Nitrogen-Rich Polyaniline-Based Activated Carbon for Water Treatment: Adsorption Kinetics of Anionic Dye Methyl Orange

In the present work, a nitrogen-rich activated carbon (PAnAC) was prepared using polyaniline (PAn) as a precursor to represent one possible conversion of nitrogen-containing polymeric waste into a valuable adsorbent. PAnAC was fabricated under the chemical activation of KOH and a PAn precursor (in a 4:1 ratio) at 650 °C and was characterized using FTIR, SEM, BET, TGA, and CHN elemental composition. The structural characteristics support its applicability as an adsorbent material. The adsorption performance was assessed in terms of adsorption kinetics for contact time (0–180 min), methyl orange (MO) concentration (C0 = 50, 100, and 200 ppm), and adsorbent dosages (20, 40, and 80 mg per 250 mL batch). The kinetic results revealed a better fit to a pseudo-second-order, specifically nonlinear equation compared to pseudo-first-order and Elovich equations, which suggests multilayer coverage and a chemical sorption process. The adsorption capacity (qe) was optimal (405.6 mg/g) at MO C0 with PAnAC dosages of 200 ppm and 40 mg and increased as MO C0 increased but decreased as the adsorbent dosage increased. The adsorption mechanism assumes that chemisorption and the rate-controlling step are governed by mass transfer and intraparticle diffusion processes.


Introduction
Currently, water pollution has become a serious crisis and a great challenge for humanity. There are several natural and man-made activities that contribute directly or indirectly to water pollution. Water dirtiness may gradually increase as a result of humans' activities, such as industrialization and civilization-related activities, which generate massive amounts of waste that is commonly discharged into water bodies and, subsequently, causes a lack of fresh water supplies [1]. About 300-400 megatons of industrial wastes are released into water bodies yearly and uncontrollably introduce various pollutants to aquatic environments, most of which are toxic to living creatures [2]. Such wastes may combine solid matter, harmful gases, toxic chemicals, and microbiological organisms. However, regarding their sources and compositions, pollutants affect our health and environment differently [3]. In particular, chemicals are known to be major water pollutants. They are present in various types, including dyes, heavy metals, pesticides, chemical compounds, cosmetics, pharmaceutics, and hormones.
Dyes, however, are among the common chemical substances largely discharged into water bodies and are considered the most dangerous pollutants due to their potential carcinogenic and mutagenic effects. Furthermore, their solubility in water affects the environment aggressively by changing the water chemistry, reducing the amounts of dissolved oxygen by obstructing sunlight penetration into the water, driving unwanted effects on aquatic creatures, and drastically disturbing their biosystems [3]. Aniline (C 6 H 7 N) (Ani, 99%), ammonium persulfate ((NH 4 ) 2 S 2 O 8 ) (APS, 98%), and potassium hydroxide (KOH) (pellets, 85%) were procured from Alfa Aesar, Karlsruhe, Germany. Hydrochloric acid (HCl) (~36%) and absolute ethanol (EtOH, 99.5%) were purchased from Fisher Chemical, Loughborough, U.K. Methyl orange sodium salt (C 14 H 14 N 3 NaO 3 S) (MO, 99.8%) powder was obtained from BDH Chemicals Ltd., Poole, England, UK. All reagents were used as received unless stated otherwise. Distilled water was used throughout the experimental process.

