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Removal-of-dyes-from-water-using-chitosan-hydrogel-SiO2-and-chitin-hydrogel-SiO2-hybrid-materials-

Removal-of-dyes-from-water-using-chitosan-hydrogel-SiO2-and-chitin-hydrogel-SiO2-hybrid-materials-
Removal-of-dyes-from-water-using-chitosan-hydrogel-SiO2-and-chitin-hydrogel-SiO2-hybrid-materials-

Journal of Hazardous Materials 186 (2011) 932–939

Contents lists available at ScienceDirect

Journal of Hazardous

Materials

j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /j h a z m a

t

Removal of dyes from water using chitosan hydrogel/SiO 2and chitin hydrogel/SiO 2hybrid materials obtained by the sol–gel method

Guillermo J.Copello a ,b ,Andrea M.Mebert a ,M.Raineri a ,Mariela P.Pesenti a ,Luis E.Diaz a ,b ,?

a Cátedra de Química Analítica Instrumental,Facultad de Farmacia y Bioquímica,Universidad de Buenos Aires (UBA),Junín 956,C1113AAD Buenos Aires,Argentina b

IQUIMEFA (UBA-CONICET),Junín 956,C1113AAD Buenos Aires,Argentina

a r t i c l e i n f o Article history:

Received 16June 2010

Received in revised form 18October 2010Accepted 23November 2010

Available online 30 November 2010Keywords:Sol–gel Chitin Chitosan Biosorbent Dye removal Hybrid material

a b s t r a c t

This work describes the synthesis of chitosan hydrogel/SiO 2and chitin hydrogel/SiO 2hybrid mesoporous materials obtained by the sol–gel method for their use as biosorbents.Their adsorption capabilities against four dyes (Remazol Black B,Erythrosine B,Neutral Red and Gentian Violet)were compared in order to evaluate chitin as a plausible replacement for chitosan considering its ef?ciency and lower cost.Both chitin and chitosan were used in the form of hydrogels.This allowed full compatibility with the ethanol release from tetraethoxysilane.The hybrid materials were characterized by Attenuated Total Re?ectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR),Scanning Electron Microscopy (SEM),Energy Dispersive X-Ray Spectroscopy (EDS),Nitrogen Adsorption Isotherms and 13C solid-state Nuclear Magnetic Resonance.Adsorption experimental data were analyzed using Langmuir,Freundlich and Dubinin–Radushkevich isotherm models along with the evaluation of adsorption energy and stan-dard free energy ( G 0).The adsorption was observed to be pH dependent.The main mechanism of dye adsorption was found to be a spontaneous charge associated interaction,except for EB adsorption on chitin/SiO 2matrix,which showed to involve a lower energy physical adsorption interaction.Aside from highly charged dyes the chitin containing matrix has similar or higher adsorption capacity than the chitosan one.

? 2010 Elsevier B.V. All rights reserved.

1.Introduction

The use of dyes in textile,plastic,food and pharmaceutical industries can generate colored ef?uents.As these dyes reach water streams they produce several effects.Coloration of water is undesirable and the reduction of sunlight transmission affects photosynthesis thereby disturbing aquatic ecosystems.In addition,many of the synthetic dyes are toxic and carcinogenic [1,2].Con-sequently,environmental regulations demand the removal of dyes before discharging industrial ef?uents into water bodies [3].

Nowadays there is a great interest in the development of low-cost sorbents for water remediation.The use of low cost sorbents has become an alternative to expensive methods such as membrane ?ltration,ion exchange or carbon adsorption [4,5].A sorbent can be assumed as “low cost”if it requires little or none processing,or if it is considered as a by-product or waste material that could be obtained in abundance [6].Recently,numerous approaches have been studied for the development of cheaper and more effective adsorbents containing polysaccharides [7].

?Corresponding author at:Cátedra de Química Analítica Instrumental,Facultad de Farmacia y Bioquímica,Universidad de Buenos Aires (UBA),Junín 956-Piso 3?(1113),Buenos Aires,Argentina.Tel.:+541149648254;fax:+541149648254.

