Separation and Purification Technology 122 (2014) 206–216

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Separation and Purification Technology

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The potential application of TiO2/hydrogel nanocomposite for removalof various textile azo dyes

1383-5866/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.seppur.2013.11.002

⇑ Corresponding author. Tel.: +381 11 3303730.E-mail address: meli@tmf.bg.ac.rs (M. Kalagasidis Krušic).

Marija Lucic a, Nedeljko Milosavljevic a, Maja Radetic a, Zoran Šaponjic b, Marija Radoicic b,Melina Kalagasidis Krušic a,⇑a University of Belgrade, Faculty of Technology and Metallurgy, Karnegijeva 4, 11120 Belgrade, Serbiab University of Belgrade, ‘‘Vinca’’ Institute of Nuclear Sciences, P.O. Box 522, 11001 Belgrade, Serbia

a r t i c l e i n f o

Article history:Received 31 July 2013Received in revised form 1 November 2013Accepted 5 November 2013Available online 13 November 2013

Keywords:Colloidal TiO2

TiO2/hydrogel nanocompositesAzo dyesPhotocatalytic degradationReproducibility

a b s t r a c t

TiO2/hydrogel nanocomposite has been utilized for investigation of photocatalytic degradation of threedifferent groups of anionic azo dyes in aqueous solutions under solar light simulating source in orderto evaluate its potential application for treatment of textile wastewaters. Hydrogel based on chitosan, ita-conic and methacrylic acid (monomers ratio Ch/IA/MAA = 1:1.56:10) was modified with synthetized0.2 M colloidal TiO2 nanoparticles and 0.2 M commercial Degussa P-25. SEM/EDX measurements con-firmed the presence of Ti in the hydrogel. TiO2 nanoparticles did not affect already formed covalent bondswithin polymer network, nor did they affect its thermal stability. However, network morphology,mechanical properties and swelling behavior were changed in the presence of TiO2 nanoparticles. Undersun-like illumination, nanocomposite with immobilized colloidal TiO2 nanoparticles completely removedC.I. Acid Red 18, C.I. Acid Blue 113, C.I. Reactive Black 5 and C.I. Direct Blue 78, while removal degree of C.I.Reactive Yellow 17 was 55%. After four cycles of illumination removal rate of C.I. Acid Red 18 was 75%,indicating that prepared TiO2/hydrogel nanocomposites could be reused without significant loses of pho-tocatalytic efficiency.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Recent growing ecological problems related to pollution ofwater resources have gained a lot of public, industrial and scientificattention. Wastewaters from various industries, factories, labora-tories, etc. have serious impact on environment. The dischargedwastewater containing dyes, even at very low concentrations, cancause waste streams to become highly colored. Aside from theirnegative aesthetic effects, certain azo dyes and biotransformationproducts have been shown to be toxic, and in some cases thesecompounds are carcinogenic and mutagenic [1].

Azo dyes are an abundant class of synthetic, colored, organiccompounds, which are characterized by the presence of one ormore azo groups (AN@NA) [2]. They can be further categorizedinto dye classes: acid, reactive, disperse, vat, metal complex, mor-dant, direct, basic and sulfur dyes. In this research, acid, reactiveand direct azo dyes were used as model pollutants. These dyesare colorants used for dyeing and printing of cellulosic fibers andtheir blends, as well as for dyeing of wool, cotton, nylon, silk, andmodified acrylics. In addition, they are used in the production ofpaper, leather, food and cosmetics [3].

The major research on wastewater treatment has been focusedon the elimination of azo dyes, essentially for three reasons: azodyes represent approximately 70% of the total dye market [1];large fractions of these dyes (10–50%) are lost during the dyeingprocess [4]; conventional wastewater treatment methods werefound to be inefficient for complete elimination of many azo dyesand often they end up in a production of secondary waste productsthat require further processing [5].

In the last two decades, advanced oxidation processes (AOPs),which rely on the high oxidizing potential particularly of hydroxylradicals (OH�), have been proposed as an alternative option fortreatment of undesirable organic pollutants, including dyestuff[6,7]. Among AOPs, heterogeneous photocatalysis seems to be anattractive method as it has been successfully used for the degrada-tion of various organic pollutants. Increased interest for the photo-catalytic degradation is likely due to the fact that this process mayuse atmospheric oxygen as the oxidant; it can be carried out underambient conditions and may lead to a total mineralization oforganics to CO2, water and mineral acids [8]. The most commonsemiconductor is titanium dioxide (TiO2) as it is inexpensive,widely abundant, photochemically stable, non-toxic and waterinsoluble under most environmental conditions [9].

The major obstacle for the practical use of photocatalysis is therequirement for the expensive liquid–solid separation, because of

M. Lucic et al. / Separation and Purification Technology 122 (2014) 206–216 207

the formation of milky dispersions after mixing the powder cata-lyst in water [10]. Therefore, development of TiO2 photocatalystsanchored to supporting materials would be of great significance.Various materials have been tested as scaffolds such as glass, acti-vated carbon, silica material and polymeric materials [11,12]. Oneof the novel matrixes exploited for immobilization of TiO2 nano-particles are hydrogels [13]. Hydrogels are three-dimensionalcrosslinked polymeric networks that are able to alter their volumeand properties in response to environmental stimuli such as pHand temperature. [14]. They are able to adsorb many pollutantswith fast response in large capacity [15].

