Preparation of Polyaniline/Emulsion Microsphere Composite for Efficient Adsorption of Organic Dyes

Surface-functionalized polymeric microspheres have wide applications in various areas. Herein, monodisperse poly(styrene–methyl methacrylate–acrylic acid) (PSMA) microspheres were prepared via emulsion polymerization. Polyaniline (PANI) was then coated on the PSMA surface via in situ polymerization, and a three-dimensional (3D) structured reticulate PANI/PSMA composite was, thus, obtained. The adsorption performance of the composite for organic dyes under different circumstances and the adsorption mechanism were studied. The obtained PANI/PSMA composite exhibited a high adsorption rate and adsorption capacity, as well as good adsorption selectivity toward methyl orange (MO). The adsorption process followed pseudo-second-order kinetics and the Langmuir isotherm. The maximum adsorption capacity for MO was 147.93 mg/g. After five cycles of adsorption–desorption, the removal rate remained higher than 90%, which indicated that the adsorbent has great recyclability. The adsorbent materials presented herein would be highly valuable for the removal of organic dyes from wastewater.


Introduction
The wide use of organic dyes in textile, leather, printing, and cosmetics industries resulted in a large amount of colored wastewater, seriously threatening ecological environment and human health [1]. Azo dyes account for about 50% of organic dyes in the world [2][3][4], they are toxic and can accumulate in the human body through the food chain, jeopardizing health and even causing cancers [5]. Methyl orange (MO), as a typical anionic azo dye, is widely used in various industrial fields. However, MO is difficult to remediate from water with chemical and biological methods due to its high chemical stability [6]. Thus, the efficient removal of MO from water is a challenge and worldwide concern.

Adsorption Experiment
The adsorption experiment was performed as follows: PANI/PSMA was dispersed in the dye solution, and the mixture was stirred in a thermostatic oscillator. The pH was adjusted with NaOH (0.1 M) and HCl (0.1 M). A series of dye solutions with different concentrations were prepared, and the absorbance values were firstly measured at an appropriate wavelength for the plot of calibration curves. Then, the absorbance was measured with an ultraviolet-visible light spectrophotometer over a specific time until the adsorption equilibrium was reached. The removal rate for dyes (% removal), adsorption capacity at time t (Qt, mg/g), and equilibrium adsorption capacity (Qe, mg/g) of PANI/PSMA were calculated with the following formulas: where C0 and Ce are the initial and equilibrium concentrations (mg/L) of dyes in the solution, Ct is the concentration (mg/L) at time t (min), V (L) is the volume of the solution, and m (mg) is the mass of PANI/PSMA adsorbent.

Selective Adsorption
Typically, 10-mL cationic dye (MB, 0.1 mM) and 10-mL MO (0.1 mM) solutions were mixed in a plastic bottle. Then, 20 mg of PANI/PSMA was added to the above solution. The mixture was oscillated for 6 h. After the adsorption process, the mixture was filtered to determine dye concentration.

Adsorption-Desorption Recycling Experiment
Firstly, 45 mg of adsorbent was added to a 30-mL MO solution (150 mg/L). The mixture was oscillated at 25 °C for 6 h. Then, the mixture was separated with a centrifuge to measure the dye concentration of supernatant. The used PANI/PSMA was immersed in NaOH solution (0.1 M) for desorption for three times; then, it was collected via centrifugation and dried at 60 °C. Finally, the treated adsorbent was used for re-adsorption. Cyclic adsorption-desorption was carried out six times.

Adsorption in Simulated Industrial Wastewater (SIW)
The SIW was prepared according to the "Integrated wastewater discharge standard" (GB8978-1996) and "Discharge standard for water pollutants of textile dyeing and finishing industry"

Adsorption Experiment
The adsorption experiment was performed as follows: PANI/PSMA was dispersed in the dye solution, and the mixture was stirred in a thermostatic oscillator. The pH was adjusted with NaOH (0.1 M) and HCl (0.1 M). A series of dye solutions with different concentrations were prepared, and the absorbance values were firstly measured at an appropriate wavelength for the plot of calibration curves. Then, the absorbance was measured with an ultraviolet-visible light spectrophotometer over a specific time until the adsorption equilibrium was reached. The removal rate for dyes (% removal), adsorption capacity at time t (Q t , mg/g), and equilibrium adsorption capacity (Q e , mg/g) of PANI/PSMA were calculated with the following formulas: where C 0 and C e are the initial and equilibrium concentrations (mg/L) of dyes in the solution, C t is the concentration (mg/L) at time t (min), V (L) is the volume of the solution, and m (mg) is the mass of PANI/PSMA adsorbent.

