Synthesis of bifunctional mesoporous silica spheres as potential adsorbent for ions in solution

Chemical Engineering Journal 214 (2013) 27–33

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Synthesis of bifunctional mesoporous silica spheres as potential adsorbent for ions in solution Elis C.C. Gomes a, Adriano F. de Sousa b, Pedro H.M. Vasconcelos a, Diego Q. Melo b, Izaura C.N. Diógenes a, Eduardo H.S. de Sousa a, Ronaldo F. do Nascimento b, Rosane A.S. San Gil c, Elisane Longhinotti b,⇑ a b c

Universidade Federal do Ceará, Departamento de Química Orgânica e Inorgânica, c.x. Postal 6021, Cep 60455-960 Fortaleza CE, Brazil Universidade Federal do Ceará, Departamento de Química Analítica e Físico-Química, c.x. Postal 6021, Cep 60455-960 Fortaleza CE, Brazil Universidade Federal do Rio de Janeiro, Instituto de Química, Laboratório Multiusuário de RMN de Sólidos, Brazil

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

" The immobilization of EDTA on Si–

APTS composite was observed to occur in a stoichiometric relation of 2(APTS):1(EDTA). " IR and NMR data suggest a cavitylike structure with four nitrogen atoms inside and two carboxylate groups outside. " Adsorption assays for ions indicated a potential applicability of the Si– APTS–EDTA spheres as a bifunctional material.

a r t i c l e

i n f o

Article history: Received 7 September 2012 Received in revised form 27 October 2012 Accepted 27 October 2012 Available online 7 November 2012 Keywords: Silica spheres EDTA functionalization Adsorption

a b s t r a c t The immobilization of EDTA on Si–APTS composite was observed to occur in a stoichiometric relation of 2(APTS):1(EDTA) with a degree of functionalization of 0.56 mmol g1. Based on IR and NMR data it was suggested that the immobilization results in a cavity with four nitrogen atoms inside and two carboxylate groups outside. Surface area, total pore volume and average pore diameter, of the Si–APTS–EDTA spheres were determined, respectively, as 177.6 m2 g1, 0.35 cm3 g1 and 73.6 Å. Zeta potential (PZC = 5.0) and adsorption assays indicated a potential applicability of the Si–APTS–EDTA spheres as a bifunctional material since it was observed the adsorption of cations and anions. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction The presence of potentially toxic substances in natural reservoirs has very large repercussions in the economy and on public health. Therefore, the global concern on environmental issues, particularly those related to heavy metals and anions, has been raised over the last decades. A superficial search in the web of science database for ‘‘removal’’ AND ‘‘heavy metals’’ results in ⇑ Corresponding author. Tel.: +55 85 3366 9052; fax: +55 85 3366 9982. E-mail address: [email protected] (E. Longhinotti). 1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2012.10.053

7000 entries for the last five years. This information reflects the relevance and how this theme is still actual for the scientific community. In addition and not less important, it is the financial concern. According to the Environmental Protection Agency of the United States, the American Recovery and Reinvestment Act (ARRA) provided $7.2 billion for environmental programs in January 2012. These concerns have motivated research groups to look for efficient materials for the treatment of liquid effluents containing toxic substances. In this sense, several species have been widely used in SLPE (Solid–Liquid Phase Extraction) method [1–7]. Selectivity, efficiency and versatility are among the most sought properties in

