Electrical Removal Behavior of Carbon Nanotube and Carbon Nanofiber Film in CuCl₂ Solution: Kinetics and Thermodynamics Study

Abstract

The kinetics, thermodynamics, and isotherms during electrical removal of Cu²⁺ by carbon nanotube and carbon nanofiber (CNT-CNF) electrodes in CuCl₂ solution were studied under different solution temperatures, initial Cu²⁺ concentrations, and applied voltages. The result shows that Langmuir isotherm can describe experimental data well, indicating monolayer adsorption, and higher Cu²⁺ removal and rate constant are achieved at higher voltage, lower initial Cu²⁺ concentration, and higher solution temperature. Meanwhile, the thermodynamics analyses indicate that the electrical removal of Cu²⁺ onto CNT-CNF electrodes is mainly driven by a physisorption process.

1. Introduction

Cupric ions (Cu²⁺) commonly exist in the waste water of several industries such as acid mine waste and acidic corrosion of pipes. The presence of excessive amounts of Cu²⁺ in drinking water may lead to accumulation in the liver and may cause gastrointestinal problems [1]. Therefore, the elimination of Cu²⁺ from water is of great importance to public health. Compared with conventional methods such as oxidation or reduction, precipitation, membrane filtration and ion exchange, electrosorption, defined as adsorption on the surface of charged electrode by applying potential or current, has been shown to be a more efficient and energy saving method to remove ions including Cu²⁺ from water since it is conducted at ambient conditions and low voltages with no secondary waste and requires no membranes, distillation columns, or thermal heaters [2–10]. Some researchers have successfully employed electrosorption to remove Cu²⁺ from solution. Oda and Nakagawa [11] investigated the removal of Cu²⁺ and Zn²⁺ using activated carbon electrodes by applying a direct voltage of 1 V. Huang and Su [12] studied the electrosorption of Cu²⁺ using activated carbon fiber cloth electrodes by imposing a low voltage of 0.3 V. Ying et al. [13] studied the electrosorption of different ions from aqueous solutions using nanostructured carbon aerogel and found a strong specific adsorption for Cu²⁺ ions. In our previous studies [14], carbon nanotube and carbon nanofiber (CNT-CNF) electrodes had been successfully used to perform the electrical removal of Cu²⁺ ions and the difficulty of their regeneration due to the electrodeposition reaction on the surface had been solved by combining reverse voltage and short circuit. When applied voltage is more than the electrodeposition potential of Cu²⁺, the electrodeposition reaction will happen together with electrosorption of Cu²⁺ ions. The term “electrical removal” is used here considering the contribution from electrodeposition during dominant electrosorption process.

Although more and more achievements have been made on the electrical removal of dangerous Cu²⁺ ions from aqueous solution, the further detailed investigation on this field is still needed. By now, extensive papers have not paid more attention on the removal kinetics and thermodynamics which is helpful to understand the removal mechanism of Cu²⁺ ions and improve their removal efficiency from aqueous solution. In this paper, we further investigated the application of CNT-CNF films as electrodes to remove Cu²⁺ ions from aqueous solution. Several experiments were conducted at different solution temperatures, applied potentials and initial Cu²⁺ concentrations. The corresponding removal isotherms, kinetics and thermodynamics were analyzed, respectively.

2. Experimental

2.1. Preparation and Characterization of CNT-CNF Electrodes

Graphite substrates were degreased and cleaned by acetone and alcohol, respectively. A layer of 20 nm-thick Ni catalysts was deposited on the surface of the graphite substrates by direct current (DC) magnetic sputtering (Shanghai Nanoking Co.). CNT-CNF film electrode was subsequently fabricated on the graphite substrates using the low pressure and low temperature thermal chemical vapor deposition (LPCVD) system (Shanghai Nanoking Co.). Acetylene-hydrogen (C₂H₂ : H₂ = 1 : 5) mixture gas was introduced into the LPCVD chamber at a flow rate of 50 and 100 sccm, respectively, at 823 K for 30 min. The surface morphology of CNT-CNF film was observed by field emission scanning electron microscopy (FESEM, JEOL S4800).

