Preparation of Chitosan－TiO2Composite and Application in the De－coloration of Reactive Dyes Wastewater

ZHANG Xiao－li(張曉莉)，TANG Ke－yong(湯克勇)，LIU Jie(劉 捷)

1 College of Materials Science and Engineering，Zhengzhou University，Zhengzhou 450052，China

2 College of Textiles，Zhongyuan University of Technology，Zhengzhou 450007，China

Preparation of Chitosan－TiO2Composite and Application in the De－coloration of Reactive Dyes Wastewater

ZHANG Xiao－li(張曉莉)1，2，TANG Ke－yong(湯克勇)1*，LIU Jie(劉 捷)1

1 College of Materials Science and Engineering，Zhengzhou University，Zhengzhou 450052，China

2 College of Textiles，Zhongyuan University of Technology，Zhengzhou 450007，China

Chitosan-TiO2composite has been prepared by the sol-gel process．Tetrabutyl titanate(TTB)was used as a precursor to obtain nano TiO2sol，which was then added into acid solution of chitosan to form titania network in the matrix．SEM，TG，and Fourier transform infrared spectroscopy(FTIR)were employed to characterize morphology and structure ofthe Chitosan-TiO2composite．The resulting hybrid has potential applications for the adsorption and sonolytic-degradation of organic dyes in wastewater．The result shows that the de-coloring ratio reaches 97．2%at the optimum conditions with the help of ultrasound．

Chitosan-TiO2;sonolytic-degradation;dyeing wastewater; de-coloration

Introduction

Nowadays，the pollution of wastewater has gained a lot of attention．The major pollution comes from industries such as textile，leathermaking，and papermaking．In these industries，dyes are usually used to endow the products with different colors．During dyeing process，about 1%to 15%of the dyestuffs can not be absorbed and would be discharged into the effluents．Besides，dyestuff－contained wastewater is known to have high salt contentand poorbiodegradation［1］．The dyestuffsin wastewater must be removed before being discharged into water bodies or on land to prevent further pollution．Reactive dyes are extensively used in textile industry，fundamentally because of the ability of their reactive groups to bind to textile fibres by covalent bonds．However，hydrolyzed reactive dyes can not be fixed on the substrate and would be lost in wastewater［2］．Therefore，reactive dye is chosen as a model pollutant in the present study．

Many different wastewater treatment methods such as flocculation［3］， activated carbon adsorption， electrocoagulation［4－5］，irradiation［6］，and oxidation［7－8］have been intensively studied and found applications in various fields．The use of cheap adsorbents to remove the dyes in wastewater is one of the most effective ways to get rid of the organic dye pollutant［9］．In recent years，many studies have been focused on looking for cheap， locally available， and effective adsorbents［10］．

Chitosan is obtained by N－deacetylation of chitin，the second most abundant natural polysaccharide after cellulose［11］．Chitosan has been reported for the high adsorption potentials of dyes［12－13］，metal ions［14］，organic acids，and pesticide［15］．It is extremely attractive，most due to its biocompatibility，biodegradability，and non toxicity as well as its fungistatic and antibacterial activity［16］．In aqueous solutions，the amine groups of chitosan can be protonated，which does well for the strong adsorption of anionic dyes by electrostatic attraction．Besides，the binding ability of chitosan with metal ion is attributed to the chelating groupsof—NH2and —OH on the chitosan． However，pure chitosan is not good in mechanical properties．Remarkable shrinkage and deformation will appear after being dried．The solubility under acidic conditions and compressibility at high pressure is usually very poor［11］．Several methods to overcome these shortcomings have been performed by the use of metal oxide complex［6，17－19］and crosslinking［20］．Chitosan－TiO2microporous materials were also used in the de－coloring of dyes solutions［21－22］．

Photocatalysisofsemiconductoris an attractive and efficient way to treat industrial wastewater［23］．Nowadays，nano inorganic material such as TiO2is recognized as one of the most efficient，non－toxic，and stable photocatalyst［24］．However，the photocatalysis of TiO2can only happen when being irradiated by solar light or ultraviolet．When the wastewater is turbid or the transparency is very low，the degradation of pollutant will be not good enough．As a new type of oxidation method，sonolytic degradation technology has been successfully used in wastewater treatment［25－26］．The penetration of ultrasound in water is much stronger than that ofvisible light or ultraviolet．So the ultrasound technology can be used instead of ultraviolet light for sewage treatment．In order to strengthen the effect of ultrasonic degradation，the research on sonolytic degradation catalyst is especially important．

