Journal of Materials
Volume 2015 (2015), Article ID 636409, 11 pages
http://dx.doi.org/10.1155/2015/636409

Research Article

Photocatalytic Degradation of Eosin Yellow Using Poly(pyrrole-co-aniline)-Coated TiO₂/Nanocellulose Composite under Solar Light Irradiation

T. S. Anirudhan and S. R. Rejeena

Department of Chemistry, University of Kerala, Kariavattom, Trivandrum 695 581, India

Received 8 June 2015; Revised 26 August 2015; Accepted 6 September 2015

Academic Editor: Carmen Alvarez-Lorenzo

Copyright © 2015 T. S. Anirudhan and S. R. Rejeena. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

The present study describes the feasibility of a novel adsorbent cum photocatalyst, poly(pyrrole-co-aniline)-coated TiO₂/nanocellulose composite (P(Py-co-An)-TiO₂/NCC), to remove eosin yellow (EY) from aqueous solutions. The removal of EY was investigated by batch adsorption followed by photocatalysis. The effect of various adsorption parameters like adsorbent dose, pH, contact time, initial concentration, and ionic strength has been optimized for treating effluents from the dye industry. Adsorption of EY reached maximum at pH 4.5 and complete removal of dye was achieved using 3.5 g/L of P(Py-co-An)-TiO₂/NCC. Adsorption equilibrium data were fitted with Langmuir and Fritz-Schlunder isotherm models and the kinetics of adsorption follows a second-order mechanism. The adsorption capacity of P(Py-co-An)-TiO₂/NCC was found to be 3.39 × 10⁻⁵ mol/g and reached equilibrium within 90 min. The photocatalytic degradation of adsorbed dye under sunlight was possible and about 92.3% of dye was degraded within 90 min. The reusability of P(Py-co-An)-TiO₂/NCC was also investigated. The results indicate that P(Py-co-An)-TiO₂/NCC is the best material for the wiping out of EY from aqueous solutions.

1. Introduction

Textile dyes are the largest group of chemicals being produced all over the world. The effluents from manufacturing and textile industries are discarded in large quantities into rivers and lakes causing water pollution and affect the ecosystem seriously [1]. So, the dye removal has been considered as a challenging problem for environmental scientists [2]. The developed dye removal strategies include biological treatment, coagulation, flotation, adsorption, oxidation, and electrochemical techniques [3]. Adsorption is a preferred method due to its high efficiency, ease of handling, and the availability of low cost adsorbents [4]. However, it has been reported that the majority of dyes are only adsorbed and are not degraded and are also ineffective that they simply transfer the pollutants to another phase rather than destroying them [5]. Photocatalysis for the purification of wastewater from industries and households has attracted much attention in recent years. Photocatalytic degradation relies on the technology of advanced oxidation processes (AOPs) in which electron-hole pair radicals able to undergo secondary reactions are created [6].

TiO₂ has been extensively used as a photocatalyst, owing to its inexpensive, nontoxic, and photoelectric properties [7]. It can also be used in harsh conditions for its high chemical and thermal stability. However, lack of recyclability and low adsorption ability for the pollutants, especially for the nonpolar organic compounds, are the disadvantages faced while using bare TiO₂. Organic molecules which can effectively adsorb to the surface of the photocatalyst will be more susceptible to direct oxidation [8]. Nano-TiO₂ is an active material which has advantages such as innocuity, resisting and decomposing bacteria, UV resistance, and superhydrophilicity [9].

Anchoring of TiO₂ on various supports including carbon, glass fibers, montmorillonite, organic materials, and zeolites may increase their photochemical stability. Organic materials with UV light resistance have been widely used as supports for TiO₂ photocatalyst, since they have the advantages such as low cost and easy separation from reaction solution. Cellulose, obtained from wood pulp and cotton, is such a material used for this purpose due to its low cost and easy availability. Cellulose has been explored as a substrate for composite materials because of the presence of functional groups that may be employed in various activation processes. Additionally, it was suggested that the holes’ scavenging ability increases with the increasing number and spatial distribution of the hydroxyl groups in the polyhydroxyl compounds [10]. When cellulose is converting to their nanocrystals, the holes’ scavenging ability increases due to the large number of hydroxyl groups exposed to the surface. The nanocellulose is mechanically strong and produced a strength of 25% that of carbon nanotubes [11].

