3.1. Adsorbent characterization
To further characterize micromorphology and molecular structure of the products, XRD, FITR, TGA and BET measurements were carried out. Fig. 2-a show the XRD patterns of the PANI, AC and PANI@AC samples, respectively. The AC shows peaks at around 2θ = 20.80º, 26.61º, 31.25º, 33.41º, 40.74º, 43.43º, and 47.85º which corresponds to (100), (101), (220), (110), (102), (200) and (112) plane of carbon, respectively. Furthermore, It can be seen that the AC pattern displayed an amorphous halo centered at 2θ = 25.32º, which indicates to the reflection of the plane (002), a common characteristic of non-crystalline structures such as AC [31]. Moreover, the PANI shows an amorphous background in their XRD patterns because polyaniline is incomplete crystalline. The crystallinity and coherence length of the pure PANI polymer chain orientation can be analyzed by the diffraction peaks at 2θ values of 8.94º (001), 16.49º (011), 20.15º (100) and 24.90º (110). All peaks are in good agreement with results by Bekhti et al. [32]. This diffraction peaks are attributed to the vertical and parallel periodicity of the PANI chain, respectively. On the other hand, the PANI@AC product consist of peaks at 2θ = 20.80º and 26.61º. Further, the peak of this sample in the observed diffraction profile is almost at around 24.68º reveals to amorphous type of PANI@AC.
Fig. 2-b. 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 1490 cm−1 and 1576 cm–1 of PANI are presented [25, 32], which makes clear that the PANI is in semi-oxidation state. The bands at 1296 cm−1 and 1242 cm−1 are attributed to C–N stretching vibration of secondary aromatic amino structures [33]. The main characteristic band at 800 cm−1 is belonging to the aromatic N–H stretching vibration of secondary aromatic amine bending vibration. Moreover, the main band at 1128 cm−1 is ascribed 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 characteristic absorption band appeared at the 1377 cm−1 is related to the bending vibrations of the C–H groups. For AC, spectra have similar shape in vibration band features of carbonaceous material and the band at 3387 cm−1 can be associated to O–H groups. The band at 1554 cm−1 can be assigned to C=O axial deformation. Whereas that, band at 1096 cm−1 can be attributed to C–OH (phenolic, ethers). Thereby, there are many functional groups for adsorbing contaminant ions on AC. These functional groups play an important role in removal of pollutant ions. Moreover, the FTIR spectra of PANI@AC are which fully match PANI spectra. For PANI@AC spectrum, the bands at 1576 and 1495 cm−1 are ascribed to vibrations of the quinoid and benzene rings, respectively. The other characteristic bands at 1301, 1126, and 816 cm−1 can be attributed with the C–N stretching of the secondary aromatic amine, aromatic C–H in-plane bending and out-of-plane bending vibration, respectively. Furthermore, the band of AC at 1096 cm−1 are shifted to 1032 cm−1, indicating the interaction between the PANI and the surface of the AC.
The TGA curve of AC, PANI and PANI@AC were showed in Fig. 2c. PANI displayed the initial weight loss (20.84%) below 194 ºC, which was attributed to the loss of water and solvents molecules. The second weight loss (32.81%) in the range from 194 ºC to 408 ºC was due to the removal of structural organic ligands from their frameworks. At 800 ºC, the total weight loss of PANI was 67.86%, while PANI@AC was 54.63%. The reason is that the presence of polymer on the surface of AC promoted the growth of the crystal. It was concluded that PANI@AC had better thermal stability than PANI, mainly due to the introduction of AC.
With measuring N2 adsorption-desorption isotherms, the pore size distribution, specific surface area and pore volume of AC, PANI and PANI@AC were calculated, and results were showed in Fig. 2-d and Table 1. It showed that curves of samples are IV-type isotherms with H3 type hysteresis loops, confirming the presence of mesoporous in the material. Comparing with PANI, the specific, pore size, and pore volume of PANI@AC were significantly increased. The specific surface PANI@AC area was 332 m2.g−1, which was substantially higher than that of PANI (17.52 m2.g−1). The pore volume increased from 0.023 cm3.g−1 to 0.038 cm3.g−1. These mesoporous structure with large surface area were more favorable to the adsorption of dyes.