In this work，we describe the synthesis of Chitosan－TiO2composite by sol－gel process．The Chitosan－TiO2composite was then characterized with SEM，TG，and Fourier transform infrared spectroscopy(FTIR)analyses．The removel of color using Chitosan－TiO2composite at different conditions was studied．The Chitosan－TiO2composite was found to have a good de－colorization by sonolytic degradation and adsorption at suitable conditions．Chitosan－TiO2composite as a new catalytic material would be widely used in sonolytic degradation treatment of dyeing wastewater．

1 Experimental

1．1 Materials

Chitosan with a deacetylation degree of 95%was obtained from Shandong AoKang Biotech Co．，Ltd．(Jinan，China)．Tetrabutyl titanate(TTB)was of chemical grade and obtained from Sinopharm ChemicalReagentCo．，Ltd．Shanghai，China．Acetic acid and sodium hydroxide were analytical reagent．Red reactive dye was purchased from the local market．

1．2 Preparation of Chitosan-TiO2composite

1．2．1 Preparation of TiO2sol

The mole ratio among TTB，distilled water，and acetic acid was 1∶73．89∶3．78．The one half acetic acid and distilled water were fully mixed as solution A and then，TTB was mixed with the other half acetic acid as solution B．In the ice bath，solution B was added into solution A drop by drop while being quickly stirred for 3 h，and then let it alone for 2 h．When twolayers were formed in the mixture，the lower layer was the TiO2sol(42 g/L of TiO2in concentration)for subsequent use．

1．2．2 Preparation of Chitosan-TiO2

Chitosan was added into 2% acetic acid solution．After being stirred for 2 h，the chitosan was completely dissolved，yielding 20 g/L chitosan acetic acid solution．After 3 mL TiO2sol was added into 10 g chitosan solution，25% by weight sodium hydroxide solution was gradually dropped into the mixture untilthe pH reached 7．So TiO2and chitosan precipitated at the same time．TiO2particles existed among the chitosan molecules and the crystallization of chitosan was avoided．Then the precipitation was filtered，and dried in vacuum at 100℃．The proposed structure of the Chitosan－TiO2composite was shown in Fig．1［27］．

Fig．1 The proposed structure of the Chitosan－TiO2composite

1．2．3 Crosslinking degree

Pure chitosan and Chitosan－TiO2composite were respectively added in 2%acetic acid solution and then stirred for 2h．Pure chitosan was completely dissolved，while Chitosan－TiO2composite was partly dissolved．The undissolved Chitosan－TiO2composite wasdried and weighed to calculate the crosslinking degree as follows

where W0is the weight of samples before dissolved in acetic acid solution and W is the weight of samples after being dissolved in acetic acid solution．

The crosslinking degree of Chitosan－TiO2composite used in the study was 55%．

1．3 De-colorization of red reactive dye

Red reactive dye(λmax=564 nm)was chosen as a model in the dyestuff wastewater studied here．Chitosan or Chitosan－TiO2composite was added into a beaker with 20 mL red reactive dye wastewater in an ultrasonic cleaningdevice with the frequency of 40 kHz and the power of 250 W．After 2 h，the mixture was centrifuged，and the upper clear liquid was subjected to visible－spectrophotometer for absorbance．The decoloring ratio was calculated by the following formula

where A0is the absorbency of the wastewater before processing，and Atis absorbency of the wastewater at the processing time of t．

1．4 Characterization of Chitosan-TiO2composite

The surface morphology of the Chitosan－TiO2composite was studied by SEM．Small pieces of the Chitosan－TiO2composite were stuck on stubs using double－sided tape．Before being studied，the samples were sputtered with a layer of gold to avoid being charged．FTIR spectra of chitosan and Chitosan－TiO2composite were obtained at the wavelength range of 500－4000 cm－1．TG of pure chitosan and Chitosan－TiO2composite was measured in pure N2at the heating rate of 20 K/min．

