Removal of heavy metal ions from wastewater by capacitive deionization using polypyrrole/chitosan composite electrode

Abstract

A polypyrrole/chitosan composite material was obtained by chemical polymerization. The adsorption performance of a hot-molded polypyrrole/chitosan composite electrode was tested by adsorption/desorption experiments. Scanning electron microscopy and Fourier-transform infrared spectroscopy both showed the deposition of polypyrrole on the chitosan surface. The specific capacitance of the polypyrrole/chitosan composite was determined by cyclic voltammetry in 1.0 M KCl at 0.01 V/s as 102.96 F/g. The adsorption/desorption experiments indicated that the specific adsorption capacity of the composite for Cu²⁺ was 99.67 mg/g, while the removal performance for other metal ions, such as Ag⁺, Pb²⁺, and Cd²⁺, was good. The results of multicycle adsorption/desorption tests showed that the adsorption rate of the polypyrrole/chitosan composite electrode for Cu²⁺ was decreased from 56.4 to 51.4% over 10 cycles, demonstrating the stable metal-ion adsorption/desorption behavior of the composite electrode. The obtained performances show that the prepared polypyrrole/chitosan composite material is an ideal electrode material for the removal of heavy metal ions.

Introduction

Heavy metal pollution has become a serious problem worldwide, due to the fact that these toxic elements easily accumulate in the food chain and then affect all living organisms in a given biological system. Heavy metals, such as copper, lead, silver, chromium, and cadmium affect the central nervous system, bones, kidneys, liver, skin, and brain. Thus, the removal of these heavy metals from wastewater has attracted research attention. Conventional methods for removing metal ions include electrochemical and chemical precipitation, ion exchange, membrane separation, coagulation–flocculation, adsorption, and capacitive deionization (CDI) (Ali et al., 2011; Fan et al., 2016; Hering et al., 1997; Mossad and Zou, 2012; Shin et al., 2011). Among these, the CDI technique is widely used because it offers simple operation, low cost, high efficiency, and no secondary pollution (Demirer et al., 2013; Hemmatifar et al., 2016; Wang et al., 2016).

The CDI process includes the two stages of adsorption and desorption. During the adsorption stage, the anions or cations are adsorbed on the cathode or anode, respectively, under an external electrostatic force when the fluid feed passes through the two electrodes of the CDI cell. When the electrode reaches its saturated adsorption point, the adsorbed ions are then desorbed by removing the primary applied electrostatic force or applying an oppositely charged one. The desorbed ions are removed by flushing with water, thus regenerating the electrode.

The performances of electrode materials, including carbon materials, conductive polymers, and metal oxides, are vital to the success of the CDI process. Recently, graphene with high surface area, good electrical conductivity, and controllable morphology structure is one of the most promising electrode material in CDI process (Ahmed and Tewari, 2018; Cao et al., 2018; Liu et al., 2017; Mubita et al., 2018). Duan and Khan et al. systematically summarize current progress on graphene nanosheets, porous graphene, graphene-based composites, surface tuned graphene, and its composites as electrodes for CDI, which all present good adsorption performance in CDI process (Duan et al., 2017; Khan et al., 2018). For example, they designed functional 3D graphene by EDTA for removing Pb²⁺ and Na⁺, and the removal efficiency was 99.9% for Pb²⁺ and 98.7% for Na⁺, and ions can be recovered in desorption process by two steps depending on different adsorption mechanism (Liu et al., 2017). Graphene or other carbon materials will be grafted other functional materials for removing heavy ions due to the adsorption mechanism.

Conductive polymers have also shown great potential in the removal of ions from water because of their ion-exchange properties between the polymer and the media, namely the electrochemically switchable ion exchange. The ion exchange of conductive polymers is a 3D process and occurs in the interior of the electrode (not just on the surface), which indicates that conductive polymers will exhibit good electrochemical and adsorption performances. Polypyrrole is one commonly studied conductive polymer with a high specific capacitance, high conductivity, and easy preparation (Dubal et al., 2013; Sun and Mo, 2013); its use has been reported for the removal of metal ions such as Cu²⁺, Pb²⁺, Cr²⁺, and Cd²⁺ (Fang et al., 2016; Ghorbani and Eisazadeh, 2013; Hasani and Eisazadeh, 2013; Hosseini et al., 2015; Karthik and Meenakshi, 2014). However, polypyrrole as an electrode for the CDI process has poor stability, low conductivity, and poor adsorption performance because of the use of adhesive agents during the electrode molding process (Xu et al., 2013). The synergistic effect of chitosan has been reported to improve the stability of polypyrrole (Bagheri et al., 2014; Huang et al., 2013; Seyed Dorraji et al., 2014). Chitosan can function as adhesive agent because of its high viscosity and molecular structure; it is also an excellent adsorbent for heavy metals, acid ions, and dyes, among other materials (Koong et al., 2013; Rangel Mendeza et al., 2009). Therefore, the chitosan-and-polypyrrole system may be suitable as an electrode for the CDI process (Karthik and Meenakshi, 2015).

