Ultrasonic-mediated synthesis and characterization of TiO₂-loaded chitosan-grafted-polymethylaniline nanoparticles of potent efficiency in dye uptake and sunlight driven self-cleaning applications

Abstract

In the present study, a new sequential process for wastewater remediation in two steps with high durability was presented. Firstly, dye uptake on TiO₂-loaded chitosan-graft-poly(2- and 3-methylaniline) nanocomposites was observed in the dark. Secondly, sunlight-assisted photocatalytic self-cleaning for regeneration of the used adsorbent was achieved. Thus a synergic effect of adsorption/photooxidation for successful removal of dye from aqueous solutions was manifested. The nanocomposites were characterized by FTIR, UV-Vis diffuse reflectance, XRD, and TEM. The data of dye adsorption were fitted to Langmuir and Freundlich isotherms. The Langmuir model gave the best fit. The pseudo-first-order and pseudo-second-order were used to analyze the kinetic data. The data fit well with the second-order kinetic model. The adsorptive removal of the dye was highly efficient. Interestingly, the adsorbents were successfully regenerated by photomineralizing the adsorbed dye via solar-assisted self-cleaning. Efficient photocatalytic ability as reflected from the estimated photodegradation rate of the dye was observed. Furthermore, the prepared materials showed good photocatalytic stability after five runs under solar light. The ˙OH detected from the photoluminescence – terephthalic acid technique along with ˙O₂⁻ species seem to be the oxidizing species responsible for the sunlight-driven photodegradation reaction.

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1. Introduction

Wastewaters containing organic pollutants such as dyes from textile industries raise much concern because of the harmful environmental and toxic effects.¹ Among the various methods available for the removal of dyestuffs, adsorption by various adsorbents is employed for wastewater treatment.^2,3 Increasing remediation possibility has also been established with photocatalytic systems.^4,5 Studies combining these two processes into a hybrid system have shown encouraging results.^6–9 Polyaniline (PANI) is one of the conducting polymers that has attracted considerable attention, due to its unique, easily controlled, and reversible properties,^10,11 and has been reported as an adsorbent for dyes.^12,13 Also conducting polymers have photocatalytic activity.¹⁴ Moreover, chitosan has demonstrated to be one of the most promising adsorbents in wastewater treatment for its high potentials in the adsorption of heavy metals,¹⁵ organic compounds¹⁶ and dyes.¹⁷ Its renewability, non-toxicity, hydrophilicity, biocompatibility, biodegradability, versatility and anti-bacterial have led to a number of applications. It is widely used in agriculture, biomedicine, biotechnology, cosmetic, food, pharmacy, and water purification.^18,19 In recent years, chitosan has exhibited multi-functional performances with TiO₂ in heterogeneous photocatalysis technologies by enhancing the adsorption–photocatalytic process.²⁰ For photodegradation, TiO₂ nanoparticles have been the most promising and well-studied material that provide efficient photocatalytic treatment of pollutants. This is because of favorable band gap, its reasonable photocatalytic activity under ultraviolet (UV)-light irradiation and the advantages of being photostable, reusable, nontoxic and comparatively inexpensive.^21,22 However, the utilization efficiency of solar light is limited severely by the wide band gap of TiO₂ and the high charge recombination rate of photogenerated electrons and holes are often two major limiting factors for wide practical applications.²³ More recently, conducting polymers with extended π conjugation, such as polyaniline, polythiophene, polypyrrole, and their derivatives have been reported as promising sensitizers to extend the spectral response of TiO₂ to visible region. This is due to their high absorption coefficients in the visible part of the spectrum, high mobility of charge carriers, and good environmental stability.^24,25 Among conducting polymers, polyaniline (PANI) has been often used as a sensitizer in polymer-sensitized TiO₂ photocatalytic system due to its unique electrical, optical and photoelectric properties and the advantages of easy synthesis and low cost.²⁶ The objective of this work was firstly: to present the ultrasonic-mediated copolymerization technique as a new avenue to fabricate chitosan/PMeANIs nanografts and TiO₂-loaded chitosan/PMeANIs nanocomposites. Secondly, to investigate in-depth the functionalities of the provided materials and compare their potency in primarily adsorb the anionic dye, remazol red 133 (RR RB-133) from aqueous solutions. Then estimating the efficiency of their solar-driven photocatalytic activity over their solid surfaces to photodegrade the pre-adsorbed dye and restoring their adsorption capacity in further adsorption runs.

2. Experimental

2.1. Materials and methods

Chitosan was extracted from the shrimp shells as described in the literature^27,28 (MW ∼ 109 000; degree of deacetylation > 85%). 2-Methylaniline (2MeANI), 3-methylaniline (3MeANI), and titanium dioxide Degussa P25 (composed of anatase and rutile) were provided from Sigma Aldrich. Ammonium persulfate and N-methyl-2-pyrolidine (NMP) was purchased from Loba Chemie (India). Hydrochloric acid, acetic acid and acetone of AR grade were provided from El-Nasr Chemical Co. (Egypt) and were used as received. The commercially available Remazol Red 133(RR RB-133) was used as received.

2.2. Synthesis of Ch-g-PMeANI/TiO₂ nanocomposites

An aqueous solution of chitosan with fixed concentration (1.0 g L⁻¹) was prepared by dissolving calculated amount of chitosan in acetic acid (2 wt%) and stirring for 2 h using a magnetic stirrer. The resulting solution was filtered to remove undissolved particles before further reaction. Different amounts (0.5–4.0 g) of TiO₂ nanoparticles were dispersed into (0.2 mol L⁻¹) HCl aqueous solution that containing methylaniline monomers with concentration (1.0 × 10⁻² mol L⁻¹), then a solution of chitosan was added to the TiO₂/HCl suspension at 5 °C, with constant stirring for 20 min to form a homogeneous solution. A ammonium persulfate (APS) in (0.2 mol L⁻¹) HCl solution with a concentration (7.5 × 10⁻² mol L⁻¹) was added drop wise into the above solution and stirring was continued in an ice bath at temperature 5 °C for 5 hours. During the polymerization time, the reaction mixture was irradiated by an ultrasonic processor by placing it in ultrasonic bath (38 kHz and 120 W, model US-2, Iuchi, Japan). The final reaction mixture was centrifuged, and then the precipitate was washed with bidistilled water. The washed product was separated and again washed with N-methyl-2-pyrrolidone (NMP) for several times to separate MeANI (homopolymer) and MeANI (oligomers) from Ch-g-PMeANI/TiO₂ via acetone extraction. The nanocomposites were dried at 50 °C for 24 hours. The samples were signed according to different mass ratios of Ch-g-PMeANI to TiO₂ were used to obtain Ch-g-PMeANI/TiO₂ nanocomposites. In this way, Ch-g-PMeANI/TiO₂ nanocomposites with different mass ratio of Ch-g-PMeANI to TiO₂ equal to 1 : 0.5, 1 : 1, 1 : 2 and 1 : 4 were prepared; these were labeled Ch-g-PMeANI/TiO₂ (1 : 0.5), Ch-g-PMeANI/TiO₂ (1 : 1), Ch-g-PMeANI/TiO₂ (1 : 2) and Ch-g-PMeANI/TiO₂ (1 : 4). Due to aid of comparison, Ch-g-PMeANI without TiO₂ loading was synthesized by the same chemical oxidative copolymerization route.

