Surface properties and chemical constitution as crucial parameters for

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Surface properties and chemical constitution as crucial parameters for the sorption properties of ionosilicas: the case of chromate adsorption Ut-Dong Thach, Benedicte Prelot, Stéphane Pellet-Rostaing, Jerzy Zajac, and Peter Hesemann ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00020 • Publication Date (Web): 27 Apr 2018 Downloaded from http://pubs.acs.org on April 28, 2018

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ACS Applied Nano Materials

Surface properties and chemical constitution as crucial parameters for the sorption properties of ionosilicas: the case of chromate adsorption

Ut Dong THACHa†, Benedicte PRELOT*a, Stéphane PELLET-ROSTAINGb, Jerzy ZAJACa and Peter HESEMANN*a a

Institut Charles Gerhardt, UMR-5253 CNRS-UM-ENSCM, Place Eugène Bataillon, F-34095 Montpellier cedex 5, FRANCE b

Institut de Chimie Séparative, UMR 5257 CNRS-CEA-UM-ENSCM, Site de Marcoule, Bâtiment 426, BP 17171, 30207 Bagnols-sur-Cèze Cedex, France

KEYWORDS: Ionosilica, hexavalent chromium, anion exchange, adsorption capacity, adsorption kinetics, isothermal titration calorimetry.

ACS Paragon Plus Environment

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ABSTRACT

We report ionosilicas with different chemistries, textures and morphologies and their use as adsorbents for chromium(VI). All studied materials are highly efficient anion exchange materials with adsorption capacities between 1.6 and 2.6 mmol/g. The ion exchange capacity of the materials reaches up to 91% of the theoretical value, i.e. the molar amount of ionic groups immobilized within the material, indicating a very high accessibility of the organo-ionic groups. Noticeable differences were found regarding the ion exchange properties in terms of capacity and kinetics according to the used material, in particular its porosity. High specific surface areas favor the adsorption process and result in high adsorption capacity. However, even a non-porous material displays high adsorption capacity of 1.7 mmol/g. This result can be attributed to the high hydrophilicity of ionosilicas that favors diffusion and mass transfer throughout the material. The adsorption kinetics are fast as 80-90% of the adsorption capacity are reached after approximately 10 min. Finally, Isotherm Titration Calorimetry (ITC) evidences the influence of the constitution of the cationic group on the displacement enthalpy, in relationship with the steric hindrance of the alkyl groups that surround the cationic center.


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INTRODUCTION In the field of functional silica based materials, so-called periodic mesoporous organosilicas (PMOs), obtained from bis- or oligosilylated organic precursor molecules, have an outstanding position. (1-3) PMOs represent a particular class of silica material, as the immobilized groups are not simply pending at the materials’ surface but incorporated within the materials’ wall. The organic (functional) groups are therefore the inherent part of the materials’ scaffold and play an important role regarding the physico-chemical properties of these materials. Additionally, PMOs contain a high number of incorporated organic groups (4-5) and find applications in domains where particular and adaptable surface properties are required, in particular adsorption (6). Metal oxyanions forming elements As, Cr, Mo, Se, such as arsenate, chromate, molybdate and selenite, are found in industrial and environmental situations (7). Among these toxic metal species, hexavalent chromium is a common hazardous pollutant coming from many industries including tanning, electroplating paints, dyes (8-9). Therefore, the development of reliable methods for its removal is of particular significance. Removal of the chromate from aqueous medium could be achieved by several processes (10) such as chemical extraction (11-12), reverse osmosis (13-14) electro-kinetic remediation (15-16) bioleaching (17) electrochemical processes (18-19), adsorption (20) or by combining several of these methods (21-22). Adsorption combines high efficiency, low energy demand, low chemical investment and reusability (23). Common adsorbent such as active carbons (24), zeolites (25), clays (26), magnetic particles (21, 27-29) low-cost materials and biosorbents (30-33) were used especially for the recovery of heavy elements. However, these materials usually display low sorption capacity, selectivity and slow kinetics because of their low quantity and limited accessibility of active sites (23, 34-36).


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Silica based materials have intensively been studied as adsorbents in solid-liquid separation processes (6, 37). Surface functionalized materials containing cationic groups have thoroughly been used for anion sorption such as arsenate, chromate (38-41), perruthenate (42), perrhenate (43) or diverse radioelements (44-45). Ammonium and imidazolium functionalized silica materials display improved sorption capacities due to the high quantity of functional groups located on the surface of silica materials (46). Additionally, ionic liquid functionalized silicabased materials appear high efficient anion exchange materials due to their high sorption capacity, selectivity, reusability, fast kinetics and efficiency in the large range of pH (40). However, only few examples of hybrid ionosilicas in anion separation processes have been reported in the literature (45, 47). Our current research interest concerns a particularly innovative and versatile class of materials based on silica with incorporation of anionic exchanging groups. These so-called ionosilicas are synthesized by hydrolysis-polycondensation reactions involving exclusively oligosilylated ionic precursors (48-53). Due to their mixed mineral-ionic nature, ionosilicas are situated at the interface of silica hybrid materials and ionic liquids (54) with very specific and unusual properties, which are currently used for various applications such as decontamination processes and drug delivery (55-58). These features can efficiently be varied both via the cation and the anion, resulting in an extraordinary versatility to fine-tune the interfacial characteristic such as high hydrophilicity and high water-affinity (59). Here, we report for the first time a systematic study about the use of ammonium-based hybrid ionosilicas as universal and highly efficient anion exchange materials. We studied in particular ammonium based ionosilicas for the adsorption of hexavalent chromium in aqueous media. The anion exchange properties of ammonium type ionosilicas were addressed with respect to the


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nature of the ionic ammonium groups and the textural properties of materials. For this purpose, materials displaying various morphologies and textures were synthesized via hydrolysis polycondensation reactions of oligosilylated ammonium precursors under different reaction conditions, i.e. in the presence of the different structure directing agents. The anion exchange properties of the resulting materials were investigated in terms of exchange capacity, kinetics and adsorption thermodynamics. The latter aspect was addressed via isothermal titration calorimetry (ITC) measurements, which gives information such as adsorption enthalpy, reflecting the affinity of chromate towards the ionosilica material together with the driving forces correlated to adsorption.