Preparation of Adsorbents
The adsorbent, PAnAC, was prepared using a previously described method [6]. Typically, a total amount equal to 0.08 mol of aniline monomer was charged into a precooled (in an ice bath of about 5 • C) 1L container of a 0.1 M HNO 3 aqueous solution and stirred for 30 min. To the monomer solution, 30 mL of a cold aqueous APS (0.12 mol) solution was added dropwise as an oxidative agent (initiator). The polymeric product immediately appeared; however, the reaction was left overnight, for completeness, at room temperature (23 ± 2 • C). The precipitate was filtered and sequentially washed with ethanol and water to obtain PAn. PAnAC was prepared using KOH as a chemical activating agent. Therefore, PAn was thoroughly mixed with KOH at a mass ratio of 1:4, respectively. Typically, KOH was individually powdered using a pestle and mortar and immediately mixed with the corresponding PAn quantity, with additional homogenization using a plastic spatula. Subsequently, the mixture was subjected to carbonization in a horizontal Carbolite MTF 12/38/250 tube furnace (Walf Laboratories, Hope, England, UK) under the following conditions: nitrogen atmosphere, heating ramp rate of 3 • C/min, activation time of 2 h, and activation temperature of 650 • C. The obtained carbon PAnAC was washed several times using 0.5 M HCl and water until neutrality, followed by drying in an oven at 100 • C overnight.
The surface charges of the adsorbents under investigation were determined using a pH drift method described elsewhere [14]. Initially, a pH series between 1 and 11 was prepared using a buffered solution of 0.1 M NaNO 3 ; the targeted pHs were obtained by the addition of 0.1 M of either HCl or NaOH and denoted as initial pH (pH i ). To each 15 mL of these solutions, 15 mg of the adsorbent was added and shaken intermittently for 24 h at room temperature. Adsorbent-free samples were also prepared and treated similarly as references. Subsequently, the adsorbents were filtered and analyzed to determine their final pHs (pH f ). The pH measurements were conducted using an Orion 3 Star benchtop pH meter from Thermo Scientific (Beverly, MA, USA). Finally, the pH at which the surface net charge of the adsorbents was zero (pH PZC ) was determined by plotting pH i vs. pH f .

Adsorbate
A stock solution of MO at a concentration of 500 ppm was prepared in distilled water, from which the working solutions were obtained using a dilution method. The pH values of the solutions were determined using a digital pH meter as described above. The concentration of the dye was measured using a U-2910 double-beam ultraviolet/visible (UV/Vis) spectrophotometer (Hitachi, Tokyo, Japan) at room temperature. The wavelength at which the dye adsorption was highest (λ max ) was determined first by scanning the full spectrum between 200 and 800 nm against distilled water as a blank, and it was found to be 465 nm. All colorimetric measurements were performed at this λ max . Then, the concentrations were calculated based on the Beer-Lambert law, with reference to a standard curve obtained using 5, 10, 15, 20, and 25 ppm (R 2 = 0.9815).

Characterization
The structural and morphological properties of the adsorbent were evaluated using various techniques. The Brunauer-Emmett-Teller (BET) specific surface area was obtained based on the nitrogen physisorption capacity at 77 K using a Gemini VII2390 V1.03 apparatus (Micromeritics, Norcross, GA, USA), with the instrument operating in single-point and multipoint modes. Prior to analysis, the samples were degassed for 3 h at 150 • C. The electron micrographs were acquired using a scanning electron microscope (SEM) (JSM-6360 LV, JEOL, Tokyo, Japan). A thermogravimetric analysis (TGA) was performed using a Mettler Toledo TGA/DSC 1 Star system (Columbus, OH, USA). The sample was heated from 25 to 800 • C at 25 • C/min under a nitrogen flow of 20 mL/min. The Fourier transform infrared (FTIR) spectra of the adsorbents before and after adsorption were recorded using a Nicolet iS10 (Thermo scientific, Madison, WI, USA) with an attenuated total reflection (ATR) diamond crystal accessory over the range of 4000-500 cm −1 , with a resolution of 4 cm −1 and a total of 16 scans per spectrum.

Modeling of Adsorption Kinetics Processes
Pseudo-first-order (PFO), pseudo-second-order (PSO), Elovich, and intraparticle diffusion (IPD) are the most applied kinetic models for the optimization of sorption process. Thus, in this study, linear and nonlinear forms for MO adsorption onto the targeted ac- tivated carbons were adopted to fit the experimental data. The kinetics profiles of the adsorbent PAnAC were compared, with variations in the initial adsorbate MO concentration (C 0 = 50, 100, and 200 ppm) and adsorbent dose (20,40, and 80 mg) at a fixed stirring rate (150 rpm), solution volume (250 mL), and temperature (23 ± 2 • C). The mathematical expressions of the applied models are given in Table 1. The accuracy of the fit of an adsorption model to the experiential data is typically assessed based on the magnitude of correlation coefficient (R 2 ) values. Thus, when approaching unity, the predicted values become closer to the experimental data, which can help in the assessment of the process mechanism and the applicability of the system.

