Enhancing the Performance and Recyclability of Polyaniline/TiO 2 Hybrid Nanocomposite by Immobilizing with Zein/Hydroxyethyl cellulose Composites for Removal of Anionic dyes

Fabricating a stable, recyclable, and eco-friendly photocatalyst for dye treatment is vital in sustaining a clean ecosystem. In this regard, polyaniline/TiO 2 (PANI/TiO 2 ) photocatalyst was immobilized by zein/hydroxyethyl cellulose (zein/HEC) adhesive to enhance recyclability and catalytic activity. The blending of zein/HEC/PANI/TiO 2 photocatalyst involves insitu oxidative polymerization, followed by immobilization with zein/HEC functionalized composites. The PANI/TiO 2 composite was successfully grafted with the adhesive through physicochemical interaction, as evidenced by field emission scanning electron microscope (FESEM), Fourier transform infrared spectroscopy (FTIR), and Powder X-ray diffractometer (XRD). The simultaneous thermal analysis (STA) results show that the photocatalyst has the best thermal stability relative to PANI and PANI/TiO 2 in the recommended range of dye degradation temperature. The effect of external factors like TiO 2 nanoparticle proportion, pH of the solution, and catalyst dosage was studied in response to dye degradation capacity. The synthesized catalyst is efficient to degrade methyl orange in a wide range of pH. The kinetics of the catalysis reaction obeys first order kinetics. The maximum degradation efficiency achieved was 97.9 and 84.3% in the presence and absence of light, respectively. The catalyst was easily recovered by decantation, and its catalytic efficacy was more than 94% after five cycles. Hence, it is a promising alternative for decolorizing anionic dyes from wastewater.


