- Email: [email protected]
Pb (II) removal from aqueous media by EDTA-modified mesoporous silica SBA-15
Journal of Colloid and Interface Science 385 (2012) 137–146
Contents lists available at SciVerse ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Pb (II) removal from aqueous media by EDTA-modified mesoporous silica SBA-15 Jin Huang, Meng Ye, Yuqi Qu, Lianfeng Chu, Rui Chen, Qizhuang He, Dongfang Xu ⇑ College of Life and Environmental Science, Shanghai Normal University, Shanghai 200234, China
a r t i c l e
i n f o
Article history: Received 21 March 2012 Accepted 18 June 2012 Available online 5 July 2012 Keywords: SBA-15 Chemical modification Adsorbent Adsorption Lead (II)
a b s t r a c t An organic–inorganic hybrid mesoporous silica material was synthesized by two-step post-grafting method of SBA-15 with 3-aminopropyltrimethoxy-silane (APTES) and thionyl dichloride (SOCl2) activated ethylenediaminetetraacetic acid (EDTA) in sequence and measured by means of Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), elemental analysis (EA), transmission electron microscopy (TEM), nitrogen (N2) adsorption–desorption analysis and back titration. The material was found having the beneficial properties of mesoporous silica SBA-15 and EDTA. Adsorption potential of the material for Pb (II) removal from aqueous solution was investigated by varying experimental conditions such as pH, contact time and initial metal concentration. The removal efficiency of Pb2+ was high under studied experimental conditions. The adsorption equilibrium could be reached within 20 min and the kinetic data were fitted well by pseudo-second-order and intraparticle diffusion model. The adsorbent exhibited a favorable performance and its maximum adsorption capacity calculated by the Langmuir model was 273.2 mg g1. Recycling experiments showed the adsorbent could be regenerated by acid treatment without altering its properties. The chemical states of the elements involved in the adsorption were analyzed by X-ray photoelectron spectroscopy (XPS). The results demonstrated that the adsorption mechanism of the material involved Na Pb ion-exchange and carboxyl group dominated surface complexation. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction A large variety of heavy metals are discharged into the environment and constitute the most significant environmental pollutants found in wastewater. Long-term exposure to those solvated metal ions and consequently the effects on human health and natural ecosystems are critical issues [1,2]. Lead is one of the common contaminants of industrial wastewaters. A large quantity of single or daily intake of lead can cause various serious disorders, such as damage to liver, kidney and reduction in hemoglobin formation, mental retardation, infertility and abnormalities in pregnant women [3]. Adsorption technology is one of the most popular methods to remove lead in contaminated water. To become an adsorbent for lead ions with best performance, a set of conditions must be met including (a) the occurrence of an open pore structure and accessible adsorption sites; (b) the adsorbent must be structurally stable under adsorption; (c) good contact should be estabilished between the lead ions and the adsorption sites. The hexagonally ordered mesoporous silica materials exhibit many attractive characteristics such as well ordered periodic pore structure, high thermal and chemical stabilities and controllable pore diameter [4]. Then, those materials with various chelating ⇑ Corresponding author. Fax: +86 21 64323350. E-mail address: [email protected] (D. Xu). 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.06.054
agents are increasingly utilized as adsorbents because of the high selectivity of the agents for metal ions adsorption [5–7]. SBA-15 is one of the most popular mesoporous silica materials because of its large adjustable pores (5–30 nm), which allow easier accessibility of target species to the inner surface of the material and lead to fast kinetics of chemical or physical processes, as well as its thick pore walls, which provide enhanced mechanical stability and can be modified with organic groups to tailor their properties and achieve specific purposes [8–10]. Thus, the development of functionalized mesoporous silica adsorbents with appropriate functional groups for heavy metal ions is interesting and promising [11]. Ethylenediaminetetraacetic acid (EDTA) has favorable chelating and ion-exchange properties for many different metal ions [12,13]. Therefore, its immobilization on the different supporting materials for the metal adsorption purposes has received wide attention. EDTA dianhydride (EDTAD), an active agent containing two anhydride groups per molecule, can react with amino groups of organic molecules so as to introduce chelating groups. The use of EDTAD to modify supports has been reported once in the literature [14–18]. In all these cases, EDTAD was observed to form stable chelates with metals. However, in their functionalization process, the two anhydrides of EDTA had the same reaction activity and both of them might react with the amino groups of the matrices, and thus decreased the amount of carboxyl groups available for adsorption of metal ions. Moreover, few interaction mechanisms involving
138
J. Huang et al. / Journal of Colloid and Interface Science 385 (2012) 137–146
both the chelation and ion-exchange for EDTA modified adsorbents have been reported in the literature because of the complexities between the modified adsorbents and metals. The objectives of this work are to synthesize mesoporous silica SBA-15 with abundant carboxyl groups, explore its application for Pb (II) removal from aqueous solutions and discuss the mechanism of Pb (II) adsorption onto the functionalized mesoporous adsorbent. For this purpose, (a) a new method for EDTA modification with stoichiometric thionyl chloride (SOCl2) as a carboxyl group activating reagent was developed, which activated only one carboxyl group per EDTA molecule and thus increased the amount of carboxyl groups available for adsorption of metal ions, (b) the preparation and characterization of the hybrid adsorbent were described, (c) a series of adsorption experiments were carried out and their kinetics and isotherms were investigated by fitting the experimental data using pseudo-first-order, pseudo-second-order, intraparticle diffusion, the Langmuir and Freundlich isotherm models, and (d) the chemical states of the atoms involved in the adsorption were analyzed using X-ray photoelectron spectroscopy (XPS) to interpret the interaction mechanism between Pb (II) and the functional groups. 2. Experimental 2.1. Materials 3-aminopropyltrimethoxy-silane (APTES, 95%) and Triblock copolymer poly (ethylene glycol)-block-poly (propylene glycol)block-poly (ethylene glycol) (Pluronic P123, MW = 5800, EO20PO70EO20) were purchased from Aldrich. Tetraethoxysilane (TEOS), hydrochloric acid (HCl, 37%), ethylenediaminetetraacetic acid (EDTA), thionyl chloride (SOCl2), toluene, dichloromethane (DCM), ether, acetone, sodium carbonate solution (Na2CO3), aqueous sodium bicarbonate (NaHCO3), nitric acid (HNO3), NaOH and lead nitrate were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All of the reagents and solvents were of analytical reagent grade and were used as received except toluene and DCM which were distilled just before use. Millipore water was used in all experiments. 2.2. Synthesis of functionalized mesoporous silica Pure silica SBA-15 was synthesized using triblock copolymer pluronic 123 as a soft template and TEOS as the silica source by following the recipe in the bibliography [19,20]. 2.0 g of Pluronic 123 was dissolved in 62 mL of 2 M HCl at the room temperature. 4.2 g of TEOS was added into this solution and the synthesis was carried out by stirring for 24 h at 40 °C, then kept aging for 24 h at 100 °C. Solid product was recovered by filtration, removal of template by using Soxhlet ethanol extraction (roughly 200 mL per gram of silica) during 24 h, and dried overnight at 80 °C under the vacuum conditions. The resulting material was simply denominated SBA15. The aminopropyl grafting of SBA-15 was implemented by applying APTES as the silylation reagent [21]. 1.0 g of SBA-15 powder was mixed with 1 mL of APTES in dry toluene solution and then the mixed solution was kept with reflux for 12 h under Ar atmosphere. The precipitate was filtered and washed adequately with toluene and dichloromethane. It was submitted to a continuous extraction running overnight in a Soxhlet apparatus with diethyl ether/dichloromethane (v/v, 1/1) at 30 °C and dried overnight at 60 °C in a vacuum container. The solid product was denoted as NH2-SBA-15. The further modification of NH2-SBA-15 was carried out in an anhydrous condition using DCM as a solvent [22]. After 0.04 mol of EDTA and 100 mL of DCM were mixed in a three-necked flask, 0.04 mol of SOCl2 was then slowly added to
the mixture through a constant pressure drop funnel under Ar atmosphere. Immediately after SOCl2 was completely dropped, 1.00 g of NH2-SBA-15 was rapidly added to the mixture. The mixture was stirred at room temperature for 2 h and the chemically modified EDTA-SBA-15 was obtained. The solid product was thus separated by filtration, washed in a row with DCM, acetone, deionized water, NaHCO3 (0.1 M), deionized water and acetone. Finally, it was dried overnight at 50 °C under vacuum conditions [23]. 2.3. Instruments and characterization FT-IR spectra (KBr pellets) in the range of 4000–400 cm1 were taken on a American Nicolet AVATAR 380 FT-IR spectrometer. Elemental analyses were performed on a Vario EL-III elemental analyzer. Small-angle XRD patterns were recorded on a Rigaku D/ max-2000 diffractometer using Cu Ka radiation (k = 1.5406 Å) at 40 kV and 20 mA. Transmission electron microscopy (TEM) images were captured on a JEOL JEM-2100 electron microscope operated at 120 kV. N2 adsorption–desorption isotherms were measured at 196 °C by using a Micromeritics Tristar II 3020 surface area and pore size analyzer. The samples were degassed for 10 h at 100 °C prior to the adsorption measurements. Specific surface area (SBET) was calculated by the BET method, the pore volume (Vp) was determined at the amount of liquid nitrogen adsorbed at P/ P0 = 0.97, and pore size distribution (Dp) was obtained from the adsorption isotherms by the Barrett–Joyner–Halenda (BJH) method. The chemical composition was analyzed using a PHI-5000 versa-probe X-ray photoelectron spectrometer (XPS). The binding energy was calibrated using the carbon 1s peak as the reference energy at 284.8 eV with a monochromatic Al Ka radiation (1486.6 eV, 25 W, 15 kV). Survey scans were performed using a 1 eV/step (binding energies ranging from 0 to 1100 eV). The sample area analyzed was about 1 mm2 and the pressure during data acquisition was typically under 1 108 Torr. 2.4. Determination of carboxyl content of EDTA-SBA-15 The concentration of carboxyl groups in EDTA-SBA-15 was measured using back titration method [24,25]. 1.0 g of EDTA-SBA-15 was mixed with 100 mL of deionized water. The pH of the mixture was lowered to 2.0 in order that the COONa groups of the EDTA assembled with SBA-15 could be transformed into COOH groups [26]. After stirring for 2 h, the mixture was separated by filtration and the obtained solid was dried at 80 °C until sample mass remained constant. 0.100 g of dried solid was dispersed in 100 mL of 0.01 M NaHCO3 standard solution and stirred for 2 h under Ar atmosphere. Soon after the suspension was filtered, the obtained filtrate was divided into three aliquots and titrated with 0.01 M HCl standard solution that the result was given by average. The concentration of carboxylic groups was calculated by the following equation:
cCOOH ¼
C NaHCO3 V NaHCO3 C HCl V HCl W
ð1Þ
where C NaHCO3 and CHCl (mM) were the content of NaHCO3 solution and HCl solution, respectively; V NaHCO3 and CHCl (mL) were the volume of NaHCO3 solution and HCl solution, respectively; W (g) was the weight of the EDTA-SBA-15. 2.5. Heavy metal adsorption experiments In order to test the metal removal ability of the synthesized material, a set of adsorption experiments were performed by stirring a certain amount of EDTA-SBA-15 in 50 mL of Pb2+ solution of the desired concentration at 25 °C for different durations with an oscillation frequency of 150 rpm. Initial pH of the solution to the
J. Huang et al. / Journal of Colloid and Interface Science 385 (2012) 137–146
desired value for these experiments were adjusted by 0.1 M HCl or 0.1 M NaOH and measured with a pH meter (pHS-3C). Immediately after the adsorption step had been completed, the adsorbent–solution mixture was filtered to obtain the final solution. The obtained solid (Pb-EDTA-SBA-15) was dried at 105 °C and then stored in desiccator for further analysis. Pb2+ concentrations in both the initial and final solutions were determined by an inductively coupled plasma optical emission spectrometry (ICP-OES). The Varian Vista MPX machine was used to take the measurements. The metal removal efficiency (r, %) was calculated by the following equation:
r¼
ðc0 ce Þ 100% c0
ð2Þ
where C0 (mg L1) and Ce (mg L1) represented the initial and the equilibrium Pb2+ concentrations, respectively. The Pb adsorption capacity was calculated according to the following equation:
qe ¼
ðc0 ce Þ V W
ð3Þ
where the equilibrium adsorption capacity qe (mg g1) was the amount of adsorbate per gram of adsorbent, W (g) was the adsorbent weight, and V (L) was the solution volume. 3. Results and discussion 3.1. Formation of EDTA-SBA-15 The synthesis route of the EDTA-SBA-15 was presented in Scheme 1. In the stage of carboxyl activation, the carboxyl groups reacted with SOCl2 and changed the carboxyl into highly reactive acyl chloride [27,28]. The SOCl2/EDTA molar ratio was fixed at
139
1:1 to guarantee only one carboxyl group per EDTA molecule could be acyl chlorinated. Then the acyl chloride groups in EDTA reacted with the amino groups of the NH2-SBA-15 to form acylamide groups and EDTA thus was grafted on the inner surface of the matrix. Compared with the EDTAD, the SOCl2 activated EDTA grafted only one carboxyl group to the NH2-SBA-15 and thus increased the amount of carboxyl groups available for metal ions adsorption. Furthermore, the reaction condition of SOCl2 activated EDTA modification was more moderate (2 h at room temperature) than the previous work (24 h at room temperature or 4 h at 60 °C) [18,29]. 3.2. Characterization of mesoporous silica materials The inner assembly details of SBA-15 and functionalized SBA-15 were illustrated by FT-IR spectroscopy. Fig. 1 showed the FT-IR spectra of unmodified SBA-15, NH2-SBA-15 and EDTA-SBA-15. The typical vibration modes of SBA-15 (OH, 3430 cm1; SiAOASi, 1081 and 804 cm1; SiAOH, 962 cm1; and SiAO, 462 cm1) were presented [30]. Comparing with the intensity of the unmodified SBA-15, the intensity of the SiAOH vibration at 962 cm1 in NH2SBA-15 decreased, which indicated most of the SiAOH bonds on the inner surface of SBA-15 had been occupied due to the modification. In addition, the bands at 3430 and 1558 cm1 in the spectrum implied the existence of the NAH stretching and bending vibrations after the modification. Both symmetric and asymmetric stretching vibrations of CH3 (tas (CH3) = 2975 cm1, ts (CH3) = 2886 cm1) and CH2 (tas (CH2) = 2935 cm1, ts (CH2) = 2858 cm1) groups [21,31] were identified. As a result, it indicated that aminopropyl groups had been grafted onto the inner surface of SBA-15 via reactions between APTES and OH groups on the channel wall. Characteristic bands were also observed in the FT-IR spectrum of the EDTA-SBA-15 material. The band at about 1645 cm1 was
Scheme 1. Synthesis route of the functionalized SBA-15.
140
J. Huang et al. / Journal of Colloid and Interface Science 385 (2012) 137–146
Fig. 2. XRD patterns of SBA-15, NH2-SBA-15 and EDTA-SBA-15 (the inset shows the amplified part of XRD patterns from 1.2° to 2.0°).
Fig. 1. FT-IR spectra of SBA-15, NH2-SBA-15 and EDTA-SBA-15.
assigned to the AC@O stretching vibration of the acylamide group, which was the principal band of the acylamide ligand [32]. The band at 1732 cm1 was determined by the AC@O stretching vibration of carboxyl group [18]. The bands at 1300–1000 cm1 were attributed to the CAN stretching and/or CAO stretching vibration. The FT-IR results confirmed that EDTA-SBA-15 had been grafted on the SBA-15 inner surface. The grafting efficiencies of EDTA to NH2-SBA-15 before and after chemical modification were evaluated by comparing the carboxyl groups and total nitrogen contents, and the results were shown in Table 1. EDTA-SBA-15 exhibited a sharp increase in carboxyl concentration by comparing with NH2-SBA-15. There was no other carboxyl source except that EDTA was introduced in during its synthesis, and then it suggested that the EDTA was well grafted to NH2-SBA-15. About 80% of the aminopropyl groups from NH2SBA-15 were reacted with acyl chlorinated EDTA. The value implied that EDTA was more difficult than APTES to incorporate on the silica framework. While elemental analysis measures the bulk quantity of a substance, XPS measures the surface composition of a depth of about 10 nm. XPS analysis gave C:Si ratios and N:Si ratios for NH2-SBA-15 and EDTA-SBA-15 in Table 1. These data indicated that the functional groups were relatively uniformly distributed on the pore walls. The experimental surface atomic ratio of C:N was found slightly larger than the theoretical value for all samples. This could be due to those carbons from ethoxy groups because of incomplete hydrolysis of TEOS and/or from surfactant P123 that was not removed during solvent extraction. The small-angle XRD patterns for SBA-15, NH2-SBA-15 and EDTA-SBA-15 were shown in Fig. 2. Each of the curves exhibited three characteristic diffraction peaks which could be indexed to (1 0 0), (1 1 0), and (2 0 0) diffractions associated with typical twodimensional hexagonal symmetry (P6mm). The results indicated that the long-range hexagonal symmetry of SBA-15 was preserved after the sequent modifications with APTES and acyl chlorinated EDTA. However, the intensities of these characteristic diffraction peaks decreased slightly after grafting of APTES with respect to the unmodified SBA-15 and decreased further after the anchoring
Table 1 Back titration, XPS and elemental analysis data of functionalized SBA-15 samples.
a b c
Material
CCOOH (mmol g1)a
N (mmol g1)
NH2-SBA-15 EDTA-SBA-15
– 0.908
1.02 1.14
Obtained by back titration. Obtained by elemental analysis. Obtained by XPS.
b
N:Sic
C:Sic
0.0770 0.216
0.479 1.067
of acyl chlorinated EDTA. The reduction of intensity was mainly caused by contrast matching between the silicate framework and organic moieties which were located inside the channels of SBA15 [33]. The TEM images in Fig. 3a–f displayed the information of the pore channel structures of SBA-15, NH2-SBA-15 and EDTA-SBA15. The TEM images of SBA-15 (Fig. 3a and b) displayed well-ordered hexagonal pore channels with a diameter of about 9.0 nm. After grafting of guest molecules, the cylindrical shape of the pores and their nearly perfect 2D hexagonal order were clearly visible (Fig. 3c and d for NH2-SBA-15 and Fig. 3e and f for EDTA-SBA15), indicating that the channel structure of SBA-15 was not destroyed by incorporation of APTES and acyl chlorinated EDTA, and the EDTA-SBA-15 sample also had a 2D (p6mm) hexagonal structure. The distance of EDTA-SBA-15 between two consecutive centers of hexagonal pores estimated from the TEM image was ca. 10 nm. The average thickness of the wall was ca. 4 nm, which was much larger than that of MCM-41, and the pore diameter was around 7 nm, which was in agreement with the N2 desorption measurements (Table 2). All N2 adsorption–desorption isotherms in Fig. 4 were of type IV according to the IUPAC classification and exhibited H1-type broad hysteresis loops, which were typical for mesoporous solids in the partial pressure range from 0.60 to 0.90 and the pores of the materials were 7–9 nm in diameter. The results revealed that the uniform mesoporous characteristic of the materials was preserved even though the grafting. The main textural properties of solids were listed in Table 2, where the BET surface and pore volume were standardized versus pure silica weights. The BET surface area and the mesopore volume decreased strongly after SBA-15 being grafted, which resulted in SBA-15 > NH2-SBA-15 > EDTA-SBA-15. These textural results confirmed that the grafted species were located inner to the mesopore. 3.3. Pb (II) adsorption onto EDTA-SBA-15 3.3.1. Adsorption kinetics of the adsorbent Adsorption kinetics was one of the most important characters which represented the adsorption efficiency. The effect of different contact time intervals ranging from 0 to 320 min on the adsorption of Pb2+ by EDTA-SBA-15 and NH2-SBA-15 was shown in Fig. 5. It could be observed that very fast absorption of Pb2+ by EDTA-SBA15 took place within 5 min and then approached an adsorption equilibrium within 20 min with the removal rate and adsorption capacity reaching 92% and 91.6 mg g1, respectively. The high initial removal rate and the short equilibration time demonstrated that the surface of the modified SBA-15 had a high density of active
141
J. Huang et al. / Journal of Colloid and Interface Science 385 (2012) 137–146
Fig. 3. TEM images of SBA-15 (a and b), NH2-SBA-15 (c and d) and EDTA-SBA-15 (e and f) (the former is parallel to the pore axis while the latter is perpendicular to the pore axis).
Table 2 Textural and structural properties of synthesized mesoporous products. Samples
SBET (m2 g1)
Dp (nm)
Vp (cm3 g1)
d100 (nm)
a0 (nm)
tw (nm)
SBA-15 NH2-SBA-15 EDTA-SBA-15
721 322 284
9.12 7.44 7.16
1.16 0.527 0.468
8.83 9.01 9.01
10.2 10.4 10.4
1.08 2.96 3.24
Dp, pore diameter; Vp, pore volume; d100 = k/(2sin (h)); a0 = 2d100/31/2; tw = a0 Dp.