PFO Model
The Lagergren kinetic equation (PFO) is the most widely used for describing a kinetic system. The model is based on the assumption that one site is available for each adsorbate molecule. Moreover, the solute uptake with time is directly proportional to the difference between the equilibrium concentration of the solute and its adsorbed amount with time [15,16]. The linear and nonlinear equations are given in Table 1. The parameters were calculated from the corresponding plot with respect to time, and thus the rate constant k 1 (min −1 ) was determined.

PSO Model
The PSO linear and nonlinear kinetic models are given in Table 1. The models assume two sites on the adsorbent for each adsorbate molecule [17,18] and suggest the dominance of the chemisorption process [19]. Moreover, the reaction rate is dependent on the amount of adsorbate on the surface of the adsorbent [20]. The parameters can be obtained by plotting the experimental q t or t/q t against t. The adsorption rate constant k 2 , as well as the initial rate (h, mg/(g·min), can be obtained by solving the PSO equations, as shown in Table 1.

Elovich Model
The Elovich equations for linear and nonlinear adsorption kinetics are also given in Table 1. The model assumes that the adsorbent sites are energetically heterogenous and that neither desorption nor interactions between the adsorbed species could substantially affect the kinetics process at low surface coverage [21]. However, the energy of adsorption increases linearly with the surface coverage according to Arrhenius equation [22]. The parameters α and β are related to the initial rate and the extent of surface coverage, respectively, as given by the model. The latter parameter can be used to assess the chemisorption process.

IPD Model
The adsorbate transfer through the internal porous structure and its actual diffusion in the adsorbate can be followed using the IPD model given by Webber and Morris, as seen in Table 1. The model constants k id and C describe the rate constant and boundary layer, which are useful for identifying the reaction pathway and adsorption mechanism and predicting the rate-controlling step. In short, if the linear slope passes through the origin, the adsorption is entirely governed by the IPD method. Otherwise, the plot may show multilinear regions, which suppose that the adsorption process is controlled by a multistep mechanism [23].
t/q t vs. t k 2 (g/mg·min); h = k 2 q e 2 (mg/g·min) [25] Elovich qt = 1 β ln(1 + αβt) q t vs. t qt = 1 β ln(αβ) + 1 β lnt q t vs. lnt α (mg/g·min); β (g/mg) [16] IPD -q t = k id t 0.5 + C q t vs. t 0.5 k id (mg/g·min 0.5 ); C (mg/g) [26] 3. Results and Discussion 3.1. Adsorbent Characterization 3.1.1. Surface Morphology and Charge Figure 1 is a schematic presentation of PAn and its corresponding nitrogen-doped graphene oxide (PAnAC) structures. According to the elemental analysis results (Table 2), PAnAC has high amounts of the heteroatoms nitrogen and oxygen, about 18 and 20 wt%, respectively. This suggests the presence of high number of active sites in the adsorbent and thus a high expected adsorption capacity.    The predicted surface charges or, more strictly, the pH at which the estimated surface charge is zero (pH PZC ) is shown in Figure 2. This property was analyzed using the basic pH drift method, and accordingly the pH PZC of PAnAC was found to be 4.4. Typically, the surface is neutral at pH PZC , below which the surface charge is positive and above which the surface charge is negative. Basically, MO is a crystalline salt that dissociates in water into an anionic MO moiety and cationic counterparts (commonly Na + ). Thus, at a pH below pH PZC , the adsorbent positive surface supported attraction with MO as a negative species, and its efficiency seemingly decreased as the pH value developed up to pH PZC , possibly due to reduced electrostatic repulsion at low pH values. As the adsorption experiments were carried out without pH adjustment (pHs were measured before the addition of the adsorbents at 20 ± 2 • C for MO concentrations of 50, 100, and 200 ppm and were found to be 6.69, 6.25, and 6.13, respectively) and the pH PZC of PAnAC is quite low, its adsorption capacity was high.   shows SEM images of the as-synthesized PAn and PAnAC. The PAn image reveals the typical nanosphere morphology of PAn, which further fused together into microsphere structures, a morphology similar to the previously reported morphology [27]. The SEM micrograph of PAnAC presents a rough structure with a peel with a surface with quite large irregular-shaped pores with a mean internal diameter of 3.7 μm. Despite the low resolution of the adsorbent image in Figure 3B, the zooming shown in the inset could predict a nanocarbon structure and pores.  Figure 3 shows SEM images of the as-synthesized PAn and PAnAC. The PAn image reveals the typical nanosphere morphology of PAn, which further fused together into microsphere structures, a morphology similar to the previously reported morphology [27]. The SEM micrograph of PAnAC presents a rough structure with a peel with a surface with quite large irregular-shaped pores with a mean internal diameter of 3.7 µm. Despite the low resolution of the adsorbent image in Figure 3B, the zooming shown in the inset could predict a nanocarbon structure and pores.