In this study, photocatalytic activity of colloidal TiO2 nanoparti-cles (TiO2 NPs), synthetized by acidic hydrolysis of TiCl4, immobi-lized onto hydrogel based on chitosan, itaconic acid andmethacrylic acid (TiO2/hydrogel nanocomposite) was investigated.The average size of synthetized TiO2 NPs was approximately 6 nm[16]. It was previously reported that commercial Degussa P-25TiO2 NPs (�30 nm) showed extraordinary photocatalytic activity[13]. Therefore, the discoloration efficiency of both types of TiO2

NPs, immobilized onto hydrogel, was simultaneously studied.Our previous research indicated that immobilization method playssignificant role in the photocatalytic efficiency of TiO2 NPs. Hence,samples of synthetized plain hydrogel were dip-coated in the col-loid/suspension containing TiO2 NPs in current study. The objectiveof this study was to highlight whether synthetized TiO2/hydrogelnanocomposites were able to remove various textile azo dyes.TiO2/hydrogel nanocomposites were characterized using a Fouriertransform infrared spectroscopy (FTIR) and field emission scanningelectron microscopy/energy dispersive X-ray (FE-SEM/EDX), ther-mogravimetric analysis (TGA), differential scanning calorimetry(DSC) and rheological measurements. Reproducibility of the TiO2/hydrogel nanocomposites on discoloration efficiency was alsoinvestigated for the dye that was the most efficiently removed byTiO2/hydrogel nanocomposite.

2. Experimental

2.1. Materials

In this study, five textile dyes were tested: two acid dyes C.I.Acid Red 18 (AR18) and C.I. Acid Blue 113 (AB113), two reactivedyes C.I. Reactive Yellow 17 (RY17) and C.I. Reactive Black 5(RB5) and one direct dye C.I. Direct Blue 78 (DB78), maximumabsorption wavelength (kmax) of each dye is given in Table 1.

For hydrogel synthesis, chitosan (Ch, Fluka, middle viscous), ita-conic acid (IA, Fluka) and methacrylic acid (MAA, Sigma A.G.) wereused. In order to remove inhibitor methacrylic acid was vacuumdistillated before use. The crosslinking agent N,N0-methylenebis-acrylamide (MBA, Acros), redox pair potassium persulfate (KPS,Merck, p.a.) and potassium pyrosulfate (KPyS, Merck p.a.), wereused without further purification. Deionized water was used forall experiments. Buffer solutions were prepared using disodiumhydrogen phosphate (Na2HPO4, Lach-Ner p.a.) and sodium dihy-drogen phosphate (NaH2PO4, Lach-Ner p.a.).

As a photocatalyst, synthetized and commercial TiO2 NPs wereemployed. Commercial TiO2 NPs were supplied by Degussa

Table 1List of anionic azo dyes used in the study.

Anionic dye Abbreviation Supplier

C.I. Acid Red 18 AR18 CasselaC.I. Acid Blue 113 AB113 CasselaC.I. Reactive Yellow 17 RY17 Höchst AG.C.I. Reactive Black 5 RB5 Höchst AG.C.I. Direct Blue 78 DB78 Bezema AG.

Corporation, Germany (commercial grade, Degussa P-25, in furthertext D).

The synthesis of colloidal TiO2 NPs (C) by acidic hydrolysis ofTiCl4 was previously described [16]. The colloid was prepared ina manner analogous to the one proposed by Rajh et al. [17]. Thesolution of TiCl4 cooled down to �20 �C was added drop-wise tocooled water (at 4 �C) under vigorous stirring and then kept at thistemperature for 30 min. The pH of the solution was between 0 and1, depending on TiCl4 concentration. Slow growth of the particleswas achieved by dialysis against water at 4 �C until the pH of thesolution reached 3.5. The concentration of colloid was determinedfrom the concentration of the peroxide complex obtained after dis-solving the particles in concentrated sulfuric acid [18]. In order toenhance the crystallinity and overall efficiency of generated TiO2

NPs the colloid was thermally treated in reflux at 60 �C for 16 h.The synthesized colloid comprised of faceted, single crystalline,anatase TiO2 NPs with an average size of 6 nm [16].

2.2. Synthesis of TiO2/hydrogel nanocomposites

Hydrogel synthesis procedure is described in detail elsewhere[19]. In short, Ch and IA were ionically crosslinked and then MAAand crosslinker MBA were added to obtain Ch/IA/MAA hydrogel.Monomers ratio was Ch/IA/MAA = 1:1.56:10. Crosslinking agent(MBA) and redox pair (KPS/KPyS) concentrations were 0.2 wt%,with respect to the total weight of the reaction mixture. Polymer-ization was carried out at 50 �C for 3 h after which hydrogel wascut into discs and left in distilled water that was changed for 7 daysto remove all unreacted monomers. Blank sample (B) was synthe-tized without photocatalyst.

