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Surface-function

Department of Chemistry, American Unive

El-Solh 11072020, Beirut, Lebanon. E-mail:

1-365217; Tel: +961-1-350000 ext. 4051

† Electronic supplementary informa10.1039/c4ra15504a

Cite this: RSC Adv., 2015, 5, 6111

Received 29th November 2014Accepted 12th December 2014

DOI: 10.1039/c4ra15504a

www.rsc.org/advances

This journal is © The Royal Society of C

alized silica aerogels and alcogelsfor methylene blue adsorption†

Naim Saad, Mazen Al-Mawla, Elias Moubarak, Mazen Al-Ghoul and Houssam El-Rassy*

Surface-functionalized silica aerogels and alcogels prepared via a two-step sol–gel process through the

combination of different silicon precursors were used in the adsorption of methylene blue dye

molecules from aqueous media. The effect on the adsorption in batch reactors of the nature of

precursors, the solvent used in the adsorbents synthesis, and the pH of the dye solution was monitored.

Phenyl-functionalized silica materials revealed the highest adsorption capacity. Two phenyl-modified

silica aerogels were widely tested in adsorption under various experimental conditions where the effect

of pH, temperature, contact time, initial dye concentration, and adsorbent dose were investigated. The

synthesis solvent was found to have a clear effect on the behavior of the adsorbent. Optimal conditions

were found at pH 8 and 9 where the adsorbent–adsorbate surface charge interactions and the p–p

stacking are most favourable. The adsorption followed a pseudo-second order kinetics, indicative of a

co-existing chemisorption and physisorption processes. The adsorption data fitted the Sips isotherm and

exhibited for the best aerogel a maximum adsorption capacity of 49.2 mg of dye per gram of adsorbent.

The thermodynamic study revealed the adsorption of methylene blue onto phenyl-functionalized silica

aerogels to be an exothermic and ordered adsorption process.

1. Introduction

Organic dyes are colored substances that selectively reect ortransmit incident daylight when applied to a substrate.1

Because of their contribution to water toxicity, the removal ofthese dyestuffs from effluent wastewater is rapidly becoming aprimary concern and important target from an environmentaland health perspective. Several limitations to reach this aimappear, of which is the large size of these molecules and theirhigh stability as non-biologically oxidizable entities.2 Methyleneblue (MB), for instance, is a heterocyclic aromatic dye moleculewidely used in the textile industry and in some medical prac-tices,3 although known to cause many well-documented threatsto both human beings and animals.4–6

Numerous physical, biological, and chemical conventionaltechniques were reported to show efficiency in removingorganic dyes from aqueous media. This includes adsorption,photocatalytic degradation, chemical decomposition by oxida-tion, and microbiological discoloration.7,8 Amongst theseprocesses, the physical adsorption at the solid–liquid interfacewas permanently a conspicuous technique owing to its easinessand low cost, in addition to the availability of several potential

rsity of Beirut, P.O. Box 11-0236, Riad

Houssam.Rassy@aub.edu.lb; Fax: +961-

tion (ESI) available. See DOI:

hemistry 2015

adsorbents of which are the metal oxide sol–gel materials.9 Theease in controlling the porosity and surface properties of theseadsorbents helps in tuning their affinity to specic adsor-bates. Although recent studies focused on the use ofalumina,10 silica,2,3,11,12 and titania13 sol–gel materials ingeneral as adsorbents for pollutants from wastewater, very fewreported the use of the highly interesting porous aerogels forthe treatment of contaminated wastewater with organicdyes.13–16

We investigate herein, for the rst time to our knowledge,the use of surface-modied silica aerogels as new class ofadsorbents for dyes from wastewater. These solids are synthe-sized from different combinations of silicon precursors alongwith various combinations of hydrolysis solvent. The effect ofthe adsorbate concentrations, adsorbent nature, quantity andparticle size, pH and temperature were studied in order toachieve optimal conditions for the adsorption of the dye, atwhich a complete kinetic and thermodynamic study wasconducted.

2. Experimental2.1. Materials

The chemicals were used in this study as received and withoutfurther purication. Tetramethylorthosilicate (C4H12O4Si,TMOS), tetraethylorthosilicate (C8H20O4Si, TEOS), and vinyl-triethoxysilane (C8H18O3Si, VTES) were purchased from Fluka

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Analytical. Phenyltrimethoxysilane (C9H14O3Si, PhTMS), phe-nyltriethoxysilane (C12H20O3Si, PhTES), and methylene blue(C16H18ClN3S, MB) were purchased form Acros Organics.Hydrochloric acid (HCl, 37%), sulfuric acid (H2SO4, 97%) andammonium hydroxide (NH4OH, 28.0–30%) were from Pan-reac, BDH and Fischer-Scientic, respectively. Methanol(CH3OH, 99.9%) and ethanol (C2H5OH, 99.8%) werepurchased from Sigma-Aldrich, acetone (CH3COCH3, 99%)from SureChem Products Ltd, and acetonitrile (CH3CN,99.9%) from Lab-Scan. Dimethyl sulfoxide (C2H6SO, DMSO,99.5%) and dimethylformamide (C3H7SO, DMF) were acquiredfrom Sigma. Double distilled water was prepared in ourlaboratory.

