Proline Functionalized Dicationic Framework of ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

L‑Proline

Functionalized Dicationic Framework of Bifunctional Mesoporous Organosilica for the Simultaneous Removal of Lead and Nitrate Ions

Manish K. Dinker,† Thalasseril G. Ajithkumar,‡ and Prashant S. Kulkarni*,† †

Energy and Environment Laboratory, Department of Applied Chemistry, Defence Institute of Advanced Technology (DU), Girinagar, Pune, 411025, India ‡ Central NMR Facility, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune, 411008, India S Supporting Information *

ABSTRACT: A novel bifunctional mesoporous organosilica, PEG-functionalized bis-prolinium chloride bridged mesoporous organosilica (BPBMO) was synthesized by reacting the precursor, PEG-functionalized bis-prolinium chloride bridged organosilane (BPRIL) with tetraethyl orthosilicate (TEOS) in the presence of surfactant. The chemical conformation of BPBMO was investigated by using Fourier transform infrared (FTIR), thermogravimentric analysis (TGA), 13C, and 29Si cross-polarization/magic angle spinning (CP/MAS) NMR techniques. The characterization represents PEG-linkedprolinium (−N+Cl−) and carboxyl (−COOH) entities, constructing the dicationic framework through siloxane (Si− O−Si) linkages. The pore-wall distribution and the periodicity of BPBMO retained during the synthesis were examined by small-angle X-ray scattering (SAXS), Brunauer-Emmett-TellerBarrett-Joyner-Halenda (BET-BJH), and transmission electron microscopy (TEM) techniques. The results revealed BPBMO as a spherical shaped solid (50−100 nm) having mesopore channels hexagonally arranged with interparticle porosity (SBET = 487 m2/ g and DBJH = 5.1 nm). The material has provided active binding sites for the simultaneous removal of NO3− and Pb2+ ions when introduced in the aqueous solutions of Pb(NO3)2 (50 mg/L, pH 6). The removal of NO3− by ion-exchange with prolinium (−N+Cl−) entities and the electrostatic interaction of Pb2+ with carboxylate (−COO−) group were characterized by using Raman spectroscopy, ion chromatography, and X-ray photoelectron spectroscopy (XPS) technique. The maximum removal of NO3− and Pb2+ ions were achieved within 1 h of the adsorption reaction. The adsorption has followed the Langmuir isotherm model with the adsorption capacities (qm) of 23.04 and 21.92 mg/g for NO3− and Pb2+ ions, respectively. The efficiency of the adsorbent was also compared with other adsorbents. Further, the BPBMO material has depicted three consecutive adsorption/ desorption cycles with negligible loss in the structural conformation. KEYWORDS: L-Proline, Ionic framework, PMO, Lead(II)nitrate, Adsorption−desorption

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INTRODUCTION Amino acids, as essential biomolecules, restrain the functionality of both amine and carboxyl groups and can be transformed into zwitterions through varying pH.1,2 These biomolecules, when immobilized on the solid surface, serve as chelating groups for the metal ion interaction via electrostatic attraction or hydrogen bonding.3−7 Among the solids, mesoporous silicas have the potential to provide better surface as it possesses high specific surface area, uniform pore size distribution, and good hydrothermal stability.8,9 Realizing these facts, researchers have designed sustainable materials, initially by grafting organosilanes and then, covalently immobilizing amino acids either by forming an amide, amino, or thiol linkage.10−13 For enhancing the selectivity of an adsorbent to a particular metal ion, a strategic selection of an ideal organosilane and an amino acid, is highly required. For instance, Bhattacharyya and the group have © 2017 American Chemical Society

carried out immobilization of L-cysteine on prederivatized 3glycidoxy propyl silica by covalent attachment of amino moiety on epoxy groups. Finally, the thiols (−SH) from L-cysteine served as binding sites for Hg(II).14 In another case, Pirngruber and the group have attempted the functionalization of histidine and glutamic acids on amino silane-grafted silicas and employed the material as ligands for Fe(II).15 Beside the development of materials by covalently immobilizing amino acids onto the functionalized organosilane, a one-step co-condensation methodology has also been equally practiced. Interestingly, the precursors were initially developed by amidification between the carboxyl groups of preprotected Received: January 18, 2017 Revised: March 18, 2017 Published: April 5, 2017 4188

