Influence of porosity and surface modification on the adsorption of both cationic and anionic dyes

Accepted Manuscript Title: Influence of porosity and surface modification on the adsorption of both cationic and anionic dyes Author: Imene Laaz Marie-Jos´e St´eb´e Abdellah Benhamou Deriche Zoubir Jean-Luc Blin PII: DOI: Reference:

S0927-7757(15)30347-2 http://dx.doi.org/doi:10.1016/j.colsurfa.2015.11.024 COLSUA 20295

To appear in:

Colloids and Surfaces A: Physicochem. Eng. Aspects

Received date: Revised date: Accepted date:

22-9-2015 11-11-2015 14-11-2015

Please cite this article as: Imene Laaz, Marie-Jos´e St´eb´e, Abdellah Benhamou, Deriche Zoubir, Jean-Luc Blin, Influence of porosity and surface modification on the adsorption of both cationic and anionic dyes, Colloids and Surfaces A: Physicochemical and Engineering Aspects http://dx.doi.org/10.1016/j.colsurfa.2015.11.024 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Influence of porosity and surface modification on the adsorption of both cationic and anionic dyes Imene Laaz1,2, Marie-José Stébé1, Abdellah Benhamou3, Deriche Zoubir2, Jean-Luc Blin1* [email protected] 1

Université de Lorraine/CNRS, SRSMC, UMR7565, F-54506 Vandoeuvre-lès-Nancy cedex, France 2 Université des Sciences et de la Technologie d’Oran -Mohamed BOUDIAF, Laboratoire Physique-Chimie des Matériaux Catalyse et Environnement, Faculté de Chimie, Oran 31000, Algérie 3 Université des Sciences et de la Technologie d’Oran -Mohamed BOUDIAF, Laboratoire d'Ingénierie des Procédés de l'Environnement, Faculté de Chimie, Oran 31000, Algérie * Corresponding author at: Université de Lorraine, SRSMC UMR 7565, Faculté des Sciences et Technologies, BP 70239, F-54506 Vandoeuvre-lès-Nancy cedex, France, Tel.: +33 3 83 68 43 70.

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Graphical Abstract Small mesopore Large mesopore

Removal of Dual-mesoporous silica

or

SBA-15

adsorbent Brilliant green Removal of adsorbent

Functionalized SBA-15

Congo red

NH2

+

NH2 NH2

3

Highlights  Peculiar efficiency of bare silica mesoporous materials for cationic dye adsorption  Strong uptake of anionic dye adsorption by functionnalization of the adsorbent  Dual mesoporosity positively affect cationic dye adsorption under acidic conditions  Adsorption kinetics determined from the pseudo-second order equation  Adsorption isotherms describe by Langmuir and Freundlich models

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Abstract We report here the use of porous materials for the adsorption of both anionic and cationic dyes. We have investigated the effect of the pH and of the presence of a dual mesoporosity on the adsorbents capacity. The results show that, regardless of porosity, the bare mesostructured silicas are particularly efficient for the adsorption of the cationic dye, brilliant green. The adsorption isotherms have been fitted by using the Langmuir and the Freundlich models. Under acidic conditions, the dual-mesoporous material presents a better affinity and a slightly higher maximum adsorption capacity for congo red than the mono-modal mesostructure. Concerning the adsorption of brilliant green, no significant effect of porosity is noted and the fitting parameters indicate that similar dye-adsorbent interactions occur.

Keywords : Ordered mesoporous materials; Dual mesoporosity; Congo red; Brilliant green; Adsorption; Functionalization

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1. Introduction There are many different types of substances that may pollute water and contamination of surface water, groundwater and soil is still a major problem [1]. Because of their carcinogenic and toxic effects on human health [2,3], among the different pollutants such as detergents, fertilizers, volatile organic compounds (VOC), halogenated organic compounds (HOCs), dyes are a major environmental concern. For example, brilliant green is toxic for lungs through inhalation and congo red, a derivative of benzidine and napthoic acid, metabolizes to carcinogenic products. Both colorants are hazardous when in contact with skin or eyes. Dyes are mainly discharged from pharmaceutics, printing, food coloring, cosmetics, paper, textiles and many others industries [4,5], involving an environmental pollution. Because of their toxicity for human health, many efforts have been devoted to remove these pollutants from the effluents and various methods such as filtration, gravity separation, flotation, enzymatic decomposition and photocatalysis have been developed [6-8]. However, due to its high efficiency, adsorption is the cheapest employed process [6] and different kinds of absorbents have been designed, in particular for the adsorption of congo red and brilliant green [9-13]. For the past few years, surfactants templated silica mesoporous materials have attracted much research attentions [14-16] and a series of compounds labeled, for example MCM (Mobil Crystalline Materials), SBA (Santa Barbara), MSU (Michigan State University), have been prepared by different groups. The synthesis of these compounds combines the sol-gel chemistry and the use of assemblies of surfactant molecules as framework templates. Surfactant micelles are commonly used as self-assembly templates for the development of new mesoporous silicas with a variety of textures and structures [14-16]. Another approach to the preparation of ordered mesostructures uses liquid crystal phases and is labeled as the direct liquid crystal templating (LCT) pathway [17,18]. In this case, larger surfactant concentration is necessary and the structure of the recovered materials can be designed a