2.2. Silica gels synthesis

The experimental procedure presented in ref. 17 and 18 wasadopted in the preparation of the materials used in the currentstudy. Tetraalkoxysilanes (Si(OR)4; R¼ CH3 or C2H5) were usedalone in the preparation of the silica gels or combined withother silicon precursors (R00Si(OR0)3; R0 ¼ CH3 or C2H5; R00 ¼C6H5 or C2H3). As well as the silicon precursors change, thesynthesis solvent was also varied where methanol, ethanol,acetone, acetonitrile, DMSO, and DMF were used. The molarratio Si(OR)4 : R00Si(OR0)3 was maintained constant at 1 : 0.25whenever R00Si(OR0)3 precursors were used. In the rst step,silicon precursors were mixed in a polypropylene vial with thesolvent, double distilled water and hydrochloric acid solution(0.2 M) under magnetic stirring for 24 h. An ammoniumhydroxide solution (0.5 M) was then added and mixed togetherfor 5 min. The vials were closed and kept for gelation andaging. The former was observed in the next 2 hours and theobtained gels were kept for aging for one day at roomtemperature. Aer this period, the wet alcogels were eitherused directly in the adsorption experiments or dried undersupercritical carbon dioxide (Tc¼ 31.1 �C; Pc¼ 73.7 bar) to obtainthe silica aerogels. The supercritical drying step was preceded by a24 h solvent exchange step, where the alcogels were soaked inacetone in order to exchange the residual water and solvent in thegel with acetone exhibiting a higher miscibility with liquid carbondioxide. The nal molar ratios Si(OR)4 : solvent : H2O : HCl : NH3

were 1 : 12 : 6 : 3 � 10�3 : 6 � 10�3 for the gels where TMOSor TEOS were used alone as silicon precursors, whereasthe ratios Si(OR)4 : R00Si(OR0)3 : solvent : H2O : HCl : NH3 were0.8 : 0.2 : 12 : 6 : 3 � 10�3 : 6 � 10�3 for the other gels. Theamount of silica was chosen to be 3 mmol for all samples, thusgiving nal gel volumes of 2.5 to 3.0 mL.

2.3. Silica gels characterization

The aerogels were characterized aer supercritical dryingwhereas the characterization of the alcogels was done aersoaking them in liquid nitrogen twice, for 10 min each, fol-lowed by a freeze-drying for 24 h. This step allows theremoval of the solvent existing within the pores withoutaffecting the network structure. Nitrogen adsorption–desorption technique was performed using a Nova 2200ehigh speed surface area analyzer (Quantachrome

6112 | RSC Adv., 2015, 5, 6111–6122

instruments) to measure the specic surface area (SSA)according to the BET theory19 while the pore size and volumewere calculated by the BJH method.20 Prior to N2 adsorption,the samples were degassed for 2 h at 120 �C. The structuralcharacterization of the silica gels was carried out using aThermo Nicolet 4700 Fourier Transform Infrared Spectrom-eter equipped with a Class 1 Laser. The measurements wereperformed in the 4000–400 cm�1 range using the trans-mission KBr pellet technique. Scanning electron microscopywas carried out on a Tescan Scanning Electron Microscope athigh voltage of 30 kV aer sputter-coating the samples with athin layer of gold.

2.4. Adsorption studies

All adsorption experiments were performed in glass vials undershaking in a Julabo SW 23 controlled-temperature water bathoperating at 160 rpm in dark. In a typical adsorption experi-ment, an initial amount of adsorbent was dropped in 30 mL of a15 mg L�1 methylene blue dye solution, at set pH and temper-ature. Aliquots were carefully withdrawn at pre-determined timeintervals over 4 h, and the MB concentration was determined bymeasuring the absorbance of the solution at the maximumabsorption wavelength (l ¼ 664 nm) using a Thermo ScienticEvolution 300 UV/VIS/NIR spectrophotometer. The aliquotswere centrifuged whenever small gel particles were presentusing a Thermo Scientic Heraeus Pico 17 centrifuge. Theamount of adsorbed dye on the solids was calculated accordingto the following equation:

qe ¼ ðCi � CeÞm

V (1)

where qe is the amount of dye adsorbed at equilibrium (mg g�1),Ci and Ce are the initial and equilibrium liquid-phase concen-trations of dye (mg L�1) respectively; V is the volume of solution(L) and m is the amount of adsorbent (g).

Because it was impossible to measure the exact weight of thealcogels due to the existence of the solvent within the pores, theas-synthesized alcogels (volume approximately 2.5–3.0 mL) weredropped in the solution. These materials were used as astepping-stone to pinpoint the best precursor/co-precursor/solvent combination preceding the more specic work on aer-ogels. The aerogels were weighed accurately and their masseswere recorded before their use as adsorbents of dye fromaqueous media.

The effect on adsorption of silicon precursors, solvent usedin the synthesis of the gels, and temperature was studied. Theeffect of pH was investigated aer the adjustment of the pH ofthe methylene blue solutions using dilute H2SO4 and NH4OHsolutions and a Corning Pinnacle 542 pH conductivity meterwith a combined pH electrode. In order to evaluate the effect ofthe size of the adsorbing material on its efficiency crushed anduncrushed adsorbents were compared. The initial dye concen-tration was studied in the range between 1 and 300 ppm, andthe dosage of the silica aerogels was investigated in the 20 to600 mg range.

