Desalination 258 (2010) 128–135

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Desalination

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Silica grafted with a silsesquioxane containing the positively charged1,4-diazoniabicyclo[2.2.2]octane group used as adsorbent for anionic dye removal

Debora S.F. Gay, Thais H.M. Fernandes, Camila V. Amavisca, Natali F. Cardoso, Edilson V. Benvenutti,Tania M.H. Costa, Eder C. Lima ⁎Institute of Chemistry, Federal University of Rio Grande do Sul (UFRGS), Av. Bento Goncalves 9500, Postal Box 15003, ZIP 91501-970, Porto Alegre, RS, Brazil

⁎ Corresponding author. Tel.: +55 51 3308 7175; faxE-mail addresses: eder.lima@ufrgs.br, profederlima@

0011-9164/$ – see front matter © 2010 Elsevier B.V. Adoi:10.1016/j.desal.2010.03.026

a b s t r a c t

a r t i c l e i n f o

Article history:Received 20 August 2009Received in revised form 8 March 2010Accepted 11 March 2010Available online 10 April 2010

Keywords:SilsesquioxaneAdsorptionNonlinear fittingOrganofunctionalized silica

The present study reports the development of an alternative adsorbent, based on silica modified with a watersoluble silsesquioxane containing a double charged 1,4-diazoniabicyclo[2.2.2]octane, employed for removal ofthedyebrilliant yellow fromaqueous solution. Thedye removal ability of this adsorbentwasdeterminedbybatchadsorption procedure. Effects such as pH value and adsorbent dosage on the adsorption capacity were studied.Four kineticmodelswere applied. The adsorptionwas bestfitted to a fractionary-order kineticmodel. The kineticdata were also adjusted to an intra-particle diffusion model resulting three linear regions, indicating that theadsorption kinetics followsmultiple sorption rates. The equilibriumdatawerefitted to Langmuir, Freundlich, Sipsand Redlich–Peterson isotherm models. The maximum adsorption capacity was 54 mg g−1.

: +55 51 3308 7304.gmail.com (E.C. Lima).

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© 2010 Elsevier B.V. All rights reserved.

1. Introduction

Industrial activity generates large volumes of hazardous species intheir waste water effluents [1]. Among these species, dyes representan undesirable class of compounds requiring special treatment, due tothe fact that the presence of these compounds in water reduces lightpenetration, precluding photosynthesis of the aqueous flora [2]. Inaddition, other features associated to dyesmay affect humans, causingallergies, dermatitis, skin irritations, cancers and mutagenicity [3–5].On the other hand, colored waters are aesthetically objectionable fordrinking and other normal purposes [6]. Therefore, purification isrequired before releasing industrial effluents containing traces of dyesinto the environment [2,6]. Brilliant yellow is an azo organic dye,mainly used in printers ink cartridges for coloring paper. Thereforethere is a real interest in its removal from aqueous solutions.

Several methods have been developed for synthetic dye removalfrom waters and waste waters aiming to decrease their impact on theenvironment. The most inexpensive and efficient procedure is basedon adsorption, with the dye species being transferred from the watereffluent to a solid phase, decreasing its concentration in the effluent toa minimum amount [7,8]. Afterwards, the adsorbent could be eitherregenerated or kept in a dry place away from the environment.

Different adsorbents have been proposed for the removal of dyesfrom aqueous solutions [9–12]. Among these sorbents, silica powdersmodified with organic groups, are being successfully employed in

removal process [13–15]. The silica supports do not shrink or swellsuch as polymeric resins and biomaterials [6,9], present mechanical,thermal and chemical resistances and may be used for several pre-concentration cycles, since the adsorption process is reversible [15].

Organofunctionalized silica materials are usually obtained bygrafting reactions between the silica surface silanol groups andorganosilanes acting as molecular precursor reagents. The resultingmaterials present the organic component covalently bonded on thesilica surface [16–18]. Although there are several commercial organo-silanes available which have been used for more than two decades, anew class of compounds, called silsesquioxanes, appeared in recentyears. They are obtained from polycondensation reactions of organo-silanes using sol–gel process [19–22]. The silsesquioxanes differ fromother hybrid materials since they are formed by oligomeric species.Silsesquioxanes are formed in polycondensation reactions withoutusing inorganicmolecular precursors like TEOS (tetraethylorthosilicate)or TMOS (tetramethylorthosilicate) [22]. The synthesis of silsesquiox-anes containing charged organic groups and presenting new propertieslike water solubility in addition to the oligomeric characteristics wasrecently reported [22–25]. This additional feature allows the immobi-lization of silsesquioxanes in solid surface by grafting reactions startingfrom aqueous environmental friendly systems [25,26].

