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. 2020 Mar 20;5(12):6651–6660. doi: 10.1021/acsomega.9b04438

Highly Efficient Removal of Cr(VI) from Aqueous Solutions by Polypyrrole/Monodisperse Latex Spheres

Linlin Du 1, Peng Gao 1, Yide Meng 1, Yuanli Liu 1, Shangwang Le 1,*, Chuanbai Yu 1,*
PMCID: PMC7114732  PMID: 32258900

Abstract

graphic file with name ao9b04438_0012.jpg

Pyrrole (Py) is easily agglomerated during the polymerization process, affecting its performance. In this paper, polypyrrole/monodispersed latex sphere (PPy/MLS) composites were prepared using in-situ polymerization for the adsorption of hexavalent chromium (Cr(VI)). The specific surface area of PPy/MLS (39.30 m2/g) was increased relative to that of PPy (24.82 m2/g), thus providing more effective adsorption sites. In addition, the adsorption properties of Cr(VI) under different conditions, including Py content, pH of the aqueous solution, and PPy/MLS dosage, were investigated to reveal the adsorption mechanism. The results showed that PPy/MLS possessed high Cr(VI) adsorption capacities when the Py content was 50 wt %. The maximum adsorption capacity was 343.64 mg/g at pH 2.0 and 25 °C. Remarkably, the adsorbents exhibited an excellent removal rate of Cr(VI) after three cycles of adsorption–desorption (over 99%), suggesting that the adsorbents had exceptional recyclability. Furthermore, the adsorption process followed quasi-second-order kinetics and Langmuir isothermal adsorption model. The high adsorption performance, sustainability, and cost-efficiency make this adsorbent a promising candidate for large-scale Cr(VI) contaminant removal.

1. Introduction

Recently, the removal of pollutants from industrial wastewater has attracted more and more attention with increasing awareness of environmental protection.1−4 The methods of removing pollutants from wastewater include ion exchange, electrochemical reduction, solvent extraction, chemical precipitation, photodegradation adsorption, or membrane separation.5−8 In contrast, the adsorption method is favored by researchers and has become one of the most promising technologies owing to its advantages of being eco-friendly, recyclable, inexpensive, and operationally simple as well as not generating waste water or secondary pollutants.9−12 Adsorbents play an important role in the process of adsorption.13−15 Among them, polypyrrole (PPy) is one of the most extensively studied conducting polymers because of its nontoxicity, biocompatibility, good electrical conductivity, moderate band gap, and reversible electrochemical properties. Moreover, it has attracted a lot of interest because of its potential applications in electrocatalysis, sensors, supercapacitors, electronic devices, and adsorption fields.16,17 Recently, PPy adsorbents have attracted great attention in wastewater treatment because of their easy synthesis, cost-efficiency, environmental stability, excellent ion exchange, and redox properties.18,19 However, PPy is easy to agglomerate in water during synthesis because of strong π–π interaction among the chains. In addition, the doped anions (such as Cl– and NO3–) are difficult to diffuse out of the polymer. Therefore, it is not easy to improve its adsorption efficiency.20,21 To overcome this shortcoming, preventing agglomeration and increasing surface area of PPy are very important during synthesis and adsorption. The PPy-based composites are considered to be one of the most effective methods, which mainly include organic-PPy composites and inorganic-PPy composites.22−27 In 2011, Chandra and co-workers28 reported a PPy-reduced graphene oxide composite for the highly selective adsorption of Hg2+. Moreover, Bhaumik et al.29 prepared the PPy/polyaniline nanofibers for the removal of Congo red from aqueous solutions. Further, Aigbe et al.30 synthesized the PPy/Fe3O4 magnetic nanocomposites for the removal of Cr(VI) in 2018. In these composites, nanocomposites have larger surface area and more adsorption sites; so they are more suitable for solving PPy aggregation.31−35 In addition, chromium (Cr) is widely used in metal smelting, leather tanning, metal anticorrosion, textile dyestuffs, and other industrial production processes.36 However, Cr pollution has gained increasing attention due to air, water, soil pollution, and concomitant negative health effects.37−40 Moreover, Cr(VI) is a highly dangerous substance with strong mobility, high toxicity, and carcinogenic and mutagenic properties.41−43 Therefore, the efficient removal of Cr(VI) from water has become a global challenge and concern of modern society.