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the development of adsorbent materials for toxic substances. Silicabased materials are very interesting because of the possibility to attach groups to produce materials with specific functionalities [8–18]. For instance, the attachment of chelant compounds has been used to produce insoluble materials with high removal capacity toward metal ions in solution [19–25]. The aim of this work is to present the results and discussions on the material produced upon the immobilization of ethylenediaminetetraacetic acid (EDTA) on Si–APTS spheres. In addition, preliminary assays of adsorption were run in order to evaluate the capacity of the functionalized spheres to simultaneously adsorb cations and anions in aqueous solution. 2. Experimental section 2.1. Chemicals The water used throughout was purified from a Milli-Q water system (Millipore Co.). Nitrogen or argon was used for the reactions carried out under air free conditions. Organic solvents (Merck and Aldrich) of spectroscopic grade, 3-aminopropyltriethoxysilane (APTS–98%), ethylenediaminetetraacetic acid (EDTA-99.4%), and diisopropylcarbodiimide (DIC-98%) from Aldrich, and Pb(NO3)2, Cd(NO3)2, Na2SO4, and KH2PO4, purchased from Merck, were used as received. 2.2. Functionalization of silica spheres The synthesis of mesoporous silica spheres was carried out as reported previously [26]. The functionalization of the mesoporous silica spheres with APTS, Si–APTS, was made as reported in the literature [18,27]. In brief, the silica spheres were thermally activated at 110 °C under vacuum for 4 h. After that, 1.0 g of activated silica was transferred to a reactional flask containing 3.54 mmol of APTS dispersed in toluene. The mixture was kept under reflux and argon for 48 h when the solid was filtered, washed with toluene and ethanol and dried under vacuum. 1.0 g of this material was added to a solution of dimethylformamide (DMF) containing 7.08 mmol of EDTA and 1.77 mmol of DIC as coupling agent. The mixture was kept under stirring at 60 °C for 24 h when the solid was filtered off, washed with DMF and ethanol and dried under vacuum.

were degassed for 2 h at 150 °C in vacuum. Specific surface areas, mesopore volume and micropore volume were calculated according by Brunauer–Emmett–Teller (BET) [28], Barret–Joyner–Halenda (BJH) [29] and Dubinin–Radushkevich (DR) [30] methods, respectively. 2.4. Zeta potential Suspensions of the sample were obtained by adding 1.0 g of Si–APTS–EDTA spheres in 100 mL of water (1% by weight), varying the pH in the range of 1.0–12.0, at 25 °C. Aliquots of these suspensions were then analyzed to determine the Zeta potential. The experiments before adsorption were performed using the commercially available equipment Zetasizer Nano from Malvern (ZEN 3500). 2.5. Adsorption assays The stock solutions (25.0 mg L1) of the ions (Pb2+, Cd2+, SO2 4 and H2 PO 4 ) used in the adsorption assays was prepared by dissolving the salts in appropriate amounts of deionized water without previous treatment. The experiments were carried out by adding 0.1 g of the Si–APTS–EDTA in 100 mL conical flasks containing 50.0 mL of the ions stock solution. The pH of the initial solutions were adjusted by adding 0.1 mol L1 NaOH or 0.1 mol L1 HCl solution and the ionic strength was kept constant at 0.01 mol L1 with KNO3. The solutions were kept stirring on shaker bath at 100 rpm for a period of 5 h at 25 °C. After the contact time, the solids were separated from the supernatants by centrifugation and the concentration was determined from the respective analytical curves. The analysis of anions were carried out by ionic chromatography using a Dionex ion chromatography ICS-3000 equipped with an anion self regeneration suppressor (ASRS 4 mm), AS18 (250 mm  4 mm) analytical column with KOH as eluent, and conductivity detection. The cations concentrations were measured using an Atomic Absorption Spectrophotometer GBC 933 plus model from Varian, Inc. Corporate (Palo Alto, CA/USA). The amount of adsorbed ions (Qt, mg g1) was calculated by the following mass balance relationship:

Qt ¼

ðC o  C t ÞV W

ð1Þ

2.3. Apparatus The transmission infrared (IR) spectra of the compounds dispersed in KBr were obtained with a ABB-BOMEM FTLA 2000-102 spectrophotometer with the resolution of ±1 cm1. Prior to the acquisition of the spectra of the samples, spectra of the non-functionalized silica spheres were acquired as blank. Cross-polarization magic-angle spinning (CPMAS) Nuclear Magnetic Resonance (NMR) spectra were performed on a Bruker DRX300 spectrometer (7.05 T), operating at frequencies of 75.4 (13C) and 59.3 (29Si) MHz and equipped with a 4 mm Bruker CPMAS probe. The samples were packed in ZrO2 rotors with a KelF cap. 13C and 29 Si CPMAS NMR spectra were acquired with 1 and 4 ms of contact time and 4 and 2 s of recycling time, respectively. The external references used for the chemical shifts were: adamantine (38.3 ppm) for 13C and Q3 Si sites of caulinite (91.5 ppm) for 29Si. Tetramethylsilane (TMS) was used as internal standard (0.0 ppm). Elemental analyses were performed on a CE Instruments EA 1110 CHNS-O analyzer model. Scanning electronic microscopy (SEM) images were obtained by Philips XL30 microscope operated at 20 kV. Textural properties were studied by N2 adsorption– desorption isotherms at 196 °C in a Micromeritics ASAP2020 apparatus between 0.1 and 0.995 mmHg. Before analyses, the samples

Fig. 1. (A) IR vibrational spectra of EDTA (dotted line) and Si–APTS–EDTA (solid line) and (B) proposed structure for the Si–APTS–EDTA composite.

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E.C.C. Gomes et al. / Chemical Engineering Journal 214 (2013) 27–33 Table 1 29 Si CPMAS NMR chemical shifts for SiO2 and Si–APTS–EDTA samples. SiO2

Si–APTS–EDTA

d (ppm)

Sites

Assignment

d (ppm)

Sites

Assignment

92 101 111

Q2 Q3 Q4

SiO2(OH)2 SiO3(OH) SiO4

59 68 101 110

T2 T3 Q3 Q4

SiO2OH)(CH2-R) SiO3(CH2-R) SiO3(OH) SiO4

where Co is the initial concentration of ions in the solution (mg L1), Ct is the concentration of ions in the solution (mg L1) at time t, V is the volume (L) of the solution and W is the mass (g) of the Si–APTS– EDTA spheres. 2.5.1. Adsorption equilibrium The adsorption of ions on Si–APTS–EDTA spheres was performed in a shaker bath, provided with hooks for 100 mL Erlenmeyer flasks. Before starting the experiment 100 mg of Si–APTS–EDTA spheres was taken in 50 mL of ions solution with different concentration (15.0; 30.0; 45.0; 50.0 and 75.0 mg L1). The flasks were then transferred to the thermostatized shaker bath at 25 °C. After 5 h the suspensions were filtered and the filtrates were analyzed for equilibrium concentration by ion chromatography or atomic absorption spectrophotometry. 3. Results and discussion 3.1. Characterization of Si–APTS–EDTA composite Fig. 1 shows the infrared vibrational spectra in the region from 2750 to 750 cm1 of the Si–APTS–EDTA composite material and of EDTA for correlation purpose. The IR spectrum of EDTA presents all bands typically assigned to this species [31,32]. The vibrational modes associated to the carboxylic fragment are observed at 1720, 1432 and 1245 cm1 being assigned, respectively, to m(C@O), d(CAOAH) and m(CAO). Upon immobilization, new bands are observed at 1648, 1590 and 1403 cm1 being assigned to the m(C@O), mas(CAO) and ms(CAO) vibrational modes of the amide and carboxylate groups, respectively. In addition, there is a decrease in the intensity of the bands