EXPERIMENTAL SECTION Chemicals. The oligosilylated precursors tris(3-(trimethoxysilyl)propyl) amine 1, methyl-(tris(3trimethoxysilyl)propyl) ammonium iodide 2 and tetrakis (3-(trimethoxysilyl) propyl)ammonium iodide 3, shown on Table 1 ,were synthesized following to previously described protocols (51). The anionic sulfate surfactants SHS (60% sodium hexadecylsulfate/40% sodium octadecyl sulfate) was purchased from ABCR. Plurionic® P123, cetyltrimethylammonium bromide CTAB (99 %), potassium chromate (99 %), ammonium hydroxide and ammonium chloride (99.5 %), sodium carbonate (99.5 %) were purchased from Aldrich. Materials’ synthesis. In this study, amine/ammonium precursors 1-3, shown on Table 1, were used for the preparation of various ionosilica type materials. All precursors were synthesized from the tris(3(trimethoxysilyl)propyl)amine 1, which is the key starting material for the ammonium precursors


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2 and 3. Protonation of 1 yielded the corresponding ammonium precursor 1-H. Alternatively, alkylation reactions with either methyl iodide or 3-iodopropyl-trimethoxysilane provided the methylated precursor 2 and the tetrasilylated precursor 3, respectively. Three different types of surfactants were also used: anionic (sodium hexadecysulfate, SHS) cationic (cetyl trimethylammonium bromide, CTAB) and non-ionic (Pluronic P123). Nanostructured ionosilica were synthesized following formerly reported procedures (49, 51). A tenth material J was synthesized under conventional sol-gel conditions, using nucleophilic activation of the silicon by fluoride ions, in the absence of surfactant.

Table 1. Amine and ammonium based precursors 1 – 3 used for ionosilica synthesis, together with the various surfactant used as structuring agents, and the name of the structuring agent.

Surfactant used as structuring agent

Precursor

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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(MeO)3Si

Anionic – SHS

Cationic – CTAB

Non-ionic – P123

A

B

C

D

E

F

CH3 N

Si(OMe)3 -

I

Si(OMe)3

2


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G

H

I

Ionosilica A, D and G were prepared following procedure reported in (51, 58). For ionosilica A: 265 mg of SHS (containing approx. 40% of sodium stearyl sulfate) was dissolved in a solution prepared from deionized water (17.9 g) and 3 ml of 1 M hydrochloric acid and stirred at 60 °C. After 1 h, a solution of precursor 1 (0.5 g, 1 mmol) dissolved in 0.5 ml of ethanol, was added to the surfactant solution. The resulting blend was vigorously stirred at 60°C for 20 min. It was then heated at 70 °C under static condition for 3 days. The resulting solid was recovered by filtration and dried under air at 70°C for 15h. The surfactant was removed by repeated washing 0.5 g of the solid with a solution of 100 ml ethanol / 5 ml conc. hydrochloric acid. Ionosilicas D and G were synthesized in a similar way from precursor 2 and 3, respectively. Ionosilica B and H were prepared following method described in (51, 57). For ionosilica B: CTAB (362 mg) was dissolved under stirring at room temperature in a solution prepared from deionized water (23.7 g) and 0.3 mL of ammonium hydroxide (25 wt% solution in water) to have a homogenous solution. Then a solution of precursor 1 (1 g, 1.98 mmol) in 0.5 mL of ethanol was added rapidly to the surfactant solution. The mixture was vigorously stirred at room temperature for 2h and then heated without stirring at 70°C for 3 days. The resulting solid product was extracted by filtration and dried under air at 70°C for 15h. The surfactant was removed by washing 0.5 g of the solid with a solution of 100 ml ethanol / 5 ml concentrated hydrochloric acid. Ionosilica E and H were synthesized in a similar way from precursor 2 and 3, respectively.


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Ionosilica C was synthesized from the route explained in (51). For ionosilica C: 4.35 g of Pluronic P123 was stirred overnight in a solution of 105 mL of water and 20 mL of concentrated hydrochloric acid (37%) at room temperature. 4.61 g of this P123 solution was rapidly added to a solution of precursor 1 (0.8 g, 1.58 mmol) in 0.5 mL ethanol at 60°C under vigorously stirring for 2 h. The mixture was kept without stirring at 70°C for 3 days. The resulting solid was extracted by filtration and dried under air at 70°C for 15h. The surfactant was removed by washing 0.5 g of the solid with a solution of 100 ml ethanol/5 ml concentrated hydrochloric acid. Ionosilica F and I were synthesized in a similar way from precursor 2 and 3, respectively. Ionosilica J: 2.18 g (4.33 mmol) of precursor 1 was dissolved in 14 mL of methanol at room temperature. 0.35 g of water was then added under vigorously stirring. Tetrabutylammonium fluoride (TBAF, 43 mg) was then added and vigorously stirred. A gel was formed after about 30 min. The mixture was left at room temperature for 2 days. The resulting solid was separated by filtration and intensively washed with acetone and ethanol. After acidic treatment with 200 mL ethanol/10 mL concentrated hydrochloric acid, the powder was finally dried under air at 70°C for 15h. Characterization. WAXS (Wide-Angle X-ray scattering) experiments were performed with an in-house setup of the Laboratoire Charles Coulomb (‘Réseau de Rayons X et Gamma’, University Montpellier, France). A high brightness low power X- ray tube, coupled with aspheric multilayer optic (GeniX3D from Xenocs) was employed. It delivers an ultralow divergent beam (0.5 mrad, λ= 1.5418 Å). The transmission electronic microscopy (TEM) images were recorded using JEOL 1200 EXII, JEOL, Japan, and scanning electronic microscopy (SEM) images were obtained with a Hitachi S-4800, Canada. The nitrogen adsorption measurements for samples were obtained at