2 Results and Discussion

2．1 Surface morphology of Chitosan-TiO2composite

Fig．2 SEM of pure chitosan and Chitosan－TiO2composite

From Fig．2，it was observed that the chitosan surface was flake and smooth．For the Chitosan－TiO2composite，however，the surface appeared very rough．It looked as some particles had filled up．The change in the surface morphology should have great effects on the de－coloring capacity．There are basically three potential sites for sono－chemical activity［26］:(1)the gaseous region of the cavitation bubble，(2)the bubble－liquid interface，and(3)the liquid bulk，with series of reactions leading to the formation of more OH radicals．The specific surface area of the composite is greater than that of pure chitosan，resulting in an increase of the gaseous region of the cavitation bubble good for sono－chemical activity．

2．2 FTIR spectra of pure chitosan and Chitosan-TiO2

FTIR spectra usually provide information on the chemical structure of samples．The FTIR spectra for chitosan and dried Chitosan－TiO2composite were shown in Fig．3．The absorption band of pure chitosan at 3423 cm－1was attributed to the stretching vibration of amine (—NH2) and hydroxyl (—OH)．The absorption bands of pure chitosan at 1654 and 1379 cm－1were attributed to the bending vibration of both functional groups of amine(—NH2)and hydroxyl(—OH)．The two groups on chitosan chains may serve as sites of coordination and reaction for the adsorption of transition metals and organic species［19］．The peak at 2879 cm－1should belong to the symmetric stretching of—CH2— of chitosan．The peak of pure chitosan around 1079 cm－1was attributed to the stretching vibration of—C—O— and—C—N—．The band change at 1079 cm－1of pure chitosan to 1020 cm－1of chitosan—TiO2should belong to the hydrogen bond of Ti—OH on—OH group of chitosan and the cormation of Ti—O—C．Absorption spectra of dried Chitosan—TiO2composite at 1411，1562，and 3433 cm－1were due to the hydroxyl groups，and absorption at 1708 and 1640 cm－1were attributed to the amine or amide groups in them．Compared with that of pure chitosan in the finger print region，the change in 805－523 cm－1is due to the presence of Ti—O—Ti．Both peaks at 642 and at 523 cm－1indicated the existence of TiO2compound．

Fig．3 FTIR spectra of pure chitosan and Chitosan－TiO2composite

2．3 TG and DTG of Chitosan-TiO2

The thermal stabilities of the pure chitosan and chitosan－TiO2composite were studied by a TG analysis to get the curves as shown in Fig．4．For the pure chitosan，the weight loss starts at about 66．5℃ and the rapidest weight loss is at 300．6℃．When the temperature reached 600℃，the residual chitosan was about 23%in weight．In both the TG and DTG curves of the Chitosan－TiO2composite，the little weight loss at 73．5℃ may be probably due to the evaporation of the water absorbed in it．Two decomposition peaks appeared at 264．1 and 434．2℃ in the TG and DTG curves of the composite．The first one may be the thermolysis of the chitosan chain not cross linked by TiO2sol．The introduction of TiO2in the molecular network of chitosan may preventthe crystallization ofchitosan．So the first thermolysis temperature of chitosan－TiO2is lower than that of pure chitosan．The second one at 434．2 ℃ may be the decomposition of the chitosan chain crosslinked by TiO2sol，which may be proved by the IR spectrum analysis．The residual weight of Chitosan－TiO2composite was 55%，much more than that of the pure chitosan，because most of the extra residual components at 600℃ are mostly titanium dioxide．So it was indicated that chitosan－TiO2composite exhibited better thermal stability than the pure chitosan．