Selective Adsorption
Typically, 10-mL cationic dye (MB, 0.1 mM) and 10-mL MO (0.1 mM) solutions were mixed in a plastic bottle. Then, 20 mg of PANI/PSMA was added to the above solution. The mixture was oscillated for 6 h. After the adsorption process, the mixture was filtered to determine dye concentration.

Adsorption-Desorption Recycling Experiment
Firstly, 45 mg of adsorbent was added to a 30-mL MO solution (150 mg/L). The mixture was oscillated at 25 • C for 6 h. Then, the mixture was separated with a centrifuge to measure the dye concentration of supernatant. The used PANI/PSMA was immersed in NaOH solution (0.1 M) for desorption for three times; then, it was collected via centrifugation and dried at 60 • C. Finally, the treated adsorbent was used for re-adsorption. Cyclic adsorption-desorption was carried out six times.

Adsorption in Simulated Industrial Wastewater (SIW)
The SIW was prepared according to the "Integrated wastewater discharge standard" ( GB8978-1996) and "Discharge standard for water pollutants of textile dyeing and finishing industry" (GB4287-2012). The compositions of SIW include aniline (1 mg/L), Na 3 PO 4 (12 mg/L), CuSO 4 ·5H 2 O (4.3 mg/L), Mg(NO 3 ) 2 ·6H 2 O (9 mg/L), and NaCl (200 mg/L), and the pH of SIW was 6.0. Then, 50 mg of adsorbent was added to 20 mL of SIW containing MO (10 mg/L). After adsorption of 3 h at 25 • C and pH 6.0, the adsorption effect of MO was measured. Thus, 20 mg of PANI/PSMA was added to 100 mL of MO-contaminated SIW and 100 mL of MO-contaminated aqueous solution (MO concentration: 200 mg/L). After adsorption of 6 h at 25 • C and pH 6.0, the adsorbents were removed, and the adsorption capacities were calculated. The adsorption capacity of prepared adsorbents toward MO in mixed dye-contaminated wastewater was tested as well, and the adsorption effect of PANI/PSMA for MO was determined in 20 mL of SIW containing 10 mg/L MO and 10 mg/L MB.

Measurements and Characterizations
Fourier-transform infrared (FT-IR) measurements were performed with a Thermo Nexus 470FT-IR spectrometer (Nicolet Company, Waltham, MA, USA). A small amount of sample was ground and pressed with KBr in a mortar. Thermogravimetric analysis (TGA) was performed with a Q500 thermogravimeter (TA Company, Newcastle, DE, USA). The temperature was increased from room temperature to 800 • C at a rate of 20 • C/min. SEM and energy-dispersive X-ray spectroscopy (EDS) were performed with a S-4800 field-emission scanning electron microscope, and the samples were sprayed with gold. Zeta potential was measured with a Zetasizer3000HS nanoparticle-size and zeta potential analyzer (Malvern Company, Malvern, UK). X-ray diffraction (XRD) analysis was performed with an X Pert PRO wide-angle X-ray scatterer (Panalytical, Almelo, the Netherlands) with a 2θ range of 5 • -80 • . Ultraviolet (UV) characterization was performed with an ultraviolet-visible light spectrophotometer (PerkinElmer Co., Ltd., Shanghai, China). Figure 2a shows the infrared spectra of PANI, PSMA, and PANI/PSMA. For PANI, the peaks at 1567, 1471, 1112, and 796 cm −1 were assigned to C=C stretching of the quinone ring, C=C stretching of the benzene ring, N=Q=N (Q = quinone rings stretching, and C-H bending of 1,4-disubstituted aromatic rings, respectively. Moreover, the bands at 1295 and 1236 cm −1 were attributed to aromatic conjugated C-N stretching. The results suggested that the PANI was successfully prepared [26]. The characteristic peaks of PSMA were as follows: 2921 cm −1 , 3025 cm −1 (=C-H stretching of benzene ring), 1452 cm −1 , 1493 cm −1 (the vibration of the benzene ring skeleton), 697 cm −1 , 756 cm −1 (the out-of-plane bending vibration of C-H bonds in the benzene ring), 1029 cm −1 (the in-plane deformation vibration of H atoms in monosubstituted benzene ring), 2848 cm −1 (absorption of methylene), 1731 cm −1 (C=O stretching), 1199 cm −1 (C-O-C stretching), and 3440 cm −1 (-OH stretching). The results confirmed that PSMA was composed of poly(styrene-methyl methacrylate-acrylic acid). In the spectrum of PANI/PSMA, the peaks at 2923 cm −1 (=C-H of benzene), 1479 cm −1 (the vibration of benzene ring skeleton), 2852 cm −1 (the vibration of methylene), and 1569 cm −1 (C=C stretching of quinone ring) indicated that PANI and PSMA were successfully combined. Figure 2b shows the TGA curves of PANI, PSMA, and PANI/PSMA. The obvious weight loss of PSMA was observed at 350-450 • C, indicating that PSMA started to decompose at about 350 • C to form small molecules. The organic matter decomposed completely at 450 • C. On the other hand, the weight loss of PANI involved three stages. The three stages at 50-250 • C, 250-458 • C, and 458-800 • C were due to the loss of water, the loss of bound water and the decomposition of PANI into small molecules, and carbonization, respectively [27]. Similarly, the weight of PANI/PSMA also decreased at 50-250 • C due to the loss of water. The PANI/PSMA exhibited a significant weight loss of about 80% at 300-516 • C owing to the decomposition of a large part of PSMA and small part of PANI. When the temperature was higher than 100 • C, slight weight loss occurred thanks to the decomposition of the PANI skeleton. The thermal decomposition temperature of PANI/PSMA (499 • C) increased by 79 • C compared with PSMA (420 • C), which was attributed to strong intermolecular interactions between PANI and PSMA on the surface, thus improving the thermal stability of the PANI/PSMA. The results further proved that PANI was successfully combined with PSMA. Figure S1 (Supplementary Materials) shows the XRD patterns of PANI and PANI/PSMA. The PANI exhibited three characteristic peaks at 2θ of 15.08 • , 20.81 • , and 25.34 • corresponding to the 011, 020, and 200 crystal planes, respectively, indicating that PANI was polycrystalline [28,29]. The PANI/PSMA exhibited similar peaks, which suggested that the crystallinity of PANI and PANI/PSMA was similar, i.e., partially crystallized.