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priori based on the corresponding surfactant phase diagram. These materials have potential applications in many fields, such as catalysts, host matrixes for electronic and photonic devices, drug delivery and sensors [19,20]. Thanks to their properties, such as large surface area and pore volume, ordered mesopores with tuned sizes, mesoporous silica materials are supports of choice for dyes adsorption, responding to both environmental and technical requirements [21-24]. For example, Lee et al. [21] have shown that MCM-41 may be an effective absorbent for basic dyes removal from aqueous solution. In addition, the nature and the strength of the interaction between the host molecules and the adsorbent can be tuned by functionalizing the surface of these supports [25,26]. The functionnalization can occur either through the post synthesis grafting of an organic compound, such as an organoalkoxysilane, on the supports or by the co-hydrolysis and polycondensation of an alkoxysilane, for instance tetramethylorthosilicate (TMOS) and an organoalkoxysilane. This second pathway involves a covalent link between both moieties. For example, Ma et al. [25] have modified MCM-41 by introducing ammonium group, according to the post-synthesis way, for the adsorption of anionic dyes. Authors have evidenced that the functionalized MCM-41 material has a high affinity for the considered dyes and that the electrostatic interaction was responsible of the colorants adsorption. Besides these mono-modal mesostructures, more recently dual mesoporous materials have attracted much attention as they are of particular interest for catalysis and for the engineering of pore systems [27]. Indeed, it was reported that a hierarchical combination of mesopores reduces transport limitations, resulting in higher activities and better controlled selectivity [27]. One strategy to prepare these bimodal mesoporous materials consists in using mixtures of templates [28-31]. In our group we have used mixtures of Pluronic P123 and fluorinated surfactant C8F17C2H4(OC2H4)9OH [RF8(EO)9] to design such bimodal mesoporous silicas [30,31]. Here we have investigated the ability of the obtained materials to be used as

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absorbents for the removal of congo red, an anionic dye and brilliant green, a cationic dye. The structure of both dyes is given in scheme 1. The efficiency of the dual-mesoporous silica is compared with the one of SBA-15, which is a mono-modal mesostructure prepared by using Pluronic P123 as template. The SBA-15 has also been functionalized with 3aminopropyl-triethoxysilane (APTES) in order to investigate the effect of the surface state on the adsorption of both the cationic and the anionic dyes.

2. Materials and methods The used fluorinated surfactant, which was provided by DuPont, has an average chemical structure of C8F17C2H4(OC2H4)9OH. It is labeled as RF8(EO)9. The hydrophilic chain moiety exhibits a Gaussian chain length distribution and the hydrophobic part is composed of well defined mixture of fluorinated tails. The selected triblock copolymer, Pluronic P123 [(EO)20(PO)70(EO)20], 3-aminopropyl-triethoxysilane (APTES), tetramethoxysilane (TMOS) and the dyes were purchased from Aldrich. Deionized water was used to prepare the various samples. Absorbents preparation : To prepare the dual-mesoporous material : 0.9 g of RF8(EO)9 and 0.1 g of P123 are dissolved in a hydrochloric acid solution (pH = 0.3) to form a micellar solution containing 10 wt.% of surfactant. Then 0.32 g of tetramethoxysilane (TMOS), used as the silica source, is added dropwise into the micellar solution at 25°C and let under gentle stirring (150 rpm) for 1 hour. The obtained samples are sealed in Teflon autoclaves and heated at 80 °C for 1 day. SBA-15 has been synthesized according to a procedure reported in literature [32] : 1.33g of P123 are dissolved in 50 mL of hydrochloric acid solution (pH = 0.3). Then 2.07g of TMOS are added. After 30 minutes under gentle stirring (300 rpm) at 25°C, the obtained samples are sealed in Teflon autoclaves and heated at 40°C for 24 hours, and then at 100°C for 24 hours.

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Whatever the material, the final product is recovered after ethanol extraction with a Soxhlet apparatus during 48 hours. Functionnalization of SBA-15 : The inorganic material (2g) was mixed with 9.42 mL of 3aminopropyl-triethoxysilane (APTES) in toluene (27 mL) at 60°C and stirred during 24 hours under reflux conditions. Then, the modified SBA-15 was filtered off and washed with toluene during 4 hours and ethanol during 10 hours. The functionalized SBA-15 is obtained after drying in air. Dyes adsorption : 10 mg of the adsorbent were added to 10 mL of the dye solution having a fixed concentration and pH. The mixture was shaken (250 rpm) at room temperature. To measure the quantity of dye adsorbed on the support as a function of time at regular intervals from 10 to 900 minutes, liquid was separated from SiO2 particles by filtration through a 0.45 µm PTFE Millipore filters. The adsorption process was monitored with a Cary 3E UV-Visspectrophotometer by determining the concentration of the colorant in the supernatant. For each dye, pH-specific standard calibration curves have been made at the maximum absorption wavelengths and use in its respective pH range. For example, at pH 7, the absorbance values were taken at 500 and 625 nm for congo red and brilliant green, respectively. At a given time t, the adsorbed quantity of dye (qt) is given by the relation: qt = (C0-Ct).V /W where C0 and V stand for the initial concentration and the volume of the solution. Ct is the dye concentration remaining in solution at time t and W is the mass of the adsorbent. The kinetic study has been performed with an initial concentration (C0) of congo red or brilliant green solution equal to 150 mg/L. To build the adsorption isotherms, the mixtures were shaken at room temperature for 240 minutes to reach equilibrium and C0 has been varied from 10 to 500 mg/L. To investigate the