E-mail addresses:ldiaz@ffyb.uba.ar ,guillejcopello@https://www.wendangku.net/doc/5817087456.html,.ar (L.E.Diaz).Chitosan and chitin,by-products of alimentary industry,are considered low cost biosorbents and have been studied for the removal of dyes from aqueous media [3,8,9].Chitin is a polysaccha-ride which structure consists predominantly of unbranched chains of ?-(1→4)-2-acetoamido-2-deoxy-d -glucose.It can be extracted from shrimp,crab shell,fungi and other crustaceans.Chitosan (?-(1→4)-2-amino-2-deoxy-d -glucose)is a hydrophilic and cationic polysaccharide product of chitin deacetylation [10].

Immobilization of biosorbents has been proposed as an advan-tage since it endows the sorbent mechanical stability and a physical support.This is crucial for its application in batch or column water remediation [11,12].Sol–gel method has been widely used to obtain polymeric hybrid materials containing inorganic and organic moieties [13–15].This is because of the mild conditions of the polymerization reaction,such as room temperature operation and polymerization of the monomers without the need of extreme pH or free radical catalyzers,among other advantages [16,17].

Chitosan/SiO 2composites have been developed with different application purposes among which the removal of toxic metals and dyes could be mentioned [11,18,19].Chitin/SiO 2hybrid mate-rials have been less studied than chitosan ones probably due to the low solubility of chitin in most common solvents.Literature reports the use of chitin/SiO 2composites for cell immobilization,as scaffolds for tissue engineering and as a chromatographic support [20–22].

0304-3894/$–see front matter ? 2010 Elsevier B.V. All rights reserved.doi:10.1016/j.jhazmat.2010.11.097

G.J.Copello et al./Journal of Hazardous Materials186 (2011) 932–939933

The objective of this work is to generate a support for chi-tosan and chitin for their use as dye sorbents in liquid media. Herein their adsorption capabilities are compared in order to eval-uate chitin as an alternative to chitosan considering its lower cost. The hybrid materials were obtained by the sol–gel method using tetraethoxysilane(TEOS)as the SiO2precursor.Four dyes were chosen as experimental models owing to their different chem-ical properties,toxicity and industrial applications:textile dye (Remazol Black B),textile and food dye(Erythrosine B),biologi-cal dye(Gentian Violet)and biological and chemical dye(Neutral Red).Conditions regarding batch optimum pHs,interaction times, and adsorption capacities were https://www.wendangku.net/doc/5817087456.html,ngmuir,Freundlich and Dubinin–Radushkevich isotherm models were analyzed together with the evaluation of adsorption energy and standard free energy ( G0).

2.Experimental

2.1.Reagents and materials

Tetraethoxysilane(TEOS),chitosan(calculated degree of acety-lation,DA:45%)and chitin(DA:92%;Mr~400,000)were purchased from Fluka(Switzerland,Israel and United States,respectively). Remazol Black B(RB,Reactive Black5)was from Sigma(St.Louis, MO,USA).Neutral Red(NR,Basic Red5)was from Riedel-de Ha?n(Seelze,Germany).Erythrosine B(EB,Acid Red51)was pur-chased from Merck(Darmstadt,Germany).Gentian Violet(GV, Basic Violet3)was purchased from Lowens Chemicals(Buenos Aires,Argentina).Water was?ltered and deionized with a Milli-Q,Millipore system(Milford,MA,USA).All other reagents were of analytical grade.

2.2.Matrix synthesis

The synthesis of a homogeneous polysaccharide hybrid mate-rial is dif?cult to achieve by direct interaction since alkoxysilanes hydrolysis releases alcohol,which precipitates these polymers.In order to by-pass this issue both polysaccharides were immobilized in the form of hydrogels,which were found to be compatible with the percentage of ethanol generated by the hydrolysis of TEOS.

A5%(w/v)chitin hydrogel was prepared as described previously [23]Brie?y,a5%(w/v)chitin solution was prepared by dissolving chitin in methanol saturated with CaCl2.Then10ml of the chitin solution was added to200ml of distilled water under vigorous stirring.Afterward the suspension was centrifuged to collect the hydrogel in10ml.Finally it was washed with0.2mol/l potassium hydrogen phthalate pH5.