2.3. Adsorption experiments

Adsorption studies for the evaluation of the Ch-g-PMeANI and Ch-g-PMeANI/TiO₂ nanocomposites in removing RR RB-133 dye from aqueous solutions were carried out using the batch contact adsorption. For these experiments, fixed amount of adsorbent was placed in a 100 mL glass Erlenmeyer flask containing 40 mL of dye solution, which were then shaken at 25 ± 2 °C on a horizontal rotary shaker. Adsorbent was then separated and the filtrate was analyzed for dye residue using an UV/Vis spectrophotometer at maximum absorbance wavelength (λ_max = 520 nm). Based on developed calibration curve, the amount of the dye uptake and percentage of removed dye by the adsorbents were calculated by applying the eqn (1)–(3), respectively:

q_t = [(C₀ − C_t) × V]/m (1)

q_e = [(C₀ − C_e) × V]/m (2)

R% = [(C₀ − C_t)/C₀] × 100 (3)

where q_t and q_e are the amount of dye taken up by the adsorbents (mg g⁻¹) at time (t) and equilibrium respectively; C₀ is the initial dye concentration (mg L⁻¹), C_t, and C_e are the dye concentration at time (t) and equilibrium (mg L⁻¹) respectively, V is the volume of dye solution used (L), and m is the mass (g) of the adsorbent.

2.4. Determination of point of zero charge (PZC)

The point of zero charge (pzc) of Ch-g-PMeANI and Ch-g-PMeANI/TiO₂ nanocomposites were determined by the solid addition method.²⁹ 10 mg of solid samples in 40 mL (0.1 M) NaCl were equilibrated at 25 °C ± 2 for 48 h. The initial pH (pH_i) of the solutions were adjusted in the range 1.5–9 with (0.1 M) NaOH or (0.1 M) HCl. The final pH_f of the suspension was measured after the 48 h equilibration. The pzc of Ch-g-PMeANI and Ch-g-PMeANI/TiO₂ nanocomposites were determined from the plot of pH_f − pH_i vs. pH_i of suspensions. The point at which pH becomes zero is called pHpzc.

2.5. Sunlight-assisted self-cleaning and photo-regeneration of spent adsorbents

A 4.0 μm thin layer of Ch-g-PMeANI and Ch-g-PMeANI/TiO₂ nanocomposites that are adsorptive to RR RB-133 were deposited on the bottom of glass Petri dishes (diameter 5 cm) and then dried in dark at room temperature for 12 h. After that, the glass Petri dishes immobilized the nanocomposites impregnated RR RB-133 were subjected to sunlight after placing it in a position achieving the highest possible irradiance of 770 W m⁻² measured using pyranometer. At different irradiation times, equal amounts of the nanocomposites bearing dye were shacked with equal volumes of NaOH (1 × 10⁻² mol L⁻¹) to completely desorb the residual dye. By measuring UV/Vis spectrophotometry for the desorbed dye samples, the self-cleaning photodegradation rate of RR RB-133 over Ch-g-PMeANI and Ch-g-PMeANI/TiO₂ nanocomposites could be determined.

On the other hand, the generation of hydroxyl radicals (˙OH) was investigated by the method of photoluminescence (PL) technique via sunlight irradiation of 10 mL of 5 × 10⁻³ mol L⁻¹ terephthalic acid (TA) and 1 × 10⁻² mol L⁻¹ NaOH that is mixed with 20 mg of either bare Ch-g-PMeANI or Ch-g-PMeANI of different TiO₂ loadings. The PL intensity was examined at maximum emission peak around 426 nm.³⁰

2.6. Characterization of composites

FTIR spectroscopy that provides direct information about the structural and bonding characteristic of molecules to be probed were performed using a Shimadzu IR Affinity-1S spectrometer.

UV-Vis diffuse reflectance spectroscopy is a more convenient technique to characterise nanomaterials than UV-Vis absorption spectroscopy, since it takes advantage of the enhanced scattering phenomenon in powder materials, for the UV-Vis method, a Shimadzu Japan 3101 p spectrophotometer was used.

Wide angle X-ray powder diffraction is used to obtain information about the crystallinity state of materials. The XRD analysis was carried out on a X-ray diffractometer (D/Max2500VB2+/Pc, Rigaku Company, Tokyo, Japan) with a Cu detector using 1.54 Å wavelength of the X-ray.

Electron microscopy is a technique used to determine the size and shape of supported particles. Transmission electron microscope (TEM) is widely used for the study of the morphology and structure of polymers and polymers composite. TEM analysis, samples were taken on a transmission electron microscopy (JEOL, JEM-2100, Japan).

UV-Vis absorption spectroscopic measurements were monitored for decolorization of RB-133 dye using Agilent Cary 60 spectrophotometer. Absorption spectra of the dye solutions were recorded in the range 200–700 nm at room temperature.

Photoluminescence (PL) is the optical emission obtained by photon excitation and is commonly observed with semiconductor materials. This type of analysis allows non-destructive characterization of semiconductors. The photoluminescence measurements were recorded using Ocean Optics USB4000 fluorescence spectrometer.

3. Results and discussion

3.1. FT-IR spectra

The FTIR spectra of chitosan, Ch-g-PMeANI and Ch-g-PMeANI/TiO₂ nanocomposite were represented in Fig. 1. For chitosan the absorption band at 3438 cm⁻¹ is due to overlapping of OH and NH₂ stretching vibrations, the bands appearing within 2870–2920 cm⁻¹ could be attributed to symmetric –CH stretching vibration, and the band appearing at 1589 cm⁻¹ is due to –NH₂ bending.³¹ The band appearing at 1426 cm⁻¹ is related to the –CH₂ bending.²⁷ The absorption bands at 1154 cm⁻¹ (anti-symmetric stretching of C–O–C bridge), and 1085 cm⁻¹ (skeletal vibration involving the C–O stretching) are characteristics of chitosan saccharide structure.³²

Fig. 1 FTIR spectra of (a) chitosan, (b) chitosan-g-poly(2-methylaniline), (c) chitosan-g-poly(3-methylaniline)/TiO₂, (d) chitosan-g-poly(3-methylaniline), and (e) chitosan-g-poly(3-methylaniline)/TiO₂.