Fig. 4. N2 adsorption–desorption isotherms and pore size distributions of SBA-15, NH2-SBA-15 and EDTA-SBA-15.
sites. The possible reasons were as follows [34]: (a) ethanol extraction retained more SiAOH groups and was more beneficial to subsequent reflections; (b) post-grafting could not break the mesoporous structure and functional groups were distributed in the pipe; and (c) the adsorption of Pb (II) onto the EDTA-SBA-15 support proceeded with the intended Pb-EDTA-SBA-15 chelate format. It also showed that no significant adsorption took place in the amino-functionalized mesoporous silica in the explored conditions. This was most likely associated with the fact that NH2SBA-15 possessed a high pHZPC value (8.62) [35]. The values of the removal efficiency and the adsorption capacity corresponding
Fig. 5. The Pb adsorption capacity and removal efficiency as functions with contact time (initial concentration 100 mg L1, solution volume 50 mL, adsorbent 50 mg, pH5.0, 150 rpm, 25 °C).
to EDTA-SBA-15 were much larger than those obtained with NH2-SBA-15, confirming the favorable role played by the EDTA functional groups. Adsorption kinetics, which could be used for predicting the rate of the adsorption as well as the rate-determining step, were studied using a pseudo-first-order model, pseudo-second-order model and intraparticle diffusion model [36,37]. The pseudo-first-order model was described by the following equation:
1 k1 1 ¼ þ qt qe t qe
ð4Þ
142
J. Huang et al. / Journal of Colloid and Interface Science 385 (2012) 137–146
where K1 (min1) was the rate constant of sorption equilibrium in the first order reaction; qt (mg g1) was the amount of lead (II) on the surface of the sorbent at any time, qe (mg g1) was the amount of Pb (II) at equilibrium state. The pseudo-second-order model was represented by the following equation:
t 1 1 ¼ þ t qt k2 q2e qe
ð5Þ
where qt (mg g1) was the amount of Pb (II) on the surface of the sorbent at any time, qe (mg g1) was the amount of Pb (II) at equilibrium state. K2 (g mg1 min1) was the rate constant of adsorption equilibrium in the second order reaction. The intraparticle diffusion model was described by the following equation:
qt ¼ kt t 1=2 þ C
ð6Þ 1
1/2
where kt (mg g min ) was the diffusion rate constant and C (mg g1) represented the thickness of the boundary layer. The linear arrangements of Eqs. (4)–(6) were used to check the validity of these models and to obtain the model parameters when the corresponding linear plot was adequate. It was clear from Fig. 6a that pseudo-first-order model was not suitable to describe the kinetic profile because of the apparent lack of linear behavior. Conversely, rate of aqueous Pb (II) adsorption over EDTA-SBA-15 material was fitted very well by the pseudo-second-order equation, as shown in Fig. 6b where experimental t/qt and t data were provided along with linear correlation (R2 P 0.99). The equilibrium adsorption capacity calculated by this model (qe, cal) was in very good agreement with those obtained from experiments (qe, exp), which showed that the metal ion adsorption on adsorbents was controlled by the chemical reactions. Due to the mesoporous structure of the adsorbent, diffusion was also expected to influence the rate of the adsorption. Fig. 6c showed the pore diffusion plot of Pb (II) adsorption on EDTASBA-15 at 25 °C. It was observed that the adsorption process tended to be followed by two linear portions which were not linear over the whole time. The linear plot (the red1 linear) indicated that the film diffusion was important during the initial stage of adsorption. The linear plot (the green linear) showed intraparticle diffusion was involved in the adsorption process. The regression did not pass through the origin which indicated that the intraparticle diffusion was not only rate controlling step and some other mechanism along with intraparticle diffusion was also involved. Such a finding was similar to that made in previous works on adsorption [38]. As a summary of the kinetic studies, even though the pseudo second-order model showed quite a good correlation with the experimental data, the porous structure of the adsorbents and intraparticle model plots indicated the significance of pore diffusion. 3.3.2. The adsorption isotherms of the adsorbent The adsorption isotherms were constructed by varying the initial Pb concentration from 25 to 600 mg L1 with a constant EDTASBA-15 dosage of 1.0 g L1 at 25 °C, pH of 5.0. As shown in Fig. 7, the isotherms showed a sharp initial slope indicating the material acts as a high efficiency adsorbent at low metal concentration; in addition, when aqueous Pb (II) concentration increased the saturation constant value was reached. The equilibrium characterization data were performed by fitting experimental data into the Langmuir Eq. (7) and Freundlich Eq. (8) isotherms [39,40].
1 For interpretation of color in Figs. 1,2 and 4–11, the reader is referred to the web version of this article.
Fig. 6. (a) Pseudo-first-order model fit for the adsorption of Pb (II) by EDTA-SBA-15; (b) pseudo-second-order model fit for the adsorption of Pb (II) by EDTA-SBA-15; (c) intraparticle diffusion model fit for the adsorption of Pb (II) by EDTA-SBA-15 (initial concentration 100 mg L1, solution volume 50 mL, adsorbent 50 mg, pH5.0, 150 rpm, 25 °C).
ce ce 1 ¼ þ qe qmax kL qmax
ð7Þ
qe ¼ K F C 1=n e
ð8Þ
where KL (L mg1) was the Langmuir constant related to the free adsorption energy and qmax (mg g1) was the maximum adsorption capacity. KF [mg g1 (L mg1)1/n] and n were the Freundlich constants related to the adsorption capacity and adsorption intensity, respectively. Parameters of the Langmuir and Freundlich models were calculated by plotting the Ce/qe versus Ce and In qe versus In Ce plots, respectively. The Langmuir isotherm assumes monolayer adsorption onto a surface containing a finite number of uniform adsorption sites of
J. Huang et al. / Journal of Colloid and Interface Science 385 (2012) 137–146
Fig. 7. Adsorption isotherms for the adsorption of Pb by EDTA-SBA-15 (initial concentration 25–600 mg L1, solution volume 50 mL, adsorbent 50 mg, contact time 5 h, pH 5.0, 150 rpm, 25 °C).
uniform strategies of adsorption with no transmigration of adsorbate in the plane of surface. Freundlich isotherm model assumes a heterogeneous surface with a non-uniform distribution of adsorption over the surface [41]. Correlating equilibrium adsorption data to these equations is not only important in the design and operation of adsorption systems, but also in the investigation of adsorptive mechanism. As shown in Fig. 7, both of these two models could well fit the equilibrium data with the coefficients (R2) higher than 0.90, and the Langmuir model was able to provide a better fit (R2 = 0.9792) to the experimental data than the Freundlich model (R2 = 0.9052). The maximum adsorption capacity calculated from the Langmuir model was 273.2 mg g1 which was in accordance with the measurement value. It indicated that the Pb adsorption onto EDTA-SBA-15 was a major monolayer adsorption process.
143
and Pb(OH)2 started at pH 3.7 and 6.8, respectively, and Pb(OH)+ became the dominant species at pH > 7.5. The effect of initial solution pH on the Pb adsorption capacity of EDTA-SBA-15 was presented in Fig. 9. As could be seen from this figure, Pb adsorption capacity increased sharply from 2.74 to 91.6 mg g1 with the pH elevation from 2.0 to 5.0, reached maximum and maintained almost constant from pH 6.0 to 8.0. At pH < 6.0, the predominant lead species was Pb2+ and the removal of Pb2+ was mainly accomplished by adsorption. The main species at pH 6.0–8.0 were Pb2+ and Pb(OH)+ thus the removal of Pb was possibly accomplished by simultaneous adsorption of Pb2+ and Pb(OH)+. In this figure, the equilibrium pH values were higher than the initial values ranging from 2.0 to 6.0, and the corresponding equilibrium pH values were slightly decreased when initial pH was 7.0 and 8.0. This trend was similar to that of Pb adsorption by other modified adsorbents [43]. The strong pH dependent adsorption suggested that the adsorption of Pb (II) was dominated by ion exchange and surface complexation. Under acidic conditions, the low Pb2+ adsorption that took place at low pH could be attributed partly to the competition between H+ and Pb2+ ions on the surface sites. Furthermore, at low pH, the functional groups were protonated forms which had electrostatic repulsion to Pb2+ and found it hard to donate their electron pairs to coordinate with Pb2+, thus weakening the complexation between them and further decreasing the Pb adsorption capacity. With the elevation of solution pH, the competition effect between Pb2+ (and/or Pb(OH)+) and H+ became weak owing to the decrease of H+ concentration, resulting in more Pb immobilized onto the EDTA-SBA-15. Besides, the combined H+ ions gradually dissociated from functional groups at high pH, enhancing the complexation between Pb and the functional groups.
3.3.3. Mechanism of Pb adsorption onto EDTA-SBA-15 3.3.3.1. Effect of initial pH on the adsorption of Pb2+. The initial pH influences significantly the adsorption processes. It does not only influence the states of the functional groups on the surface of the adsorbent, but also the existing form of the metal ions in solution. In the solution of pH 2.0–8.0, it is known that lead species exist in three forms of Pb2+, Pb(OH)+ and Pb(OH)2. The relative distribution of Pb species (i.e. Pb2+, Pb(OH)+ and Pb(OH)2) in the aqueous solution was calculated using a visual MINTEQ software [42], and the result was represented in Fig. 8. It could be seen that the dominant species was Pb2+ at pH < 6.0, the hydrolysis of Pb2+ to form Pb(OH)+
3.3.3.2. XPS analysis. In order to further understand the mechanism of Pb adsorption, binding energy values of EDTA-SBA-15 and PbEDTA-SBA-15 (initial concentration 100 mg L1, pH8.0) were measured by XPS and their results were shown in Fig. 10. Fig. 10a given an overview of the chemical composition of the two samples analyzed based on the XPS survey scans. The spectrum of EDTA-SBA15 clearly showed two peaks at around 1070 and 497 eV which were attributed to Na1s and Na Auger photoelectrons, respectively, whereas the two peaks were not observed in the spectrum of PbEDTA-SBA-15. The disappearance of Na peaks was mainly due to the exchange of Na ions on the surface of EDTA-SBA-15 by the Pb ions during adsorption process, which was in accordance with others’ work [44,45]. Fig. 10b and d showed the N1s spectra of EDTA-SBA-15 and PbEDTA-SBA-15. In Fig. 10b, a peak at 398.8 eV was attributed to
Fig. 8. The distribution of Pb (II) species as a function of pH based on the equilibrium constants (total concentration 100 mg L1, 25 °C).
Fig. 9. Effect of pH on the uptake of Pb2+ by EDTA-SBA-15 (initial concentration 100 mg L1, solution volume 50 mL, adsorbent 50 mg, contact time 5 h, 150 rpm, 25 °C).
144
J. Huang et al. / Journal of Colloid and Interface Science 385 (2012) 137–146
Fig. 10. XPS spectra of EDTA-SBA-15 and Pb-EDTA-SBA-15: (a) overall spectra of EDTA-SBA-15 and Pb-EDTA-SBA-15; (c) Pb4f spectrum of Pb-EDTA-SBA-15; N1s spectra of (b) EDTA-SBA-15 and (d) Pb-EDTA-SBA-15; O1s spectra of (e) EDTA-SBA-15 and (g) Pb-EDTA-SBA-15; C1s spectra of (f) EDTA-SBA-15 and (h) Pb-EDTA-SBA-15.