The Structural Characterization of PANI/PSMA
Polymers 2020, 12, x FOR PEER REVIEW 5 of 15 that PANI was successfully combined with PSMA. Figure S1 (Supplementary Materials) shows the XRD patterns of PANI and PANI/PSMA. The PANI exhibited three characteristic peaks at 2θ of 15.08°, 20.81°, and 25.34° corresponding to the 011, 020, and 200 crystal planes, respectively, indicating that PANI was polycrystalline [28,29]. The PANI/PSMA exhibited similar peaks, which suggested that the crystallinity of PANI and PANI/PSMA was similar, i.e., partially crystallized.  Figure 3 shows the SEM images of PSMA, PANI, and the PANI/PSMA composite. In Figure 3a, the microspheres have uniform particle size, regular shape, high sphericity, and good monodispersity (coefficient of variability (CV) = 4.7%). Based on the particle size measurement and Gauss fitting, the mean particle size of PSMA microspheres was determined to be about 317 ± 15 nm. In Figure 3b, PANI was composed of intertwined fibers and a small number of agglomerates. From Figure 3c, it can be seen that PANI/PSMA was covered by reticulate PANI with an enlarged surface area [28]. The N2 adsorption-desorption curves ( Figure S2, Supplementary Materials) also showed that PANI/PSMA (41.7 m 2 /g) had a larger specific surface area than PANI (22.8 m 2 /g), further demonstrating that PANI/PSMA could provide more effective adsorption sites than PANI.   Figure 3 shows the SEM images of PSMA, PANI, and the PANI/PSMA composite. In Figure 3a, the microspheres have uniform particle size, regular shape, high sphericity, and good monodispersity (coefficient of variability (CV) = 4.7%). Based on the particle size measurement and Gauss fitting, the mean particle size of PSMA microspheres was determined to be about 317 ± 15 nm. In Figure 3b, PANI was composed of intertwined fibers and a small number of agglomerates. From Figure 3c, it can be seen that PANI/PSMA was covered by reticulate PANI with an enlarged surface area [28]. The N 2 adsorption-desorption curves ( Figure S2, Supplementary Materials) also showed that PANI/PSMA (41.7 m 2 /g) had a larger specific surface area than PANI (22.8 m 2 /g), further demonstrating that PANI/PSMA could provide more effective adsorption sites than PANI.
Polymers 2020, 12, x FOR PEER REVIEW 5 of 15 that PANI was successfully combined with PSMA. Figure S1 (Supplementary Materials) shows the XRD patterns of PANI and PANI/PSMA. The PANI exhibited three characteristic peaks at 2θ of 15.08°, 20.81°, and 25.34° corresponding to the 011, 020, and 200 crystal planes, respectively, indicating that PANI was polycrystalline [28,29]. The PANI/PSMA exhibited similar peaks, which suggested that the crystallinity of PANI and PANI/PSMA was similar, i.e., partially crystallized.  Figure 3 shows the SEM images of PSMA, PANI, and the PANI/PSMA composite. In Figure 3a, the microspheres have uniform particle size, regular shape, high sphericity, and good monodispersity (coefficient of variability (CV) = 4.7%). Based on the particle size measurement and Gauss fitting, the mean particle size of PSMA microspheres was determined to be about 317 ± 15 nm. In Figure 3b, PANI was composed of intertwined fibers and a small number of agglomerates. From Figure 3c, it can be seen that PANI/PSMA was covered by reticulate PANI with an enlarged surface area [28]. The N2 adsorption-desorption curves ( Figure S2, Supplementary Materials) also showed that PANI/PSMA (41.7 m 2 /g) had a larger specific surface area than PANI (22.8 m 2 /g), further demonstrating that PANI/PSMA could provide more effective adsorption sites than PANI.