Conducting polymers such as polypyrrole, polyaniline, and polythiophene have functioned as dopants which shift the border of the TiO₂ particles to longer wavelengths, thereby improving the optical absorption in the visible region. These hybrid conducting polymer/TiO₂ composites exhibit excellent properties unlike those of the individual materials, such as controlled conductivity and thermal or mechanical stability, and these properties have made them potentially applicable as anode materials for lithium-ion batteries [12], anode electrodes for dye solar cells [13], or photocatalytic materials [14]. The conjugated polymers could be separated from the aqueous phase by using simple gravity settling and could be recycled easily. The conjugated polymers have an extended conjugation system and behave like semiconducting materials with low charge carrier mobility. Polyaniline (PAn) has been deposited on various materials to perform desired applications [15]. Polypyrrole (PPy) has a potential application in the field of composite materials due to their appreciable environmental stability, higher electrical conductivity, easier synthesis, and solubility in different solvents [16].

As we know, eosin yellow (EY) is widely used for staining purposes; however, it is listed as a carcinogen. The present work is aimed at wiping out the textile dye, EY, from the environment for the well-being of the organisms. In the present work, a novel polymer composite, poly(pyrrole-co-aniline)-coated TiO₂/nanocellulose composite [P(Py-co-An)-TiO₂/NCC] was prepared for the adsorptive removal of EY from aqueous solutions, followed by photocatalytic degradation under sunlight. For the preparation of this polymeric hydrogel, glutaraldehyde, the agent widely used for industrial water treatment and a preservative, was selected as a crosslinker.

2. Materials and Methods

2.1. Materials

Sawdust of Mangifera indica was a kind gift from Local Saw mill, Trivandrum. Titanium(IV) isopropoxide (97%), eosin yellow (EY), pyrrole (Py), aniline (An), and glutaraldehyde were purchased from Sigma Aldrich. H₂S₂O₄, NaOH, HNO₃, FeCl₃, H₂O₂, ethanol, HCl, and ammonium persulfate (APS) were received from E. Merck, India. All the chemicals were used without further purification. Deionized water was used in preparing the aqueous stock solutions for the adsorption-photocatalytic experiments.

2.2. Preparation of P(Py-co-An)-TiO₂/NCC

2.2.1. Extraction of Nanocellulose (NC) from Sawdust

Cellulose has been extracted from sawdust and was converted to nanocrystals following the procedure reported earlier [17]. Briefly, sawdust was pretreated with 10% H₂SO₄ solution (120°C, 10 min) and centrifuged to remove rich pentosans solution. Delignification was achieved by the subsequent treatment with 1% NaOH (100°C, 1 h). The obtained brown mass upon bleaching with 5% H₂O₂ (80°C, 1 h) yielded white cellulose. About 5 g of cellulose was dispersed in 250 mL distilled water under magnetic stirring (20 min). 140 mL 98% sulfuric acid was dropped to the homogenized mixture, without causing heating. After complete addition, the mixture was heated at 50°C for 2 h. The hot mixture was diluted ten times with ice cooled distilled water. The obtained white colloid was centrifuged, washed many times with water, and freeze-dried.

2.2.2. Preparation of TiO₂/NC Composite (TiO₂/NCC)

NC (0.2 g), HNO₃ (5 mL, 0.1 M), and absolute ethanol (100 mL) were mixed with each other and the mixture was ultrasonically dispersed for 1 h. After dispersion, 1 mL of titanium(IV) isopropoxide was added to this mixture slowly with a constant pressure funnel under magnetic stirring. The reaction temperature was kept at 40°C for 4 h. Finally, the suspension was diluted fivefold with water, centrifuged, and then washed with water repeatedly. The precipitate was dialyzed against deionized water for 2 days and then freeze-dried.