A silsesquioxane containing the double charged group 1,4-diazoniabicyclo[2.2.2]octane was developed. This silsesquioxanepresents interesting properties such as self-organization, opticalbirefringence [27] and sorption properties towards inorganic andorganic species [28–30]. Important applications of materials pre-pared from this silsesquioxane was the adsorption of brilliant yellow(BY) by anion exchange mechanism and the development of an

129D.S.F. Gay et al. / Desalination 258 (2010) 128–135

electrochemical sensor for ascorbic acid using BY as electron-mediated species [30].

In the present work, an alternative adsorbent for BY removal fromaqueous solutions was developed considering the affinity betweenthe dye BY and the silsesquioxane containing the charged group 1,4-diazoniabicyclo[2.2.2]octane. The synthesis of the new adsorbent isbased on the grafting reaction of the charged silsesquioxane onto acommercial silica surface previously modified with aluminum.Chemical features associated with the structure of the material arepresented as well as the application of the adsorbent for removal of BYdye from aqueous solutions. The kinetic, equilibrium and thermody-namic parameters of the batch dye adsorption method are presented.Up to the best of our knowledge this is the first paper dealing with theadsorption of brilliant yellow (BY).

2. Materials and methods

2.1. Solutions and reagents

Deionized water was used throughout the experiments for allsolution preparations. The dye, brilliant yellow (BY), also known asdirect yellow 4, (C.I. 24890, C26H18N4S2O8Na2, purity of 70%) waspurchased from Sigma Chemical Co (St. Louis, MO, United States) andused without further purification. The dye stock solution was preparedby dissolving an accurately weighed amount of the dye in deionizedwater in order to obtain a concentrationof 5000 mg L−1. Other solutionswere prepared by diluting the dye stock solution to the requiredconcentration. In order to adjust thepH, 0.10 mol L−1 sodiumhydroxideor hydrochloric acid solutions were used, using a bench pH meter(Hanna HI 255; Padova, Italy).

For the synthesis of the adsorbent the following reactants wereemployed: 1,4-diazabicyclo[2.2.2]octane (Dabco), furnished by Aldrich,Steinheim,Germany, purity grade 98%; 3-chloropropyltrimethoxysilane(CPTMS) by Adrich, Steinheim, Germany, purity grade 97%; aluminumisopropoxide furnished Acros Organics, New Jersey, USA, purity grade98%.

The solvents utilized in the synthesiswere: N,N-dimethylformamide(DMF) furnished by Adrich, Steinheim, Germany, purity grade 99.8%;methyl alcohol from Merck, Darmstadt, Germany, purity grade 99.9%;formamide from Merck, Darmstadt, Germany, purity grade 99.5%;ethanol furnished by Merck, Darmstadt, Germany, purity grade 99.9%;toluene furnished Merck, Darmstadt, Germany, purity grade 99.9%;ethyl ether from Merck, Darmstadt, Germany.

For adjustments of the acidity of the media for preparation of theadsorbents the following acids were employed: HF (48% weight)furnished by Synth, São Paulo, Brazil.

Silica gel (Merck, Darmstadt, Germany) particle size 0.02–0.05 mmwas employed as support for preparation of the adsorbent.

For keeping the inert atmosphere, argon (White Martins, purity99.998%) was employed in the synthesis of the adsorbent.

2.2. Synthesis of the adsorbent

2.2.1. Synthesis of the water soluble silsesquioxane containing the1,4-diazoniabicyclo[2.2.2]octane charged group

Firstly, 8 mmol (0.90 g) of the 1,4-diazabicyclo[2.2.2]octane(Dabco), was dissolved in 20 ml N,N-dimethylformamide (DMF). Thesolution was then added to 16 mmol (3.0 ml) of 3-chloropropyltri-methoxysilane (CPTMS). The mixture was stirred for 72 h, under argonat 75 °C. The white solid obtained was filtered andwashed withmethylalcohol and then dried at 70 °C for 2 h. The solid, 1,4-bis-(3-trimethoxysilylpropyl)diazoniabicyclo[2.2.2]octane chloride (3.06 g)was dissolved in a mixture containing 25 ml of formamide, 0.65 ml ofwater and 0.1 ml of HF (48% weight) at 60 °C. This solution was left forgelation and solvent evaporation during 30 days near 40 °C. After thistime, the mixture became a white solid. This solid is the water soluble

silsesquioxane precursor containing the charged 1,4-diazoniumbicyclo[2.2.2]octane group. This solid was washed with ethanol and finallydried in an oven at 90 °C for 2 h and designated as Dabcosil. This is animportant step in sol–gelmethod, because it involves the hydrolysis andpolicondensation reactions to obtain the silsesquioxane material.