Effects of pH
Because the intermolecular interactions between adsorbents and dye molecules are significantly affected by the pH, pH is a key parameter for dye adsorption [30,31]. The initial pH of solution determines the charges on the surface of the adsorbent. Figure S3 (Supplementary Materials) shows the zeta potential profiles of PSMA, PANI, and PANI/PSMA at different pH. The values of zeta potential of PSMA were about −60 in the pH range of 4-10, which indicated that PSMA possessed excellent stability.
The pH values of PANI and PANI/PSMA were 6.0 and 4.7 at their isoelectric points, respectively. When the pH was lower than the isoelectric point, the imines and amines in PANI and PANI/PSMA had positive charges. Figure 4a shows the adsorption capacities of PANI and PANI/PSMA for MO at different pH. The results showed that the adsorption capacities decreased gradually with the increase of pH. This was because the basic imino and amino groups of adsorbents produced positive charges in an acidic environment, resulting in improved electrostatic attraction between adsorbents and anionic dye MO. With the increase of pH, the imino and amino groups were deprotonated, causing a smaller number of positive charges. As a result, the electrostatic attraction between adsorbents and MO decreased gradually, and the adsorption capacities decreased accordingly. Moreover, the adsorption capacity of PANI/PSMA was greater than that of PANI, indicating that the combination of PANI and PSMA exhibited a better adsorption performance. The adsorption capacities of PANI and the PANI/PSMA composite were greater at a lower pH.

Effects of pH
Because the intermolecular interactions between adsorbents and dye molecules are significantly affected by the pH, pH is a key parameter for dye adsorption [30,31]. The initial pH of solution determines the charges on the surface of the adsorbent. Figure S3 (Supplementary Materials) shows the zeta potential profiles of PSMA, PANI, and PANI/PSMA at different pH. The values of zeta potential of PSMA were about −60 in the pH range of 4-10, which indicated that PSMA possessed excellent stability. The pH values of PANI and PANI/PSMA were 6.0 and 4.7 at their isoelectric points, respectively. When the pH was lower than the isoelectric point, the imines and amines in PANI and PANI/PSMA had positive charges. Figure 4a shows the adsorption capacities of PANI and PANI/PSMA for MO at different pH. The results showed that the adsorption capacities decreased gradually with the increase of pH. This was because the basic imino and amino groups of adsorbents produced positive charges in an acidic environment, resulting in improved electrostatic attraction between adsorbents and anionic dye MO. With the increase of pH, the imino and amino groups were deprotonated, causing a smaller number of positive charges. As a result, the electrostatic attraction between adsorbents and MO decreased gradually, and the adsorption capacities decreased accordingly. Moreover, the adsorption capacity of PANI/PSMA was greater than that of PANI, indicating that the combination of PANI and PSMA exhibited a better adsorption performance. The adsorption capacities of PANI and the PANI/PSMA composite were greater at a lower pH.

Effects of the Dye Type
Five representative dyes were selected to study the adsorption behavior toward PANI/PSMA. The initial concentration of these dyes was 100 mg/L. As shown in Figure 4b, the adsorption capacities of PANI/PSMA for anionic dyes (MO, TTZ, and ART) were much higher than those for cationic dyes (MB and MV). The structure of the dyes is shown in Figure S4 (Supplementary Materials). For example, the adsorption capacity for MO reached 152.9 mg/g, about 47 times higher than that for cationic MB and MV dyes, respectively. The images of different dyes before and after adsorption by PANI/PSMA are shown in Figure S5 (Supplementary Materials). The excellent adsorption performance of PANI/PSMA toward anionic dyes could be attributed to the strong interactions between the positively charged amino groups in PANI/PSMA and negatively charged SO3 − groups in the anionic dyes. In contrast, MB and MV molecules contain positively charged amino groups, which electrostatically repulse PANI/PSMA, resulting in a poor adsorption performance [32].