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77 K with a Micromeritics Tristar, USA. Samples were outgassed for 10 hours at 100°C under vacuum (0.1 mBar) before the measurements. The specific surface area was calculated based on the BET model, taking a cross-sectional area of 0.162 nm2 per nitrogen molecule. The pore size distributions were obtained from the desorption branch of the nitrogen isotherms using the Barrett-Joyner-Halenda (BJH) method. The mesoporous and the microporous parameters (external + mesoporous surface, total pore volume, mesoporous volume, microporous volume) have been calculated from the linear segment of the αS-plot. Adsorption of Cr(VI). The adsorption isotherms onto ionosilica were determined by equilibrating about 10 mg of the solid with 20 ml of aqueous solution at a given composition in Nalgene® reactors. The initial solute concentrations were varied within the following range: 0.04 – 3.0 mmol L-1. The pH of initial solution Cr(VI) was adjusted at 4 by 0.1 mol L-1 HCl. The reactors were slowly shaken for 2 h in a thermostated cage (± 0.1 deg) at 25 °C. The effect of pH on the uptake of Cr(VI) was studied by contacting 10 mg of ionosilica and 20 mL of 1 mmol L-1 Cr(VI) solution at pH 2–8 and shaking for 2 h. To adjust the pH, 0.1 mol L-1 HCl and 0.1 mol L-1 NaOH were used. The kinetic study was carried out by shaking 10 mg of the ionosilicas and 20 mL of 1 mmol L-1 Cr(VI) solution for different intervals of time from 2 min to 240 min. The amount adsorbed was determined by following the general procedure: After filtration, the equilibrium concentration of Cr(VI) species in the supernatant was determined by ion chromatography (Shimadzu HPLC apparatus equipped with a Shim-pack ICA1 column in 40 °C oven, and a UV detector at 371 nm) with 4 mmol L-1 sodium carbonate at flow rate of 1.5 mL min−1 as mobile phase. The corresponding amount adsorbed (Qads) was calculated as follows:


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=

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Eq (1)

where Ci and Ce represent, respectively, the initial and final (after attaining the equilibrium) concentrations of the adsorbed species, Vo is the initial volume of the aqueous solution and ms is the mass of the ionosilica sample. The experimental error of the sorption measurements was evaluated by following a previously detailed procedure for some selected systems (60); the maximum error was within 5% and concerned mostly the amounts adsorbed in the adsorption plateau region. Desorption study. A 0.2 g sample of material A was contacted with 200 mL of 1 mmol L-1 Cr(VI) solution, and the mixture was shaken for 1 h. The Cr(VI)-loaded ionosilica was then separated by filtration and slightly rinsed with a small quantity of deionized water to remove the excess of Cr(VI) in interstitial water. The Cr(VI)-loaded ionosilica was dried for 10 h at room temperature under vacuum (0.1 mBar). Then 10 mg of dried Cr(VI)-loaded ionosilicas was shaken with 20 mL of desorption solution for 2 h. Various mixtures of NH3.H2O and NH4Cl were employed as desorption media. The amount of Cr(VI) desorbed was determined by Ionic Chromatography. The percentage of desorption was calculated as follow: Desorption (%) =

∗ 100

Eq (2)

with Cr(VI)des and Cr(VI)ads the amount desorbed and adsorbed of Cr(VI) respectively. Isothermal titration calorimetry (ITC). A TAM III differential microcalorimeter operating in a heat flow mode was used to measure the enthalpy of displacement accompanying adsorption of Chromium onto ionosilica from aqueous solutions at 298 K. The stainless-steel measuring ampoule containing about 5 mg of powder suspended in 0.8 mL of ultrapure of water was placed in the microcalorimeter, and


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equipped with a Gold paddle stirrer. The operational parameters were kept the same for all calorimetry experiments, namely: 25 pulse injections (10 µL) of an appropriate Cr(VI) stock solution (50 mmol L-1) during 10 s, a stirring speed of 90 rpm and time of equilibration between two successive injections equal to 45 min. Further procedures for data processing were described previously (60). The experimental enthalpy changes were subsequently corrected for dilution effects. The dilution experiments were carried out under the same experimental conditions but without an ionosilica sample in the measuring ampoule. The enthalpy changes corresponding to the successive injection steps were finally summed up to obtain the cumulative enthalpy of displacement, ∆ H!" , per unit mass of the solid material. The measurements were at least repeated in triplicate (between 3 to 9 times). The repeatability of the molar enthalpy of displacement for low surface coverage and including the data processing procedures was evaluated at ± 2 J g-1. The calculation of the slope was performed in this adsorption capacity range. The average value was calculated and the error varied from ± 0.4 to 1.5 kJ mol-1.

RESULTS AND DISCUSSION Characterization of ionosilica materials. Nine materials A - I were synthesized from three precursors 1-3 in the presence of three different structure directing agents, as shown in Table 1. A tenth material J was synthesized in the absence of surfactant. The synthesis procedures include washing steps for the elimination of the surfactants (materials A - I) or acidic treatment for the protonation of the amine groups (material J). These procedures were performed with ethanolic hydrochloric acid and afforded ionosilica materials with chloride counter-anions. The obtained materials were characterized using Wide-Angle X-ray Scattering (WAXS), nitrogen sorption experiments and electron


