Investigation of Photocatalytic Activity of Chitosan / Poly ( vinyl alcohol ) / TiO 2 / Boron Nanocomposites

In the current work, A series of chitosan/poly(vinyl alcohol)/TiO 2 /boron nanocomposites (CS/P/Ti-B) were prepared and their activity in the photocatalytic removal of non-steroidal anti-inflammatory drug (ibuprofen (IBP)) were evaluated for the first time. The nanocomposites were characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectrum (FT-IR), and thermogravimetric analysis (TG-DTG). TGA thermograms showed the enhanced thermal stability of nanocomposites with increasing Ti-B content. In the photocatalytic experiments, the effects of Ti-B content, catalyst dosage and initial concentration were discussed and a suitable photodegradation process for enhanced removal was proposed. The photocatalytic removal of IBP increased with increasing Ti-B ratio and the kinetic rate constant of Ti-B increased from 4.516 × 10 −2 min -1 (for raw Ti-B) to 5.346 × 10 −2 min -1 (for CS/P/Ti-B/1). This increment could be attributed to the decreased recombination rate of the photo-generated electron-hole pairs.


Introduction
In recent years, the presence of pharmaceutical residues in water has become a global concern.The non-steroidal anti-inflammatory drugs (NSAIDs) are pharmaceuticals which have been commonly detected in effluents from hospitals and urban wastewater treatment plants.Ibuprofen (IBP) was classified as one of eight pharmaceutical chemicals with fatal or chronic risks by the Organization for Economic Co-operation and Development [1].
Although IBP has hazard for human health, conventional treatment methods are ineffective to remove this pollutant [2].Therefore, efficient treatment method should be used to remove this NSAID from the aquatic environment.
Heterogeneous photocatalysis has been applied extensively for the oxidation of various organic pollutants.Titanium dioxide (TiO2) is the most suitable catalyst due to its chemical stability, low cost and high photoactivity [3].However, the photocatalytic activity of TiO2 is limited by fast recombination of photogenerated electron (e -) and hole (h + ) [4].Many studies have been conducted to enhance the catalytic efficiency of TiO2 by doping with metals or nonmetals [5].Another way is to suppress the recombination of excited e -and h + by supporting TiO2 on substrates such as zeolite, silica, activated carbon, alumina, and chitosan [6][7][8].
Due to its high mechanical strength and non-toxicity, chitosan is an effective substrate for TiO2 in both environmental and economical ways.Chitosan is a linear polysaccharide including amino and hydroxyl functional groups.Immobilizing TiO2 nanoparticles with chitosan could enhance the specific surface area and pore size [3,6,[9][10][11][12][13][14].Zainal et al. [3] showed that TiO2-Chitosan/Glass catalyst was efficient for removing monoazo dye via the combined effect of photocatalysis-adsorption. Afzal et al. [6] synthesized TiO2 supported chitosan nanocomposites by in-situ sol gel method and found that the reactive NH2 and OH -functional groups of chitosan were responsible for the methyl orange adsorption-photodecolorization process.Nawi et al. [11] stated that immobilized TiO2/chitosan catalyst had attractive features like long-term stability and super oxidizing agents.Preethi et al. [12] demonstrated that the chitosan as bio template offered controlled growth of titanium via Lewis base reaction.As a result, the existence of chitosan is favorable for TiO2 with the potential of increasing the photocatalytic activity via adsorption or via decreasing the recombination rate of electron-hole pairs.
Poly(vinyl alcohol) (PVA) is a water-soluble material with a large number of hydroxyl groups in it.Due to the distinctive intermolecular interactions and formation of hydrogen bonds between PVA and chitosan, chitosan/PVA composites have good mechanical property, adjustable pore size, favorable film-and particle-forming property and have been utilized as ideal adsorbents for removal of metal ions and organic dye from aqueous solutions [15].On the other hand, titanium dioxide immobilized with PVA have been used as effective catalysts for photocatalytic applications [15][16][17][18][19]. Jiang et al. [15] synthesized mesoporous titania spheres using chitosan and PVA hydrogel and evaluated the phenol degradation.Lei et al. [19] prepared PVA/TiO2 hybrid films as a promising recyclable TiO2 hybrid catalysts with high photocatalytic activity.
In the current work, TiO2 was initially doped with boron to enhance visible light response.Then, TiO2-Boron nanoparticles were immobilized into chitosan/PVA matrix.The photocatalytic activities of composites were compared via pharmaceutical degradation under visible light.The effects of catalyst dosage and initial concentration were discussed and a suitable photodegradation process for enhanced removal was proposed.