J. Huang et al. / Journal of Colloid and Interface Science 385 (2012) 137–146 Table 3 Peak area ratios (%) of OAC@O (288.4 eV) for the EDTA-SBA-15 with and without Pb adsorption. Material
EDTA-SBA-15 Pb-EDTA-SBA-15
Peak area ration (%) pH 6.0
pH 8.0
16.7 11.8
18.3 10.9
nitrogen in CAN, which shifted to 399.3 eV after the Pb2+ adsorption in Fig. 10d. The increase in the binding energy after Pb adsorption was due to the formation of the coordination bond of N-metal ions, in which the nitrogen atoms shared electrons with Pb and thus electron densities of nitrogen atoms were reduced. The same trend was observed in the O1s spectra of EDTA-SBA-15 and EDTASBA-15 (Fig. 10e and g). In Fig. 10e, the O1s spectrum consists of three peaks components with binding energies at about 531.4, 532.0, and 532.9 eV, which could be ascribed to NaAOAC, C@O, CAOAH [46], respectively. After Pb2+ adsorption, the O1s spectrum changed. As shown in Fig. 10g, the peak at 531.4 eV increased to 532.1 eV. According to the above analysis, the main functional groups involved in the Pb adsorption were carboxyl groups and tertiary amino groups. The C1s spectrum of EDTA-SBA-15 was composed of four peaks with differentiated binding energy values via deconvolution (Fig. 10f). These peaks were assigned to CAC and CAH (284.4 eV), CAN (286.3 eV), NAC@O (286.7 eV) and OAC@O (288.4 eV), respectively [47,48]. In the spectrum of Pb-EDTA-SBA-
145
15 (Fig. 10h), the peaks of CAC (284.5 eV), NAC@O (286.8 eV) kept almost unchanged, while the binding energy of CAN (285.9 eV) and OAC@O (287.7 eV) decreased. The result was indicative of the formation of Pb complexation, in which the nitrogen and oxygen shared electrons with Pb2+ and hence the electron density at the adjacent carbon atoms increased and the binding energy of the carbons were reduced [49]. The XPS Pb4f spectrum of PbEDTA-SBA-15 was also determined. For Fig. 10c, doublet characteristics of Pb appear at 138.2 eV (assigned to Pb4f7/2) and 143.0 eV (assigned to Pb4f5/2). The binding energy of Pb4f7/2 in Pb(NO3)2 was about 139.1–139.5 eV [50,51], and the significant chemical shift was assigned to the interaction of Pb2+ with EDTA-SBA-15. Table 3 showed the OAC@O peak area ratios of the EDTA-SBA15 before and after Pb adsorption. The area ratio for the peak decreased 29% and 40% between pH 6.0 and pH 8.0, respectively. The dominant species was Pb2+(>97%) in the aqueous solution at pH 6.0, which deduced Pb onto the EDTA-SBA-15 support proceeds with the Pb2+-carboxyl group per EDTA molecule complex format in 1:1 molar ratio. At pH 8.0, the main species were Pb(OH)+ (70%) and Pb2+ (27%). The reduced value of area ratio was consistent with the intended Pb2+-carboxyl group per EDTA molecule complex format in 1:2 molar ratio and Pb(OH)+-carboxyl group per EDTA molecule complex format in a 1:1 molar ratio. In general, per molecule Pb (II) could react with per grafted molecule in the experimental conditions. In summary, the XPS analysis above suggested that removal of Pb2+ from aqueous solution by adsorption onto EDTA-SBA-15 was
Fig. 11. The mechanism for Pb adsorption onto EDTA-SBA-15.
146
J. Huang et al. / Journal of Colloid and Interface Science 385 (2012) 137–146
a multiplex process which involved ion exchange as well as surface complexation (Fig. 11).
Academic Discipline Project (S30406), Key Subject of the Shanghai Normal University (DZL806) and the Shanghai Normal University (No. SK201048).
3.4. Regeneration of EDTA-SBA-15 adsorbent References In addition to excellent adsorption capacity, it was highly desirable that an adsorbent could be regenerated and reused repeatedly with regard to the cost. The regeneration capacity of the EDTASBA-15 material was evaluated by acid treatment. 50 mg of PbEDTA-SBA-15 complex sample (91.6 mg g1 Pb) was treated with 100 mL of a 0.1 M HCl solution, under stirring for 4 h, at room temperature. After the acid regeneration and neutralization (with a 0.1 M NaHCO3), the solid was reused in three successive adsorption-regeneration cycles. The adsorption capacity of the EDTASBA-15 after four cycles was 87.5 mg g1, indicating a loss in the adsorption capacity of only 4.4%, compared to the initial one. These data indicated the very good regeneration capacity of the SBA-15 modified material. 4. Conclusions Ordered functionalized mesoporous silica EDTA-SBA-15 used for the removal of Pb2+ from polluted water was successfully prepared via post-grafting pathway and characterized. The adsorbent presented high sensitivity and efficiency for removing Pb (II) from wastewater, which was a critical requirement for practical applications. The following conclusions could be drawn: (a) EDTA could be effectively grafted to the SBA-15 matrix using SOCl2 as an activating reagent in moderate conditions, and the carboxyl group content of the adsorbent was 0.908 mmol g1 after modification. (b) The hybrid mesoporous adsorbent had the advantages of both mesoporous silica SBA-15 and EDTA showed an excellent performance for Pb (II) adsorption and its maximum adsorption capacity calculated from the Langmuir model was 273.2 mg g1. (c) The Pb adsorption by EDTA-SBA-15 could reach the equilibrium within 20 min, and the kinetic data were well described by pseudo-second-order model with the correlation coefficient (R2) more than 0.99, confirming that the chemical sorption was the rate limiting step in the adsorption process. Further analyses revealed that the pore diffusion process also played a role in adsorption kinetics, which could be attributed to the porous structure of the synthesized adsorbent. (d) The adsorption was a highly pH dependant process and pH 5.0 was the optimum pH for Pb adsorption onto EDTASBA-15. According to the XPS analysis, the adsorption mechanism of EDTA-SBA-15 involved Na Pb ion-exchange and surface complexation. (e) The chemical stability of the functionalized material SASBA-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.
Acknowledgments This work was financially supported by the National Nature Science Foundation of China (20671063), Key Laboratory of Resource Chemistry of the Ministry of Education, Shanghai Leading
[1] A. Celik, A. Demirba, Energy Sour. 27 (2005) 1167. [2] R. Say, E. Birlik, A. Denizli, A. Ersoz, Appl. Clay Sci. 31 (2006) 298. [3] M. Nadeem, R. Nadeem, H.U. Nadeem, S.S. Shah, Pak. J. Sci. Res. 57 (2005) 71. [4] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834. [5] O. Zaporozhets, N. Petruniock, O. Bessarabova, V. Sukhan, Talanta 49 (1999) 899. [6] E. Soliman, Mahmoud M. Ahmed, Talanta 54 (2001) 243. [7] Y. Yamini, J. Hassan, R. Mohandesi, N. Bahramifar, Talanta 56 (2002) 375. [8] Dongyuan Zhao, Jianglin Feng, Qisheng Huo, Nicholas Melosh, Glenn H. Fredrickson, Bradley F. Chmelka, Galen D. Stucky, Science (1998) 548. [9] A. Stein, B.J. Melde, R.C. Schroden, Adv. Mater. 12 (2000) 1403. [10] A. Vinu, K.Z. Hossain, K. Ariga, J. Nanosci. Nanotechnol. 3 (2005) 347. [11] A. Sayari, S. Hamoudi, Chem. Mater. 13 (2001) 3151. [12] J. Rämö, M. Sillanpää, V. Vickackaite, M. Orama, L. Niinistö, J. Pulp Pap. Sci. 26 (2000) 125. [13] B. Sen Gupta, M. Curran, Shameem Hasan, T.K. Ghosh, J. Colloid Interf. Sci. 90 (2009) 954. [14] E. Repo, T.A. Kurniawan, J.K. Warchol, M.E.T. Sillanpää, J. Hazard. Mater. 171 (2009) 1071. [15] E. Repo, J.K. Warchol, T.A. Kurniawan, M.E.T. Sillanpää, Chem. Eng. J. 161 (2010) 73. [16] O. Karniz Júnior, L.V.A. Gurgel, R.P. Freitas, L.F. Gil, Bioresour. Technol. 98 (2007) 1291. [17] F.V. Pereira, L.V.A. Gurgel, L.F. Gil, J. Hazard. Mater. 176 (2010) 856. [18] J.X. Yu, M. Tong, X.M. Sun, B.H. Li, Bioresour. Technol. 99 (2008) 2588. [19] Dongyuan Zhao, Qisheng Huo, Jianglin Feng, Bradley F. Chmelka, Galen D. Stucky, J. Am. Chem. Soc. 120 (1998) 6024. [20] José Aguado, Jesús M. Arsuaga, Amaya Arencibia, J. Hazard. Mater. 163 (2009) 213. [21] T. Martin, A. Galarneau, D. Brunel, V. Izard, V. Hulea, A.C. Blanc, S. Abramson, F. Di Renzo, F. Fajula, Stud. Surf. Sci. Catal. 135 (2001) 178. [22] Mukthavaram Rajesh, Joyeeta Sen, Marepally Srujan, Koushik Mukherjee, Bojja Sreedhar, Arabinda Chaudhuri, J. Am. Chem. Soc. 129 (2007) 11408. [23] W.J. Liu, F.X. Zeng, Chem. Eng. J. 170 (2011) 21. [24] H.P. Boehm, Carbon 32 (1994) 759. [25] I. Ghodbane, L. Nouri, O. Hamdaoui, M. Chiha, J. Hazard. Mater. 152 (2008) 148. [26] J.A. Dean, Lange’s Hand Book of Chemistry, 15th ed., McGraw-Hill Book Company, USA, 1999. [27] C.S. Li, Y.H. Tsai, W.C. Lee, W.J. Kuo, J. Org. Chem. 75 (2010) 4004. [28] R. Chicharro, M. Alonso, V.J. Aran, B. Herradon, Tetrahedron Lett. 49 (2008) 2275. [29] S. Nagib, K. Inoue, T. Yamaguchi, T. Tamaru, Hydrometallurgy 51 (1999) 73. [30] P.P. Yang, S.S. Huang, D. Kong, J. Lin, H.G. Fu, Inorg. Chem. 46 (2007) 3203. [31] D. Brunel, A. Cauvel, F. Fajula, F. DiRenzo, Stud. Surf. Sci. Catal. 97 (1995) 173. [32] J. Losada, I. Del Peso, L. Beyer, Inorg. Chim. Acta. 321 (2001) 107. [33] S. Gago, J.A. Fernandes, J.P. Rainho, R.A. Ferreira, S.M. Pillinger, A.A. Valente, T.M. Santos, L.D. Carlos, P.J.A. Ribeiro-Claro, Chem. Mater. 17 (2005) 5077. [34] S. Lagergren, Ksver Veterskapsakad Handl. 24 (1898) 1. [35] E. Da’na, A. Sayari, Chem. Eng. J. 166 (2011) 445. [36] Y.S. Ho, G. Mckay, Adsorpt. Sci. Technol. 16 (1998) 243. [37] E. Repo, J.K. Warchol, A. Bhatnagar, J. Colloid Interf. Sci. 358 (2011) 261. [38] M.A. Al-Ghouti, M.A.M. Khraisheh, M.N.M. Ahmad, S. Allen, J. Hazard. Mater. 165 (2009) 589. [39] I. Langmuir, J. Am. Chem. Soc. 40 (1918) 1361. [40] H.M.F. Freundlich, Z. Phys. Chem. 57A (1906) 385. [41] F. Haghseresht, G. Lu, Energy Fuel 12 (1998) 1100. [42] J.P. Gustafsson, Visual MINTEQ: Department of Land and Water Resources Engineering (Ver. 3.0), The Royal Institute of Technology, Sweden,
Contents lists available at SciVerse ScienceDirect
Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Pb (II) removal from aqueous media by EDTA-modified mesoporous silica SBA-15 Jin Huang, Meng Ye, Yuqi Qu, Lianfeng Chu, Rui Chen, Qizhuang He, Dongfang Xu ⇑ College of Life and Environmental Science, Shanghai Normal University, Shanghai 200234, China
a r t i c l e
i n f o
Article history: Received 21 March 2012 Accepted 18 June 2012 Available online 5 July 2012 Keywords: SBA-15 Chemical modification Adsorbent Adsorption Lead (II)
a b s t r a c t An organic–inorganic hybrid mesoporous silica material was synthesized by two-step post-grafting method of SBA-15 with 3-aminopropyltrimethoxy-silane (APTES) and thionyl dichloride (SOCl2) activated ethylenediaminetetraacetic acid (EDTA) in sequence and measured by means of Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), elemental analysis (EA), transmission electron microscopy (TEM), nitrogen (N2) adsorption–desorption analysis and back titration. The material was found having the beneficial properties of mesoporous silica SBA-15 and EDTA. Adsorption potential of the material for Pb (II) removal from aqueous solution was investigated by varying experimental conditions such as pH, contact time and initial metal concentration. The removal efficiency of Pb2+ was high under studied experimental conditions. The adsorption equilibrium could be reached within 20 min and the kinetic data were fitted well by pseudo-second-order and intraparticle diffusion model. The adsorbent exhibited a favorable performance and its maximum adsorption capacity calculated by the Langmuir model was 273.2 mg g1. Recycling experiments showed the adsorbent could be regenerated by acid treatment without altering its properties. The chemical states of the elements involved in the adsorption were analyzed by X-ray photoelectron spectroscopy (XPS). The results demonstrated that the adsorption mechanism of the material involved Na Pb ion-exchange and carboxyl group dominated surface complexation. Ó 2012 Elsevier Inc. All rights reserved.
