Aniline-p-Phenylenediamine Copolymer for Removal of Hexavalent Chromium from Wastewater

Abstract

Hexavalent chromium, one of the heavy metal pollutants in water, harms the ecological environment and human health. In this work, an aniline-p-phenylenediamine copolymer has been prepared by chemical oxidative polymerization to remove the hexavalent chromium (Cr(VI)) from wastewater. The results show that when the initial Cr(VI) concentration is 1.5 mg·L⁻¹, the removal percentage (RP%) of Cr(VI) could reach 94.84% after 180 s of treatment. The RP% of Cr(VI) increases with the dosage of copolymers and decreases with an increase in the initial Cr(VI) concentration. Additionally, the RP% of Cr(VI) removal reaches a maximum of 97.70% with a pH value of 1.0. The Cr(VI) removal kinetics of the copolymers follows a pseudo-first-order chemical reaction model. The X-ray photoelectron spectroscopy (XPS) results demonstrate that the Cr(VI) removal mechanism by the aniline-p-phenylenediamine copolymer is a redox reaction. The positive value of ΔH° and negative value of ΔG° affirm that the Cr(VI) removal process by aniline-p-phenylenediamine copolymer is endothermic, thermodynamically achievable, and spontaneous.

1. Introduction

Water is a crucial resource in human life, and its quality directly impacts human health. The rapid industrial development has led to the production of wastewater entering water bodies, causing a growing issue of heavy metal pollution in water. Heavy metal pollution, particularly hexavalent chromium (Cr(VI)), is concerning due to its strong oxidizing properties, biotoxicity, and non-biodegradability [1,2]. Prolonged consumption of contaminated water can lead to serious health problems, affecting vital organs such as the kidneys, liver, nervous system, and digestive system and posing a threat to human life [3,4]. The World Health Organization (WHO) has established regulations to restrict the levels of Cr(VI) present in drinking water to safeguard the health of the general population [5]. Various treatment methods for heavy metal wastewater, including traditional methods [6,7], biological methods [8,9,10], membrane separation [11,12,13], etc., aim to alter the physical location or chemical form of pollutants. For instance, porous solid adsorbents have been exploited to capture Cr(VI) from wastewater [14,15].

Polyaniline (PANI), a non-toxic polymer material, is well-known for its excellent physicochemical properties, such as non-toxic, readily available raw materials, and excellent stability [16,17]. However, the rigid structure of molecular chains results in strong interactions between chains, posing challenges to dissolution and processing and limiting its applications [18,19]. It is reported that the introduction of other groups (e.g., methyl or amino groups) into the benzene ring could create PANI derivatives or copolymers [20]. This modification could reduce the molecular chain rigidity and minimize inter-chain interactions, enhancing its solubility, dispersibility, and processability. Aniline-p-phenylenediamine copolymer (P(ANI-PDA)), a derivative of PANI, has received limited attention in its potential application for removing heavy metals from wastewater.

In this study, P(ANI-PDA) has been synthesized via a chemical oxidative polymerization process and analyzed using scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and specific surface area testing (BET). Furthermore, P(ANI-PDA) has been used to evaluate the Cr(VI) removal performance. The various factors influencing the Cr(VI) removal process have been investigated, such as pH value, initial Cr(VI) concentration, treatment time, and P(ANI-PDA) dosages. Moreover, the Cr(VI) removal kinetics and removal mechanism have also been explored. This research proposes an innovative approach for developing new materials to treat heavy metal-contaminated wastewater.

2. Materials and Methods

2.1. Materials

The concentrated sulfuric acid (H₂SO₄, 98.0 wt%), hydrochloric acid (HCl, 36.0–38.0 wt%), phosphoric acid (H₃PO₄, 85 wt%), sodium hydroxide (NaOH), acetone (CH₃COCH₃), diphenylcarbamoyl dihydrazide (C₁₃H₁₄N₄O), aniline (C₆H₅NH₂), p-phenylenediamine (p-PDA, C₆H₄N₂H₄), and potassium dichromate (K₂Cr₂O₇) were supplied by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) Amine persulphate (APS, (NH₄)₂S₂O₈, ≥98%) was purchased from Shanghai Aladdin Biochemical Science and Technology Co., Ltd. (Shanghai, China), and p-toluenesulfonic acid monohydrate (PTSA, p-CH₃C₆H₄SO₃H, 99%) was obtained from Shanghai Macklin Biochemical Technology Co., Ltd. (Shanghai, China). All chemical reagents were utilized in their original form without additional processing.