Effects of the Dye Type
Five representative dyes were selected to study the adsorption behavior toward PANI/PSMA. The initial concentration of these dyes was 100 mg/L. As shown in Figure 4b, the adsorption capacities of PANI/PSMA for anionic dyes (MO, TTZ, and ART) were much higher than those for cationic dyes (MB and MV). The structure of the dyes is shown in Figure S4 (Supplementary Materials). For example, the adsorption capacity for MO reached 152.9 mg/g, about 47 times higher than that for cationic MB and MV dyes, respectively. The images of different dyes before and after adsorption by PANI/PSMA are shown in Figure S5 (Supplementary Materials). The excellent adsorption performance of PANI/PSMA toward anionic dyes could be attributed to the strong interactions between the positively charged amino groups in PANI/PSMA and negatively charged SO 3 − groups in the anionic dyes. In contrast, MB and MV molecules contain positively charged amino groups, which electrostatically repulse PANI/PSMA, resulting in a poor adsorption performance [32].

Adsorption Kinetics
Adsorption rate is one of the most important indices of adsorptive performance for an adsorbent. With two different initial MO concentrations, the effect of adsorption time on the adsorption of MO is shown in Figure 5a. The adsorption capacity increased with the increase of adsorption time until adsorption equilibrium was reached. Because the adsorption process was slow, the increase of adsorption time was favorable to the interactions between MO molecules and PANI/PSMA adsorbent. The adsorption capacity of PANI/PSMA increased sharply within 60 min and reached an equilibrium state after about 120 min. The high adsorption rate, resulting from the strong electrostatic attraction between the positively charged amines in PANI/PSMA and negatively charged anionic dyes, has obvious advantages in practical applications. For the understanding of the adsorption mechanism, the pseudo-first-order model, pseudo-second-order model, and intraparticle diffusion model were separately fitted with the adsorption data [33,34]. The kinetic parameters were calculated according to the corresponding slope and intercept values in the fitting (Figure 5b-d), as depicted in Table 1.
As illustrated in Figure 5b-d, the correlation coefficients (R 2 ) of the pseudo-second-order kinetic model were 0.9995 (20 mg/L) and 0.9984 (30 mg/L), higher than those of the pseudo-first-order kinetic model and intraparticle diffusion model. The results show that the adsorption process conformed with the pseudo-second-order kinetic model (Figure 5c). In addition, the Q e values based on the pseudo-second-order kinetic model were 81.30 (20 mg/L) and 116.41 mg/g (30 mg/L), in good agreement with the experimental data (79.37 and 112.10 mg/g, respectively). Because the pseudo-second-order kinetic model is based on the hypothesis of chemical adsorption, the interaction between amino groups in PANI/PSMA and anionic dye molecules was the main driving force for adsorption in the present study.

Adsorption Kinetics
Adsorption rate is one of the most important indices of adsorptive performance for an adsorbent. With two different initial MO concentrations, the effect of adsorption time on the adsorption of MO is shown in Figure 5a. The adsorption capacity increased with the increase of adsorption time until adsorption equilibrium was reached. Because the adsorption process was slow, the increase of adsorption time was favorable to the interactions between MO molecules and PANI/PSMA adsorbent. The adsorption capacity of PANI/PSMA increased sharply within 60 min and reached an equilibrium state after about 120 min. The high adsorption rate, resulting from the strong electrostatic attraction between the positively charged amines in PANI/PSMA and negatively charged anionic dyes, has obvious advantages in practical applications. For the understanding of the adsorption mechanism, the pseudo-first-order model, pseudo-second-order model, and intraparticle diffusion model were separately fitted with the adsorption data [33,34]. The kinetic parameters were calculated according to the corresponding slope and intercept values in the fitting (Figure 5b-d), as depicted in Table 1.
As illustrated in Figure 5b-d, the correlation coefficients (R 2 ) of the pseudo-second-order kinetic model were 0.9995 (20 mg/L) and 0.9984 (30 mg/L), higher than those of the pseudo-first-order kinetic model and intraparticle diffusion model. The results show that the adsorption process conformed with the pseudo-second-order kinetic model (Figure 5c). In addition, the Qe values based on the pseudo-second-order kinetic model were 81.30 (20 mg/L) and 116.41 mg/g (30 mg/L), in good agreement with the experimental data (79.37 and 112.10 mg/g, respectively). Because the pseudosecond-order kinetic model is based on the hypothesis of chemical adsorption, the interaction between amino groups in PANI/PSMA and anionic dye molecules was the main driving force for adsorption in the present study.  For a better understanding of rate-determining factors in the adsorption process, the adsorption data were analyzed on the basis of the intraparticle diffusion model [35,36] as described in Table 1. The correlation coefficients (R 2 ) were 0.8077 (20 mg/L) and 0.9173 (30 mg/L). In the fitting of the intraparticle diffusion model, the fitting curve did not pass through the origin (Figure 5d), indicating that intraparticle diffusion was not the only rate-determining factor. It is supposed that electrostatic attraction, ion exchange, and some other factors may also affect the adsorption rate.