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microscopies in order to determine the surface properties and to monitor the influence of reaction conditions on the textures and morphologies of materials. The nitrogen sorption isotherms for the materials A, D and G, all prepared from different precursors, in the presence of anionic surfactant, are shown in Figure 1 A. The results obtained for materials A, B, C, all prepared from precursor 1 using different templates are displayed in Figure 1 B. The shape of the isotherms indicate that the materials display various porosity. The complete textural parameters (specific surface area, pore diameter and pore volume) are given in Table 2. For materials A, D and G prepared with SHS, the specific surface areas are in the range of 750-860 m2 g-1. The porosity and the templating effect of the surfactant are clearly visible in the sorption isotherms via the inflection at low pressures (p/p0 = 0.2), resulting in high porous volumes. The BJH pore size distribution (Figure S1) is narrow and displays a clear maximum between 2.2 and 2.4 nm for materials A and D. The ordered architecture on the mesoporous length scale is confirmed by Wide-Angle X-ray scattering. Diffraction patterns in Figure 2 exhibit (1 0 0), (1 1 0) and (2 0 0) diffraction rays, characteristic for phases with a 2D hexagonal pore arrangement, for materials A and D. The presence of a large (1 0 0) diffraction ray combined with the absence of diffraction rays of higher order in the diffractogram of material G indicates absence of long range order in this material and hence a ‘worm-like’ architecture (61). The structuration is clearly observed on TEM obtained on materials A and D (Figure 3). As evidenced in our previous study(53), these results confirm strong precursor-surfactant interactions, formation of ion pairs in the hydrolysis-polycondensation reaction mixture, and thus highly ordered architectures of the ionosilicas.


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Figure 1. a. Nitrogen adsorption-desorption isotherm of materials A, D and G, all formed in the presence of anionic surfactant SHS. b. Nitrogen adsorption-desorption isotherm of materials A,


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B and C, all formed in the presence of amine based precursor 1. Close and open symbol represent the adsorption and desorption, respectively.

Figure 2. Small-angle X-ray diffraction patterns of ionosilicas A, D and G, all formed in the presence of anionic surfactant SHS.


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Figure 3. SEM image of ionosilica A (a) and ionosilica D (b); TEM image of ionosilica A (c) and ionosilica D (d).

As shown in Figure 1 B for materials prepared from precursor 1, the shape of the nitrogen sorption isotherms for materials B and C, is different compared to material A. The isotherm indicates nitrogen uptake over a wide range of relative nitrogen pressure. This outcome suggests broad pore size distribution and considerably larger pore diameters. The SBET are lower, in the 200-500 m2 g-1 range, except for the material B obtained from precursor 1 and CTAB which has a specific surface area of 713 m2 g-1. This finding demonstrates that the materials formed in the presence of cationic (CTAB) or neutral (P123) surfactants (for material B and C respectively) result in non-structured materials, combined with lower specific surface area.


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Table 2. Textural parameters of ionosilicas materials prepared with one of the precursors (the amine based, or the ammonium methylated or the tetrasilylated precursor), and one of the structuring agent (SHS, CTAB, P123), or the non-porous solid prepared with TBAF). dBJH is the Mode obtained after the BJH data treatment.

Sext

SBET Solid

m2g-1

CBET

Smes

Vmeso

Vmicro

dmes dBJH

m2g-1 m2g-1 cm3g-1 cm3g-1 nm nm

Structure

A

859

163

58

643

0.53

0.05

3.3

1.9

2D hexag.

B

713

58

8

543

0.68

0.02

5.1

3.4

Amorphous

C

399

73

-

-

-

-

-

-

Amorphous

D

751

141

10

671

0.48

0.02

2.9

1.8

2D hexag.

E

213

81

-

-

-

-

-

-

Amorphous

F

221

115

4

142

0.17

0.02

4.7

3.5

Amorphous

G

863

112

313

302

0.24

0.06

3.2

1.7

Worm-like

H

421

96

285

99

0.21

0.004

8.2

3.8

Amorphous

I

522

122

22

335

0.43

0.05

5.2

3.5

Amorphous

J

34

59

-

-

-

-

-

-

Non porous

Finally, hydrolysis-polycondensation reaction carried out in the absence of surfactant yields the non-porous material J with very low specific surface area (isotherm shown in Figure S5). It clearly appears that materials with regular architectures on the mesoscopic length scale are only obtained in the presence of anionic surfactants, whereas amorphous materials are formed in the presence of cationic and non-ionic structure directing agents or in the absence of template. These findings and these discrepancies between various synthesis conditions indicate that a specific


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combination of precursor + template leads to different interactions and to different formation modes of the ionosilica.

Chromate sorption. Similar materials have been studied as versatile exchangers for the retention of radionuclides or organic pollutants. Here, chromate is chosen as model for the study of oxo-anions sorption onto a series of ionosilicas with tuned physicochemical properties, and also as pollutants with high environmental impact. The chromate adsorption properties of the 10 ionosilicas were evaluated in terms of sorption capacity, displacement enthalpy and kinetics. For this purpose, the chromate sorption isotherms were recorded to determine the maximum adsorption capacity of the materials. Sorption experiments were combined with calorimetric measurements to get a complete thermodynamical description, thus giving information related to the affinity of chromate towards ionosilica. Kinetics and effect of pH were also evaluated. The chromate adsorption on the materials was followed via the depletion method, i.e. the concentration difference of the aqueous supernatant phase between the initial and the equilibrium concentration. In this work, we did not perform in-depth characterization of the materials after chromate adsorption, as the presence of interstitial water within the material may result in artefacts due to the presence of weakly bound species. However, in any case, we observed the formation of chromium(III) species during the adsorption process. Sorption capacity of Cr(VI). Chromate sorption isotherms and corresponding cumulative exchange enthalpies are shown in Figure 4 and Figure 5 for two different series of materials. The whole results are summarized in Table 3. The ionosilica materials of our study display a high sorption capacity in the range from


