Spectacular Selectivity in the Capture of Methyl Orange by Composite

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Spectacular Selectivity in Capture Methyl Orange by Composite Anion Exchangers with Organic Part Hosted by Daisogel Microspheres Ecaterina Stela Dragan, and Maria Valentina Dinu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04498 • Publication Date (Web): 25 May 2018 Downloaded from http://pubs.acs.org on May 25, 2018

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ACS Applied Materials & Interfaces

Spectacular Selectivity in Capture Methyl Orange by Composite Anion Exchangers with Organic Part Hosted by Daisogel Microspheres Ecaterina Stela Dragan,* Maria Valentina Dinu a

“Petru Poni” Institute of Macromolecular Chemistry, Grigore Ghica Voda Alley 41 A, Iasi

700487, Romania

CORRESPONDING AUTHOR FOOTNOTE. Ecaterina Stela Dragan; Telephone number: +40.232217454; Fax number: +40.232211299, e-mail address: [email protected];

KEYWORDS. Daisogel silica; Chicago Sky Blue 6B; Methyl Orange; Silica/Anion exchanger composites; Reusability; Selectivity.

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ABSTRACT. There is a paramount need in finding sorbents endowed with selectivity in sorption of certain dyes from their mixture with other dyes from the same family. In this context, novel composite anion exchangers (CANEXs) were fabricated here by an innovative approach consisting of using silica Daisogel as host for an anion exchanger bearing vinylbenzyl N,N-diethyl 2-hydroxyethyl ammonium moieties. Information about the outer surface versus in-pore generation of ANEX as a function of silica morphology was acquired by scanning electron microscopy. It was demonstrated that the CANEX microspheres were able to selectively capture Methyl Orange (MO), in binary mixtures with either methylene blue (MB), as cationic dye, or even with Chicago Sky Blue 6B (CSB) as competing azo dye. The adsorption kinetics of MO and CSB were well fitted by pseudo-second order model indicating that chemisorption controlled the sorption process. Isotherms of “H” type characterized the sorption of MO, while “L” type isotherms described the sorption of CSB. Langmuir and Sips isotherms were the most suitable models to describe the sorption process at equilibrium. Even if only about 10 wt.% of the CANEX sorbents was involved in the sorption process, the maximum sorption capacity was 180.25 mg MO/g composite, and 153.86 mg CSB/g sorbent. Moreover, the CANEX sorbents exhibited a spectacular preference for MO molecules in competition with CSB, at pH 5.5. Selectivity coefficient for MO in the mixture with either MB or CSB was 370, and 38.4, respectively. Removal efficiency of MO remained up to 100% after 10 consecutive sorption/desorption cycles.

INTRODUCTION Among azo dyestuffs, the water soluble dyes are the most widely used in industries, such as textile, leather, cosmetics, pulp and paper, and even in food production. They have a high capacity of coloration and therefore the amount of dye necessary to get a certain color is small. However, the presence of dyes in the wastewaters, even in low concentrations, is


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forbidden because the photosynthetic activity in aquatic systems is strongly affected by the presence of dyes associated with the reduced light penetration.1 Furthermore, the azo dyes of benzidine type (4,4'-diarylazobiphenyl dyes) are recognized as carcinogenic and therefore their use is restricted.2,3 Biological treatment, coagulation/flocculation,4 chemical oxidation,5 complex formation,6 chemical precipitation,7 and adsorption have been employed to remove dyes from the wastewaters. Among these strategies, sorption is recognized as one of the most feasible approach owing to the large variety of sorbents, which allow to choose the sorbent adequate for a certain category of dyes, cationic8-12 or anionic,1,13-20 ease of operation, and minimal generation of by-products. Methyl Orange (MO) is one of the most utilized azo dye for laboratory studies and industrial applications, its efficient removal being a very stringent issue in the wastewater processing.14-20 Numerous investigations have been focused on the sorption performances of the various sorbents,15,16,18-20 and only scarcely on the selective separation of MO from its mixture with other dyes, of a high interest being the dyes from the same category.14,17 In this context, the scientific interest is lately focused on designing novel organic/inorganic composite sorbents endowed with fast adsorption/desorption rate, selectivity, and preferable reusability.9,10,12,15,18 Among the multitude of known composites, those consisting of an organic part and an inorganic oxide have been widely developed last decades.15,18,21-29 At a first glance, the most often used composites could be considered polymer/silica, polymer/clays, and polymer/sand. Preparation of polymer/silica composites usually occurs by mixing monomers and silica precursors followed by the simultaneous polymerization via non-interfering routes,27,30,31 or by preparing nanocomposite derived from polysaccharides and silica precursors.32,33 Preparation of polymer/silica composites has also been performed by coating silica with thin layers of various polymers, using different strategies, such as: sorption of thin layers of preformed polymers,34-36 layer-by-layer (LbL) deposition of water soluble polymers,37-40



plasma polymerization of organic monomers on silica surface to get hydrophobic silica surface for oil removal,41 plasma polymerization of thiophene to get composite sorbents for heavy metal removal,42 or by generation of anion exchanger layers on a silica core.43,44 Similar ways have been successfully used in the preparation of organic polymer/sand45,46 and organic polymer/clays composites.29,47 The main advantage of composite anion exchangers consisting of inorganic component as a core coated with organic polymers, which possess ion exchange properties, is a fast uptake of the ionic solutes and mass transfer properties more favorable for rapid separations compared with the uncoated material.33,43,46,47 However, the sorption capacity for ionic species is low in many cases because the most part of the composite is occupied by the inorganic component, which has modest uptake capacity for the ionic solutes. To overcome these drawbacks, a novel recyclable composite sorbent is developed in this work by an innovative approach consisting of the synthesis of a strong base anion exchanger (ANEX) into the pores of Daisogel silica microspheres (Daisogel/ANEX) for efficient and selective removal of certain azo dyes. Macroporous Daisogel silica phases are known for various applications including medicine and separations of pharmaceuticals, food ingredients, biomolecules, and functional dyes.48,49 Thus, to prepare stationary phases for hydrophobic interaction chromatography, Danilevicius et al. have uniformly hydrophobized the surface of Daisogel microspheres by coating with cellulose derivatives followed by cross-linking.48 Another strategy used to tailor the properties of Daisogel microspheres consists of the construction of LbL thin films on the silica surface.36,38 The novel approach presented herein for the synthesis of the composite sorbents (CANEXs type) is consisting of: (i) impregnation of Daisogel silica with poly(vinylbenzyl chloride) (PVBC); (ii) cross-linking of PVBC with N,N,N’,N’-tetramethyl-1,3-propanediamine (TMPDA) at a ratio of 0.2 moles TMPDA:1 mole –CH2Cl; (iii) reaction of nonreacted -