DOI: 10.1021/acssuschemeng.7b00132 ACS Sustainable Chem. Eng. 2017, 5, 4188−4196

ACS Sustainable Chemistry & Engineering

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amino acids and the amino groups of organosilanes.16 Subsequently, the precursors were introduced with silica templates via hydrolysis and co-condensation to produce functionalized mesoporous silicas.17,18 The utilization of commercially available precursor was also reported when (S)N-(3-(triethoxysilyl) propyl) pyrrolidine-2-carboxamide and sodium silicate were co-condensed over amphiphilic surfactant to yield L-proline functionalized mesoporous silica.19 Recently, researchers have strategically followed a sol−gel methodology for the development of (poly)amino-acids modified mesoporous silica by avoiding organosilane. The co-condensation of silicates with the side groups of (poly)amino-acids in basic media resulted in the formation of functionalized silica adsorbents having moderate pore size distribution and organic contents.20,21 Initially, three independent groups reported the designing of highly functionalized silica materials with a periodic arrangement of the organic contents for constructing the pore-wall framework.22−24 It was possible due to the synthesis of bridged organosilanes [(R′O)3Si−R−Si(OR′)3] as a precursor and their self-organization over the surfactant through hydrolysis and polycondensation. The arrangement produces periodic mesoporous organosilicas (PMOs). The involvement of amino acids in designing PMOs is rarely practiced. Kuschel and co-workers created PMOs by applying 1,3-bis-tri-isopropoxysilylbenzoic acid over the surfactant P-123 in acidic medium, and further modified them by grafting amino acids. They employed the materials for absorption of a chiral gas.25 Beretta and the group have developed divinylaniline-bridged PMO material wherein the amino group was covalently functionalized with L-alanine. The peptide linkages (−CO-NH−) are formed within the mesopore channels.26 Recently, Voort and the group have modified PMO containing ethylene bridges by immobilizing cysteine and cystamine, and obtained bifunctional acid−base catalysts.27 The formation of solid framework of mesoporous organosilicas using ionic liquid (IL) containing bridged organosilane is seldom explored.28−31 Recently, our group has synthesized PEG-linked bis-imidazolium bridged framework of mesoporous organosilicas as ion exchangers.32 However, to the best of our knowledge, construction of the main pore-wall framework of PMO by using amino acid functionalized organosilane as a precursor is never reported. In the present work, the objective was to design a bifunctional ionic framework of mesoporous organosilica, BPBMO, by organizing novel PEG-linked Lproline functionalized bridged-organosilane over the surfactant. The prepared material was endowed with both ammonium (−N+Cl−) and carboxyl groups (−COOH) within its solid pore-wall entity. It was characterized by using solid NMR, Fourier transform infrared (FTIR), and thermogravimetric analysis (TGA) techniques. Interestingly, these organic groups behaved as cationic (−N+−) and anionic (−COO−) moieties (zwitterions) at neutral pH. Consequently, the material has provided adsorption sites for NO3− and Pb2+ ions by following ion-exchange mechanism as well as electrostatic interaction. Moreover, three successive regenerations of the material were attempted for the adsorption/desorption of Pb2+ and NO3− ions. The effect of the desorbent on the covalent linkages of Si−O−Si and Si−C were also examined. Considering the health hazards of Pb2+ and NO3− ions in the surface water from various industrial and agricultural activities, development of such a sustainable material is beneficial.33−36