Introduction
The worldwide development of industries contributes to an increase in the release of different hazardous wastes to the environment. Among the harmful wastes, dyes are the primary toxic and persistent organic chemicals released from textile, pharmaceutical, food, paper, and cosmetic industries. From the overall quantity of dyes produced globally, over 10% are released to the water body as waste [1]. This reduces dissolved oxygen levels [2] and photosynthesis rate in the aquatic system [3]. In addition, they are also poisonous and carcinogenic agents to living organisms [4].
Different approaches like adsorption [5], coagulation [6], filtration [7], ozonation [8], biodegradation [9], photocatalytic degradation [10], etc., have been developed to address these problems. Most of these methods have drawbacks, such as low-performance capacity, producing secondary pollutants, low stability, require huge investment, and complex separation processes [11]. Many studies showed that photocatalytic degradation had become the most preferred choice in the removal of dyes. Because it is economical [12], and the final degradation products have no negative impact on the ecosystem [13]. Thus, it is essential to develop a recyclable, efficient, and stable photocatalyst to remove wastewater dyes.
In the current study, TiO2 nanoparticle is selected for the photocatalytic agent due to its chemical stability, non-toxicity, compatibility, and conductivity [14]. However, it was reported that the agglomeration of nanoparticles and recombination of excited electrons due to strong coulombic interaction strongly affect its catalytic activity during the reaction [15][16]. Hybridizing it with conductive polymers can reduce the instability of charge separation between the valence and conduction band and agglomeration of nanoparticles [17]. Polyaniline is the one that was exclusively used to boost the photocatalytic function of TiO2. Because it possesses lower bandgap energy, and can easily excite electrons by absorbing light. Thus, TiO2 nanoparticles in the hybrid composite trapped excited electrons from polyaniline to its conduction band, and hence the catalytic reaction is enhanced due to electron-hole stabilization [18]. In addition, polyaniline is commonly used for these purposes due to its biocompatibility, high electrical conductivity, chemical stability, and ease of synthesis [19]. However, it is not easy to recover the slurry of polyaniline/TiO2 from the final product, particularly in large scale treatment [20][21]. Furthermore, these suspensions from the reaction medium prevent the incoming light from penetrating the entire part of the reaction system [22]. This effect limits the performance capacity of the photocatalyst in addition to the challenge of separation. Accordingly, it is essential to enhance the photocatalytic efficiency and recyclability of polyaniline/TiO2 (PANI/TiO2) photocatalyst. Thus, this study deals with improving and immobilizing PANI/TiO2 photocatalyst by zein/hydroxyethyl cellulose (zein/HEC) functionalized composites.
Zein is composed of prolamine containing more than 50% hydrophobic residues of amino acids.
Its hydrophilic nature can be enhanced by blending with water-soluble polymers due to hydrogen bonding between constituent species [23]. Composites of zein with hydrophilic polymer also possess a rough structure with high surface area microholes due to the self-aggregation of hydrophobic amide groups favored by hydrophilic group repulsion [24]. Besides, many active groups disclosed and positively charged functionalized networked porous structure of zein formed in acidic condition [25][26]. The microholes, functionalized porous structures, and different functional groups produce synergetic properties towards absorption [23,27]. This property is essential to support the adsorption of dye pollutants to the heterogeneous catalyst surface.
Furthermore, the strong interaction between zein and polyaniline makes the photocatalyst immobilization effective [28].
Hydroxyethyl cellulose is a well-known non-ionic stabilizer, binder, and water retainer in various composite products due to its high viscosity and stability in all pH ranges [29][30]. Hence, the composite of zein and hydroxyethyl cellulose is therefore suggested to be a stable and absorbing composite. Based on these facts, blending PANI/TiO2 with zein/HEC composite immobilizes and increases adsorptive-assisted catalytic function due to the combined effects of factionalized structures and various active groups.
In the past, materials like polyacrylonitrile [31], polystyrene cubes [32], Poly (vinyl alcohol) [33], diazonium salt [34], polyvinyl chloride (PVC), and epoxidized natural rubber (ENR-50) [35], etc., were used to immobilize and increase the recyclability of polyaniline/TiO2 composite. But some of the immobilizers lower the efficiency of the catalyst [32], and others are toxic, such as polyvinyl chloride and epoxides [36]. Thus, it is reasonable to immobilize PANI/TiO2 photocatalyst with nontoxic adhesive materials without affecting its catalytic activity. In this regard, glutaraldehyde crosslinked zein/HEC composite was used for anchoring PANI/TiO2 and enhancing the photocatalytic activity. So far, there was no report on polyaniline/TiO2 photocatalyst immobilized by zein/HEC composite polymer. Therefore, the objective of this work was to study the combined effect of zein and hydroxyethyl cellulose composite on PANI/TiO2 adsorptive assisted photocatalytic property for the degradation of anionic dye wastes. The newly synthesized zein/HEC/PANI/TiO2 photocatalyst was characterized by FESEM connected with energy dispersive X-ray (EDX), STA, XRD, and FTIR instruments. The TiO2 nanoparticles proportion, catalyst dosage, and pH on the photocatalytic degradation reaction were optimized. The kinetics of the reaction was also evaluated using first order kinetic models. Furthermore, the photocatalyst's feasibility study was assessed by recycling and comparing it with previous reports.