FTIR Analysis
The spectra of PAn, PAn-based activated carbons before (PAnAC) and after the adsorption of MO (PAnAC-MO), and MO are shown in Figure 4. The spectra of MO, PAn, PPyAC, and PPyAC-MO agreed with the literature [4,28,29]. The MO spectrum displayed peaks at 3626 and 3431 cm −1 for adsorbed water-OH, at 3030 cm −1 (=CH) and 2900-2811 cm −1 (CH 3 ) for stretching vibrations, and at 1519 cm −1 for C-H bending. The aromatic structure of MO was confirmable via ring deformation modes at 1034, 1005, and 846 cm −1 , while benzene substitution was assured by the characteristic peak at 816 cm −1 [28]. The Azo moiety was proven by the presence of stretching bands at 1599 (-N=N-) and 1112 cm −1 (-C-N). The sulfonic nature was affirmed by peaks at 1363 and 692 cm −1 , assigned to S=O and -C-S-stretching vibrations, respectively. In the spectrum of PAn, the peaks at 1538 and 1456 cm −1 were typical of C=N stretching vibrations.

Thermal Analysis
A thermal analysis of PAn, PAnAC, and PAnAC-MO was also carried out. The corresponding thermograms and the predicted decomposition steps are given in Figure 5 and Table 3. As can be seen, the decomposition curves can be generalized in five steps for polymers, four steps for PAnAC, and two steps for MO-loaded PAnAC. The initial TGA decomposition step, with mass losses of 7.4-11.2, was predictably due to the evaporation of volatiles, including adsorbed water and gas molecules, which peaked at 95, 86, and 98 °C in the DTG curves of PAn, PAnAC, and PAnAC-MO, respectively. The next step could be assigned to weak functional groups in the target materials, which were slightly absent in the PAnAC and MO-loaded PAnAC, showing DTG peaks at 221, 248, and 381 °C, respectively, and were supported by the extra steps for PAn. The analysis also revealed residual masses of 43.4, 64.5, and 62.3 %, respectively, at 950 °C , which is consistent with the chemical structures. After adsorption, the trend in the residual carbon content remained the same. However, the values were higher after adsorption, which may have been due to the higher content of adsorbed volatiles in the PAnACs, as assessed by the mass loss percentage in the first step. Additionally, the observed high residue of PAnAC before adsorption may have indicated an incomplete carbonaceous property, suggesting high amounts of heteroatoms. The characteristic peaks of PAn could be seen at (cm −1 ) 3219 (NH stretching, reported for protonated form [30]), 3048 (=CH stretching), 2917 (CH 3 stretching), 2328 (C=NH-C, immonium), 1569 and 1484 (C=N and C=C stretching in quinoid and benzenoid rings, respectively), 1289 and 1241 cm −1 (C-N-C and C-N stretching), 1036 and 876 cm −1 (C-H bending; its broadening may indicate contributions of peaks from the dopant NO 3− ), and 792 cm −1 (1,4-disubstituted phenyl ring [31,32]). After adsorption, the absorption of C=N (1557 cm −1 ) and C-O (1035 cm −1 ) were shifted to 1539 and 1157 cm −1 , respectively, suggesting contributions of nitrogen and oxygen groups in the adsorption mechanism. Furthermore, the additional peaks on the fingerprint region were assigned to MO and supported its attachment to the adsorbent [4].