Research Article

EXPERIMENTAL SECTION

Chemicals. All the reagents and solvents were of analytical grade and used as purchased without any further purification. MCM-41 silica (hexagonal), L -proline (≥99.0%), p-toluenesulfonyl chloride (≥98.0%), 3-chloropropyl trimethoxysilane (CPTMS) (≥97.0%), tetraethyl orthosilicate (TEOS) (≥99.0%), dichloromethane (DCM) (≥99.0%), tetrahydrofuran (THF) (≥99.9%), and lead(II)nitrate (≥99.0%) were obtained from Sigma-Aldrich. Cetyltrimethylammonium bromide (CTAB) (≥97.0%), polyethylene glycol (PEG)-200, sodium hydrogen carbonate (≥99.0%), sodium hydroxide pellets, and other reagents were purchased from Merck Chemicals. Preparation of the Precursor BPRIL. The precursor BPRIL [(PEG-functionalized bis-prolinium chloride bridged silsesquioxane or 1,1′-(oxy bis(ethane-2,1-diyl))bis(2-carboxy-1-(3-(trimethoxysilyl)propyl)pyrrolidin-1-ium) chloride)] was synthesized according to our reported procedure.32 In a reaction vessel, a biphasic system was created by dissolving PEG-200 (2.4 g, 11.5 mmol) in a mixture of THF (12 mL) and 2.5 M NaOH. Further, p-toluenesulfonyl chloride (5 g, 26.2 mmol) was added to the system which is incubated in an ice bath. Then, 1 M HCl (1 mL) was added to the mixture by vigorous stirring at room temperature for 4 h. Later, the material was extracted using DCM (25 mL), 5% NaHCO3 (25 mL), and water. The removal of solvent was carried out using vacuum evaporator to obtain PEGditosylate, which is later dissolved (2.3 g, 7.2 mmol) in a mixture of THF (5 mL) and 4 M NaOH. To this solution, L-proline (1.5 g, 13 mmol) was batch-wise added while stirring at 50−60 °C for 4 h. Afterward, sodium tosylate and NaOH were rinsed off from the material using DCM and water (25 mL) and filtered. The material was dried using vacuum evaporator to remove THF and DCM and obtained as a viscous orange liquid (PEG-functionalized bis-proline). Next, in a round-bottom flask, a mixture was prepared by adding CPTMS (7.4 g, 37.2 mmol) and PEG-functionalized bis-proline (4.1 g) in dry toluene (10 mL). It was kept on stirring to reflux under N2 atmosphere at 60 °C for 24 h. Thereafter, the biphasic liquid system was separated using a separating funnel and the bottom fraction was washed with dry toluene. Later, the product was kept in a vacuum oven for 4 h, which yielded 53% (6.1 g) of pink colored viscous liquid. Synthesis of BPBMO Material. Initially, CTAB (1.2 g, 3.3 mmol) and NaOH (0.5 g, 12.5 mmol) were dissolved in a reaction vessel containing distilled water (50 mL). Thereafter, TEOS (0.5 g, 2.4 mmol) was batch-wise added to the solution at 40 °C for 1 h. After getting a homogeneous solution, BPRIL (2.8 mL) was added to it with continuous stirring and then, the mixture was kept in a round-bottom flask at 70 °C, and refluxed for 20 h. The entire sol solution was then incubated in a Teflon-lined stainless steel autoclave and aged at 100 °C for 24 h. An orange precipitate was formed which is filtered and dried at 50 °C in a vacuum oven. The solid was extracted using ethanol in a Soxhlet extractor, for 5 h to remove the surfactant and unreacted organic units. Finally, it was air-dried to get BPBMO. Adsorption Experiments. Batch-mode experiments were carried out by using BPBMO for the adsorption of Pb2+ and NO3− ions from the stock solution of Pb(NO3)2 having a concentration 10−50 mg/L. The efficiency of BPBMO was compared by using neat MCM-41. The amount of adsorbent was kept constant, i.e., 50 mg in 25 mL (2 g/L) of each stock solution. The adsorption reaction was performed for 60 min by keeping the reaction flask on the orbital shaker at room temperature. Thereafter, the mixture was centrifuged to separate the adsorbent from the solution. The filtrate was analyzed using an ion chromatograph to measure the final concentration of Pb2+ and NO3− ions in stock solution. Thus, the adsorption capacity of the material was estimated. The effect of pH on the removal of Pb2+ and NO3− ions was observed by adjusting the pH of the stock solution using NaOH and HCl (each 0.1 M). The regeneration of the adsorbent was carried out using 0.1 M of NH4OH solution. Characterization Techniques. The FTIR study of the material was carried out using a Bruker ALPHA ATR instrument with ZnSe crystal and gold coated mirror and monitored at 4 cm−1 resolution with 25 scans. 1H and 13C NMR spectroscopic analyses of BPRIL were performed on Bruker AV 300 NMR at the frequencies of 400 and 100 4189