The successful grafting of (poly-methylaniline) PMeANI onto chitosan was confirmed by the appearance of absorption bands around 1608 cm⁻¹, that attributed to the characteristic peaks of nitrogen quinone (Q) and the bands appeared at 1493 cm⁻¹ associated to the benzenoid ring present in the PMeANI. The absorption peaks appeared at 1327 cm⁻¹ related to the stretching vibration of C–N and C=N in PMeANI.^33,34 Moreover, the absorption bands of N=Q=N (Q representing the quinoid ring) bending vibrations of PMeANI were observed around 1150 cm⁻¹.^35,36 Further, the shift of these bands toward lower wavenumber 1107 cm⁻¹ for Ch-g-PMeANI, were attributed to hydrogen bonding between the chitosan and PMeANI.³⁷

Fig. 1b and d indicate that the main characteristic peaks of Ch-g-PMeANI appear in the Ch-g-PMeANI/TiO₂ nanocomposite. Also, Fig. 1b and d reveals that the maximum peak of TiO₂ appear in the nanocomposite at 400–700 cm⁻¹. However, all bands shift slightly, and quinonoid to benzenoid band intensity ratio has also change due to the action of hydrogen bonding between the surfaces of the TiO₂ and the N–H groups in the Ch-g-PMeANI. In addition, a new peak around 1092 cm⁻¹ appears in Ch-g-PMeANI/TiO₂ nanocomposite. These results indicate that a strong interaction exists at the interface of Ch-g-PMeANI and nano-TiO₂.³⁸

3.2. UV-Vis diffuse reflectance spectra

Fig. 2 shows the UV-Vis diffuse reflectance spectra of TiO₂ nanoparticles, Ch-g-PMeANI, and Ch-g-PMeANI/TiO₂ nanocomposites. Conductive polymers have a conjugate system of double bonds on their backbone. The conductive polymers have some of the conventional transfers in the UV range as well as in the visible region, such as π–π*, polaron–π*, and π–polaron transitions, respectively. The Ch-g-PMeANI (with aniline pattern) is expected to have three peaks in ≈320 nm, 400 nm, and 600–620 nm, the first transfer may be due to the excitation of the nitrogen in the benzenoid ring (π–π* transition), while the second and the third have been ascribed to the polaron-π*, and π-polaron transitions of the quinonoid ring, respectively, which suggesting the prepared polymer are in the doped state, associated with the presence of protonated amine.³⁹ But, an increase in the chain length and subsequently, an increase in conjugated double bonds, a decrease in energy difference π–π* occurs that causes a red shift (higher wavelengths). So from Fig. 2, the absorption bands of π–π* transition have been transferred from 320 nm to 346 nm, 338 nm, 354 nm, and 347 nm for Ch-g-P2MeANI, Ch-g-P3MeANI, Ch-g-P2MeANI/TiO₂ and Ch-g-P3MeANI/TiO₂ respectively, moreover, the peaks at ≈400 nm were transferred to 402 nm, 390 nm, 410 nm, and 404 nm for Ch-g-P2MeANI, Ch-g-P3MeANI, Ch-g-P2MeANI/TiO₂ and Ch-g-P3MeANI/TiO₂ respectively.^40,41 The peak at 600–620 nm was shifted to 645 nm, 641 nm for Ch-g-2NMeANI and Ch-g-2NMeANI, while for Ch-g-PMeANI/TiO₂ nanocomposites the π-polaron peak disappears, and displaced by a broad and strong absorption band with a long tail characterization in the visible-light region called “free-carrier-tail” due to the presence of TiO₂. This tail is consistent with the delocalization of electrons in the polaron band promoted by an extended conformation of the polymer chains. In Ch-g-PMeANI the strong interaction of chains make π-conjugation defects, and always lead to “compact coil” conformation. While in the nanocomposites, the TiO₂ not only eliminates the interaction of different polymer chains, but also limits the contraction of the chains. Therefore, the polaron band becomes more dispersed in energy. As a result the localized polaron band disappears and the “free-carrier tail” appears. Therefore, Ch-g-PMeANI/TiO₂ nanocomposites can be excited to produce more electron–hole pairs under visible-light illumination, this is expected to increase photocatalytic activities.^39,42 As shown in Fig. 2, the Ch-g-PMeANI/TiO₂ nanocomposites exhibit different optical behavior from that observed for pristine TiO₂ nanoparticles. Ch-g-PMeANI/TiO₂ nanocomposites not only absorbs the UV light but also significantly absorb the visible and near-IR. Whereas pristine TiO₂ nanoparticles can absorb light with wavelengths below 400 nm only (the UV region and a small part of visible light). The expected band of chitosan at 330 nm due to the glucopyranose components of chitosan was shown in UV-Vis spectrum by overlapping effect of benzenoid rings of grafted PMeANI around 342 nm.⁴³

Fig. 2 UV-Vis diffuse reflectance spectra of (a) chitosan-g-poly(2-methylaniline), (b) chitosan-g-poly(3-methylaniline), (c) chitosan-g-poly(2-methylaniline)/TiO₂, and (d) chitosan-g-poly(3-methylaniline)/TiO₂ [insets UV-Vis diffuse reflectance spectra of TiO₂].

The valance band or highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) band are separated by an energy gap that is called band gap which is of fundamental importance, because the energy gap determines the electrical conductivity and optical absorption character of the Ch-g-PMeANI and Ch-g-PMeANI/TiO₂. Fig. 3 shows the estimated band gap values (obtained by extrapolating the linear region to (αhν)^1/2 = 0) for Ch-g-PMeANI and Ch-g-PMeANI/TiO₂, calculated with the Kubelka–Munk function (eqn (4)):

(αhν) = A(hν − E_g)^1/2 (4)

where α is the extinction coefficient, h is Planck's constant, ν is the frequency of vibration A is the absorption constant, and E_g is the band gap (eV). The plot of (αhν)^1/2 versus (hν) is linear function existence of indirect allowed band transition in Ch-g-PMeANI and Ch-g-PMeANI/TiO₂. Extrapolating of linear dependence of the relation to abscissa yields the corresponding band gap E_g. The band gap of Ch-g-P2MeANI, Ch-g-P3MeANI, Ch-g-P2MeANI/TiO₂ and Ch-g-P3MeANI/TiO₂ were estimated to be 2.76, 2.82, 2.69, and 2.79 eV, respectively (Fig. 3), clearly showing a band gap narrowing as compared to 3.18 eV of the bare TiO₂.