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1.6 to 2.6 mmol g-1. The highest capacity is observed with the ionosilica A displaying the highest specific surface area together with an ordered pore architecture. The adsorbed chromate amount corresponds to more than 135 mg of Cr(VI) adsorbed by 1 g of ionosilica material. This value is also two times higher compared to commercial strong acid resins with trimethylammonium as functional exchange groups (62). The bibliographic survey reported in Table 4 shows that the results presented in this study are least similar and often superior compared to those observed with conventional surface functionalized silicas (38, 41). In their review, Dinker and Kulkarni reported chromate adsorption capacities lower than 1.8 mmol g-1 using organically functionalized silica materials (23). For amino functionalized silica prepared via post synthesis grafting (41). the sorption capacity increases when shifting from mono- to triamino functionalized silica. The maximum capacity of 1.2 mmol g-1 was found for a triamino functionalized SBA 1 silica material. In another study, Liu et al described porous silica modified with quaternary ammonium and quaternary phosphonium ionic liquids (63). The resulting materials exhibit very low sorption capacity and the highest value 0.36 mmol g-1 was obtained for Cyphos IL 104 porous material (Table 4, entry 6). This result is largely inferior compared to that achieved with an methylimidazolium functionalized mesoporous silica that was directly synthesized via a cocondensation reaction (40). In this last example, the highest sorption capacity reaches 1.74 mmol g-1 for a specific surface area SBET of 418 m2 g-1 (table 4, entry 7). The generally lower adsorption capacity of functionalized silica is likely due to a lower number of cationic exchange groups on the surface of the support material.

Table 3. Theoretical and experimental sorption capacity together with the calculated accessibility and the corresponding cumulative displacement enthalpy of Cr(VI) on ionosilicas.


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∆exp Hdpl

Qtheo

Qmax

Qmax / Qtheo

mmol g-1

mmol g-1

%

2.6

87

-16.4 ± 0.4

2.5

83

-22.7 ± 1.5

C

2.0

67

-15.7 ± 1

D

2.4

83

-11.7 ± 0.5

2.0

69

-12.6 ± 0.5

F

2.4

83

-13.1 ± 0.4

G

2.0

83

-8.2 ± 0.5

1.6

67

-15.0 ± 0.8

2.2

91

-12.5 ± 0.8

1.7

57

-13.2 ± 1.5

Material A B

E

H

3.0

2.9

2.4

I J

3.0

kJ mol-1

Table 4. The maximal removal capacities (Qmax) of various functionalized and hybrid silica used for Cr(VI) removal. (AP = amino-propyl functionalized silica; N- = mono, NNN- = triamino functionalized silica; SBA15Im = methylimidazolium functionalized mesoporous silica directly synthesized by co-condensation). (nd: not determined). Entry 1 2 3 4 5 6 7 8

Adsorbents Qmax (mmol g-1) SBET (m2 g-1) References AP-MCM41 0.76 nd (38) AP-SBA15 0.93 nd (38) N-MCM41 0.4 1037 (41) NNN-MCM41 1 481 (41) NNN-SBA1 1.2 126 (41) Cyphos IL 104 0.36 344 (63) porous silica SBA15Im0.25Cl1.74 418 (40) 3h Cu-EDA porous 2.3 nd (64) silica


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The repercussions of the textural properties of the materials on their sorption properties were investigated via the comparison of ionosilicas D, E and F, all synthesized from precursor 2 in the presence of difference templates. The results are given in Figure 4. We observed that the adsorption capacity within this series of materials decreases with both decreasing SBET and porosity (Table 2 and Table 3). Similar results were obtained with the ionosilicas A, B, C, and G, H, I, synthesized from the precursors 1 and 3, respectively. The highest adsorption capacities were observed for the materials with the highest specific surface areas. Nevertheless, the adsorption capacity is not strictly proportional to the specific surface area. Indeed, the materials with the highest and the lowest specific surface areas (SBET = 859 m2/g for material A, 213 m2/g for material E,) gave rise to adsorption capacities of 2.0 (material A) and 2.6 mmol/g (material E) respectively. Whereas the specific surface area of material E is four times lower, its adsorption capacity is only 30 % inferior compared to that of material A. This means that the accessible surface is not the only contribution to the exchange. Moreover, the adsorption capacity for material J is still relatively high (1.7 mmol g-1) though its specific surface area is relatively low (SBET = 34 m2 g-1). This result indicates that even in the absence of porosity or high specific surface area, ionosilica materials are efficient chromate exchangers. The fact that even non-porous materials, prepared via a very simple and straightforward synthesis procedure, still exhibit high sorption performance is a real advantage of this approach.


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Figure 4. Sorption isotherms of Cr(VI) onto the ionosilica D, E and F synthesized from precursor 2 with the surfactant SHS, CTAB, and P123 (left) and the corresponding cumulative displacement enthalpies (right).

The high anionic adsorption capacity of the ionosilica materials may be compared with the theoretical amount of incorporated ionic groups, Qtheo. Taking into account the amount of the precursor, Qtheo is calculated for the various materials. Table 3 indicates that the theoretical anionic exchange capacity (AEC) is 3, 2.9 and 2.4 mmol g-1 for materials prepared from the precursor 1, 2 and 3, respectively. The effective amount adsorbed can be expressed as Q max / Q theo,

i.e. the percentage of sorption relative to the theoretical AEC. In the case of materials

prepared with SHS, the maximum sorption is 2.6, 2.4 and 2.0 mmol g-1 for material A, D and G respectively. This points out that ca. 85 % of cationic precursor have exchanged their primary chloride species by chromate anions independently of the nature of the cationic ammonium groups. This 0.85 ratio is obtained for the three structured materials and is the highest in the


21


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present series of ionosilicas. Then the ratio decreases for materials displaying lower porosity. For the slightly porous material J, almost 60 % of the theoretical capacity is reached. These findings prove that a high number of ionic sites are available for the ion exchange in all samples and even in less porous ones. This can be assigned to the high hydrophilicity of the present ionosilicas already evidenced using a calorimetric approach based on the competitive adsorption of butanol (59). This accessibility is observed for all materials and do not depend on the nature of the immobilized ammonium group. Our results are improved compared to PMO with post-synthesis modifications. For example, ethylenediamine (EDA)-functionalized mesoporous ethylenesilica exhibit a relative arsenate adsorption ratio per amount of amino group in the range of 38 to 42% (65).