This study aimed to prepare PPy-coated monodisperse latex spheres (MLS) with a large specific surface area, good dispersibility, and controlled surface functional groups, using a simple preparation method to provide effective adsorption sites for the adsorption of Cr(VI). In addition, part of toxic Cr(VI) could be spontaneously reduced to less toxic Cr(III) by PPy/MLS composites. The effects of the Py content on the structure and morphology of the PPy/MLS composites were studied. The removal rate and maximum adsorption capacity of Cr(VI) by PPy/MLS were determined using adsorption kinetics, isotherms, thermodynamics, and cyclic adsorption properties of Cr(VI). Furthermore, the preparation process and adsorption mechanism of PPy/MLS for Cr(VI) were investigated.

2. Results and Discussion

2.1. Characterization of PPy/MLS

The Fourier transform infrared (FT-IR) spectra of MLS, PPy, and PPy/MLS are presented in Figure 1a. The main characteristic peaks of MLS were at 2921 and 3025 cm–1 (C–H stretching vibrations of benzene), 2848 cm–1 (methylene adsorption bands), 1452 and 1493 cm–1 (benzene skeleton vibrations), and 1731 cm–1 (carbonyl stretching). For PPy, the stretching peaks at 1544, 1296, and 1201 cm–1 were related to C=C, C–C, and C–N, respectively. The peaks at 1460, 1400, 916, and 780 cm–1 were attributed to Py ring stretching.19,44,45 PPy/MLS had not only the main characteristic peaks of MLS at 2921, 1452, and 2848 cm–1 but also the characteristic peaks of Py at 1296 and 780 cm–1.

Figure 1.

Figure 1

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FT-IR (a), TGA (b), DTG (c), and N2 adsorption–desorption (d) spectra of MLS, PPy, and PPy/MLS.

Figure 1b,c shows the thermogravimetric analysis (TGA) and DTG curves of MLS, PPy, and PPy/MLS. The weight losses of MLS and PPy were 99.5 and 32 wt % at 800 °C, respectively. However, PPy/MLS had two stages of weight loss. The mass loss of 66 wt % between 200 and 455 °C was caused by the decomposition of MLS. At a temperature more than 455 °C, the weight loss was mainly due to the decomposition of PPy. In addition, the maximum decomposition temperature of PPy/MLS was 12.39 °C higher than that of MLS because the molecular chains of PPy were entangled on the surface of MLS, which improved the thermal stability of PPy/MLS.44,46 This result further confirmed that PPy and MLS were successfully combined.

The specific surface area is a primary characteristic of adsorbents. Figure 1d shows the N2 adsorption–desorption curves of PPy, MLS, and PPy/MLS. The specific surface area of PPy/MLS (39.30 m2/g) was increased by 58.34% relative to that of PPy (24.82 m2/g). The large specific surface area increased the number of adsorption sites for Cr(VI) ions, improving the adsorption capacity of the composites.

In addition, the model diagram and scanning electron microscopy (SEM) and transmission electron microscopy (TEM) micrographs of MLS and PPy/MLS composites are shown in Figure 2. The MLS possessed uniform particle size (226 ± 9 nm), excellent monodispersity and an obvious core–shell structure with a smooth surface, excellent homogeneity, and complete sphericity. The PPy/MLS also possessed outstanding sphericity with an average particle size of 255 ± 6 nm and poor monodispersity.47 TEM micrographs showed that the outer surface of PPy/MLS became rough due to bonding and agglomeration.48 Elemental mapping graphs (Figure 2e–i) illustrated the presence of carbon, oxygen, and nitrogen in PPy/MLS, indicating that PPy was wrapped on the surface of the MLS, which was consistent with the results of FT-IR, TGA, and N2 adsorption–desorption analyses.

Figure 2.

Figure 2

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Model diagrams of MLS (I) and PPy/MLS (II); SEM and TEM micrographs of MLS (a,b) and PPy/MLS (c,d); and the elemental mapping graphs of the overall content (e,f), carbon (g), oxygen (h), and nitrogen (i).