Fig. 3 Relation between (αhν)^1/2 and hν of (a) chitosan-g-poly(2-methylaniline), (b) chitosan-g-poly(3-methylaniline), (c) chitosan-g-poly(2-methylaniline)/TiO₂, and (d) chitosan-g-poly(3-methylaniline)/TiO₂.

3.3. X-ray diffraction patterns

X-ray diffraction patterns of Ch-g-PMeANI and Ch-g-PMeANI/TiO₂ nanocomposites are shown in Fig. 4. The XRD pattern of Ch-g-PMeANI showed some crystallinity from 20°–32° due to the grafting of PMeANI onto the chitosan backbone. While in the XRD pattern of Ch-g-PMeANI/TiO₂ nanocomposite with different loadings of TiO₂ can be seen that the nano-TiO₂ of major peaks at 25.4°, 27.6°, 36.2°, 37.9°, 48.1°, 54.1°, 55.1°, 62.8° and 69° present in these composites can be indexed to the mixed phase anatase and rutile structure of nano-TiO₂ with high crystallinity.^44,45 Moreover, different loadings of TiO₂ in nanocomposite practically do not cause any change in peak positions. Also it can be noted that, when TiO₂ be introduced in the Ch-g-PMeANI/TiO₂ nanocomposite, the peaks of diffraction for Ch-g-PMeANI were disappear or became weaker. The result suggests that the addition of nano-TiO₂ hampers the crystallization of the Ch-g-PMeANI molecular chain.⁴¹ Furthermore, as the nano-TiO₂ content in the composites increases, the diffraction peak of Ch-g-PMeANI around 25° becomes sharper and more intense, revealing that the nanocomposites are more crystalline than Ch-g-PMeANI.

Fig. 4 XRD spectra of (a) chitosan-g-poly(3-methylaniline), (b) chitosan-g-poly(2-methylaniline), (c) chitosan-g-poly(3-methylaniline)/TiO₂ (1 : 0.5), (d) chitosan-g-poly(3-methylaniline)/TiO₂ (1 : 1), (e) chitosan-g-poly(3-methylaniline)/TiO₂ (1 : 2), (f) chitosan-g-poly(3-methylaniline)/TiO₂ (1 : 4), (g) chitosan-g-poly(2-methylaniline)/TiO₂ (1 : 0.5), (h) chitosan-g-poly(2-methylaniline)/TiO₂ (1 : 1), (i) chitosan-g-poly(2-methylaniline)/TiO₂ (1 : 2), and (j) chitosan-g-poly(2-methylaniline)/TiO₂ (1 : 4).

3.3.1. Calculation of average crystallite size from XRD pattern. The average crystallite size is calculated using Debye–Scherrer eqn (5):

D = Kλ/β cos θ (5)

where D is the crystallite size in nanometers, k is a constant known as Scherrer's constant and its value is (0.89), λ is the wavelength of X-ray for this analysis (λ = 1.54056 Å), β is full width at half maximum (FWHM) (in radian) of the particular peak and θ is the Bragg's angle (in degrees).^46–48 The average crystallite sizes calculated were listed in Table 1.

Table 1 Values of density, average crystallite sizes, and specific surface area (SSA) of Ch-g-PMANI and their TiO₂ composites of different loadings

Material	Density (g cm^−3)	Size (nm)	SSA (m^2 g^−1)
Ch-g-P2MANI	0.654	38.1	240.79
Ch-g-P3MANI	0.636	41.4	227.87
Ch-g-P2MANI/TiO2 (1 : 0.5)	0.737	44.99	180.95
Ch-g-P3MANI/TiO2 (1 : 0.5)	0.714	47.82	175.73
Ch-g-P2MANI/TiO2 (1 : 1)	0.809	37.9	195.69
Ch-g-P3MANI/TiO2 (1 : 1)	0.786	40.4	188.95
Ch-g-P2MANI/TiO2 (1 : 2)	0.861	32.51	214.35
Ch-g-P3MANI/TiO2 (1 : 2)	0.866	32.61	212.46
Ch-g-P2MANI/TiO2 (1 : 4)	0.834	35.1	204.96
Ch-g-P3MANI/TiO2 (1 : 4)	0.901	43.98	151.42

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3.3.2. Calculation of specific surface area. It has been reported that the relation between crystallite size and surface area has been used in calculating the specific surface area (SSA) via the X-ray diffraction method, depending on the assumption that crystallites are spheres. The X-ray diffraction technique provides rapid surface area values which are in good agreement with those obtained by the classical low temperature nitrogen adsorption (BET) method.^46,47,49 The specific surface area can be calculated by:

S = 6000/P × D (6)

where S is the specific surface area, D is the size of the particles and P is the density of the synthesized material. The bulk density of a powder was measured using the experimental method that reported in literature.⁵⁰ The surface area of the synthesized materials is found to be very large and hence the tested materials have a positive response Table 1.

As it is known, high specific surface area comes from employing smaller nanoparticles, which ensures high dye loading. It is noted that the specific surface area of Ch-g-P2MeANI or Ch-g-P2MeANI/TiO₂ nanocomposites are higher than their counterparts of Ch-g-P3MeANI or Ch-g-P3MeANI/TiO₂. Moreover, for TiO₂ loaded Ch-g-PMeANI nanocomposites, it firstly increased from (1 : 0.5) to (1 : 2) and then decreased due to the larger addition of TiO₂ nanoparticles causing strong aggregation of particle, and hence reducing the surface area of the nanocomposites.^51,52

3.4. Transmission electron microscopy (TEM)

TEM images of Ch-g-PMeANI and Ch-g-PMeANI/TiO₂ composites are shown in Fig. 5. It can be observed that Ch-g-PMeANIs are aggregated in nanoparticles have nearly spherical shape, smooth surface and size range of about 6–28 nm. The morphology of the TiO₂ containing composite particles is also aggregated and the size is in the range 8–32 nm. The aggregations of both kinds of particles are caused by high surface energy; however, the agglomeration of Ch-g-PMeANI/TiO₂ one is alleviated obviously compared with that of Ch-g-PMeANI. This could be due to the formation of Ch-g-PMeANI on the surface of TiO₂ nanoparticles which causes repulsion forces between nanoparticles and prevents their agglomeration. Ch-g-PMeANI/TiO₂ nanocomposite photocatalysts with small size can exhibit much larger efficient areas and be a more efficient active surface, which is favorable for the pre-adsorption of organic pollutants and subsequent photodegradation.^53,54 Further insights in TEM images revealed that, the growing layers of Ch-g-PMeANI on TiO₂ surfaces are attached together and generate the Ch-g-PMeANI/TiO₂ nanocomposites. In fact, such structure has been created due to the presence of TiO₂ in the course of grafting copolymerization process. The particles of nanocomposites have variety in size, because ultrasonic processor can break the aggregates of nano TiO₂ via the immense shock wave and microstream produced by ultrasonic cavitations.⁵⁵

Fig. 5 TEM images of (a) chitosan-g-poly(2-methylaniline), (b) chitosan-g-poly(3-methylaniline), (c) chitosan-g-poly(2-methylaniline)/TiO₂, and (d) chitosan-g-poly(3-methylaniline)/TiO₂.