This journal is © The Royal Society of Chemistry 2015

Fig. 1 SEM images of TEOS-PhTMS aerogel synthesized inacetonitrile.

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3. Results and discussion3.1. Adsorbents characterization

Nitrogen adsorption–desorption isotherms were performed onprepared alcogels and aerogels. However, the data corre-sponding to the alcogels is not discussed here as we believe thatthe surface properties and porosity are very similar for bothmaterials with the latter being only lled with the solvent andliquids used in its synthesis. The aerogels exhibited type IVisotherms as per the IUPAC classication of isotherms,21,22

typical for mesoporous materials with H2 hysteresis loopsassociated with the occurrence of pore condensation, or “inkbottle” pores.18,23

The FTIR spectra performed for phenyl-functionalized andnon-functionalized silica aerogels revealed a noticeable differ-ence attributed to the existence of phenyl groups in the solidnetwork. Table 1 shows the vibration frequencies obtained forboth categories of aerogels where characteristic peaks of surfacephenyl groups appear for functionalized samples in contrast tothe non-functionalized ones.17 This indicates the successfulsurface phenyl-functionalization whenever phenyl-trialkoxysilane precursors are used in the synthesis. Charac-teristic vibrations corresponding to the bulk siliceous structureappear for both categories without any noticeable difference.

Scanning electron microscopy (SEM) micrographs reveal theporous structure of the various phenyl-modied aerogels wheresilica aggregates and large interconnected pores appear. Themicrographs clearly show a “spongy” porous structured mate-rial, conrming the high porosity of the obtained silica aero-gels. SEMmicrographs obtained for a representative aerogel areshown in Fig. 1.

3.2. Effect of pH and silicon precursors on adsorption

Determining the effect of the surface functionalization of silicagels and the variation of the silicon precursors on the adsorp-tion of methylene blue was studied. Three different gels madeaccording to the procedure described in previous sections were

Table 1 Characteristic vibration frequencies (cm�1) in FTIR spectra of diffaerogels

Non functionalized gels Phenyl-functionalized gels T

3460 3440 O

2910 2900 ns2840 2830 na1630 1630 d

1420 nC�1200 1130 na1070 1070 na951 947 nb789 785 ns

739 u

696 F

531 544 nS�476 F

457 450 d

This journal is © The Royal Society of Chemistry 2015

synthesized in order to understand the correlation between MBadsorption and surface functionalization of the silica gels inwhich TEOS was solely used as silicon precursor or coupled withVTES or PhTES as co-precursors. Methanol was used for all gelsas synthesis solvent. A study of the pH effect on the adsorptionof the dye onto those three alcogels was conducted at 30 �Cwhere the pH was varied between 2 and 10. It is worthmentioning at that point that, as mentioned in Section 2.4, alladsorption tests were performed in dark in order to eliminateany doubt of MB photodegradation being responsible of the MBdecrease in themedium. Furthermore, MB solutions kept underthe same conditions without the existence of any adsorbent,showed no changes in the MB concentration over time.

The difference between alcogels and xerogels was alsoinvestigated. Alcogels were used aer aging at room tempera-ture for 48 h whereas the xerogels were le for 10 days beforeuse resulting in denser, shrunk, glass-like solids. As expected,the xerogels showed very little adsorptive capacity with respectto the alcogels due to the decrease in their porosity; thereforestudying the adsorption capacity of the xerogels was found notto be of interest. The UV-Vis measurements showed anincreasing percent removal of the dye with increasing pH aer4 hours, with a maximum adsorption at pH 8 and 9 (Fig. 2). The

erent synthesized non-functionalized and phenyl-functionalized silica

ype of vibration Structural Unit

–H and SiO–H H–O–H/H2O and ^SiO–H/H2O

C–H –CH3

sC–H –CH2

H–O–H H–O–H]C + dC–H in plane 2nSi–C Si–PhsSi–O–Si (LO mode) ^Si–O–Si^sSi–O–Si (TO mode) ^Si–O–Si^Si–O ^Si–OSi–O ^Si–O–Si^d,gC–H Si–PhC–H Si–Phi–O SiO2 defectsC–H Si–PhO–Si–O –O–Si–O–

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surface functionalization of the silica alcogels also had a crucialimpact on the adsorptive capacity with the TEOS-PhTES alcogelbeing the best at almost all pHs, with clear maxima at pH 8and 9.

The difference in the adsorption capacity at different pHscan easily be attributed to the surface charge of the adsorbent.The point of zero charge (PZC) of silica is around 2.5,24 meaningthat for lower pH it exhibits a positive zeta potential resulting ina positively charged surface of the adsorbent. This has a nega-tive impact on the dye adsorption due to the electrostaticrepulsion between the adsorbent surface and the positivelycharged methylene blue monomer MB+. Following the samereasoning, the adsorption capacity of the solids should behigher with an increase in the pH due to the abundance ofnegative surface charges on the adsorbent which favors theelectrostatic attraction between the negatively charged Si–O�

surface groups and the positively charged MB+ dye monomer.On the other hand, the difference in the adsorption capac-

ities of the three different silica alcogels can be explained by thedifference in the intermolecular forces between the differentsurface groups and the dye monomer. Fig. 3 shows the surfaceof the gels, where TEOS is solely used as silicon precursor (a), orcoupled with VTES (b) and PhTES (c) as co-precursors. WhenTEOS alcogels are used in adsorption, the surface silanol groups(Si–OH) make for a strong hydrogen bonding between the geland the MB+ dye monomer, whereas in the case of TEOS-VTESalcogels, the presence of vinyl surface groups in addition tosilanols allow for an additional good p electrons attractions andthus a superior attraction between the dye molecules and thealcogel surface groups. The aromatic rings on the surface ofphenyl-functionalized gels (TEOS-PhTES) offer a superiordegree of delocalization due to the p–p stacking of the phenylsurface groups and the aromatic rings of the dye molecule.Thus, comparing these three surface–dye interactions, the latestwas expected to show the highest adsorption capacity; this wasconrmed experimentally where we found the phenyl-