Adsorption Isotherms
For a better understanding of the interactions between dye molecules and the adsorbent surface, the adsorption isotherms were studied as shown in Figure 6. In Figure 6a, the adsorption capacity of PANI/PSMA for MO increased gradually with the increase in initial dye concentration and reached an equilibrium state. When the dye concentration was high, the driving force was strong in the adsorption process, thus overcoming the mass transfer resistance of dye molecules from the aqueous to solid phase and improving the contact between dye molecules and binding sites on the adsorbent. For the study on the maximum adsorption capacity, Langmuir, Freundlich, and Temkin isothermal adsorption models were selected to analyze the adsorption data [37][38][39]. The fitting results and calculated adsorption isotherm constants are shown in Figure 6b-d and Table 2. The correlation coefficient (R 2 ) of the Langmuir model was 0.9999, higher than that of the other two models. Therefore, the adsorption process agreed well with the Langmuir isothermal adsorption model. After further calculation, the maximum adsorption capacity Q m of PANI/PSMA for MO was found to be 147.93 mg/g, higher than most of the reported PANI-containing adsorption materials (Table S1, Supplementary Materials), showing that the PANI/PSMA adsorbent has great potential in the adsorption of MO from aqueous solutions.

Adsorption Thermodynamics
Temperature has a significant influence on the adsorption process, and the influence can be reflected by thermodynamic parameters, such as Gibbs free energy change (∆G 0 ), enthalpy change (∆H 0 ), and entropy change (∆S 0 ). After the adsorption process for MO, ∆G 0 (kJ·mol −1 ), ∆S 0 (J·mol −1 ·K −1 ), and ∆H 0 (J·mol −1 ) were calculated according to the van't Hoff equation [40,41] as follows: where Qe (mg/g) denotes the amount of dye adsorbed per gram of PANI/PSMA, Ce (mg/L) represents the equilibrium concentration of dyes, R stands for the ideal gas constant (8.314 J.mol −1 ·K −1 ), and T (K) denotes the reaction temperature. As shown in Figure 7a, the adsorption capacity of PANI/PSMA for MO increased with the increase of reaction temperature. Figure 7b shows that the correlation coefficient (R 2 ) between ln(Qe/Ce) and 1/T was 0.9998, which indicates that the adsorption data were in good agreement with the van′t Hoff equation. ∆G 0 (kJ.mol −1 ) can be calculated according to the following formula:

Adsorption Thermodynamics
Temperature has a significant influence on the adsorption process, and the influence can be reflected by thermodynamic parameters, such as Gibbs free energy change (∆G 0 ), enthalpy change (∆H 0 ), and entropy change (∆S 0 ). After the adsorption process for MO, ∆G 0 (kJ·mol −1 ), ∆S 0 (J·mol −1 ·K −1 ), and ∆H 0 (J·mol −1 ) were calculated according to the van't Hoff equation [40,41] as follows: where Q e (mg/g) denotes the amount of dye adsorbed per gram of PANI/PSMA, C e (mg/L) represents the equilibrium concentration of dyes, R stands for the ideal gas constant (8.314 J.mol −1 ·K −1 ), and T (K) denotes the reaction temperature. As shown in Figure 7a, the adsorption capacity of PANI/PSMA for MO increased with the increase of reaction temperature. Figure 7b shows that the correlation coefficient (R 2 ) between ln(Q e /C e ) and 1/T was 0.9998, which indicates that the adsorption data were in good agreement with the van t Hoff equation. ∆G 0 (kJ.mol −1 ) can be calculated according to the following formula: The calculated thermodynamic parameters are shown in Table 3. Both ∆H and ∆S were positive, indicating that the adsorption on PANI/PSMA was endothermic, and that a higher temperature should be more favorable to the adsorption, which is consistent with the experimental results shown in Figure 7a. In addition, the negative ∆G value indicates that the adsorption process was spontaneous.
The calculated thermodynamic parameters are shown in Table 3. Both ΔH and ΔS were positive, indicating that the adsorption on PANI/PSMA was endothermic, and that a higher temperature should be more favorable to the adsorption, which is consistent with the experimental results shown in Figure 7a. In addition, the negative ΔG value indicates that the adsorption process was spontaneous.