2.2. Adsorption Behaviors

2.2.1. Effect of the Py Concentration

The content of the Py monomer had a substantial influence on the adsorption process (see the details in Text S1). Figure 3 shows the SEM images and adsorption properties of PPy/MLS for Cr(VI) with different concentrations in the Py monomer. The adsorption capacity of Cr(VI) increased with the increase in Py concentration and reached the maximum (364.19 mg/g) at 50 wt %. However, the adsorption capacity decreased gradually when the Py concentration further increased. These results indicated that the amount of PPy and active adsorption sites on the surface of MLS increased with the increase in the Py monomer content, thus enhancing the adsorption capacity of Cr(VI). In fact, MLS were introduced to increase the dispersion of PPy. In Figure 3a, PPy began to agglomerate when Py concentration was more than 50 wt %. Therefore, PPy/MLS prepared with 50 wt % Py was used in the adsorption experiments.

Figure 3.

Figure 3

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SEM micrographs (a) and adsorption properties (b) of PPy/MLS with different Py concentrations for Cr(VI).

2.2.2. Effect of pH

As shown in Figure 4a, the zero-charge point (pHzcp) of PPy/MLS was at pH 4.6. The nitrogen atoms of PPy were protonated in the presence of H+ at lower pH (lower than pHzcp), which made the surface of PPy/MLS positively charged. This allowed the surface to interact with HCrO4– by electrostatic attraction, thereby improving the ion exchange performance of the composites. The zeta potentials of MLS were all less than −30 mV at the tested pH, proving that MLS was highly stable in solution.49

Figure 4.

Figure 4

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(a) Zeta potentials of PPy/MLS at different pH; (b) adsorption performance of PPy, MLS, and PPy/MLS for Cr(VI) at different pH; (c) adsorption properties for Cr(VI) with different Py concentrations; and (d) effect of coexisting ions on Cr(VI) adsorption by PPy/MLS.

The ionic forms of Cr(VI) were different at different pH. The predominant species were H2CrO4 (pH was less than 1), HCrO4–, Cr2O72– (at pH 2–6), Cr2O72–, CrO42– (at pH 6–8), and CrO42– (more than pH 8).49,50Figure 4b shows the effect of pH on the removal efficiency for Cr(VI) by PPy, MLS, and PPy/MLS (details in Text S2). The removal rate of three adsorbents decreased gradually with increasing pH. The removal efficiency reached 98.13% with PPy/MLS at pH 2.0, whereas those of PPy and MLS were only 24.85 and 17.30%, respectively. These results were due to the larger specific surface area and greater number of adsorption active sites on the composites after the polymerization of Py on the surface of MLS. The decrease in the removal efficiency was attributed to the competition between hydroxyl (OH–) and CrO42– with increasing pH (higher than pHzcp), which competed for the same adsorption sites on the adsorbent surface.51 Therefore, pH 2.0 was ideal for the removal of Cr(VI) by PPy/MLS.

2.2.3. Effect of Adsorbent Dosage

The effects of the adsorbent dosage on the removal rate (%) and adsorption capacity of Cr(VI) from aqueous solutions are shown in Figure 4c (details in Text S3). The removal rate improved from 38.8 to 100% with increasing adsorbent dosage. This could be attributed to the greater number of adsorption active sites.52 When the dosage was greater than 80 mg, the Cr(VI) was completely removed. These results further confirmed that PPy/MLS exhibited excellent adsorption efficiency for Cr(VI).

2.2.4. Effect Coexisting Ions

Industrial wastewater contains not only Cr(VI) but also other ions. Therefore, studying the adsorption properties of PPy/MLS for Cr(VI) in the presence of other ions is important. Figure 4d shows the effects of the binary adsorption systems on the adsorption of Cr(VI). The initial concentration of all ions and the dose of adsorbent were 100 mg/L and 20.0 mg, respectively. The removal rate of Cr(VI) by PPy/MLS did not change significantly in the presence of Cl–, NO3–, SO42–, Zn2+, Fe3+, Sn4+, and Cu2+, indicating no competitive relationship between the coexisting ions and HCrO4– on the surface of the adsorbent.50,53,54 The reason was that the PPy/MLS was positively charged at pH 2.0. In the binary system of cations (Zn2+, Fe3+, Sn4+, and Cu2+), the adsorption centers with positive charge were repulsive to cations, and the main existence form of Cr(VI) was HCrO4– at pH 2.0; so the cations were not competitive for the adsorption of Cr(VI). For the binary system of anions (Cl–, NO3–, SO42–), it is well known that the interaction of Cl– and NO3– with adsorption sites of the adsorbent was weak than HCrO4–. The hydration energy of SO42– was higher than that of HCrO4–; so SO42– was not easy to bind to the adsorption sites. On the other hand, PPy/MLS can reduce Cr(VI) to Cr(III), so that Cr(VI) was adsorbed until equilibrium.55,56