Thermal Analysis
A thermal analysis of PAn, PAnAC, and PAnAC-MO was also carried out. The corresponding thermograms and the predicted decomposition steps are given in Figure 5 and Table 3. As can be seen, the decomposition curves can be generalized in five steps for polymers, four steps for PAnAC, and two steps for MO-loaded PAnAC. The initial TGA decomposition step, with mass losses of 7.4-11.2, was predictably due to the evaporation of volatiles, including adsorbed water and gas molecules, which peaked at 95, 86, and 98 • C in the DTG curves of PAn, PAnAC, and PAnAC-MO, respectively. The next step could be assigned to weak functional groups in the target materials, which were slightly absent in the PAnAC and MO-loaded PAnAC, showing DTG peaks at 221, 248, and 381 • C, respectively, and were supported by the extra steps for PAn. The analysis also revealed residual masses of 43.4, 64.5, and 62.3 %, respectively, at 950 • C, which is consistent with the chemical structures. After adsorption, the trend in the residual carbon content remained the same. However, the values were higher after adsorption, which may have been due to the higher content of adsorbed volatiles in the PAnACs, as assessed by the mass loss percentage in the first step. Additionally, the observed high residue of PAnAC before adsorption may have indicated an incomplete carbonaceous property, suggesting high amounts of heteroatoms.

Effect of pH
The effect of the solution pH was analyzed under the following conditions: adsorbate MO initial concentration: 200 ppm, MO volume: 25 mL, adsorbent dose: 15 mg, shaking speed: 150 rpm, temperature: 25 • C, and contact time: 24 h. The pH solution was varied between 2 and 10. The results are the averages of two independent experiments. The dependency of the sorption process on the solution pH is depicted in Figure 6. It was found that the adsorption had minimum and maximum values at pHs around 5 and 7, respectively. However, pH 7.1 was close to the untreated pollutant solution (pH 6.4). Therefore, the next experiments were performed without pH adjustments. As can be seen in Figure 6, the adsorption efficiency steadily decreased with a pH increase from 2 to 5.2, then increased and peaked at about pH 7.1 before decreasing rapidly over the remaining pH range of 7.1-10. At a low pH, the adsorbent was highly protonated. Thus, electrostatic attraction with the anionic dye was expected, which decreased as the pH developed up to pH PZC . However, with continued increases in pH, the efficiency increased, and this was mostly due to the contributions of other mechanisms, including physical interactions, rather than electrostatic interactions. This case was again observed beyond the peak of pH 7.1; that is, a drop in efficiency occurred due to the electrostatic repulsion between the negatively charge surface and the anionic dye. Furthermore, it is likely that the competition between dye molecules and hydroxide ions for adsorption onto PAnAC may explain the sharp decline in the adsorption in the basic range of pHs [33].
pHPZC. However, with continued increases in pH, the efficiency increased, and this was mostly due to the contributions of other mechanisms, including physical interactions, rather than electrostatic interactions. This case was again observed beyond the peak of pH 7.1; that is, a drop in efficiency occurred due to the electrostatic repulsion between the negatively charge surface and the anionic dye. Furthermore, it is likely that the competition between dye molecules and hydroxide ions for adsorption onto PAnAC may explain the sharp decline in the adsorption in the basic range of pHs [33].