In a similar way,a5%(w/v)chitosan hydrogel was prepared by dissolving chitosan in0.3mol/l HCl.Then10ml of the chitosan solu-tion was added to200ml of0.2mol/l K2HPO4pH7under vigorous stirring.Afterward the suspension was centrifuged to collect the hydrogel in10ml.Finally it was washed with0.2mol/l potassium hydrogen phthalate pH5.

A TEOS sol was prepared by sonicating(35kHz,Transsonic Tl-H-5,Elma,Germany)a mixture of10ml TEOS,0.6ml of0.05mol/l HCl and2ml of water for30min at25?C.For hybrid matri-ces synthesis equal amounts of TEOS sol and the correspondent hydrogel were mixed.Thus,the hybrid matrices obtained contain equal SiO2:polysaccharide mass ratios:14:1.Polymerization took place at25?C within10min.Then the matrix was sectioned in 1mm×1mm beads.

The TEOS sol obtained consisted of silicic acid monomers in an ethanol solution.Its polymerization lead to a SiO2polymeric net-work.Blank beads were obtained by mixing equal amounts of TEOS sol and0.2mol/l potassium hydrogen phthalate pH5.The three matrices were named according to their composition:(1)TEOS, (2)TEOS-Chitosan(TeChito),and(3)TEOS-Chitin(TeChitin).The Washed-TeChito matrices were obtained by the same procedure than the TeChito ones but adding two incubations of1h in2%acetic acid and a?nal incubation of1h in water.

2.3.Adsorption experiments

Adsorption experiments were carried out by a batch method at room temperature(25?C)with constant stirring(120rpm).A weighted mass of the matrices(0.2g)was added to an aqueous solution(5ml)of each dye,ranging from0.05to100mmol/l.The effect of pH,interaction times and adsorption isotherms were determined by dye absorbance decay in the solution supernatant. All measurements were carried out at each dye characteristic absorption peak with an UV–Vis Spectrophotometer(Cecil CE3021, Cambridge,England).All experiments and their corresponding measurements were conducted in triplicate under identical con-ditions and statistically analyzed by one-way ANOVA.In all cases, the differences were considered to be signi?cant when p<0.05.

2.4.Characterization

ATR-FTIR transmission spectra were acquired in the range of 4000–650cm?1using a Fourier Transform Infrared spectrometer (FT-IR)with a Flat-plate Attenuated Total Re?ectance(ATR)(Perkin Elmer,Spectrum One IR).All samples were previously dried for 24h at60?C to avoid water related bands interference.The degree of acetylation(DA)was determined by the method proposed by Brugnerotto et al.,which is based on the relationship between a ref-erence band at1420cm?1and the amide III band at1320cm?1by applying the following equation:A1320/A1420=0.3822+0.03133DA [24].Samples were analyzed using a Zeiss Supra40micro-scope for Scanning Electron Microscopy(SEM),while elemental analyses were carried out by using an Energy Dispersive ana-lyzer(EDS)(Oxford Instruments).Before performing the N2gas adsorption–desorption isotherms,all samples were degassed for 24h at50?C under high vacuum.N2adsorption and desorption isotherms were performed in duplicate at77.7K employing N2 spectroscopic grade.The speci?c surface area(S BET)and total pore volume(TPV)were estimated by the Brunauer–Emmett–Teller (BET)and Barrett–Joyner–Halenda(BJH)methods,respectively. Matrices water content was determined with a moisture ana-lyzer at constant temperature(105?C)(MX-50,A&D Company, Tokyo,Japan).Solid-state13C CP-MAS NMR(cross-polarization-magic angle spinning Nuclear Magnetic Resonance)spectra were performed on a Bruker Avance II-300spectrometer equipped with a4-mm MAS probe.All the NMR experiments were performed at room temperature.The operating frequency for carbons was 75.46MHz and the spinning rate was10kHz.AQ:41ms,CT:2ms, repetition rate5s.Decoupler sequence SPINAL6478.2kHz(equiv-alent to1H /2pulse of3.2?s).