Research Article

ACS Sustainable Chemistry & Engineering MHz, using DMSO-d6 solvent. Solid-state 13C and 29Si crosspolarization/magic angle spinning (CP/MAS) NMR measurements of BPBMO were done using Bruker AV 300 NMR at the frequencies 75.47 and 59.63 MHz. The small-angle X-ray diffraction pattern of the material was recorded using a Rigaku RINT-2200 diffractometer with Cu Kα radiation from the 2θ range of 0.5 to 7° in 0.03 steps. The transmission electron microscopy (TEM) technique was employed to obtain the particle images of BPBMO using the TEM instrument of Philips CM 200. The specific surface area and pore-size distribution of the material were recorded using a Thermo Scientific Surfur BET-BJH instrument at 77 K within the P/P0 range of 0.05−0.3. The samples were degassed at 150 °C for 4 h under vacuum prior to the N2 adsorption/desorption measurements. Thermogravimetrical analyses (TGA) of the precursor and the solid material were carried out using a PerkinElmer 1061608 instrument within a temperature range of 25− 800 °C. Analysis of Pb(NO3)2 solution before and after the adsorption were performed by using a Raman spectrophotometer (Olympus BX41, 2 cm−1 of spectral resolution with 400 laser beam scans), X-ray photoelectron spectrophotometer (Thermo Scientific with MnSi/Al Kα (hν = 1486.68 eV) manual X-ray source), and ion chromatograph (Metrohm 883 Basic IC Plus). The regeneration of the BPBMO material was characterized by using a FTIR instrument.

in silsesquioxanes, which further remains as silicon oxide after 626 °C. The chemical structure of the bridged organosilane (BPRIL) was investigated by using FTIR and NMR techniques. Figure 1a depicts the FTIR spectra of BPRIL with the stretching

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RESULTS AND DISCUSSION Structural Analysis of BPBMO Material. The preparation of mesoporous organosilica BPBMO was carried out by crosslinking BPRIL with TEOS in the presence of CTAB through a complete hydrolysis−condensation process. Scheme 1 demon-

Figure 1. FTIR spectra of (a) precursor BPRIL, and (b) as-synthesized BPBMO material.

vibrations of −OH, CO, and C−N bonds at 3435, 1739, and 1194 cm−1, respectively, which confirms the proline entity. The stretching vibrations of C−H bonds of the precursor could be due to intense bands at about 2943−2841 cm−1. The characteristic absorption peaks at 1070 and 815 cm−1 correspond to the stretching of Si−O−C and Si−C bonds, respectively.38 The absorption peak at 755 cm−1 belongs to the stretching of C−Cl bond which could be attributed to the presence of a small amount of unreacted CPTMS units. The 1H NMR spectra of the precursor (Figure S2) also depicts the proton peak of the same organic group (−CH2−Cl) at chemical shift (δ) 3.6 ppm. The existence of this entity was also seen in 13C NMR spectra of BPRIL at the chemical shift 47.8 ppm (Figure S3). Apart from this, the intense NMR peak at 68.3 ppm depicts the ether bridging (CH2−O−CH2) between the two prolinium units. The C atom existing adjacent to the carboxyl group represents at δ 78.2 ppm while the characteristic NMR peak of the terminal Si−OCH3 unit can be seen at δ 49.05 ppm. The aforementioned chemistry of the precursor was tailored into a solid framework of BPBMO though cross-linking BPRIL with TEOS over the surfactant CTAB (Scheme 1). The transformation of terminal Si−OCH3 entities into the covalent siloxane linkages constructing the pore-wall of the material was determined by the FTIR. As depicted in Figure 1b, the absorption peaks at 3346 and 1060 cm−1 correspond to the stretching vibrations of surface silanols (Si−OH) and siloxane (Si−O−Si) bonds, respectively.38 It indicates the formation of a solid-framework after the hydrolysis−condensation process. Nonetheless, to further explore the chemistry of BPBMO material, a solid carbon NMR analysis was carried out. Figure 2a shows the peaks C1, C2, and C5 at 10.6, 17.8, and 60.7 ppm, respectively, which belong to the carbon of the alkyl chain (N− CH2−CH2−CH2−Si) existing in the pore-wall. The NMR peak C3 at 26.8 ppm represents the β-carbon of the proline ring while the peak C8 at 175.1 ppm denotes the carbon from the

Scheme 1. Preparation of Mesoporous Organosilica BPBMO by Cross-Linking BPRIL with TEOS in Basic Medium

strates the development of an organosilica material by using suitable organic moieties (precursor) within its mesoporous framework. Among the covalent Si−O and Si−C linkages present in BPRIL, the polyethylene glycol linkage may provide additional polarizability and thermal stability to the material.37 The thermal stability of the precursor was investigated by performing TGA. Figure S1 shows a major decomposition of the material, observed between 164−385 °C with almost 48% weight loss. It is attributed to the cleavage of ether bridging and alkyl chain associated with the material. The decomposition at 385−626 °C illustrates 11.8% loss from the Si−OCH3 existing 4190


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Figure 2. (a) 13C and (b) 29Si CP/MAS NMR of as-synthesized BPBMO material (nonactivated).