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CH2Cl with N,N-diethyl 2-hydroxyethylamine (DEHEA) in excess to generate ammonium salt groups. The Daisogel/ANEX composites were characterized by thermogravimetric analysis (TGA), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDX), and equilibrium uptake of water. The performances of our newly developed composites as sorbents of azo dyes were investigated by the sorption of two highly toxic anionic azo dyes, i.e. MO and one benzidine type dye, Chikago Sky Blue 6B (CSB), in batch mode. The sorbent selectivity for MO in a mixture with either a cationic dye (methylene blue, MB) or CSB was also explored. The sorption mechanism of the anionic dyes onto CANEXs was discussed in dependence on the dye structure based on the sorption kinetics. Finally, the adsorption performances of the novel CANEXs were analyzed in comparison with the adsorption capacity of other sorbents recently reported in literature.

MATERIALS AND METHODS Materials. Two types of Daisogel silica (SP300 and SP1000), different by morphological characteristics (specific surface area, pore volume and average pore radius), were purchased from the Daiso Co. (Nishi-Ku, Osaka, Japan). Dioxan (DOX), THF and TMPDA, purchased from Sigma-Aldrich, were used as received. The tertiary amine DEHEA, from Fluka Chemical Co. (Buchs, Switzerland), was used after distillation under reduced pressure. 2,2'Azo-bis(isobutyronitrile) (AIBN), from Sigma-Aldrich, was purified by recrystallization three times from methanol. VBC, from Sigma-Aldrich, was used after distillation under reduced pressure (about 4 mm Hg), being kept at +4 oC. MO (dye content 85%), and CSB (dye content 80%), were purchased from Sigma-Aldrich. To avoid the uncontrolled interference of other ions with the anionic species under investigation, during the sorption experiments, the neutral salts were removed from azo dyes by three times recrystallization, as 5 Environment ACS Paragon Plus


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follows: MO from 8 wt.% solution in a methanol:water (1:1, v/v) mixture, and CSB from methanol:water, 70:30 (v/v). The chemical formula and the maximum wavelengths of the dyes are shown in Table 1. Table 1. Chemical Formula and the Maximum Wavelengths of the Azo Dyes. Azo Dye

Methyl Orange

Chikago Sky Blue 6B (Direct Blue 1) SO3Na

Structural formula N

NH2

OH

NaO3S

N

N N

H3C

H3CO

N

2

CH3

λmax, nm

462

SO3Na

621

Methods and Apparatus PVBC was prepared by free radical polymerization of VBC with AIBN as initiator, at a concentration of 1.12 g/100 g CMS. Thus, 0.244 g AIBN were dissolved in 20 mL VBC, and then 60 mL DOX were added. Polymerization was conducted at 70 oC for 10 h. PVBC was precipitated in methanol twice, and then dried in air over night, and in the oven at 40 oC for two days. The molecular weight of PVBC was evaluated by Gel Permeation Chromatography relative to polystyrene standards (580-467000 Da), instrument WGE SEC-3010 multidetection system, in CHCl3, with two PL gel columns (PLgel 5 µm Mixed C Agilent and PLgel 5 µm Mixed D Agilent) dual detector RI/VI (Refractometer/Viscometer), flow rate of 1 mL/min, at 30 oC. The device was provided with a UV detector WGE SEC-3010 and Bi-MwA Brookhaven multi-angle SLS detector. The PARSEC Chromatography software was used to analyse the data. The samples used in this work had: Mw = 31900 g/mol and Mw/Mn = 1.74.


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Fabrication of CANEXs For the synthesis of CANEX based on Daisogel SP300 (CANEX300), 1 g PVBC dissolved in 10 mL THF was added to 5 g silica microspheres, under stirring and allowed the PVBC solution to enter in silica pores for 2 h, at room temperature. To remove the excess of PVBC from the silica surface, 8 mL of THF were added under manually fast stirring, for 2 min, and then the supernatant containing the excess of PVBC was removed and the polymer was recovered by precipitation in methanol p.a., filtration and let in air over night to evaporate methanol. At this end, the polymer was further dried 24 h in the oven at 40 oC, and weighed. The amount of PVBC adsorbed into silica was 0.643 g. Silica containing PVBC was let over night in air to remove THF. The next step of the synthesis was consisting in the crosslinking of PVBC with TMPDA in a mole ratio of 0.2 moles TMPDA/1 mole –CH2Cl, in this case the volume of the cross-linker was 0.143 mL in 10 mL THF added under fast stirring, and allowed to react at room temperature, 18 h, in a well closed bottle. Then, the THF was completely removed from the sample by evaporation (about 2 h). In the 3rd step, the fraction of -CH2Cl groups remained available after cross-linking was reacted with an excess of DEHEA (1.5 mL), in DMSO, at 60 oC, 15 h, the reaction being conducted in a thermostated shaker. To remove the solvent and the amine in excess, the composite silica/ANEX microspheres were washed several times with methanol and water, as follows: first with methanol p.a. (1:1), under fast stirring, for 10 min; four times with distilled water (2:1), each 1 h; two times with methanol p.a., each 1 h, and finally the composite was recovered and let over night in the air to loose methanol. The composite anion exchanger was then dried under vacuo, in the oven, four days at 40 oC. For the synthesis of CANEX based on SP1000 (CANEX1000), the reaction steps were similar with those presented above for the synthesis of CANEX300, with the difference that the amount of PVBC immobilized in 5 g of silica was 0.67 g, and the volume of the cross-linker (TMPDA) was 0.148 mL.