2.2.3. Preparation of P(Py-co-An)-TiO₂/NCC

P(Py-co-An)-TiO₂/NCC was synthesized by chemical oxidation of its respective monomers, Py and An, by keeping their molar ratios constant at 1 : 1. The aqueous solution of TiO₂/NCC (240 mg in 100 mL) was mixed with a solution of pyrrole (3 mL), aniline (3 mL), and HCl (37%, 0.16 mL) for 30 min where after APS (0.46 g) and of FeCl₃ (8.0 g) was added, respectively, in the solution for polymerization initiation, and stirred for 3 h. The obtained product with a layer of poly(pyrrole-co-aniline) was filtered, washed with ethanol, and dried at 50°C. The dried product, designated as P(Py-co-An)-TiO₂/NCC, was sieved to get an average particle size of 0.096 mm and used for adsorption followed by photocatalytic experiments.

2.3. Characterization of P(Py-co-An)-TiO₂/NCC

Surface morphology of the adsorbents was investigated by SEM micrographs recorded with a JEOL JSM 6390 LA scanning electron microscope. The Brunauer-Emmett-Teller (BET) surface area was determined using a model Q7/S surface area analyzer (Quantasorb, USA). The XRD patterns were recorded using X’Pert Pro X-ray diffractometer. Patterns were recorded in the range of 10–70 at a scan rate of 2 counts/s. The FTIR spectra of adsorbents were recorded on KBr pellets with FTIR spectrophotometer in the wavelength range of 550–4000 cm⁻¹ (Shimadzu, Japan). Dried sample weighing 10 mg was dispersed in 200 mg of spectroscopic grade KBr to record the spectra at a resolution of 4 cm⁻¹. JASCO UV-visible (model V-550) spectrophotometer was used for the estimation of dye concentration in the solution and gel.

2.4. Adsorption of EY

Adsorption experiments were conducted to evaluate adsorption capacities of P(Py-co-An)-TiO₂/NCC. A stock solution of EY (10−2 M) was prepared in 1 L of double distilled water. The desired concentrations of the dye solution were obtained by proper dilution of the stock solution. 0.1 g of P(Py-co-An)-TiO₂/NCC was added into a stoppered bottle containing 50 mL of EY solution (10−5 M). pH of the solution was adjusted to the desired value with dilute HCl or NaOH. The mixture was shaken at 30°C for 90 min for adsorption, and then the mixture was filtered. The concentration of the dye solution before and after adsorption was estimated using UV-visible spectroscopic method. The adsorption (%) and the amount adsorbed (mol/g) were calculated as follows:where and (mol/L) are the concentrations of EY solution before and after adsorption, respectively, is the adsorption capacity of the adsorbent, (mL) is the volume of the EY solution, and (g) is the mass of the adsorbent. Adsorbent dose experiments were conducted by shaking 10⁻⁵ M dye solution with 0.1–1.0 g of P(Py-co-An)-TiO₂/NCC. Adsorption kinetics was obtained by determining the adsorption capacity at different time intervals from 5 to 240 min. Adsorption isotherm was studied by changing the concentrations of the dye solution from 10⁻⁵ to 10⁻⁴ M.

2.5. Photocatalytic Degradation of EY and the Regeneration of the Photocatalyst

Photocatalytic degradation of the dye was studied using P(Py-co-An)-TiO₂/NCC after adsorption experiment. The dye loaded swollen hydrogels were exposed to sunlight (at noon) and withdrawn at different time intervals from 5 to 120 min. Each sample is then analyzed spectrophotometrically at a wavelength of 515 nm. Degradation rate was calculated from the amount of the dye in the swollen gels as follows:where and are the initial and equilibrated concentrations of the dye (mol/L).