2.2. Preparation of Aniline-p-Phenylenediamine Copolymer (P(ANI-PDA))

Firstly, 0.0972 g of p-PDA, 2.85 g of PTSA, and 1.64 mL aniline were put into a 300 mL beaker with 80 mL of deionized water. Then, 2.15 g of APS dissolved in 20 mL of deionized water was added into the above solution for polymerization for 2 h. Afterward, the products were filtered and freeze-dried for 12 h to obtain P(ANI-PDA) powder.

2.3. Characterizations

The chemical structure of samples was conducted on an FTIR instrument (NICOLET IS10, Thermo Fisher Scientific Inc., Waltham, MA, USA). The microstructure and elemental mapping analysis of materials were observed by an SEM (S-4800, Hitachi Ltd. (Tokyo, Japan)). TGA was performed in air conditions with a temperature range from 30 to 750 °C at a heating rate of 10 °C∙min⁻¹ using a TGA55 (TA Instruments, New Castle, DE, USA). The specific surface area was analyzed using a specific surface area porosity analyzer (TriStar II 3020, Micromeritics Instrument Corporation, Norcross, GA, USA). X-ray photoelectron spectroscopy (XPS) analysis was carried out on a Thermo Scientific K-Alpha instrument with Al Kα (hν = 1486.6 eV) radiation as the excitation source at an anode voltage of 12 kV and an emission current of 10 mA. Zeta potential was attained by the microelectrophoresis measurements (JS94, POWEREACH Co., Ltd., Shanghai, China).

2.4. Cr(VI) Removal Evaluation

The copolymer powder was added to the Cr(VI) solution and then ultrasonicated to treat the Cr(VI) solution. After that, the dispersion solution was centrifuged, and the supernatant was taken to determine the residual Cr(VI) concentration in the solution using an ultraviolet-visible spectroscopy (UV-Vis) in every trial. The reported value for each sample represented the mean of three measurements with a deviation of 5%. The removal percentage (RP%) of Cr(VI) was computed according to Equation (1).

$$RP\%={\displaystyle \frac{C_0-C_t}{C_0}}\times 100\%$$

(1)

where the Cr(VI) initial concentration was denoted as C₀ (mg∙L⁻¹), and the final concentration of Cr(VI) in the solution after removal was indexed as C_t (mg∙L⁻¹).

2.4.1. Effect of P(ANI-PDA) Dosage

The different P(ANI-PDA) dosages from 5 to 15 mg were chosen to study the effect of P(ANI-PDA) dosage on the Cr(VI) removal under the condition of 293 K, 20.0 mL of Cr(VI) solution with an initial Cr(VI) concentration of 1.5 mg∙L⁻¹ for 45 s. Meanwhile, the controlled trials were conducted using PANI powder under the same conditions.

2.4.2. Effect of Initial Cr(VI) Concentration

The effect of initial Cr(VI) concentration on Cr(VI) removal by 10 mg of P(ANI-PDA) was performed in the 20.0 mL of Cr(VI) solution with an initial Cr(VI) concentration from 0.4 to 2.0 mg L⁻¹ at 293 K for 45 s.

2.4.3. Effect of pH Value

The effect of solution pH value on Cr(VI) removal was taken in 20.0 mL of Cr(VI) solution (1.5 mg L⁻¹) with pH values of 1.0, 3.0, 5.0, 8.0, and 12.0 at 293 K for 45 s using a pH meter (PHS-3C-s, INESA Scientific Instrument Co., Ltd., Shanghai, China). The pH value of Cr(VI) solutions was manipulated by 1.0 mol∙L⁻¹ of NaOH solution and 1.0 mol∙L⁻¹ of HCl solution. The concentrated sulfuric acid (98.0 wt%) was used to alter the Cr(VI) solution to achieve a pH value of 1.0.

2.4.4. Kinetic and Thermodynamic Study

During the kinetic study, 10 mg of P(ANI-PDA) was dispersed in 20.0 mL of Cr(VI) solution containing an initial Cr(VI) concentration of 1.5 mg∙L⁻¹ at 293 K for varying treatment durations ranging from 30 to 300 s. In the thermodynamic study, 10 mg of P(ANI-PDA) was dispersed in 20 mL of Cr(VI) solution with initial Cr(VI) concentrations of 0.4, 0.8, 1.2, 1.6, and 2.0 mg L⁻¹ at 293, 303, 313, and 323 K, respectively.

2.4.5. Regeneration and Recyclability

After treating 20.0 mL of 1.5 mg L⁻¹ Cr(VI) solution with 10 mg of P(ANI-PDA) for 45 s at 293 K, a solid sample was attained by centrifugation and drying. After that, the regenerated copolymer, indexed as PAP-R, was achieved by 20.0 mL of 1 mol L⁻¹ PTSA for 300 s at 293 K. This process was conducted five times to study the recyclability of samples.