Synthesis of zein/HEC/PANI/TiO 2 composite
PANI/TiO2 hybrid nanocomposites were fabricated by oxidative in-situ polymerization. An equimolar concentrations of ammonium persulphate and aniline were prepared using 1M HCl.
Both solutions were stirred in a separate container until a clear, transparent solution was formed.
Presonicated aqueous solution of TiO2 nanoparticles was added to the polyaniline solution and stirred for half an hour. This solution temperature was adjusted to 0 -5 o C in an ice bath. With continuous stirring, ammonium persulphate solution was added, and stirring continued for five hours. This green acidic solution pH was adjusted to three to prevent zein breakdown during mixing [37].
Zein functionalization was performed by dissolving zein powder in dilute hydrochloric acid using the previous standard method with a slight modification [25]. Typically, HCl (0.4M) was prepared in 70% ethanol solution. Then zein powder was transferred to this solution. The reaction mixture was stirred for 12 hours at 70 o C to convert hydrophobic amine groups of zein to hydrophilic ones.
Hydroxyethyl cellulose solutions were also dissolved separately in 50% ethanol solution. The hydroxyethyl cellulose and zein solution were mixed with a glutaraldehyde crosslinker and stirred overnight. The green polyaniline/TiO2 composite solution was added to the zein/HEC gel solution and stirred for 12 hours to immobilize photocatalyst. Then it was neutralized by adding NaOH until the pH becomes above six and precipitated by adding excess water. It was allowed to settle all the precipitates. After decanting the supernatant, it was washed with water, followed by acetone.
Zein/HEC/PANI/TiO2 composite obtained was transferred to an oven set at 60 o C for drying in 12 hrs. Finally, it was crushed into small size. In the same procedure, the hybrid composite with different proportions of TiO2 was prepared for further characterization.

Characterization
The elemental composition and morphology of the hybrid composite were determined by FESEM

Adsorptive assisted photocatalytic study
The adsorptive assisted photocatalytic study was performed using batch photoreactors. In this experiment, 0.03 g of the photocatalyst was immersed in a catalytic bath containing 100 ml of 125 µM of methyl orange (MO) and stirred continuously. After 30 minutes of adsorption in the dark environment, it was irradiated with an 18 W UV light source with a wavelength of 395 nm. In a predetermined interval, 4 ml of the sample was withdrawn, and the analyte concentration was determined by UV-Vis spectrophotometer (UV-1800, Shimadzu, Japan). For recycling, zein/HEC/PANI/TiO2 composite was allowed to settle for half an hour and then isolated from the first phase by decantation. After that, it was washed with acidic water followed by distilled water repeatedly and rinsed with acetone. Finally, it was dried in an oven set at 60 o C before using for the second cycle with a freshly prepared MO solution. A similar procedure was used for the rest of the cycles. The catalytic efficiency (ɳ) of the hybrid composite was calculated from equation 1.
where Co and Ce are the initial and equilibrium dye concentrations of the solution respectively.

Morphology Analysis
Zein/HEC/PANI/TiO2 composite morphology was described in comparison with pure polyaniline and PANI/TiO2 composite, as shown in fig. 1. Most of the polyaniline molecules had long roadlike fibers, as shown in fig. 1 (a). The addition of TiO2 nanoparticles changed this structure and formed short length fibers as evidenced by fig.1(b). This shows that TiO2 nanoparticles interacted with polyaniline molecules and preventing long chain road-like structure formation. In fig.1(c) respectively. In addition to morphological changes, it is also essential to describe major constituent species present in the composite using EDX. The elements, O, C, N, and Ti were expected components in the composite. From the EDX spectrum shown in fig.1(a), there were no spectral peaks related to Ti, while Ti peaks were observed in fig. 1(b) and (c) confirming that TiO2 nanoparticles were involved in the hybrid composite. Both the weight and atomic percent of nitrogen were also negative in pure polyaniline and PANI/TiO2 composite, confirming that nitrogen content was less and could not be detectable. However, as shown in fig. 1(c) of EDX graph, nitrogen appeared due to zein/HEC composite addition. This was the indication for nitrogen-containing groups (zein) in the composite [28]. As shown in fig.1(b) and (c), the percentage composition of components in polyaniline and PANI/TiO2 changed with the addition of TiO2 nanoparticle and zein/HEC composite, respectively. This also confirmed that TiO2 nanoparticles and zein/HEC composites took part in the hybrid composite formation.