3.5. Point of zero charge (pzc)

The plots of ΔpH versus pH_i are shown in Fig. 6. The pH point of zero charge (pHpzc) values of Ch-g-P2MeANI, Ch-g-P3MeANI, Ch-g-P2MeANI/TiO₂ and Ch-g-P3MeANI/TiO₂ are 4.37, 4.30, 4.32, and 4.21 respectively. This means that the Ch-g-PMeANI and Ch-g-PMeANI/TiO₂ are positively charged below (pHpzc) values, and negatively charged above (pHpzc) values.

Fig. 6 Plot of pH point zero charge (pHpzc) of (a) chitosan-g-poly(methylaniline), and (b) chitosan-g-poly(methylaniline)/TiO₂ (1 : 2).

3.6. Effect of titanium loading

To study the effect of TiO₂ loaded in Ch-g-PMeANI/TiO₂ nanocomposites on dye removal, TiO₂ loading was varied from 0.5 to 4.0 g, Fig. 7. In a batch-based study, 40 mL of 40 mg L⁻¹ aqueous solutions of RR RB-133 were added to 40 mg of either TiO₂ or Ch-g-PMeANI/TiO₂ nanocomposites. It was observed that an increase in titanium loading from (1 : 0.5) to (1 : 2) has improved dye removal. Further increasing in titanium loading has a negative effect on dye adsorption and this is due to reduction in the surface area of nanocomposites. Therefore, further adsorption studies were carried out for Ch-g-PMeANI/TiO₂ (1 : 2). The effect of bare TiO₂ on dye adsorption was shown in Fig. 7c that illustrates the least adsorption removal of dye via TiO₂ comparing to other adsorbents under study.

Fig. 7 Effect of titanium loading on adsorption of RR RB-133 dye onto (a) chitosan-g-poly(2-methylaniline)/TiO₂, and (b) chitosan-g-poly(3-methylaniline)/TiO₂. Effect of contact time on adsorption of RR RB-133 dye onto (c) bare TiO₂.

3.7. Effect of initial dye concentration

Dye uptake results at different initial concentrations are shown in Fig. 8. In a batch-based study, 40 mL of different initial concentrations of RR RB-133 were added to 10 mg of either Ch-g-PMeANI or Ch-g-PMeANI/TiO₂ nanocomposites. It is seen that the adsorption capacity of Ch-g-PMeANI and Ch-g-PMeANI/TiO₂ nanocomposites were dependent on the concentration of dye. Moreover, it can be observed that the adsorption capacity increased with an increase in the initial dye concentration to a certain level. This may be due to the increase in the number of dye molecules competing for the available binding sites in the surface of the adsorbents, which become saturated at RR RB-133 concentration of 70 mg L⁻¹.

Fig. 8 Effect of initial concentration of RR RB-133 dye on adsorption capacity of (a) chitosan-g-poly(methylaniline), and (b) chitosan-g-poly(methylaniline)/TiO₂.

3.8. Effect of contact times

The contact time was also evaluated as one of the most important factors affecting the adsorption efficiency. In a batch-based study, 40 mL of 20 mg L⁻¹ aqueous solutions of RR RB-133 were added to 10 mg of either Ch-g-PMeANI or Ch-g-PMeANI/TiO₂ composites. At different contact times, samples were taken and analyzed to check the progress of adsorption. As shown in Fig. 9, the dye removal increase with the increase of contact time. The rapid removal was observed during the first 5 minutes and gradually decreased with laps of time until equilibrium. The increased activity at initial stage could be due to the availability of more adsorption sites on Ch-g-PMeANI and Ch-g-PMeANI/TiO₂ nanocomposites, where the gradual occupancy of these sites reduced the reaction rate and the adsorption becomes less efficient. At this point, the amount of dye being adsorbed onto the adsorbents was in a state of dynamic equilibrium with the amount of dye desorbed from the adsorbents.

Fig. 9 Effect of contact time on adsorption of RR RB-133 dye onto (a) chitosan-g-poly(methylaniline), and (b) chitosan-g-poly(methylaniline)/TiO₂.

3.9. Effect of pH on the adsorption behavior

The pH of the solution influences not only the solution dye chemistry but also the surface charge of the adsorbents. Fig. 10 illustrates the effect of pH ranging from 1.5 to 9.0 on the removal of RR RB-133 dye. The dye removal increased with decreased initial pH value, whereas the highest sequestered amount of dye was reached at pH = 1.5. The increase in the removal at lower pH could be explained by protonation properties of the adsorbents that are positively charged at acidic pH. RR RB-133 is an anionic dye containing the sulfonic group that has negative charges. Consequently, it enhances the electrostatic attractions between dye anions and positively charged adsorption sites of Ch-g-PMeANI thus increases dye adsorption.⁵⁶ Furthermore, the impact of pH on the efficiency of TiO₂ loaded in the nanocomposites in adsorbing excessive amounts of RR RB-133 can be understood from the ionization state of the surface of metal oxide and ionization state of dye. TiO₂ in water is of amphoteric nature and is known to have an acid–base equilibrium.⁵⁷ The surface of TiO₂ will be positively charged at low pH. Since the dye is anionic in nature, under acidic conditions, the surface of TiO₂ will be positively charged and dye molecules will be readily absorbed by the surface of TiO₂ due to electrostatic attraction. This feature has been further supported by studies about pH point zero charge (pHpzc). Below the pHpzc values, the surface of Ch-g-PMeANI and Ch-g-PMeANI/TiO₂ acquired positive charge and hence have found a possibility of electrostatic attraction between dye anions and positively charged adsorption sites of adsorbents. At pH > pHpzc, the electrostatic attraction between negatively charged anionic dye and adsorbents decreases. However, even at high pH levels the removal of RR RB-133 dye occurred via the ion exchange mechanism.⁵⁸

Fig. 10 The effect of pH on adsorption of RR RB-133 dye onto (a) chitosan-g-poly(methylaniline), and (b) chitosan-g-poly(methylaniline)/TiO₂.