3. Results and Discussion

3.1. Characterization

The chemical structure of PANI and P(ANI-PDA) copolymers was determined by FTIR spectroscopy. In the FTIR spectrum of Figure 1A (a), the absorption peaks at 1569 and 1497 cm⁻¹ are the stretching vibrations of quinone and benzene structures, respectively. The absorption peak of 1299 cm⁻¹ corresponds to the stretching vibration of aromatic amine. The absorption peaks at 817 and 1130 cm⁻¹ are the out-of-plane bending vibration and in-plane bending vibration of benzene rings, separately. The absorption peak at 817 cm⁻¹ is the out-of-plane bending vibration of C-H in the benzene rings, demonstrating the presence of a phenazine-like structure in the copolymer backbone. The absorption peak at 1130 cm⁻¹ is the stretching vibration of S=O in the dopant acid PTSA [17,21]. These characteristic peaks are also observed in the FTIR spectrum of P(ANI-PDA), Figure 1A (b). However, a weak absorption peak around 1037 cm⁻¹ appeared in the FTIR spectrum of P(ANI-PDA), which is assigned to the phenazine structure of PDA. This confirms the successful copolymerization of aniline and p-PDA. The SEM images of PANI and P(ANI-PDA), Figure 1B,C, display the different morphologies. The unique morphology of P(ANI-PDA) allows the heavy metal ions to contact the active sites by capillary action or diffusion, which may improve the material’s ability to remove heavy metals [22].

3.2. Cr(VI) Removal Evaluation

The relationship between Cr(VI) concentration in solution and absorbance in UV-Vis spectra is fitted in Figure S2, and the fitting results are shown in Equation (S1). Then, experiments were carried out to test the effect of different factors on the removal of Cr(VI) by P(ANI-PDA). To begin with, the effect of P(ANI-PDA) dosage on Cr(VI) removal was investigated, and the results are listed in Figure 2A, in which the varying amounts of P(ANI-PDA) from 5 to 15 mg were introduced into 20.0 mL of solution with an initial Cr(VI) concentration of 1.5 mg∙L⁻¹ at 293 K for treatment of 45 s. The RP% of Cr(VI) removal by P(ANI-PDA) powder is measured at 43.47, 66.24, 81.23, 88.08, and 92.39%. The effect of PANI dosage on the Cr(VI) removal was also investigated for comparison, and the results are listed in Figure 2A. The RP% of Cr(VI) removal by pure PANI powder is measured at 11.80, 25.55, 36.55, 44.59, and 50.27%, which are much lower than those of P(ANI-PDA) powder. The initial Cr(VI) concentration also plays a role in the Cr(VI) removal, as shown in Figure 2B. The RP% is87.46, 83.71, 81.51, 76.60, and 71.46% for 20.0 mL of solution with initial Cr(VI) concentrations of 0.4, 0.8, 1.2, 1.6, and 2.0 mg∙L⁻¹, respectively, using 10 mg of P(ANI-PDA) for treatment for 45 s. The effect of solution pH value on the Cr(VI) removal with 10 mg of P(ANI-PDA) was examined in a 1.5 mg∙L⁻¹ Cr(VI) solution within the pH range of 1.0 to 12.0 for 45 s at 293 K. The results in Figure 2C indicate a substantial correlation between the RP% of Cr(VI) and the pH value of the solution. The RP% of Cr(VI) is found to be 92.35, 84.71, 80.10, 60.13, and 36.23% for pH values of 1.2, 2.5, 5.5, 9.7, and 11.8, respectively, and the optimal pH value for Cr(VI) removal by P(ANI-PDA) was 1.2. RP% comparison of various materials used for Cr(VI) removal from literature is shown in Table S1.

3.3. Removal Mechanism

The Cr(VI) removal mechanism has been explored by XPS, EDS, and zeta potential. The high-resolution C 1s XPS spectrum of PAP-Cr in Figure 3A is deconvoluted into three peaks at 285.5, 284.8, and 284.0 eV, corresponding to the C-N, C=C, and C-C, respectively. The peak at 284.0 eV is the charge-calibrated C sp³ peak, while the peak of 284.8 eV is the C sp² peak, confirming the presence of the benzene ring, which in turn indicates the presence of benzene ring and quinone ring structures in PAP-Cr. In the high-resolution Cr 2p XPS spectrum in Figure 3B, two obvious peaks at 586.48 and 576.91 eV corresponding to the Cr 2p_1/2 and Cr 2p_3/2 orbitals prove the existence of Cr(III) on the surface of PAP-Cr. This implies that Cr(VI)with a higher oxidation potential (+1.33 V), undergoes a redox reaction [23] and is reduced to Cr(III) after treatment.