Chemicals and reagents
All chemicals were obtained commercially and used without further purification.

Preparation of TiO2/boron catalyst
Visible light-activated boron doped TiO2 catalysts were synthesized according to the procedure as described in the previous study [20].The weight content of boron was selected as 8.0 wt.%.Briefly, the desired amount of boric acid was dissolved in ethanol, the PEG-600 and concentrated nitric acid were added to the solution.Then, tetrabutyl titanate in ethanol was dripped into the boric acid solution and the gel was treated hydrothermally in an autoclave at 180 o C and calcined at 500 o C. The resultant sample was coded as Ti-B.The Ti-B catalyst was of well-developed anatase phase with 90.24 m 2 /g specific surface area and the mean pore diameter was around 10.64 nm [20].

Preparation of chitosan based TiO2/boron nanocomposites
Chitosan (CS) was dissolved in 2 wt.% acetic acid solution at 25 o C and left overnight with continuous mechanical stirring.Polyvinyl alcohol (PVA) with a concentration of 5 g/100 mL was dissolved in distilled water by heating the mixture at 70 °C on hot plates and stirring for about 1 h.The two solutions were then mixed at a 1:1 ratio and at room temperature using the magnetic stirrer.TiO2-Boron powder was added into the above solution and continuously stirred to avoid aggregation of the particles.The blend solution was ultrasonically degassed to remove air bubbles, cast onto the petri dishes and dried in freeze-dryer.(TiO2-Boron):(Chitosan/PVA) ratio was chosen as 0.5, 1.0 and 2.0 (wt.%).The composites are designated as CS/P/Ti-B/0.5,CS/P/Ti-B/1 and CS/P/Ti-B/2 according to the mass ratio of Ti-B to chitosan/PVA.The neat chitosan/PVA films were also prepared without adding Ti-B powder and coded as CS/P.

Characterization studies
The morphology of composite catalysts was examined by scanning electron microscope (SEM) equipped with a Bruker energy dispersive X-Ray (EDX) detector (Philips XL30 ESEM-FEG/EDAX).The surface functional groups were analyzed by Fourier transfer infrared (FTIR) spectrum (Perkin Elmer Spectrum One).The elemental composition of the catalysts was determined by X-ray photoelectron spectroscopy (XPS) measurements (Thermo Scientific K-Alpha X-ray Photoelectron Spectrometer).Thermogravimetric analysis (TG-DTG) was conducted in air flow with a Simultaneous TGA-DTA Instrument (SEIKO).

Photocatalytic degradation experiments
Photocatalytic degradation studies were performed in a laboratory scale square shaped reactor equipped with two visible metal halide lamps (λ: 400-800 nm).The system is cooled through a fan at the bottom of the cabinet.For each degradation experiment, IBP solution with concentration of 10 mg/L was prepared in 50 mL double distilled water followed by adjusting the pH.Before the photocatalytic experiments, the solution was stirred for 1 h in dark to achieve adsorption equilibrium.Aliquots (5 mL) were collected at defined time intervals and filtered with 0.45 µm syringe membrane filters.The IBP concentration was analyzed by High Performance Liquid Chromatography (Dıonex Ultimate 3000 UHPLC) equipped with Acclaim C18 column (4,6 mm × 150 mm, 3 μm, 120 A o ).The eluent used was acetonitrile/water (80/20, v/v) at a flow rate of 0.2 mL/min and the adsorption wavelength was selected as 214 nm.The effects of catalyst dosage (1.0, 2.0, 3.0 g/L), and initial concentration (10, 20, 30 mg/L) on the photocatalytic degradation were examined.

Catalysts characterization
Figure 1 shows the SEM images of CS/P and CS/P/Ti-B nanocomposites.The smooth surface of CS/P was altered as rough surface with increasing TiO2-Boron ratio.The Ti-B nanoparticles with dimension in the range of 16-25 nm are found as agglomerated and merged into the CS/P matrix.