3.10. Adsorption isotherms

The interaction behavior between the dye and adsorbents can be best described by two commonly used isotherm models, namely Freundlich and Langmuir under varying initial dye concentrations from 2.0 × 10⁻⁵ mol L⁻¹ to 7.0 × 10⁻⁵ mol L⁻¹, while other parameters were kept constant.

The Freundlich adsorption model assumes that adsorption takes place on heterogeneous surfaces. The linear form of the Freundlich equation is:

log(q_e) = log K_F + 1/n log(C_e) (7)

here K_F and 1/n are Freundlich constants, related to adsorption capacity and adsorption intensity, respectively. The values of 1/n and log K_F were obtained from the slope and intercept of the linear Freundlich plot of log q_e vs. log C_e.

The Langmuir model describes the formation of a monolayer adsorbate on the outer surface of the adsorbent, and no more adsorption takes place once a saturation value is reached. The linear form of the Langmuir isotherm is given by:

C_e/q_e = 1/(q_mK_L) + C_e/q_m (8)

where q_m is maximum monolayer adsorption capacity (mg g⁻¹), C_e is the equilibrium concentration of dye in solution (mg L⁻¹) and K_L is Langmuir isotherm constant that related to the affinity of the binding sites. q_e represents equilibrium dye adsorption amounts (mg g⁻¹). The values of Langmuir parameters 1/q_m and 1/(q_mK_L) were calculated from the slope and intercept of the linear plots of C_e/q_e vs. C_e.

The fitting parameters for dye adsorption isotherms based on Freundlich and Langmuir equations are shown in Table 2. The fitting results show that the correlation coefficient of Langmuir isotherm was higher than that of Freundlich isotherm, indicating that the adsorption of RR RB-133 on Ch-g-PMeANI and Ch-g-PMeANI/TiO₂ can be better fitted using Langmuir model than Freundlich model, and the adsorption of dye on the adsorbents is a monolayer adsorption. Based on the Langmuir isotherm model, the maximum monolayer adsorption capacities (q_m) for Ch-g-PMeANI and Ch-g-PMeANI/TiO₂ were compared to other adsorbents reported in previous literatures^59–62 in Table 3. In general, the developed Ch-g-PMeANI and Ch-g-PMeANI/TiO₂ nanocomposites exhibited higher adsorption capacity than other adsorbents. Hence, it can be concluded that the prepared Ch-g-PMeANI and Ch-g-PMeANI/TiO₂ nanocomposites and thus could be preferentially used for removal of anionic dyes. Furthermore, the Ch-g-P2MeANI or Ch-g-P2MeANI/TiO₂ nanocomposites exhibit better adsorption capacity over the Ch-g-P3MeANI or Ch-g-P3MeANI/TiO₂ counterparts. According to the oxidation state, substituted polyanilines moiety in the Ch-g-PMeANI is correlated well with its adsorption capacity. Based on the UV-Vis and IR data in which the quinoid (Q) and benzenoid (B) units were identified, the intensity ratio of these two absorption bands Q/B is indicative of the extent of oxidation state of the copolymer.^63,64 Ch-g-P2MeANI has the highest Q/B value. This means it may exhibit the highest oxidation state, and therefore higher the adsorption capacity comparing to Ch-g-P3MeANI.

Table 2 Parameters for Freundlich and Langmuir adsorption isotherms

Adsorbent	Freundlich adsorption isotherm			Langmuir adsorption isotherm
	KF	1/n	R^2	qm	KL	RL	R^2
Ch-g-P2MeANI	20.3985	0.3858	0.86	104.17	0.0917	0.3468	0.98
Ch-g-P3MeANI	17.6929	0.3912	0.85	94.34	0.0856	0.3627	0.98
Ch-g-P2MeANI/TiO2	27.5740	0.3340	0.93	109.9	0.1243	0.2816	0.99
Ch-g-P3MeANI/TiO2	26.9773	0.3023	0.93	99.01	0.1239	0.2822	0.99

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Table 3 Comparison of maximum adsorption capacities for RR RB-133 on various adsorbents

Adsorbent	qm (mg g^−1)	Reference
TiO2 on polyvinyl pyrrolidone/acrylic acid	6.66	45
MCM-41	45.9	46
Fly ash	47.26	47
Activated carbon	59.88	48
Ch-g-P3MeANI	94.34	This study
Ch-g-P3MeANI/TiO2	99.01	This study
Ch-g-P2MeANI	104.17	This study
Ch-g-P2MeANI/TiO2	109.9	This study

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For the Langmuir-type adsorption process, the influence of the isotherm shape on whether adsorption is favorable or unfavorable can be classified by a separation factor R_L, which is considered as a more reliable indicator of the adsorption capacity. This constant is evaluated as:

R_L = 1/(1 + K_LC₀) (9)

where C₀ is the initial dye concentration and K_L is the Langmuir adsorption constant. The calculated value of R_L for adsorption of RR RB-133 dye less than unity, thus the adsorption of dye onto adsorbents is favorable.

3.11. Adsorption kinetics

Kinetics is important for adsorption studies because it provides valuable data for understanding the mechanism of adsorption reactions. Two kinetic models including pseudo-first-order and pseudo-second order models were selected to fit the kinetic data. The first-order rate expression of Lagergren based on solid capacity is generally expressed as follows:

log(q_e − q_t) = log q_e − (K₁/2.303)t (10)

where q_e and q_t (both in mg g⁻¹) are the amount of dye adsorbed per unit mass of adsorbent at equilibrium and time t (min), respectively. k₁ is the adsorption rate constant of pseudo first-order adsorption (min⁻¹). The slope (–k₁/2.303), and intercept log(q_e) are given from the linear plot of log(q_e − q_t) vs. time (t).

The pseudo-second-order kinetic model is also widely used. The most popular linear form can be expressed by:

t/q_t = 1/h + t/q_e (11)

where q_e and q_t are the amounts of dye adsorbed (mg g⁻¹) at equilibrium and at time t (min), k₂ is the rate constant of pseudo-second order adsorption (mg g⁻¹ min), t is the adsorption time and the initial adsorption rate h = k₂q_e². The slope (1/q_e) and intercept (1/h) of the linear plot of t/q_t versus t, gives the values of q_e and K₂, respectively.

The adsorption correlation coefficient was closer to pseudo-second-order, which fits the experimental data better than the pseudo first-order for the entire adsorption process Table 4. Therefore, it can be concluded that pseudo-second-order equation is better in describing the adsorption kinetics in this study and the rate of reaction appeared to be controlled by the chemical process. Other workers have also shown that pseudo-second-order model fits well in describing the adsorption process.⁶⁵ Furthermore, the q_e values obtained by pseudo-second-order model were closer to the experimental values. The pseudo-second order model suggests that the adsorption depends on the adsorbate as well as the adsorbent and involves chemisorption process.