Kinetic Studies: Effect of Adsorbate Concentration
The adsorption kinetic courses of MO at initial concentrations of 50, 100, and 200 ppm onto PAnAC at a dosage of 40 mg were evaluated over the predetermined contact time of 0-180 min at a fixed agitation speed of 150 rpm, a temperature of 23 °C, and a pH of 6.4. In all experiments, 250 mL batches of MO were used, and a 3 mL portion was withdrawn at predefined intervals, then centrifuged, measured for the remaining MO concentration,

Kinetic Studies: Effect of Adsorbate Concentration
The adsorption kinetic courses of MO at initial concentrations of 50, 100, and 200 ppm onto PAnAC at a dosage of 40 mg were evaluated over the predetermined contact time of 0-180 min at a fixed agitation speed of 150 rpm, a temperature of 23 • C, and a pH of 6.4. In all experiments, 250 mL batches of MO were used, and a 3 mL portion was withdrawn at predefined intervals, then centrifuged, measured for the remaining MO concentration, suspended again, and returned to the main adsorption solution. The results were modeled using the linear and nonlinear equations of PFO, PSO, and Elovich kinetics for comparison. In addition, IPD was utilized to help with the adsorption mechanism assessment. Figures 7 and 8 show the applied kinetic models at varied MO initial concentrations, and the corresponding parameters are summarized in Tables 4 and 5. As can be seen, the adsorption rates in the initial period, assessed before 30 min, for the three C 0 values were fast, where more than two thirds of the adsorbents were occupied with MO molecules. This reveals that the adsorption occurred on the adsorbent surface and was governed by charge interactions [4]. Following the first stage, the adsorption developed slowly but quickly reached the equilibrium at ca. 60 min.
By increasing the MO adsorbate concentration from 50 to 200, the uptake rate become slower, and the equilibrium point was delayed accordingly. The capacity was found to be increased with an initial MO concentration increase. For example, the experimental capacity (q e,exp ) of PAnAC was increased from 264.7 to 405.0 (mg/g) as MO C 0 increased from 50 to 200 ppm. These results are in accordance with the physical properties observed for the adsorbent, including the surface area and nitrogen contents. Furthermore, it was found that the lower the dye concentration, the quicker the equilibrium was reached and the lower the capacity of the adsorbent.
As noted in Tables 4 and 5, in which the kinetic parameters of the four models (PFO, PSO, Elovich, and IPD) are presented, the values of q e,exp were closer to q e,calc from PSO than that from PFO. In addition, the coefficient of determination (R 2 ) indicated the goodness of fit to be in the order of PSO > Elovich > PFO. Comparing the linear and nonlinear models, it was clear that the linear model established a poor R 2 and an overestimation of the capacity compared to the nonlinear equations. This suggested that the nonlinear PSO model was more appropriate for describing the experimental data. Further, the k 2 constant increased with an MO increase, a result that was consistent with the justified thought that the adsorption rate constants are inversely proportional to the initial adsorbate concentration; however, this was not the case for some results, as previously noted [34].     The IPD model is depicted in Figure 8, and the corresponding data are summarized in Table 5. It is clear that the data linearity is poor, indicating less effect of the model on the rate-controlling step. Furthermore, the intercept deviation from the origin suggests another mechanism, such as film diffusion. The overall pattern of the IPD data supports a two-stage mechanism. A first stage with lower values for the intercepts compared to the second stage indicates adsorption controlled by the boundary layers, while the slower rates in the second stage suggest intraparticle diffusion and an equilibrium process. The effect of the IPD mechanism may be higher at a higher C 0 concentration, as revealed by both C 1 and C 2 values being higher for C 2 (Table 5) and increasing as C 0 increased.

