Synthesis of PANI@ZnO Hybrid Material and Evaluations in Adsorption of Congo Red and Methylene Blue Dyes: Structural Characterization and Adsorption Performance

In this research, a simple oxidation chemical process was applied for the synthesis of novel PANI@ZnO nanocomposite. The prepared nanocomposites were characterized by XPS, XRD, FTIR, SEM, TGA and N2 adsorption–desorption isotherms. Thereby, PANI@ZnO highest SBET values (about 40.84 m2 g−1), total mesoporous volume (about 3.214 cm3 g−1) and average pore size (about 46.12 nm). Afterwards, the prepared nanomaterial was applied as novel nanoadsorbent for the adsorption of Congo Red and Methylene Blue dyes from aqueous solutions at 298 K and pH 5.0. Besides, the pseudo-second-order model was obtained the best for the adsorption of both dyes. In the case of isotherm models, the Freundlich model showed the best fit. After removal, the spent adsorbent was regenerated. With the regeneration repeated five cycles, the PANI@ZnO regeneration efficiency remained at a very adequate level. These results are heartening in respect with the objective to utilize them in the field of sensors technology and research related to the photoluminescence sensor application.


Introduction
Dye effluents are one of the most dangerous chemical product classes found in industrial textile waste to the environment. These dyes can cause dermatitis, allergy and too provoke cancer [1][2][3]. Therefore, its elimination from different aqueous wastes is required and necessary to protect environment and human health. Generally, membrane filtration process, precipitation, coagulation, biological treatment, precipitation, adsorption, ion exchange, electrochemical process, photocatalytic degradation and ozonation are adequate candidates for such purpose [4][5][6][7][8][9][10][11]. In addition, adsorption treatment is the most effective method because providing the simplicity in employment, flexibility and low cost [12].
Similarly to conducting polymer, polyaniline (PANI) is a popular. This conductive product also has other appealing features such as good environmental stability, simple synthesis, ability to dope with protonic acids, and the being of amine groups in its structure and higher electrical conductivity [13][14][15]. The polyaniline has been prepared via chemical oxidative polymerization and electropolymerization process [16][17][18]. Generally, the usage of PANI is presently limited, due to its poor mechanical property. The PANI nanocomposites with different inorganic materials greatly ameliorate their physical and structural properties for pertinent applications [19][20][21].
Among the inorganic nanomaterials, ZnO nanoparticles have received vast interest in exclusive electrical, photocatalytic, adsorption, electronic and optical properties due to their wide bandgap (3.31 eV) as well as their low cost [22]. The wurtzite-based ZnO nanorods (NRs) showed high photoluminescence in UV range (370-390 nm), which is related to excitonic emission band [23]. For example, Balevicius et al. [24] have evaluate TIRE-based sensitivity of several different Al 2 O 3 /ZnO nanolaminates and single Al 2 O 3 or ZnO layers and Viter et al. [25] have synthesized ZnO-NRs/ MIP-Ppy as photoelectrochemical sensor for determination of bisphenol S. Several synthesis methods of PANI@ZnO and their application have been widely used in the environment field, e.g. Turemis et al. [26], recorded PANI@ZnO as photoluminescence sensors for the determination of acetic acid vapors at room temperatures and Saravanan et al. [27], examined to degrade methyl orange (MO) and methylene blue (MB) dyes by PANI/ZnO. Additionally, PANI@ inorganic hybrid is already an established adsorbent of organic pollutants in aqueous solution, such as Orange G, Methyl orange, Methylene blue, Malachite green, Congo red, Amido black 10B [28][29][30], etc.
This investigation aims to demonstrate the synthesis and application of PANI@ZnO by in-situ chemical oxidative process. The adsorbent product was characterized by different techniques XPS, XRD, FT-IR, SEM, TGA and BET. The removal of Congo red (CR) and Methylene blue (MB) using PANI@ZnO adsorbent was studied. The process of adsorption was established through the kinetics, isotherm and regeneration results.