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Structural and Morphological Characterization The specific surface area (Ssp) of the composites along with pore volume were measured by N2 adsorption-desorption performed at 77 K by an Autosorb-1-MP surface area analyzer (NOVA, Quantachrome Company, USA). The isotherms of adsorption/desorption were registered at the relative pressure p/po in the domain 0.01-1.0. The Ssp was evaluated from the linear portion of the adsorption isotherms by Brunauer-Emett-Teller (BET) method. The adsorbed volume was determined at saturation, respectively at values of the relative pressure p/po nearest to unity (0.95). The organic material immobilized by silica composites was quantified by TGA, under air at a heating rate of 10 °C/min with a device NETZSCH STA 449F1, with 5-8 mg composite. The structure of the composite anion exchangers was investigated by FTIR spectroscopy, with a Bruker Vertex FTIR spectrometer, resolution of 2 cm-1, by KBr pellet technique, with 5 mg composite. The samples were scanned in the range of 4000 - 400 cm-1. SEM was used to observe the surface morphology of the silica microspheres, before and after the generation of the ion exchanger into the pores of Daisogel, using an Environmental Scanning Electron Microscope (ESEM) type Quanta 200,8,11 coupled with EDX (SEM-EDX) for determination of the elemental composition. The evaluation of the water uptake (WU) of the Daisogel/ANEX composites was performed as previously described.28 The WU (g/g) was defined by Eq. (1): WU = (Wt – Wd)/Wd

(1)

where: Wd is the weight (g) of the dried microspheres, and Wt is the weight (g) of the hydrated composite at time t. Sorption of Azo Dyes Sorption experiments of anionic dyes from aqueous solution were conducted using a batch equilibrium procedure carried out on a water bath temperature controlled shaker (GFL 1083,


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Gemini BV). The influence of the initial pH on the sorption of MO and CSB, at equilibrium, on the CANEX300 was investigated in the range of pH of 1.5 – 11, using dye solutions with the same initial concentration (about 70 mg/L), adjusting the initial pH with 1 M HCl or 1 M NaOH. Ten mL dye solution of desired pH were added to about 0.01 g of sorbent and let to equilibrate 24 h at 25 oC. The concentration of the dyes in supernatant was measured after 24 h, by UV-Vis spectroscopy with a SPECORD 200 Analytic Jena apparatus, as previously shown.17 The sorption capacity, qe, and the removal efficiency (RE), as a function of the sorbent dose, were evaluated, at 25 oC, as presented below. Increasing amounts of composite microspheres were weighed into a flask and then 10 mL of the aqueous solution of the dye were added, at a shaking rate of about 180 rpm. The amount of solute captured by the CANEX samples, at equilibrium, qe (mg dye/g composite), was calculated with Eq. 2:

qe =

(C0 − Ce )V W

(2)

where: Co and Ce represent the dye concentration (mg/L) before and after the interaction with the composite microspheres, respectively; V - the volume of aqueous phase (L); W - the mass of composite sorbent (g). Removal efficiency (RE, %), was evaluated using Eq. (3):

RE =

C0 − Ce ×100 C0

(3)

where: Co and Ce have the same meaning as in Eq. 2. The sorption kinetics were investigated by adding 10 mL aqueous solution of dyes with a concentration of about 70 mg/L in a flask containing about 0.01 g of dried composite ANEX, and allowed in contact at 25 oC, and 180 rpm for contact times increasing from 5 min to 24 h. The experimental isotherms for the sorption of azo dyes onto CANEX microspheres were determined at 25 oC, the dye concentration increasing from 10 to 2000 mg/L, the initial pH of



the dye solution being 5.5. Desorption of the dye was performed with a solution of 0.5 M KCl in H2O:C2H5OH (1:1).50 To test the reusability of the CANEX in the sorption of MO, sorption/desorption was repeated up to 10 cycles, both with CANEX300 and CANEX1000. To demonstrate the selectivity for MO of the sorbent CANEX300, two mixtures of MO with either MB as cationic dye or with CSB as competitor anionic dye, in a ratio of 2:1 were prepared, the total concentration of dyes being around 138 mg/L. The sorption experiments were performed in batch mode. The UV-Vis spectra of the dyes and their concentrations were determined as a function of contact time, at 25 oC.

RESULTS AND DISCUSSION Synthesis of CANEX with Organic Part Hosted by Daisogel Microspheres Figure 1 presents the main steps of the CANEX synthesis. As Figure 1 shows, the first step in the synthesis of these composite anion exchangers was consisting of the partial filling the pores of Daisogel with PVBC. Subsequently, PVBC was cross-linked with TMPDA, two quaternary ammonium salt groups being generated for each molecule of cross-linker.


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Figure 1. Schematic presentation of CANEX fabrication. In the 3rd step, all the -CH2Cl groups remained free after the cross-linking step were reacted with DEHEA thus obtaining CANEX300 and CANEX1000, when silica was SP300 and SP1000, respectively.

Characterization of CANEX Sorbents In order to demonstrate the formation of the CANEX with the ANEX hosted in the pores of Daisogel microspheres, the TGA measurements were performed up to 800 oC. The organic content in the composite microspheres was evaluated as the difference between the percentage of solid remained at 800 oC and 100%.28 The TG and DTG curves for the CANEX based on either SP300 or SP1000 are presented in Figures 2A, and 2B, respectively. As can be seen, the composite derived from Daisogel SP300 present five stages of degradation. The 1st stage of TG curve in Figure 2A corresponds to a mass change of 4.03%, being located at a



maximum of weight loss at 53.36 oC. This mass change was attributed to the loss of residual water.

100 weight loss, %

A

1 Deriv. weight, % min

Weight loss, %

95

0

90 85 deriv. weight, % min

C

-1

80 75 70

0

-2 100 200 300 400 500 600 700 Temperature, oC

100

1 weight loss, %

95 90

B 0

85 80

deriv. weight, % min

-1

75 70

Deriv. weight, % min

Weight loss, %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-2

0

100 200 300 400 500 600 700 Temperature, oC

Figure 2. Thermograms of CANEX300 (A) and CANEX1000 (B), and FTIR spectra of the pristine silica and of the composite sorbents (C). The 2nd stage, with a maximum located at 172.31 oC and a weight loss of 2.48%, was assigned to the dealkylation of the quaternary ammonium salt groups; the 3rd stage, with a maximum at 388.71 oC and a mass change of 2.64%, the 4th stage with a maximum situated at 517.45 oC and a mass change of 1.61%, and the 5th stage with a maximum located at 653.74 o

C and a weight loss of 1.32%, support the complex thermal degradation of the ANEX hidden

in the pores of silica. As Figure 2A shows, the residual mass of CANEX300 was 88.08%. As the thermogram of the CANEX1000 in Figure 2B shows, the 1st stage with a maximum of weight loss of 3.36% was located at 41.8 oC. The 2nd stage with a maximum of weight loss of