Kinetic Studies: Effect of Adsorbent Dose
Kinetic studies with variation in the adsorbent dosage (20,40, and 80 mg per 250 mL of MO solution volume) were also conducted. The adsorption profiles for the kinetic capacity over time are depicted in Figure 9. As can be seen, the adsorption was initially fast in a manner similar to that illustrated in Figure 7, e.g., up to 30 min. Then, adsorption developed slower up to equilibrium. The calculated experimental capacity of PAnAC was found to decrease as the dosage quantity increased, with values of 320.09, 319.73, and 287.73 g/mg when the PAnAC doses were 20, 40, and 80 mg, respectively, as summarized in Table 6.  Kinetic studies with variation in the adsorbent dosage (20,40, and 80 mg per 250 mL of MO solution volume) were also conducted. The adsorption profiles for the kinetic capacity over time are depicted in Figure 9. As can be seen, the adsorption was initially fast in a manner similar to that illustrated in Figure 7, e.g., up to 30 min. Then, adsorption developed slower up to equilibrium. The calculated experimental capacity of PAnAC was found to decrease as the dosage quantity increased, with values of 320.09, 319.73, and 287.73 g/mg when the PAnAC doses were 20, 40, and 80 mg, respectively, as summarized in Table 6.

Adsorption-Based Mechanism
Adsorption kinetics provide insight into the reaction rate and the sorption mechanism, involving mass transfer, diffusion, and the reaction that occurred on the adsorbent surface during adsorption. It is a time-dependent process that consists of several phases, such as (i) an outer diffusion stage during which the sorbate molecules transfer across the liquid film to the adsorbent exterior surface; (ii) intraparticle diffusion, involving the transportation of adsorbate particles from the adsorbent exterior surface to its internal pores; and (iii) the interaction process, with the formation of physical or chemical bonds at the active centers in the pores. However, the interaction step is commonly fast and cannot be treated as rate-limiting [4,35]. Thus, the mechanism is an important descriptor for better understanding the adsorption process. As supposed by data related to kinetic adsorption, the adsorption was mainly PSO. Thus, multilayer coverage and a chemisorption process are supported. The initial sorption rates could be assessed using the PSO and