Table 1. Chemical and Textural Parameters of BPBMO Material 29 2

Si CPMAS NMR peaks integration (%) 3

3

textural properties 4

T (−60 ppm)

T (−68 ppm)

Q (−102 ppm)

Q (−112 ppm)

SBET [m /g]

V [cm3/g]

DBJH [nm]

a0 [nm]

22.62

45.12

13.26

19.00

487

0.56

5.1

6.33

carboxyl group (COOH). The solid NMR peak C6 at 70.5 ppm corresponds to the carbon of ether linkages (CH2−O−CH2) whereas, the peak C4 at 47.4 ppm can be denoted to the carbon of −CH2−Cl units. As discussed earlier, the minor unreacted CPTMS molecules existing in the precursor may have embedded within the solid framework of BPBMO and can be seen as solid NMR peak C4. The disappearance of chemical shift (49.05 ppm) of BPRIL from the solid NMR spectra of BPBMO indicates the formation of Si−O−Si and Si−OH linkages from the hydrolysis of terminal Si−OCH3 entities. It can be further elaborated from the solid silica NMR spectra of BPBMO, as given in Figure 2b. The spectra depicts four peaks in which two of them belong to weak T2 [RSi(OSi)2(OH)] and strong T3 [RSi(OSi)3] signals, where R represents PEG-functionalized bis-prolinium groups. The Q3 [Si(OSi)3(OH)] and Q4 [Si(OSi)4] signals were attributed to the presence of typical Si−O−Si and Si−OH groups, respectively, comprising the pore-wall of BPBMO.39 Also, the weak signals of T2 and Q3 indicate the presence of fewer amounts of surface silanols (Si−OH). It has been observed that some bulky ionic disilanes having electronegative groups may led to the breaking of Si−C bonds during hydrolysis and causes partial condensation of self.32,40−42 Therefore, deconvolution of 29 Si CPMAS NMR data was carried out to calculate the quantitative assessment (%) of Tn and Qm peaks via Tn/(Tn + Qm) (Figure S4 and Table 1). The T-signals integration (ΣTn = 67.7%) confirmed the loading of most of the molecules from the PEG-linked bis-prolinium bridged organosilane. Beside this, the rest of the siliceous species of BPBMO are from covalently cross-linked TEOS. Interestingly, these covalent siliceous linkages (Si−O−Si, Si−OH, and Si−C bonds) of BPBMO can be accounted for the high thermal stability. The thermal stability of the material is revealed from the TGA studies as reported in Figure S5. The initial decomposition from 257 °C with almost 13.5% weight loss of the material until 594 °C may have been due to the initial Si−C isolations and the decay of ionic moieties. The mesoporous configuration of the material was measured by the small angle XRD technique. Figure 3a shows the SAXS pattern of BPBMO which exhibited a strong reflection indexed (100) at q = 0.115 A−1 and a weak reflection (110) at q = 0.191

2

Figure 3. (a) Small angle X-ray scattering (SAXS) pattern, (b) TEM image, (c) N2 adsorption−desorption isotherm (BET), and (d) poresize distribution (PSD) curve of as-synthesized BPBMO material.

A−1. These two hkl reflections indicate the homogeneous covalent arrangement of the organic linkers and a hexagonal P6mm space group in the material. The calculated value of the lattice constant a = 2d100/√3 (where d is the interlayer distance, d = 2π/q) is given in Table 1. The porosity and particle morphology of the material were further corroborated by using transmission electron microscopy (TEM), as shown in Figure 3b. The particle size of BPBMO lies between 50−100 nm and seems to have a contour of spherical shaped solids. A high-resolution TEM image represents the existence of ordered mesopore channels in BPBMO, and the structure resembles the pore-structure of functionalized MCM-41.43 Moreover, the inset image of Figure 3b shows some interruptions in the mesoporous arrangement, indicating the existence of interparticle porosity. It may be due to the minor unreacted moieties of CPTMS in the precursor which later forms organic linkages within the pore-wall framework of BPBMO solid (Figure S6). The mesoporous character of BPBMO was further investigated by N2 physisorption measurement, and the resulting isotherm is shown in Figure 3c. The material depicted a type IV isotherm 4191


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Figure 4. (a) Adsorption mechanism for the removal of Pb2+ and NO3− ions using BPBMO and (b) Raman spectra of BPBMO material before and after the treatment of Pb(NO3)2 solution (pH 6 and concentration 50 mg/L).