Figure 1. SEM images of samples
The EDX element mapping results of CS/P/Ti-B/2 are shown in Figure 2. The oxygen and carbon atoms formed the main structure of composite where titanium dispersed over a wide area and slight agglomeration occurred in the composite structure.Previous study [20] showed that the boron formed via Ti-O-B bonds in the TiO2 structure.The mapping analysis (Figure 2) revealed that the boron was homogenously dispersed in the Ti/B catalyst.The oxygen, titanium, carbon and boron contents were found as 15.7%, 22.7%, 53.8% and 7.6%, respectively.In the FTIR spectra (Figure 3), the characteristic absorption bands at appeared both in raw chitosan and in nanocomposites.The broad peaks at about 3160 cm −1 are ascribed to the amine N-H symmetrical vibrations.The absorption peaks near 2800 cm -1 correspond to asymmetric stretching of C-H bonds of chitosan.The peaks at 1550 cm -1 ascribe to N-H deformation groups while the broad peaks at 1057 cm -1 are due to C-O bending vibrations.The absorption bands are also detected at 1021 cm −1 and 1401 cm −1 which are C-C and C-N stretching vibrations, respectively.Compared with raw CS/P, a new peak around 750 cm −1 appeared in the FTIR spectra of CS/P/Ti-B/1 and CS/P/Ti-B/2 samples corresponding to Ti-O-Ti groups.Moreover, for nanocomposite samples, the new bands at 1260 cm -1 were assigned to the Ti-O-C groups [19], indicating that TiO2/Boron nanoparticles are chemically bonded to the surface of CS/P membrane rather than being adsorbed on the surface [21].The formation of covalent Ti-O-C bonds also revealed the effective immobilization of Ti/B nanoparticles in CS/P matrix [19].When temperature increased, the raw CS/P was not thermally stable and 75% of the sample decomposed before 600 o C. For CS/P/Ti-B/2 nanocomposite, a weight loss of 5.2% at 50-100 o C is due to the evaporation of water.The second stage starts at 200-400 o C with a weight loss of 29.7% due to the thermal and oxidative decomposition of composite.The residual mass at 800 o C were found as 32.9%, 38.1% and 51.7% for CS/P/Ti-B/0.5,CS/P/Ti-B/1 and CS/P/Ti-B/2 catalysts, respectively.With the increase in Ti-B content, the weight-loss decreased gradually indicating the enhanced thermal stability of nanocomposites.This could be attributed to the crystalline regions and Ti-O-C chemical bonds which act as crosslinking sites to keep CS/PVA stable [19].
In order to examine the surface compositions of raw CS/P and CS/P/Ti-B composites, X-ray photoelectron spectroscopy (XPS) was conducted.As seen in Figure 5, all samples have main peaks at 531.9 eV, 285 eV and 399 eV corresponding O 1s, C 1s and N 1s chemical states, respectively.After doped with Ti-B nanoparticles, Ti 2p peak was located at about 458 eV confirming the presence of Ti 4+ in the chitosan/PVA matrix.However, no peaks of boron were detected owing to the small quantity of boron in the structure.