2.3. Adsorption Kinetics, Isotherms, Thermodynamic Investigations, and Regeneration Analyses

2.3.1. Adsorption Kinetics

The adsorption kinetics are mainly concerned with the time at which the adsorbents reach adsorption equilibrium in the adsorption process. The effect of the adsorption time on the adsorption of Cr(VI) at three different initial concentrations of Cr(VI) is shown in Figure 5a. The adsorption capacity of Cr(VI) increased with increasing adsorption time before reaching adsorption equilibrium. The adsorption rate was fast at the beginning of the adsorption process. However, it gradually slowed due to the decreased number of active sites available on the adsorbent as the reaction approached equilibrium. When the initial concentrations of Cr(VI) were 25, 50, and 75 mg/L, the adsorption capacities of PPy/MLS for Cr(VI) were 172.41, 323.62, and 371.75 mg/g, respectively, indicating that the concentration gradient was a driving factor in the adsorption process.56,57

Figure 5.

Figure 5

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(a) Effects of the adsorption time on the adsorption capacity for Cr(VI) with different initial concentrations of solution; (b) quasi-first order kinetic model; (c) quasi-second-order kinetic model, and (d) intraparticle diffusion model.

The pseudo-first-order, pseudo-second-order, and intraparticle diffusion models were applied to better understand the adsorption process (details in Text S4). The fitting results are shown in Figure 5 and Table 1; the correlation coefficient (R2) of the pseudo-second-order kinetic model was larger than that of the pseudo-first-order kinetic model. The experimental capacity was in agreement with the calculated values of pseudo-second-order kinetic model, indicating that the adsorption process of PPy/MLS for Cr(VI) followed pseudo-second-order kinetics. Furthermore, in the intraparticle diffusion model, the adsorption process of Cr(VI) by PPy/MLS was divided into two parts. The initial rapid removal rate might be achieved by the influence of boundary layer diffusion, and the slow removal rate was due to intraparticle pore diffusion.21,29

Table 1. Kinetics Parameters of Cr(VI) Adsorption by PPy/MLS.
  pseudo-first order model
 pseudo-second order model
 intraparticle diffusion model
C0/(mg/L) K1 Qe R2 K2 Qe R2 Kip C R2
25 16.1 × 10–3 152.48 0.625 3.12 × 10–4 172.41 0.997 10.185 160.84 0.774
50 7.21 × 10–3 182.20 0.964 1.70 × 10–4 323.62 0.997 9.617 119.35 0.833
75 9.74 × 10–3 200.28 0.925 2.10 × 10–4 371.75 0.998 6.146 44.60 0.893
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2.3.2. Adsorption Isotherms

The adsorption isotherms at different temperatures are shown in Figure 6. The adsorption capacity increased rapidly and then attained equilibrium with increasing concentration. The adsorption capacities at 25, 35, and 45 °C were 348.67, 387.99, and 428.56 mg/g, respectively. The adsorption data were fitted using the Freundlich and Langmuir models (see the details in Text S5). The fitting results are shown in Figure 6b,c and Table 2. The results showed that the R2 of the Langmuir isotherm model was greater than 0.99 at different temperatures, indicating that the Cr(VI) adsorption process was better fitted by the Langmuir adsorption model. In addition, the maximum adsorption capacities (Qmax) were 343.64, 389.11, and 429.18 mg/g at 25, 35, and 45 °C, which were higher than those of most other reported adsorbents at 25 °C (Table 3). The value of RL was between 0 and 1, confirming that the adsorption process using PPy/MLS was advantageous. In addition, the 1/n parameter of the Freundlich model was also between 0.1 and 0.5, suggesting that the adsorption was easy.50,56

Figure 6.

Figure 6

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Adsorption isotherms (a), Freundlich model (b), Langmuir model (c), and plot to determine thermodynamic parameters (d) of Cr(VI) adsorption onto PPy/MLS.