3.Results and discussion

3.1.Matrix characterization

ATR-IR spectra of TEOS,TeChito and TeChitin matrices are shown in Fig.1.As it was expected,the silicon oxide matrix is detected in both polysaccharide containing matrices as well as in TEOS matrix.This is evidenced by the characteristic silicon oxide broad bands at790cm?1,950cm?1and1070cm?1cor-responding to symmetric Si–O–Si bond stretching,Si–OH bond stretching and asymmetric Si–O–Si bond stretching,respectively. The1070cm?1band accounts for condensed polymeric precursors forming oligomeric units typical of SiO2polymerization[25,26].

934G.J.Copello et al./Journal of Hazardous Materials

186 (2011) 932–939

Fig.1.ATR-FTIR spectra of TEOS,TeChito and TeChitin matrices. TeChitin spectrum shows a less intense band at1660–1620cm?1 corresponding to the amide I band indicating the presence of chitin (Fig.1).This band corresponds to a doublet at1655and1625cm?1 (C O and C–N stretching,respectively)which appears unresolved in ATR spectra[24].Amide I band is expected to decrease with deacetylation,thus being hardly detected for highly deacetylated chitosans[27].This is probably the reason why this band does not appear in TeChito spectrum while amide II band(1540cm?1,N–H stretching)is detectable together with amide III band(1390cm?1).

EDS spectra were used to con?rm the presence of the N contain-ing polysaccharides(see Supplementary data).In both chitosan and chitin containing matrices the peaks corresponding to the presence of nitrogen and carbon atoms are observed while they are absent in the TEOS EDS spectrum.On the other hand,in the three spec-tra the peaks corresponding to oxygen and silicon atoms of the hybrid materials are present.SEM images show the topography of the hybrid matrices where sharp differences between the three sur-faces can be seen(Fig.2).The addition of molecules or polymers to the polymerization mixture is known to in?uence the condensa-tion process accounting for the difference in the topography of the matrices[28,29].

The nitrogen sorption isotherms con?rm that the introduction of chitosan or chitin in the matrices is responsible for the varia-Table1

Data obtained from Nitrogen Adsorption Isotherms and water content analysis. Matrix Nitrogen sorption isotherms Water content(%)

S BET a(m2/g)TPV b(cm3/g)

TEOS1030.11280.1

TeChito1540.17578.4

TeChitin1420.29578.3

a Surface area for nitrogen adsorption,as determined by BET equation.

b Total pore volume.

tions in their structures.The three materials show the same type of isotherm(type IV)which is typical of mesoporous materials(see Supplementary data).Moreover it could also be seen the hysteresis loop present in some mesoporous materials[30,31].The matrices structure dissimilarities are evidenced in their surface area and total pore volume.Table1summarizes the parameters obtained from the nitrogen sorption isotherms and the water content anal-ysis.It can be noticed that the addition of the biopolymers leads to matrices with high surface area and high total pore volume. As mentioned above,the interaction of the monomers with the polysaccharide during polymerization directs its condensation and greatly in?uences the material?nal structure.However,the intro-duction of polysaccharides within the SiO2network does not show to have a strong effect on matrix water content since all the matrices show water contents around80%.

In an attempt to evaluate the mechanism involved in the polysaccharides immobilization,IR spectra and13C CP-MAS NMR spectra of TeChitin and TeChito were compared with free chitin and chitosan spectra(see Supplementary data).IR spectra comparison do not showed the appearance of extra bands in either TeChitin or TeChito spectra.Moreover,no band shift could be observed.In addi-tion,13C CP-MAS NMR spectra showed no chemical shift differences after the entrapment within the SiO2matrix(see Supplementary data).All signals showed concordance with literature carbon signal assignments[32].Thus,both IR and NMR spectra showed that the interaction between the immobilized polymers does not account for the formation of a new chemical bonding detectable at the experimental conditions.

In order to con?rm this,the adsorption of a1mmol/l solution of Remazol Black at pH4by TEOS,TeChito and Washed-TeChito

matri-Fig.2.SEM images of TEOS(a),TeChito(b)and TeChitin(c)matrices.