Fig. 2 (a) % removal of methylene blue as a function of time for TEOS, TInitial methylene blue concentration ¼ 15 mg L�1; contact time ¼ 4 h;methylene blue onto silica alcogels. Initial methylene blue concentration

6114 | RSC Adv., 2015, 5, 6111–6122

functionalized alcogel to be the best adsorbent in the wholepH range whereas the non-functionalized solid exhibited theweakest adsorption capacity. Based on these results, phenyl-functionalized gels were selected for adsorption study in thefollowing sections.

These ndings conrm the superiority of phenyl-functionalized silica adsorbents recently reported in the litera-ture for various applications.25–28 However, the current work isunique in terms of use of phenyl-functionalized silica aerogelsfor dye adsorption, which are reported herein for the rst time.

3.3. Effect of synthesis solvent

In order to explore the full potential of the phenyl co-precursor,alcogels with combinations of TMOS, TEOS, PhTMS and PhTESas precursors were synthesized. Along with these precursorscombinations, various solvents such as methanol, ethanol,acetone, acetonitrile, DMSO, and DMF were used in thesynthesis of the gel. The experiment showed that only three ofthese solvents resulted in successful gelation of the sol, namelymethanol, acetone and acetonitrile. Consequently, the rest ofthe study was conducted with gels prepared with one of thesethree solvents, while the others were discarded. A comprehen-sive listing of the synthesized gels is presented in Table 2.

Adsorption experiments were conducted on each of theaforementioned 18 alcogels at pHs between 4 and 10 in order toreassert the pH effect in combination with the different solventsand precursor combinations, as a rst step for the forthcomingwork on aerogels in this study. The pH range was chosen so thatthe methylene blue is in its cationic monomeric form (MB+) andthe silica solids are above their PZC.

The solvent was found to play a crucial role in the resultingadsorptive capacities of the gels (Fig. 4), implying a directcorrelation between the surface properties of the materials andthe polarity of the used solvent. Out of the three solvents beingstudied, the aprotic acetonitrile is known to be the most polar,followed closely by the protic methanol and the aprotic

EOS-VTES and TEOS-PhTES alcogels synthesized in methanol solvent.solution volume ¼ 30 mL, pH ¼ 6. (b) Effect of pH on adsorption of¼ 15 mg L�1; contact time ¼ 4 hours; solution volume ¼ 30 mL.

This journal is © The Royal Society of Chemistry 2015

Fig. 3 Surface-functionalized silica gels. (a) TEOS used as only silicon precursor, (b) TEOS-VTES co-precursors, (c) TEOS-PhTES co-precursors.

Table 2 Precursor/co-precursor/solvent composition of the differentsurface-modified silica gels

Sample Composition Solvent

M1 TMOS MethanolM10 TMOS AcetoneM100 TMOS AcetonitrileM2 TMOS-PhTMS MethanolM20 TMOS-PhTMS AcetoneM200 TMOS-PhTMS AcetonitrileM3 TMOS-PhTES MethanolM30 TMOS-PhTES AcetoneM300 TMOS-PhTES AcetonitrileE1 TEOS MethanolE10 TEOS AcetoneE100 TEOS AcetonitrileE2 TEOS-PhTMS MethanolE20 TEOS-PhTMS AcetoneE200 TEOS-PhTMS AcetonitrileE3 TEOS-PhTES MethanolE30 TEOS-PhTES AcetoneE300 TEOS-PhTES Acetonitrile

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acetone.29 These characteristics play an important role in thesynthesis process, having an impact on the precursors' hydro-lysis and condensation rate constants kH and kC, respectively.9,30

During hydrolysis, it is known that kH increases linearly with theconcentration of [H+] in the medium.31 Moreover, it was alsoshown through 29Si NMR that the solvent has a secondaryimportant effect on the hydrolysis rate constant, being thehighest in acetonitrile, then in order of decreasing kH: meth-anol, dimethylformamide, dioxane, and formamide.31

The nature of the solvent not only plays a role in thehydrolysis step, but it also affects the sol condensation andgelation. However, scientists seem to agree that a trend existsdictating that the larger the size of the solvent molecule is, thelonger is the gelation time.30 Brinker and Scherer9 argue thatpolar protic solvents, such as methanol in the current study,offer the possibility of hydrogen bonding to the anionic nucle-ophile SiO� involved in the condensation reaction thus makingit less nucleophilic. This is not the case for polar aproticsolvents such as acetone and acetonitrile. However, due to theirrelatively high polarity, this nucleophile is stabilized withrespect to the activated complex, which results in slowing downthe reaction.