2.2. Batch-Mode Experiments

Batch-mode experiments were carried out for the Cu²⁺ removal in a continuously recycling system, as depicted in Figure 1. The system consisted of a peristaltic pump, removal cell, measuring cell, temperature meter and conductivity meter. The assembly of the removal cell was in the order: retaining plate/rubber gasket/CNT-CNF electrode/spacer/CNT-CNF electrode/rubber gasket/retaining plate. Retaining plate was made of polymethyl methacrylate. The spacing between the electrodes was maintained by rectangular nylon spacer and rubber spacer. Before the experiments, the as-grown CNT-CNF film electrodes were immersed into acid solution to dissolve the Ni particles and were packed in holders with an area of 8 cm × 8 cm and a distance of 2 mm between the electrodes. The solution was continuously pumped from the peristaltic pump into the removal cell and the effluent turned to the removal cell via measuring cell. The solution flow rate was around 40 mL/min. The analytical pure cupric chloride (CuCl₂) was used for the aqueous solutions. The electrical removal was performed by a DC power supply and the variation of Cu²⁺ concentrations was continuously monitored and measured in the measuring cell by using a DDS-308 ion conductivity meter (Shanghai Precision and Scientific Instrument Co. Ltd.). The relationship between conductivity and concentration was obtained according to a calibration table made prior to the experiments.

Figure 1 

Schematics of batch-mode experiment.

2.3. Data Analysis

2.3.1. Removal Kinetics

Generally, removal kinetics is an important characteristic of adsorbents. During the electrical removal, ion concentration gradually decreases until equilibrium is reached. This process is expected based on the large number of vacant surface sites available for electrical removal during the initial stage, and, after certain amount of time, the remaining vacant surface sites are difficult to occupy due to repulsive forces between ions on electrode and in solution. The pseudofirst-order kinetic model [15, 16] (Lagergren’s equation) was employed to fit the experimental data to investigate the influence of the applied potential, solution temperature, and initial Cu²⁺  concentration on removal behavior of CNT-CNF electrodes in Cu²⁺ solution. The form of this model equation can be formulated as 𝐶ln^𝑡−𝐶_𝑒𝐶₀−𝐶_𝑒=−𝑘𝑡, (1) where 𝑘 (min⁻¹) is the reaction rate constant, 𝐶₀, 𝐶_𝑒, and 𝐶_𝑡 (mg/L) are initial concentration, equilibrium concentration, and the concentration at time 𝑡 (min), respectively. The kinetics parameters can be obtained by fitting the experimental data using least square method.

The change in reaction rate constant 𝑘 is described by the Arrhenius equation [16, 17]: 𝐸ln𝑘=ln𝐴−^𝑎RT, (2) where 𝐴 is the pre-exponential factor, 𝑅 is the gas constant (8.314 Jmol⁻¹K⁻¹), and 𝐸_𝑎 is the activation energy. 𝐸_𝑎 and 𝐴 are determined from the slope and intercept of the plot of ln𝑘 versus 1/𝑇.