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5.26% was located at 209.2 oC, and the 3rd stage with a weight loss of 5.64% was located at 409.0 oC, the residual mass being 85.6%. FTIR spectra of the CANEX sorbents and those of the pristine silica (SP300 and SP1000, before reaction) are presented in Figure 2C. The presence of the anion exchanger in CANEX300 is confirmed by the peaks at 2927 cm-1, 2856 cm-1, and the shoulder at 1450 cm1

, attributed to the stretching vibrations of -CH2 groups. The characteristic peaks of silica are

located at: 3446 cm-1, a broad band at 1200–1000 cm-1, and 803 cm-1 attributed to the stretching vibrations of Si-OH bonds, asymmetric stretching vibrations of Si-O-Si bonds, and bending vibrations of Si-O bonds, respectively.28 In the FTIR spectrum of CANEX1000 can be seen peaks at 2926 cm-1, 2854 cm-1, and 1451 cm-1 assigned to the stretching vibrations of -CH2 groups. The characteristic peaks of silica are present at 1132 cm-1 and 804 cm-1. The peaks at 709 and 712 cm-1 in the CANEX300 and CANEX1000, respectively, were assigned to the presence of anion exchanger (stretching vibrations of aromatic ring). The surface morphology of silica microspheres, before and after the generation of ANEX inside of pores (image A and the inset for the pristine silica SP300, and image B and the inset for CANEX300; image C for the pristine silica SP1000, and image D and the inset for CANEX1000) is supported by the SEM images in Figure 3. Image B and the inset show that almost no surface changes can be seen in the case of the synthesis of CANEX300 compared with the pristine silica (image A and the inset), while in the case of silica SP1000, part of the organic polymer is deposited on the silica surface (image D and the inset). The main difference between these two composites, at the end of synthesis, was the smooth and clean microspheres in the case of CANEX300, and the presence of some agglomerations of microspheres in the case of CANEX1000.



A

C

B

D

Figure 3. SEM images of bare Daisogel microspheres (A and C) and of the CANEX300 (B) and CANEX1000 (D). The interior of the composites could be seen by broken the CANEX microspheres (Supporting Information, Figure S1). The organic part (ANEX) is visible in both images (CANEX300 on the left and CANEX1000 on the right) at a magnification of x5000. Table S1 presents the elemental analysis by SEM-EDX of the surface of Daisogel microspheres before and after the synthesis of the silica/ANEX composite. The SEM-EDX data in Table S1 confirm the presence of the elements brought by the organic part of the composite (C, N, Cl), i.e. the anion exchanger, more C and N being found in the case of the CANEX1000 than in the case of CANEX300. The difference is attributed to the presence of more ANEX on the surface of microspheres than inside of pores in the case of CANEX1000, while all the organic


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part was confined inside the pores in the case of CANEX300. The higher content in organic part of the CANEX1000 also supports the TGA results (Figure 2). Table 2 shows that the specific surface area decreased after the synthesis of the anion exchanger inside the silica microspheres, in a higher level for the bare silica having a lower initial surface area (SP1000 compared with SP300). The values of WU given in Table 2 for the CANEX sorbents compared to the bare silica indicate the changes in the hydrophile/hydrophobe balance after the silica modification. Table 2. Specific Surface Area (Ssp), Average Pore Radius (rp), and Water Uptake (WU) of the Daisogel/ANEX Composites Compared with Pristine Samples of Silica. Sample code

Ssp, m2/g

rp, nm

WU, g/g

SP300

95

26.7

1.98

SP1000

24

122.2

1.88

CANEX300

43.36

-

3.867

CANEX1000

6.25

-

4.7195

As Table 2 shows, the values of Ssp decreased after the construction of ANEX inside the pores of silica, more for CANEX1000 than for CANEX300. The values of WU augmented compared with bare silica, increasing more for the composite derived from SP1000 than for that derived from SP300, because the amount of ANEX was higher in the first case, as demonstrated by the TGA results (Figure 2). That is because the formation of ANEX was not only inside of silica but also on the surface (see Figure 3). Sorption Characteristics of Anionic Dyes in Aqueous Solution Efficiency in the removal of dyes is a function of some essential factors, such as solution pH, sorbent dose, contact time, dye concentration, and temperature, whose influence will be



inspected here. Figure 4 presents the influence of pH (A) and of the sorbent dose (B and C) on the sorption of MO and CSB onto CANEX300. As Figure 4A shows, the effect of pH on the sorption of the dyes at equilibrium, was strongly dependent on the dye structure.

50

CANEX300+MO CANEX300+CSB

40

qe, CANEX300 qe, CANEX1000

140 120 100

30 20

RE, CANEX300 RE, CANEX1000

80 60

10

A

40

0 1

2

3

4

5

6

7

8 pH

9 10 11 12

B

20 0.000 0.004 0.008 0.012 0.016 0.020 sorbent dose, g

110 100 90 80 70 60 50 40 30 20 10 0

30

100 90 80 70 20 qt, CANEX1000 60 qt, CANEX300 50 15 40 10 30 20 5 RE, CANEX1000 10 RE, CANEX300 0 0 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 sorbent dose, g

25

RE, %

160

qe, mg/g

180

60

RE, %

70

qe, mg/g

qe, mg/g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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C

Figure 4. Effect of pH (A), and sorbent dose (B and C) on the sorption capacity (qe) and removal efficiency (RE) of CANEX sorbents for the dyes MO and CSB: initial dye concentration about 71 mg/L for both dyes; sorbent dose for the influence of pH 1 g/L; pH 5.5 for the influence of sorbent dose; temperature 25 oC; contact time 24 h; shaking rate 180 rpm. While the sorption capacity of MO increased when pH of the dye solution increased from 2.5 to 4.5 and remained almost constant up to pH 11, the sorption capacity of CSB abruptly decreased from a maximum located at pH 1.5 down to a minimum situated at pH 5.5 -6.0, and remained almost constant up to pH 11. The maximum sorption capacity of MO onto CANEX300 in the range of pH 4.5-11.0 could be attributed to the ion exchange between the counterion of the quaternary ammonium groups and SO3Na groups of the dye.51-54 In the acidic pH (pH < 3.1, the structure of MO changed to quinonoidic form, which diminishes the electrostatic interaction between the dye with positive groups of the anion exchanger. In the case of CSB, in the acidic range, the quaternary ammonium groups of the ion exchanger have Cl- as counterion, while the four SO3Na groups of the dye are shifted to SO3H by protonation. As a result, enhanced affinity of the sorbent for this dye at lower pH was observed, because