Aer a thorough analysis of the signicant amount of resultsobtained, the study was narrowed down to nine siliconprecursors-solvent combinations to be tested as aerogels in

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adsorption at the best 2 pHs. The selection was made based onthe results obtained at the best two pHs (8 and 9) where weselected the best nine solids that exhibit the highest percentadsorption aer 4 h of contact time between the adsorbent andadsorbate. The selected aerogels are: TMOS-PhTMS inmethanol(M2) and acetonitrile (M200), TMOS-PhTES in methanol (M3),TEOS in acetone (E10), TEOS-PhTMS in methanol (E2), acetone(E20), and acetonitrile (E200), TEOS-PhTES in methanol (E3) andin acetonitrile (E300). The difference in the adsorptive capacity ofthe studied aerogels (Fig. 4) indicates that the synthesis solventstill plays a crucial role in the adsorption process, even aer ithas been completely removed during the CO2 supercriticaldrying step. The comparison of the obtained adsorption resultsrevealed one silicon precursors combination (TEOS-PhTMS) tobe the best at both pHs, where the synthesis solvent wasacetonitrile for the best aerogels at pH 8 (E200) whereas acetonewas used in the synthesis of the aerogel having the highestadsorption capacity at pH 9 (E20).

3.4. Effect of crushing the gels on adsorption

The effect on adsorption of crushing or uncrushing the gel wasstudied for alcogels and aerogels alike. To do so, TEOS-PhTMSin acetonitrile (E200) and TEOS-PhTMS in acetone (E20) wereprepared as alcogels and aerogels to be used in adsorption.The rst adsorbent was tested at pH 8 whereas the second wasused at pH 9 according to the results reported in the previoussections. To study the effect of crushing the adsorbent (eitheralcogel or aerogel) on its adsorptive capacity, the gels wereeither dropped whole in the solution or crushed into smallpieces (powder in the case of aerogels). The masses of theadsorbents were kept constant in the adsorption experimentswhere 100 mg of the aerogels were used. However since it isextremely difficult to get an accurate mass reading for thealcogel due to the large amount of solvent and water present inthe gel matrix (TGA measurements were not possible becauseof the fast evaporation of the solvents from the alcogel pores),the full amount of gel prepared in one propylene vial was usedper experiment. The results showed that the physical aspect ofthe adsorbent, crushed or uncrushed, has a very pronouncedeffect on the adsorptive capacity of both alcogels and aerogels(Fig. 5), with the effect being more noticeable when aerogelsare used. Crushing the gel affects the kinetics of the adsorp-tion process, making it signicantly faster as well asincreasing the nal adsorptive capacity of the gel at equilib-rium. This behavior is due to the drastic increase of thecontact surface area in the case of crushed adsorbents making

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Fig. 4 % removal of methylene blue after 4 h for alcogels synthesized in different solvents with different silicon precursors, at different pHs: (a)pH¼ 4; (b) pH¼ 5; (c) pH¼ 6; (d) pH¼ 7; (e) pH¼ 8; (f) pH¼ 9 and (g) pH¼ 10. Initial methylene blue concentration¼ 15 mg L�1; contact time¼4 hours; solution volume ¼ 30 mL.

6116 | RSC Adv., 2015, 5, 6111–6122 This journal is © The Royal Society of Chemistry 2015

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Fig. 5 Comparison of the % adsorption of E200 at pH 8 and E20 at pH 9crushed and uncrushed (a) alcogels and (b) aerogels at t ¼ 5 min andt ¼ 4 hours. Initial aerogel dose ¼ 100 mg; initial methyleneblue concentration ¼ 15 mg L�1; contact time ¼ 4 hours; solutionvolume ¼ 30 mL.

Fig. 6 Plot of the effect of initial concentration variation, relating boththe equilibrium adsorptive capacity (left y-axis, in mg g�1) and the %adsorption after 4 hours (right y-axis) to the initial concentration ofmethylene blue solution (mg L�1). Initial aerogel dose ¼ 100 mg;contact time ¼ 4 hours; solution volume ¼ 30 mL.

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the adsorption sites more easily accessible by the methyleneblue molecules; a very simple yet very important observation.The comparison of the adsorption capacities of crushed aer-ogels to that of crushed alcogels revealed a much higheradsorption for the former adsorbents. This can be due to theeasier diffusion of the dye molecules within the free pores ofthe aerogel whereas the diffusion is not easy when the poresare lled with the solvent and water used in the alcogelsynthesis. Accordingly, aerogels aer being crushed wereselected to conduct the adsorption experiments in thefollowing sections.

3.5. Effect of initial methylene blue concentration

The methylene blue adsorption capacity of the best twoaforementioned aerogels has been studied as a function of thedye initial concentration. The methylene blue solutionconcentration was adjusted between 1 and 300 ppm, the pH tooptimal conditions (8 for E200 aerogel and 9 for E20 aerogel),and the temperature was set at 30 �C. The MB concentrationswere found to be almost unchanged aer 4 h of experiments,thus the reactions are considered to reach equilibrium aer

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4 h of contact time. It is worth mentioning that the studiedmethylene blue concentrations were below the dye solubilitylimit in water, which implies that the decrease in dyeconcentration over time is due solely to its adsorption on thesurface of the gel and not to the recrystallization of the MBsalt. Fig. 6 shows the change of percent adsorption of methy-lene blue at equilibrium while increasing the initial dyeconcentration, as well as the adsorptive capacity qe of bothaerogels. An interesting observation is that even though theE200 aerogel at pH 8 presents a higher percent adsorptionespecially at relatively low concentrations, the equilibriumadsorption capacity qe has been found to be very similar forboth aerogels at relatively low concentrations, whereas forconcentrations greater than 100 ppm, a clear detachment ofthe two curves can be observed. For instance, at 300 ppm, thedifference is even more pronounced with the E20 aerogel atpH 9 showing a much higher adsorptive capacity than the E200

aerogel at pH 8 (39.1 and 29.7 mg g�1, respectively).