2.2.2. Synthesis of aluminum oxide grafted onto silica gel surface(Al-SiO2)

Silica gel of particle size 0.02–0.05 mmwas activated at 423 K undervacuum for 8 h. The aluminum isopropoxide (Aldrich) (4.37 g) wasdissolved in 50 mlof toluene and the activated silica (21.4 g)was added.The mixture was stirred for 24 h under argon atmosphere, at solvent-reflux temperature. The modified silica was filtered under argon in aSchlenk apparatus, washed with toluene, ethyl alcohol, distilled waterand ethyl ether. The resulting product, assigned asAl-SiO2, was dried for4 h under vacuum at 393 K.

2.2.3. Grafting reaction of Dabcosil onto a Al-SiO2 surfaceThe water soluble silsesquioxane precursor Dabcosil (1.59 g) was

dissolved indistilledwater (40 ml) at 333 Kandadded to 19 gofAl-SiO2.The mixture was stored for 3 days and finally dried under vacuum at393 K. The resulting material was designated as Dabcosil-Al-SiO2.

2.3. Elemental analysis and thermogravimetric analysis

The elemental analysis of the organic groups incorporated in theAl-SiO2 matrix was made in a CHN Perkin Elmer (Waltham, MA,United States) M CHNS/O Analyzer, model 2400. The analyses weremade in duplicate, after heating the material at 373 K under vacuumfor 1 h.

To analyze the chloride contents in the adsorbent material, anamount of the solid was immersed in HNO3 solution, under stirring,exchanging the chloride for nitrate. The exchanged chloride ion waspotentiometrically measured by titration with silver nitrate.

The Dabcosil-Al-SiO2was submitted to thermogravimetric analysis(TGA), using argon atmosphere in a Shimadzu (Kyoto, Japan) analyzermodel TA50. The sample was heated from room temperature to800 °C at a rate of 20 °C min−1.

2.4. Adsorption studies

2.4.1. Nitrogen adsorption–desorption isothermsThe N2 adsorption–desorption isotherms were determined at

liquid nitrogenboiling point, using a homemadevolumetric apparatus,connected to a turbo molecular Edwards vacuum (London, UnitedKingdom) line system, fitted with an Hg capillary barometer. Theapparatus is frequently checked with alumina standard reference. Theadsorbent materials were previously degassed at 150 °C undervacuum for 2 h. The pore size distribution was obtained by using theBJH (Barret, Joyner and Halenda) method [30] and the specific surfacearea was determined by the BET (Brunauer, Emmett and Teller) [31]multipoint technique.

2.4.2. Batch procedureAdsorption studies to evaluate the removal of the dye BY from

aqueous solutions were carried out in triplicate using a batchadsorption procedure. For these experiments, an amount of adsorbent,varying from 20.0 to 200.0 mg, was suspended in a series of 20.0 mldye solution with concentrations varying from 2.00 to 1000.0 mg L−1

using 50 ml glass flasks. The suspensions were stirred for suitabletimes ranging from 5 to 300 min. After 300 min the isotherms showeda well-established plateau indicating that the equilibrium wasestablished. The equilibrium studies were made using the optimizedcondition of 300 min at temperatures ranging from 298 up to 323 K.The pH of the dye solutions ranged from 3.0 to 8.0. Subsequently, inorder to separate the adsorbent from the aqueous solutions, the flasks

Table 2Isotherm models.

Isotherm model Equation

Langmuir qe = Qmax⋅KL ⋅Ce1 + KL ⋅Ce

Freundlich qe=KF∙Ce1/nF

Sips qe = Qmax⋅Ks ⋅C1 = nse

1 + Ks⋅C1 = nseRedlich–Peterson

qe = KRP⋅Ce1 + aRP⋅Cg

ewhere 0≤g≤1

130 D.S.F. Gay et al. / Desalination 258 (2010) 128–135

were centrifuged at 3600 rpm for 10 min and aliquots of 1.0 to 10.0 mlof the supernatant liquidwere analyzed by visible spectrophotometry,using a Femto spectrophotometer (São Paulo, Brazil) with 1.0 cm pathlength optical-glass cells. Absorbance measurements were carried outat λ=403 nm were the light absorption of BY is maximum. The BYdetection limit using the spectrophotometric method, determinedaccording to IUPAC [32], was 0.11 mg L−1. The dye uptake and thepercentage of dye removal by the adsorbent were calculated byEqs. (1) and (2), respectively:

q =Co � Cfð Þ

m⋅V ð1Þ

%Removal = 100⋅ Co−Cfð ÞCo

ð2Þ

where q is the dye uptake by the adsorbent (mg g−1); Co is the initialBY concentration (mg L−1), Cf is the dye concentration at equilibrium(mg L−1), V is the volume of the dye solution (L) in contact with theadsorbent and m is the mass (g) of adsorbent.