The samples obtained after photocatalytic degradation were washed thoroughly and adsorption-photocatalytic degradation experiments were repeated for four cycles in order to perform their regeneration capacity.

3. Results and Discussion

3.1. Preparation of the Photocatalyst, P(Py-co-An)-TiO₂/NCC

The photocatalyst, P(Py-co-An)-TiO₂/NCC, was prepared from TiO₂/NCC via chemical oxidation polymerization technique. Firstly, Cellulose was extracted from the value added product, sawdust, by acid-alkali treatment followed by bleaching with H₂O₂. The cellulose was made into nanocrystals via acid hydrolysis which improves its crystallinity and hydrophilicity. TiO₂/NCC was prepared from NC and the precursor Ti(OiPr)₄ using chemical precipitation method. PPy and PAn were coated onto TiO₂/NCC by chemical oxidation polymerization of pyrrole and aniline with FeCl₃. APS will generate free radicals and the polymer chain gets attached to it. The presence of the crosslinker, glutaraldehyde, will create a three-dimensional network structure, thereby increasing the water or solute holding capacity of the material.

3.2. Characterization Studies

3.2.1. Surface Morphology Analysis

The surface morphology of the samples is clearly displayed in Figure 1. The SEM image of the cellulose was observed to be smooth and fibrous and to have a layered structure, which provided a good condition for the adsorption of the dye pollutants. NC seems to be hydrophilic and behaves as gel structure. It may increase the swelling and diffusion of solute molecules from aqueous phase. TiO₂/NCC composite showed TiO₂ particles to be well dispersed and so reside equally on all regions on the surface. The polymerization onto TiO₂/NCC (Figure 1(d)) led to a heterogeneous rough surface, and this surface roughness leads to different prominent adsorption phases during the adsorption process [18].

Figure 1: SEM photographs of (a) cellulose, (b) NC, (c) TiO₂/NCC, and (d) P(Py-co-An)-TiO₂/NCC.

3.2.2. XRD Analysis

Figure 2 shows the XRD patterns of cellulose, NC, TiO₂/NCC, and P(Py-co-An)-TiO₂/NCC. The XRD pattern of cellulose shows peaks at values of 22.1 and 34.2° correspond to crystalline domain of cellulose structure, whereas the broad hump at 15.8° indicates the amorphous nature of cellulose. The small shift and the increase in broadness of the characteristic peaks of NC may be due to the small particle size of NC. The sharper diffraction peaks of NC were aroused due to the partial removal of the amorphous regions during the acid hydrolysis treatment of cellulose [19]. The identified XRD patterns of TiO₂/NCC gave distinctive peaks at ~25.6, 38.2, 48.2, 54.6, and 63.4° (JCPDS card number 21-1272), which resemble the characteristic peaks of TiO₂, as given in earlier reports [7]. From Figure 2, it is observed that the crystalline peaks are relatively broad compared to those normally obtained for bulk material, indicating that the crystal sizes are smaller. The Debye-Scherrer equation [20] is used to determine the average crystal size of the NC and it is found to be in the range of 21.6 nm. The appearance of new peaks and the shifting of the characteristic peaks of TiO₂ in P(Py-co-An)-TiO₂/NCC clearly indicate that proper grafting had been occurring on the surface of TiO₂/NCC.

Figure 2: XRD patterns of cellulose, NC, TiO₂/NCC, and P(Py-co-An)-TiO₂/NCC.