3.6. Methylene blue adsorption isotherms

Collected adsorption data at various methylene blue concen-trations ranging between 1 and 300 ppm were tted with sevenadsorption models, namely Langmuir,32 Freundlich,33 Tem-kin,34 Dubinin–Radushkevich,35 Redlich–Peterson,36 Toth,37 andSips38,39 isotherm models.

The Langmuir isotherm equation is represented by:

qe ¼ qmaxKLCe

1þ KLCe

(2)

where qmax is the monolayer adsorption capacity of the adsor-bent (mg g�1) and KL is the Langmuir equilibrium constant(L mg�1).

The Freundlich isotherm is represented by the equation:

qe ¼ KFCe1/n (3)

where n and KF are the heterogeneity factor (generally greaterthan 1 and an indication of the linearity of the adsorptionisotherm; the greater the value the more non-linear the

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Table 3 Langmuir, Freundlich, Temkin, Dubinin–Radushkevich, Red-lich–Peterson, Toth and Sips isotherm constants compiled for E200 andE20 aerogels at their respective optimum pHs

Langmuir isotherm

qmax (mg g�1) KL (L mg�1) R2

E200 aerogel pH 8 43.36 0.009723 0.984E20 aerogel pH 9 68.15 0.005269 0.992

Freundlich isotherm

n KF (L g�1) R2

E200 aerogel pH 8 1.781 1.443 0.938E20 aerogel pH 9 1.475 0.9552 0.971

Temkin isotherm

bT (J mol�1) KT (L g�1) R2

E200 aerogel pH 8 392 0.2677 0.887E20 aerogel pH 9 328.5 0.2343 0.854

Dubinin–Radushkevich isotherm

b (mol2 kJ�2) qmax (mg g�1) E (kJ mol�1) R2

E200 aerogel pH 8 0.0002628 28.86 43.62 0.953E20 aerogel pH 9 0.0004606 36.52 32.95 0.949

Redlich–Peterson isotherm

KR (L g�1) aR (L mg�1)bR bR R2

E200 aerogel pH 8 0.2969 9.33 � 10�5 1.754 0.987E20 aerogel pH 9 0.277 2.03 � 10�5 1.922 0.988

Toth isotherm

Kt (L g�1) at t R2

E200 aerogel pH 8 9056 279.2 0.5588 0.991E20 aerogel pH 9 2436 375.2 0.6665 0.994

Sips isotherm

qmax as ns R2

E200 aerogel pH 8 33.69 0.01641 1.47 0.994E20 aerogel pH 9 49.18 0.00997 1.337 0.997

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isotherm and the greater the heterogeneity of the system) andthe Freundlich constant (L g�1) respectively, both are temper-ature dependent.

The Temkin isotherm equation is represented by:

qe ¼ RT

bTln Ce þ RT

bTlnKT (4)

where R is the universal gas constant (8.3145 J mol�1 K), T is theabsolute temperature, bT is the variation of the adsorptionenergy (J mol�1) and KT is the equilibrium binding constant(L g�1).

Similar to the Langmuir isotherm but without assuming ahomogeneous surface or constant sorption potential, theDubinin–Radushkevich (D–R) equilibrium isotherm is repre-sented by the following equation:

qe ¼ qmax exp(�b32) (5)

where qmax is the D–R monolayer capacity (mg g�1), b is aconstant related to the adsorption energy (mol2 kJ�2), and 3 isthe Polanyi potential related to the equilibrium concentrationas follows:

3 ¼ RT ln

�1þ 1

Ce

�(6)

D–R isotherm allows for the determination of the mean freeenergy of adsorption (E, J mol�1), which can be calculatedaccording to the following relationship:

E ¼ 1ffiffiffiffiffiffi2b

p (7)

A slightly more complex isotherm is the Redlich–Peterson(R–P) equilibrium isotherm that can be expressed as follows:

qe ¼ KRCe

1þ aRCebR

(8)

where KR and aR are Redlich–Peterson isotherm constants(L g�1 and (L mg�1)bR respectively, and bR is the R–P isothermexponent. It is worth mentioning that this isotherm is a hybridmodel featuring Langmuir and Freundlich isotherms. At lowconcentrations, the Redlich–Peterson isotherm approximatesthe ideal Langmuir model, while at high concentration itbehaves similarly to the Freundlich isotherm.40

Derived from potential theories, the Toth isotherm can beexpressed by:

qe ¼ KtCe

ðat þ CeÞ1=t(9)

where Kt (L g�1) and at are the Toth isotherm constants and t isthe Toth isotherm exponent. The main premise of that equi-librium isotherm is that it assumes that most sites have asorption energy less than the mean value.41

The last isotherm that the experimental data were tted withwas the generalized Sips equilibrium isotherm, expressed by thefollowing equation:

6118 | RSC Adv., 2015, 5, 6111–6122

qe ¼ qmaxðasCeÞns1þ ðasCeÞns (10)

where as is the Sips isotherm constant and nS is the Sipsisotherm exponent.