3.2. Adsorption of MO
3.2.1. Influence of pH
The effect the pH solution has on the MO removal was investigated by changing the reaction solution pH from 3 to 11 and conserving all other parameters constant by PANI and PANI@AC adsorbents, respectively. Fig. 3-a shows the effect pH has on removal efficiency. It is clear that PANI@AC performed better relative to PANI in the PANI in the adsorption of MO from aqueous solution at various values of pH studied. In addition, it was showed that both adsorbents realized the best results at pH 6. The elimination rate was low at both lower and higher pH values. As observed, the PANI has a high potential for MO removal on the pH between 6 and 8. Further, the MO ions possesses negatively charged at pH 7 (or practically neutral pH values, between 6 and 8) and display maximum MO removal. Furthermore, as Emeraldine-Salt (ES) and Emeraldine-Base (EB) formulas of the PANI occur at lower acidic and higher basic pH values respectively, the ES form get passed to EB near pH 7 [34]. Likewise, the MO exist in cationic form and at the same time the PANI has positive charge in the pH value between 5 and 8, the maximum removal has been presented in this range of pH values, thanks to the formation of electrostatic force to the gravitational between MO and adsorbent used.
3.2.2. Effect of adsorption time on adsorption efficiency
Fig. 3-b. exhibits a comparison of the MO adsorption capacity and removal efficiency with time. The results showed that the % removal of dye by PANI increased with increasing time from 10 to 40 min where reached 17.64 % at 40 min. Thereafter, the % removal of MO increased to 76.18 % when the time changed from 40 to 60 min. Also, the influence of time on the adsorption capacity of PANI@AC sample toward MO was performed in the time range of 10-120 min. The results showed that the adsorption capacity of the hybrid adsorbent toward dye increased with increasing time from 10 to 60 min where reached 192.52 mg.g−1 (77.14 %) at 60 min. Hence, 60 min is considered the optimum time for the removal of MO using the PANI@AC.
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. Pseudo first order and pseudo second order models have widely used for investigation of the adsorption process. In Table 2, the parameters related to studied kinetic models are presented. The correlation coefficient R2 represents how good these kinetic models fit the removal process. The R2 values obtained from kinetic models reveal that the removal process complies more with the pseudo second order kinetic model indicating that chemical adsorption is ratio controlling and adsorption is the result of interaction betwixt functionally groups on the nanoadsorbent surface. Also, the calculated value of Qeq.Cal obtained from the pseudo second order model is close to the experimental value of Qeq.Exp. Hence, the kinetics of adsorption is best defined by the pseudo second order kinetic model for two adsorbents used in this study.
3.3. Adsorption isotherms of MO
Fig. 3-c display the adsorption isotherms of MO by various adsorbents calculated at 298 K. The matching result of sorption isotherms using Langmuir and Freundlich models are summarized in Table 3. The data show that the removal process of dye was fitted well with the Langmuir isotherm. Further, the removal capacity of PANI and PANI@AC adsorbents toward MO are 49.50 mg.g−1 and 192.30 mg.g−1, respectively. The adsorption performance of the PANI@AC product toward MO was compared with that of other adsorbent materials in the literature as clarified in Table 4. Clearly, the PANI@AC product outperformed most of the adsorbents because it has the highest adsorption capacity value.
3.4. Reuse of adsorbent
Regeneration and reusability of an adsorbent 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 C2H5OH and distilled H2O was used to regenerate the adsorbent PANI@AC. As shown in Fig. 3-d, the adsorbent show 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 ethanol is an exceptional detergent for adsorbent recovery. Moreover, the calculated regeneration efficiencies were 75.79%, 69.75%, 58.13% and 40.34%, respectively. The continuous decrease in quantity adsorbed and reusability efficiency could suggest that some MO remained on PANI@AC after every reusability or that the adsorbent structure was modifying or changing.