Figure 5. Sorption isotherms of Cr(VI) onto the ionosilica A, D and G synthesized from precursor 1, 2 and 3 with SHS as structuring agent (left) and the corresponding cumulative displacement enthalpies (right).


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All materials exhibit also vertical increase in the adsorption isotherm at low concentrations (Figure 4 and Figure 5). This indicates that Cr(VI) has high affinity towards ionosilica. To support this conclusion, the cumulative enthalpies upon Cr(VI) adsorption, obtained via isothermal titration calorimetry (ITC), were determined. Figure 4 shows the cumulative enthalpies of displacement for the materials D, E, F and J prepared with the same precursor and different structuring agent. Figure 5 illustrates the results obtained for the ionosilicas A, E and G, all displaying similar textural properties but constituted of different ammonium building blocks. In all cases the enthalpy of displacement is exothermic. First the signal decreases then passes by an inflexion point. At the end, the total cumulative enthalpy effect reaches a plateau when the exchange is complete. At this stage, it is not possible to differentiate in details the various contributions to the overall enthalpy change upon displacement. Indeed, the cumulative enthalpy represents a global effect combining all various contributions involved in the sorption process. This includes such different contributions as the insertion-sorption of the adsorbed species, the displacement of other species (e.g. release of chloride), hydration / dehydration of the exchanged species, and also potential swelling (expansion or compression) of the material (66). In Figure 4 (c.f. also Figures S7 and S8), the initial slope seems to be similar for a given series of ionosilica prepared from the same precursor. This is truly obvious for precursor 2, where the curves are superposed for the first exchanged Cr(VI). To analyze quantitatively the obtained curves, the initial slope of the plot of ∆#$ %&'( vs. amount adsorbed corresponding to the molar enthalpy was calculated. Table 3 and the histogram in Figure 6 summarize the observed results. It is here challenging to show a clear trend taking into account all ionosilicas since the materials


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are made of different precursors and structuring agents, and thus presenting various surface reactivity, various structuration and porosity.

Figure 6. Displacement effect in the initial part of the Cr(VI) for given series of precursor, or for a given structuring agent. Ionosilica A, B, C and J, are prepared with precursor 1, with SHS, CTAB and P123 as structuring agent respectively. Ionosilica D, E and F are prepared with precursor 2, with SHS, CTAB and P123 as structuring agent respectively. Ionosilica G, H and I are prepared with precursor 3, with SHS, CTAB and P123 as structuring agent respectively.

These results clearly show that the nature of the cationic group has a deep impact on the sorption thermodynamics. This is obvious for the SHS and the P123 templated materials, with decreasing displacement enthalpies when going from protonated ammonium cations to methylated and tetrasilylated precursor. The enthalpy is the most exothermic for the precursor 1, and the lowest in intensity for the precursor 3. Tetra-alkylammonium substructure shows weaker interaction with the Cr(VI) anion and therefore these materials display a lower sorption enthalpy. For SHS materials (A, D, G), they exhibit similar structuration and textural properties. The


24

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nature / constitution of the ionic groups influences significantly the adsorption process. This has to be correlated with the increase in the steric shielding of the ammonium functions, which limits the electrostatic interaction between the anionic species and the cationic precursor. The same type of interaction was also reported between the anionic surfactant and cationic precursor during template directed hydrolysis polycondensation procedures (51). The same trend and similar effect due to steric hindrance is observed for the P123 series. Nevertheless, for materials prepared with P123, and also with CTAB, there is probably an additional parameter to be taken into account, that is the porosity of the materials, with the presence or not of meso- and / or microporosity. This could explain why the material E prepared from the methylated precursor 2 and CTAB shows lower displacement enthalpy compared to material H synthesized with the same template but from the tetrasilylated precursor 3. For a given precursor, the tendency is more complex. For methylated cationic substructure, the displacement enthalpy is similar, with enthalpy values being -11.7, -12.6 and -13.1 kJ mol

-1

for

SHS, CTAB and P123 templates respectively. The interaction seems to be the same in this series prepared with precursor 2 and with various templates. The comparison of materials prepared with precursor 1 or precursor 3 is not so clear. The following tendency is observed: B > A ≈ C > J, and H > I > G. For precursor 1 and 3, the highest value is obtained with the CTAB template materials. No clear correlation with textural, structural or physicochemical properties was observed. At the end, it has to be mentioned that for materials prepared from the precursor 3, the shape of the second part of the curve is slightly different. After a linear decrease of the enthalpy, there is an inflexion and a small increase of the displacement effect. This means that there is an additional endothermic contribution, indicating an entropy-driven process for the second part of the sorption isotherm up to the plateau. This could be correlated with the steric hindrance


25


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between adsorbed anions, in addition with steric hindrance between the inorganic anion and the cationic precursor. At this state, we only focused on the enthalpic determination of the adsorption process via calorimetric measurements. We did not address the determination of other thermodynamic parameters such as entropy or free energy, as these values are based on adsorption models such as Freundlich or Langmuir equations. We believe that these oversimplified models are not applicable on surfaces developing strong electrostatic interactions and would therefore give misleading results. Finally, a selected material was characterized via FT-IR spectroscopy before and after chromate adsorption (Figure S10). The spectrum of material A is dominated by adsorption bands at 1030 and 915 cm-1, which can be attributed to the polysilsesquioxane network. After chromate adsorption, the adsorption band at 915 cm-1 splits up to give two new bands at 940 and 888 cm-1. The latter band can be attributed to the chromate anion (67). However, at this state, not exact information about the speciation effects of the chromate anions within the ionosilica matrix can be deduced.