Figure 5. (a) XPS wide scan of BPBMO material before and after the treatment of Pb(NO3)2 solution (pH 6 and concentration 50 mg/L), and (b) deconvoluted Pb 4f spectrum. Deconvolution of the O 1s spectra obtained (c) before and (d) after the adsorption process.

633.4 and 967.7 cm−1 represent the stretching modes of Si− O−Si and Si−C bonds, respectively, while the peak at 1123.7 cm−1 belongs to the bending of C−H bond (alkyl group).44 After the adsorption treatment, a new peak appears at the Raman shift 1042.3 cm−1 that corresponds to the symmetric stretching mode of the adsorbed NO3− ions on the solid surface.45 Ren and group have similarly analyzed amine-crosslinked reed (ACR) after nitrate loading and obtained a spectra of the Raman shift at 1043.9 cm−1 for the NO3− ions.46 The ion chromatography analyses of pre and post-treated aqueous solutions of Pb(NO3)2 also demonstrates the ion exchange between NO3− and Cl− ions (Figure S7). An XPS analysis was carried out for the detailed investigation into the adsorption of Pb2+ ions on the surface of BPBMO. Figure 5a depicts the XPS spectra after the adsorption treatment where the peaks 536, 403, 288, 160, 142, and 107 eV belong to the elements O, N, C, Si, and Pb, respectively.47 Considering the XPS peak of Pb element, the spectrum was deconvoluted into two Gaussian peaks centered at 141.9 and

with a large capillary condensation step at partial pressure range P/P0 of 0.4−0.9. This moderate hysteresis loop seems to be associated with the presence of mesopores and interparticle porosity. It reveals the pore-wall arrangement of BPBMO which is constructed by the covalent linkages of organosilane moieties.43 The specific surface area (SBET) and the pore size (DBJH) of the material derived from the aforementioned isotherms were calculated as 487 m2/g and 5.1 nm (Table 1). Treatment of Lead(II)nitrate Solution. A reasonable pore-size distribution and high organic functionalities of BPBMO could serve as a proficient adsorbent for removal of both cations and anions. Figure 4a demonstrates the probable adsorption mechanism, while treating the material with model aqueous solution of Pb(NO3)2. Two methods exist for the removal, an ion-exchange for NO3− ions with Cl− from the dicationic framework and the electrostatic interaction of Pb2+ with the carboxylate groups (−COO−). The Raman spectra of BPBMO obtained before and after the treatment of Pb(NO3)2 solution are shown in Figure 4b. The peaks at the Raman shifts 4192


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−H2O+ surface charges, utmost generated at pH 8 and 4, respectively.57,58 A high pH condition generates stronger negative charges on the carboxyl group (COO−) and fewer surface silanols (−OH−) available on BPBMO; therefore, more Pb2+ ions get electrostatically attracted toward them (Figure 6a). Dakova and Liu groups have obtained similar results for the removal of Pb2+ ions at the pH range 6−8 by using Lcysteine modified silica and Fe3O4 nanoparticles.59,60 On the contrary, the low pH condition was found responsible to retain high protonation within the bis-prolinium entity of BPBMO which ultimately led to having high uptake of NO3− via ionexchange (Figure 6b). Nesterenko et al. has examined the role of L-proline-bonded silica (L-Pro−SiO2) as a stationary phase for the separation of NO3− ions and has achieved high ionexchange kinetics at pH ∼4.61 Nonetheless, adjusting the lower and higher pH conditions led to increasing the concentration of H+ and OH− ions in the solution which competes with simultaneous removal of Pb2+ and NO3− ions. In order to optimize the role of the material for simultaneous removal of cation and anion, the adsorption was carried out from the Pb(NO3)2 solutions at pH 6. As a result, the removal of NO3− ions decreased to 82.5 from 92%. Similar findings were reported by Yu and group during the simultaneous removal of Pb2+ and NO3− ions at pH 5 by using bifunctional mesoporous silica adsorbent.62 The adsorption isotherms were considered to understand the adsorption affinity and to calculate the capacity of the material for both cation and anion. Hence, following equations were considered to evaluate the experimental (qe) and theoretical (qm) adsorption capacities of the material.