Measurements
FTIR Spectrometer (Bruker-Inc., Model-Alpha spectrophotometer) was applied to determine the functional groups in the range of 500−4000 cm −1 . XRD (Bruker-CCD Apex instrument) with X-ray wavelength at 1.542 Å (CuK α radiation; 40 kV; 100 mA), diffraction angles at 0.2-70º, test rate of 10º min -1 and test step of 0.02° was applied to determine the structural properties of samples. Calculation of the specific surface area was performed by N 2 adsorption and desorption analysis (Autosorb-6-Quantachrome equipment). UV-Spectrophotometer (Hitachi U-3000 Spectrophotometer) was applied to measure the MO concentration. (SEM) images of the products were taken using (FEI Quanta 400 FEG). Thermogravimetry Analyze (TGA) (Hitachi STA7200 Instrument) was used to determine the thermal and/or oxidative stabilities of materials as well as their. X-ray photoelectron spectroscopy analysis (XPS) was controlled by (VG Microtech Multilab 3000-electron) spectrometer. [19,31].

Porous Texture Characterization
The microscopic pore structure property (BET analysis) of adsorbent was characterized by physical adsorption of gases (nitrogen at 77 K and Carbon dioxide at 273 K) using Micromeritics ASAP-2020 M system. Correspondingly, N 2 adsorption is mainly to gain the information on total micropores volume (V DR ) applying the Dubinin-Radushkevich (DR) formula and to determine the specific surface area according to the BET law (S BET ) [25,32].

Synthesis of Adsorbent Material
Adsorbent material was synthetized by insitu polymerization of ANI 220 mol in HCl dispersions of ZnO nanoparticles. 1.0 g of ZnO was added to a 0.5 M HCl and sonicated using probe ultrasound for 30 min. Thereafter, the ANI was added, and the solution was sonicated also 30 min to prepare stable suspension. Finally, APS dissolved in 0.5 M of HCl was added dropwise to solution of ANI with ZnO under constant stirring (the molar ratio of ANI to APS was 1:1). The preparation was carried out at 298 K for 24 h. The final produced were filtered, washed with deionized H 2 O and then dried in oven at 333 K for 24 h [19,[31][32][33]. The PANI was synthetized in similar way mentioned above but in absence of ZnO.

Adsorption Tests
The 1000 ppm stock dyes (CR or MB) solution was synthesized through dissolving 1.0 g of CR powder in 1 L of deionized H 2 O. Then, through dilution, the solution was synthesized with the required initial concentrations of dyes from 5 to 200 ppm.

Batch Adsorption Experiments
Kinetics of batch adsorption experiments were considered to investigate the time to reach equilibrium and carried out at dyes (CR or MB) solution concentrations of 5 ppm, 10 ppm, 50 ppm, 100 ppm, 150 ppm and 200 ppm with 10 mg of the PANI@ZnO adsorbent in the ambient temperature. The CR and MB solution concentration was identifying by UV-Vis spectrophotometer in wavelength of maximum absorbance of 497 nm and 664 nm, respectively. A calibration curve of concentration-vs-absorbance was determined by Beer-Lambert's equation. The quantity of dyes adsorbed at equilibrium, Q eq (mg g -1 ), was established applied formula below: Here, m (g) is the amount of adsorbent; C 0 and C eq (mg L -1 ) are the initial and equilibrium concentrations of dye, respectively; V (mL) is dye volume; Q eq (mg g −1 ) is the quantity of dye adsorbed by adsorbent at time t.
The needful properties of the Langmuir model can be proved in terms of a dimensionless separation factor (R L ) are obtained by the following: Here, C 0 : the dye concentration at equilibrium (mg L −1 ) and K L : the Langmuir constant (L mg −1 ). R L shows the state of isotherm to be either unfavorable for R L > 1, favorable (0 < R L < 1), linear for R L = 1 or irreversible for R L = 0 [34].
In conformity with the Freundlich isotherm, the adsorbed molecules cannot be greater than of active sites number, and the layer formed on nanoadsorbent surface authorized development of following layers [35]. The isotherm is determined by the next law: Here, K F : Freundlich-constant (L mg −1 ); 1∕n : intensity of adsorption constant; Q eq : adsorption capacity of dye adsorbed at equilibrium (mg g −1 ) and C eq : equilibrium concentration of dye (mg L −1 ).
Adsorption kinetic isotherms were used to describe adsorptive molecules transfer behavior and investigate factors affecting reaction rate. In the current work, pseudo first order (PFO) and pseudo second order (PSO) kinetics models were applied to research the batch adsorption performance. PFO: PSO: Here, Q eq is quantity of dye adsorbed (mg g −1 ); Q t is dye quantity on adsorbent surface at every time t (mg g −1 ) and k is equilibrium speed-constant of the PFO.