G.J.Copello et al./Journal of Hazardous Materials186 (2011) 932–939935

Table2

Effect of pH in the adsorption of a0.1mmol/l solution of dye.

Dye Matrix Adsorption(%)

pH

345678

RB TEOS N.T.a 1.1±0.4 4.3±0.14±1 2.2±0.23±1 TeChito N.T.98.7±0.597.1±0.398.2±0.596.9±0.794.7±0.6

TeChitin N.T.81.7±0.574±260±163±273±2

EB TEOS N.T.N.T.26±117±114.7±0.17±1 TeChito N.T.N.T.95.2±0.396.0±0.493.9±0.988±3

TeChitin N.T.N.T.79±168±254±136±3

NR TEOS86±175.9±0.687±385±287.36±0.0183±1 TeChito80.3±0.290.6±0.288.94±0.0693.6±0.390±192.7±0.1

TeChitin81.0±0.488±186.05±0.0191.6±0.288±289.8±0.7

GV TEOS91.3±0.689±290.6±0.791.0±0.194.06±0.0894±1 TeChito90±488.48±0.0792.5±0.891.3±0.694.6±0.395.0±0.2

TeChitin92.4±0.191.5±0.194.2±0.493.8±0.494±294.7±0.9

a N.T.:not tested due to dye precipitation.

ces was compared.The adsorption showed to be2.1±0.6,27±2 and99.4±0.2%for TEOS,Washed-TeChito and TeChito,respec-tively.The drop of adsorption evidenced by the Washed-TeChito matrices in comparison with the TeChito ones is due to a chi-tosan dissolution and subsequent diffusion out of the SiO2matrix during the acetic acid incubation step.This assay could not be con-ducted for the TeChitin matrices due to its lack of solubility in most common solvents.However,the same result of Washed-TeChito matrices could be expected for the chitin containing matrices.

These results suggest that at the matrix synthesis conditions the interaction between the SiO2network and the polysaccharides do not involve a covalent bonding,or at least it is not the main mecha-nism of immobilization.Probably the hybrid material presents the form of an interpenetrated network between the organic and the inorganic polymers[33].Literature suggest that at the conditions of the matrices synthesis,SiO2and the polysaccharides could inter-act by hydrogen bonding which has a strong in?uence on the?nal nanostructure of the hybrid material[34].

3.2.Effect of pH on adsorption behavior

The in?uence of aqueous solution pH on the adsorption of the four dyes was investigated in the pH range3–8(Table2).For all dyes high adsorption percentages are observed.RB and EB are acid dyes and their major adsorption percentage is evidenced in solu-tions with low pH.For TeChitin matrices higher adsorption was found to be signi?cantly different at pH4and pH5for RB and EB,respectively(signi?cantly different,p<0.05).For TeChito matri-ces no signi?cantly different adsorption percentages(p<0.05)were observed below chitosan protonation pH.At low pH amino groups of the chitosan in TeChito matrices are protonated allowing the interaction with the deprotonated form of the sulphonate group of RB and the carboxylate group of EB(Fig.3).This is less evident for TeChitin which has lower adsorption percentages probably due to the lack of highly protonable groups in chitin structure.Instead, N-acetylated groups of chitin should have af?nity for EB in its neu-tral carboxylic form which is capable to interact by hydrophobic forces.The SiO2skeleton has negligible adsorption for these dyes because its surface has negative charge density above pH3show-ing repulsion for the negative form of the dyes[16].This property of the SiO2matrix is evidenced in the adsorption of the protonable NR and the positive dye GV where the Si–O?groups could interact with the dye positive charges(Fig.3).Thus,SiO2contributes to the interaction of deprotonated polysaccharide with the basic dyes by its negative surface charge.For NR the major adsorption occurs at higher pHs than for the acid dyes(Table2).Both hybrid matrices present at pH6their maximum adsorption(signi?cantly different, p<0.05).This is probably due to a reduction of charge repulsion as long as the polysaccharides and the dye become less positively charged(pH6–7).GV has a positive charge in spite of the medium pH and this effect is not evidenced,being adsorption slightly higher at pH8.Therefore the differences in adsorption percentages among all three matrices(TEOS,TeChito and TeChitin)are not signi?cant (p<0.05),probably because of the great in?uence of SiO2in the GV adsorption.