1. Introduction A large variety of heavy metals are discharged into the environment and constitute the most significant environmental pollutants found in wastewater. Long-term exposure to those solvated metal ions and consequently the effects on human health and natural ecosystems are critical issues [1,2]. Lead is one of the common contaminants of industrial wastewaters. A large quantity of single or daily intake of lead can cause various serious disorders, such as damage to liver, kidney and reduction in hemoglobin formation, mental retardation, infertility and abnormalities in pregnant women [3]. Adsorption technology is one of the most popular methods to remove lead in contaminated water. To become an adsorbent for lead ions with best performance, a set of conditions must be met including (a) the occurrence of an open pore structure and accessible adsorption sites; (b) the adsorbent must be structurally stable under adsorption; (c) good contact should be estabilished between the lead ions and the adsorption sites. The hexagonally ordered mesoporous silica materials exhibit many attractive characteristics such as well ordered periodic pore structure, high thermal and chemical stabilities and controllable pore diameter [4]. Then, those materials with various chelating ⇑ Corresponding author. Fax: +86 21 64323350. E-mail address: [email protected] (D. Xu). 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.06.054
agents are increasingly utilized as adsorbents because of the high selectivity of the agents for metal ions adsorption [5–7]. SBA-15 is one of the most popular mesoporous silica materials because of its large adjustable pores (5–30 nm), which allow easier accessibility of target species to the inner surface of the material and lead to fast kinetics of chemical or physical processes, as well as its thick pore walls, which provide enhanced mechanical stability and can be modified with organic groups to tailor their properties and achieve specific purposes [8–10]. Thus, the development of functionalized mesoporous silica adsorbents with appropriate functional groups for heavy metal ions is interesting and promising [11]. Ethylenediaminetetraacetic acid (EDTA) has favorable chelating and ion-exchange properties for many different metal ions [12,13]. Therefore, its immobilization on the different supporting materials for the metal adsorption purposes has received wide attention. EDTA dianhydride (EDTAD), an active agent containing two anhydride groups per molecule, can react with amino groups of organic molecules so as to introduce chelating groups. The use of EDTAD to modify supports has been reported once in the literature [14–18]. In all these cases, EDTAD was observed to form stable chelates with metals. However, in their functionalization process, the two anhydrides of EDTA had the same reaction activity and both of them might react with the amino groups of the matrices, and thus decreased the amount of carboxyl groups available for adsorption of metal ions. Moreover, few interaction mechanisms involving
138
J. Huang et al. / Journal of Colloid and Interface Science 385 (2012) 137–146
both the chelation and ion-exchange for EDTA modified adsorbents have been reported in the literature because of the complexities between the modified adsorbents and metals. The objectives of this work are to synthesize mesoporous silica SBA-15 with abundant carboxyl groups, explore its application for Pb (II) removal from aqueous solutions and discuss the mechanism of Pb (II) adsorption onto the functionalized mesoporous adsorbent. For this purpose, (a) a new method for EDTA modification with stoichiometric thionyl chloride (SOCl2) as a carboxyl group activating reagent was developed, which activated only one carboxyl group per EDTA molecule and thus increased the amount of carboxyl groups available for adsorption of metal ions, (b) the preparation and characterization of the hybrid adsorbent were described, (c) a series of adsorption experiments were carried out and their kinetics and isotherms were investigated by fitting the experimental data using pseudo-first-order, pseudo-second-order, intraparticle diffusion, the Langmuir and Freundlich isotherm models, and (d) the chemical states of the atoms involved in the adsorption were analyzed using X-ray photoelectron spectroscopy (XPS) to interpret the interaction mechanism between Pb (II) and the functional groups. 2. Experimental 2.1. Materials 3-aminopropyltrimethoxy-silane (APTES, 95%) and Triblock copolymer poly (ethylene glycol)-block-poly (propylene glycol)block-poly (ethylene glycol) (Pluronic P123, MW = 5800, EO20PO70EO20) were purchased from Aldrich. Tetraethoxysilane (TEOS), hydrochloric acid (HCl, 37%), ethylenediaminetetraacetic acid (EDTA), thionyl chloride (SOCl2), toluene, dichloromethane (DCM), ether, acetone, sodium carbonate solution (Na2CO3), aqueous sodium bicarbonate (NaHCO3), nitric acid (HNO3), NaOH and lead nitrate were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All of the reagents and solvents were of analytical reagent grade and were used as received except toluene and DCM which were distilled just before use. Millipore water was used in all experiments. 2.2. Synthesis of functionalized mesoporous silica Pure silica SBA-15 was synthesized using triblock copolymer pluronic 123 as a soft template and TEOS as the silica source by following the recipe in the bibliography [19,20]. 2.0 g of Pluronic 123 was dissolved in 62 mL of 2 M HCl at the room temperature. 4.2 g of TEOS was added into this solution and the synthesis was carried out by stirring for 24 h at 40 °C, then kept aging for 24 h at 100 °C. Solid product was recovered by filtration, removal of template by using Soxhlet ethanol extraction (roughly 200 mL per gram of silica) during 24 h, and dried overnight at 80 °C under the vacuum conditions. The resulting material was simply denominated SBA15. The aminopropyl grafting of SBA-15 was implemented by applying APTES as the silylation reagent [21]. 1.0 g of SBA-15 powder was mixed with 1 mL of APTES in dry toluene solution and then the mixed solution was kept with reflux for 12 h under Ar atmosphere. The precipitate was filtered and washed adequately with toluene and dichloromethane. It was submitted to a continuous extraction running overnight in a Soxhlet apparatus with diethyl ether/dichloromethane (v/v, 1/1) at 30 °C and dried overnight at 60 °C in a vacuum container. The solid product was denoted as NH2-SBA-15. The further modification of NH2-SBA-15 was carried out in an anhydrous condition using DCM as a solvent [22]. After 0.04 mol of EDTA and 100 mL of DCM were mixed in a three-necked flask, 0.04 mol of SOCl2 was then slowly added to
the mixture through a constant pressure drop funnel under Ar atmosphere. Immediately after SOCl2 was completely dropped, 1.00 g of NH2-SBA-15 was rapidly added to the mixture. The mixture was stirred at room temperature for 2 h and the chemically modified EDTA-SBA-15 was obtained. The solid product was thus separated by filtration, washed in a row with DCM, acetone, deionized water, NaHCO3 (0.1 M), deionized water and acetone. Finally, it was dried overnight at 50 °C under vacuum conditions [23]. 2.3. Instruments and characterization FT-IR spectra (KBr pellets) in the range of 4000–400 cm1 were taken on a American Nicolet AVATAR 380 FT-IR spectrometer. Elemental analyses were performed on a Vario EL-III elemental analyzer. Small-angle XRD patterns were recorded on a Rigaku D/ max-2000 diffractometer using Cu Ka radiation (k = 1.5406 Å) at 40 kV and 20 mA. Transmission electron microscopy (TEM) images were captured on a JEOL JEM-2100 electron microscope operated at 120 kV. N2 adsorption–desorption isotherms were measured at 196 °C by using a Micromeritics Tristar II 3020 surface area and pore size analyzer. The samples were degassed for 10 h at 100 °C prior to the adsorption measurements. Specific surface area (SBET) was calculated by the BET method, the pore volume (Vp) was determined at the amount of liquid nitrogen adsorbed at P/ P0 = 0.97, and pore size distribution (Dp) was obtained from the adsorption isotherms by the Barrett–Joyner–Halenda (BJH) method. The chemical composition was analyzed using a PHI-5000 versa-probe X-ray photoelectron spectrometer (XPS). The binding energy was calibrated using the carbon 1s peak as the reference energy at 284.8 eV with a monochromatic Al Ka radiation (1486.6 eV, 25 W, 15 kV). Survey scans were performed using a 1 eV/step (binding energies ranging from 0 to 1100 eV). The sample area analyzed was about 1 mm2 and the pressure during data acquisition was typically under 1 108 Torr. 2.4. Determination of carboxyl content of EDTA-SBA-15 The concentration of carboxyl groups in EDTA-SBA-15 was measured using back titration method [24,25]. 1.0 g of EDTA-SBA-15 was mixed with 100 mL of deionized water. The pH of the mixture was lowered to 2.0 in order that the COONa groups of the EDTA assembled with SBA-15 could be transformed into COOH groups [26]. After stirring for 2 h, the mixture was separated by filtration and the obtained solid was dried at 80 °C until sample mass remained constant. 0.100 g of dried solid was dispersed in 100 mL of 0.01 M NaHCO3 standard solution and stirred for 2 h under Ar atmosphere. Soon after the suspension was filtered, the obtained filtrate was divided into three aliquots and titrated with 0.01 M HCl standard solution that the result was given by average. The concentration of carboxylic groups was calculated by the following equation:
cCOOH ¼
C NaHCO3 V NaHCO3 C HCl V HCl W
ð1Þ