147 eV which can be assigned to Pb 4f7/2 and Pb 4f5/2 (Figure 5b).36 Previously, these two peaks representing the oxidation states of Pb2+ ions have been found centered at 144 and 139 eV.48,49 However, in our case, such shifts can be vindicated on the basis of C 1s spectra obtained before and after the treatment of Pb(NO3)2 solution, as shown in Figure S8. The deconvoluted C 1s spectra of BPBMO material have shown three Gaussian peaks at 284.4, 286.8, and 289.9 eV which correspond to the C atoms in C−C, C−O, and O−C=O, respectively.50,51 On the contrary after the treatment, the C 1s spectra obtained from the material has depicted two idle Gaussian peaks at 284.5 and 286.7 eV, and a shifted peak at 289.3 eV (Figure S8). The decrease in the binding energy from 289.9 to 289.3 eV signifies toward the chelation of Pb2+ ions with the carboxylate group (−COO−), and a change in the oxidation state of the adjacent C atom.47 Moreover, the O 1s spectra obtained from the material before and after the treatment have shown a shift in the peak from 536.3 to 536.8 eV (Figure 5c,d). It evidently proves the electrostatic interaction of Pb2+ with oxygen from the carboxylate group which ultimately reduces the electron density on the O atom and thus, a shift in the peak occurred.52,53 In the meantime, the dicationic sites (−N+Cl−) simultaneously adsorbs NO3− via the ion-exchange mechanism. This caused an appearance of a new peak in N 1s spectra at 412.6 eV (for NO3−) along with the shift in a peak from 403.8 to 403.2 eV belonging to the cationic N atom (Figure S8).54 In a similar findings, the peak assigned to NO3− ions were spotted at 404.2 and 406.2 eV which does depend on the type of adsorption and the adsorption sites.46,54 In adsorption process, a solution pH has an important role in creating active binding sites on the material which ultimately decides the maximum removal of metal ions.55,56 Figure 6a,b

qe =

(C0 − Ce)V M

(1)

Ce C 1 = e + qe qm KLqm

ln qe = ln KF +

(2)

1 ln Ce n

(3)

The values C0 and Ce stand for the concentration (mg/L) of Pb(NO3)2 solution at initial and the equilibrium stage, V is the volume (L) of the solution, and M is the amount (g/L) of BPBMO. The constants KL (Langmuir), KF, and n (Freundlich) represents the energy, capacity, and intensity (favorability, if n > 1) of the adsorption.63 As shown in Figure S9, the adsorption of Pb2+ and NO3− ions seem to get more linearized when plotted as Ce/qe versus Ce (Langmuir isotherm) than lnqe versus lnCe (Freundlich isotherm). It suggests the homogeneous monolayer adsorption for both the ions on the pore-wall surface of BPBMO. The theoretically estimated maximum adsorption capacities of the material for the removal of Pb2+ and NO3− ions are given in Table 2. Regeneration Studies of BPBMO. The desorption of Pb2+ and NO3− ions and subsequent regeneration of the material

Figure 6. Effect of pH on the removal of (a) Pb2+ and (b) NO3− ions using BPBMO material and neat MCM-41 (time 60 min).

reveals the effect of solution pH on the removal (%) of Pb2+ and NO3− ions using BPBMO and bare silica (MCM-41) within 1 h of the reaction. The maximum removal of Pb2+ (85%) was observed at the pH range 6−8, whereas the solution pH 4 was found to be the ideal for the maximum uptake of NO3− ions (92%) by using BPBMO. On the contrary, neat MCM-41 material could achieve only 8.2 and 9.0% removal of Pb2+ and NO3− via electrostatic interaction with −OH− and

Table 2. Langmuir and Fruendlich Isotherm Models for the Removal of Pb2+ and NO3− Ions from Pb(NO3)2 Solution Using BPBMO Material Langmuir isotherm adsorbate 2+

Pb NO3−

Freundlich isotherm 2

qe (mg/g)

qm (mg/g)

KL (L/mg)

R

19.4 20.5

21.92 23.04

0.551 0.705

0.9997 0.9984 4193

KF

n

R2

10.763 7.088

1.857 1.868

0.9557 0.9542


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In another report, amino-functionalized silica submicrospheres were modified with L-cysteine for the removal of Pb2+ and the adsorption capacity attained was 0.1 mg/g.59 Further, Yongsheng and group have carried out the removal of Pb2+ ions by using organosilane-functionalized ion-imprinted nano-TiO2 (Pb-IP) and observed adsorption capacity of 22.7 mg/g.68 On the contrary, for the removal of NO3−, Hamoudi and group have reported adsorption capacity of 20.5 mg/g by using MCM-41−NH3+ material within 3 h.69 Comparatively, the BPBMO material has achieved 23.0 mg/g of NO3− removal capacity within 1 h of equilibrium time. Thus, the development of bifunctional (ion-exchange and electrostatic) L-proline-based silica adsorbent which helps in minimizing the energy demand thereby simultaneously removing toxic ions such as Pb2+ and NO3− is never reported.