3.3.Adsorption times

Adsorption kinetic experimental results are shown in Fig.4. These plots show aqueous dye concentration over time.In order to analyze the dyes uptake rates a simple kinetic analysis using the pseudo-?rst-order and pseudo-second-order equations was per-formed in their non-linear forms[35–37]:

C t=C eq+C x·e?k1·t(1) C t=C eq+

C x

1+k2·t·C x

(2)

where C t and C eq are aqueous dye concentration at time t and at equilibrium,respectively(mmol/l),C x is the difference between dye initial concentration(C0)and C eq(mmol/l)and k1(h?1)and k2(l mmol?1h?1)are the sorption rate constants for the pseudo-?rst and pseudo-second order models,respectively.The modeled kinetic parameters are summarized in Table3.

Kinetic parameters show good adjustment to the pseudo-second-order model,which indicates that adsorption is being the rate controlling step.Despite in most cases in the literature the pseudo-?rst-order equation does not?t well for the whole range of contact time,kinetic data present a good agreement for both models[37].For all dyes tested adsorption was monitored for72h in order to let the solutions to achieve equilibrium.

3.4.Adsorption isotherms

Adsorption isotherm models can describe the interaction of a sorbent with a given adsorbate which is essential for its effec-tive application.In this work data for adsorption isotherms were obtained after equilibrium time was reached at25?C at a given pH for each dye.Adsorption capacities(q eq)are expressed as the moles of adsorbed dye per mass unit of sorbent(mmol/g)and determined

936G.J.Copello et al./Journal of Hazardous Materials

186 (2011) 932–939

Fig.3.Structures of the dyes:Remazol Black (RB),Erythrosine B (EB),Neutral Red (NR)and Gentian Violet (GV).

as follows:q eq =

(C 0?C eq )V

m

(3)

where C 0and C eq are the initial and the equilibrium dye concentra-tions of the incubation solution,respectively (mmol/l),V is volume of solution (l)and m is the hybrid matrix mass (g).

Langmuir and Freundlich isotherms were modeled in order to evaluate its application in the hybrid matrices characterization.

Langmuir and Freundlich adsorption isotherms can be expressed using Eqs.(4)and (5),respectively [38]:q eq =

q m ·K a ·C eq 1+K a ·C eq

(4)q eq =k ·C eq n

(5)

where K a is the adsorption equilibrium constant (l/mmol),q m is the maximum adsorption capacity (mmol/g)and k and n are

arbi-

Fig.4.Aqueous dye concentration over time for initial concentrations of 0.1mmol/l at pH 4for RB (a),pH 5for EB (b),pH 6for NR (c)and pH 8for GV (d).Pseudo-?rst order plot is represented.

G.J.Copello et al./Journal of Hazardous Materials186 (2011) 932–939937

Table3

Kinetic parameters for dye adsorption by the hybrid matrices.

Dye Matrix pH Pseudo1st order Pseudo2nd order

C x(?mol/l)k1(h?1)C eq(?mol/l)R2C x(?mol/l)k2(l mmol?1h?1)C eq(?mol/l)R2

RB TeChito493±2153±82±10.99498±3 1.9±0.22±20.987 TeChitin472±262±521±20.99091±20.76±0.074±20.995

EB TeChito594±4110±106±30.96995±5 1.1±0.25±50.965 TeChitin573±4190±2024±20.96083±4 3.2±0.517±20.968

NR TeChito689±3220±108±10.98898±2 3.3±0.32±20.991 TeChitin683±4240±1012±20.96994±2 3.9±0.45±10.989

GV TeChito875±2560±30 4.8±0.70.99080±213±12±10.987

TeChitin873±2400±306±10.98279±18.7±0.1 1.9±0.6

0.995

Fig.5.Adsorption isotherms for RB at pH4(a),EB at pH5(b),NR at pH6(c)and GV at pH8(d).Langmuir plots are represented.

trary parameters.The dimension of k depends on the value of n.The adsorption isotherms are shown in Fig.5,where Lang-muir plots are represented.Parameters obtained for the nonlinear regression of Langmuir and Freundlich models are summarized in Table4.