Adsorbent Regeneration Test
From the operational perspective and environmental goals, adsorbent regeneration and reuse constitute one of the important and innovative aspects of economic feasibility. To investigate PANI@ZnO adsorbent regeneration performance, 0.1 g of the prepared adsorbent was poured in 25 ml of dye (CR or MB) solution with a concentration of 1 Q eq t 150 ppm at 298 K under stirring for 2 h. The residue dye solution concentration was determined by UV-Vis spectrophotometer and dye adsorption capacity by PANI@ZnO was investigated.
After that, the adsorbent was removed from the solution and placed in 40 ml of nitric acid 0.05 molar as elution solvent on a stirrer for 10 min. Afterwards, the PANI@ZnO adsorbent was washed with deionization H 2 O and placed in dye solution with the same condition and these stages were repeated for 5 cycles.

Adsorbent Characterization
To further characterize micromorphology and molecular structure of the products, XPS, XRD, FITR, TGA, SEM and BET measurements were carried out.  [36]. As demonstrated in Table 1, the high resolution XPS spectrum of Zn2p3 presented shows three evident signals located at 1021.69 eV, 1022.45 eV and 1023.62 eV are arising from the Zn metal, Zn-O and Zn(OH) 2 , respectively. Moreover, the XPS spectrum of  [22]. Likewise, the XPS spectra confirm that Zn and N elements exists mainly in the form PANI@ZnO surfaces, this indicating to successful formation of hybrid materials. Figure 2. display the FT-IR spectrum of synthesized PANI, it can be seen a series of characteristic peaks including C=C stretching vibration of benzenoid units at 1500 cm −1 and 1589 cm -1 of PANI are presented [19,20], which makes clear that the PANI is in semi-oxidation state. The band at 1239 cm −1 is attributed to C-N stretching vibration of secondary aromatic amino structures [37]. The main characteristic band at 750 cm −1 is belonging to the aromatic N-H stretching vibration of secondary aromatic amine bending vibration. Moreover, the main band at 1111 cm −1 is attributed to the out-of-plane bending vibration of C-H within the stretching vibration of C-N of the secondary aromatic amine structures bending vibration. Besides, the ZnO nanoparticles display one maximum at 537 cm −1 [38]. Additional absorption bands were attributed to organic impurities originating from reaction intermediates, whilst one at 3438 cm −1 was ascribed to the OH-group on ZnO surface. Moreover, the FTIR spectra of PANI@ZnO are which fully match PANI spectra. Therefore, the bands at 1602 and 1500 cm −1 are ascribed to vibrations of quinoid & benzene rings, respectively. The other characteristic bands at 1302, 1104, 792 and 697 cm −1 can be attributed with the C-N stretching of the secondary aromatic amine, aromatic C-H in-plane and out-of-plane bending, respectively. Furthermore, the band of ZnO at 537 cm −1 are