where C NaHCO3 and CHCl (mM) were the content of NaHCO3 solution and HCl solution, respectively; V NaHCO3 and CHCl (mL) were the volume of NaHCO3 solution and HCl solution, respectively; W (g) was the weight of the EDTA-SBA-15. 2.5. Heavy metal adsorption experiments In order to test the metal removal ability of the synthesized material, a set of adsorption experiments were performed by stirring a certain amount of EDTA-SBA-15 in 50 mL of Pb2+ solution of the desired concentration at 25 °C for different durations with an oscillation frequency of 150 rpm. Initial pH of the solution to the
J. Huang et al. / Journal of Colloid and Interface Science 385 (2012) 137–146
desired value for these experiments were adjusted by 0.1 M HCl or 0.1 M NaOH and measured with a pH meter (pHS-3C). Immediately after the adsorption step had been completed, the adsorbent–solution mixture was filtered to obtain the final solution. The obtained solid (Pb-EDTA-SBA-15) was dried at 105 °C and then stored in desiccator for further analysis. Pb2+ concentrations in both the initial and final solutions were determined by an inductively coupled plasma optical emission spectrometry (ICP-OES). The Varian Vista MPX machine was used to take the measurements. The metal removal efficiency (r, %) was calculated by the following equation:
r¼
ðc0 ce Þ 100% c0
ð2Þ
where C0 (mg L1) and Ce (mg L1) represented the initial and the equilibrium Pb2+ concentrations, respectively. The Pb adsorption capacity was calculated according to the following equation:
qe ¼
ðc0 ce Þ V W
ð3Þ
where the equilibrium adsorption capacity qe (mg g1) was the amount of adsorbate per gram of adsorbent, W (g) was the adsorbent weight, and V (L) was the solution volume. 3. Results and discussion 3.1. Formation of EDTA-SBA-15 The synthesis route of the EDTA-SBA-15 was presented in Scheme 1. In the stage of carboxyl activation, the carboxyl groups reacted with SOCl2 and changed the carboxyl into highly reactive acyl chloride [27,28]. The SOCl2/EDTA molar ratio was fixed at
139
1:1 to guarantee only one carboxyl group per EDTA molecule could be acyl chlorinated. Then the acyl chloride groups in EDTA reacted with the amino groups of the NH2-SBA-15 to form acylamide groups and EDTA thus was grafted on the inner surface of the matrix. Compared with the EDTAD, the SOCl2 activated EDTA grafted only one carboxyl group to the NH2-SBA-15 and thus increased the amount of carboxyl groups available for metal ions adsorption. Furthermore, the reaction condition of SOCl2 activated EDTA modification was more moderate (2 h at room temperature) than the previous work (24 h at room temperature or 4 h at 60 °C) [18,29]. 3.2. Characterization of mesoporous silica materials The inner assembly details of SBA-15 and functionalized SBA-15 were illustrated by FT-IR spectroscopy. Fig. 1 showed the FT-IR spectra of unmodified SBA-15, NH2-SBA-15 and EDTA-SBA-15. The typical vibration modes of SBA-15 (OH, 3430 cm1; SiAOASi, 1081 and 804 cm1; SiAOH, 962 cm1; and SiAO, 462 cm1) were presented [30]. Comparing with the intensity of the unmodified SBA-15, the intensity of the SiAOH vibration at 962 cm1 in NH2SBA-15 decreased, which indicated most of the SiAOH bonds on the inner surface of SBA-15 had been occupied due to the modification. In addition, the bands at 3430 and 1558 cm1 in the spectrum implied the existence of the NAH stretching and bending vibrations after the modification. Both symmetric and asymmetric stretching vibrations of CH3 (tas (CH3) = 2975 cm1, ts (CH3) = 2886 cm1) and CH2 (tas (CH2) = 2935 cm1, ts (CH2) = 2858 cm1) groups [21,31] were identified. As a result, it indicated that aminopropyl groups had been grafted onto the inner surface of SBA-15 via reactions between APTES and OH groups on the channel wall. Characteristic bands were also observed in the FT-IR spectrum of the EDTA-SBA-15 material. The band at about 1645 cm1 was
Scheme 1. Synthesis route of the functionalized SBA-15.
140
J. Huang et al. / Journal of Colloid and Interface Science 385 (2012) 137–146
Fig. 2. XRD patterns of SBA-15, NH2-SBA-15 and EDTA-SBA-15 (the inset shows the amplified part of XRD patterns from 1.2° to 2.0°).
Fig. 1. FT-IR spectra of SBA-15, NH2-SBA-15 and EDTA-SBA-15.
assigned to the AC@O stretching vibration of the acylamide group, which was the principal band of the acylamide ligand [32]. The band at 1732 cm1 was determined by the AC@O stretching vibration of carboxyl group [18]. The bands at 1300–1000 cm1 were attributed to the CAN stretching and/or CAO stretching vibration. The FT-IR results confirmed that EDTA-SBA-15 had been grafted on the SBA-15 inner surface. The grafting efficiencies of EDTA to NH2-SBA-15 before and after chemical modification were evaluated by comparing the carboxyl groups and total nitrogen contents, and the results were shown in Table 1. EDTA-SBA-15 exhibited a sharp increase in carboxyl concentration by comparing with NH2-SBA-15. There was no other carboxyl source except that EDTA was introduced in during its synthesis, and then it suggested that the EDTA was well grafted to NH2-SBA-15. About 80% of the aminopropyl groups from NH2SBA-15 were reacted with acyl chlorinated EDTA. The value implied that EDTA was more difficult than APTES to incorporate on the silica framework. While elemental analysis measures the bulk quantity of a substance, XPS measures the surface composition of a depth of about 10 nm. XPS analysis gave C:Si ratios and N:Si ratios for NH2-SBA-15 and EDTA-SBA-15 in Table 1. These data indicated that the functional groups were relatively uniformly distributed on the pore walls. The experimental surface atomic ratio of C:N was found slightly larger than the theoretical value for all samples. This could be due to those carbons from ethoxy groups because of incomplete hydrolysis of TEOS and/or from surfactant P123 that was not removed during solvent extraction. The small-angle XRD patterns for SBA-15, NH2-SBA-15 and EDTA-SBA-15 were shown in Fig. 2. Each of the curves exhibited three characteristic diffraction peaks which could be indexed to (1 0 0), (1 1 0), and (2 0 0) diffractions associated with typical twodimensional hexagonal symmetry (P6mm). The results indicated that the long-range hexagonal symmetry of SBA-15 was preserved after the sequent modifications with APTES and acyl chlorinated EDTA. However, the intensities of these characteristic diffraction peaks decreased slightly after grafting of APTES with respect to the unmodified SBA-15 and decreased further after the anchoring
Table 1 Back titration, XPS and elemental analysis data of functionalized SBA-15 samples.
a b c
Material
CCOOH (mmol g1)a
N (mmol g1)
NH2-SBA-15 EDTA-SBA-15
– 0.908
1.02 1.14
Obtained by back titration. Obtained by elemental analysis. Obtained by XPS.
b
N:Sic
C:Sic
0.0770 0.216
0.479 1.067
of acyl chlorinated EDTA. The reduction of intensity was mainly caused by contrast matching between the silicate framework and organic moieties which were located inside the channels of SBA15 [33]. The TEM images in Fig. 3a–f displayed the information of the pore channel structures of SBA-15, NH2-SBA-15 and EDTA-SBA15. The TEM images of SBA-15 (Fig. 3a and b) displayed well-ordered hexagonal pore channels with a diameter of about 9.0 nm. After grafting of guest molecules, the cylindrical shape of the pores and their nearly perfect 2D hexagonal order were clearly visible (Fig. 3c and d for NH2-SBA-15 and Fig. 3e and f for EDTA-SBA15), indicating that the channel structure of SBA-15 was not destroyed by incorporation of APTES and acyl chlorinated EDTA, and the EDTA-SBA-15 sample also had a 2D (p6mm) hexagonal structure. The distance of EDTA-SBA-15 between two consecutive centers of hexagonal pores estimated from the TEM image was ca. 10 nm. The average thickness of the wall was ca. 4 nm, which was much larger than that of MCM-41, and the pore diameter was around 7 nm, which was in agreement with the N2 desorption measurements (Table 2). All N2 adsorption–desorption isotherms in Fig. 4 were of type IV according to the IUPAC classification and exhibited H1-type broad hysteresis loops, which were typical for mesoporous solids in the partial pressure range from 0.60 to 0.90 and the pores of the materials were 7–9 nm in diameter. The results revealed that the uniform mesoporous characteristic of the materials was preserved even though the grafting. The main textural properties of solids were listed in Table 2, where the BET surface and pore volume were standardized versus pure silica weights. The BET surface area and the mesopore volume decreased strongly after SBA-15 being grafted, which resulted in SBA-15 > NH2-SBA-15 > EDTA-SBA-15. These textural results confirmed that the grafted species were located inner to the mesopore. 3.3. Pb (II) adsorption onto EDTA-SBA-15 3.3.1. Adsorption kinetics of the adsorbent Adsorption kinetics was one of the most important characters which represented the adsorption efficiency. The effect of different contact time intervals ranging from 0 to 320 min on the adsorption of Pb2+ by EDTA-SBA-15 and NH2-SBA-15 was shown in Fig. 5. It could be observed that very fast absorption of Pb2+ by EDTA-SBA15 took place within 5 min and then approached an adsorption equilibrium within 20 min with the removal rate and adsorption capacity reaching 92% and 91.6 mg g1, respectively. The high initial removal rate and the short equilibration time demonstrated that the surface of the modified SBA-15 had a high density of active
141
J. Huang et al. / Journal of Colloid and Interface Science 385 (2012) 137–146
Fig. 3. TEM images of SBA-15 (a and b), NH2-SBA-15 (c and d) and EDTA-SBA-15 (e and f) (the former is parallel to the pore axis while the latter is perpendicular to the pore axis).
Table 2 Textural and structural properties of synthesized mesoporous products. Samples
SBET (m2 g1)
Dp (nm)
Vp (cm3 g1)
d100 (nm)
a0 (nm)
tw (nm)
SBA-15 NH2-SBA-15 EDTA-SBA-15
721 322 284
9.12 7.44 7.16
1.16 0.527 0.468
8.83 9.01 9.01
10.2 10.4 10.4
1.08 2.96 3.24
Dp, pore diameter; Vp, pore volume; d100 = k/(2sin (h)); a0 = 2d100/31/2; tw = a0 Dp.