In this work, polypyrrole was deposited on chitosan by chemical oxidative polymerization in an aqueous solution. The morphology and electrochemical performance of the composite polymer were tested by scanning electron microscopy (SEM), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The efficiency of the polypyrrole/chitosan composite material in the CDI process for the removal of heavy metal ions from aqueous solutions was tested, and the desorption mechanism of the composite electrode was investigated for the recovery of metal ions and recycling of the electrode material.

Experimental

Preparation of polypyrrole/chitosan composite electrode

Chitosan was dissolved in 2% acetic acid solution, to which 0.1 M sodium dodecyl benzene sulfonate solution, 0.5 M FeCl₃ solution, and pyrrole were added. Polymerization was performed for 24 h with electromagnetic mixing at ambient temperature (25°C), and the solution was adjusted to a neutral pH by adding NaOH solution. The product was obtained by filtering, washing, and vacuum drying.

A polypyrrole/chitosan composite electrode of 30 mm in diameter and 1.0 mm in thickness was prepared by hot molding a mixture of the polypyrrole/chitosan composite material (95% w/w) and active carbon (5% w/w) at the curing temperature of 150°C under 10 MPa for 5 min.

Adsorption experiment

The adsorption performance of the polypyrrole/chitosan composite electrode was characterized using inorganic metal salt (e.g. CuCl₂, Pb(NO₃)₂, AgNO₃, and Cd(NO₃)₂) solutions with the concentration of 1000 ppm at room temperature under moderate stirring and the adsorption voltage of 1.5 V for 60 min. When the adsorption process was completed, an opposite potential of −1.5 V was applied for 30 min for the desorption process. The conductivity of the feed solution was recorded continuously during the adsorption/desorption experiment by a conductivity meter, which indicated the concentration changes of the solution.

Results and discussion

Morphology of polypyrrole/chitosan composite material

The morphology of polypyrrole/chitosan composite material is tested by SEM, with images of the composite material shown in Figure 1. Figure 1(a) shows that the chitosan is lamellar in structure, with lamellae agglomerated together. The polypyrrole and polypyrrole/chitosan composite materials both have particle-like morphologies, but the diameters of the polypyrrole/chitosan particles (0.9–1.5 µm) are larger than those of the polypyrrole particles (0.4–0.8 µm), as seen from Figure 1(b) and (c). And compared with the different morphology of chitosan and polypyrrole/chitosan from Figure 1(a) and (c), it can said that polypyrrole may be coated on the chitosan surface.

Figure 1.

SEM images of (a) chitosan, (b) polypyrrole, and (c) polypyrrole/chitosan composite material.

The Fourier-transform infrared (FT-IR) spectra of polypyrrole/chitosan composite material are shown in Figure 2. The intensities of the characteristic absorption peaks of chitosan at 1091, 1033, and 895 cm⁻¹ are reduced by the changes in the chitosan molecular structure during the polymerization process. Compared with the chitosan spectra, the spectrum of composite electrode shows peaks at 1552 cm⁻¹, corresponding to pyrrole ring adsorption, and at 1306 cm⁻¹, indicating the C–N stretching vibration. The pyrrole ring plane-bending vibration peaks are located at 904 and 793 cm⁻¹, which also indicated that polypyrrole is polymerized on the surface of chitosan.

Figure 2.

The FT-IR spectra of the polypyrrole/chitosan composite material and chitosan.

The wettability of the polypyrrole/chitosan composite material was tested. The contact angle of the composite material is 83.73° as shown in Figure 3, indicating that the composite material has a better hydrophilicity than that of polypyrrole alone (104.5°, shown in Figure 3(a)). This finding is observed because the surfaces of the polypyrrole/chitosan composite material have some hydrophilic functional groups as –OH and –CO–, indicated by the FT-IR spectra shown in Figure 2.