Adsorption-Based Mechanism
Adsorption kinetics provide insight into the reaction rate and the sorption mechanism, involving mass transfer, diffusion, and the reaction that occurred on the adsorbent surface during adsorption. It is a time-dependent process that consists of several phases, such as (i) an outer diffusion stage during which the sorbate molecules transfer across the liquid film to the adsorbent exterior surface; (ii) intraparticle diffusion, involving the transportation of adsorbate particles from the adsorbent exterior surface to its internal pores; and (iii) the interaction process, with the formation of physical or chemical bonds at the active centers in the pores. However, the interaction step is commonly fast and cannot be treated as rate-limiting [4,35]. Thus, the mechanism is an important descriptor for better understanding the adsorption process. As supposed by data related to kinetic adsorption, the adsorption was mainly PSO. Thus, multilayer coverage and a chemisorption process are supported. The initial sorption rates could be assessed using the PSO and Elovich models through the constants h and α, which were calculated to be 28.5-49.5 and 101.2-223.1 (mg/(g·min)), respectively, and their values increased as MO C 0 increased. The IPD, according to the Weber-Morris model, illustrated that if it is the rate-controlling factor, the plot of q t vs. t 0.5 will go through the origin. However, the IPD deviation from the origin indicated that IPD is not the sole rate-controlling step and that two processes had taken place [35]. As seen in Figure 8, two linear portions can be identified in the IPD plot, which supports the participation of other mechanisms, including the film diffusion mechanism. Furthermore, IPD intercept C showed a faster process in the initial stage of adsorption, as indicated by the smaller C 1 compared to C 2 , and both increased as C 0 increased, suggesting a concentration-dependent process. Generally, beside the diffusion process, many other factors can play essential roles in the adsorption process, including the adsorbate structure and the associated functional groups, the textural and surface properties of the adsorbents, and the nature of the adsorbate-adsorbent interaction [36].
The proposed mechanism for MO adsorption onto PAnAC is depicted in Figure 10. As can be seen, the structural properties could support electrostatic attraction the most. The adsorption was performed in the yellow phase of MO at a working solution pH of 7.1. The MO indicator is on its azo-structure, which has a salty structure and is therefore anionic in solution. This is consistent with the results of the pH effect, in which the adsorption decreased as pH increased from 2 to 5. With the solution pH varying from 5 to 7, beside the electrostatics caused by the weak acidic nature of the solution, the molecular form of MO may drive the adsorption capacity increase as well and support the contribution of physical and some other types of adsorption mechanisms (e.g., mechanical) via hydrogen bonding and possible pore tracing events. Above pH 7, the PAnAC surface became more negative, while the OH − molecules became dominant and effectively competed with the anionic MO molecules for adsorption onto PAnAC, leading to a substantial drop in the adsorption capacity at the end.
As in Figure 4, additional peaks emerging between 1150 and 820 cm −1 for MO rings confirmed adsorption. The peak at PAnAC 1690 cm −1 assigned to C=O stretching (or OH − bending) was down-shifted to 1575 cm −1 due to an interaction with MO. This suggests contributions of such functional groups (e.g., C=O, OH, NH, N=N, and N=C) in the adsorption mechanism, and thus chemosorption cannot be ignored. As the adsorption kinetics supposed a PSO mechanism, some MO-MO interactions could be suggested as well. These types of interactions include ionic and π-π interactions, as shown in Figure 10. Table 7 compares the adsorption performance of PAn and some of the commercial and synthetic activated carbons reported in the literature with PAnAC in terms of their adsorption capacities. For appropriate and easy comparison, adsorbents from the literature were selected based on their similar structures and close adsorption conditions to PAnAC, and their application was also the removal of azo dyes. It is noteworthy that the listed capacities were primarily based on kinetic data. As can be seen, the adsorption performances of commercial ACs in the removal of azo dyes, e.g., reactive violet 5 [37,38] and MO [39], was enhanced after modification, suggesting better performance for synthetic ACs. Furthermore, Khattabi et al. [40] also demonstrated the moderate performance of commercially obtained activated carbon with a capacity (q e ) of 96 mg/g; however, the capacity of PAnAC is seemingly well positioned in the list and surpasses most adsorbents. The adsorption performance of PAn and PAn/metal oxide composites [41][42][43] was low (22-111 mg/g) compared to the listed ACs (commercials: 96-267 mg/g; synthetics: 400-482 mg/g) and PAnAC (406 mg/g). Interestingly, the nitrogen-doped AC revealed a better performance than the undoped one, with capacities of 135 and 120 mg/g, respectively, supporting the argument that activated carbons doped with nitrogen are more active, as nitrogen may provide the AC with additional effective N-based functional groups, thus increasing the number of active sites to adsorb ionic substances, including organic dyes.

Conclusions
A nitrogen-rich activated carbon (PAnAC) was prepared from a PAn precursor. Its structural and morphological properties, confirmed by FTIR, SEM, BET, TGA, and elemental analysis, evidently support its applicability as an efficient adsorbent for water treatment. The adsorption performance was evaluated in terms of the kinetic behavior of methyl orange (MO) adsorbing onto PAnAC at various MO C 0 values and PAnAC doses. The results indicated better fit to the PSO model than the PFO and Elovich models, suggesting chemisorption and a multilayer process with a rate-limiting step controlled by both film and intraparticle diffusions. Under the applied conditions of pH 7.1, room temperature, and 150 rpm agitation, the kinetic profile revealed fast adsorption over the first 30 min. Then, the rate decreased and reached equilibrium after nearly 60 min. The adsorption capacity (q e ) was found to be solute-and sorbent-dependent, being higher at higher MO concentrations and lower PAnAC dosages. It was optimal at a pH of 7.1, an MO C 0 of 200 ppm, and a PAnAC dose of 40 mg, with a value of 405.6 mg/g. Data of the kinetics favored PSO rather than PFO or Elovich, assuming a chemisorption process on a heterogenous surface, and IPD supported a partial contribution of mass transfer and IPD mechanisms, with a limiting step partially controlled by mass transfer and IPD processes. Therefore, the method of preparation can be extended to similar polymeric wastes for their conversion into valuable materials, including in the water treatment field.

Data Availability Statement:
The data that support the findings of this study are included within the article.