Selective Adsorption Experiments
In wastewater treatment, adsorption selectivity is a key index of the performance of adsorbents [42,43]. Although many materials can adsorb MO, few materials show adsorption selectivity for MO. Thanks to the numerous positively charged amine groups on the surface of PANI/PSMA, anionic dyes can be exclusively adsorbed on PANI/PSMA from dye mixtures. The selectivity in the adsorption of a mixture of MO/MB on PANI/PSMA was tested, and the results are shown in Figure  8. Figure 8b shows that MB and MO dyes exhibited two intense absorption bands independently before the adsorption. After the addition of PSMA/PANI, the intensity of the absorption band corresponding to anionic dye MO decreased significantly, while that of cationic dye MB did not change significantly, indicating that PANI/PSMA can selectively adsorb anionic dyes from dye mixtures. As shown in Figure 8a, after the adsorption, the color of anionic dye MO faded, and the color of the mixture was close to that of cationic dye MB, which indicated that PANI/PSMA has high selectivity for anionic dyes.

Selective Adsorption Experiments
In wastewater treatment, adsorption selectivity is a key index of the performance of adsorbents [42,43]. Although many materials can adsorb MO, few materials show adsorption selectivity for MO. Thanks to the numerous positively charged amine groups on the surface of PANI/PSMA, anionic dyes can be exclusively adsorbed on PANI/PSMA from dye mixtures. The selectivity in the adsorption of a mixture of MO/MB on PANI/PSMA was tested, and the results are shown in Figure 8. Figure 8b shows that MB and MO dyes exhibited two intense absorption bands independently before the adsorption. After the addition of PSMA/PANI, the intensity of the absorption band corresponding to anionic dye MO decreased significantly, while that of cationic dye MB did not change significantly, indicating that PANI/PSMA can selectively adsorb anionic dyes from dye mixtures. As shown in Figure 8a, after the adsorption, the color of anionic dye MO faded, and the color of the mixture was close to that of cationic dye MB, which indicated that PANI/PSMA has high selectivity for anionic dyes.

Adsorption-Desorption Recycling Experiments
In practice, the adsorption-desorption recyclability is also a key index of the performance of adsorbents. In the adsorption process, electrostatic attraction was the main reason for the adsorption of MO onto PANI/PSMA. Under alkaline conditions, the zeta potential of PANI/PSMA was negative

Adsorption-Desorption Recycling Experiments
In practice, the adsorption-desorption recyclability is also a key index of the performance of adsorbents. In the adsorption process, electrostatic attraction was the main reason for the adsorption of MO onto PANI/PSMA. Under alkaline conditions, the zeta potential of PANI/PSMA was negative ( Figure S3, Supplementary Materials), indicating weak electrostatic attraction between MO and PANI/PSMA. Therefore, the desorption of MO from the surface of PANI/PSMA can be achieved by treating it with an alkaline solution [25]. As shown in Figure 9, the removal rate of MO remained unchanged in the first four runs. In the fifth run, the removal rate slightly decreased to 91.13%, possibly due to the cleavage of polymer chains caused by the repeated treatment with acid/alkali during the cycles. Thus, PANI/PSMA showed good reusability for the removal of MO.

Adsorption-Desorption Recycling Experiments
In practice, the adsorption-desorption recyclability is also a key index of the performance of adsorbents. In the adsorption process, electrostatic attraction was the main reason for the adsorption of MO onto PANI/PSMA. Under alkaline conditions, the zeta potential of PANI/PSMA was negative ( Figure S3, Supplementary Materials), indicating weak electrostatic attraction between MO and PANI/PSMA. Therefore, the desorption of MO from the surface of PANI/PSMA can be achieved by treating it with an alkaline solution [25]. As shown in Figure 9, the removal rate of MO remained unchanged in the first four runs. In the fifth run, the removal rate slightly decreased to 91.13%, possibly due to the cleavage of polymer chains caused by the repeated treatment with acid/alkali during the cycles. Thus, PANI/PSMA showed good reusability for the removal of MO.

EDS Analysis before and after Adsorption
The adsorption of MO onto PANI/PSMA was confirmed by energy-dispersive X-ray spectroscopy ( Figure 10). Before the adsorption, C and N elements were observed at 0.27 keV in the PANI/PSMA sample, and O was observed at 0.54 keV, indicating that PANI and PSMA were successfully combined. After the adsorption, S element was observed at 2.31 keV, confirming that MO was successfully adsorbed onto the surface of PANI/PSMA.