2.4.3. Kinetic and equilibrium modelsKinetic equations corresponding to the Avrami, the pseudo-first-

order, the pseudo-second-order, the Elovich and the intra-particlediffusion models [33] are given in Table 1. The Langmuir, theFreundlich, the Sips and the Redlich–Peterson isotherm models [34]are listed in Table 2.

2.4.4. Statistical evaluation of the kinetic and isotherm parametersThe kinetic and equilibrium models were fitted employing the

nonlinear method, of the softwareMicrocal Origin 7.0. In addition, themodels were also evaluated by an error function, using the differencesbetween the amount of dye uptake predicted by the models and theactual q measured experimentally [35].

Ferror =

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi∑p

i

qimodel � qi experimental

qi experimental

!2

⋅ 1p� 1

� �vuut ð3Þ

where qi model is the value of q predicted by the fitted model,qi experimental is the value of q measured experimentally and p is thenumber of experiments.

3. Results and discussion

3.1. Adsorbent synthesis and characterization

A schematic representation of the Dabcosil-Al-SiO2 material isshowed in Fig. 1, illustrating the aluminum coverage that allows theattaching of silane stable films on the silica surface as already reportedfor similar systems [36,37]. Fig. 1 also shows the presumable bonds ofthe Dabcosil silsesquioxane grafted on the surface, emphasizing thedouble charged 1,4-diazoniumbicyclo[2.2.2]octane organic group andits exchangeable chloride counter ions.

The carbon and chloride elemental analyses and the TGA resultsare shown in Table 3. The results confirm that the charged Dabcosil

Table 1Kinetic adsorption models.

Kinetic model Non-linear equation

Avrami qt=qe.{1−exp[−(kAV∙ t)]nAV}

Pseudo-first-order qt=qe.[1−exp(−kf∙t)]Pseudo-second-order qt =

ks⋅q2e ⋅t

1 + qe⋅ks ⋅tho=kS∙qe

2 initial sorption rateElovich qt = 1

β Ln α⋅β� �

+ 1β Ln tð Þ

Intra-particle diffusion qt¼kid⋅ffiffiffiffit

p+ C

silsesquioxane was grafted onto the matrix Al-SiO2. The amount oforganic group immobilized is satisfactory considering its large size,estimated in 1.4 nm [26], making important the diffusion process inthe grafting reaction. It is important to point out that each grouppresents two charged sites allowing specific adsorption by ion-exchange process.

Specific surface area values are also presented in Table 3. Thealuminum grafting reaction produces a reduction in the surface areafrom 280 to 230 m2g−1. However, the subsequent immobilization ofDabcosil silsesquioxane does not produce any additional surface areareduction.

The pore size distribution curves are presented in Fig. 2. It ispossible to observe that the curves of Al-SiO2 and Dabcosil-Al-SiO2 arevery similar, showing a reduction in the larger diameter pores whencompared to the pure silica curve. The results are in accordance withthe BET surface area results showing that the insertion of Dabcosilsilsesquioxane did not produce significant changes in the texture,different from the aluminum grafting reaction.

3.2. Effects of pH on adsorption

One of the most important adsorption features is the effect of themedium acidity [38]. Different species adsorbed will present differentranges of suitable pH for adsorption. The effect of the initial pH on BYadsorption capacity using Dabcosil-Al-SiO2 was evaluated within a pHrange from 2 to 10, as shown in Fig. 3A. Dye removal was practicallyconstant for a pH ranging from 2.0 to 5.0 decreasing abruptly for pHN5.5 and becoming practically null at pH 10.0.

Taking into account that industrial effluents containing dyes areusually neutralized with caustic soda [35], presenting pH values from4.5 to 5.5, the initial pH of the adsorbate solutions was fixed at 5.0.

The BY is an anionic dye containing two sulfonic groups, showed inthe Fig. 4, which present negative charge even at higher acidicsolutions because their pKa values are very low [39]. On the otherhand, the adsorbent Dabcosil-Al-SiO2 presents formal positive charges(see Fig. 1), therefore the mechanism of interaction adsorbent–adsorbate should be an ion-exchange mechanism.