2.3.2. Removal Isotherms

Isotherm models are used to describe the equilibrium of the adsorbate between the aqueous solution and the CNT-CNF solid phase. Langmuir isotherm model (3) and Freundlich isotherm model (4) [18] were used to fit the experimental data. Langmuir isotherm assumes that the single adsorbate binds to a single site on the adsorbents, and that all surface sites on the adsorbents have the same affinity for the adsorbate. The Freundlich isotherm can be derived from the Langmuir isotherm by assuming that there exists a distribution of sites on the adsorbents for different adsorbates with each site behaving accordingly to the Langmuir isotherm [19, 20] 𝑞_𝑒=𝑞_𝑚𝐾_𝐿𝐶_e1+𝐾_𝐿𝐶_e,𝑞_𝑒=𝐾_𝐹𝐶_𝑒^1/𝑛, (3)(4) where 𝑞_𝑒 (mg/g) is the amount of adsorbed CuCl₂ per unit mass of CNT-CNF film and 𝑞_𝑚 (mg/g) is the maximum removal capacity corresponding to complete monolayer coverage. 𝐾_𝐿 and 𝐾_𝐹 are Langmuir constant that relates to the affinity of binding sites and Freundlich constant, respectively. 𝑞_𝑒 is calculated from the following equation: 𝑞_𝑒=𝐶₀−𝐶_𝑒𝑉𝑚, (5) where 𝑉(𝐿) is the solution volume and 𝑚(𝑔) is the mass of CNT-CNF film.

2.3.3. Removal Thermodynamics

The thermodynamic parameters, free energy change Δ𝐺⁰ (KJ/mol), enthalpy change Δ𝐻⁰, (KJ/mol), and entropy change Δ𝑆⁰ (Jmol⁻¹K⁻¹), provide in-depth information on inherent energetic changes associated with electrical removal and can be determined using the following equations: Δ𝐺⁰=−RTln𝐾_𝐿,Δ𝑆⁰=Δ𝐻⁰−Δ𝐺⁰𝑇,ln𝐾_𝐿=Δ𝑆⁰𝑅−Δ𝐻⁰,RT (6) where 𝑇(𝐾) is absolute temperature. Δ𝐻⁰ and Δ𝑆⁰ are determined from the slope and intercept of the van’t Hoff plots of ln𝐾_𝐿 versus 1/𝑇 [21, 22].

2.3.4. Cu²⁺ Removal and Surface Coverage

The Cu²⁺ removal is defined as follows: Cu²⁺(%)=𝐶Removal⁰−𝐶_𝑒𝐶₀×100%. (7)

The surface coverage, defined as the ratio of the Cu²⁺ coverage per unit mass of CNT-CNF film (𝑆_𝑐, m²/g) to the BET surface area of CNT-CNF film (𝑆_𝑝, m²/g), is helpful in understanding the interaction between adsorbates and the surfaces of adsorbents and can be calculated as follows [23]: 𝑆_𝑐𝑆_𝑝(%)=𝑁_𝐴𝐴_𝑚𝑞_𝑚1000𝑆_𝑝𝑀_𝑤×100%, (8) where 𝐴_𝑚 (m²) is the maximum cross-section area of adsorbate molecule or ion. 𝑀_𝑤 is the molecular weight of adsorbate.

3. Results and Discussion

Figures 2(a)–2(c) show the FESEM images of CNT-CNF film, CNFs, and CNTs, respectively. It can be found that CNF with a diameter of around 600 nm consists of core palpus and a carbon layer sheath outside and serves as the frame for the CNT growth. The CNTs and CNFs are entangled and form a continuous electroconducting network microstructure. There are many mesoporous pores between CNTs and CNFs and their sizes are distributed from 4 nm to 30 nm [24]. Such a network structure ensures a low mass-transfer and allows hydrated ions easily to enter through the pores of the CNT-CNF film which were very helpful for the ion adsorption [25, 26].

(a)

(b)

(c)

Figure 2 

(a) FESEM images of CNT-CNF film, (b) CNFs, and (c) CNTs.