3.2.3. FTIR Analysis

The FTIR patterns of cellulose, NC, TiO₂/NCC, and P(Py-co-An)-TiO₂/NCC are presented in Figure 3. The peaks observed at ~3441 and 3417 cm⁻¹ are derived from hydroxyl groups in cellulose and water. The bands at ~2922 and 2899 cm⁻¹ (C-H stretching of CH₂) and 1426 cm⁻¹ (CH₂ symmetric bending) can be assigned to the stretching and bending modes of hydrocarbons in cellulose backbone [21]. The characteristic peaks of cellulosic material also appeared at 1112 and ~1033 cm⁻¹ (skeletal vibrations involving C-O stretching) [22]. The peak at ~539 cm⁻¹ clearly shows the presence of discrete anatase particles in TiO₂/NCC structure [23]. The two characteristic peaks at 1554 and 1492 cm⁻¹ in P(Py-co-An)-TiO₂/NCC correspond to the frequencies of aromatic ring in PAn, which are stretching of quinine ring and benzene rings, respectively [24]. Peaks attributed to out-of-plane bending vibration of C-H band of parasubstituted benzene ring were observed near 755 cm⁻¹ and the stretching of C-N band of benzene ring was confirmed by the peak of 1267 cm⁻¹ [25]. The band at 1457 cm⁻¹ may be attributed to C-N stretching modes of vibration in pyrrole ring. The peak nearer to ~3318 cm⁻¹ found in TiO₂/NCC was not observed after polymerization which suggests that polymerization had occurred due to the breakage of intermolecular hydrogen bond present in TiO₂/NCC.

Figure 3: FTIR spectra of cellulose, NC, TiO₂/NCC, and P(Py-co-An)-TiO₂/NCC.

3.3. Adsorption Studies

3.3.1. Effect of Adsorbent Dose

The amount of the spent adsorbent determines the economic value of the adsorption process. The performances of the adsorbent P(Py-co-An)-TiO₂/NCC and the precursor material cellulose were evaluated by varying their amounts from 0.5 to 5.0 g/L. As seen in Figure 4, the adsorption percentage of the sorbents increases with increase in amount because of the increased available surface sites for adsorption [26]. The complete removal of EY was achieved with an amount of TiO₂/NCC, 9.0 g/L, and P(Py-co-An)-TiO₂/NCC, 3.5 g/L, respectively. It means that the sorption efficiency of P(Py-co-An)-TiO₂/NCC is 2.6 times more than the untreated cellulose at pH 4.5. The variation in the adsorption capacity between the various adsorbents could be related to the nature and concentration of surface groups responsible for the interaction with the dyes.

Figure 4: Effect of adsorbent dose on EY adsorption onto P(Py-co-An)-TiO₂/NCC.

3.3.2. Effect of Solution pH

pH value of the solution is an important controlling parameter in the adsorption process, since it affects the surface charge of the adsorbent and the degree of speciation of adsorbate. As depicted in Figure 5, adsorption of EY on P(Py-co-An)-TiO₂/NCC was strongly affected by the solution pH. As pH of the solution was increased from 2.0 to 9.0, the percentage adsorption increases and reaches maximum at pH 4.5 and thereafter decreases. The maximum dye removal (%) at pH 4.5 was found to be 91.7% for an initial EY concentration of 1.0 × 10−5 M. The increased uptake at much higher acidic solution can be attributed to the increasing electropositive charge of the adsorbent which favored the adsorption of dye anions through electrostatic attraction (Figure 6). At pH < 4.5, the aniline surfaces were positively charged and, at pH > 4.5, the surfaces were negatively charged. Aniline can exist as nondissociated and/or dissociated species in aqueous solutions. Aniline is an ionizable organic compound and is a weak base that can protonate to form anilinium ion. Thus, the high efficient removal of EY by P(Py-co-An)-TiO₂/NCC is due to its high porosity, great surface area, and intrinsic positive charge of P(Py-co-An)-TiO₂/NCC as a type of n-doping polymer in which the anionic dopant (Cl⁻) is exchanged by the anionic dye.

Figure 5: Effect of solution pH on EY adsorption onto P(Py-co-An)-TiO₂/NCC.

Figure 6: Possible interactions between EY and P(Py-co-An)-TiO₂/NCC.