3.3. Adsorbent dosage

The optimal amount of adsorbent for dye removal from aqueoussolution was determined using quantities of both adsorbents rangingfrom 20.0 to 200.0 mg. In all experiments 20 ml of a 100.0 mg L−1 dye

Fig. 1. Schematic representation of the Dabcosil-Al-SiO2 material.

Table 3Elemental analysis, TGA and surface area results.

Sample Carbona/mmolg−1

Chloridea/mmolg−1

TGAab/mmolg−1

BET surfacearea/m2 g−1

Pure silica – – 280±15Al-SiO2 – – 230±12Dabcosil-Al-SiO2 0.16±0.03 0.15±0.01 0.19±0.03 230±12

a Coverage Dabcosil/g of adsorbent.b Calculated from weight loss between 423 and 873 K.

131D.S.F. Gay et al. / Desalination 258 (2010) 128–135

solution with pH=5 was used. The highest amount of dye removalwas attained for adsorbent quantities of at least 50.0 mg as shown inFig. 3B. For adsorbent quantities higher than those, dye removalremained almost constant. The increase in dye removal with theadsorbent mass can be attributed to the increase in the total surfacearea, augmenting the number of adsorption sites [40,41]. On the otherhand, increasing the adsorbent mass promotes a decrease in theamount of dye uptake per gram of adsorbent (q), as shown in Fig. 3C,an effect that can be mathematically explained by combining Eqs. (1)and (2) to give:

q =% Removal⋅Co⋅V

100⋅m ð4Þ

As observed from Eq. (4), the amount of dye uptake (q) is inverselyproportional to mass of adsorbent (m). A similar behavior has beenreported for the removal of Cr(VI) from aqueous solutions [42]. Basedon these results, the adsorbent mass was fixed at 50.0 mg of Dabcosil-Al-SiO2.

3.4. Kinetic studies

Adsorption kinetics is an important feature to be considered inaqueous effluent treatments as it provides valuable information onthe mechanism of adsorption. Attempting to describe the dyeadsorption by both adsorbents, four liquid-phase kinetic modelswere tested, and its kinetic parameters obtained for these models arelisted in Table 4.

The Avrami fractionary kinetic order was suitably fitted, present-ing low error function values and also high R2 values, for two initialdye concentration levels. The lower the error function is, the lowerwill be the difference of the q calculated by the model from theexperimentally measured q [42]. Additionally, it was verified that theqe values found using fractionary-order were in better agreementwith the experimental data. For all other models, the q values werenot so coincident with the qe experimental values. These results

Fig. 2. Pore size distribution curves of Dabcosil-Al-SiO2 adsorbent obtained from BJHmethod.

Fig. 3. A— Effect of pH on brilliant yellow (BY) adsorption (Co 100 mg L−1, massof adsorbent 30.0 mg); B— Effect of adsorbent mass on the percentage of BY removal(Co 100 mg L−1, pH 5.0) from aqueous effluents; C— Effect of the adsorbent mass on theamount of dye adsorbed (Co 100 mg L−1, pH 2.0). Contact time was fixed at 240 minthroughout. The temperature was fixed at 298 K throughout.

indicate that the fractionary-order kinetic model is the best approachto the adsorption of BY by Dabcosil-Al-SiO2. Taking into account thatthe best kinetic liquid-phase model fitted to the experimental datawas the Avramimodel, the curves of this model are presented on Fig. 5for the two levels of concentration of BY dye.

The kinetic parameters presented in Table 4 show that the kAVvalues give a better description of the kinetic process than the pseudo-second-order kinetic constant (kS). The kAV values are independent ofthe initial concentration while the kS values are strongly depended of

Fig. 4. Molecular structure of brilliant yellow dye (BY).

132 D.S.F. Gay et al. / Desalination 258 (2010) 128–135

that concentration. The variation of kAV was lower than 3.2% using twodifferent initial concentrations of BY compared with a kS variation of28%. Therefore, the fractionary kinetic model provides a rate constantparameter which is much better for comparison of different kineticconstants, using several adsorbates and adsorbents. It should also bepointed out that the pseudo-first-order constant (kf) rate does notpresent significant variations when the initial dye concentration wasincreased from 75.0 to 100.0 mg L−1 (0.37%), since its units is min−1.Therefore, this explanation reinforces that adsorption rate constantsthat do not present the amount adsorbed in their units will notdepend on the initial dye concentration. The Avrami kinetic equationhas been successfully employed to explain several adsorption kineticprocesses [43–46]. The Avrami exponent (nAV) is a fractionarynumber which reflects mechanism changes that may take placeduring the adsorption process [43–46].