Table 4 Results revealed after applying the pseudo-first-order and pseudo-second order adsorption kinetic models^a

Adsorbent	Lagergren first order kinetic model				Pseudo second order kinetic model
	qe-cal	k1	qe-exp	R^2	qe-cal	k2	qe-exp	R^2
a qe-exp, amount of dye adsorbed at equilibrium (mg g^−1); qe-cal, calculated amount of adsorption capacity (mg g^−1); k1, rate constant of first order adsorption; k2, rate constant pseudo second-order sorption; R^2, the correlation coefficients.
Ch-g-P2MeANI	36.35	0.0723	43.41	0.99	49.26	0.0024	43.41	0.996
Ch-g-P3MeANI	34.31	0.0624	40.34	0.98	45.04	0.0025	40.34	0.992
Ch-g-P2MeANI/TiO2	41.50	0.0796	51.99	0.98	59.52	0.0023	51.99	0.994
Ch-g-P3MeANI/TiO2	40.34	0.0746	50.59	0.98	57.14	0.0019	50.59	0.992

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3.12. Photocatalytic renovation of the spent Ch-g-PMeANI and Ch-g-PMeANI/TiO₂ adsorbents

The photocatalytic activities of Ch-g-PMeANI and Ch-g-PMeANI/TiO₂ nanocomposites were investigated as shown in Fig. 11. The temporal changes in absorption spectra of the adsorbed dye onto the Ch-g-PMeANI/TiO₂ (1 : 0.5) due to sunlight-assisted photocatalytic process are shown in Fig. 11a and b. The decrease of the absorption peak of RR RB-133 at 520 nm indicates a rapid degradation of the dye. After 120 min of sunlight irradiation, the RR RB-133 was degraded almost completely over the nanocomposite photocatalysts.

Fig. 11 Temporal absorption spectral variation of RR RB-133 during the self-cleaning photocatalytic degradation under sunlight irradiation in the presence of (a) chitosan-g-poly(2-methylaniline)/TiO₂ (1 : 0.5) and (b) chitosan-g-poly(3-methylaniline)/TiO₂ (1 : 0.5), [insets show images that illustrate the self-cleaning occurred at a sunlight irradiated and screened polymer surfaces after 120 min]. Photodegradation first order kinetics of RR RB-133 using (c) chitosan-g-poly(2-methylaniline) and chitosan-g-poly(2-methylaniline)/TiO₂ (d) chitosan-g-poly(3-methylaniline) and chitosan-g-poly(3-methylaniline)/TiO₂ with different loadings of TiO₂ under sunlight irradiation.

Moreover, the kinetic of photocatalytic degradation have been established and can be described well by the apparent first order reaction:

ln C₀/C_t = k_appt (12)

where k_app is the apparent rate constant, C₀ is the concentration of dye adsorbed onto adsorbents and C_t is the concentration of residual dye after time t of sunlight-irradiation. Fig. 11c and d shows the linear relation of ln C₀/C_t versus irradiation time for degradation of RR RB-133. Also, the determined apparent rate constants (k_app), life time (t_1/2), linear regression coefficient, and degradation efficiency of dye after irradiating for 120 min are presented in Table 5. From Table 5, it can be seen that the photocatalytic activity of nanocomposite photocatalysts increases at first and then decreases with the increase of the TiO₂ content from (1 : 0.5) to (1 : 4). A synergetic effect between Ch-g-PMeANI and TiO₂ on the photocatalytic degradation of RR RB-133 exists for the nanocomposites with different TiO₂ content from (1 : 0.5) to (1 : 4), and an optimum of the sensitized effect is found at Ch-g-PMeANI/TiO₂ (1 : 0.5) in nanocomposites. Because a large amount of TiO₂ and adsorbed dye will hinder the light absorption by Ch-g-PMeANI that act as a photosensitizer for TiO₂, resulting in the decrease of the photocatalytic degradation performance due to increase in recombination rate and decrease in the production of electron–hole pairs per unit time under sunlight-irradiation and the photocatalytic activity of nanocomposite photocatalysts becomes lower. From experimental results, it is clear that almost complete photodegradation of RR RB-133 dye has been achieved after 120 min of sunlight irradiation using Ch-g-PMeANI/TiO₂ nanocomposite as a photocatalyst. The degradation efficiencies of the Ch-g-PMeANI are lower than that of the corresponding Ch-g-PMeANI/TiO₂ (1 : 0.5) catalyst in the same conditions. It is clear that the TiO₂ nanoparticles play an important role to increase the photocatalytic degradation activity of the Ch-g-PMeANI matrix under sunlight irradiation, the similar observation reported for PANI and PANI/CdO.⁶⁶

Table 5 Apparent rate constants (k_app), half life time (t_1/2), linear regression coefficients (R²) of RR RB-133 photodegradation first order kinetics from a plot of ln(C₀/C_t) = k_app versus t and degradation efficiency (%) of dye after 120 min illumination under sunlight

Photocatalyst	kapp, min^−1	t1/2 (min)	R^2	Degradation (%)
Ch-g-P2MeANI	0.0191	36.3	0.99	88.81
Ch-g-P2MeANI/TiO2 (1 : 0.5)	0.0214	32.4	0.99	92.27
Ch-g-P2MeANI/TiO2 (1 : 1)	0.0111	62.4	0.98	74.05
Ch-g-P2MeANI/TiO2 (1 : 2)	0.0059	117.4	0.97	51.53
Ch-g-P2MeANI/TiO2 (1 : 4)	0.0040	173.2	0.99	37.63
Ch-g-P3MeANI	0.016	43.3	0.99	85.60
Ch-g-P3MeANI/TiO2 (1 : 0.5)	0.0181	38.3	0.98	87.83
Ch-g-P3MeANI/TiO2 (1 : 1)	0.0102	67.9	0.98	69.54
Ch-g-P3MeANI/TiO2 (1 : 2)	0.0057	121.5	0.99	47.63
Ch-g-P3MeANI/TiO2 (1 : 4)	0.0038	182.3	0.99	35.63

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3.13. Detection of ˙OH by photoluminescence-terephthalic acid (PL-TA) technique