Desorption study. In view of the reusability of the materials, the desorption and the regeneration steps of the adsorbents are crucial issues. The recyclability of the ionosilicas was therefore evaluated via desorption tests. NaCl and NaOH or their mixed solution are usually used for desorption of Cr(VI) in organic polymer anion exchangers. In most cases, the higher efficiency is obtained for NaOH solution.(20, 68) For materials prepared from precursor 1, this procedure induces the deprotonation of the ammonium groups and therefore the loss of the ion exchange function of the


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material, thus facilitating the Cr(VI) release. However, the ion exchanger based silica materials show limited stability in strongly alkaline solution due to the lability of the Si-O-Si bonds towards the hydroxide anion. The release of the Cr(VI) loaded ionosilica (material A) was determined in slightly basic conditions. The good desorption ability of buffer solution was evidenced for imidazolium functionalized silica. (40) We therefore worked with ammonium buffers (NH4OH and NH4Cl) at slightly basic pH (range pH 8-10) for the leaching experiments. Various molar ratios of NH4OH: NH4Cl (Table 5) were tested. However, our results indicate that only 55 % of the adsorbed Cr(VI) were released in each case. The Cr(VI) desorption is low and seems independent of the basicity of the solution. This rather low percentage may be explained by partial collapse of the porosity of the ionosilica mesophase during the adsorption/desorption process, as already observed after diclofenac or sulindac sorption (56).

Table 5. Evaluation of desorption for various NH4OH / NH4Cl solutions on ionosilica A. Desorption solution

pH

Desorption %

0.5 mol L-1 NH4OH: 0.5 mol L-1 NH4Cl

9.4

56


9.2

55


8.7

53


10.8

53


11.0

56

Effect of pH on the sorption efficiency. pH plays an important role in anion exchange and has a direct impact on the sorption properties. The effect of the pH on the sorption capacity of ionosilicas A and D is shown in


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Figure 7. The maximum adsorption capacity was observed in slightly acidic solution (pH 3.54.5). For material A, the sorption amount decreases for higher or lower pH. For material B, it seems stable until pH 5.5 and then decreases. The sorption amount depends on the surface properties of the adsorbent materials and on the type of Cr(VI) species. Material A contains protonated ammonium cations (R3NH+), whereas material D is constituted of quaternary R4N+ units. Cr(VI) exists in different forms in aqueous solution. Under slightly acidic conditions (2 < pH < 6), the HCrO4- ion is the predominant form. At higher pH, the CrO42- ion becomes the major species (10, 68). Furthermore, the presence of competing anions such as chloride or hydroxide may interfere with the chromate adsorption and lower the adsorption capacity. At low pH, the adsorption efficiency is lowered mainly due to increasing chloride concentration added for acidification. Another effect may be the formation of neutral H2CrO4 at pH ≤ 3.5. On the other side, at pH higher than 4.5, the decrease in the adsorption efficiency can be explained by the formation of divalent CrO42- ions, which probably occupy two exchangeable sites in ionosilica (40). Our materials still exhibit high sorption capacity at neutral and slightly basic conditions compared to the primary amine and imidazole functionalized silicas with low sorption capacity due to the deprotonation of primary amine at pH ˃ 3.5 or imidazole at pH ˃ 5 (40, 6970). It should be mentioned that the formation of amine groups via deprotonation of ammonium groups within material A occurs at much higher pH (pKa ~ 10.6) and has no influence on the adsorption efficiency in the investigated pH range.


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Figure 7. Effect of pH on the Cr(VI) uptake onto ionosilicas A and D.

Kinetics of the ion exchange with ionosilicas. Rapid adsorption kinetics is an important feature and a prerequisite in view of potential applications of ionosilicas in wastewater treatment. Using ionosilica materials, rapid adsorption kinetics is expected due to the high amount of immobilized ionic species, their localization and finally the high affinity of Cr(VI) towards the material. The influence of porosity on the adsorption kinetics was mainly studied. For this purpose, we used the materials A, B, C and J for ion exchange (Figure 8). All these materials contain identical ammonium substructures and are characterized by different specific surface area and porosity. The materials with the highest porosity show the fastest kinetics. The ion exchange is fast and 80-90 % of Cr(VI) exchange capacity is reached after 10 minutes for materials A, B and C. This behavior has to be compared to other surface functionalized silicas. Mesoporous ionosilicas show fast kinetics of the sorption


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of Cr(VI) compared to the imidazolium functionalized mesoporous silica (68). This rate of kinetics is faster than the amino, imidazole functionalized silica (69, 71) and significantly faster compared to organic anion exchangers (68) or modified carbons (24). In the case of the nonporous material 80 % of sorption is reached after 40 minutes and the saturation after complete ion exchange is reached after 75 minutes. The comparison of the various ammonium substructures for a given structuring agent is rather tricky since both the surface exchangeable sites and the textural properties (microporosity), are simultaneously modified.


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Figure 8. Effect of contact time on adsorption of Cr(VI) onto the materials A, B, C and J synthesized from precursor 1 with the surfactant SHS, CTAB, P123 and without surfactant (a), and, for ionosilica A, D and G synthesized from precursor 1, 2 and 3 with SHS as a structuring agent (b), together with the fitted data from the pseudo-second-order model simulation. The kinetic parameters of the sorption process were calculated using Lagergren-pseudo-first order model (72), pseudo-second-order model (73-74) and Weber’s intraparticle diffusion model (75). The best fit parameters (Table 6) are obtained for the pseudo-second-order model, and the experimental data points correspond well to the calculated values. This means that the sorption of Cr(VI) on ionosilicas is governed by an electrostatic sorption mechanism suggesting that the electrostatic attraction plays a key role in the sorption processes (24, 69). The sorption process occurs through several steps i.e. external surface diffusion, intraparticle diffusion, and sorption on the internal sites (74, 76). The kinetic plots following the Weber’s intraparticle diffusion


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model exhibit multi-linear plots. This indicates that the intraparticle diffusion is not the ratecontrolling step (69) and the sorption rate depends on two or more rate-controlling steps (74).