Taking into account that the kinetic results presented in Table 4and Fig. 5A and B fit very well into the Avrami fractionary kineticmodel, the intra-particle diffusion model [33] was plotted in order toverify the influence of mass transfer resistance on the binding of BY tothe adsorbents (Fig. 5C and D).

The possibility of intra-particle diffusion resistance affectingadsorption was explored using the appropriate model [33]. Theintra-particle diffusion constant, kid (mg g−1min−0.5), can be obtainedfrom the slope of the plot of qt (uptake at any time,mg g−1) against thesquare root of time. Fig. 5C and D shows plots of qt versus t1/2, with

Table 4Kinetic parameters for BY removal using Dabcosil-Al-SiO2 adsorbent. Conditions:temperature 298±1 K at pH 5.0 and mass of adsorbent 50.0 mg.

Dabcosil-Al-SiO2

75 mg L−1 100 mg L−1

Fractionary orderkAV (min−1) 0.0197 0.0191qe (mg g−1) 33.0 42.7nAV 0.651 0.639R2 0.9998 0.9996Ferror 5.14×10−3 2.37×10−2

Pseudo-first-orderkf (min−1) 0.0272 0.0273qe (mg g−1) 29.9 38.3R2 0.9740 0.9715Ferror 0.137 0.132

Pseudo-second-orderkS (g mg−1 min−1) 9.60×10−4 7.50×10−4

qe (mg g−1) 34.4 44.1ho (mg g−1 min−1) 1.14 1.46R2 0.9953 0.9953Ferror 7.29×10−2 6.22×10−2

Chemisorptionα (mg g−1 min−1) 2.99 3.80β (g mg−1) 0.151 0.117R2 0.9955 0.9978Ferror 5.29×10−2 3.61×10−2

Intra-particle diffusionki (mg g−1 min−0.5)a 1.56 1.73

a Second stage.

multi-linearity for the dye using Dabcosil-Al-SiO2. These results implythat the adsorption processes involve more than one single kineticstage or sorption rate [34]. For instance, the adsorption exhibited threestages, which can be attributed to each linear portion of the Fig. 5Cand D. The first linear portion is the fastest step and is attributed to thediffusion of BY towards adsorbent. The second linear portion is a delayprocess that corresponds to intra-particle diffusion. The third stage isthe diffusion through small pores and is followed by the establishmentof equilibrium [34].

It can be observed in the Fig. 5 that, the minimum contact time ofBY with the Dabcosil-Al-SiO2 required to reach the equilibrium wasnear 250 min (see Fig. 5A and B), although Dabcosil-Al-SiO2 presentsmesopores, with pore diameter ranging from 4 to 8 nm (Fig. 2) and anappreciable adsorption surface area (320 m2g−1). This could beattributed to the large size of the BY molecule (Fig. 4). Thus, thediffusion of BY from the solution to the adsorption sites inside theDabcosil-Al-SiO2 pores was limited, delaying the adsorption process.This interpretation is corroborated by the intra-particle diffusionconstant (kid) reported on Table 4.

3.5. Equilibrium and thermodynamic studies

Several equations have been proposed for analyzing experimentaladsorption equilibrium data. The equation parameters often providesome insight into the adsorption mechanism, the surface propertiesand the affinity of the adsorbent. For this purpose the Langmuir, theFreundlich, the Sips and the Redlich–Peterson isotherm models wereemployed in the present work.

The adsorption of BY on Dabcosil-Al-SiO2 was carried out using50.0 mg of the adsorbent, pH=5.0 and temperatures ranging from298 K to 323 K, are presented in Table 5. It can be seen that, with theexception of the Freundlich model, the data fit very well in allisotherm models.

It should be pointed out that in order to establish a given model,the sole analysis of R2 values is not enough, because the error functionevaluates the differences between each point fitted by the model(qi model) and the corresponding experimental point (qi experimental).On the other hand the R2 value measures the differences betweeneach theoretical point (qi model) and the arithmetical average ofexperimental amount adsorbed (q̄) [47]. This analysis could bevisualized by observing the R2 and the Ferror values at 308 K for theLangmuir and Sips isotherms. The Langmuir isotherm, presented a R2

value of 0.9997, was slightly worse than the Sips isotherm(R2=0.9998). On the other hand, the Ferror value for Langmuirisotherm (1.02×10−2) was remarkably worse than that for Sipsisotherm (6.44×10−3). These results reinforce the need of using astatistical error function to better evaluate the adsorption models,since the lower the value error function is, the lower will be thedifference between the q calculated by the model and the qmeasuredexperimentally [47].

