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

Authors: Yankun Zhan; Chunyang Nie; Likun Pan; Haibo Li; Zhuo Sun
Journal: International Journal of Electrochemistry
 (2011)

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## 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.


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## Body

## 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
(1)ln⁡[Ct-CeC0-Ce]=-kt,
where k (min−1) is the reaction rate constant, C0, Ce, and Ct (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 constantk is described by the Arrhenius equation [16, 17]:
(2)ln⁡k=ln⁡A-EaRT,
where A is the pre-exponential factor, R is the gas constant (8.314 Jmol−1K−1), and Ea is the activation energy. Ea 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]
(3)qe=qmKLCe1+KLCe,(4)qe=KFCe1/n,
where qe (mg/g) is the amount of adsorbed CuCl2 per unit mass of CNT-CNF film and qm (mg/g) is the maximum removal capacity corresponding to complete monolayer coverage. KL and KF are Langmuir constant that relates to the affinity of binding sites and Freundlich constant, respectively. qe is calculated from the following equation:
(5)qe=(C0-Ce)Vm,
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 changeΔG0 (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:
(6)ΔG0=-RTlnKL,ΔS0=ΔH0-ΔG0T,ln⁡KL=ΔS0R-ΔH0RT,
where T(K) is absolute temperature. ΔH0 and  ΔS0 are determined from the slope and intercept of the van’t Hoff plots of  lnKL versus 1/T [21, 22].


#### 2.3.4. Cu2+ Removal and Surface Coverage

The Cu2+ removal is defined as follows:
(7)Cu2+Removal(%)=C0-CeC0×100%.The surface coverage, defined as the ratio of the Cu2+ coverage per unit mass of CNT-CNF film (Sc, m2/g) to the BET surface area of CNT-CNF film (Sp, m2/g), is helpful in understanding the interaction between adsorbates and the surfaces of adsorbents and can be calculated as follows [23]:
(8)ScSp(%)=NAAmqm1000SpMw×100%,
where Am (m2) is the maximum cross-section area of adsorbate molecule or ion. Mw is the molecular weight of adsorbate.


## 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
(1)ln⁡[Ct-CeC0-Ce]=-kt,
where k (min−1) is the reaction rate constant, C0, Ce, and Ct (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 constantk is described by the Arrhenius equation [16, 17]:
(2)ln⁡k=ln⁡A-EaRT,
where A is the pre-exponential factor, R is the gas constant (8.314 Jmol−1K−1), and Ea is the activation energy. Ea 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]
(3)qe=qmKLCe1+KLCe,(4)qe=KFCe1/n,
where qe (mg/g) is the amount of adsorbed CuCl2 per unit mass of CNT-CNF film and qm (mg/g) is the maximum removal capacity corresponding to complete monolayer coverage. KL and KF are Langmuir constant that relates to the affinity of binding sites and Freundlich constant, respectively. qe is calculated from the following equation:
(5)qe=(C0-Ce)Vm,
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 changeΔG0 (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:
(6)ΔG0=-RTlnKL,ΔS0=ΔH0-ΔG0T,ln⁡KL=ΔS0R-ΔH0RT,
where T(K) is absolute temperature. ΔH0 and  ΔS0 are determined from the slope and intercept of the van’t Hoff plots of  lnKL versus 1/T [21, 22].


### 2.3.4. Cu2+ Removal and Surface Coverage

The Cu2+ removal is defined as follows:
(7)Cu2+Removal(%)=C0-CeC0×100%.The surface coverage, defined as the ratio of the Cu2+ coverage per unit mass of CNT-CNF film (Sc, m2/g) to the BET surface area of CNT-CNF film (Sp, m2/g), is helpful in understanding the interaction between adsorbates and the surfaces of adsorbents and can be calculated as follows [23]:
(8)ScSp(%)=NAAmqm1000SpMw×100%,
where Am (m2) is the maximum cross-section area of adsorbate molecule or ion. Mw is the molecular weight of adsorbate.