EDS Analysis before and after Adsorption
The adsorption of MO onto PANI/PSMA was confirmed by energy-dispersive X-ray spectroscopy ( Figure 10). Before the adsorption, C and N elements were observed at 0.27 keV in the PANI/PSMA sample, and O was observed at 0.54 keV, indicating that PANI and PSMA were successfully combined. After the adsorption, S element was observed at 2.31 keV, confirming that MO was successfully adsorbed onto the surface of PANI/PSMA. Polymers 2020, 12, x FOR PEER REVIEW 12 of 15 Figure 10. Energy-dispersive X-ray spectroscopy (EDS) spectra of the PANI/PSMA before and after adsorption (the inset in EDS spectra shows the structure of methyl orange dye).

The Adsorption of SIW
In order to study the adsorption effect of PANI/PSMA adsorbent in industrial wastewater, we studied the adsorption effect in SIW. As can be seen from Figure 11b, the adsorption peak of MO almost completely disappeared in SIW after adsorption by PANI/PSMA. The photographs before and after adsorption also exhibited the same phenomenon (Figure 11a). The adsorption capacity in SIW (140 mg/g) slightly changed compared to that in aqueous solution (150 mg/g). This confirmed that Figure 10. Energy-dispersive X-ray spectroscopy (EDS) spectra of the PANI/PSMA before and after adsorption (the inset in EDS spectra shows the structure of methyl orange dye).

The Adsorption of SIW
In order to study the adsorption effect of PANI/PSMA adsorbent in industrial wastewater, we studied the adsorption effect in SIW. As can be seen from Figure 11b, the adsorption peak of MO almost completely disappeared in SIW after adsorption by PANI/PSMA. The photographs before and after adsorption also exhibited the same phenomenon (Figure 11a). The adsorption capacity in SIW (140 mg/g) slightly changed compared to that in aqueous solution (150 mg/g). This confirmed that PANI/PSMA was effective for the adsorption of MO in SIW. Figure 11c,d show the photographs and absorption spectra of a mixture dye (MO/MB) in SIW before and after adsorption by PANI/PSMA. After adsorption, the peak of MO decreased significantly, while the peak of MB remained almost unchanged, suggesting that PANI/PSMA had good adsorption selectivity toward MO in SIW. Thus, the PANI/PSMA adsorbent has potential application value in actual industrial wastewater. Figure 10. Energy-dispersive X-ray spectroscopy (EDS) spectra of the PANI/PSMA before and after adsorption (the inset in EDS spectra shows the structure of methyl orange dye).

The Adsorption of SIW
In order to study the adsorption effect of PANI/PSMA adsorbent in industrial wastewater, we studied the adsorption effect in SIW. As can be seen from Figure 11b, the adsorption peak of MO almost completely disappeared in SIW after adsorption by PANI/PSMA. The photographs before and after adsorption also exhibited the same phenomenon (Figure 11a). The adsorption capacity in SIW (140 mg/g) slightly changed compared to that in aqueous solution (150 mg/g). This confirmed that PANI/PSMA was effective for the adsorption of MO in SIW. Figure 11c,d show the photographs and absorption spectra of a mixture dye (MO/MB) in SIW before and after adsorption by PANI/PSMA. After adsorption, the peak of MO decreased significantly, while the peak of MB remained almost unchanged, suggesting that PANI/PSMA had good adsorption selectivity toward MO in SIW. Thus, the PANI/PSMA adsorbent has potential application value in actual industrial wastewater.

Conclusions
A PANI/PSMA composite was prepared through emulsion polymerization and in situ polymerization. FT-IR, TGA, and SEM results showed that PANI was uniformly coated on the surface of PSMA, and a three-dimensional reticulate structure was successfully fabricated. The as-prepared composite exhibited a fast adsorption rate, high adsorption capacity, excellent adsorption selectivity, and exceptional reusability, and the adsorption process was endothermic and spontaneous, in good agreement with the pseudo-second-order kinetic model and Langmuir isothermal adsorption model. PSMA improved both the dispersion of PANI and the adsorption performance toward MO. Therefore, the PANI/PSMA composite proved to be an excellent adsorbent for dye-contaminated wastewater treatment.
Author Contributions: X.X. directed the research, C.Y. designed the experiments, L.S., L.D., and Y.L. performed the experiments, P.G. and N.L. analyzed the data, Y.L. and S.W. prepared the manuscript, and T.M. and L.Z. edited the manuscript. All authors have read and agreed to the published version of the manuscript.