Taking into account that the equilibrium results were practically thesame for Langmuir, Sips and Redlich–Peterson isotherms, themaximumamount of BY uptake was 54.1 mg g−1 at 298 K, corresponding to0.087 mmol g−1. From the point of view of adsorption interaction at thesolid/liquid interface, the negatively charged BY has the ability toexchange with chloride ions, remaining bonded to the positively

Fig. 5. Kinetic curves for brilliant yellow (BY) adsorption. A) Liquid-phase diffusion kinetic models (70 mg L−1); B) Liquid-phase diffusion kinetic 100 mg L−1; C) Intra-particlediffusion kinetic model (70 mg L−1); D) Intra-particle diffusion kinetic model (100 mg L−1). The pH of the solutions was kept at pH 5.0, the amount of adsorbent was fixed at50.0 mg. The temperature was fixed at 298 K throughout.

Table 5Isotherm parameters for BY adsorption using Dabcosil-Al-SiO2 as adsorbent. Conditions: contact time 300 min, at pH 5.0 and mass of adsorbent 50.0 mg.

Temperature (K)

298 303 308 313 318 323

LangmuirQmax (mg g−1) 54.1 56.0 55.0 42.5 46.3 44.8KL (L g−1) 161 280 488 949 1269 2401R2 0.9996 0.9972 0.9997 0.9999 0.9998 0.9998Ferror 5.00×10−3 3.96×10−2 1.02×10−2 5.21×10−3 5.99×10−3 2.01×10−3

FreudlichKF (mg g−1 (mg L−1)−1/n

F) 22.8 23.1 25.4 23.5 26.4 33.3nF 5.56 5.14 5.83 7.46 7.33 13.7R2 0.9039 0.8771 0.8525 0.8161 0.7839 0.7378Ferror 8.11×10−2 0.213 0.576 0.237 0.489 0.103

SipsQmax (mg g−1) 54.5 57.3 55.2 42.5 46.2 44.8KS ((g L−1)−1/ns) 172 306 496 948 1274 2407nS 1.04 1.12 1.03 1.01 0.990 0.994R2 0.9999 0.9991 0.9998 0.9999 0.9998 0.9998Ferror 2.77×10−3 1.71×10−2 6.44×10−3 5.50×10−3 4.12×10−3 1.91×10−3

Redlich–PetersonKRP (L g−1) 9.01 17.3 27.6 40.5 58.7 107aRP (mg L−1)−g 0.173 0.342 0.515 0.958 1.27 2.40g 0.992 0.978 0.994 0.999 1.00 1.00R2 0.9998 0.9985 0.9998 0.9999 0.9998 0.9998Ferror 3.30×10−3 2.66×10−2 5.00×10−3 5.34×10−3 5.99×10−3 2.01×10−3

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charged sites on matrix as represented in Fig. 1. It is important to pointout that the maximum BY uptake, 0.087 mmol g−1, corresponds to anoccupation of near 60% of the adsorption sites (see Table 3). This is anacceptable result for this kind of material, even considering mesoporeswith average size of 6 nm, since a fraction of sites cannot be available forthe large species such BY dye.

Thermodynamic parameters related to the adsorption process, i.e.,Gibb's energy (ΔG°, kJ mol−1), enthalpy (ΔH°, kJ mol−1), and entropy(ΔS°, J mol−1 K−1) were determined by the following equations:

ΔG 0 = ΔH 0−TΔS 0 ð5Þ

ΔG 0 = −RTLn Kð Þ: ð6Þ

By combining Eqs. (5) and (6):

Ln Kð Þ = ΔS 0

R−ΔH 0

R×

1T: ð7Þ

where R is the universal gas constant (8.314 JK−1 mol−1), T is thetemperature (K) and K is the adsorption constant of the isotherm(KL — Langmuir and KS — Sips equilibrium constant) obtained fromthe isotherm plots. The ΔH° and ΔS° values can be calculated from theslope and intercept of a linear plot of Ln(K) versus 1/T.