The generation of ˙OH due to the sunlight irradiation of either Ch-g-PMeANI or Ch-g-PMeANI/TiO₂ was detected based on the PL-TA technique.³⁰ Fig. 12 shows a variation in the PL intensity after 2 h of sunlight-irradiation of TA/NaOH aqueous solution mixed with either bare Ch-g-PMeANI or Ch-g-PMeANI of different TiO₂ loadings. Clearly, the PL intensity and therefore the photogenerated ˙OH radicals at the surface of Ch-g-P2MeANI or Ch-g-P2MeANI/TiO₂ (0.5) is higher than that of Ch-g-P3MeANI or Ch-g-P3MeANI/TiO₂ (0.5). This corresponds to the higher sunlight photocatalytic activity of Ch-g-P2MeANI or Ch-g-P2MeANI/TiO₂ (0.5) compared to the Ch-g-P3MeANI counterparts. Additionally, the variation of photocatalytic efficiency of Ch-g-P2MeANI/TiO₂ over Ch-g-P3MeANI/TiO₂ could be attributed to the lowering of bandgap of Ch-g-P2MeANI comparing to Ch-g-P3MeANI that enhancing the charge separation efficiency of electron–hole pairs and in turn promoting the photocatalytic performance of the photocatalysts. From UV-Vis data. The longer the conjugation length, the lower the band gap (difference between HOMO and LUMO levels of the polymer), and the lowering of bandgap energy is a favorable factor for more photons can be absorbed, meanwhile leads to a delay in the recombination rate and enhance photocatalytic activity.^14,25,66 On the other hand, a markedly decrease in the PL output due to Ch-g-PMeANI/TiO₂ (0.5–4.0) was estimated. This may be attributed to morphological surfaces that hinder electron/hole separation in case of excessive loadings of TiO₂ than 0.5 g.

Fig. 12 The comparison plot of ˙OH-trapping PL spectra of chitosan-g-poly(2-methylaniline)/TiO₂ and chitosan-g-poly(3-methylaniline)/TiO₂ (TiO₂ loadings ranges from 0 to 4.0 g) after 120 min of sunlight irradiation.

3.14. Mechanism of photocatalysis

The proposed mechanism of photodegradation of organic pollutants by Ch-g-PMeANI and Ch-g-PMeANI/TiO₂ nanocomposite photocatalysts under sunlight irradiation has been illustrated in Fig. 13. Owing to PMeANI part in Ch-g-PMeANI copolymers, a charge-transfer excitation-like transition from HOMO to LUMO can lead to itself excited photogenerated electrons (e_CB⁻) and holes (h_VB⁺), which enables it to act as semiconductor and good photocatalysts.^67,68 On the other hand, TiO₂ can be excited under sunlight irradiation, but only a few electrons and holes were generated because UV light makes up merely 5% of sunlight.⁶⁹ To overcome this problem and achieving efficient solar-driven photocatalytic self-cleaning reaction over the surface of Ch-g-PMeANI/TiO₂ loaded with the RR RB-133, Ch-g-PMeANI can absorb visible light to induce π–π* transition which transfers the photogenerated electrons to LUMO orbital of Ch-g-PMeANI. Since this LUMO orbital matches well in energy and has chemical bond interaction with the d-orbital i.e. conduction band (CB) of semiconductor TiO₂, a synergic effect is created. The photogenerated holes can oxidize the organic molecule, since it could help in production of ˙OH radicals. Furthermore, the excited electrons can efficiently migrate from Ch-g-PMeANI to TiO₂. Subsequently, these photogenerated electrons can be transferred to the surface of the photocatalysts to react with oxygen to yield highly active superoxide anion radicals (˙O₂⁻) leading to oxidation of the adsorbed organic contaminants.⁷⁰ In short, under sunlight illumination, the synergic effect of Ch-g-PMeANI causes rapid charge separation, slow charge recombination and thus an enhanced photocatalytic effect of the prepared Ch-g-PMeANI/TiO₂ nanocomposites. Therefore, the Ch-g-PMeANI/TiO₂ nanocomposite can be subjected to solar-driven excitation to produce more active oxidizing species, which could result in an efficient photocatalytic activity.

Fig. 13 Proposed mechanism for the charge transfer of the photogenerated electrons and holes by chitosan-g-poly(methylaniline)and chitosan-g-poly(methylaniline)/TiO₂ nanocomposite under sunlight irradiation.

3.15. Reusability of photocatalyst

Besides activity, renewable catalytic activity is another important factor for photocatalysts. To evaluate and compare the photocatalytic stability of the Ch-g-PMeANI and Ch-g-PMeANI/TiO₂ (1 : 0.5) (the highest active TiO₂-loaded grafts) nanocomposites, they were subjected to several photocatalytic runs after a pre-adsorption process and each run lasted 120 min. After the first adsorption/self-cleaning photo-regeneration cycle, the photo-regenerated catalysts were used subsequently for further runs without any treatment. The results in Fig. 14, show that there is not much change in degradation profiles of the dye. Only a gradual loss of photocatalytic activity can be observed after each run, but it is still achieved after five runs. The decrease of photocatalytic activity after each run is probably due to slight aggregation of nanoparticles and the gradual photocatalytic degradation of catalysts during the photocatalytic process.^71,72 These results indicate that the Ch-g-PMeANI and Ch-g-PNMeANI/TiO₂ (1 : 0.5) are reusable and maintain high activity.

Fig. 14 Effect of number of runs on dye degradation over (a) Ch-g-P2MeANI/TiO₂ (1 : 0.5) [blue column] and Ch-g-P2MeANI [red column] (b) Ch-g-P3MeANI/TiO₂ (1 : 0.5) [blue column] and Ch-g-P3MeANI [red column] under visible light irradiation.

4. Conclusion

Composites of TiO₂-loaded chitosan-grafted-polymethylanilines (Ch-g-PMeANI/TiO₂) were successfully prepared by ultrasonic-mediated chemical oxidative copolymerization. XRD of Ch-g-PMeANI showed some crystallinity due to the grafting of PMeANI onto chitosan. Introducing nano-TiO₂ in different loadings to polymerization medium hampers the crystallization of the Ch-g-PMeANI molecular chain. TEM images shows less agglomeration in Ch-g-PMeANI/TiO₂ compared to neat Ch-g-PMeANI. FT-IR analysis suggests that there are Ch-g-PMeANI and TiO₂ in the composites. Moreover, the Ch-g-PMeANI and Ch-g-PMeANI/TiO₂ nano-composites can be used for the removal of anionic dye via synergistic adsorption–photodegradation effect. Introducing of TiO₂ in the nano-composites can increase both adsorption and photocatalytic degradation performance. Optimization of adsorption process was carried out. On the other hand, UV-Vis diffuse reflectance measurements suggest the photosensitizing role of Ch-g-PMeANI to improve TiO₂ response to sunlight. Interestingly, Ch-g-PMeANI/TiO₂ composite that is up taken to RR RB-133 dye was successfully regenerated by solar-driven self-cleaning photocatalytic reaction. Therefore, Ch-g-PMeANI/TiO₂ may be effectively used for removal of dye from aqueous systems for water remediation purpose.

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