The Langmuir, Freundlich, Temkin, Dubinin–Radushke-vich, Redlich–Peterson, Toth and Sips isotherm constants werecomputed by tting the experimental data with the corre-sponding model. These constants as well as their non-linearcorrelation coefficients (R2) are regrouped in Table 3. The datacomparison shows clearly that the three-parameter Sips equi-librium isotherm represents the best t of the experimentalresults for both adsorbents. Theoretically calculated maximummonolayer capacities (qmax) from Sips isotherm model werefound to be 33.7 mg g�1 for E200 aerogel in a methylene bluesolution at pH 8, and 49.2 mg g�1 for E20 aerogel in a pH 9 dyesolution.

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3.7. Effect of adsorbent dose

The effect of the quantity of aerogel used for adsorption wasstudied at 30 �C. Adsorbent masses ranged between 20 and 600mg per experiment. The pH was adjusted to the optimal values 8(for E200 aerogel) and 9 (for E20 aerogel). UV-VIS spectroscopicmeasurements showed that the maximum adsorption valuesaer 4 hours were obtained at doses of 50 and 100 mg ofadsorbent for both gels.

The plot of the relative amount of dye qe (in mg of dye per g ofadsorbent) versus the quantity of adsorbent shows a decrease inqe as the quantity of adsorbent increases (Fig. 7a). The plot of1/qe versus the adsorbent quantity gave a linear plot (Fig. 7b),which reveals that adsorption sites are available when largeadsorbent quantities are used as well as when small quantitiesare used.13 For instance, 50mg of TEOS-PhTMS in acetonitrile atpH 8 and 50 mg of TEOS-PhTMS in acetone at pH 9 are able toremove 94 and 91% of methylene blue molecules existing in30 mL of a 15 mg L�1 dye solution, respectively.

3.8. Adsorption kinetics

The adsorption kinetics were studied in the 30–60 �C temper-ature range. However, because the results obtained are similar,we selected to present only those corresponding to the experi-ments performed at 30 �C. The dye adsorption kinetics wasmonitored over 4 h and the experimental data were tted to fourkinetic models, namely rst, second, pseudo-rst, and pseudo-second orders.

The integrated rst order equation is

Ct ¼ Cie�k1t (11)

with Ct being themethylene blue concentration (mg L�1) at timet (min), Ci the initial MB concentration (mg L�1) and k1 the rstorder rate constant (min�1).

The integrated second order equation is:

1

Ct

¼ 1

Ci

þ k2t (12)

with k2 being the second order rate constant (L mg�1 min�1).The pseudo-rst order equation can be expressed as:

Fig. 7 (a) Plot of qe vs. m; (b) plot of 1/qe vs. m for E200 at pH 8 and E20 atpH 9 aerogels. Initial aerogel dose ¼ 20–600 mg; initial methyleneblue concentration ¼ 15 mg L�1; contact time ¼ 4 hours; solutionvolume ¼ 30 mL.

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qt ¼ qe(1 � e�k10t) (13)

where qe is the adsorption capacity of the silica aerogel atequilibrium (mg g�1), qt is the amount of dye adsorbed (mg g�1)at time t (min) and k10 is the pseudo-rst order rate constant(min�1).

The pseudo-second order equation can be written as follows:

qt ¼ k20qe2t

1þ k20qet(14)

where k20 is the pseudo-second order rate constant (g mg�1

min�1).The linear tting of the results to the rst and second order

models, and the non-linear tting to the pseudo-rst andpseudo-second models (Fig. 8) indicate beyond doubt that thepseudo-second order model ts perfectly with the experimentaldata with correlation coefficients (R2) greater than 0.9968. Thisresult gives some insight into the adsorption mechanism, sug-gesting that the adsorption depends on the adsorbate as well asthe adsorbent, involving both chemisorption and physisorptionprocesses.

3.9. Effect of temperature

Adsorption experiments were performed at 4 different temper-atures: 30, 40, 50 and 60 �C in order to study the effect oftemperature on the MB removal. The activation energy (Ea) forthe adsorption of dye onto the surface of selected aerogels wascalculated from the rate constants (k2) obtained for the reac-tions performed at each temperature. Arrhenius equation wasused for this purpose:

Fig. 8 (a) Linear first order; (b) linear second order; (c) non-linearpseudo-first order and (d) non-linear pseudo-second order kineticsplots for adsorption of methylene blue by E200 at pH 8 and E20 at pH 9aerogels. Initial aerogel dose ¼ 100 mg; initial methylene blueconcentration¼ 15mg L�1; contact time¼ 4 hours; solution volume¼30 mL.

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Fig. 10 Plot of ln KC vs. 1/T for E200 at pH 8 and E20 at pH9 aerogels.