Table 2. Isotherm Parameters for Cr(VI) Adsorption onto PPy/MLS at 25, 35, and 45 °C.
  Langmuir model
 Freundlich model
T (K) b Qm R2 RL KF 1/n R2
298 0.590 343.64 0.999 0.0635 176.23 0.141 0.825
308 1.125 389.11 0.999 0.0175 263.94 0.093 0.837
318 2.178 429.18 0.999 0.00608 295.62 0.092 0.789
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Table 3. Comparison of the Performances of PPy/MLS and other Adsorbents for Removing Cr(VI) at 25 °C.
adsorbents Qm (mg/g) equilibrium time (min) optimum pH references
PPy-polyaniline nanofibers 227–294 30–180 2.0 Bhaumik50
PPy/cellulose composite 130 60 4.8 Lei58
PPy/wood sawdust 3.4 15 5.0 Ansari59
PPy/graphene oxide composite 497.1 1440 3.0 Li57
PAN/PPy core/shell mats 61.8 60 2.0 Wang52
PPy-organically modified montmorillonite clay nanocomposite 119.3 180 2.0 Setshedi56
surfactant-modified zeolite 5.07 180 6.0 Ramos60
PPy/MLS 343.64 160 2.0 [Present study]
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2.3.3. Thermodynamic Investigations

The thermodynamic parameters, such as the Gibbs free energy change (ΔG0), enthalpy change (ΔH0), and entropy change (ΔS0), are shown in Figure 6d and Table 4 (see the details in Text S6). The values of ΔH0 and ΔS0 were 17.50 kJ·mol–1 and 0.062 kJ·mol–1 K–1, respectively. The positive value of ΔH0 indicated that the adsorption process was endothermic, and the positive value of ΔS0 suggested the randomness in the solid/liquid interface system increased during the adsorption process. However, the negative values of ΔG0 at all temperatures illustrated the feasibility and spontaneity of the adsorption process.21,56

Table 4. Adsorption Thermodynamic Parameters of PPy/MLS.
temperature (°C) ΔG0 (kJ·mol–1) ΔH0 (kJ·mol–1) ΔS0 (kJ·mol–1 K–1)
25 –0.98 17.50 0.062
35 –1.63    
45 –2.22    
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2.3.4. Regeneration Analyses

The cyclic adsorption performance of adsorbents is key to their potential applicability in wastewater treatment. Figure 7 shows the removal efficiency of Cr(VI) by PPy/MLS over five adsorption–desorption cycles (details in Text S7). The removal rate remained the same (100%) in the first three cycles and gradually decreased in the fourth (86.47%) and fifth cycles (76.37%). The decrease in adsorption capacity might be because of the breakage of the polymer chains due to the repeated oxidation reactions with highly oxidized Cr(VI), which would reduce the effective adsorption mass and number of active sites on the adsorbents.20,29,53

Figure 7.

Figure 7

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Removal efficiency of PPy/MLS nanohybrids over five cycles of adsorption.

2.4. Adsorption Mechanism

To verify the ion exchange mechanism of PPy/MLS on Cr(VI) and PPy/MLS before and after adsorption were characterized by energy dispersive spectrometry (EDS). In Figure 8, two new characteristic peaks appeared at 5.4 and 5.9 keV after adsorption. In addition, the adsorption intensity of Cl decreased and that of O increased. According to elemental mapping, the Cr was dispersed on the surface of the composite. These results showed that Cr was successfully adsorbed on the surface of PPy/MLS based on to the exchange of the chloride ions (Cl–) of PPy/MLS for Cr.50,53,54

Figure 8.

Figure 8

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EDS spectra (A) of PPy/MLS before and after adsorption and the elemental mapping graphs (B) of the overall content (a,b), carbon (c), oxygen (d), nitrogen (e), and chromium (f).

The adsorption mechanism of Cr(VI) on PPy/MLS was further verified, using X-ray photoelectron spectroscopy (XPS) to investigate the elemental composition and content before and after adsorption. Figure 9a shows that the characteristic peak of Cr appeared after adsorption, and the peak intensity of Cl in PPy/MLS decreased significantly, while that of O increased. This phenomenon was due to the ion exchange reaction between Cl– and HCrO4– in the adsorption process of Cr(VI) by PPy/MLS, resulting in the obvious enhancement of the characteristic peaks of Cr and O in PPy/MLS after adsorption. In the Cr 2p spectra (Figure 9b), the adsorption peaks of Cr appeared with binding energies of 576.7, 578.1, 586.3, and 588.1 eV, which corresponded to the characteristic peaks of Cr(III) and Cr(VI). The appearance of Cr(III) indicated that PPy/MLS possessed strong reducing ability and converted Cr(VI) into Cr(III).49,52,57

Figure 9.