assigned to the carboxylic fragment. This is an expected result since, as can be seen in the structures illustrated in Fig. 1, the immobilization is proposed to occur through two of the four carboxylate groups. The structure of the functionalized silica spheres were also characterized by 29Si and 13C CPMAS NMR spectra. Tables 1 and 2 present the assignments of the observed signals. The original spectra are available as Figs. S1 (29Si) and S2 (13C) in the Supporting information. Tn and Qn sites are used to label, respectively, organic groups directly attached to the silica and all silica containing species (SiO2(OH)2; SiO3(OH) and SiO4). In comparison to the TMS standard compound (0.0 ppm) the 29Si chemical shifts of the non-functionalized material, SiO2, are observed at lower frequencies in consequence of the electropositivity character of Si in relation to H atoms. Based on the relative intensities of the peaks of the 29Si CPMAS NMR spectrum of the precursor material, one can conclude that the SiAOH groups (Q3), observed at 101 ppm, is the most abundant site in relation to Si(OH)2, which is observed at 92 ppm. In the spectrum of the Si–APTS–EDTA composite, the peaks observed at 59 and 68 ppm are assigned to T2 (CASiAOH) and T3(CASiAO) sites indicating the functionalization of the precursor material [33–35]. As can be ascertained in Table 2, no signal at c.a. 57 ppm, which is assigned to ethoxyl groups, was observed in the 13C CPMAS NMR spectrum of the Si–APTS composite indicating that these groups were hydrolyzed to SiAOASi. On the other hand, the chemical shifts of the carbon atoms in the neighboring of the carboxylic group are indicative of the immobilization of EDTA on the Si–APTS composite. This result is reinforced by the observation of a new signal at 43.3 ppm, which is assigned to the carbon atoms of the ACH2ANHAC@O fragment of EDTA molecule. The observation of the signals at 177.3 and 179.8 ppm, which are assigned to COO, indicates the presence of carboxylic groups not immobilized on the Si–APTS–EDTA composite as illustrated in Fig. 1B. This conclusion is in accordance with the IR results from which it was suggested the presence of carboxylate groups in the Si–APTS–EDTA composite. Accordingly, each APTS–EDTA unit contains four nitrogen atoms producing a ring-like structure in the modified surface that may be associated with a cavity as a macrocyclic providing binding sites for ‘‘guest’’ ions. With this idea in mind, the results show that two carboxylate groups are free of immobilization.

Table 2 13 C CPMAS NMR chemical shifts for Si–APTS, EDTA and Si–APTS–EDTA samples. Chemical shifts are for bold underlined atoms. Si–APTS d (ppm)

EDTA Assignment

Si–APTS–EDTA

d (ppm)

Assignment

d (ppm)

Assignment

SiACH2A

53.9

NACH2ACH2AN

10.7

SiACA

SiACACH2A

59.2

HOOCACH2AN

22.2

SiACAC

61.7

ACH2ANHAC@O

11.1 26.4 44.6

SiACH2ACH2ANH2

HOOCACH2AN

43.3

169.1

COOH

51.9

AH2CACH2AN

175.5

COOH

59.5

O@CACH2AN

61.9

HCOOACH2AN O@CAN COO COO

173.3 177.3 179.8

Table 3 Percentages (%) of C, Si and N of SiO2, Si–APTS and Si–APTS–EDTA. Sample

SiO2 Si–APTS Si–APTS–EDTA

Carbon

Nitrogen

C/N

FD

%

mmol g1

%

mmol g1

Calc.