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k ¼ Ae�Ea/RT (15)

where A is the Arrhenius frequency factor, R is the ideal gasconstant (8.314 J mol�1 K�1) and T is the adsorption tempera-ture (K). Plotting ln k2 vs. 1/T (Fig. 9) allowed us to calculate thevalue of Ea which was found to be 49.4 kJ mol�1 for E200 at pH 8,and 32.2 kJ mol�1 for E20 at pH 9. Both values exist at theinterface between physisorption (5–40 kJ mol�1) and chemi-sorption (40–800 kJ mol�1) ranges42 which suggests that bothadsorption phenomena do take place, a revelation that is inexcellent agreement with what was discussed in previoussections.

Initial aerogel dose ¼ 100 mg; initial methylene blue concentration ¼15 mg L�1; contact time ¼ 4 hours; solution volume ¼ 30 mL.

3.10. Thermodynamic study

The change in Gibbs free energy DG� of the adsorption can becalculated according to the following equation:

DG� ¼ �RT ln KC (16)

where KC is the equilibrium constant, calculated as the ratio ofthe concentration of dye on adsorbent at equilibrium (qe) to theremaining concentration of dye at equilibrium (Ce). The equi-librium was considered attained aer 4 hours.

KC ¼ qe

Ce

(17)

It is worth mentioning that a slight decrease in the value ofKC was noted with increase in temperature, indicating that theadsorption is thermodynamically stable.43

On the other hand, DG� can also be calculated from thefollowing equation:

DG� ¼ DH� � TDS� (18)

Fig. 9 Plot of ln k vs. 1/T for E200 at pH 8 and E20 at pH 9 aerogels. Initialaerogel dose ¼ 100 mg; initial methylene blue concentration ¼ 15 mgL�1; contact time ¼ 4 hours; solution volume ¼ 30 mL.

6120 | RSC Adv., 2015, 5, 6111–6122

where DH� is the change in the enthalpy of adsorption and DS�

is the change in the entropy. The combination of eqn (16) and(18) leads to the Van't Hoff equation which can be expressed asfollows:

ln KC ¼ � DH�

R

1

Tþ DS�

R(19)

Plotting ln KC vs. 1/T allows the calculation of DH� and DS�

from the slope and the intercept of the linear plot, respectively(Fig. 10). The results show that the enthalpy of adsorption DH�

was �86.9 kJ mol�1 and DS� was �264.6 J mol�1 K�1 for E200 atpH 8, and �17.6 kJ mol�1 and �51.1 J mol�1 K�1, respectivelyfor E20 at pH 9. The standard free energy change DG� wascalculated, using eqn (19), to be �6.7 kJ mol�1 for the rst and�2.1 kJ mol�1 for the second at 30 �C. These value indicate thatthe adsorption is spontaneous at that temperature, the negativevalue of DH� is a reection of an exothermic process and afurther indication that adsorption is favored at low tempera-ture, and the negative entropy suggests that the methylene bluemolecules were orderly adsorbed on the surface of the silicaaerogel. The negative values of DH� and DS� indicate thatthe adsorption will be spontaneous even at lowertemperatures.44

4. Conclusion

A complete and detailed adsorption study of methylene bluefrom a simulated wastewater onto highly porous phenyl-functionalized silica aerogels was performed where theeffect of various parameters was tested and resulted insuggestions of kinetic and thermodynamic models for theprocess. The characterization of these materials revealed theirmesoporous character and that functionalization by addingsurface functional groups without altering the core structureof the gel was successful. The study of various siliconprecursors showed that the combination with the highestadsorption capacity was TEOS-PhTMS, as the phenyl surfacefunctional group offers a high degree of delocalization, withp–p stacking of the aromatic rings on the surface of the geland the aromatic groups of the methylene blue dye. Varying

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the solvent in which the gel is synthesized had a notable effecton its adsorption capacity, however no clear trend could befound. Adsorption studies showed that these surface func-tionalized silica aerogels are highly efficient methylene blueadsorbents. Out of this preliminary all-inclusive study, twoaerogels and two pHs came out on top and proved to be thebest adsorbents: TEOS-PhTMS in acetonitrile solvent at pH 8and TEOS-PhTMS in acetone solvent at pH 9. Extremely fastadsorption of the dye was noticed within the rst 30 minutesof the experiment (more than 70% of the initial dye amounton 100 mg of gel). Adsorbent bulk size has proven to be acrucial factor in the adsorption, as crushing the aerogelparticles into a ne powder made the adsorption extremelyfaster due to the increase in contact surface area between thegel and the dye solution. The experimental adsorption datatted with theoretical models met the Sips adsorptionisotherm model and showed that the TEOS-PhTMS in aceto-nitrile at pH 8 gel is able to adsorb up to 33.7 mg of methyleneblue solution per gram of adsorbent, and that the TEOS-PhTMS in acetone at pH 9 gel is able to adsorb up to 49.2mg of dye per gram of adsorbent. The kinetics of the experi-ments showed that they follow a pseudo-second order kineticmodel indicating the concomitance of chemisorption andphysisorption in the adsorption process. The thermodynamicstudy revealed an exothermic and ordered adsorptionprocess. In conclusion, this study showed the phenyl-functionalized silica aerogel to be a highly efficient adsorb-ing material for methylene blue, highly customizable, ne-tunable and economically very attractive.

Acknowledgements

The authors gratefully acknowledge the nancial support of thiswork by the Lebanese National Council for Scientic Research(CNRS) through a research grant. The authors are also thankfulfor the Kamal A. Shair Central Research Science Lab (KAS CRSL)of the Faculty of Arts and Sciences at AUB.

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