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effect of pH on the adsorption, the pH of the dye solution was adjusted to 3 with hydrochloric acid (HCl) or to 10 with sodium hydroxide (NaOH). Characterization : SAXS measurements were carried out using SAXSess mc2 (Anton Paar) apparatus. It is attached to a ID 3003 laboratory X-Ray generator (General Electric), equipped with a sealed X-ray tube (PANalytical, λCu (Kα) = 0.1542 nm) operating at 40 kV and 50 mA. A multilayer mirror and a block collimator provide a monochromatic primary beam. A translucent beam stop allowed the measurement of an attenuated primary beam at q = 0. Mesoporous materials were introduced into a powder cell before being placed inside an evacuated chamber equipped with a temperature controlled sample holder unit. Acquisition times were typically in the range of 1 to 5 minutes. Scattering of X-ray beam was recorded by a CCD detector (Princeton Instruments, 2084 x 2084 pixels array with 24 x 24 µm² pixel size) placed at 309 mm from the sample holder in the q range from 0.09 to 5 nm-1. Scattering data, obtained with a slit collimation, contain instrumental smearing. Therefore, the beam profile has been determined and used for the desmearing of the scattering data. All data were corrected for the background scattering from the empty cells. N2 adsorption and desorption isotherms were determined on a Micromeritics TRISTAR 3000 sorptometer at –196 °C. The pore diameter and the pore size distribution were determined by the BJH (Barret, Joyner, Halenda) method applied to the adsorption branch of the isotherm [33]. Diffuse reflectance infrared spectra were obtained with a FTIR Perkin Elmer 2000 spectrometer and a deuterated triglycine sulfate DTGS detector. Powder Diffuse Reflectance data were collected with a Harrick DRA-3XX-PE9 attachment and a HVC-DRP heatable and evacuable cell. This accessory allowed to analyze the sample under vacuum (pressure below 10-4 mbar). The spectrum resolution was 4 cm-1. Spectroscopic grade KBr (Prolabo, for infrared spectroscopy) was used as background. The sample was prepared by mixing mesoporous silica with potassium bromide (silica mass fraction of 0.10 in KBr), without any pressure.

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3. Results and discussion 3.1 Adsorbents characterization The SAXS pattern of the dual mesoporous material exhibits reflection lines at 11.2, 6.4, 5.6, 5.2 and 3.1 nm (Fig. 1Aa). The three first ones can be attributed to the (100), (110) and (200) reflections of a first hexagonal structure having a cell parameter a0 equal to 12.9 nm (a0 = 2d100/ 3 ) and the relative position 1,

3 of the two last peaks characterizes a second

hexagonal mesopore network with a smaller cell parameter (a0 = 6.0 nm). Thus, the SAXS analysis shows the coexistence of two mesostructures. Looking at the patterns of both SBA15 and SBA-15 functionalized with APTES, the hexagonal pore ordering is evidenced by the presence of three peaks at 10.4 nm, 6.1 and 5.2 nm (Fig. 1Ab,c). All compounds exhibit a type IV isotherm (Fig. 1B), characteristic of mesoporous materials according to the IUPAC classification [34]. For the dual-mesoporous material, two distinct capillary condensation steps are clearly seen at p/p0 values of about 0.50 and 0.75 (Fig. 1Ba), respectively. The desorption branch also displays two distinct steps. This suggests the presence of two pore systems with different diameters. The BJH model analysis of this material provides two narrow peaks centered at 4.0 nm and 9.6 nm (insert of Fig. 1Ba). The specific surface area and pore volume values of this adsorbent are 832 m²/g and 0.82 cm3/g, respectively (Table 1). It is also interesting to note that the un-functionalized SBA-15 presents the values of pore volume and specific surface area, of 1.07 cm3·g-1 and 887 m2·g-1 and that, by introducing APTES, the pore volume and the specific surface area decrease to reach 0.57 and 359, respectively (Table 1). This phenomenon can be attributed to the occupancy of the mesopore channels by the organic groups. The obtained pore size distributions are rather narrow (Insert of Fig. 1Bb and 1Bc). For SBA-15, free of APTES, the pore size is close to 9 nm and after the introduction of the organic group, the pore diameter is decreases to 8.1 nm. The efficiency of the functionnalization is further confirmed by infrared spectroscopy (Fig. 2). On the spectrum of