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beside the electrostatic interaction between the dye molecules and CANEX positive groups, a multitude of hydrogen bonds could be generated. The same behaviour has been observed for the sorption of CSB onto magnetized calixarene,24 and β-cyclodextrin-based polymers.55 The strong difference between the sorption of MO and CSB, around the neutral pH creates an excellent opportunity for the selective sorption of MO in the presence of CSB. The influence of pH on the sorption capacity of CANEX1000, for both dyes, was similar with that found for CANEX300. The effect of sorbent dose on the amount of dye sorbed at equilibrium and on the RE for the sorption of MO and CSB onto the CANEX300 and CANEX1000 is presented in Figures 4B and 4C, respectively. Figure 4B shows that only 8 mg of sorbent were enough to completely remove the MO molecules from 10 mL of the dye solution with a concentration of 71 mg/L. This spectacular efficiency in the uptake of MO occurs due to the high affinity of the CANEX sorbents for this dye. Figure 4C shows that in the case of CSB, almost all the dye molecules were removed when the dose of CANEX1000 was about 6 g/L of CSB solution, and higher than 7 g/L in the case of CANEX300. For the economical reasons, the sorbent dose used in the next soption experiments was of 1 g/L dye solution, for both dyes.

Sorption Kinetics To get insights into the sorption mechanism of MO and CSB onto the sorbents of CANEX type, the sorption kinetics were inspected (Figure 5). Three kinetic models were applied on the experimental data to evaluate the influence of the dye structure on the sorption mechanism, the initial solution pH being 5.5: pseudo-first-order (Lagergren) (PFO) kinetic model, pseudo-second-order (PSO) kinetic model, and Elovich model. The PFO kinetic model considers the adsorption rate proportional with the difference between the equilibrium adsorption capacity and the adsorbed amount.56 The PSO kinetic model is based on the



assumption that the rate-limiting step is chemisorption, involving valence forces through sharing or exchange electrons between sorbent and sorbate.57 The Elovich kinetic model is one of the most useful models used to confirm chemisorption as the sorption mechanism. The value of 1/β indicates the number of sites accessible for the dye adsorption.

40

80

A

70

qt, mg/g

50

CANEX300 + MO CANEX1000 + MO PFO PSO Elovich

40 30

CANEX300 + CSB CANEX1000 + CSB PFO PSO Elovich

35

60 qt, mg/g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 41

30 25

B

20 15

20

10

10

5 0

0 0

50

100

150 200 Time, min

250

0

300

200 400 600 800 1000120014001600 Time, min

Figure 5. Sorption kinetics data of MO (A) and CSB (B) onto CANEX300 and CANEX1000 fitted by PFO, PSO and Elovich kinetic models: initial concentration of the dye 72 mg/L; temperature 25 oC; sorbent dose 1 g/L; shaking rate 180 rpm. The equations of the three kinetic models and the kinetic parameters are presented in Table 3. The values of the regression coefficient of determination, R2, and of the Chi-square, χ2, which are the highest and the lowest, respectively, for the PSO kinetic model, indicate that this model provides an adequate fitting of the kinetic data, for both dyes. Table 3. Kinetic Parameters for the Sorption of Azo Dyes onto CANEX Sorbents, at 25 oC. Kinetic parameters

qe exp. (mg/g)

MO

CSB

CANEX300

CANEX1000

CANEX300

CANEX1000

72

72.28

21

25

PFO kinetic model: qt = qe (1- e-k1t) qe calc. (mg/g)

71.57

71.36

20.09

23.86

k1 (min-1)

0.04628

0.0403

0.0029

0.00381


Page 19 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


R2

0.9792

0.9862

0.9412

0.9508

χ2

11.94

8.2

2.5

3.06

2

k 2 qe t PSO kinetic model: q t = 1 + k 2 qet qe calc. (mg/g)

79.56

80.56

24.44

28.16

k2 (g/mg min)

8.0767x10-4

6.56 x 10-4

1.294x10-4

1.5828x10-4

R2

0.9902

0.9909

0.9647

0.9747

χ2

5.62

5.4

1.5

1.57

Elovich kinetic model: = ln ( ∙ ∙ )

α (mg/g.min)

18.96

12.75

0.2386

0.3653

β (g/mg)

0.0742

0.06817

0.2231

0.1885

R2

0.9766

0.9747

0.9584

0.9778

χ2

13.44

14.97

1.8

1.38

Thus, the sorption rate seems to be dependent on the availability of sorption sites and, therefore, the rate limiting step would be chemisorption.1,13,19,33 Furthermore, the high values of the correlation coefficients and the low values of χ2 in Table 3 support the suitability of the Elovich kinetic model. The values of the kinetic constant, k2, which are 6.24 and 4.14 times higher for the sorption of MO onto CANEX300 and CANEX1000, respectively, compared with CSB, indicate the potential in selective sorption of MO from its mixtures with CSB. The higher ratio between the rate constants in the case of CANEX300 compared with CANEX1000 shows that selectivity augments if the ion exchanger is completely hidden inside the silica particles. To get information about the diffusion during the sorption process of MO and CSB onto the porous sorbents of CANEX type, Weber and Morris plot was used to explore the contribution of intraparticle diffusion (IPD) on the dye sorption (Figure S2, Table S2). If the plot of qt



versus t0.5 exhibits multi-linear plots, then two or more steps influence the sorption process.33,58-60 Because the plots of qt vs t0.5 are three- or two-linear, the IPD is not the only rate limiting step controlling the whole process. The first region is connected with the external resistance to mass transfer from dye solution to the external surface of the CANEX, while the second region would be dominated by the IPD of the dye molecules through the pores of the sorbent. The values of kid, Ci and R2 for the first and the second region in Figure S2 are presented in Table S2. According to the R2 values for the first region, the IPD kinetic model would be suitable to describe the sorption of both dyes. However, there is a clear difference between the constants of IPD, and the values of the term Ci, which are much higher for the sorption of MO than for the sorption of CSB, on both sorbents. This would indicate that the contribution of the boundary layer is stronger for the sorption of MO molecules than for the CSB dye.

