groups grafted on this material was 2.02 molecules nm2. Moreover, the number of SA functions was only 1.01 molecules nm2, indicating that the second grafting step was incomplete. This low grafting level may be attributed to the non-optimum experimental conditions and/or the steric hindrance effect during the second reaction of the functionalization process. The XRD measurements confirmed the SBA-15 structure for both unmodified and grafted samples. The SBA-15 material exhibited a strong (1 0 0) reflection peak (at 2h = 0.7°) and smaller (1 1 0), (2 0 0), (2 1 0) diffraction peaks, which are characteristic of a well ordered SBA-15 type material (Zhao et al., 1998a). No significant changes upon amine and SA immobilization were observed, except for the expected decrease in XRD peak intensity, providing evidence that functionalization occurred mainly inside the mesopore channels. All materials exhibited irreversible type IV adsorption–desorption isotherms (not shown here) with a H1 hysteresis loop in the partial pressure range from 0.65 to 0.75, characteristic of materials with 7–8 nm pore diameter. This result reveals that the uniform mesoporous nature of the material is preserved even though the grafting has occurred. The main textural properties of solids are listed in Table 1. We note that for the functionalized SBA-15 materials the BET surface and volume were standardized versus pure silica weights. As expected, the BET surface area and the mesopore volume strongly decreased after grafting, according to the sequence SBA15 > NH2–SBA-15 > SA–SBA-15. For example, for the aminopropyl functionalized mesoporous silica (NH2–SBA-15) the BET surface area and the mesopore volume were diminished by 53% and 52%, respectively, comparatively to the SBA-15 support. These textural results confirm that the grafted species are located inside the mesopores and not only on the outer surface. FTIR spectra of the SBA-15 precursor and organic–inorganic hybrid SBA-15 materials previously heated at 423 K under vacuum are illustrated in Fig. 1. At this temperature only the physisorbed water was removed from the samples, as was determined from the TGA curves. The FTIR spectrum of SBA-15 shows typical bands at 3500–3750 cm1 due to the presence of silanol groups. After functionalization with aminopropyl groups, the intensity of the bands corresponding to these groups decreased with a concomitant increase of bands characteristic of the immobilized aminopro-

Fig. 1. FTIR spectra for unmodified SBA-15 and functionalized SBA-15 silica.

pyl groups (NH2–SBA-15 spectrum), indicating that the reaction between the OH groups of the silica network with the ethoxy groups of the organic precursor has taken place. As previously reported (Liu et al., 2000; Wang et al., 2005, 2006; Gao et al., 2007), the new bands can be attributed to both symmetric and asymmetric stretching of CH3 and CH2 groups (mas (CH3) = 2975 cm1, mas (CH2) = 2928 cm1, ms (CH3) = 2886 cm1, ms (CH2) = 2870 cm1) and of NH2 function (mas = 3450 cm1, ms = 3400 cm1). A band at 1560 cm1 (dN–H) was also identified. These results confirm the successful functionalisation of SBA-15 with aminopropyl groups. Characteristic bands can also be observed in the FTIR spectrum of the SA–SBA-15 material. The first one, at about 1650 cm1 was assigned to –[email protected] stretching vibration of the imine group, which is the principal band for the Schiff base ligand (Losada et al., 2001). The second one, at 1250 cm1 can be attributed to the phenolic C–O bond. The medium intensity bands at 2922 and 2885 cm1 correspond to the mas (CH2) and ms (CH2) vibrations. This FTIR result confirms the formation of the Schiff base ligands on the SBA-15 surface. 3.2. Chemical stability of functionalized SBA-15 silica

Table 1 Textural properties of calcined SBA-15 and modified SBA-15 samples Sample

SBET (m2 g1)

Vmeso (mL g1)

D (nm)