Figure 3.

The water contact angle of polypyrrole (a) and polypyrrole/chitosan composite material (b).

Electrochemical performances of polypyrrole/chitosan composite electrode

The electrochemical performances of the polypyrrole/chitosan composite electrode are tested by CV and EIS, with results presented in Figures 4 and 5, respectively. The CV curve in Figure 4 shows a complete and smooth rectangular shape, indicating the fast diffusion of ions and ideal capacitive behavior. The CV of the composite electrode shows a redox peak, which is characteristic of pseudocapacitance, due to the oxidation–reduction reaction at the scanning potential. The specific capacitance is 102.96 F/g, as calculated by equations (1) and (2)

$$C=\frac{\overline{i}}{v}=\frac{\frac{1}{E_2-E_1}{\displaystyle {\int }_{E_1}^{E_2}i(E)\text{d}E}}{v}$$

(1)

$$C_m=\frac{C}{m}$$

(2)where C is the capacitance of the polypyrrole/chitosan composite material (F); C_m is the specific capacitance of the polypyrrole/chitosan composite material (F/g); i(E) is the scanning current as a function of voltage for the polypyrrole/chitosan composite material (A); E₁ and E₂ are the lower and upper limit potentials (V), respectively; v is the scanning rate (V/s); and m is the mass of the polypyrrole/chitosan composite material (g).

Figure 4.

CV curve of the polypyrrole/chitosan composite material in 1.0 mol/l KCl solution at 10 mV/s and −0.1–0.8 V versus saturated calomel electrode.

Figure 5.

Nyquist plot of polypyrrole/chitosan composite material in 1.0 mol/l KCl solution at scanning frequencies from 10 mHz to 100 kHz.

The Nyquist plot of the polypyrrole/chitosan composite material is shown in Figure 5. By analyzing the curves, two well-separated patterns are observed: a semicircle in the high-frequency region reveals that the electrostatic absorption mechanism at the interface between the electrolyte and the electrode material is electric double-layer capacitance, and the radius of the semicircle provides the ion diffusion resistance between the electrolyte and electrode materials. In the low-frequency region, an inclined line is observed. The angle between the inclined line and the real axis is between 45° and 90°, which correspond to the ion diffusion mechanisms of Warburg diffusion and ideal capacitive ion diffusion, respectively. Therefore, the adsorption mechanism in the low-frequency regime is pseudocapacitance. From the curve in Figure 5, the low-frequency straight line leans toward the imaginary axis, indicating good capacitive behavior.

Adsorption performance

The adsorption processes of the polypyrrole/chitosan composite electrode at different concentrations of CuCl₂ solution are shown in Figure 6. The adsorption curves show that the adsorption rate of the polypyrrole/chitosan composite electrode for CuCl₂ increases as the solution concentration is increased for the first 60 min. The adsorption capacity of the composite electrode changes only slightly after 60 min, indicating that the electrode is saturated in adsorption capacity after 60 min of adsorption. The concentration of the CuCl₂ solution after adsorption was investigated, with results listed in Table 1. The concentrations of the CuCl₂ solutions with the initial concentrations of 50, 100, and 200 mg/l after 120 min adsorption time are all decreased to 0 mg/l. This shows that the adsorption capacity of the polypyrrole/chitosan composite electrode is not completely saturated. Meanwhile, the CuCl₂ solutions with initial concentrations of 500 and 1000 mg/l after 120 min adsorption are decreased to 84.51 and 494.61 mg/l, respectively; these show only slight decreases relative to the concentrations measured after 60 min. This indicates that the adsorption capacity of the composite electrode is nearly saturated at 60 min for these higher initial concentrations. The saturated adsorption capacity of the composite electrode is 99.67 mg/g, as calculated following equation (3)

$$q=\frac{(c_0-c_1)v}{m}$$

(3)where q is the saturated adsorption capacity of the polypyrrole/chitosan composite electrode (mg/g), c₀ and c₁ are the initial and final concentrations of the CuCl₂ solutions (mg/l), v is the volume of the CuCl₂ solution (ml), and m is the mass of the polypyrrole/chitosan composite electrode (g).

Figure 6.

Adsorption capacities of polypyrrole/chitosan composite electrode over time for different initial concentrations of CuCl₂.