Found

mmol g1

0.107 6.83 10.45

– 5.69 ± 0.01 8.71 ± 0.02

0.162 2.52 3.13

– 1.80 ± 0.006 2.23 ± 0.007

– 3.00 4.00

– 3.16 ± 0.01 3.91 ± 0.02

– 1.8 0.56

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The percentages of C, Si and N were evaluated by elemental analysis and are presented in Table 3. The functionalization degree (FD) was determined based on the C/N ratio. For the non-functionalized material (SiO2) there is a low percentage of C and N indicating that the calcination results in the virtual removal of the chitosan, which was used as template for the formation of the spheres. For the functionalized materials, the C/N ratio increases with the immobilization of organic species on the silica spheres, as expected. The amount of immobilized EDTA on the Si–APTS composite was determined based on the C/N ratio and admitting that for each EDTA molecule two ACOOH groups bind to two NH2 groups of the surface of the composite Si–APTS, resulting in a stoichiometric relation of 2(APTS):1(EDTA), according to IR and NMR data (see Fig. 1B for the proposed structure). Therefore, the degree of functionalization of EDTA on the Si–APTS composite was calculated as 0.56 mmol g1. 3.2. Morphologic characterization of Si–APTS–EDTA composite The textural properties of the Si–APTS–EDTA composite were determined by N2 adsorption isotherms which are illustrated in Fig. 2. As can be ascertained in Fig. 2, the isotherm profile is consistent with a type IV with a distinct hysteresis loop of type H3, characteristic slit-type pores with average pore diameter 73.6 Å resulted from the aggregation of plate-like particles [36]. This conclusion is reinforced by the curve of the distribution of pore diameter from which it was determined that the majority of mesoporous is in the range of 20–80 Å. Furthermore, the isotherm shows an inflection at low relative pressures, suggesting the presence of micropores. The isotherm profile for the samples Si and Si–APTS (not shown) are very similar to those observed in Fig. 2 for Si–APTS–EDTA. Table 4 shows the textural properties of the silica spheres before (Si) and after (Si–APTS and Si–APTS–EDTA) modification. The results suggest that the functionalization reactions promote a decrease gradually in specific area, diameter and volume of pores, suggesting a diffusion and graft reaction of the modify molecules into interior of pores. The grafted functional groups (APTS and EDTA) cause a decrease in pore size and consequently the steric hindrance block the adsorption of N2 molecules. This result is in concordance with for similar materials reported by the literature [37–39]. The average diameter of the Si–APTS–EDTA spheres was determined as 1 mm according to the SEM (Scanning Electron Microscopy) micrographs illustrated in Fig. 3. 3.3. Zeta potential and adsorption assays on Si–APTS–EDTA spheres 3.3.1. Zeta potential Fig. 4 shows the plot of Zeta potential in function of pH. As can be seen in the plot of Zeta potential, there is a decrease in the charge density of the material with the increase in the pH. The isoelectric point (PZC) was determined as 5.0 indicating positive charge density below this value while negative charge density is observed over 5.0. This result corroborates with the structural characterizations (NMR and IR) in which the existence of two carboxylate non-immobilized groups is proposed. Therefore, these groups are deprotonated at pH higher than 5.0 increasing the negative charge density of the material. This negative charge density is still more increased at pH values higher than 9.0 due to the deprotonation of the nitrogen atoms. Correlating this data with those of the structural determinations, it is suggested that, depending on the pH, the Si–APTS–EDTA spheres can present regions of different charge densities. At pH 2.0 the cavity is positively charged and the carboxylate groups are protonated while at pH 5.0 the carboxylate groups are deprotonated neutralizing part of the positive charge of the cavity. At pH values higher than 9.0 all sites are deprotonated thus increas-

Fig. 2. N2 adsorption isotherms of Si–APTS–EDTA composite and plot of distribution of pore diameter.

Table 4 Textural properties of the silica spheres determined by N2 adsorption. Sample

Vpmicro (cm3 g1)

Vpmeso (cm3 g1)

Dp (Å)

S (m2 g1)

SiO2 Si–APTS Si–APTS–EDTA

0.13 0.074 0.049

0.85 0.52 0.35

92.6 82.8 73.6

382.7 251.2 177.6

S = specific surface area, Vp = micro and mesopore volume, Dp = pore diameter.

ing the negative charge density of the whole material. This aspect makes the Si–APTS–EDTA spheres a bifunctional material since it can be simultaneously used as adsorbent for cations and anions. 3.3.2. Adsorption isotherm The adsorptions process includes the transference of solute in the solution phase onto the surface of adsorbent. The two most com-

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Fig. 3. SEM micrographs of Si–APTS–EDTA.

Table 5 Langmuir adsorption isotherm parameters for ions adsorption on Si–APTS–EDTA spheres. Ions

Pb2+

Cd2+

SO2 4

H2 PO 4

K (L mg1) Qmax (mg g1) KL (L g1) R2

0.376 7.15 2.69 0.999

0.166 5.54 0.919 0.998

0.175 19.02 3.33 0.999

0.054 17.95 0.970 0.986

monly methods used to interpret the experimental data are Freundlich [40] and Langmuir [41] isotherms models, so these models ware used to interpret the experimental data obtained to ions adsorption onto Si–APTS–EDTA spheres. The adsorption isotherms were carried  2+ out at pH 3.0 for SO2 and Cd2+, mono4 and H2 PO4 and 5.5 for Pb elemental solutions of the ions. The linearized form to Langmuir model (Eq. (2)) provided the best fit to the experimental.