For all dyes tested both TeChito and TeChitin matrices present good?tting to Langmuir model.This model is based on the assump-tion that adsorption exists up to the formation of a homogeneous monolayer of the adsorbate interacting with the sorbent[39].Thus, a tendency to saturate the interaction sites of the hybrid matrices could be deducted from these results.On the other hand,the equi-librium data are also consistent with the Freundlich model which presents a better adjustment to materials with heterogeneous adsorption sites.This is probably because multiple interactions are present in these complex systems where dyes could interact with different sites of the hybrid matrices.Moreover,interactions with these materials could vary according to the dye protonation equi-librium.

As it is shown in Table4the q m of TeChito for RB is near ten times greater than the TeChitin one.This could be explained because RB negative charges would interact with positive charges in the matrices.Therefore,being chitosan the chitin deacetylation prod-

Table4

Adsorption parameters for RB,EB,NR and GV adsorptions by the hybrid matrices at pH4,5,6,and8,respectively.

Dye Matrix Langmuir Freundlich

q m(mmol/g)K a(l/mmol)R2k n R2

RB TeChito0.081±0.004 2.4±0.40.9350.044±0.0020.28±0.020.926 TeChitin0.0062±0.000418±60.8540.0048±0.00020.22±0.020.932

EB TeChito0.080±0.005 1.8±0.90.9130.044±0.0040.18±0.030.883 TeChitin0.15±0.010.05±0.010.9610.015±0.0030.47±0.060.931

NR TeChito0.88±0.020.66±0.090.9840.27±0.020.31±0.020.958 TeChitin 1.06±0.020.58±0.060.9890.30±0.030.32±0.020.959

GV TeChito0.17±0.025±10.8960.140±0.0070.45±0.050.928 TeChitin0.14±0.015±10.9270.107±0.0050.42±0.040.921

938G.J.Copello et al./Journal of Hazardous Materials186 (2011) 932–939

Table5

D–R parameters,E DR and G0.

Dye Matrix D–R E DR(kJ/mol) G0(kJ/mol)

q DR(mg/g)K DR(mol2/kJ2)R2

RB TeChito124±80.0033±0.00030.94612.3±0.5?19.32±0.08 TeChitin9.8±0.60.0023±0.00020.95314.7±0.6?24.3±0.2

EB TeChito87±50.0023±0.00030.94114±1?18.5±0.2 TeChitin169±140.010±0.0010.9547.0±0.4?9.5±0.1

NR TeChito347±140.0036±0.00020.98111.7±0.3?16.08±0.06 TeChitin427±180.0038±0.00020.98211.4±0.3?15.76±0.05

GV TeChito256±460.0040±0.00040.93011.2±0.6?20.9±0.1 TeChitin181±270.0037±0.00030.93311.5±0.5?12.17±0.03

uct it has in its structure a higher number of protonable amino

groups(DA:45%)readily available to interact with RB.The oppo-

site could explain why TeChitin q m for EB is almost twice TeChito

q m for this dye.Chitin has an acetyl group in most saccharide unit of

its structure(DA:92%)which would interact with the neutral form

of EB at pH5by physical adsorption.In the case of TeChito matri-

ces two types of interactions are probable.The main interaction

would involve protonated amino groups of chitosan and the neg-

ative form of EB.Nevertheless,the interaction between remaining

acetyl groups in the polysaccharide structure and the neutral form

of the dye could not be discarded.

The difference among matrices q m s was expected to be less

marked for the basic dyes because of SiO2owns af?nity.For NR

adsorption it could be seen a slightly greater q m for TeChitin than

for TeChito(Table4).The lack of repulsion for the positive form of

the dye in TeChitin,which is present in the protonated chitosan,

would contribute to chitin af?nity for the neutral form of NR,rising

TeChitin q m in comparison with TeChito ones.This is not the case

for GV which positive charge is not repelled by TeChito or TeChitin

matrices at pH8.This could explain the similarities among their

q m.