Table 1.

Concentrations of CuCl₂ solutions after different adsorption times.

Initial concentration (mg/l)	50	100	200	500	1000
Solution concentration after 60 min (mg/l)	4.84	1.12	1.63	93.42	501.69
Solution concentration after 120 min (mg/l)	0	0	0	84.51	494.61

The adsorption performances of the polypyrrole/chitosan composite electrode for the different metal ions of Cu²⁺, Pb²⁺, Ag⁺, and Cd²⁺ were tested at 1.5 V working voltage as shown in Figure 7. The curves show that the adsorption rates of the composite electrode increase as the adsorption time is increased, but the increased speed decreases. The final adsorption rates of the composite electrode for Cu²⁺, Pb²⁺, Ag⁺, and Cd²⁺ are 52.9, 49.3, 55.7, and 51.1%, respectively. This shows that the adsorption performance of the composite electrode differs somewhat for different metal ions.

Figure 7.

The adsorption performance of the polypyrrole/chitosan composite electrode for different mental ions.

Adsorption kinetics

To investigate the adsorption ratios of the polypyrrole/chitosan composite electrode for different metal ions, pseudo-second-order kinetic models were applied to the adsorption processes of the composite electrode for different metal ions, with results shown in Figure 8. The pseudo-second-order model is represented in equation (4)

$$\frac{t}{q_t}=\frac{1}{k_2{q}_e^2}+\frac{t}{q_e}$$

(4)where q_t is the adsorption capacity at time t (mg/g), k₂ is the rate constant of the pseudo-second-order model (g/(mg min)), and q_e is the equilibrium adsorption capacity (mg/g).

Figure 8.

The pseudo-second-order adsorption kinetics of the polypyrrole/chitosan composite electrode for different metal ions.

The slopes and intercepts of the plots of t/q_t versus t were obtained to determine k₂ and q_e. The correlation coefficients (R²) of the pseudo-second-order kinetic model for different ions all exceed 0.9926, demonstrating that the adsorption kinetics of the polypyrrole/chitosan composite electrode for different ions follow the pseudo-second-order kinetic model with good accuracy. Thus, the adsorption process of the polypyrrole/chitosan composite electrode for metal ions includes two steps. The first step is surface diffusion and adsorption, and the second is internal diffusion and electrochemical adsorption (ion exchange). The second-order kinetic model assumes that the rate-limiting step is the second process of electrochemical adsorption (ion exchange), which is reflected by the slope of the adsorption rate at different adsorption times in Figure 7. This indicates that all stages contribute to the adsorption, by mechanisms of external surface diffusion, intraparticle diffusion and sorption, and ion-exchange equilibrium.

The adsorption stability of the polypyrrole/chitosan composite electrode under a working voltage of 1.5 V and 100 mg/l CuCl₂ solution was tested for 10 cycles, with experimental results as shown in Figure 9(a). Figure 9(b) shows the adsorption rates and regeneration rates of the composite electrode in the different cycles. The adsorption rate of the polypyrrole/chitosan composite electrode decreases from 56.4 to 51.4% over the 10 cycles, a loss of approximately 8%. The regeneration rate of the composite electrode is 97% or higher in every cycle, indicating good and stable regeneration performance.

Figure 9.

(a) Cyclic adsorption/desorption process and (b) adsorption rate over cycling of the polypyrrole/chitosan composite electrode.

Conclusions

A polypyrrole/chitosan composite material with a high specific capacitance has been prepared and used as an electrode in a CDI cell. SEM images and FT-IR spectra clearly show that the polypyrrole is successfully deposited on the surface of chitosan. The specific capacitance of the composite material, determined by CV in 1.0 M KCl at a scanning rate of 0.01 V/s, reaches 102.96 F/g. The results of the adsorption experiment show the saturated adsorption capacity of 99.67 mg/g for the composite electrode toward Cu²⁺. The composite electrode also shows good removal performance for other metal ions of Ag⁺, Pb²⁺, and Cd²⁺. The results of multicycle adsorption/desorption experiments show that the adsorption rate of the polypyrrole/chitosan composite electrode decreases from 56.4 to 51.4% over 10 consecutive cycles, which indicates that the composite material as an electrode for adsorbing metal ions has stable adsorption and desorption behavior, indicating long lifetime for use in a CDI cell.