shifted to 569 cm −1 , indicating the formation of PANI@ZnO hybrid. Generally, this results show a clear shifting of wavenumbers indicating the interaction between PANI and the surface of the ZnO. On the other hand, a remarkable shift was observed for all bands which shifted to new values, the FT-IR spectra of PANI@ZnO after dyes adsorption also displays novel peaks. This can be considered as further evidence for the interaction between PANI@ZnO adsorbent and the (CR and MB) dyes cation.
The XRD patterns of the PANI, ZnO and PANI@ZnO samples, respectively (Fig. 3a)  showed two sharp bands centered at 2θ = 7.43º and 24.17º corresponding to (002) and (200) crystal planes, with and a broad band indicating that the majority of PANI chains were oriented in these crystal planes. Also, broad a peak are observed between 2θ = 13.18º to 18.28º indexed to (011) and (020). On the other hand, PANI@ZnO shows two peaks at 2θ = 31.81º and 36.24º corresponding to the reflections due to the ZnO (100) and (101) planes. The peak at 2θ = 7.78º of hybrid material also coincided with the (002) peak of pure PANI, but in contrast to PANI, the XRD peak of PANI@ ZnO converted to a broad amorphous peak that is observed between 2θ = 15.54º to 30.53º. By comparing the XRD patterns of the hybrid adsorbent and ZnO, it is assured that ZnO nanoparticles has retained its structure even though it is dispersed in polymer matrix during synthesis reaction. The TGA curve of PANI, ZnO and PANI@ZnO were showed in Fig. 3b. PANI displayed the initial weight loss (10.21%) below 266 ºC, which was attributed to the loss of H 2 O and solvents molecules. The second weight loss (42.51%) in the range from 266 to 447 ºC was due to the removal of structural organic ligands from their frameworks. At 900 ºC, the total weight loss of PANI was 66.21%, while PANI@ZnO was 50.51%. The reason is that the presence of PANI on the surface of ZnO promoted the growth of the crystal. It was concluded that PANI@ZnO had better thermal stability than polymer, mainly due to the introduction of ZnO in PANI matrix [39].
The SEM images of samples are shown in Fig. 4. The ZnO nanoparticles were grain and it has the shape of faceted crystals. This material is characterized by adequate porosity. While, PANI shows a classically cauliflower-like morphology [40]. Whereas that, the SEM images clearly exhibit the dual structure of PANI@ZnO hybrid that is comprised of spherical particles are surrounded by polymer matrix and hence it appears as agglomerated macromolecules [41].
The textural properties of PANI and PANI@ZnO were calculated by BET Nitrogen adsorption-desorption isotherms determined and the obtained values is described in Table 2 and Fig. 5a. It is found that the measurement S BET of PANI@ZnO was increased in some extent due to existence of ZnO nanoparticles, indicating large ratio of macropores in