Figure 9

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Survey XPS spectra (a) before and after adsorption of Cr(VI) and binding energies of Cr 2p (b) after the adsorption of Cr(VI) by PPy/MLS.

Based on the aforementioned analysis, the preparation of PPy/MLS and adsorption mechanism for Cr(VI) are shown in Figure 10. First, the surface of MLS contained a large number of carboxyl groups with negative charges under acidic conditions, and the N+ sites were positively charged in the Py monomers. Py monomers were enriched on the surface of MLS via electrostatic interactions. After that, the Py monomers were oxidized and polymerized on the surface of MLS through intermolecular hydrogen bonding or electrostatic interactions to afford the PPy/MLS composites after adding FeCl3, as the N+ and Cl– ions were attracted by electrostatic interactions. When K2Cr2O7 was added to the system at low pH, the HCrO4– ions exchanged with Cl– and associated with N+ through electrostatic interactions, thereby achieving the adsorption of Cr(VI). In the desorption process, OH– ions exchanged with HCrO4– after adding NaOH solution and were associated with N+ by electrostatic interactions, achieving the desorption of Cr(VI).41,50,55

Figure 10.

Figure 10

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Schematic of the preparation process of PPy/MLS and the adsorption–desorption mechanism for Cr(VI).

3. Conclusions

The MLS and PPy/MLS were prepared by emulsion polymerization and in-situ polymerization, respectively. The FT-IR, TGA, SEM, and TEM analyses confirmed that PPy was uniformly coated on the surface of MLS, which solved the agglomeration of PPy and increased the adsorption sites. The adsorption properties of PPy/MLS for Cr(VI) were studied under different conditions. The adsorption capacity was best at pH 2.0 with a Py content of 50 wt %. The adsorption process followed quasi-second-order kinetic and Langmuir isothermal adsorption models. The maximum adsorption capacity was 343.64 mg/g at 298 K. The removal rate of Cr(VI) remained constant over three adsorption–desorption cycles. PPy/MLS showed selective adsorption. In addition, the removal mechanism of Cr(VI) was confirmed by EDS and XPS. The dispersion of PPy was successfully improved by MLS, and the adsorption performance for Cr(VI) was increased. Therefore, the PPy/MLS composite is a promising adsorbent for Cr(VI).

4. Experimental Procedure

4.1. Materials

Styrene (St), methyl methacrylate (MMA), acrylic acid (AA), sodium dodecylbenzene sulfonate (SDBS), ammonium persulfate (APS), ammonium bicarbonate (NH4HCO3), pyrrole (Py), ferric chloride hexahydrate (FeCl3·6H2O), potassium dichromate (K2Cr2O7), alkaline aluminum oxide, sodium chloride (NaCl), sodium nitrate (NaNO3), sodium sulfate (Na2SO4), zinc chloride (ZnCl2), stannic chloride (SnCl4), copper nitrate trihydrate (Cu(NO3)2·3H2O), sulfuric acid (H2SO4), phosphoric acid (H3PO4), diphenyl carbamide (C13H14N4O), acetone (C3H6O), hydrochloric acid (HCl), and sodium hydroxide (NaOH) were purchased from Aladdin (China, Shanghai). All chemicals were of analytical grade. Styrene (St) was purified by passing through an alkaline aluminum oxide column, and all other chemicals were used without further purification.

4.2. Preparation of PPy/MLS Composites

PPy/MLS were prepared by an in situ oxidative polymerization (Figure 11). First, the MLS of poly(St-MMA–AA) were prepared by an emulsion polymerization using a traditional approach.47 Briefly, NH4HCO3 (0.498 g) and SDBS (0.003 g) in 100 mL of water were transferred to a 250 mL three-necked round-bottomed flask equipped with a water segregator, dropping funnel, and a magnetic stirrer. Then, St (19.018 g), AA (1.001 g), and MMA (1.001 g) were added. The reaction was carried out at 70 °C for 0.5 h with continuous agitation. Then, an aqueous solution of APS (0.484 g) was added dropwise, and the polymerization was carried out at 80 °C for 10 h with continuous stirring. The obtained latex spheres were used directly without purification. The sample was denoted as MLS.