The thermodynamic results are showed on Table 6. Langmuir andSips equilibrium constants give similar results for adsorption enthalpyand adsorption entropy, reinforcing that the equilibrium data werepractically coincident for the Langmuir, the Sips and the Redlich–Peterson isotherm models. Furthermore, the enthalpy magnitude isconsistent with the electrostatic interaction between the BY dye andthe charged sites on the matrix surface. Enthalpy changes (ΔH°)indicate that adsorption followed endothermic processes. Negativevalues of ΔG indicate that the BY reactive dye adsorption by Dabcosil-Al-SiO2 adsorbent is spontaneous and favorable processes for allstudied temperatures. The positive values of ΔS° confirm a highpreference of BY molecules for the Dabcosil-Al-SiO2 adsorbent surfaceand also suggest the possibility of some structural changes orreadjustments in the dye-adsorbent adsorption complex [48]. Besides,it is consistent with the dehydration of dye molecule before itsadsorption to silica surface, and the releases of these water moleculesto the bulk solution.

4. Conclusion

The synthesized Dabcosil-Al-SiO2, obtained by grafting reaction ofthe soluble silsesquioxane containing the charged 1,4-diazoniabicyclo[2.2.2]octane chloride group onto a silica surface, previously modifiedwith aluminum oxide, represents a good alternative adsorbent for

Table 6Thermodynamic parameters of the adsorption of BY on Dabcosil-Al-SiO2 adsorbent.Conditions: contact time 300 min, at pH 5.0 and mass of adsorbent 50.0 mg.

Temperature (K)

298 303 308 313 318 323

LangmuirKL (L g−1) 161 280 488 949 1269 2401ΔG (kJ mol−1) −12.6 −14.2 −15.9 −17.8 −18.9 −20.9ΔH° (kJ mol−1) 85.6ΔS° (J K−1 mol−1) 330R2 0.9973

SipsKS ((g L−1)−1/ns) 172 306 496 948 1274 2407ΔG (kJ mol−1) −12.8 −14.4 −15.9 −17.8 −18.9 −20.9ΔH° (kJ mol−1) 82.9ΔS° (J K−1 mol−1) 321R2 0.9974

brilliant yellow (BY) dye removal from aqueous solutions. Thismaterial has the ability to electrostatically adsorb the anionic BY dye,on the solid/liquid interface, when suspended in water. The bestconditions were established as follows: pH=5.0, shaking time of300 min, time necessary to saturate the available centers located on theexchange surface and provide a well-defined isotherm. Four kineticmodels were used and the best fit was obtained by employing theAvrami fractionary-order model. However, the intra-particle diffusionmodel gave three linear regions, suggesting that the adsorption can alsobe followed by multiple sorption rates. The maximum adsorptioncapacity of BY by Dabcosil-Al-SiO2 was 54 mg g−1 (0.087 mmol g−1).

Acknowledgements

The authors are grateful to Ministério de Ciência e Tecnologia(MCT), and to Conselho Nacional de Desenvolvimento Científico eTecnológico (CNPq) for financial support and fellowships.

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Glossary

α: initial adsorption rate (mg g−1 min−1) of the Elovich equation.β: Elovich constant related to the extent of surface coverage and also to the activation

energy involved in chemisorption (g mg−1).aRP: the Redlich–Peterson constant (mg L−1)−g.C: constant related to the thickness of the boundary layer (mg g−1).Cf: dye concentration at the end of the adsorption (mg L−1).Ce: dye concentration at equilibrium (mg L−1).Co: initial dye concentration (mg L−1).dq: differential of q.g: dimensionless exponent for Redlich–Peterson equation.ho: initial sorption rate (mg g−1 min−1) of the pseudo-second-order equation.kAV: Avrami kinetic constant (min−1).kf: pseudo-first-order rate constant (min−1).KF: Freundlich constant related to adsorption capacity [mg g−1 (mg L−1)−1/n

F].kid: intra-particle diffusion rate constant (mg g−1 min−0.5).KL: Langmuir equilibrium constant related to adsorption process (L g−1).KRP: Redlich–Peterson equilibrium constant (L g−1).KS: Sips equilibrium constant related to adsorption process (g L−1)−1/ns.kS: pseudo-second-order rate constant (g mg−1 min−1).m: mass of adsorbent (g).nAV: fractionary reaction order (Avrami).nF: dimensionless exponent of Freundlich equation.nS: dimensionless exponent of Sips equation.q: amount of the dye adsorbed by the adsorbent (mg g−1).qe: amount of adsorbate adsorbed at the equilibrium (mg g−1).Qmax: the maximum adsorption capacity of the adsorbent (mg g−1).qt: amount of adsorbate adsorbed at time t (mg g−1).t: time of contact (min).V: volume of dye in contact with the adsorbent (L).