11 the bare SBA-15 (Fig. 2a), below 1800 cm-1, the broad and intense band at 1080 cm-1 and the shoulder at 1185 cm-1 are characteristic of the antisymmetric stretching vibrational mode of the Si-O-Si siloxane bridges. The less intense absorption at 978 cm-1 is assigned to the Si-O stretching of free silanols. The sharp band at 3741 cm-1 is due to the free silanols. Upon the functionalization, the characteristic vibration of the silanol groups disappears (Fig. 2b). This reflects the reaction between the APTES and the free OH of the SBA-15. Simultaneously, vibrations attributed to the asymmetric NH2 stretching, symmetric NH2 stretching and NH2 bending of hydrogen bonded to the amino group appear at 3365, and 3280 and 1635 cm-1, respectively. Bands due to the C-H stretching are also observed at 2932 and 2893 cm-1. It should be noted that before analyzes, the samples have been outgassed under vacuum to remove physisorbed water, as its presence can forbid the observation of the vibrations due to the amino group.

3.2 Kinetic study For a colorant solution having an initial concentration of 150 mg/mL, Figure 3 depicts the adsorption, under different pH conditions, of congo red and brilliant green onto both dualmesoporous silica (Fig. 3A) and SBA-15(Fig. 3b) versus time. Since the simplest way to describe dye adsorption is using a pseudo-second order model, the experimental data have been fitted by using this model. It can be expressed by the following equation : k s q e2 t qt = 1  ks qe t

(1)

where ks is the constant rate of the second order model (g/mg.min), qt and qe are the amounts of adsorbed dye on adsorbent at equilibrium and at time t, respectively. Equation 1 can also be written as : t 1 t   2 q t ks qe qe

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The kinetic parameters, i.e qe and ks, obtained from the straight plots of

t versus t are qt

gathered in Table 2. Concerning congo red, whatever the supports, the adsorption capacity increases when the pH is decreased (Fig. 3Aa and 3Ba), while the opposite effect is noted for brilliant green (Fig. 3Ab and 3Bb). For example, using the dual-mesoporous silica as adsorbent, after 100 minutes, when the pH is changed from 10 to 3, for congo red qt varied from 17.3 to 25.5 mg/g, whereas within the same period for brilliant green the adsorption capacity decreased from 142.3 to 56.3 mg/g. Regarding to the effect of pH, the difference of behavior between the two colorants can be explained by considering the point of zero charge of the silica surface of the mesoporous material which is around 2-3 [35]. So, at pH above 3, the surface of mesoporous silica is negatively charged. At pH below its isoelectric point, a dye exists predominantly in the molecular form, while above the isoelectric point it exists with a higher proportion in its dissociated form. Congo red is an anionic azo dye with sulfonic acid group and, in aqueous solution, it dissociates to the sodium ion and sulfonate anion. Since the isoelectric point of congo red is 3 [36], at pH above 3, both the dye and the adsorbent are negatively charged and repulsive interactions occur between them. Higher the pH of the solution is, higher the repulsion between congo red and the silica surface is and lower the loading of dye is. The opposite effect is noted when brilliant green, a cationic dye (pKa = 4.93 and 2.62 [37]) is adsorbed onto the silica adsorbents. In that case, the dye cations are positively charged (pH > pKa) and electrostatic attractions between this dye and the adsorbent take place. Higher the pH is, higher the negatively charge density of the adsorbent is and due to the significant increase in electrostatic attraction between the colorant and the support, the adsorption capacity is strongly enhanced. The pH effect on the adsorption capacity also explains why a higher amount of brilliant green, even at pH = 3, is adsorbed on both SBA-15 and dual-mesoporous silica. Indeed, the investigated range of pH is more favorable for the

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adsorption of a cationic dye. For example, using the dual mesoporous as adsorbent, at pH = 3, the qe values are 30.8 and 61.45 mg/g for congo red and brilliant green, respectively (Table 2). Looking at the kinetic constant ks, it also appears that for a given adsorbent and at a fixed pH, the adsorption of brilliant green is faster than the one of congo red. For example, for adsorption onto the dual-mesoporous material at pH = 3, ks values are 3.3 10-3 and 7.9 10-3 g/mg.min for the anionic and the cationic colorants, respectively. Still by companying the values of ks, in a general way, for both a given dye and pH the adsorption is slightly faster onto SBA-15 than onto the dual-mesoporous material (Table 2). For the latter, the adsorption can be slowed down because of the diffusion of the dyes molecules in the two mesopores channels arrays (proportion mesopores with a smaller size). The intra-particle diffusion model, described by qt = k id0.5 + C (where kid is the intra-particle diffusion rate constant and C is a constant directly proportional to the boundary layer), has been applied to investigate the mechanism of dyes adsorption. Figure 4 shows that whatever the dyes, the adsorbents and the pH, the plot of the amounts of adsorbed dye at time t versus the square root of time are not linear over the whole time range and they can be separated into two linear portions. The initial one can be attributed to bulk diffusion. The adsorbate molecules diffuse through the solution to the external surface of the adsorbent. The second portion of the plot is due to adsorption stage where the dye entered into the pores of the silica materials [38]. However, since none of the plots depicted in Figure 4 passed the origin, we can conclude that the adsorption of both congo red and brilliant green onto the dualmesoporous silica and SBA-15 are not controlled by the intra-particle diffusion and that more than one process affect the adsorption [39].