Fig. 4. N2 adsorption–desorption isotherms and pore size distributions of SBA-15, NH2-SBA-15 and EDTA-SBA-15.
sites. The possible reasons were as follows [34]: (a) ethanol extraction retained more SiAOH groups and was more beneficial to subsequent reflections; (b) post-grafting could not break the mesoporous structure and functional groups were distributed in the pipe; and (c) the adsorption of Pb (II) onto the EDTA-SBA-15 support proceeded with the intended Pb-EDTA-SBA-15 chelate format. It also showed that no significant adsorption took place in the amino-functionalized mesoporous silica in the explored conditions. This was most likely associated with the fact that NH2SBA-15 possessed a high pHZPC value (8.62) [35]. The values of the removal efficiency and the adsorption capacity corresponding
Fig. 5. The Pb adsorption capacity and removal efficiency as functions with contact time (initial concentration 100 mg L1, solution volume 50 mL, adsorbent 50 mg, pH5.0, 150 rpm, 25 °C).
to EDTA-SBA-15 were much larger than those obtained with NH2-SBA-15, confirming the favorable role played by the EDTA functional groups. Adsorption kinetics, which could be used for predicting the rate of the adsorption as well as the rate-determining step, were studied using a pseudo-first-order model, pseudo-second-order model and intraparticle diffusion model [36,37]. The pseudo-first-order model was described by the following equation:
1 k1 1 ¼ þ qt qe t qe
ð4Þ
142
J. Huang et al. / Journal of Colloid and Interface Science 385 (2012) 137–146
where K1 (min1) was the rate constant of sorption equilibrium in the first order reaction; qt (mg g1) was the amount of lead (II) on the surface of the sorbent at any time, qe (mg g1) was the amount of Pb (II) at equilibrium state. The pseudo-second-order model was represented by the following equation:
t 1 1 ¼ þ t qt k2 q2e qe
ð5Þ
where qt (mg g1) was the amount of Pb (II) on the surface of the sorbent at any time, qe (mg g1) was the amount of Pb (II) at equilibrium state. K2 (g mg1 min1) was the rate constant of adsorption equilibrium in the second order reaction. The intraparticle diffusion model was described by the following equation:
qt ¼ kt t 1=2 þ C
ð6Þ 1
1/2
where kt (mg g min ) was the diffusion rate constant and C (mg g1) represented the thickness of the boundary layer. The linear arrangements of Eqs. (4)–(6) were used to check the validity of these models and to obtain the model parameters when the corresponding linear plot was adequate. It was clear from Fig. 6a that pseudo-first-order model was not suitable to describe the kinetic profile because of the apparent lack of linear behavior. Conversely, rate of aqueous Pb (II) adsorption over EDTA-SBA-15 material was fitted very well by the pseudo-second-order equation, as shown in Fig. 6b where experimental t/qt and t data were provided along with linear correlation (R2 P 0.99). The equilibrium adsorption capacity calculated by this model (qe, cal) was in very good agreement with those obtained from experiments (qe, exp), which showed that the metal ion adsorption on adsorbents was controlled by the chemical reactions. Due to the mesoporous structure of the adsorbent, diffusion was also expected to influence the rate of the adsorption. Fig. 6c showed the pore diffusion plot of Pb (II) adsorption on EDTASBA-15 at 25 °C. It was observed that the adsorption process tended to be followed by two linear portions which were not linear over the whole time. The linear plot (the red1 linear) indicated that the film diffusion was important during the initial stage of adsorption. The linear plot (the green linear) showed intraparticle diffusion was involved in the adsorption process. The regression did not pass through the origin which indicated that the intraparticle diffusion was not only rate controlling step and some other mechanism along with intraparticle diffusion was also involved. Such a finding was similar to that made in previous works on adsorption [38]. As a summary of the kinetic studies, even though the pseudo second-order model showed quite a good correlation with the experimental data, the porous structure of the adsorbents and intraparticle model plots indicated the significance of pore diffusion. 3.3.2. The adsorption isotherms of the adsorbent The adsorption isotherms were constructed by varying the initial Pb concentration from 25 to 600 mg L1 with a constant EDTASBA-15 dosage of 1.0 g L1 at 25 °C, pH of 5.0. As shown in Fig. 7, the isotherms showed a sharp initial slope indicating the material acts as a high efficiency adsorbent at low metal concentration; in addition, when aqueous Pb (II) concentration increased the saturation constant value was reached. The equilibrium characterization data were performed by fitting experimental data into the Langmuir Eq. (7) and Freundlich Eq. (8) isotherms [39,40].
1 For interpretation of color in Figs. 1,2 and 4–11, the reader is referred to the web version of this article.
Fig. 6. (a) Pseudo-first-order model fit for the adsorption of Pb (II) by EDTA-SBA-15; (b) pseudo-second-order model fit for the adsorption of Pb (II) by EDTA-SBA-15; (c) intraparticle diffusion model fit for the adsorption of Pb (II) by EDTA-SBA-15 (initial concentration 100 mg L1, solution volume 50 mL, adsorbent 50 mg, pH5.0, 150 rpm, 25 °C).
ce ce 1 ¼ þ qe qmax kL qmax
ð7Þ
qe ¼ K F C 1=n e
ð8Þ
where KL (L mg1) was the Langmuir constant related to the free adsorption energy and qmax (mg g1) was the maximum adsorption capacity. KF [mg g1 (L mg1)1/n] and n were the Freundlich constants related to the adsorption capacity and adsorption intensity, respectively. Parameters of the Langmuir and Freundlich models were calculated by plotting the Ce/qe versus Ce and In qe versus In Ce plots, respectively. The Langmuir isotherm assumes monolayer adsorption onto a surface containing a finite number of uniform adsorption sites of
J. Huang et al. / Journal of Colloid and Interface Science 385 (2012) 137–146
Fig. 7. Adsorption isotherms for the adsorption of Pb by EDTA-SBA-15 (initial concentration 25–600 mg L1, solution volume 50 mL, adsorbent 50 mg, contact time 5 h, pH 5.0, 150 rpm, 25 °C).
uniform strategies of adsorption with no transmigration of adsorbate in the plane of surface. Freundlich isotherm model assumes a heterogeneous surface with a non-uniform distribution of adsorption over the surface [41]. Correlating equilibrium adsorption data to these equations is not only important in the design and operation of adsorption systems, but also in the investigation of adsorptive mechanism. As shown in Fig. 7, both of these two models could well fit the equilibrium data with the coefficients (R2) higher than 0.90, and the Langmuir model was able to provide a better fit (R2 = 0.9792) to the experimental data than the Freundlich model (R2 = 0.9052). The maximum adsorption capacity calculated from the Langmuir model was 273.2 mg g1 which was in accordance with the measurement value. It indicated that the Pb adsorption onto EDTA-SBA-15 was a major monolayer adsorption process.
143
and Pb(OH)2 started at pH 3.7 and 6.8, respectively, and Pb(OH)+ became the dominant species at pH > 7.5. The effect of initial solution pH on the Pb adsorption capacity of EDTA-SBA-15 was presented in Fig. 9. As could be seen from this figure, Pb adsorption capacity increased sharply from 2.74 to 91.6 mg g1 with the pH elevation from 2.0 to 5.0, reached maximum and maintained almost constant from pH 6.0 to 8.0. At pH < 6.0, the predominant lead species was Pb2+ and the removal of Pb2+ was mainly accomplished by adsorption. The main species at pH 6.0–8.0 were Pb2+ and Pb(OH)+ thus the removal of Pb was possibly accomplished by simultaneous adsorption of Pb2+ and Pb(OH)+. In this figure, the equilibrium pH values were higher than the initial values ranging from 2.0 to 6.0, and the corresponding equilibrium pH values were slightly decreased when initial pH was 7.0 and 8.0. This trend was similar to that of Pb adsorption by other modified adsorbents [43]. The strong pH dependent adsorption suggested that the adsorption of Pb (II) was dominated by ion exchange and surface complexation. Under acidic conditions, the low Pb2+ adsorption that took place at low pH could be attributed partly to the competition between H+ and Pb2+ ions on the surface sites. Furthermore, at low pH, the functional groups were protonated forms which had electrostatic repulsion to Pb2+ and found it hard to donate their electron pairs to coordinate with Pb2+, thus weakening the complexation between them and further decreasing the Pb adsorption capacity. With the elevation of solution pH, the competition effect between Pb2+ (and/or Pb(OH)+) and H+ became weak owing to the decrease of H+ concentration, resulting in more Pb immobilized onto the EDTA-SBA-15. Besides, the combined H+ ions gradually dissociated from functional groups at high pH, enhancing the complexation between Pb and the functional groups.
3.3.3. Mechanism of Pb adsorption onto EDTA-SBA-15 3.3.3.1. Effect of initial pH on the adsorption of Pb2+. The initial pH influences significantly the adsorption processes. It does not only influence the states of the functional groups on the surface of the adsorbent, but also the existing form of the metal ions in solution. In the solution of pH 2.0–8.0, it is known that lead species exist in three forms of Pb2+, Pb(OH)+ and Pb(OH)2. The relative distribution of Pb species (i.e. Pb2+, Pb(OH)+ and Pb(OH)2) in the aqueous solution was calculated using a visual MINTEQ software [42], and the result was represented in Fig. 8. It could be seen that the dominant species was Pb2+ at pH < 6.0, the hydrolysis of Pb2+ to form Pb(OH)+
3.3.3.2. XPS analysis. In order to further understand the mechanism of Pb adsorption, binding energy values of EDTA-SBA-15 and PbEDTA-SBA-15 (initial concentration 100 mg L1, pH8.0) were measured by XPS and their results were shown in Fig. 10. Fig. 10a given an overview of the chemical composition of the two samples analyzed based on the XPS survey scans. The spectrum of EDTA-SBA15 clearly showed two peaks at around 1070 and 497 eV which were attributed to Na1s and Na Auger photoelectrons, respectively, whereas the two peaks were not observed in the spectrum of PbEDTA-SBA-15. The disappearance of Na peaks was mainly due to the exchange of Na ions on the surface of EDTA-SBA-15 by the Pb ions during adsorption process, which was in accordance with others’ work [44,45]. Fig. 10b and d showed the N1s spectra of EDTA-SBA-15 and PbEDTA-SBA-15. In Fig. 10b, a peak at 398.8 eV was attributed to
Fig. 8. The distribution of Pb (II) species as a function of pH based on the equilibrium constants (total concentration 100 mg L1, 25 °C).
Fig. 9. Effect of pH on the uptake of Pb2+ by EDTA-SBA-15 (initial concentration 100 mg L1, solution volume 50 mL, adsorbent 50 mg, contact time 5 h, 150 rpm, 25 °C).
144
J. Huang et al. / Journal of Colloid and Interface Science 385 (2012) 137–146
Fig. 10. XPS spectra of EDTA-SBA-15 and Pb-EDTA-SBA-15: (a) overall spectra of EDTA-SBA-15 and Pb-EDTA-SBA-15; (c) Pb4f spectrum of Pb-EDTA-SBA-15; N1s spectra of (b) EDTA-SBA-15 and (d) Pb-EDTA-SBA-15; O1s spectra of (e) EDTA-SBA-15 and (g) Pb-EDTA-SBA-15; C1s spectra of (f) EDTA-SBA-15 and (h) Pb-EDTA-SBA-15.