The distribution of Cr element on PAP-Cr was analyzed using an energy dispersive spectrometer (EDS) to elucidate the removal mechanism. The SEM image in Figure 4 depicts the elemental mappings of C, O, N, S, and Cr in different colors, highlighting the elemental positions in PAP-Cr. C and N element maps, shown as red and yellow, respectively, in Figure 4B,C, are from P(ANI-PDA). O and S, shown as blue and cyan in Figure 4D,E, are from the doped acid PTSA. Figure 4F provides the distribution of Cr element, which is evenly distributed on the surface of P(ANI-PDA).

The zeta potential measurement is laid out in Figure 3C. It is observed that the surface of P(ANI-PDA) is positively charged when the pH values are below 4, and the zeta potential is decreased gradually with increasing pH values. The surface of P(ANI-PDA) is negatively charged as the pH value is higher than 4. The zeta potential is changed from positive to negative when the solution pH value is about 5.0, which verifies that P(ANI-PDA) reaches the isoelectric point when the pH value is approximately equal to 5.0. The change in charge carried on the surface of P(ANI-PDA) is consistent with the effect of pH value on the Cr(VI) removal performance. This means that as the solution pH is acidic, P(ANI-PDA) carries a positive charge, which is beneficial for the electrostatic attraction of Cr(VI) on its surface with subsequent redox reactions. On the contrary, with the increasing solution pH, P(ANI-PDA) starts to carry the negative charge, leading to repulsion between P(ANI-PDA) and Cr(VI) and decreasing the RP% of Cr(VI). It is concluded that the removal process of Cr(VI) is a multi-step process: (1) the Cr(VI) anion converges on the active site of P(ANI-PDA) by electrostatic attraction in an acid solution or the anion doped on P(ANI-PDA) (dehydro-PTSA anion) converges to the active site by ion exchange with Cr(VI)-containing anions in solution; (2) Cr(VI)-containing anions are reduced to Cr(III) cations by electron-rich nitrogen-containing groups on P(ANI-PDA), while the amines are oxidized to quinone amines and the Cr(III) further bind to N atoms containing lone-pair electrons. Additionally, it should be noted that not all the active sites of P(ANI-PDA) are oxidized during the removal process since the Cr(VI) content in solution is only around 0.03 mg with a 1.5 mg L⁻¹ initial concentration and 10 mg dosage of P(ANI-PDA), which means that the dosage of P(ANI-PDA) is far higher than that of Cr(VI) content in the solution. The proposed mechanism is shown in Figure 5.

3.4. Removal Kinetics and Thermodynamics

The effect of treatment time on Cr(VI) removal was studied to determine the Cr(VI) removal kinetics by P(ANI-PDA), and the results are depicted in Figure 3D. The RP% of Cr(VI) is 84.24, 89.49, 91.43, 93.09, 94.84, and 95.30% for treatment times of 30, 60, 90, 120, 180, and 300 s, respectively. According to the redox mechanism during the treatment process above, a linear fit of ln([HCrO₄⁻])~t is achieved using Equation (S3), and the results are expressed in Figure 3D. The results suggest that the removal process follows the pseudo-first-order chemical reaction kinetic model. As the temperature increases, the RP% of Cr(VI) is increased, suggesting that the Cr(VI) removal by P(ANI-PDA) is a heat-absorbing process [24,25].

The thermodynamic results were captured at 293, 303, 313, and 323 K, respectively. Choosing 180 s as the equilibrium time and Cr(VI) concentrations ranging from 0.4 to 2.0 mg∙L⁻¹, the results are laid out in Figure 3E. The thermodynamic parameters such as standard Gibbs free energy change (ΔG°, kJ∙mol⁻¹), standard enthalpy change (ΔH°, kJ∙mol⁻¹), and standard entropy change (ΔS°, J∙mol⁻¹∙K⁻¹) are calculated by Equations (S4)–(S6) and listed in Table S2. A positive value of ΔH° of 94.67 kJ∙mol⁻¹ affirms that the Cr(VI) removal process by P(ANI-PDA) is endothermic, and the negative value of ΔG° (−447.55, −465.49, −484.19, and −503.01 kJ∙mol⁻¹ for temperature of 293, 303, 313, and 323 K, respectively) suggests that this process is thermodynamically achievable and spontaneous. The positive value of ΔS° of 1.85 kJ∙mol⁻¹∙K⁻¹ illustrates that the randomness at the solid-liquid interface is increased during the Cr(VI) removal process.