Figures 3(a) and 3(b) show the variation of Cu²⁺ concentration and a linear plot of the first-order kinetic equation between the concentration terms versus time during batch mode experiments at five different applied voltages of 0, 1.0, 1.2, 1.6, and 2.0 V, respectively. All the charge processes are carried out for 60 minutes. The solution temperature is 290 K and the initial solution conductivity is 50 μS/cm. Table 1 summarizes the Cu²⁺ removal and the estimated coefficients of kinetic equation at different applied voltages. Under the potential ranges between 0 and 2.0 V, Cu²⁺ removal and rate constant increase from 0.0285 min⁻¹ and 10% to 0.1055 min⁻¹ and 90%, respectively. Nevertheless, hydrolysis of water is not found when the voltage between the two electrodes is less than 2.0 V because of the existence of resistance in the whole circuit. Obviously, the electrical removal is dependent on applied voltage and higher Cu²⁺ removal and rate constant are achieved at higher voltage due to stronger electrostatic force to drive Cu²⁺ onto CNT-CNF film.

Table 1 

Cu²⁺ removal, the reaction rate constant 𝑘 and correlation coefficients 𝑟² of kinetic equation at different applied voltages.

(a)

(b)

(c)

(d)

(e)

(f)

Figure 3 

(a) Variation of Cu2+ concentration and (b) a linear plot of the first-order kinetic equation at different applied voltages; (c) variation of Cu2+ concentration and (d) a linear plot of the first-order kinetic equation at different initial solution conductivities; (e) variation of Cu2+ concentration and (f) a linear plot of the first-order kinetic equation during batch mode experiments at different solution temperatures.

Figures 3(c) and 3(d) display the variation of Cu²⁺ concentration and a linear plot of the first-order kinetic equation between the concentration terms versus time during batch mode experiments with different initial solution conductivities of 25, 100, 200, and 400 μS/cm, respectively. All the charge processes are carried out for 60 minutes. The solution temperature is 300 K and the applied voltage is 1.2 V. Table 2 summarizes the Cu²⁺ removal and the estimated coefficients of kinetic equation with different initial solution conductivities. It can be observed that the Cu²⁺ removal and rate constant decrease with the increase in initial solution conductivity, that is, initial Cu²⁺ concentration. The lower rate constant in solution with higher initial Cu²⁺ concentration may be due to higher coulomb repulsion, while the lower Cu²⁺ removal is ascribed to the lack of available active sites on the CNT-CNF surface which is quickly saturated with ion species [27].

Table 2 

Cu²⁺ removal, the reaction rate constant 𝑘 and correlation coefficients 𝑟² of kinetic equation at different initial solution conductivities and different solution temperatures.

Figures 3(e) and 3(f) show the variation of Cu²⁺ concentration and a linear plot of the first-order kinetic equation between the concentration terms versus time during batch mode experiments at different solution temperatures of 280, 290, and 300 K, respectively. All the charge processes are carried out for 60 minutes. The applied voltage is 1.2 V and the initial solution conductivity is 100 μS/cm. As shown in Table 2, the Cu²⁺ removal and rate constant increase with the increase in solution temperature. This should be due to the availability of more active sites on the surface of CNT-CNF at higher temperature [28].

Electrical removal experiments with different initial solution conductivities (25–400 μs/cm) at 280, 290, and 300 K were carried out to obtain the isotherm. The applied voltage is 1.2 V. Figures 4(a)–4(c) show the removal isotherms at 280, 290 and 300 K, respectively. The determined parameters and regression coefficients 𝑟², 𝐾_𝐿, and 𝐾_𝐹 of Langmuir and Freundlich isotherms are given in Table 3. The 𝑟² of Langmuir model at each temperature is larger than that of Freundlich model, which indicates Langmuir isotherm better describes the experimental data and suggests that for modeling purposes, monolayer coverage of the CNT-CNF film surface area and equal activation energy during electrical removal process can be assumed [29].

Table 3 

Parameters determined from Langmuir and Freundlich isotherms of CNT-CNF film electrodes in CuCl₂ solution (𝑞_𝑒 is the amount of adsorbed CuCl₂ per unit mass of CNT-CNF film; 𝑞_𝑚 is the maximum removal capacity corresponding to complete monolayer coverage; 𝐾_𝐿 and 𝐾_𝐹 are Langmuir constant and Freundlich constant; 𝑟² is correlation coefficients).