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3.3. Equilibrium adsorption isotherms. Two isotherms models, the Langmuir one and the Freundlich one, have been used to describe the adsorption of congo red at pH =3 and pH= 10 onto both supports (Fig. 5) and the adsorption of brilliant green at pH 3 (Fig. 6). Concerning the adsorption of the cationic dye at pH 10, since the adsorption is very fast and the adsorption capacity is very high in the investigated range of concentrations, no saturation is obtained (Fig. 7), thus the experimental data reported have not been fitted, most of all the dyes molecules are adsorbed onto the materials. The Langmuir isotherm assumes a monolayer adsorption onto a surface containing a finite number of active sites. It is represented as following :

q max [C]eq [C]eq  KL where qe and qmax respectively stand for the quantity of adsorbed dye at equilibrium and the qe 

maximal monolayer adsorption capacity (mg/g). [C]eq corresponds to the concentration of the dye (mg/L) in the solution at adsorption equilibrium and KL (L/mg) is the Langmuir constant. The Freundlich isotherm takes heterogeneous systems into account and is not restricted to the formation of a monolayer. It is describes by the following equation : qe = KF C 1/n e where KF is the Freundlich constant [(mg/g) (L/mg)1/n] and 1/n is the heterogeneity factor. Isotherms parameters and the correlation coefficients (R²) are gathered in Table 3. The values of the Freundlich constant n are greater than 1, this value is in accordance with a physical adsorption process of congo red and brilliant green onto both the dual-mesoporous silica and the SBA-15. Concerning brilliant green, whatever the adsorbent, similar values of KF are obtained at pH = 3, indicating similar dye-adsorbent interactions. This is also reflected by the closed values of KL, 19.4 and 21.5 for the dual-mesoporous silica and SBA-15, respectively. Indeed, the

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Langmuir constant provides information about the affinity between the adsorbent and the dyes. Looking at the congo red at pH = 3, both KL and KF suggest a better affinity for the dual-mesoporous materials (KL= 10.8; KF = 11.4) than for SBA-15 (KL= 4.7; KF = 7.6) and the maximum adsorption capacity is also slightly higher 30.5 mg/g against 17.4 mg/g for SBA-15, this could be due to the presence of the two mesopores channel arrays, which favors the adsorption of the dye. In a general way, concerning the pH effect on the dyes adsorption, the qmax values confirm the trend observed for the kinetic study led with C0 = 150 mg/mL, i.e. the adsorption of the cationic dye is favored under alkaline conditions (Fig. 6 and Fig. 7), while the one of the anionic dye is better when the pH is decreased (Table 3). However, the decrease of qmax is less pronounced for the SBA-15 than for the dual-mesoporous silica. As reported in Table 3, in that case the monolayer adsorption capacity varies from 30.5 to 5.1 mg/g when the pH is increased from 3 to 10 against 17.4 to 15.7 mg/g for SBA-15. This difference in behavior can be explained by a better stability of SBA-15 under alkaline conditions. Indeed, it was reported by Brinker et al. [40] that between pH 5 and 10, silica undergoes many rearrangements as the dissolution rate sharply increases. It varies by three orders of magnitude when the pH passes from 3 to 8. At higher pH values, with an increase in the concentration of OH- ions (or their hydrated state, H3O2-), nucleophilic attack of the silicon atoms on the surface siloxane bonds is increasingly efficient, occurring according to the following equation: Si-O-Si + H3O2- = Si-OH + Si-O- + H2O The formed silanol groups will be attacked by H3O2- anions leading to silanodiol [=Si(OH)2] and polysilicate anions [Si-O-]. Then silanotriol [-Si(OH)3] and orthosilicic acid [Si(OH)4] will be formed respectively by the attack of [=Si(OH)2] and [-Si(OH)3] by H3O2-. Then, the recondensation of orthosilicic and polysilicate molecules will occur: Si-O- + Si(OH)4 = Si-O-Si(OH)3 + OH-

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Thus, many dissolution-reprecipitation processes will occur. In a paper dealing with the use of ordered mesoporous materials containing Mucor Miehei lipase as biocatalyst for transesterification reaction [41], we have shown that for the material prepared by using only RF8(EO)9 as template, the structure begins to collapse when the pH of the enzyme solution is increased from 3 to 10. By contrast, thanks to a higher level of silica condensation, SBA-15 is not affected by the pH variation. Since, in this work, RF8(EO)9 surfactant is mixed with Pluronic P123 to synthesize the dual-mesoporous material, we cannot exclude a partial damage of the mesostructure when the pH of the dye solution is increased. In addition for a given adsorbent comparing the adsorption of brilliant green and congo red, we can assume that contrary to what is reported for bimodal mesoporous carbon [42], here the spatial effect of dye does not play a significant role in the adsorption. Indeed, even if brilliant green has a larger molecular size than congo red (scheme 1) the maximum adsorption capacity of the dual-mesoporous for brilliant green is 59 mg/g against 30.5 mg/g for congo red (Table 3). The same trend is observed when SBA-15 is used as adsorbent. In that case, its maximum adsorption capacity is 72 mg/g for brilliant green and 17.4 mg/g for congo red (Table 3). These observations clearly show that for the investigated dyes the adsorption is governed by the interaction between the dye and the adsorbent rather than by the structure of the dye. The possible heterogeneity of adsorption has also been investigated by considering the Sips model (Fig. 8), which is a combined form of Langmuir and Freundlich isotherms, used to predict the heterogeneous adsorptions systems. qt =