3.5.Evaluation of adsorption energy and standard free energy

The equilibrium data shown as points in Fig.5have been

analyzed by Dubinin–Radushkevich(D–R)equation(6).For

liquid–solid phase adsorption the amount adsorbed correspond-

ing to any adsorbate concentration is assumed to be a Gaussian

function of the Polanyi potential,ε[40,41]:

q eq=q DR e?K DRε2(6)

with

ε=RT ln

1+

1

C eq

(7)

where q DR is the maximum adsorption capacity(mg/g),K DR is a constant related to sorption energy(mol2/kJ2),R the gas constant (kJ/mol K)and T the absolute temperature(K).

If the adsorbent surface is heterogeneous and homogeneous subregions are considered,an average free energy value could be calculated using equation(8)[42]:

E DR=(2K DR)?1/2(8) where E DR is the mean free energy of adsorption(kJ/mol).This mag-nitude is useful for estimating the type of adsorption interaction. If the sorption process is primarily physical in nature,such as van der Waals forces,the average free energy is typically in the range of1–8kJ/mol,while in the case of charge associated interactions is in the range of8–16kJ/mol related to[43,44].

The standard free energy( G0)of the process is related to the adsorption equilibrium constant(K a)by the following equation(9) [45]:

G0=?R·T ln K a(9) Table5summarizes the D–R parameters together with E DR and G0values.In all cases the adsorption of the dyes tested shows good agreement to D–R isotherm model.For almost all dyes tested the mean free energy of adsorption calculated for the interaction is between11and15kJ/mol,which account for charge associated interactions as the main sorption mechanism.Chitin(DA:92%),in a less extent than chitosan(DA:45%),has in its structure a certain amount of free amino groups,product of its deacetylation dur-ing the isolation process.Thus,both polysaccharides are capable of opposite charge attraction with the dyes which lead to E DR in the range of8–16kJ/mol.On the other hand,the TeChitin mean free energy of adsorption of EB accounts for physical adsorption as the primary mechanism.This supports the hypothesis of an interaction of the neutral form of EB to uncharged chitin structure,as sug-gested after Langmuir isotherm model results.The negative values obtained for the standard free energy for all dye tested demonstrate the spontaneity of the process at these conditions.

4.Conclusion

The present work describes a suitable method to obtain two types of hybrid mesoporous materials which contain chitosan or chitin as the organic moieties.The use of the polysaccharides in the form of hydrogels allowed full compatibility with the ethanol release from tetraethoxysilane.Both matrices demonstrated to maintain their capability of dye removal within the SiO2network with an improved mechanical behavior and adsorption properties than the free polysaccharides.The adsorption was observed to be pH dependent.The main mechanism of dye adsorption was found to be a spontaneous charge associated interaction,except for EB adsorption on TeChitin matrix,which showed to involve a lower energy physical adsorption interaction.

Aside from highly charged dyes,the chitin containing matrix has similar adsorption capacity than the chitosan one,or even higher in some cases.This could be considered as an advantage of the hybrid material since it has both the polysaccharide and the SiO2network dye af?nity.Moreover,chitin has lower cost since it is the source of chitosan industrial production.

The immobilization matrix endows the sorbents a physical sup-port with mechanical stability to easily accomplish both interaction with the pollutant and its removal from the liquid medium.This, in addition to the fact that chitin and chitosan are considered low-cost biosorbents,arises the interest in the scale up of the hybrid materials application in the removal of pollutant dyes from natural waters and industrial ef?uents.

G.J.Copello et al./Journal of Hazardous Materials186 (2011) 932–939939

Acknowledgements

M.P.P.is grateful for her undergraduate fellowship granted by Universidad de Buenos Aires.G.J.C.is grateful for his postdoctoral fellowship granted by CONICET.This work was supported with grants from Universidad de Buenos Aires(UBACYT B049).

Appendix A.Supplementary data

Supplementary data associated with this article can be found,in the online version,at doi:10.1016/j.jhazmat.2010.11.097. References

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