Figure 11.

Figure 11

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Schematic of the preparation process and adsorption effect of PPy/MLS.

In a typical procedure, PPy/MLS were prepared by an in situ oxidative polymerization. In detail, the MLS were dispersed in an aqueous solution of Py with different concentrations (10 wt % ≈ 90 wt %) for 10 min by ultrasonic dispersion. The polymerization was carried out at 25 °C for 12 h by slowly adding a solution of FeCl3·6H2O in HCl (0.5 mol/L) as the oxidant. During the oxidative polymerization of Py, the MLS turned from white to gray and finally turned black within a few minutes, which confirmed that the MLS were filled with PPy. After polymerization, the PPy/MLS composite was washed with distilled water several times to remove the byproducts and residues of the reaction. Finally, the product was dried in a vacuum oven at 80 °C for 12 h. The Py/FeCl3·6H2O molar ratio used in the chemical synthesis was 1:2.3. The sample was denoted PPy/MLS.

The preparation method of pure PPy is as follows: pyrrole monomer (0.6 g) was dispersed in the aqueous solution for 10 min by ultrasonic dispersion. Then, the mixture was carried out at 25 °C for 12 h by slowly adding a solution of FeCl3·6H2O (5.556 g) in HCl (100 mL, 0.5 mol/L). The solid was washed with distilled water 3 times. Finally, the product was dried in a vacuum oven at 80 °C for 12 h to obtain the pure PPy powder.

4.3. Adsorption Capacity

Cr(VI) solutions were prepared by dissolving K2Cr2O7 in deionized water. A solution with a Cr(VI) concentration of 1000 mg/L was prepared by dissolving 2.8287 g of K2Cr2O7 in 1000 mL of water. Adsorption experiments were carried out by diluting the original solution to different concentrations. The pH of each solution (from 2 to 10) was adjusted using a SX-610 pH meter (Sanxin, China) by adding HCl or NaOH.

A series of adsorption experiments were conducted to investigate the effect of Py concentration, pH, and reaction time on the adsorption capacity. The PPy/MLS composites were added into the Cr(VI) solutions. Then, the above mixtures were placed in a thermostatic shaker (Tianjin Saidriss Experimental Analytical Instrument Factory SHA-B, China) at ambient temperature (25 °C) for 24 h at 200 rpm. Finally, the solution was filtered through 0.22 mm filter paper (micropore, 47 mm in diameter). The analyses of Cr(VI) in the aqueous solutions were performed using a UV–vis spectrophotometer at 540 nm after complexation with 1,5-diphenylcarbazide.

4.4. Characterization

FT-IR spectra were obtained using a Thermo Nexus 470 FT-IR instrument by grinding and tableting the sample with KBr. The microphotographs of the materials were observed through field emission SEM (S-4800) and TEM (JEM-2100F). TGA was performed using a thermogravimetric analyzer (TA Q500) under N2 with a temperature ramp rate of 10 °C/min from 20 to 800 °C. The zeta potential and particle size of the samples were measured by a Zetasizer instrument (3000HS, Malvern Ltd., UK). X-ray diffraction analysis was performed using small-angle X-ray scattering (X’Pert Pro) with a scanning range of 5–80°, a speed of 3°/min, and Kα(Cu) radiation. The specific surface area, pore volume, and pore size of the samples were determined using an automatic physical adsorption instrument (Tristar3000, Micromeritics Ltd., UK). EDS, and the sample concentrations were assessed by electron microscopy (SEM, S-4800) and dual-beam ultraviolet–visible spectrophotometry (TU-1901), respectively. The elements in the materials were analyzed by XPS (ESCALAB 250Xi).

Acknowledgments

The research is financially supported by the National Natural Science Foundation of China (grant nos: 51563004 and 51203029) and Collaborative Innovation Center for Exploration of Hidden Nonferrous Metal Deposits and Development of New Materials in Guangxi.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.9b04438.

  • Details of the adsorption experiment (effect of Py concentration, pH and adsorption dosage, adsorption kinetics, adsorption isotherms, thermodynamic investigations, and regeneration analyses) (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao9b04438_si_001.pdf (87.5KB, pdf)

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