q max K s C1/n e 1/n 1  K s Ce

where Ks is the Sips constant (L. mg-1) and 1/n is the Sips model exponent. When n is equal to 1, Sips isotherm equation reduces to the Langmuir equation and it implies a homogeneous adsorption process. The Sips’ isotherm parameters are given in Table 3. Looking at the R²

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values, the Sips model is the best fit isotherm to explain the adsorption of both dyes onto both the dual-mesoporous silica and the SBA-15. This confirms the heterogeneity of the adsorption process for both colorants.

3.4. Effect of surface modification on dyes adsorption Another method to tune the adsorption capacity of the materials consists in modifying their surface. Since, among the considered supports, SBA-15 is the most stable under both acidic and alkaline conditions, its surface has been functionalized with APTES to modify the charge density of this adsorbent. The adsorption isotherms, fitted with the Langmuir model, are shown for both dyes in Figure 9 and the isotherms parameters are given in Table 4. Comparing the adsorption of congo red onto un-modified (Fig. 5A) and functionalized SBA15 (Fig. 9A), a strong uptake in the adsorption is noted both at pH 3 and 10 after modification of the adsorbent. The value of the maximum adsorption capacity increases from 17.4 to 234.9 mg/g at pH = 3 and from 15.7 to 221.1 mg/g at pH = 10. Under acidic conditions, the enhancement of the anionic colorant adsorption can be attributed to more favorable interactions between the adsorbent and congo red. Indeed, at pH = 3 the surface of the SBA15-APTES is positively charged thanks to the presence of the protonated amine groups (NH3+), and a strong ionic interaction between the support and the anionic dyes occurs, resulting in a better adsorption. By contrast, the positively charged surface disfavored the adsorption of brilliant green as in that case, repulsive interactions take place between the adsorbent and the cationic dye. As shown in Table 4, qmax decreases from 72.0 to 57.7 mg/g when SBA-15 is replaced by the APTES functionalized SBA-15. Increasing the pH, less and less NH3+ groups are present at the surface, replaced by NH2 and the maximum adsorption capacity for brilliant green increases to reach 201.09 mg/g at pH 10 (Table 4). The appreciable amount of adsorption,

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when the pH is increased, suggests a strong involvement of physical force such as hydrogen bonding or Van der Waals in the adsorption process [43]. However, comparing the data reported in Figures 7 and 9B, it can be noted that for this colorant the conditions are less favorable when using functionalized SBA-15 as adsorbent instead of the bare SBA-15. Indeed, for the latter, which exhibits a negatively charged surface under alkaline conditions, the dye adsorption is governed by strong electrostatic interactions instead of weaker ones, such as hydrogen bonding. As depicted in Figure 7, for this support, in the considered range of initial concentration C0, the plateau was not reached at pH 10. Concerning congo red, and comparing with the efficiency of SBA-15 for its adsorption, when the pH increased, hydrogen bonding interactions between the oxygen nitrogen atom containing functional groups of congo red and the surface of the SBA-15 modified by the amine group (NH2) could be responsible of the enhancement of the adsorption capacity of the modified adsorbent. The variations of the maximum adsorption capacity of the mono-modal mesoporous materials for congo red and brilliant green after functionnalization supports the fact that the structure of the dye does not significantly affect the adsorption.

3.5. Performance evaluation Table 5 compares the maximum adsorption capacity of the investigated adsorbents with the one of some typical adsorption materials reported previously. Data comparison revealed that the mesoporous silica materials used in the present study are potential adsorbents for the red congo and brilliant green adsorption from aqueous solutions. Indeed, for the cationic dye, their maximum adsorption capacity is higher or in the same range of order than the one of most of the classical adsorbents. It also appears that functionalized SBA-15 is particularly efficient for the adsorption of red congo.