= __________________________________________2
Where d is the diameter of the crystal, λ is the wave length of X-ray radiation, k is a constant = 0.15418, B is the line width at the half maximum intensity, and θ is the Bragg diffraction angle.
The proximate average crystal size calculated from equation (2)

FTIR Analysis
The characteristics FTIR peaks of PANI, PANI/TIO2, zein/HEC/PANI/TiO2, and glutaraldehyde crosslinked zein/HEC/PANI/TiO2 were elucidated in fig.3. The quinoid (C=N) and benzoid (C=C) unit ring stretching vibrations were observed in the wave number of 1557 cm -1 and 1462 cm -1, respectively, as shown in fig.3(a) [44]. These two units confirmed the formation of emeraldine polyaniline. The peak corresponding to C-N stretching in the benzenoid unit was observed at the wave number of 1285 cm -1 [45]. At 1230 cm -1 , protonated conductive polyaniline emeraldine salt characteristic was also observed due to C-N + stretching vibration [46]. The peaks around 791 and 1020 cm -1 were due to out plane and in-plane bending vibrations of =C-H, respectively [47]. In fig. 3(b), the features polyaniline peaks of C=C and C-N (1462 cm -1 and 1285 cm -1 ) were transformed to higher wave numbers (1472 cm -1 and 1291 cm -1 ) due to Ti-N interaction in the composite [48] while the peak corresponding to quinone remains unchanged. All the characteristics peaks of pristine polyaniline observed between 450 cm -1 and 2000 cm -1 were also present in PANI/TiO2 composite, and new peaks appeared at 501 and 572 cm -1 due to TiO2 nanoparticles as shown in fig. 3(b) [49]. But some spectra shifted to higher wave number and decrease in intensity in PANI/TiO2 composite due to the surface interactions of TiO2 nanoparticles with hydrogen bonds and N-H groups in polyaniline [50]. In fig.3(c), almost all the spectral peaks appeared in polyaniline, and PANI/TiO2 were also observed with additional spectrum in 1652 cm -1 due to amide I. More peaks around 1557 & 1285 cm -1 due to amide II and III were also expected but overlapped with the peaks of quinone and benzene ring stretching vibrations, respectively. The strong and sharp spectral peaks observed in fig.3(c) became diminished, as shown in fig. 3(d) revealing that zein/HEC crosslinked with PANI/TiO2 through glutaraldehyde crosslinker.

Thermal Analysis
As shown in fig.4(a), in the decomposition of polyaniline emeraldine salt, two nonlinear weight losses were observed from 25 to 100 o C and from 100 to 310 o C. The first weight loss was due to the evaporation of water and a small amount of HCl. In the second case, from 100 to 310 o C, the primary weight change was due to the dopant (HCl) removal. Additionally, some water molecules firmly attached to the polymer surface were also released [51][52]. Similar to the previous report, the weight loss from 300 to 800 o C showed a linear degradation curve [50]. Most of the molecules removed from 300 to 700 o C were ammonia and aniline molecules. Above 700 o C temperature, ammonia and acetylene were the possible degradation products [53]. But, under similar temperature change, the decay of PANI/TiO2 was less than pure polyaniline, as shown in fig. 4(b). This proved that PANI/TiO2 thermogravimetric stability was higher than pure PANI due to TiO2 nanoparticle strong interaction with polyaniline [50].
In the thermal degradation of zein/HEC/PANI/TiO2, three major weight loss changes were observed; as shown in fig.4(c), the first weight change from 25 to 120 o C was due to the removal of water from the hybrid nanocomposite surface. In the second phase, around 23% weight loss was due to the composite degradation from 220 to 375 o C temperature ranges [55][56]. In the third stage,

Effect of TiO 2 on zein/HEC/PANI/TiO 2 hybrid composite
The hybrid composite catalytic efficiency was evaluated by changing the proportions of TiO2 nanoparticles under fixed methyl orange concentration, catalyst dosage, pH, and reaction time.
The degradation efficiency of zein/HEC/PANI/TiO2 hybrid nanocomposite with TiO2 proportions of 0%, 5%,10%, 15% and 20% relative to aniline is shown in fig.5. The catalytic reaction efficiency increased with increasing TiO2 nanoparticles, and the highest dye degradation efficiency was achieved at 10% load of TiO2. Further increasing the nanoparticle decreases catalytic degradation. This is because the catalyst's surface area decreased due to nanoparticles' spontaneous agglomeration at higher proportions of TiO2 [16]. In addition, a high proportion of TiO2 prevents the UV light source from striking to polyaniline surface; thus, polyaniline was unable to sensitize the reaction, which leads to decreased degradation [33]. Thus, the amount of TiO2 nanoparticles in the composite should be lower than polyaniline. Because a high concentration of polyaniline is not only sensitizing the reaction but also effectively stabilizing the nanoparticles of TiO2 by preventing nanoparticle aggregation [58].