3.3.3. Effect of Adsorption Time

Figure 7 shows the effect of contact time on the batch adsorption of EY at 30°C and pH 4.5. It is obvious that the increase in contact time from 5 to 90 min increased the percentage removal of EY. This is due to the large number of active binding sites for adsorption during the initial stage and gradual occupancy of them makes the sorption less efficient in the later stages [27]. A further increase in contact time had a negligible effect on the percentage removal of EY. It implies that the sorption process is considerably fast and more than 91.7% and 88.1% of dye solution with initial concentrations of 1.0 × 10⁻⁵ and 5.0 × 10−5 M, respectively, were adsorbed within 90 min.

Figure 7: Kinetic modeling for the EY adsorption onto P(Py-co-An)-TiO₂/NCC.

Adsorption Kinetics. To perform the adsorption process on a larger scale, the elucidation of kinetic parameters and the sorption characteristics of the adsorbent are required. With that purpose, we analyzed the experimental kinetic data with pseudo-first-order and pseudo-second-order equations (see (3)). The first-order kinetics model assumes mass transport as the rate limiting mechanism, whereas the second-order one assumes chemical adsorption as the rate limiting mechanism. Considerwhere and (mg/g) are the amount of dye adsorbed on P(Py-co-An)-TiO₂/NCC at equilibrium and time (min), respectively. (min⁻¹) is the rate constant for the pseudo-first-order adsorption process. (g mol−1 min⁻¹) is the rate constant for the pseudo-second-order kinetics.

Additionally, the initial adsorption rate (mol g⁻¹ min⁻¹) can be calculated from pseudo-second-order kinetic model as follows:

Kinetic parameters for different concentrations were calculated by a nonlinear curve fitting method using ORIGIN program (version 7.5) and are listed in Table 1. The computed values of correlation coefficient () and chi-square () values measure the degree of fitness of the model with the experimental kinetic data. Higher (>0.99) and lower values obtained for the pseudo-second-order model indicate that this model could define the experimental results more precisely than pseudo-first-order model which indicates that the rate-limiting step may be a chemical sorption involving valence forces through exchange of ions between the adsorbent and adsorbate. As the initial dye concentration increases, the initial adsorption rate () increases. The values of were found to be decreased with increase of initial concentration and may be due to lowering the probability of collision between the dye molecules which faster the movement of dye towards the adsorbent surface at lower concentrations.

Table 1: Kinetic parameters for the adsorption of EY onto P(Py-co-An)-TiO₂/NCC (±standard deviation; ).

(mol L⁻¹)
, exp × 10⁻⁵ (mol g⁻¹)
Pseudo-first-order
Pseudo-second-order

(min⁻¹)
, cal × 10⁻⁵ (mol g⁻¹)

(g mol⁻¹ min⁻¹)
, cal × 10⁻⁵ (mol g⁻¹)
  ×  

1
0.458
0.270
0.447
0.974
0.0003
1.234
0.461
0.262
0.995
7 × 10⁻⁵

5
2.202
0.279
2.157
0.977
0.007
0.003
2.218
0.015
0.996
0.0014

3.3.4. Effect of Solute Concentration

Adsorption isotherm measures the adsorption efficiency of a polymer over a range of analyte concentrations. It describes the interactive behavior between adsorbate and adsorbent and is important for predicting the adsorption capacity of adsorbent, which is the main parameter required for design of an adsorption system. The increase in concentration of dye solution caused an increase in adsorption capacity (Figure 8), since the increase in initial dye concentration facilitates the dye movement from bulk to the surface of the adsorbent.

Figure 8: Isotherm modeling for the EY adsorption onto P(Py-co-An)-TiO₂/NCC.