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Conclusion In our group, we have recently developed a simple and effective route for synthesizing, through the self assembly mechanism, these dual-mesoporous materials that have two ordered interconnected pore networks. The aim of the study reported here is to evaluate their adsorption capacity to remove both congo red, an anionic dye, and brilliant green, a cationic dye, from water. The efficiency of the adsorbents is compare with the one of a mono-modal mesoporous silica. The latter has also been efficiently functionalized by amine group. Langmuir and Freundlich models were applied to fit adsorption equilibrium data. The results show that, regardless of porosity, the bare mesostructured silicas are particularly efficient for the adsorption of the cationic dye. Indeed, in the investigated range of pH, whatever the support porosity, the adsorption of brilliant green onto the un-modified absorbent is favored and it increased with pH, whereas the one of congo red is rather limited. Results also show that for the investigated dyes the adsorption is governed by the interaction between the dye and the adsorbent rather than by the structure of the dye. In addition, by tuning the adsorbent/colorant interactions, the anionic dye adsorption can be strongly enhanced by introducing amine groups at the surface of the silica

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26

Figure Captions Scheme 1 :

Molecular structure of congo red (A) and brilliant green (B).

Figure 1 :

SAXS patterns (A) and nitrogen adsorption-desorption isotherm (B) with the corresponding pore size distribution (insert B) of the adsorbents a: dualmesoporous material, b : SBA-15 and c : functionalized SBA-15.

Figure 2 :

FTIR spectra of the bare (a) and functionalized (b) SBA-15.

Figure 3 :

Adsorption as a function of time of congo red (a) and brilliant green (b) onto dual-mesoporous material (A) and onto SBA-15 (B). The scatters correspond to the experimental data and the solid lines to the fitted curves with a pseudosecond order model. Adsorption occurs at pH = 3 (); 7() or 10 (). The initial dye concentration is 150 mg/mL.

Figure 4 :

Intra-particle diffusion kinetic for congo red (a) and brilliant green (b) adsorption onto dual-mesoporous material (A) and onto SBA-15 (B). The dash lines are just a guide for the eyes.

Figure 5 :

Adsorption isotherm at 298 K of congo red onto dual-mesoporous material () and onto SBA-15 ().The scatters correspond to the experimental data and the solid lines to the fitted curves with the Langmuir (A) or Freundlich (B) models. The dye adsorption is carried out at pH 3 (a) or 10 (b).

Figure 6 :

Adsorption isotherm at 298 K of brilliant green onto dual-mesoporous material () and onto SBA-15 ().The scatters correspond to the experimental data and the solid lines to the fitted curves with the Langmuir (A) or Freundlich (B) models. The dye adsorption is carried out at pH 3

Figure 7 :

Adsorption isotherm at 298 K of brilliant green onto dual-mesoporous material () and onto SBA-15 ().The dye adsorption is carried out at pH 10. The dash lines are just a guide for the eyes.

27

Figure 8 :

Sips adsorption isotherms at 298 K of both congo red (A) and brilliant green (B) onto dual-mesoporous material () and onto SBA-15 (). The scatters correspond to the experimental data and the solid lines to the fitted curves.The dye adsorption is carried out at pH 3 (a) or 10 (b).

Figure 9 :

Adsorption isotherm at 298 K of congo red (A) and brilliant green (B) onto functionalized SBA-15. Adsorption occurs at pH = 3 (); 7() or 10 ().The scatters correspond to the experimental data and the solid lines to the fitted curves with the Langmuir model.

28

Scheme 1

A

B

29

Figure 1

B

A

c 200 6.1 nm 5.2 nm

0.2 0.1 0.0 5 10 15 Pore diameter (nm)

20

a 2 q (nm ) -1

3

b

0.4

3

-1

dV/dD (cm /g nm )

9.1 nm

500

0.2 0.0

5 10 15 Pore diameter (nm)

20

250

0

600

a

0.4 4.0 nm

-1

3.1 nm

750

3

5.6 nm 6.4 nm 5.2 nm

0

dV/dD (cm /g nm )

b

11.2 nm

1

0.3

100

Volume adsorbed (cm3/g-STP)

Intensity (a.u.)

10.4 nm

-1

300

8.1 nm

3

dV/dD (cm /g nm )

6.1 nm 5.2 nm

c

0.4

400

10.4 nm

0.3

0.1 0.0

400

9.6 nm

0.2

5 10 15 Pore diameter (nm)

20

200 0.0

0.2

0.4

0.6

0.8

Relative pressure p/p0

1.0

30

Intensity (a.u.)

Figure 2

1635

2893

2932 3280

3365

1080 1185

b

978 3741

a 1000

2000 3000 -1 Wavenumber (cm )

4000

31

Figure 3

A

200

B

200

b

b 150

qt (mg/g)

qt (mg/g)

150 100

100

50 0

50 0

0

100

200

300

400

500

0

100

t (min)

a

20 10

400

500

a

30

qt (mg/g)

qt (mg/g)

30

200 300 t (min)

20 10

0

0 0

100

200 300 t (min)

400

500

0

100

200 300 t (min)

400

500

32

Figure 4

A

B 200

a

30

b

20

qt (mg/g)

qt (mg/g)

150

10

100

0

0 0

5

10 t

0.5

15

20

25

0

5

0.5

(min )

10 t

a

30 20 10

0

0.5

15

20

25

0.5

(min )

a

30

qt (mg/g)

qt (mg/g)

50

20 10

0 0

5

10 t

0.5

15 0.5

(min )

20

25

0

5

10 t

0.5

15 0.5

(min )