It was found in our previous research that samples with the bestphotocatalytic activity were those that were prepared by dip-coat-ing method. This method relied on the immobilization of TiO2 NPsonto hydrogel by immersing the blank hydrogel discs into photo-catalyst colloid solution/suspension. 0.2 M colloid solution of syn-thetized TiO2 NPs (0.2Ci) and 0.2 M suspension of Degussa P-25(0.2Di) were used for hydrogel modification. Discs were left toswell for 2 h. Subsequently, they were dried at room temperatureuntil the constant weight was obtained. Afterwards, samples wereleft 30 min at 80 �C and then were rinsed twice (5 min) with deion-ized water and dried at room temperature.

2.3. Hydrogel characterization

Presence of Ti was determined in energy-dispersive X-ray (EDX)spectroscopy mode using JEOL JSM-5800 scanning electron micro-scope. Sample morphology was characterized by field-emissionscanning electron microscopy (FE-SEM, Tescan Mira 3XMU). Priorto analysis, samples were swollen to equilibrium and then lyophi-lized. Lyophilized hydrogel discs were sputter coated with Au/Pdalloy.

Fourier transform infrared (FT-IR) spectra were recorded by Bo-mem MB 100 FT-IR spectrophotometer in the region 4000–400 cm�1. The samples were prepared in the form of pellets withKBr at room temperature and measured in the transmission mode.

kmax (nm) M.W. (g mol�1) pH of dye solution

507 604.47 6.1567 681.85 6.3430 682.77 7.5590 991.82 6.2603 1055.9 5.6

Fig. 1. FT-IR spectra of the samples (a) B, (b) 0.2Ci and (c) 0.2Di.

Fig. 2. TG (a) and DTG (b) curves of samples B, 0.2Ci and 0.2Di.

208 M. Lucic et al. / Separation and Purification Technology 122 (2014) 206–216

Thermo-gravimetric analyses (TGA) were conducted on a ther-mo microbalance (Netsch GmbH TG 209C) under a nitrogen atmo-sphere within the temperature range from 25 to 850 �C at a heatingrate of 10 �C min�1. All analyses were performed with approxi-mately 12 mg of sample in Al2O3 pans under the dynamic nitrogenatmosphere.

Differential scanning calorimetry (DSC) was performed on aDSC 7 equipped with a TAC 7/DX and CCA 2 (Perkin Elmer, Wal-tham, USA). DSC analysis was performed under a nitrogen flow of30 mL min�1 with a linear temperature programming of5 �C min�1.

Mechanical properties of synthetized samples were also inves-tigated. Frequency sweeps on hydrogel discs swollen to equilib-rium were recorded using plate–plate rheometer (Rheotec, HaakeMARS Thermo Fischer) equipped with RheoWin software using aPeltier element of 20 mm diameter. The complex shear moduliwere measured as a function of frequency, from 0.01 to 15 Hz, at25 �C.

Swelling studies were performed using dry samples (xerogels),which were immersed in excess of buffer solution (pH 5.6, 6.2 and7.5) at 25 �C. Progress of the swelling process was monitored gravi-metrically until equilibrium was reached. Swelling degree (q) wascalculated according to following equation:

q ¼ wt

w0ð1Þ

where w0 and wt present the initial weight of the xerogel andweight of the swollen hydrogel at time t, respectively. The equilib-rium degree of swelling was calculated as follows:

qe ¼we

w0ð2Þ

where we is the weight of hydrogel swollen to equilibrium.Pore size of the synthesized hydrogels was calculated by apply-

ing equilibrium swelling theory [20], according to followingequation:

n ¼ m�1=32;s � 2CnMc

Mr

� �12

� l ð3Þ

where m2,s is polymer volume fraction in the equilibrium state, Mc

average molar mass between the network crosslinks, Cn is the Florycharacteristic ratio and l is the carbonAcarbon bond length(0.154 nm). Detailed equations are given in literature [21].

2.4. Dye sorption from the solution in the dark

Experiment in the dark was performed in order to evaluatesorption abilities of the blank sample and TiO2/hydrogel nanocom-posites for selected azo dyes and to exclude photodegradationinfluence. 0.2 g of xerogel was placed in beaker with 25 mL ofdye solutions (10 mg L�1), after which beaker was transferred inwater bath with mechanical agitation and shaked throughout sorp-tion experiment. In the first 8 h (1, 2, 3, 4, 6 and 8 h), 3 mL of solu-tion were taken to monitor the remaining dye concentration by anUV/VIS spectrophotometer Cary 100 Scan (Varian), at a maximumabsorption wavelength, kmax, of each dye. The aliquot was then re-turned to the beaker. Afterwards, sample was left in the dark forthe next 16 h. Each measurement was done in duplicate. The aver-age percentage of dye removal from solution due to adsorption wascalculated by following equation:

Dð%Þ ¼ Co � Ct

Co� 100 ð4Þ

where C0 and Ct (mg L�1) are the liquid phase concentrations of dyeinitially and at time t.