For evaluating the maximum adsorption capacity and adsorption mechanism, the experimental isotherm data were interpreted using the equilibrium isotherm models, such as Langmuir, Freundlich, and Fritz-Schlunder isotherm equations, as follows: where (mol/L) is the amount adsorbed at equilibrium, (mol/L) is the equilibrium concentration of the dye on adsorption, (mol/g) is the maximum adsorption capacity at complete monolayer, and (L/mol) is the Langmuir constant related to the affinity of binding sites and is a measure of the energy of adsorption. (mol1−1/n L1/n/g) and are the Freundlich constants related to adsorption capacity and intensity of adsorption, respectively. is the Fritz-Schlunder maximum adsorption capacity (mol/g), is the Fritz-Schlunder equilibrium constant (L/mol), and is the Fritz-Schlunder exponent.

The Langmuir isotherm [28] is based on assumption of structurally homogeneous adsorbent and monolayer coverage with no interaction between the sorbate molecules. According to this model, once a dye molecule occupies a site, no further adsorption can take place at that site [29]. The Langmuir parameters can be used to predict the affinity between the sorbate and sorbent using dimensionless separation factor ():where is the Langmuir isotherm constant relating to binding energy and is the initial solute concentration. The values of at 30°C were determined and were found to be 0.2874, 0.1678, 0.1185, 0.0916, 0.0746, 0.0629, 0.0545, 0.0479, 0.0429, and 0.0387 at an initial dye concentration of 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 × 10−5 mg/L, respectively. values for the present experimental data fell between 0 and 1, which is indicative of favorable adsorption of EY onto P(Py-co-An)-TiO₂/NCC. The maximum monolayer adsorption capacity, based on Langmuir model, was 3.397 mol/g. The adsorption capacity of the adsorbent P(Py-co-An)-TiO₂/NCC towards EY was found to be much higher as compared to the conjugated polymer-grafted sawdust [30].

Freundlich isotherm equation can be applied to nonideal sorption on heterogeneous surfaces as well as to multilayer sorption. The value of Freundlich exponent (0.326) is in the range of 0 to 1, indicating a favorable adsorption. Fritz-Schlunder isotherm model [31] is a widely accepted isotherm model in which Fritz and Schlunder the equations of Langmuir and Freundlich were developed empirically. The isotherm plots and the isotherm parameters are represented in Figure 8 and Table 2, respectively.

Table 2: Isotherm parameters for the adsorption of EY onto P(Py-co-An)-TiO₂/NCC at 30°C (±standard deviation; ).

Langmuir
Freundlich
Fritz-Schlunder

(mol g⁻¹)
(L mol⁻¹)

()

(mol g⁻¹)
(L mol⁻¹)

3.397
2.480
0.996
0.005
2.160
0.326
0.949
0.062
2.504
3.385
0.997
0.996
0.005

As suggested by Figure 8 and Table 2, the equilibrium sorption data were best presented by Fritz-Schlunder isotherm. According to and values, the adsorption isotherm data follow the order: Fritz-Schlunder > Langmuir > Freundlich.

3.4. Photocatalytic Studies

3.4.1. Photocatalytic Degradation of EY and Its Mechanism

The photocatalytic activities of P(Py-co-An)-TiO₂/NCC were evaluated by the degradation of EY in an aqueous solution under sunlight irradiation. No degradation of EY was observed in the absence of a photocatalyst under visible light illumination. It was observed that photocatalytic degradation of EY increases with increase in irradiation time and 92.3% of the dye was degraded within 90 min. Photocatalytic degradation was found to follow first-order rate equation, for lower concentrations, as follows: where is the concentration of the dye at time (mol/L), is the illumination time in minutes, is the reaction rate constant (min⁻¹), and is the concentration of the dye before irradiation (mol/L). Figure 9 shows the relationship between of EY and photodegradation time in the presence of P(Py-co-An)-TiO₂/NCC for 90 min under visible light irradiation. The plot of versus time represents a straight line, the slope of which upon linear regression equals the apparent first-order rate constant “” and was calculated to be 0.00575 min⁻¹ in the present study.

Figure 9: Kinetic data for the photodegradation of EY onto P(Py-co-An)-TiO₂/NCC.