20

25

33

Figure 5

A

B

20

20

b

b 15

qe (mg/g)

qe (mg/g)

15 10 5 0

5 0

0

50

100 Ce (mg/L)

150

0

a

30 20 10

0

50

100 Ce (mg/L)

150

50

100 Ce (mg/L)

150

a

30

qe (mg/g)

qe (mg/g)

10

20 10

0 0

50

100 Ce (mg/L)

150

0

34

Figure 6

B

60

60

40

40

qe (mg/g)

qe (mg/g)

A

20

0

20

0 0

50

100 Ce (mg/L)

150

0

50

100 Ce (mg/L)

150

35

Figure 7 250 200

qe (mg/g)

150 100 50 0 0.0

0.5

1.0 1.5 Ce (mg/L)

2.0

2.5

36

Figure 8

A

B

20 b

qe (mg/g)

15 10 5 0 0

100 Ce (mg/L)

150

a

30

a

60

20

qe (mg/g)

qe (mg/g)

50

10

0

40

20

0 0

50

100 Ce (mg/L)

150

0

50

100 Ce (mg/L)

150

37

Figure 9 250

200 B

A 200 150

qe (mg/g)

qe (mg/g)

150 100

100 50

50 0

0 0

1

2 Ce (mg/L)

3

0

50

100 Ce (mg/L)

150

200

38

Tables Table 1 : Specific surface area (SBET), pore volume (Vp) and pore diameter (Ø) of the adsorbents Adsorbent Dual mesoporous material SBA-15 Functionalized SBA-15

S BET (m²/g) 832 887 359

Vp (Cm3/g) 0.82 1.07 0.57

Ø (nm) 4.0-9.6 9.1 8.1

39

Table 2 : Pseudo-second order kinetic parameters obtained for C0 = 150 mg/L qe (mg/g)

ks (g/mg.min)

R²

pH = 3 pH = 7 pH = 10 pH = 3 pH = 7 pH = 10

30.8 14.9 21.3 27.9 10.1 20.7

3.3 10-3 1.3 10-3 1.5 10-3 5.4 10-3 2.6 10-3 1.1 10-3

0.9983 0.9854 0.9938 0.9980 0.9914 0.9868

pH = 3 pH = 7 pH = 10 pH = 3 pH = 7 pH = 10

61.5 53.1 149.0 59.6 13.7 146.8

7.9 10-3 3.3 10-3 4.0 10-3 0.010 0.013 6.9 10-3

0.9998 0.9997 0.9999 0.9990 0.9985 0.9999

Congo red Bimodal

SBA-15 Brilliant green Bimodal

SBA-15

40

Table 3 : Langmuir, Freundlich and Sips isotherms constants for congo red and brilliant green adsorbed onto dual mesoporous material and onto SBA-15 at different pH

Adsorbent Bimodal Adsorption pH 3 Langmuir model KL (L/mg) 10.8 qmax (mg/g) 30.5 R² 0.8775 Freundlich model KF 11.4 (mg/g)/(L/mg)1/n N 5.5 R² 0.7719 Sips model KS (L/mg) 1.4 10-3 qmax (mg/g) 30.0 N 0.38 R² 0.9199

Congo red SBA-15 Bimodal 3 10

Brilliant green Bimodal SBA-15 3 3

SBA-15 10

4.7 17.4 0.8716

10.2 5.1 0.8792

5.1 15.7 0.9788

19.4 59 0.9956

21.5 72 0.9598

7.6

3.9

9.1

14.8

14.3

5.6 0.9147

13.2 0.8537

0.97 0.9405

3.9 0.9924

3.2 0.9325

5.1 10-2 18.0 0.57 0.9995

2.3 10-4 5.3 0.37 0.8771

5.2 10-4 14.8 0.28 0.9908

7.8 10-2 63.5 1.2 0.9949

2.3 10-5 55.9 0.28 0.9995

41

Table 4 : Langmuir isotherms constants for congo red and brilliant green adsorbed onto functionalized SBA-15 at different pH

Congo red pH = 3 pH = 10 Brilliant green pH = 3 pH = 7 pH = 10

KL (L/mg)

qmax (mg/g)

R²

0.2 0.4

234.9 221.1

0.9798 0.9620

5.5 1.0 2.0

57.7 173.8 201.1

0.8028 0.9387 0.9725

42

Table 5 : Congo red and brilliant green maximum adsorption capacities of various adsorbents

Adsorbent Dual-mesoporous silica SBA-15 Functionalized SBA-15 Kaolin Red clay RHS-MCM-41 Rice husk ash Ni-SBA-16 Chitosan hydrobeads Activated carbon Anilinepropylsilica xerogel Ca-bentonite Mesoporous activated carbons

Maximum adsorption capacity for congo red qmax (mg/g) 30.5 17.4 234.9 5.5

Maximum adsorption capacity for brilliant green qmax (mg/g) > 200 > 200 201.1 65.4 125.0 232.6 26.2 322.5

Reference

92.6 52.2

This work This work This work 12, 44 45 46 47 48 43 38

40.9

49

107.4

50

189.0

38