Fig．4 TG and DTG curves of pure chitosan and Chitosan－TiO2composite

2．4 Comparing ofadsorption and sonolyticdegradation

From Fig．5，it was observed that the de－coloring ratio of pure chitosan was higher than Chitosan－TiO2composite for normal adsorption．The result is because the adsorption of dyestuff depended on the amount of—NH2group，and—NH2group of pure chitosan was more than Chitosan－TiO2composite．While sonolytic－degradation was composed of adsorption and degradation，the de－coloring ratio of sonolytic－degradation washigher than that of adsorption from Fig．5．It was interesting that for the sonolytic method the de－coloring ratio of Chitosan－TiO2composite was higher than that of pure chitosan．This was due to that TiO2was semiconductor material usually used as photocatalyst．The TiO2sol was crosslinked between large moleculesofchitosan． The crosslinking degree forthe composite was 55%by weight method．Then the crystallization of chitosan was limited，resulting in the structure shown in Fig．1 and rough surfaces from SEM．Besides，by sonolytic method，the water may be decomposed into lively hydroxyl radicals(·OH)at the surface of the Chitosan－TiO2composite，and these lively hydroxyl radicals would attack the color groups of dye［28］．As a result，the destruction of chromophores appeared to increase the de－coloring ratio．

Fig．5 Effect of de－coloring method on de－coloring ratio

2．5 Factors affecting the de-coloring of Chitosan-TiO2composite by sonolysis

Different amounts of Chitosan－TiO2composite were added into wastewater containing 100 mg/L red reactive dyestuff．Various amounts of Chitosan－TiO2composite on the de－coloring ratio，such as 2．5，5．0，7．5，10．0，and 12．5 g/L were studied．The pH of the solution was 7．After being ultrasonic treated for 2 h，the mixture was centrifuged，and the clear liquid was measured for absorbency to calculate the de－coloring rate．

In Fig．6，it was indicated that with the increasing of Chitosan－TiO2concentration in the wastewater，the de－coloring effect was obviously increased．No obvious de－coloring rate increase was found when the Chitosan－TiO2concentration reached more than 10 g/L．This may be because that the adsorption and degradation ofChitosan－TiO2to dyestuffs reached equilibrium and the most dyestuffs were removed．

Fig．6 Effects of Chitosan－TiO2concentration on de－coloring ratio

Chitosan－TiO2composite of 10 g/L was put in wastewater with 100 mg/L red reactive dyestuff for different times，i．e．，0．5，1．0，1．5，2．0，and 2．5 h at the pH of 7 to yield the results as shown in Fig．7．From Fig．7，the de－coloring ratio reached 91．1%in 0．5 h．The longer the ultrasonic processing time is，the higher the de－coloring ratio is．The de－coloring ratio may reach equilibrium，97．2%，at the ultrasound processing time of 2 h．No more increase was found for the ultrasonic processing time beyond 2 h． This illustrated thatthe decolorizing efficiency by sonolysis was high．The rapid speed and high rate of the treatment were mainly due to the adsorption and the sonolytic－degradation of Chitosan－TiO2composite．

Fig．7 Effects of de－coloring time on de－coloring ratio

Chitosan－TiO2composite of 10 g/L was put in the wastewater containing 100 mg/L red reactive dyestuff at various pH，i．e．，2，5，7，8．5，and 11．The mixtures were ultrasonic processed for 2 h to yield the results as shown in Fig．8．The best pH to get a high de－coloring ratio was at neutral，and the second came the weak acidity or weak alkali condition．At pH=2，the de－coloring effect is the worst，only 59．7%．In acid solution，there were a lot of H+．Hydroxyl free radical (·OH)would react with H+，resulting in a decrease of hydroxyl free radical，which was not benefit for the degradation of dyes．At a high pH，the Chitosan－TiO2surface would be negatively charged．There would be repulsive force between Chitosan－TiO2composite and dyes．So the appropriate pH was 7．

Fig．8 Effects of pH on de－coloring ratio

3 Conclusions

By sol－gel method Chitosan－TiO2composite was produced．From SEM，the Chitosan－TiO2surface was rough．And Ti—O—C bond may be formed in the composite from FTIR and TGanalyses．The synergistic effect of adsorptive and sonolyticdegradation ability ofprepared Chitosan－TiO2on dyeing wastewater was identified．At the optimum conditions with the help of ultrasound，the de－coloring rate may reach as high as 97．2%，which means that the wastewater becomes nearly clear．Chitosan－TiO2composite as a new catalytic material would be widely used in sonolytic degradation treatment ofdyeing wastewater．

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X791

A

1672－5220(2015)03－0390－05

date:2014－02－26

s:National Natural Science Foundations of China(Nos．50973097，21076199)

*Correspondence should be addressed to TANG Ke－yong，E－mail:keyongtang@yahoo．com．cn