(a)

(b)

(c)

Figure 4 

Equilibrium isotherms of CNT-CNF film electrodes in CuCl2 solution at (a) 280 K, (b) 290 K and (c) 300 K.

The activation energy 𝐸_𝑎 was calculated to be 12.256 KJ/mol from the slope of linear plot of ln𝑘 versus 1/𝑇, as shown in Figure 5(a). Such a value (5–40 KJ/mol) is characteristic for a diffusion and physisorption controlled process [30], suggesting that the Cu²⁺ ions are removed mainly due to the electrical double layer effect although the existence of electrodeposition reaction. Figure 5(b) shows the van’t Hoff plot for the electrical removal of CuCl₂ onto CNT-CNF electrodes. Table 4 presents the thermodynamic parameters at 280, 290, and 300 K. The Δ𝐺⁰ values are negative at all testing temperatures, verifying that the electrical removal of CuCl₂ onto CNT-CNF is thermodynamically favorable. In other words, a more negative Δ𝐺⁰ implies a greater driving force, resulting in an increased electrical removal performance. As temperature increases from 280 K to 300 K, Δ𝐺⁰ decreases negatively, suggesting that the electrical removal is more favorable at high temperatures. The negative Δ𝐻⁰ indicates that electrical removal of CuCl₂ onto CNT-CNF is an exothermic process [28, 31, 32] and the positive Δ𝑆⁰ represents that the degrees of freedom increase at the solid-liquid interface during electrical removal [33]. Physisorption and chemisorption can be classified, to a certain extent, by the magnitude of enthalpy changeΔ𝐻⁰. Δ𝐻⁰of <40 kJ/mol are typically considered as those of physisorption bonds [34, 35]. Generally, Δ𝐺⁰ for physisorption is less than that for chemisorption. The former is between −20 and 0 kJ/mol and the latter is between −80 and −400 kJ/mol [36]. Therefore, both of Δ𝐻⁰ and Δ𝐺⁰ suggest that electrical removal of CuCl₂ onto CNT-CNF is mainly driven by a physisorption process although the existence of electrodeposition reaction.

Table 4 

The free energy changeΔ𝐺⁰, enthalpy change Δ𝐻⁰ and entropy change Δ𝑆⁰ for electrical removal of CNT-CNF film electrodes in CuCl₂ solutions.

(a)

(b)

Figure 5 

(a) Linear plot of ln𝑘 versus 1/𝑇 and (b) van’t Hoff plot for the electrical removal of CuCl2 onto CNT-CNF film electrodes.

The BET specific surface area 𝑆_𝑝 of CNT-CNF is 210 m²/g. 𝐴_𝑚 is calculated based on the hydrated Cu²⁺ ion radius (6×10⁻¹⁰ m). Therefore, the monolayer coverage area of the CNT-CNF surface by Cu²⁺ at 280, 290, and 300 K, calculated from (8) are 5.6%, 13%, and 21.7%, respectively. The surface coverage of CNT-CNF by hydrated Cu²⁺ ions increases with the increase of temperature, indicating that more Cu²⁺ ions are removed at high temperature.

4. Conclusion

The CNT-CNF electrodes were fabricated using LPCVD method and their electrical removal behaviors including kinetics, thermodynamics, and isotherms in CuCl₂ solution were studied. The result shows that (i) the electrical removal follows Langmuir isotherm, indicating monolayer adsorption; (ii) higher voltage, lower initial Cu²⁺ concentration and higher solution temperature can enhance Cu²⁺ removal and facilitate rate constant; (iii) electrical removal of CuCl₂ onto CNT-CNF can be mainly ascribed to physisorption process.

Acknowledgments

This paper was supported by Special Project for Nanotechnology of Shanghai (no. 1052nm02700) and the Scientific Research Foundation for the Returned Overseas Chinese Scholars.