## 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
(1)ln⁡[Ct-CeC0-Ce]=-kt,
where k (min−1) is the reaction rate constant, C0, Ce, and Ct (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 constantk is described by the Arrhenius equation [16, 17]:
(2)ln⁡k=ln⁡A-EaRT,
where A is the pre-exponential factor, R is the gas constant (8.314 Jmol−1K−1), and Ea is the activation energy. Ea 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]
(3)qe=qmKLCe1+KLCe,(4)qe=KFCe1/n,
where qe (mg/g) is the amount of adsorbed CuCl2 per unit mass of CNT-CNF film and qm (mg/g) is the maximum removal capacity corresponding to complete monolayer coverage. KL and KF are Langmuir constant that relates to the affinity of binding sites and Freundlich constant, respectively. qe is calculated from the following equation:
(5)qe=(C0-Ce)Vm,
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 changeΔG0 (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:
(6)ΔG0=-RTlnKL,ΔS0=ΔH0-ΔG0T,ln⁡KL=ΔS0R-ΔH0RT,
where T(K) is absolute temperature. ΔH0 and  ΔS0 are determined from the slope and intercept of the van’t Hoff plots of  lnKL versus 1/T [21, 22].


## 2.3.4. Cu2+ Removal and Surface Coverage

The Cu2+ removal is defined as follows:
(7)Cu2+Removal(%)=C0-CeC0×100%.The surface coverage, defined as the ratio of the Cu2+ coverage per unit mass of CNT-CNF film (Sc, m2/g) to the BET surface area of CNT-CNF film (Sp, m2/g), is helpful in understanding the interaction between adsorbates and the surfaces of adsorbents and can be calculated as follows [23]:
(8)ScSp(%)=NAAmqm1000SpMw×100%,
where Am (m2) is the maximum cross-section area of adsorbate molecule or ion. Mw is the molecular weight of adsorbate.


## 3. Results and Discussion

Figures2(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) FESEM images of CNT-CNF film, (b) CNFs, and (c) CNTs.
(a)(b)(c)Figures3(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.
Voltage (V)01.01.21.62.0Removal (%)1063728490k (min−1)0.02850.05540.070.10220.1055r20.95010.99540.98550.99260.9967(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.
(a)(b)(c)(d)(e)(f)Figures3(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.
Initial solution conductivity (μS/cm)Temperature (K)25100200400280290300Removal (%)77706458354270k (min−1)0.06240.06090.05240.04350.04570.05250.0626r20.99670.99520.96940.97730.99180.98300.9954Figures3(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 (25–400μs/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, KL, and KF 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 (qe is the amount of adsorbed CuCl2 per unit mass of CNT-CNF film; qm is the maximum removal capacity corresponding to complete monolayer coverage; KL and KF are Langmuir constant and Freundlich constant; r2 is correlation coefficients).
IsothermModel equationParameter280 K290 K300 KLangmuirqe=(qmKLCe)/(1+KLCe)qm (mg/g)2.305.388.96KL (L/mg)0.03450.03260.0305r20.99630.98520.9918Freundlichqe=KFCe1/nKF0.38480.340.93091/n0.31220.44150.4172r20.95840.97280.9483Equilibrium isotherms of CNT-CNF film electrodes in CuCl2 solution at (a) 280 K, (b) 290 K and (c) 300 K.
(a)(b)(c)The activation energyEa 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.
T(K)ΔG0 (KJ/mol)ΔH0 (KJ/mol)ΔS0 (J/molK)280−19.80−4.77453.27290−20.30300−20.76(a) Linear plot oflnk versus 1/T and (b) van’t Hoff plot for the electrical removal of CuCl2 onto CNT-CNF film electrodes.
(a)(b)The BET specific surface areaSp of CNT-CNF is 210 m2/g. Am 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.



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*Source: 572862-2011-04-18.xml*