2.5. Photocatalytic degradation of anionic azo dyes

Studies on photocatalytic activity of immobilized TiO2 NPs wereperformed in the same manner as those carried out in the dark, butin this case all experiments have been performed under the illumi-nation provided by ULTRA-VITALUX lamp, 300 W (Osram). The ap-plied lamp simulated the sun-like irradiation with a spectralradiation power distribution at wavelengths between 300 and1700 nm. The distance between the lamp and sample was set to

M. Lucic et al. / Separation and Purification Technology 122 (2014) 206–216 209

30 cm. Optical power was measured using R-752 Universal Radi-ometer Readout with sensor model PH-30, DIGIRAD and it was30 mW cm�2. The photodegradation process was followed for

Fig. 3. SEM images of cross-section of samples: (a) B; (b) 0.2Ci and (c) 0.2Di.

24 h. During the illumination the temperature increased from25 �C to 33 �C. Discoloration of dyes solution was measured at indi-cated time intervals (1, 2, 3, 4, 6 and 8 h) by a UV/VIS spectropho-tometer at kmax for each dye. After the first photodegradation cyclehas been finished, the nanocomposites discs were collected andleft to dry to the constant weight. The reproducibility of TiO2/hydrogel nanocomposite photocatalytic activity was tested threetimes under the same conditions in the fresh 25 mL of dye solutionthat showed the best results in the first photodegradation cycle.

3. Results and discussion

3.1. The influence of TiO2 NPs on the hydrogel structure and networkproperties

In order to confirm the presence of titanium in the hydrogels,EDX analysis of the samples B, 0.2Ci and 0.2Di was investigated.Typical EDX spectra of the samples with and without TiO2 aregiven as Supplementary data. The EDX spectrum of the blanksample did not show the characteristic peaks corresponding toTi. On the other hand, the EDX spectra of TiO2/hydrogel

Table 2Influence of pH value of surrounding media on the groups in chitosan, methacrylicand itaconic acid and TiO2 NPs.

pH Hydrogel Group on the surfaceof TiO2 NPs

Chitosan Acids

5.6 Predominantly NHþ3 COO� OHþ26.2 NH2; NHþ3 COO� OH7.5 Predominantly NH2 COO� O�

Table 3Equilibrium degree of swelling and pore size of TiO2/hydrogel nanocomposites.

Sample pH qe n (nm)

B 5.6 15.8 12.66.2 25.2 22.07.5 21.3 17.9

0.2Ci 5.6 13.0 10.06.2 21.8 18.37.5 21.5 18.1

0.2Di 5.6 15.3 12.16.2 23.0 19.67.5 22.4 19.0

Fig. 4. Shear modulus (G0) for B and TiO2/hydrogel nanocomposites at 25 �C.

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nanocomposites clearly showed the peaks related to Ti confirmingthe existence of Ti on the prepared hydrogel.

FT-IR spectra of the samples B, 0.2Ci and 0.2Di are presentedin Fig. 1. The FT-IR spectrum of the sample B shows characteris-tic peaks at 896 and 1155 cm�1 (asymmetric stretching of theCAOAC bridge) corresponding to saccharide structure [22,23].Sharp peaks at 1379 and 1421 cm�1 are assigned to the CH3

symmetric deformation mode, while the broad peaks at 1074and 1030 cm�1 are assigned to the CAO stretching vibration inchitosan [23]. The band around 3440 cm�1 corresponds to thestretching vibration of OAH and/or NAH [22], while peaks at3000 and 2927 cm�1 are assigned to the vibration of CH2 group,which are present in chitosan and acids [24]. The characteristicpeak of the carboxylic groups can be observed at 1718 cm�1. Apeak at 1541 cm�1, found in the FT-IR spectrum of B correspondsto the ionic interaction between chitosan and the acids [25]. Acomparison between spectra of sample B and samples containingTiO2 NPs implied that significant changes in peaks positionscould not be observed. After immobilization of TiO2 NPs ontohydrogel, characteristic peaks are found at the same wavenum-bers without substantial shifts.

The influence of TiO2 NPs presence on the thermal properties ofthe nanocomposite was examined by TGA and DSC under an inertatmosphere. The thermal stability of the TiO2/hydrogel nanocom-posites was compared with the thermal stability of the sample B.The thermogravimetric (TG) and differential thermogravimetric(DTG) curves of samples B, 0.2Ci and 0.2Di are shown in Fig. 2.

Fig. 5. (a and a0) Removal of dye AR18 in the dark and corresponding photographs of sillumination and corresponding sample photographs after 8 h and 24 h. Initial dye conceAR18 dye.