Table 6. Kinetic parameters for sorption of Cr (IV) onto ionosilicas Lagergren-first order

Pseudo-second order

qe. Ionosilica

mmol g-1

qe.cal/

qe.cal/

k 1/

mmol g-1

min

Intra-particle mass transfer diffusion

2

-1

R

mmol g-1

k2/mmol g-1min-1

R2

kid/mmol g-1min-1

R2

A

1.91

0.98

1.1219

0.9783

1.94

0.1205

0.9999

0.0235

0.9912

B

1.57

0.98

1.1823

0.9809

1.60

0.1189

0.9999

0.0115

0.9616

C

1.92

0.98

1.0357

0.9979

1.92

0.1422

0.9997

0.0329

0.9970

J

1.65

0.98

0.1813

0.9876

1.72

0.0430

0.9974

0.0714

0.9874

D

1.90

0.99

2.6478

0.9308

1.89

0.5251

0.9999

0.0041

0.8644

G

1.60

0.99

1.4499

0.9934

1.60

0.1058

0.9998

0.0141

0.9928

CONCLUSION

We synthesized a set of ionosilicas starting from three different oligosilylated ammonium precursors. Hydrolysis-polycondensation reactions of these precursors in the presence of various types of surfactant (anionic, cationic or neutral) or without addition of a structure directing agent led to materials displaying significant differences in terms of chemistry, texture, morphology and porosity. The whole series of materials exhibit excellent anion exchange properties. All materials appeared as highly efficient adsorbents for Cr(VI) with adsorption capacities > 1.6 mmol g-1. The highest exchange capacity was found for highly porous materials, whereas the lowest values were observed for low-porous ones. The exchange rate of ionic sites for chromate exchange was


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between 67 and 87% based on the theoretical exchange capacity using porous materials (SBET > 200 m2/g), and of 57% for a low-porous material. These results suggest that the ion exchange took place not only at the surface, but also within the walls of the materials. This feature is specific for ionosilicas and probably related to the high hydrophilicity of these materials. Diffusion and mass transfer within the material are favored in aqueous solution, thus leading to enhanced accessibilities of the anion exchange sites. The displacement enthalpy measured via Isotherm Titration Calorimetry was negative in all cases. Its intensity was mainly governed by the nature of the cationic substructure and the bulkiness of the substituents attached to the ammonium center. Materials exhibit quite similar sorption capacity, but the strength of the interaction was influenced directly by the nature of the cationic substructure and the confinement effects inside the porosity. All materials showed efficient chromate sorption in a pH range between 2-8. The adsorption kinetics are generally fast and reach 80-90% of the maximum adsorption capacity after only 10 min. Preliminary desorption tests indicate that recyclability has to be improved before considering practical uses and the design of closed separationregeneration cycles.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website.


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BJH desorption pore size distribution; Nitrogen adsorption-desorption; Scanning electron microscopy pictures; Sorption isotherms and displacement enthalpy from ITC measurements; Models for the kinetic study.

AUTHOR INFORMATION Corresponding Author * [email protected] * [email protected]

Present Addresses † Department of Polymer Chemistry, University of Science, VNU-HCM, 227 Nguyen Van Cu St, District 5, HoChiMinh City, Viet Nam

Author Contributions The manuscript was written through contributions of all authors. All authors have given their approval to the final version of the manuscript. FUNDING SOURCES The authors acknowledge the LabEx Chemistry of Molecular and Interfacial Systems (LabEx CheMISyst) (ANR-10-LABX-05-01) for financial support.


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ACKNOWLEDGMENT The authors are grateful to Mr. Amine Geneste for his valuable help with calorimetric experiments.

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Figure 1. a. Nitrogen adsorption-desorption isotherm of materials A, D and G, all formed in the presence of anionic surfactant SHS. 119x84mm (300 x 300 DPI)



Figure 1. b. Nitrogen adsorption-desorption isotherm of materials A, B and C, all formed in the presence of amine based precursor 1. Close and open symbol represent the adsorption and desorption, respectively. 119x84mm (300 x 300 DPI)


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Figure 2. Small-angle X-ray diffraction patterns of ionosilicas A, D and G, all formed in the presence of anionic surfactant SHS. 119x84mm (300 x 300 DPI)


ACS Applied Nano Materials 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

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b

2 μm

c

1.2 μm

d

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50 nm

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Figure 4. Sorption isotherms of Cr(VI) onto the ionosilica D, E and F synthesized from precursor 2 with the surfactant SHS, CTAB, and P123 (left) and the corresponding cumulative displacement enthalpies (right). 297x127mm (300 x 300 DPI)



Figure 5. Sorption isotherms of Cr(VI) onto the ionosilica A, D and G synthesized from precursor 1, 2 and 3 with SHS as structuring agent (left) and the corresponding cumulative displacement enthalpies (right). 159x72mm (300 x 300 DPI)


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Figure 7. Effect of pH on the Cr(VI) uptake onto ionosilicas A and D. 279x215mm (300 x 300 DPI)


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Figure 8. Effect of contact time on adsorption of Cr(VI) onto the materials A, B, C and J synthesized from precursor 1 with the surfactant SHS, CTAB, P123 and without surfactant (a), and, for ionosilica A, D and G synthesized from precursor 1, 2 and 3 with SHS as a structuring agent (b), together with the fitted data from the pseudo-second-order model simulation. 119x84mm (300 x 300 DPI)



Figure 8. Effect of contact time on adsorption of Cr(VI) onto the materials A, B, C and J synthesized from precursor 1 with the surfactant SHS, CTAB, P123 and without surfactant (a), and, for ionosilica A, D and G synthesized from precursor 1, 2 and 3 with SHS as a structuring agent (b), together with the fitted data from the pseudo-second-order model simulation. 119x84mm (300 x 300 DPI)


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Surface properties and chemical constitution as crucial parameters for

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