SBA-15 NH2–SBA-15 SA–SBA-15

697 368 317

1.49 0.77 0.53

8.3 7.8 6.9

The chemical stability of the functionalized SBA-15 was examined in acidic and basic solutions. Each sample was mixed with 1, 3 and 6 M HCl or NaOH solutions and stirred at room temperature for 24 h. The change in the degree of functionalization was calculated by elemental analysis of the samples before and after the chemical treatment. After acid treatment the percentage of N and C in the functionalized mesoporous silica was not modified. More-


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3.3.2. Metal ion adsorption studies Adsorption in homoionic solutions. The amount of metal ions adsorbed by SA–SBA-15 (Q, mmol g1), the distribution coefficient (Kd, mL g1), and the removal level for each ion are summarized in Table 2. The metal loading capacities of different adsorbents for an initial ion concentration of 4 mmol L1 are included in Table 3. Each value in these tables is the mean of three determinations with an accepted relative standard deviation of ±3%. Based on these results, some main remarks can be made: (i) the Q values corresponding to the functionalized SA–SBA-15 and SA– SG samples were much larger than those obtained with the unmodified SBA-15 (for this material, the Q values were less than 0.17 mmol g1), confirming the favorable role played by the organic functional group; (ii) the mesostructured SA–SBA-15 material is more efficient than the modified silica gel SA–SG sample; (iii) both SA–SBA-15 and SA–SG adsorbents show very good adsorption capacity for the Cu2+ ions, which is much higher comparatively to that of other metal ions; (iv) the loading capacities decrease in the following order: Cu2+ > Zn2+ > Ni2+ > Co2+. For the maximum loading level of the Cu2+-containing complex (0.92 mmol g1), the copper/nitrogen molar ratio was 0.42 which is in agreement with the expected value of 0.5 (Singh et al., 2005; Lagadic, 2006). This confirms that the functionalization of copper onto the SA–SBA-15 support proceeds with the intended Cu–Schiff base complex format in a 1:2 (Cu–Schiff base) molar ratio (Scheme 2). The extent of the functional group participation to the metal ions complexation for the tested adsorbent was calculated based on the amount of SA groups grafted on the silicas, considering the metal/ligand ratio of ½. Furthermore, the amount of complexed metal ions was determined by subtraction from the total adsorbed metal quantity, the amount of metal adsorbed by the non-functionalized silica. The values (%) obtained for the two adsorbents were: (i) SA–SBA-15: 83.6 (Cu2+), 35.5 (Ni2+), 27.3 (Co2+), 34.6 (Zn2+) and (ii) SA–SG: 100 (Cu2+), 61.4 (Ni2+), 31.8 (Co2+), 13.6 (Zn2+). These results can be related to the formation constants (Kf) of the metal ion–ligand complexes. Indeed, according to the Irving–Williams series (Shriver et al., 1992), the bond strength between a ligand (despite its nature) and a series of metal cations depends on the Kf parameter. For the metal ions investigated in

over, the XRD patterns and the N2 adsorption–desorption isotherms were mostly similar with those of the parent materials, confirming a high stability in mineral acid media. On the contrary, the functionalized SBA-15 was found to be unstable in alkaline solutions due to the breaking of the Si–O–Si bonds by hydroxide ions. 3.3. Heavy metal ion adsorption studies 3.3.1. Evidence of metal ions complexation Transition metal ions (Cu2+, Zn2+, Ni2+ and Co2+) were incorporated in the N-propylsalicylaldimine functionalized SBA-15 silica, by treating this material with solutions containing metal salts. As known, pH is an important parameter in the ion adsorption process. In order to prevent the precipitation of metallic cations, the solution pH must be adjusted in the range 1.76–6.2 (Jiang et al., 2007). The best adsorption performances were obtained in the pH range of 4–5.5 (Girish Kumar and Saji John, 2006; Jiang et al., 2007). Based on these results, in this study we performed the adsorption tests at pH 4.8. The resulting Me2+–Schiff base SBA-15 solids were characterized by FTIR spectroscopy. In order to establish the nature of the complexes bands, the FTIR bands were compared with those of the free ligands. The band assigned to –[email protected] stretching vibration of the imine group (which was observed at 1650 cm1 for the free ligand) appeared at lower frequencies in the spectra of Me2+–Schiff base complexes (Fig. 1), indicating the coordination of the imine nitrogen with the Me2+ ion (Refat et al., 2006). This feature can be explained by the withdrawal of electrons from the nitrogen atom to the metal ions. The band assigned to C–O group (at 1250 cm1 for the free ligand) also shifted to lower frequencies after metal adsorption. The new bands at about 500 and 400 cm1 appearing for the complexes are assigned to the m(Me–O) and m(Me–N) vibration modes, respectively (Golcuk et al., 2004).