The thermal degradation mechanism of the synthetized blanksample and TiO2/hydrogel nanocomposites consists of three stepsinitiated by different modes. The weight loss that emerged around180 �C is a consequence of loss of physically absorbed moistureand/or evaporation of trapped solvent [26]. The second stage ap-pears in the range between 245 and 255 �C. This step could be as-signed to degradation of poly(methacrylic acid) chain [27]. Themain decomposition temperature occurs between 420 and 445 �Cwith a weight loss of approximately 45%. This corresponds to thesynergetic process of thermal decomposition of chitosan anddecarboxylation of previously formed anhydride groups and thebreaking of the main polymer backbone within polyitaconic acid.Pyrolysis of polysaccharides, namely chitosan, commences withthe random scission of glycosidic bonds, followed by a furtherdecomposition forming acetic and butyric acids and a series of low-er fatty acids, where C2, C3 and C6 predominate [28].

DSC thermograms of samples B, 0.2Ci and 0.2Di were recordedin the temperature interval between �20 and 150 �C. Glass transi-tion temperatures (Tg) for the samples B, 0.2Ci and 0.2Di werefound to be 59.6, 59.0 and 58.7 �C, respectively. It should bestressed that the Tg of the nanocomposites polymer matrixes wasslightly reduced by the addition of the TiO2 NPs. However, negligi-ble decrease of Tg indicated a lack of TiO2 NPs influence on the heatresistance in an inert atmosphere.

Based on FT-IR, TGA and DSC analyses of blank sample and TiO2/hydrogel nanocomposites, it could be concluded that the presenceof TiO2 NPs did not affect already formed covalent bonds within

amples B, 0.2Ci and 0.2Di after 8 h and 24 h; (b and b0) removal of dye AR18 underntration was 10 mg L�1, pH = 6.1, ULTRA-VITALUX lamp, 300 W. (c) The structure of

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polymer network, nor did they affect its stability. Obtained resultsdemonstrated that all three samples showed the same thermal sta-bility, regardless of the presence or type of immobilized photocat-alyst. In addition, FT-IR analysis did not reveal any shift in peaksposition in samples spectra indicating that TiO2 NPs were notinvolved in the formation of polymer network bonds.

The FE-SEM micrographs of samples cross-sections with andwithout TiO2 NPs are shown in Fig. 3. It can be observed that thepresence of TiO2 NPs affected porosity of the hydrogel. Pore sizeof the B is larger compared to the samples 0.2Ci and 0.2Di, indicat-ing that they decrease after immobilization of TiO2 NPs ontohydrogel by dip-coating method. Potential sites for interactionwith TiO2 NPs are available carboxyl groups on the surface ofhydrogel, originating from itaconic and methacrylic acids [29].Therefore, it is likely that after immobilization of nanoparticlesand interaction between carboxylic groups and TiO2 swelling ofhydrogel and the pore size are reduced.

Swelling behavior of blank hydrogel and TiO2/hydrogel nano-composites was investigated at three different pH values: 5.6, 6.2and 7.5, which were chosen because the pH values of aqueoussolution of studied dyes were in the selected range (Table 1). ThepKa of chitosan has been reported to be in the range of 6–6.5[30]. Thus, the aim was to investigate the influence of aminogroups from chitosan and its protonated and non-protonated formto swelling behavior of samples. The influence of the charge on thesurface of TiO2 NPs has been evaluated as well, since it was foundthat the point of zero charge of TiO2 is 6.2 [31].

Fig. 6. (a and a0) Removal of dye AB113 in the dark and corresponding photographs of sillumination and corresponding sample photographs after 8 h and 24 h . Initial dye conceAB113 dye.

Carboxylic groups in itaconic acid become ionized at pH above3.85 and 5.44 (pKa1 and pKa2, respectively) and in methacrylic acidabove its pKa = 4.66 [32], therefore at the investigated pH valuesthese groups are in COO� form (Table 2). Amino groups of chitosanare protonated below its pKa value, thus at pH 5.6 and 7.5 allgroups are either protonated or non-protonated, while at 6.2 thecharge changes so that both forms are present.

Change of charge on the surface of TiO2 NPs also occurs in theselected range of pH due to point of zero charge (Table 2). At pHbelow the pHpzc the surface of TiO2 would have a positive charge,while at pH above pHpzc negative charge is present. However, atthe pH 6.2 OH groups are present on the surface of TiO2 NPs, andthis state is called zero surface charge.

According to the results presented in Table 3, equilibrium de-gree of swelling of samples (Eq. (2)) was similar at pH 5.6 and6.2, where certain amount of the amino groups of chitosan werestill protonated. At these pH values the equilibrium degree ofswelling of sample B was higher compared to samples 0.2Ci and0.2Di. It is possible that during the immobilization of TiO2 NPs ontohydrogel surface, pores become covered and closed, as was con-firmed by SEM images (Fig. 3), due to interaction between TiO2

NPs and carboxylic groups. On the other hand, at pH above pKa,swelling behavior alters. At pH 7.5 there is no significant differencebetween swelling behaviors of all three samples. At investigatedpH value, amino groups from chitosan are no longer completelyprotonated, while all carboxylic groups are in their anionic form,so their contribution to the swelling degree of samples is the same.

amples B, 0.2Ci and 0.2Di after 8 h and 24 h; (b and b0) removal of dye AB113 underntration was 10 mg L�1, pH = 6.3, ULTRA-VITALUX lamp, 300 W. (c) The structure of

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Obtained results indicate that amino groups from chitosan signifi-cantly affect the swelling behavior of samples.