were performed by using 0.1 M NH4OH. The material was regenerated and reused three successive times for the treatment of Pb(NO3)2 solutions (50 mg/L), and the results are depicted in Figure 7a. It shows gradual decrease on the removal

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CONCLUSIONS The PEG-functionalized bis-prolinium chloride bridged mesoporous organosilica (BPBMO) was successfully developed by reacting the newly synthesized viscous ionic precursor BPRIL (PEG-functionalized bis-prolinium chloride bridged organosilane) with TEOS over CTAB. The dicationic pore wall framework of the material consists PEG-linked bis-prolinium chloride units covalently attached through Si−O−Si linkages with the hexagonal arrangement. It has mesoporous channels with interparticle porosity and high surface area. Therefore, the material possesses bifunctionality (−N+Cl− and −COOH entities) and, hence, served as an adsorbent for the simultaneous removal of cations and anions. The material has provided adsorption sites for the removal of NO3− via ionexchange with Cl− and Pb2+ through electrostatic interaction with −COO−, within 1 h of the reaction at pH 6. The Langmuir isotherm model has well-defined the adsorption mechanism and the material has achieved 23.04 and 21.92 mg/ g of maximum adsorption capacities for NO3− and Pb2+ ions, respectively. The successive regeneration and reusability of BPBMO material have been attained three times. The chemical and textural conformations of BPBMO have evidently demonstrated the sustainability of the adsorbent, and thus, it can be employed in the simultaneous removal of Pb2+ and NO3− ions from the industrial wastewater (e.g., electroplating, explosives, matches, etc.). Moreover, the material can also be utilized as an in vivo drug-carrier, for chiral recognition, and as a heterogeneous catalyst in high-temperature reactions.

Figure 7. (a) Removal efficiency (%) and (b) FTIR spectra of BPBMO material obtained after the 3rd cycle of regeneration.

percentages of both Pb2+ and NO3− ions after each cycle of regeneration. It could be due to the material handling and structural inadequacy while using 0.1 M NH4OH solution. Therefore, FTIR studies of the BPBMO material were performed after the third attempt of recycle and compared with the original (Figure 7b). It was observed that the intensity of the absorption peaks of silanol (3420 cm−1) and siloxane (1075 cm−1) are descended compared with the neat. The stretching vibration of C−H bond (2915 cm−1) from an alkyl group showed similar pattern. All these consequences demonstrate the need to search for a better desorbing agent which is currently underway in our laboratory. Comparison with Other Silica Based Adsorbents. The role of BPBMO as a bifunctional material for the simultaneous removal of Pb2+ and NO3− ions can be compared with reported silica based materials.58,59,62−69 Particularly, Yu and group have synthesized the acid−base bifunctional (dual electrostatic characteristics) mesoporous silica (Al-MS2) for the simultaneous removal of Pb2+ and NO3− and the removal capacity was found to be higher than the BPBMO (Table S1).62 However, the mechanism was not confirmed by the instrumentation techniques. The lower removal capacity of the BPBMO material could be attributed to the functional groups covalently integrated within the pore walls. However, it imparts substantial stability to the material in the adverse conditions and hence helped in carrying out the regeneration studies.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00132. 1 H and 13C NMR spectra of BPRIL, TG analytical curves of BPRIL and BPBMO, 29Si CP/MAS NMR spectral deconvolution of BPBMO, chromatograms of Pb(NO3)2 sample before and after the treatment, and XPS spectra of C 1s and N 1s from BPBMO before and after the treatment (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: ps_kulkarni@rediffmail.com, [email protected]. Phone: +91 20 24304161. Fax: +91 20 24389509. 4194


Research Article

ACS Sustainable Chemistry & Engineering ORCID

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Prashant S. Kulkarni: 0000-0001-7730-7287 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support from DRDO (Grants ERIP/ER/1003883/M/01/908/2012/D, R&D/1416, dated, March 28, 2012) New Delhi, India. Also, we would like to thank Dr. P. R. Rajamohanan of NCL, Pune, India, for his support in providing 13C and 29Si CP/MAS NMR analyses.

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