Sorption Isotherms The presence of strong base ANEX inside the pores of Daisogel recommends these organic/inorganic hybrids for the sorption of anionic species like anionic dyes. The equilibrium data obtained for the sorption of MO and CSB onto the CANEX300 and CANEX1000 are presented in Figure 6A and 6B, respectively. The shape of sorption isotherms of MO on both sorbents (Figure 6A) indicates isotherms of “H” type, where the initial slope is very high suggesting a very high affinity of both sorbents for MO molecules, while the sorption isotherms of CSB onto both sorbents shows isotherms of “L” type (Figure 6B), where the ratio between the concentration of the dye remained in solution and that sorbed onto the CANEX is a concave curve.


Page 20 of 41

Page 21 of 41

A

200

B

160 140

qe, mg/g

150

120

qe, mg/g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100

CANEX300 + MO CANEX1000 + MO Langmuir Freundlich Sips D-R

100

50

80

CANEX300+CSB CANEX1000+CSB Langmuir Freundlich Sips D-R

60 40 20

0 0

400

800

1200 1600 Ce, mg/L

2000

0

0

400

800

1200 1600 2000 2400 Ce, mg/L

Figure 6. Equilibrium sorption isotherms for the sorption of MO (A) and CSB (B) onto CANEX300 and CANEX1000; sorption conditions: pH 5.5; temperature 25 oC; sorbent dose 1 g/L; shaking rate 180 rpm. The striking difference between the shape of sorption isotherms of MO and CSB supports the possibility to selectively adsorb MO in the mixture with CSB. Sorption of the anionic dye C.I. Basic Blue 3 onto the cation exchanger Lewatit MonoPlus SP 112,1 and of MB onto the semi-IPN composite cryogels having anionically modified potato starch entrapped in a PAAm matrix8 are two examples of recently reported “H type” isotherms. Fitting different isotherm models onto the experimental equilibrium data of sorption was performed to establish the most appropriate model to design the sorption of anionic dyes onto the composites under study. Therefore, the adsorption data were fitted, by the non-linear regression method, to four isotherm models: Langmuir, Freundlich, Sips, and DubininRadushkevich presented in Table S3. The goodness-of fit for the isotherm models was estimated by the coefficient of determination (R2) and non-linear Chi-square test (χ2) (Supporting Information, eq. 1). The values of the model isotherm parameters, R2 and χ2 are presented in Table S4. The values of R2, located in the range 0.9585 - 0.9713, and the low values of χ2 show that the Langmuir isotherm is suitable for modeling the sorption process of MO and CSB onto the CANEX type sorbents. The value of qm for the sorption of MO on the



Page 22 of 41

CANEX300 is lower than for the sorption on CANEX1000, probably because the content in organic part is higher in the case of CANEX1000 than for CANEX300, all positively charged sites being able to interact with the sulfonic groups of MO. The lower values of qm found for the sorption of CSB molecules onto CANEX1000 compared with CANEX300 could be attributed to the large contribution of the non-electrostatic interactions between the CSB molecules and all functional groups of the sorbents, the specific surface area of the CANEX having a high contribution in this case. As can be seen in Table 2, the Ssp of CANEX300 is much larger than that of CANEX1000 (43.36 m2/g compared with 6.25 m2/g). The adsorption feasibility was evaluated as a function of the constant separation factor or equilibrium parameter, RL, defined by Hall et al. by Eq. 4:61

=

(4)

where: KL is the Langmuir adsorption constant, L/mg, and Ci is the initial concentration of the dye, mg/L. The RL values in Table S4 are greater than zero and lower than unity, for all systems, indicating the sorption of both dyes onto the CANEX sorbents was favorable and reversible. Freundlich isotherm assumes the binding sites are not equivalent and is used to model the adsorption process on heterogeneous surfaces.60 The values of R2 and χ2, which are lower and respectively much higher compared to Langmuir isotherm, indicate that the sorption of MO molecules onto CANEX300 and CANEX1000 is not well described by the Freundlich isotherm. Contrary, the sorption of CSB onto both sorbents seems to be well described by the Freundlich isotherm, the values of R2 and χ2, being higher and respectively lower compared to those found for the Langmuir isotherm. The values of 1/n, all lower than unity, support the feasibility of the dye sorption on both sorbents. The values of qm calculated by fitting the Sips isotherm on the equilibrium data are higher than those given by the Langmuir isotherm, and


Page 23 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the values of R2 and χ2 are comparable with those found by fitting the Langmuir isotherm, indicating the Sips isotherm is also suitable to model the sorption process of both dyes onto the sorbents of CANEX type. The lowest values of R2 and the highest values of χ2 found by fitting the D-R isotherm on the experimental data indicate that this isotherm does not describe well the sorption of the two azo dyes onto the CANEX sorbents. However, the D-R constant, β, was used to calculate the mean free energy of adsorption, E (kJ/mol), defined as the free energy when one mole of ion is transferred to the surface from infinity in solution, and computed by Eq. 5: E = 1/(2β)1/2

(5)

The values of E are usually used to discriminate among different types of adsorption, as follows: physical sorption, when E < 8 kJ/mol, and ion exchange when 8 < E < 16 kJ/mol. Chemisorption is the most probable mechanism when E > 40 kJ/mol.62 As Table S4 shows, the values of E found for the sorption of MO onto CANEX300 and CANEX1000 were 38.1 and 113.85 kJ/mol, respectively. This recommends the chemisorption as more probable mechanism of sorption, in agreement with the sorption mechanism indicated by the kinetic study. In the case of the sorption of CSB on both sorbents, the adsorption process proceeds mainly by physical sorption and ion exchange. Table 4 presents an overview on the sorption of MO and CSB onto recently reported sorbents and the sorption capacity of the composite sorbents synthesized in this study. Table 4. Overview of Azo Dye (MO and CSB) Uptake with Recently Reported Adsorbents. Initial

qma, mg/g

pH

Sorbent dose, g/L

25

-

40

15.38

35

7

8

30.33± 6

T, oC

Sorbent [ref] Methyl Orange Cellulose hydrogel nanoparticles [15]

assisted

Chitosan/Alumina composite [18]

by

Fe2O3



Activated carbon nanotubes (CNTs-A) [52]

Page 24 of 41

25

7

0.75

149

Chitosan based semi-IPN composite hydrogel 25 [53]

2

0.6

180.24

Core-Shell structured Graphene oxide-chitosan 45 beads [54]

7

0.5

353

CANEX300 [This study]a

25

5.5

1

160.2 (1982)

CANEX1000 [This study]a

25

5.5

1

180.2 (1653)

CANEX300 [This study]

25

5.5

1

153.86

CANEX1000 [This study]

25

5.5

1

139.77

Chicago Sky Blue

a

The values given in parentheses represent the theoretical sorption capacity if the dye

adsorbed at equilibrium is reported only to the ANEX hosted in silica microspheres, as determined by TGA, i.e. 8.08% for CANEX300 and 10.9% for CANEX1000.