Further, the pore size of the investigated hydrogels, calculatedaccording to the Eq. (3), is given in Table 3. As it was expected, poresize of the blank hydrogel was the largest at pH 6.2 due to the factthat the equilibrium degree of swelling of the blank sample wasthe highest at this pH value.

It can be observed that pore size of TiO2/hydrogel nanocompos-ites is dependent on both, the pH value of the surrounding buffersolution and on the surface charge, as well as type (synthetizedcolloidal TiO2 and Degussa P-25) of immobilized TiO2 NPs. Thesmallest pores for sample B and nanocomposites are found at pH5.6. Smaller pores in sample B are due to interaction between pro-tonated amino groups from chitosan and carboxylic groups fromacids. In addition to these changes, interactions between positivecharge on the surface of TiO2 NPs and negatively charged carbox-ylic groups in nanocomposites occurred.

Equilibrium degree of swelling and pore size of TiO2/hydrogelnanocomposites were the highest at pH 6.2. However, it can be no-ticed that difference between those two parameters at pH 6.2 and7.5 is smaller compared to the blank sample. At pH 6.2 TiO2 NPs arein the state of zero surface charge, so there is no additional inter-action between either negatively charged carboxylic groups or pos-itively charged amino groups. However, at pH 7.5 O� are present

Fig. 7. (a and a0) Removal of dye RY17 in the dark and corresponding photographs of sillumination and corresponding sample photographs after 8 h and 24 h. Initial dye conceRY17 dye.

on the surface of TiO2 NPs forcing hydrogel to swell. Additionally,its pores increased compared to pore size of blank sample. At thesame time, these O� also might interact with few remaining NHþ3groups from chitosan suppressing hydrogel swelling. Conse-quently, the equilibrium degree of swelling at pH 7.5 is slightlylower and pore size is smaller than at pH value of 6.2. Dependingon the size of hydrogel pores, they can be conventionally classifiedas (1) macro-porous (0.1–1 lm); (2) micro-porous (10–100 nm)and (3) non-porous (1–10 nm). Based on the pore size, the investi-gated hydrogels are classified as micro-porous [14].

For environmental application and wastewater treatment con-sideration, hydrogels must be in swollen state. Photodegradationexperiments in this study were performed with mechanical aggita-tion in a laboratory shaker. Keeping in mind that mechanical prop-erties of swollen hydrogels are weaker compared to the xerogels, abreakage of samples or their destruction may occur. Therefore, theshear storage moduli (G0) was measured in order to determinewhether the samples are stable for this specific application. The ef-fect of TiO2 NPs on mechanical properties of the hydrogels was alsoevaluated, because it was shown that it affected swelling behaviorof samples, which is in direct correlation with mechanicalproperties.

The shear storage moduli (G0) was measured as a function offrequency (f), by applying an oscillatory shear strain and the

amples B, 0.2Ci and 0.2Di after 8 h and 24 h; (b and b0) removal of dye RY17 underntration was 10 mg L�1, pH = 7.5, ULTRA-VITALUX lamp, 300 W. (c) The structure of

M. Lucic et al. / Separation and Purification Technology 122 (2014) 206–216 213

dependence of G0 vs. f for samples B, 0.2Ci and 0.2Di, swollen toequilibrium, is given in Fig. 4. Although, mechanical propertieswere measured in the range from 0.01 to 15 Hz, the data pointsafter 7 Hz are not shown and could not be taken into account be-cause sample behavior was no longer in the viscoelastic region.Compared to sample B, immobilization of TiO2 NPs onto hydrogelby dip-coating method resulted in increase of G0 and thus im-proved mechanical properties of nanocomposites. Furthermore, itis evident that G0 also depends on the type of immobilized NPs.G0 values are higher for the samples with the colloidal TiO2 NPsthan for the samples modified with the Degussa P-25 NPs. The ob-tained results are in good correlation with SEM analysis and swell-ing experiments, where it was found that pore size and swellingdegree decreased during the immobilization of photocatalyst NPs.

3.2. Removal of anionic azo dyes by TiO2/hydrogel nanocomposite

3.2.1. Photolysis of azo dyesBefore the photodegradation experiments, the stability of dyes

and possible photolysis were investigated under illumination

Fig. 8. (a and a0) Removal of dye RB5 in the dark and corresponding photographs of sillumination and corresponding sample photographs after 8 h and 24 h. Initial dye conceRB5 dye.

provided by ULTRA-VITALUX lamp (300 W). 25 mL of each dyesolution was placed under lamp without addition of photocatalystand change of concentration was monitored using UV/VIS spectro-photometer. The decrease of concentration was detected only fordyes RB5 and AB113, while concentration of AR18, RY17 andDB78 remained constant during 8 h of illumination. It was foundthat after 8 h long illumination photolysis degradation rates forRB5 and AB113 were around 10% and 21%, respectively.