Influence of pH
The effect the pH solution has on the dyes (CR or MB) removal was investigated by modifying the reaction solution pH from 4 to 11 and conserving all other parameters constant by PANI@ZnO adsorbent. Herein, lower pHs were not tested due to the instability of CR below pH 5 [43]. Figure 5b shows the effect pH has on removal efficiency. It is clear that PANI@ZnO performed better in the adsorption of CR from aqueous solution at various values of pH compared with MB dye. As observed, the adsorbent hybrid has a high potential for both dyes removal on the pH between 5 and 6. Furthermore, as Emeraldine-Salt (ES) and Emeraldine-Base (EB) formulas of the PANI in adsorbent hybrid occur at lower acidic and higher basic pH values respectively, the ES form get passed to EB about pH 7 [44]. Accordingly, the decrease in the dyes removal efficiency is may be related to this phenomenon that the surface of the PANI@ZnO adsorbent becomes less positive when pH increased from 5 to 11. The negative charge on the surface of adsorbents promotes repelling of negatively charged dyes. Therefore, pH 5 was selected for further experiments. To prepare information about factors affecting reaction rate, it is necessary to determine mechanisms that control the adsorption process such as surface adsorption, chemical reaction, and kinetics assessment infiltration mechanisms. PFO and PSO models have widely used for investigation of the adsorption process. In Table 3, the parameters related to studied kinetic models are presented. The results showed that the PFO kinetic model could not fit the experimental data well, and there was a great deviation between the linear fitting value and the experimental value, indicating that The pseudo second-order kinetic plots of dyes (CR and MB) adsorption on PANI@ZnO the adsorption of PANI@ZnO for dyes (CR and MB) did not conform to the first order kinetics. However, as showed in (Fig. 6b), the fitting results obtained by the PSO kinetic model were highly consistent with the experimental values. Accordingly, this kinetic model provided the best results and the correlation coefficient was very close to 1 which proposes a physiochemical adsorption process and an intraparticle diffusion mechanism for both dyes. The obtained R 2 values for PSO model were 0.977 and 0.991 for CR and MB dyes, respectively. Also, the calculated value of Q eq.Cal obtained from the PSO model is close to the experimental value of Q eq.Exp . Hence, the kinetics of adsorption is best defined by the PSO kinetic model for both dyes used in this study.  Table 4. The data show that the removal process of dyes was fitted well with the Langmuir isotherm. Likewise, this isotherm model indicates heterogeneous and multilayer adsorption sites. Further, the removal capacity toward CR and MB by PANI@ZnO are 69.82 mg g −1 and 56.23 mg g −1 , respectively. To compare the present method with other reported studies for the adsorption of CR or MB, adsorption capacity of the methods is summarized in Table 5. Thus, the present study provided better adsorption capacity in comparison with other reported methods using adsorbent. One possible reason for the better performance of the prepared adsorbents is the adsorption mechanism, which occurs in both anion exchange and surface adsorption by H-bonding with π-π liaison.

Reuse of Adsorbent
Regeneration and reusability of an adsorbent material is an important factor to assess the feasibility for workable applications. Therefore, this adsorbent product was used for several adsorption-regeneration cycles with removal over 60 min. In this study, washing of employed adsorbent with C 2 H 5 OH and distilled H 2 O was used to regenerate the adsorbent PANI@ZnO. As shown in Fig. 7b. the adsorbent presents suitable capabilities for recovery and reuse. Besides, the adsorbent recovery at some steps showed a stable adsorbent capacity in dye removal which this result can illustrate that C 2 H 5 OH is an exceptional detergent for adsorbent recovery. On the other hand, the continuous decrease in quantity adsorbed and reusability efficiency could suggest that some dyes remained on adsorbent material next each reusability or that the adsorbent structure was changing. In addition, this high performance of PANI@ZnO adsorbent can be further utilized as a sensor or photoluminescence sensor [45].

Conclusions
In this research, PANI@ZnO nanocomposite was facilely prepared by in-situ oxidative polymerization. After the characterization of the synthesized nanocomposite by XPS, XRD, FTIR, TGA, SEM and BET analyses, this hybrid material were applied as novel adsorbent for removal of CR and MB dyes from aqueous solution. The effect of important experimental parameters including solution pH, contact time, dyes concentration, kinetics, and isothermal analysis was investigated. The novel prepared nanoadsorbent showed significant adsorption performance toward both dyes. Moreover, the kinetic analysis detected that PSO rate formula executed better than PFO rate law, this promoting the formation of physiochemical adsorption process and an intraparticle diffusion mechanism. For isothermal studies, the Freundlich model showed the best fit based on R 2 values.
Finally, maximum adsorption capacities of 69.82 mg g −1 and 59.23 mg g −1 were obtained for CR and MB dyes, respectively, by PANI@ZnO adsorption. Additionally, the obtained nanoadsorbent exhibited an adequate cyclability to a range between 77.14 and 40.34% after 5 cycles. Besides, the present work provides new insights into the synthesis of PANI@ZnO hybrid material as an efficient adsorben, because of the high adsorption capacity, rapid removal, excellent reusability, and adequate stability through the electrostatic attraction, hydrogen bonding, π-π interaction, and dipole-dipole interaction, and this developed nanocomposite would have potential applications in environmental remediation related fields.