As can be seen in Table 4, the novel composite anion exchangers are endowed with a sorption capacity comparable with that of the best composite sorbents reported in literature for MO, even if the percentage of the organic part responsible for the dye binding, as determined by the TGA was 8.08% for CANEX300 and 10.9% for CANEX1000. Equilibrium sorption data about CSB were difficult to find in literature, and therefore the values of qm were not discussed in comparison with other sorbents. However, Yilmaz et al. have reported high removal of CSB molecules by the cyclodextrin-immobilized iron oxide magnetic nanoparticles,55 and by cyclodextrin polymers63 with 99% removal at pH 3.0. The adsorption of anionic dyes onto CANEX300 was also qualitatively supported by FTIR spectroscopy (Supporting Information, Figure S3). Spectrum of CANEX300 loaded with MO shows the presence of MO by the peaks located at: 1605.67 cm-1, 1517 cm-1 and 1420.51 cm-1 attributed to p-disubstituted benzene; 1368.44 cm-1 [aryl–N(CH3)2]; 1184 cm-1 and 1118 cm-1, characteristic to SO3- are screened by the stretching vibrations of Si-OH bonds and the asymmetric stretching vibrations of Si-O-Si bonds;28 the new peaks at 696.27 cm-1 and


Page 25 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


622.98 cm-1 were attributed to the stretching vibrations of C-S bonds. After loading of CANEX300 with CSB, the following peaks are visible: 1623 cm-1, 1497.66 cm-1, 1419.55 cm-1, 683.74 cm-1, 662.52 cm-1 attributed to primary OH in plane bending, the aromatic C-C stretch, p-disubstituted benzene, C-S bonds, and O-S bonds, respectively. The possible interactions between MO and CSB and CANEX300 are schematically presented in Figure 7.

Figure 7: Types of interactions between MO and CSB and CANEX sorbents.



Page 26 of 41

As can be seen, the interaction of MO molecules with CANEX functional groups is mainly electrostatic, while the binding of CSB dye is much more complex, the hydrogen bonds having an important contribution.

Effect of Temperature on the Sorption of MO and CSB Sorption temperature could strongly influence the efficiency and selectivity in the sorption of a certain dye. Van’t Hoff equation (Eq. 6) was used to calculate the free energy of adsorption (∆G°), enthalpy (∆H°), and entropy change (∆S°) for the sorption of MO and CSB onto CANEX300 and CANEX1000:

ln =

∆

−

∆

(6)

where Kc is the sorption equilibrium constant (L/g) calculated with Eq. 7.

=

!

(7)

!

where: qe is the amount of dye sorbed at equilibrium, mg/g, and Ce is the concentration of dye at equilibrium, mg/L. The slope and intercept of the plot ln Kc vs. 1/T (Figure 8) were used to evaluate ∆H° and ∆S°, while the values of ∆G° were calculated with Eq. 8:

∆" # = −$ %&

(8)

The values of the thermodynamic parameters for the sorption of MO and CSB onto the two composite sorbents are presented in Table 5.


Page 27 of 41

7 6 5 ln Kc

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4

CANEX300+MO CANEX1000+MO CANEX300+CSB CANEX1000+CSB , Fit linear , Fit linear

3 2 1 0 3.1

3.2

3.3

3.4 3.5 1000/T

3.6

3.7

Figure 8. Plot of ln Kc vs. 1000/T for the sorption of MO and CSB onto CANEX sorbents. Table 5. Thermodynamic Parameters for the Sorption of MO and CSB onto CANEX Sorbents. ∆H°,

∆S°,

J/mol

kJ/mol·K

CANEX300+MO

22.63

CANEX1000+MO

Sorbent/Dye

∆G°, kJ/mol 277 K

293 K

298 K

308 K

318 K

0.119

- 10.2

- 12.37

- 12.94

- 13.8

-

17.84

0.112

-13.55

- 14.97

- 15.46

- 16.64

-

CANEX300+CSB

92.34

0.311

-

-

- 1.041

- 3.893

- 7.143

CANEX1000+CSB

71.89

0.241

-

-

- 0.125

-2.832

- 4.781

As Table 5 shows, the sorption process of both dyes was spontaneous and thermodynamically favorable, the values of ∆G° change being negative, irrespective of temperature. The more negative values of ∆G° with the increase of temperature support the increase of the degree of spontaneity for the sorption of both dyes onto both sorbents. The affinity of both sorbents for MO is reflected in the more negative values of ∆G° in the case of



MO sorption compared with the sorption of CSB, at the same temperature. The positive values of ∆H° show the sorption process was endothermic for both dyes, but the much lower values of ∆H° in the case of MO suggest the sorption is less influenced by temperature than in the case of CSB. In other recently reported studies, the sorption of MO has been either not influenced by temperature,64 or even exothermic.16,65 The positive values of ∆S° indicate the increase of the randomness at the solid-liquid interface and support the interaction between dyes and the active sites on the sorbent surface.

Selectivity for Methyl Orange Selective sorption of a certain dye from a dye mixture represents a great challenge of high interest for practical applications. The most part of the recent information are focused on the separation of cationic dyes from their mixture with anionic dyes or conversely.12,33,59,65,66 Selective separation of the dyes which belong to the same category, such as azo dyes is scarcely found.17 Figure 9A presents the selective sorption of MO from its mixture with MB, the mass ratio between MO and MB being 2:1, at a total concentration of dyes in aqueous solution of 138 mg/L.

1.0

A

MO+MB, 0 min after 30 min after 60 min after 130 min

0.8

B

0.9 0.8 Absorbance, a.u.