3.5. Recycle and Regeneration

The regeneration is intricately linked to economic viability and environmental sustainability, making it a crucial factor in widespread application [26]. As shown in Figure 6, once the copolymer has captured the contaminant, the treated solution is separated from the copolymer through filtration [27,28]. Subsequently, the contaminated copolymer undergoes regeneration through interaction with 1.0 mol L⁻¹ of PTSA solution since the presence of Cr(III) on P(ANI-PDA) is from the chelation between Cr(III) with doped N atom, followed by separation via centrifugal treatment. The dried sample is ready for the next cycle. The removal of pollutants by copolymers is notably influenced by the pH value of the solution [29,30]. In this experiment, PAP-Cr has been regenerated using 1 mol∙L⁻¹ PTSA solution, and Cr(VI) solution (20 mL, 1.5 mol∙L⁻¹) has been treated with regenerated P(ANI-PDA), as shown in Figure 3F. Without regeneration treatment, PAP-Cr is extremely ineffective in treating Cr(VI) solution, with an RP% of only 0.09%, which evinces that PAP-Cr can no longer remove Cr(VI). The results also show that the RP% of PAP-R obtained after one regeneration is 68.95% for the Cr(VI) solution. Under the same conditions, the freshly prepared P(ANI-PDA) signifies an RP% of 72.43% for Cr(VI) solution with only 3.48% decay. After five regeneration cycles, the RP% is still 65.02%. This verifies that our P(ANI-PDA) has a good regeneration and reusability capability.

Generally, PANI has three different forms, i.e., “leucoemeraldine” (LEB), “emeraldine” (EB), and “pernigraniline” (PB). The acid could control the reduction and oxidation of PANI because of its unique doping and de-doping process [31]. In fact, the oxidation of PANI is associated with proton doping since it consists of alternative amine and imine groups in its polymer backbone. As proton is doped to the amine and imine groups, the PANI has changed to the reduced form (EB salt form). When the proton is dissociated from the amine and imine groups of PANI, it is transformed to the oxidation form (PB form). Meanwhile, the main removal sites of Cr(VI) are amine and imine groups in the molecular chain of P(ANI-PDA). N atoms have lone-pair electrons, so they can effectively bind Cr(III) through shared electron pairs to form the complexes. During the regeneration process, the PTSA solution contains a high concentration of H⁺, which competes with Cr(III) at the N atom on the surface of the copolymer. H⁺ dominates the competition due to its higher concentration, and the removal sites are protonated, thus releasing Cr(III) [32]. Moreover, the high-resolution N 1s XPS spectra of copolymer, copolymer after treatment with Cr(VI), and copolymer regenerated by PTSA are shown in Figures S4A, S4B, and S4C, respectively. It is found that the N1s XPS spectrum can be deconvoluted into four peaks, −N−, −N=, −NH⁺−, and −N⁺=, Figure S4A. After treatment with Cr(VI), Figure S4B, the content of −N= increased from 11.94 to 24.94%, indicating the redox reaction occurs between Cr(VI) and copolymers. After regeneration by PTSA, Figure S4C, the contents of −N= and −N− are like the original copolymer, Figure S4A, suggesting the successful regeneration of the copolymer by PTSA. Furthermore, the Cr 2p XPS spectrum of the copolymer after third-cycle Cr(VI) treatment (Figure S5) has no difference in the oxidation state of Cr from that observed in the first cycle in Figure 3B. Based on the above, it can be demonstrated that PTSA plays a crucial role in regeneration. It can reduce the copolymer from the PB form to the EB salt form [33,34] so that the copolymer can regain the ability to reduce Cr(VI).

4. Conclusions

The experimentally synthesized P(ANI-PDA) is a material with many amino groups. It exhibits an excellent ability to remove Cr(VI) from wastewater. The longer the treatment time and copolymer dosage, the higher the RP% of Cr(VI) is obtained. When the treatment time is 300 s, the RP% of Cr(VI) can reach 95.30%. The RP% is 92.17% at a P(ANI-PDA) dosage of 15 mg. The high Cr(VI) concentration is not conducive to the removal process. The RP% decreases to 61.30% when the Cr(VI) concentration is 2.0 mg∙L⁻¹. The RP% is increased to 82.35% when the pH value is 1.0. The removal process follows the pseudo-first-order chemical reaction kinetic model. Thermodynamic results with a negative ΔG° demonstrate that the removal process is thermodynamically feasible and spontaneous. Furthermore, the removal process is a redox process. Meantime, the copolymers demonstrate excellent reuse and regeneration capability. In summary, P(ANI-PDA) has impressive potential for heavy metal removal.