Table 2 Adsorption of Cu2+, Co2+, Ni2+ and Zn2+ ions by SA–SBA-15 from homoionic solutionsa Metal ions

Co (mmol L1)

Ce (mmol L1)

Qb (mmol g1)

Me2+ removal (%)

Kdc (mL g1)


2 4 5 6

0.01 0.35 0.5 1.4

0.40 0.73 0.90 0.92

99.5 91.3 90.0 76.7

39 800 2086 1800 657


2 3 4

0.6 1.5 2.4

0.28 0.30 0.32

70.0 50.0 40.0

467 200 133


3 4 5

1.1 2.1 3.2

0.38 0.38 0.36

63.3 47.5 36.0

345 181 113


2 3 4

0.4 1.2 2.0

0.32 0.36 0.40

80.0 60.0 50.0

800 300 200

a b c

Conditions: 0.05 g adsorbent, 10 mL, solution, pH 4.8. V Q ¼ ðC 0  C e Þ  W ; V = solution volume (L), W = adsorbent weight (g). 3 K d ¼ 10C eQ .

Scheme 2. Suggested model of the possible mode of Cu2+ ion complexation by the SA–SBA-15 sample.

Table 3 Metal ion-loading capacities of SBA-15 type adsorbentsa Adsorbent

Cu2+ (mg g

SBA-15 SA–SBA-15 SA–SG a

10.5 46 33

Ni2+ 1


(mmol g 0.17 0.73 0.51



Co2+ 1

(mg g 8.3 22 16



(mmol g


0.14 0.38 0.27

Conditions: initial metal concentration 4 mmol L1, 0.05 g adsorbent, 10 mL solution, pH 4.8.

(mg g 5.8 19 8.3

Zn2+ 1


(mmol g 0.09 0.32 0.14



(mg g1)

(mmol g1)

6.9 26 17

0.11 0.40 0.26


M. Mureseanu et al. / Chemosphere 73 (2008) 1499–1504 Table 4 Competitive adsorption of Cu2+, Co2+, Ni2+ and Zn2+ ions by different adsorbentsa Adsorbent

Cu2+ (mg g

SBA-15 SA–SBA-15 SA–SG a

8.2 44 29

Ni2+ 1


(mmol g



0.13 0.70 0.45

Co2+ 1

(mg g 7.6 3.5 25



(mmol g 0.13 0.06 0.42


Zn2+ 1

(mg g 4.7 2.9 13



(mmol g 0.08 0.05 0.22


(mg g1)

(mmol g1)

5.5 2.6 18

0.08 0.04 0.28

Conditions: 0.1 g adsorbent, 100 mL solution, pH 4.8.

this study, the Kf values, and implicitly the bond strength between the metal ion and the ligand, increase in the following order: Zn2+ < Co2+ < Ni2+ < Cu2+. Our results are completely in agreement with this rule. Ion competitive adsorption studies. The competitive adsorption of Cu2+, Ni2+, Co2+ and Zn2+ ions on SA–SBA-15 and SA–SG materials are summarized in Table 4. Both hybrid materials showed a higher adsorption capacity for Cu2+ than for the other metal ions; the adsorbents affinity for different ions decreased in the order: Cu2+  Ni2+ > Co2+ > Zn2+. The complexation selectivity coefficients (ki) of the Cu2+ in the presence of other Me2+ ions (Me2+ = Ni2+, Co2+ and Zn2+) were calculated according to Eq. (1)