0.9 Absorbance, a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 41

0.7 0.6 0.5 0.4 0.3

MO+CSB, 0 min after 30 min after 60 min after 130 min

0.7 0.6 0.5 0.4 0.3

0.2

0.2

0.1

0.1 0.0

0.0 300

400

500

600 λ, nm

700

800

300

400

500

600

700

800

λ, nm

Figure 9. UV-Vis spectra as a function of contact time for the selective sorption of MO from its mixture with MB (A) and CSB (B): sorbent dose 2 g/L; temperature 25 oC; pH 5.5.


Page 29 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


As Figure 9B shows, after 130 min almost all molecules of MO were removed from the solution, a small part of CSB being adsorbed. The quantitative evaluation of the selectivity of CANEX300 for MO molecules, expressed by the selectivity coefficient when MO was in the same solution with MB or CSB, is presented in Table 6. Table 6. Distribution Constants and Selectivity Coefficients for the Sorption of MO from Its Mixture with MB or CSB.

Sorbent

Dyes

qe(MO), qe(MB), qe(CSB), KD(MO) KD(MB) KD(CSB) k mg/g mg/g mg/g

CANEX300

MO+MB

33.630 0.01

-

4.078

0.011

-

370

CANEX300

MO+CSB

37.437 -

4.623

5.616

-

0.146

38.4

' = ! ; ()%)* +,+ - *.)//+*+)& , 1 = !

2(34)

2(35 67 895)

As Table 6 shows, the selectivity coefficients for the sorption of MO from its mixture with MB or CSB were 370, and 38.4, respectively. These results demonstrate the high potential of the CANEX sorbents in separation of MO not only from its mixtures with cationic dyes but also from its mixtures with other azo dyes. Separation of MO from its mixture with Congo red (CR) was also tested. The issue which arises in this case is that the quantitative evaluation of each dye is not accurate because of the narrow distance between the maximum wavelength of each dye (462 nm for MO and 497 nm for CR). However, as Figure S5A shows, after 150 min, only CR was present in the supernatant, and this demonstrate the applicability of CANEX sorbents for the separation of MO from its mixture with CR. The selective sorption of CSB from its mixture with MB, these dyes having the maximum wavelengths very close (621 nm for CSB and 666 nm for MB), was also conducted.



However, as Figure S5B shows (Supporting Information), the initial solution of the two dyes has a maximum wavelength located at 591 nm, i.e., the maximum wavelength of the mixture was lower than the characteristic wavelength of the individual dyes. Therefore, it was not possible to make accurate evaluation of the separation of these two dyes. The shift supports the strong complex formed by the interaction between CSB (having four sulfonic groups) and MB. After 13 hours in contact with the CANEX1000, the maximum wavelength was shifted at 550 nm and the absorbance significantly decreased. This indicated the sorption of CSB molecules in excess, those involved in the complex with MB being found in solution even after 20 hours of sorption. Reusability of CANEX300 and CANEX1000 after the Repetitive Sorption of MO From the economical point of view, the reuse of a sorbent in as many as possible repetitive sorption/desorption cycles is a very important requirement. Figure S5 presents sorption capacity of CANEX300 and CANEX1000 in 10 consecutive sorption/desorption cycles of the dye MO. Judging from the values of the sorption capacity of the two composite sorbents after each cycle, an excellent recycling performance was demonstrated, the sorption capacity being almost unchanged at the end of the 10th cycle. This behavior might be attributed to the completely desorption of MO after each cycle, the sorption mechanism of this dye being mainly based on the electrostatic interactions (ion exchange) between the sulfonic group of the dye and the quaternary ammonium groups (exchange of the Cl- counterion) of the sorbents.

CONCLUSIONS Novel organic/inorganic composite sorbents were prepared by the synthesis of a strong base anion exchanger in the pores of Daisogel silica, i.e. SP300 and SP1000. The evaluation


Page 30 of 41

Page 31 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of structural and textural changes of silica particles after the synthesis of ANEX were followed by FTIR spectroscopy and SEM-EDX measurements, respectively. It was found that the morphology of the bare Daisogel influenced the outer surface vs. in-pore generation of the ANEX. The CANEX sorbents were endowed with a very fast uptake of the MO molecules, the sorption process being characterized by “H” type isotherms, while the sorption of CSB molecules followed “L” type isotherms. The maximum sorption capacity given by the Langmuir isotherm was: 160.2 mg MO and /g CANEX300, 180.25 mg MO/g CANEX1000, 153.86 mg CSB/g CANEX300 and 139.77 mg CSB/g CANEX1000. The most striking performance of the novel composite anion exchangers is consisting of the excellent selectivity in the sorption of MO in its mixture with a cationic dye, in this case MB, and even in its mixture with competing anionic dye CSB. Furthermore, the composite sorbent proposed herein featured a high level of recyclability in the sorption of MO, the sorption capacity being almost unchanged after 10 sorption desorption cycles. To sum up, this work proposed a novel approach for the synthesis of recyclable composite sorbents of CANEX type, with a fast sorption and selective separation of MO molecules in aqueous solutions.

ACKNOWLEDGMENT The results presented in this manuscript have been financed by a grant of the Romanian National Authority for Scientific Research, CNCS – UEFISCDI, project number PN-II-IDPCE-2011-3-0300. The authors thank to Dr. Florica Doroftei for the SEM/EDX measurements.

SUPPORTING INFORMATION Supplementary data associated with this article, present: SEM images of the interior of the CANEX ion exchangers (Figure S1); SEM-EDX analysis-Table S1; Fitting of IPD kinetic



model- Figure S2; Parameters of IPD model-Table S2; Table S3-isotherm models; Table S4Isotherm parameters; Figure S3-FTIR of CANEX300 loaded with MO and CSB; Figure S4Reusability; Figure S5-UV-vis spectra of the mixtures MO-CR and CSB-MB.

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Table of Contents Graphic

S O 3Na N

0.9 0.8 Absorbance, a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.7

N

H3C

NH2

N

0 min 30 min 60 min 130 min

CH3

OH

NaO3S

N

N H3C O 2

S O 3N a

0.6 0.5

NH2

OH

N a O 3S

N

0.4

N H 3C O 2

S O3Na

0.3 0.2 0.1 0.0 300

400

500

600 λ , nm


700

800

Spectacular Selectivity in the Capture of Methyl Orange by Composite

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