Ce 1 Ce ¼ þ Q e KQ max Q max Fig. 4. Zeta potential of an aqueous suspension of Si–APTS–EDTA spheres at different pH values.

ð2Þ

were Qmax is the maximum adsorption capacity (mg g1) and K (L mg1) is a constant related to the affinity between the adsorbent and the adsorbate. The linearization of the adsorption isotherm according to the Langmuir model is shown in Fig. 5. In Eq. (2), Qmax

Fig. 5. Linearization of the adsorption isotherm according to the Langmuir model for anions (A) and cations (B). Insert: the adsorption isotherm of ions by Si–APTS–EDTA spheres. Conditions: 100 mg of the adsorbent, initial ions concentration 15–75 mg L1, contact time 5 h at 25 °C, pH 3.0 for anions and 5.5 for cations.

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represents the monolayer coverage and is related to the Langmuir equilibrium constant, KL (L g1) by the Eq. (3) [42].

K L ¼ Q max  K

ð3Þ

Table 5 presents the adsorption parameters for adsorption of ions onto Si–APTS–EDTA spheres. The data presented in Table 5 hint that the Si–APTS–EDTA spheres are, indeed, a potential bifunctional material since it is showed a good adsorption performance for the cations and anions removal in solution. Therefore, the maximum adsorption capacity (Qmax) showed more favorable for anions, as expected, because the Zeta potential presents a surface with high potential positive charge. There is a greater adsorption capacity 1 for the SO2 ) compared to H2PO4- (17.95 mg g1) un4 (19.02 mg g der the conditions studied. This selectivity may be attributed to the more favorable interaction of the sulfate with cavity that is protonated at pH 3.0. For the cations adsorption the relationship of the polarizability for both adsorbate and adsorbent groups, resulting in better adsorption capacity for Pb2+ than for Cd2+ since both the Pb2+ and the carboxylate group are less polarizable [43]. The selectivity for lead ions is also reinforced by the values of the adsorption equilibrium constants (K). The K values of 0.376 L mg1 for Pb2+ and 0.166 L mg1 for Cd2+, suggesting a more effective coordination between the Pb2+ and COO groups of the adsorbent, making the process more irreversible in this case [44]. The adsorption assays indicated a real potential applicability of the Si–APTS–EDTA spheres as a bifunctional material since it was observed the adsorption of cations and anions, making it very attractive adsorbent. This is an important characteristic when compared with the other of adsorbents containing silica and which have similar adsorption capacities [24,25,45,46] or even with materials that are more efficient for cations or anions [20,21,39]. 4. Conclusions EDTA molecules were immobilized on silica spheres previously functionalized with APTS, thus forming Si–APTS–EDTA spheres. This material was characterized by means of IR, 29Si and 13C CPMAS NMR, SEM, elemental analysis, N2 adsorption isotherm and Zeta potential. According to the results, the immobilization of EDTA on the Si–APTS composite occurs in a stoichiometric relation of 2(APTS):1(EDTA) with a degree of functionalization of 0.56 mmol g1. IR and NMR data suggest the presence of carboxylic groups not immobilized on the Si–APTS–EDTA composite. In such case, there must be considered that each APTS–EDTA unit contains four nitrogen atoms inside the cavity and two carboxylate groups outside the cavity. The textural properties, surface area, total pore volume and average pore diameter, of the Si–APTS–EDTA spheres were determined, respectively, as 177.6 m2 g1, 0.35 cm3 g1 and 73.6 Å. Zeta potential (PZC = 5.0) and adsorption assays indicated a real potential applicability of the Si–APTS–EDTA spheres as a bifunctional material since it was observed the adsorption of cations and anions. Acknowledgements The authors gratefully acknowledge the ‘‘Universidade Federal do Ceará’’ and the Brazilian agencies, CNPq and FUNCAP (PRONEM PRN-0040-00065.01.00/10 SPU No.: 10582696-0), for the financial support. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cej.2012.10.053.

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