ki ¼

K d Cu2þ K d Me2þ


3.3.3. Regeneration of SA–SBA-15 adsorbent The regeneration capacity of the SA–SBA-15 material was evaluated by acid treatment. 100 mg of Cu2+–SA–SBA-15 complex sample (0.92 mmol g1 Cu) was treated with 100 mL of a 0.3 M HCl solution, under stirring for 4 h, at room temperature. The chemical analysis indicated that the metal was integrally removed. XRD and FTIR spectroscopy measurements of the regenerated SA–SBA-15 solid confirmed that its structure was not altered by acid treatment. After the acid regeneration and neutralization (with a 0.3 M NaHCO3 solution), the solid was reused in three successive adsorption–regeneration cycles. The adsorption capacity of the SA–SBA-15 after four cycles was 0.9 mmol g1, indicating a loss in the adsorption capacity of only 3.3%, compared to the initial one. These data indicate the very good regeneration capacity of the SBA-15 modified material.

The ki values obtained for the SA–SBA-15 adsorbent were 11.9, 15.6 and 10.4 for Ni2+, Co2+ and Zn2+, respectively. These values were much higher than those obtained for the unmodified SBA15 sample (1.0, 1.6 and 1.6 for Ni2+, Co2+ and Zn2+, respectively), indicating that the N-propylsalicylaldimine group complexed selectively the Cu2+ ions. We consider that the adsorption mechanism of metal cations is different for the organically functionalized SBA-15 type mesoporous silica and the silica gel. In the first case, the complexation is the major mechanism for the metal ion adsorption. This mechanism is governed by the formation constants of the metal complexes and consequently the Q values are affected by the competition between different ions in solution and are limited by the amount of surface grafted ligand. Contrarily, the mechanism of cation adsorption onto amorphous functionalized silica is based not only on the complexation between the metal ions and the ligand, but a physisorption mechanism is also involved. In this case the Q value increases when the total metal ion concentration increases.

An effective adsorbent for metal ions has been prepared by immobilization of N-propylsalicylaldimine groups on the surface of the SBA-15 mesoporous silica. The high density of the organic groups grafted on the SBA-15 surface resulted in remarkable adsorption capacities for Cu2+, Co2+, Ni2+ and Zn2+ ions. The hybrid material showed higher adsorption selectivity for Cu2+ compared to other metal ions present in a mixed metal ions solution. These results suggest the possibility of employing this material in the selective recovery of the Cu2+ ions from a mixed metal ions solution. The chemical stability of the functionalized material SA– SBA-15 in acidic media, as well as the possibility for the regeneration by washing with HCl, allowed the reuse of the adsorbent material for several cycles. Comparison with other studies. In the present study, the best adsorption capacity of SA–SBA-15 material was 59 mg g1 for Cu2+ ions. For comparison, the adsorption capacities for Cu2+ showed by different common adsorbents (reviewed in Babel and Kurniawan (2003) and O’Connell et al. (2008)) were as follows: 19–75 mg g1 (modified cellulose materials); 9–38 mg g1 (activated carbon); 13–222 mg g1 (chitosan); 1–25 mg g1 (zeolites); 3–11 mg g1 (clays); 10–66 mg g1 (agricultural waste). The hybrid materials showed different potential for copper adsorption: aminopropylMCM-41: 30.5 mg g1 (Algarra et al., 2005); mercaptopropyl-functionalized porous silica: 13 mg g1 (Lee et al., 2001b); EDTA modified SBA-15: 13.2 mg g1 (Jiang et al., 2007). It can be observed that the adsorption efficiency of SA–SBA-15 is higher than that of most adsorbents. Additionally, if we take into account the particular characteristics of the SBA-15 support (excellent textural properties, high hydrophilicity, their surface silanol groups can be easily functionalized by using various organic species), we can state that the adsorbents based on modified SBA-15 mesoporous silica are interesting candidates for applications in heavy metal removal from wastewater.

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4. Conclusions



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