J. Huang et al. / Journal of Colloid and Interface Science 385 (2012) 137–146 Table 3 Peak area ratios (%) of OAC@O (288.4 eV) for the EDTA-SBA-15 with and without Pb adsorption. Material
EDTA-SBA-15 Pb-EDTA-SBA-15
Peak area ration (%) pH 6.0
pH 8.0
16.7 11.8
18.3 10.9
nitrogen in CAN, which shifted to 399.3 eV after the Pb2+ adsorption in Fig. 10d. The increase in the binding energy after Pb adsorption was due to the formation of the coordination bond of N-metal ions, in which the nitrogen atoms shared electrons with Pb and thus electron densities of nitrogen atoms were reduced. The same trend was observed in the O1s spectra of EDTA-SBA-15 and EDTASBA-15 (Fig. 10e and g). In Fig. 10e, the O1s spectrum consists of three peaks components with binding energies at about 531.4, 532.0, and 532.9 eV, which could be ascribed to NaAOAC, C@O, CAOAH [46], respectively. After Pb2+ adsorption, the O1s spectrum changed. As shown in Fig. 10g, the peak at 531.4 eV increased to 532.1 eV. According to the above analysis, the main functional groups involved in the Pb adsorption were carboxyl groups and tertiary amino groups. The C1s spectrum of EDTA-SBA-15 was composed of four peaks with differentiated binding energy values via deconvolution (Fig. 10f). These peaks were assigned to CAC and CAH (284.4 eV), CAN (286.3 eV), NAC@O (286.7 eV) and OAC@O (288.4 eV), respectively [47,48]. In the spectrum of Pb-EDTA-SBA-
145
15 (Fig. 10h), the peaks of CAC (284.5 eV), NAC@O (286.8 eV) kept almost unchanged, while the binding energy of CAN (285.9 eV) and OAC@O (287.7 eV) decreased. The result was indicative of the formation of Pb complexation, in which the nitrogen and oxygen shared electrons with Pb2+ and hence the electron density at the adjacent carbon atoms increased and the binding energy of the carbons were reduced [49]. The XPS Pb4f spectrum of PbEDTA-SBA-15 was also determined. For Fig. 10c, doublet characteristics of Pb appear at 138.2 eV (assigned to Pb4f7/2) and 143.0 eV (assigned to Pb4f5/2). The binding energy of Pb4f7/2 in Pb(NO3)2 was about 139.1–139.5 eV [50,51], and the significant chemical shift was assigned to the interaction of Pb2+ with EDTA-SBA-15. Table 3 showed the OAC@O peak area ratios of the EDTA-SBA15 before and after Pb adsorption. The area ratio for the peak decreased 29% and 40% between pH 6.0 and pH 8.0, respectively. The dominant species was Pb2+(>97%) in the aqueous solution at pH 6.0, which deduced Pb onto the EDTA-SBA-15 support proceeds with the Pb2+-carboxyl group per EDTA molecule complex format in 1:1 molar ratio. At pH 8.0, the main species were Pb(OH)+ (70%) and Pb2+ (27%). The reduced value of area ratio was consistent with the intended Pb2+-carboxyl group per EDTA molecule complex format in 1:2 molar ratio and Pb(OH)+-carboxyl group per EDTA molecule complex format in a 1:1 molar ratio. In general, per molecule Pb (II) could react with per grafted molecule in the experimental conditions. In summary, the XPS analysis above suggested that removal of Pb2+ from aqueous solution by adsorption onto EDTA-SBA-15 was
Fig. 11. The mechanism for Pb adsorption onto EDTA-SBA-15.
146
J. Huang et al. / Journal of Colloid and Interface Science 385 (2012) 137–146
a multiplex process which involved ion exchange as well as surface complexation (Fig. 11).
Academic Discipline Project (S30406), Key Subject of the Shanghai Normal University (DZL806) and the Shanghai Normal University (No. SK201048).
3.4. Regeneration of EDTA-SBA-15 adsorbent References In addition to excellent adsorption capacity, it was highly desirable that an adsorbent could be regenerated and reused repeatedly with regard to the cost. The regeneration capacity of the EDTASBA-15 material was evaluated by acid treatment. 50 mg of PbEDTA-SBA-15 complex sample (91.6 mg g1 Pb) was treated with 100 mL of a 0.1 M HCl solution, under stirring for 4 h, at room temperature. After the acid regeneration and neutralization (with a 0.1 M NaHCO3), the solid was reused in three successive adsorption-regeneration cycles. The adsorption capacity of the EDTASBA-15 after four cycles was 87.5 mg g1, indicating a loss in the adsorption capacity of only 4.4%, compared to the initial one. These data indicated the very good regeneration capacity of the SBA-15 modified material. 4. Conclusions Ordered functionalized mesoporous silica EDTA-SBA-15 used for the removal of Pb2+ from polluted water was successfully prepared via post-grafting pathway and characterized. The adsorbent presented high sensitivity and efficiency for removing Pb (II) from wastewater, which was a critical requirement for practical applications. The following conclusions could be drawn: (a) EDTA could be effectively grafted to the SBA-15 matrix using SOCl2 as an activating reagent in moderate conditions, and the carboxyl group content of the adsorbent was 0.908 mmol g1 after modification. (b) The hybrid mesoporous adsorbent had the advantages of both mesoporous silica SBA-15 and EDTA showed an excellent performance for Pb (II) adsorption and its maximum adsorption capacity calculated from the Langmuir model was 273.2 mg g1. (c) The Pb adsorption by EDTA-SBA-15 could reach the equilibrium within 20 min, and the kinetic data were well described by pseudo-second-order model with the correlation coefficient (R2) more than 0.99, confirming that the chemical sorption was the rate limiting step in the adsorption process. Further analyses revealed that the pore diffusion process also played a role in adsorption kinetics, which could be attributed to the porous structure of the synthesized adsorbent. (d) The adsorption was a highly pH dependant process and pH 5.0 was the optimum pH for Pb adsorption onto EDTASBA-15. According to the XPS analysis, the adsorption mechanism of EDTA-SBA-15 involved Na Pb ion-exchange and surface complexation. (e) The chemical stability of the functionalized material SASBA-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.
Acknowledgments This work was financially supported by the National Nature Science Foundation of China (20671063), Key Laboratory of Resource Chemistry of the Ministry of Education, Shanghai Leading
[1] A. Celik, A. Demirba, Energy Sour. 27 (2005) 1167. [2] R. Say, E. Birlik, A. Denizli, A. Ersoz, Appl. Clay Sci. 31 (2006) 298. [3] M. Nadeem, R. Nadeem, H.U. Nadeem, S.S. Shah, Pak. J. Sci. Res. 57 (2005) 71. [4] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834. [5] O. Zaporozhets, N. Petruniock, O. Bessarabova, V. Sukhan, Talanta 49 (1999) 899. [6] E. Soliman, Mahmoud M. Ahmed, Talanta 54 (2001) 243. [7] Y. Yamini, J. Hassan, R. Mohandesi, N. Bahramifar, Talanta 56 (2002) 375. [8] Dongyuan Zhao, Jianglin Feng, Qisheng Huo, Nicholas Melosh, Glenn H. Fredrickson, Bradley F. Chmelka, Galen D. Stucky, Science (1998) 548. [9] A. Stein, B.J. Melde, R.C. Schroden, Adv. Mater. 12 (2000) 1403. [10] A. Vinu, K.Z. Hossain, K. Ariga, J. Nanosci. Nanotechnol. 3 (2005) 347. [11] A. Sayari, S. Hamoudi, Chem. Mater. 13 (2001) 3151. [12] J. Rämö, M. Sillanpää, V. Vickackaite, M. Orama, L. Niinistö, J. Pulp Pap. Sci. 26 (2000) 125. [13] B. Sen Gupta, M. Curran, Shameem Hasan, T.K. Ghosh, J. Colloid Interf. Sci. 90 (2009) 954. [14] E. Repo, T.A. Kurniawan, J.K. Warchol, M.E.T. Sillanpää, J. Hazard. Mater. 171 (2009) 1071. [15] E. Repo, J.K. Warchol, T.A. Kurniawan, M.E.T. Sillanpää, Chem. Eng. J. 161 (2010) 73. [16] O. Karniz Júnior, L.V.A. Gurgel, R.P. Freitas, L.F. Gil, Bioresour. Technol. 98 (2007) 1291. [17] F.V. Pereira, L.V.A. Gurgel, L.F. Gil, J. Hazard. Mater. 176 (2010) 856. [18] J.X. Yu, M. Tong, X.M. Sun, B.H. Li, Bioresour. Technol. 99 (2008) 2588. [19] Dongyuan Zhao, Qisheng Huo, Jianglin Feng, Bradley F. Chmelka, Galen D. Stucky, J. Am. Chem. Soc. 120 (1998) 6024. [20] José Aguado, Jesús M. Arsuaga, Amaya Arencibia, J. Hazard. Mater. 163 (2009) 213. [21] T. Martin, A. Galarneau, D. Brunel, V. Izard, V. Hulea, A.C. Blanc, S. Abramson, F. Di Renzo, F. Fajula, Stud. Surf. Sci. Catal. 135 (2001) 178. [22] Mukthavaram Rajesh, Joyeeta Sen, Marepally Srujan, Koushik Mukherjee, Bojja Sreedhar, Arabinda Chaudhuri, J. Am. Chem. Soc. 129 (2007) 11408. [23] W.J. Liu, F.X. Zeng, Chem. Eng. J. 170 (2011) 21. [24] H.P. Boehm, Carbon 32 (1994) 759. [25] I. Ghodbane, L. Nouri, O. Hamdaoui, M. Chiha, J. Hazard. Mater. 152 (2008) 148. [26] J.A. Dean, Lange’s Hand Book of Chemistry, 15th ed., McGraw-Hill Book Company, USA, 1999. [27] C.S. Li, Y.H. Tsai, W.C. Lee, W.J. Kuo, J. Org. Chem. 75 (2010) 4004. [28] R. Chicharro, M. Alonso, V.J. Aran, B. Herradon, Tetrahedron Lett. 49 (2008) 2275. [29] S. Nagib, K. Inoue, T. Yamaguchi, T. Tamaru, Hydrometallurgy 51 (1999) 73. [30] P.P. Yang, S.S. Huang, D. Kong, J. Lin, H.G. Fu, Inorg. Chem. 46 (2007) 3203. [31] D. Brunel, A. Cauvel, F. Fajula, F. DiRenzo, Stud. Surf. Sci. Catal. 97 (1995) 173. [32] J. Losada, I. Del Peso, L. Beyer, Inorg. Chim. Acta. 321 (2001) 107. [33] S. Gago, J.A. Fernandes, J.P. Rainho, R.A. Ferreira, S.M. Pillinger, A.A. Valente, T.M. Santos, L.D. Carlos, P.J.A. Ribeiro-Claro, Chem. Mater. 17 (2005) 5077. [34] S. Lagergren, Ksver Veterskapsakad Handl. 24 (1898) 1. [35] E. Da’na, A. Sayari, Chem. Eng. J. 166 (2011) 445. [36] Y.S. Ho, G. Mckay, Adsorpt. Sci. Technol. 16 (1998) 243. [37] E. Repo, J.K. Warchol, A. Bhatnagar, J. Colloid Interf. Sci. 358 (2011) 261. [38] M.A. Al-Ghouti, M.A.M. Khraisheh, M.N.M. Ahmad, S. Allen, J. Hazard. Mater. 165 (2009) 589. [39] I. Langmuir, J. Am. Chem. Soc. 40 (1918) 1361. [40] H.M.F. Freundlich, Z. Phys. Chem. 57A (1906) 385. [41] F. Haghseresht, G. Lu, Energy Fuel 12 (1998) 1100. [42] J.P. Gustafsson, Visual MINTEQ: Department of Land and Water Resources Engineering (Ver. 3.0), The Royal Institute of Technology, Sweden,