Effect of pH
The catalyst surface and dye solution's charges are affected by the variation of pH in the reaction path. Therefore, it is vital to optimize the pH of the reaction condition to obtain maximum degradation products. Thus, the photocatalytic degradation reaction was performed with different pH values ranging from 2 to 12. The degradation efficiency was decreased when pH goes towards a lower acidic pH below four and higher basic pH above ten. But in the mild acidic and basic pH degradation, efficacies were higher, as shown in fig. 6. This is because the zein surface in a mild acidic condition is positively charged due to side amine groups' protonation on glutamine residues [59]. Hence the anionic methyl orange adsorption on the surface of the catalyst was enhanced, supporting catalytic reaction. But in a strongly acidic solution, it decreased due to the loss of positively charged groups by the change of glutamine into glutamic acid. Similarly, a dramatic decrease in higher basic pH was caused by the generation of negatively charged groups due to deprotonation of carboxylic groups on glutamate. This creates strong electrostatic interaction with anionic dyes, which caused a decrease in the reaction [60]. Different amino acid groups were disclosed in a mildly alkaline solution, and thus, catalytic degradation was enhanced. Also, PANI/TiO2 is positively charged in acidic media and negatively charged in higher pH. Hence, the electrostatic force of attraction between the catalyst and dye molecules favored an acidic environment but diminished in a strong alkaline condition [32]. With a 95% confidence level, the photocatalytic degradation of methyl orange was strongly affected by acidic (pH<4) and basic (pH>10) conditions. However, there were no significant changes in the photocatalytic degradation of methyl orange with the pH ranging from 4 to 10, as shown in fig. 6. This result depicted that the newly synthesized photocatalyst was efficient in a wide range of pH, which is essential to treat anionic dye wastes without adjusting pH. Even though no substantial change, 97.5165% was the maximum degradation efficiency achieved at neutral pH.
Hence, it was this pH used for all investigations carried out in this study. With an increasing amount of catalyst, the number of photons adsorbed on it increased due to the increased number of available active sites. Hence the number of dye molecules adsorbed on the photoactivated surface increased, and thus the rate of degradation is enhanced [62]. However, the degradation dependency of dyes with the catalyst higher than 0.03 g was an insignificant increment, as shown in fig.7. When the amount of photocatalyst is beyond the optimum limit, the probability of dye molecules to bind on the photocatalyst surface becomes equal since there are sufficient active sites. Thus, the photocatalytic degradation rate becomes nearly constant, whatever the catalyst increased [63].

Efficacy and kinetic study
The photocatalytic activity of zein/HEC/PANI/TiO2 hybrid nanocomposite was evaluated with and without light. The degradation of MO by zein/HEC/PANI/TiO2 in the presence of light was considerably higher than in the dark condition, as shown in fig.8 fig. 9, the straight line or the correlation coefficient R 2 = 0.992 proved that the photocatalytic degradation of methyl orange by zein/HEC/PANI/TiO2 obeys the kinetics of 1 st order reaction.

Recyclability Evaluation
Recoverability and reusability are essential metrics used to assess the economic viability of the

Conclusion
In this investigation, zein/HEC/PANI/TiO2 hybrid nanocomposite was produced by insitu oxidative polymerization followed by immobilization with zein/HEC functionalized composite.
In summary, the newly synthesized photocatalyst is an invaluable alternative for removing dyes (especially anionic dyes) from wastewater.