3.2.2. Removal of acid azo dyesThe photocatalytic activity of the TiO2/hydrogel nanocompos-

ites was evaluated from the degradation rate of different azo dyesin an aqueous solution with an initial concentration of 10 mg L�1

and measured by UV/VIS spectrophotometer. Remaining dye con-centrations in the solution were determined using a calibrationcurve and then percentage of dye removal was calculated accord-ing to Eq. (4). The results for dye AR18 removal in the dark and un-der illumination are demonstrated in Fig. 5 along with photographsof samples after 8 h and 24 h for both experiments.

Blank sample showed very low sorption affinity for dye AR18both, in the dark (Fig. 5a) and under illumination provided by

amples B, 0.2Ci and 0.2Di after 8 h and 24 h; (b and b0) removal of dye RB5 underntration was 10 mg L�1, pH = 6.2, ULTRA-VITALUX lamp, 300 W. (c) The structure of

Fig. 9. (a and a0) Removal of dye DB78 in the dark and corresponding photographs of samples B, 0.2Ci and 0.2Di after 8 h and 24 h; (b and b0) removal of dye DB78 underillumination and corresponding sample photographs after 8 h and 24 h. Initial dye concentration was 10 mg L�1, pH = 5.6, ULTRA-VITALUX lamp, 300 W. (c) The structure ofDB78 dye.

214 M. Lucic et al. / Separation and Purification Technology 122 (2014) 206–216

ULTRA-VITALUX lamp, 300 W (Fig. 5b). As expected, sample B can-not provide photodegradation of the dyes (Fig. 5b). These resultsare also illustrated on the photographs of the samples taken after8 and 24 h. It can be noticed that under illumination sample sorbedmore dye after 24 h, but the dye did not degrade as no photocata-lyst was present.

Samples 0.2Ci and 0.2Di both contained TiO2 NPs, but their sorp-tion/photodegradation behavior is shown to be totally different.When the experiment was conducted in the dark, the sample0.2Ci removed around 50% of dye from solution, while 0.2Di didnot remove dye at all. Possible explanation for such behavior isthat during the immobilization of colloidal TiO2 NPs larger activesurface is available for sorption of the dye.

Unlike in the dark, sample 0.2Di removed 20% of dye from thesolution and photodegraded it when illuminated as was confirmedby the image after 8 h. On the other hand, complete discolorationof the dye AR18 solution by 0.2Ci occurred within 6 h of illumina-tion, but from the image of nanocomposite it can be seen that discswere slightly colored. After 24 h of illumination dye completelyphotodegraded on the nanocomposite, indicating that in the last16 h photodegradation became dominant process.

Another investigated acid dye was AB113 (Fig. 6). Compared tothe AR18, sorption of AB113 in dark by samples B, 0.2Ci and 0.2Di

significantly increased. Keeping in mind that pH of both dye solu-tions was very similar (Table 1), it could be expected that surfacecharge of nanocomposites was the same (Table 2). Thus, thedifference in photocatalytic degradation efficiency of these dyeshas to be explained by difference in their molecular structures. Un-like AR18, AB113 has one sulfonic group less, which results in de-crease of dye solubility in water compared to AR18 and easierinteraction with protonated amino groups from chitosan is ob-tained [33].

As reported earlier, this dye is prone to photolysis, so there issynergetic effect of photolysis, sorption and photodegradation un-der illumination. For the samples B and 0.2Di similar amount of thedye was removed in the dark and under illumination, while 0.2Ci

removed dye to a much greater extent under illumination.

3.2.3. Removal of reactive azo dyesThe comparison between the curves corresponding to experi-

ments performed in the dark and those under illumination forthe dye RY17 (Fig. 7) indicates the similar removal trends. Dyesorption by sample B was low in the dark (8%) and under illumina-tion (13%). The pH value of the dye solution was 7.5. Hence, themost of the amino groups were not protonated, so they could notestablish interaction with dye that is in anionic form, while TiO2

Fig. 10. Reproducibility study of photodegradation of dye C.I. Acid Red 18 after 4cycles of UV illumination by sample 0.2Ci (8 h of illumination, initial concentration10 mg L�1). Light was supplied by ULTRA-VITALUX lamp, 300 W (Osram). (Forinterpretation of the references to colour in this figure legend, the reader is referredto the web version of this article.)

M. Lucic et al. / Separation and Purification Technology 122 (2014) 206–216 215

NPs were negatively charged, causing repulsion between nanopar-ticles surface and dye molecules.

Nanocomposites with both types of immobilized TiO2 NPs re-moved dye at the same percent in the dark and under illumination.This implies that maximum capacity for the removal of the dyewas achieved. However, under illumination a certain amount ofdye on the hydrogel photodegraded, which is illustrated on thephotographs of samples after 24 h long illumination.