3.4.2. Mechanism of Photocatalysis

TiO₂ particles can absorb UV light from the sunlight to create electrons (e⁻) in the conduction band and holes (h⁺) in the valence band, respectively. If the electrons and holes cannot be captured in time, they will recombine with each other within a few nanoseconds, which will reduce the photocatalytic efficiency of TiO₂. However, due to the existence of the interface between polymer and TiO₂, separated electrons and holes have little possibility to recombine again in the case of composites. This ensures higher charge separation efficiency and better photooxidation capacity for the nanocomposite. In addition, the conjugated polymer can absorb the visible light and produces an electron (e⁻) that transfers to the conduction band of TiO₂ [32, 33] and then to an adsorbed electron acceptor like dye, yielding oxygen radicals to degrade pollutants into mineral end products [34].

Thus, the heterogeneous photocatalytic degradation of an azo compound can be summarized by the following reactions [35]:

Several authors make use of these types of conjugated polymers in the photodegradation of dye under visible light. The conjugated polymers (CPs) with extended p-conjugated electron systems showed the relatively high photoelectric conversion efficiency and charge transfer due to their high absorption coefficients in the visible part of the spectrum, high mobility of charge carriers, and good stability [36]. pH adjustments using acid (H⁺) or alkali () during adsorption processes may increase the photocatalytic reaction rate, in accordance with and . According to previous literature, the hydroxyl radical will oxidize eosin yellow to its leuco form, which may ultimately degrade to environmentally benign products such as water, carbon dioxide, and inorganic salts like bromides. It was confirmed that radical participates as an active oxidizing species in the degradation of eosin yellow as the rate of degradation was appreciably reduced in presence of hydroxyl radical scavenger [37].

3.4.3. Photorecylability of P(Py-co-An)-TiO₂/NCC

To examine the photocatalytic stability, adsorption of EY onto P(Py-co-An)-TiO₂/NCC hydrogel was carried out at optimum conditions and the adsorbed dye material was exposed to sunlight for degradation. This process was repeated for four cycles and the results obtained were depicted in Figure 10. It can be found that the adsorption capacity and photodegradation efficiency of EY slightly decrease with the recycling runs up to four successive cycles. The above results indicate that P(Py-co-An)-TiO₂/NCC shows excellent photocatalytic stability.

Figure 10: Adsorption-photocatalytic degradation cycle of EY on P(Py-co-An)-TiO₂/NCC.

4. Conclusions

Eosin yellow (EY) is a carcinogenic dye and its exposure may cause adverse effects. Here we prepared a conductive conjugated polymer/semiconductor system, poly(pyrrole-co-aniline)-coated TiO₂/nanocellulose composite (P(Py-co-An)-TiO₂/NCC), for the removal of EY from aqueous solutions. The samples under preparation stage were characterized by SEM, XRD, and FTIR analyses. The systematic adsorption cum photocatalytic studies were carried out to explore the optimum conditions for the dye removal. Adsorption of dye was highly dependent on pH and 91.7% of the dye was adsorbed at pH of 4.5 within 90 min. An adsorbent dose of 3.5 g/L was needed for the complete adsorption of EY from aqueous solutions. Adsorption follows both Langmuir and Fritz-Schlunder isotherm models and pseudo-second-order kinetics, suggesting that chemisorption may be the mechanism behind the adsorption process. Photocatalytic degradation of EY under sunlight produced reliable results. P(Py-co-An)-TiO₂/NCC shows excellent photocatalytic stability and recyclability. Thus, the present investigation shows that the photocatalyst, P(Py-co-An)-TiO₂/NCC, is a valuable material for the adsorption and photocatalytic degradation of EY dye from aqueous solutions and can be developed as a support for the removal of EY dye from industrial effluents.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgment

The authors would like to thank the University Grants Commission, Government of India, New Delhi, for financially supporting this research under Major Research Project (MRP) F. no. 37-425/2009 (S. R. Rejeena).

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