Photocatalytic studies
Photocatalytic degradation experiments were conducted with raw CS/P and CS/P/Ti-B composites under visible light illumination.The IBP solution with concentration of 10 mg/L were illuminated with 2 g/L catalyst at natural pH∼6.5 and the results are represented in Figure 6.Under visible light irradiation no degradation of IBP was observed in the absence of catalyst over a period of 240 min.To ensure the adsorption performance, IBP was treated with raw CS/P sample and at the end of 4 h, 20.4% removal was achieved.After 3 h treatment with composites under visible light, the degradation rate of CS/P/Ti-B/0.5,CS/P/Ti-B/1 and CS/P/Ti-B/2 were 62.3%, 93.1% and 95.7%, respectively.The high photocatalytic activity of CS/P/Ti-B/2 composite due to the synergistic effect of the photocatalysis and adsorption [9,11,22].Since the adsorption efficiency of CS/P can result in higher concentration of IBP molecules near the catalyst, the adsorption performance of CS/P plays an important role for the enhancement in the photodegradation over CS/P/Ti-B catalyst.In addition, the main factor affecting the enhanced degradation could be due to the decreased recombination rate of the photo-induced electronhole pairs which was achieved with introducing TiO2/B into the chitosan/PVA matrix.
It is observed that CS/P/Ti-B/2 nanocomposite shows higher photocatalytic efficiency when compared with Ti/B powder.At the end of 2 h, the removal efficiency of IBP by CS/P/Ti-B/2 is calculated to be 93.9%,which is about 1.05 times of Ti/B (89.1%).On the other hand, CS/P/Ti-B/1 shows similar photocatalytic performance with Ti/B powder.Due to the practical drawbacks of using powder catalyst, such as agglomeration during utility and difficulty in separating from treated effluent [15], CS/P/Ti-B composite is a good alternative to remove organic pollutants from aqueous solution.The Langmuir-Hinshelwood kinetic model was used in order to examine the photocatalytic removal: where r is the rate of degradation, KC and kc reflect the adsorption equilibrium and kinetic constants, respectively.At low initial concentrations, the term KCC in the denominator can be neglected with respect to unity and the rate becomes, apparently, first order: where kapp (min -1 ) is the apparent rate constant.After integration of Eq.( 2 where C and C0 are the IBP concentrations at time t and at the start of the degradation, respectively.The rate constants for IBP degradation are shown in Table 1.The IBP photodegradation was enhanced significantly with increasing the (Ti-B): (CS/P) ratio from 0.5 to 2.0.The highest kinetic constant (6.650 × 10 −2 min -1 ) was obtained with CS/P/Ti-B/2 nanocomposite.Moreover, the kapp value of Ti-B increased from 4.516 × 10 −2 min -1 to 5.346 × 10 −2 min -1 when the CS/P/Ti-B/1 catalyst was used.

Effect of catalyst dosage
In the photocatalytic process, the catalyst dosage should be optimized for maximum degradation since it may effect in unfavorable light scattering and decrease the photon absorption efficiency.The effect of catalyst dosage on the degradation of IBP was studied with the dosages varied from 1.0 g/L to 3.0 g/L.degradation efficiency increased from 78.3% to 95.7%This increase could be ascribed to the increment in the active sites available for IBP degradation.However, further increase has a slight effect on the IBP removal which could be attributed to the difficult light penetration with catalyst agglomeration [23].Consequently, 2.0 g/L catalyst loading was chosen as optimum for further experiments.

Effect of initial concentration
Figure 8 shows the effect of varying initial concentration on IBP degradation by CS/P/Ti-B/2 catalyst.IBP solution with 10 mg/L concentration was degraded at a faster rate than 20 mg/L and 30 mg/L solutions.The IBP removal decreased from 98.6% to 76.8% when the initial concentration increased from 10 mg/L to 30 mg/L.The decrease can be ascribed to the saturation of the photocatalytic active sites by higher concentration of substrate and reduction the photons interaction [12,24].Due to the saturation, the drug molecules cannot easily contact with the photo-induced holes of catalyst [25].On the other hand, with increasing initial concentration, more IBP molecules were adsorbed on the catalyst surface, and the generation of OH radicals at the catalyst surface was reduced since the active sites were occupied by IBP molecules [24,26].

Discussion and Conclusion
Photocatalytic degradation of a non-steroidal antiinflammatory drug, ibuprofen (IBP) was first investigated by using chitosan/poly(vinyl alcohol)/TiO2/Boron nanocomposites.Boron doped TiO2 catalysts were prepared by solvothermal method and then immobilized into chitosan/poly(vinyl alcohol) matrix.SEM images revealed that the smooth surface of CS/P was altered as rough surface with increasing TiO2-Boron ratio.EDX and SEM mapping analysis demonstrated the presence of titanium and boron elements in the polymer matrix.TGA thermograms showed the enhanced thermal stability of nanocomposites with increasing Ti-B content.The photocatalytic experiments were conducted under visible light irradiation.The IBP photocatalytic removal was found higher with increasing Ti-B ratio which could be attributed to the decreased recombination rate of the photo-generated electron-hole pairs.

Figure 3 .
Figure 3. FTIR spectra of samples As the thermogravimetric analysis is considered as the most essential method for studying thermal stability of polymers, the influence of the TiO2/B crystal structure on the polymer template was examined by TGA.The results are shown in Figure 4.The weight loss of 9.5% at 50-100 o C is due to the moisture vaporization.The other weight loss at 200-300 o C is due to the decomposition of chitosan chain.

Figure 4 .
Figure 4. TGA results of samples

Figure 8 .
Figure 8.Effect of IBP concentration on the photocatalytic degradation (Catalyst dose: 2 g/L)