Like in the case of AR18 and RY17, the only sample with highaffinity towards dye RB5 was 0.2Ci, while the other two samplesdid not remove any dye from the solution (Fig. 8a0). Photographsof the samples are in good correlation with the results presentedin Fig. 8a and b, and only sample with colloidal TiO2 NPs was inten-sively colored.

It was shown that dye RB5 removal after 8 h long photolysiswas found to be 10%. On the other hand, when photocatalysis isperformed by 0.2Ci complete degradation is much faster and itoccurs after 2 h. The degradation is also confirmed by the ab-sence of dye in the samples 0.2Ci (Fig. 8b0). Due to surface chargeof the nanocomposites (Table 2), OH groups at the surface TiO2

nanoparticles could interact with OH and NH2 groups in RB5.Larger removal of the dye RB5 in comparison with the RY17could be explained by the fact that it has two azo and two vinyl-sulfone groups, which are susceptible to photolytic and photo-catalytic degradation [34].

3.2.4. Removal of direct azo dyeExperiments in the dark pointed out that the dye DB78 could

not be removed without the illumination and sorption rate ofdye from the solution was low for all samples (Fig. 9a). There arefour sulfonic groups in the dye molecule that are enhancing solu-bility in water. Poor sorption could be also attributed to the mole-cule size. Among all studied dyes, molecular weight of DB78 is thelargest. Therefore, its diffusion into the polymer network is moredifficult.

However, the TiO2/hydrogel nanocomposites under illumina-tion provided the photodegradation of DB78 (Fig. 9b). Sample Bsorbed the dye and did not degrade it, hence it was colored atthe end of photodegradation process. On the other hand, bothTiO2/hydrogel nanocomposites removed dye from solution approx-imately at the same rate within 8 h of illumination, but did notcompletely degrade dye sorbed on the hydrogel. After 24 h of illu-

mination 0.2Di was discolored, while certain amount of dye still re-mained on the 0.2Ci.

3.3. Reproducibility of the TiO2/hydrogel nanocomposite for repetitiveuse

As mentioned before, a major advantage of TiO2 NPs immobili-zation onto hydrogels as a support is easy and inexpensive recov-ery. Since the dye AR18 is not prone to photolysis and the bestphotodegradation results in the first cycle were obtained by thesample 0.2Ci, this nanocomposite has been chosen for the repro-ducibility study. After the first illumination cycle was finished,the samples were collected and left to dry. Afterwards, the samesamples were used in fresh batch of dye solution and illuminatedagain. The results are presented in Fig. 10. In the first and the sec-ond photodegradation cycle complete discoloration of dye solutionand degradation of dye sorbed onto hydrogel occurred. After thethird cycle, decrease in photodegradation efficiency was evident,but it was still at satisfactory level of 85%. Further decrease wasseen in the forth cycle. Such decline for photodegradation effi-ciency could be attributed to generated by- and end-product ofphotocatalysis on nanocomposites surface in previous cycles,which blocked the TiO2 NPs activity.

4. Conclusion

TiO2/hydrogel nanocomposites were synthetized in order toevaluate their ability to photodegrade different textile azo dyes:acid dyes C.I. Acid Red 18, C.I. Acid Blue 113, reactive dyes C.I. Reac-tive Yellow 17, C.I. Reactive Black 5 and direct dye C.I. Direct Blue78 from aqueous solution. Two different types of TiO2 nanoparti-cles were immobilized: colloidal TiO2 nanoparticles synthetizedby acidic hydrolysis of TiCl4 and commercial Degussa P25 TiO2

nanoparticles.The presence of Ti in nanocomposites was confirmed by EDX

analysis. FT-IR, TGA and DSC analyses showed that presence ofTiO2 nanoparticles did not affect polymer network formation.SEM analysis, swelling and rheology studies revealed that poresize, swelling behavior and mechanical properties depend on thepresence and type of the immobilized TiO2 nanoparticles.

It was found that dyes AB113 and RB5 are prone to photolysis.Under illumination, nanocomposite with immobilized colloidalTiO2 nanoparticles successfully removed and photodegraded dyeAR18, AB113, RB5 and DB78, while the highest removal of dyeRY17 was 55%. Reproducibility study was performed for the dyeAR18 and after four cycles of illumination removal rate was 75%,indicating that prepared TiO2/hydrogel nanocomposites could bereused without significant loses of photocatalytic efficiency.

Acknowledgements

The authors acknowledge funding from the Ministry of Educa-tion, Science and Technological Development of the Republic ofSerbia, Project Nos. 172056 and 172062. The authors gratefullyacknowledge Dr. Mirjana Mihajlovic (University of Belgrade, Insti-tute for Biological Research ‘‘Siniša Stankovic’’, Serbia) for her helpin the sample lyophilization and Dr. Bojan Jokic (University of Bel-grade, Faculty of Technology and Metallurgy, Serbia) for providingFE-SEM measurements.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.seppur.2013.11.002.

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