Abstract

In this work, the study of copper particles deposition on to carbon felt was presented by pulse electrodeposition method to electrochemically degrade methyl iodide (CH3I, 1 mg L−1) in aqueous solution. In order to solve the problems linked to the heterogeneous potential distribution in the 3-D porous structure, which lead to the so-called ‘black core’, we successfully used low concentration of copper salt (1 mM) and negative deposition potential (−2.5 V) to obtain Cu-nanoparticles/carbon felt (Cu-nano/CF) electrode, the copper coating improved the specific surface area of carbon felt from ∼0.07 to 0.7 m2 g−1 with high catalytic activity. Results show that 98.1% of CH3I can be removed with the Cu-nano/CF electrode in 120 min.

INTRODUCTION

Prospects for the development of nuclear power engineering are tightly associated with the solution of such important problems as safe operation of nuclear power plants (NPPs) and nuclear waste release. When an accident occurs in a nuclear power plant such as the Fukushima Daiichi nuclear power plant accident in Japan (Masamichi et al. 2011), the release of radioactive waste will cause serious harm to the environment and nearby residents, where the radioactive iodine-131 (131I) is greatly concerned (Bukrinskii & Fedulov 1991). Radioactive iodine released from NPPs can be classified into inorganic iodine and organic iodine; the former is mainly iodine-129 (129I2), and the latter is mainly methyl iodide (CH3131I) (Kepák 1990). Because of its high volatility and relatively low reaction activity, CH3131I is more difficult to be captured and removed than other radioactive iodine (Bučko et al. 2017). In recent years, the studies on the treatment of radioactive CH3131I focused on activated carbon adsorption (González-García et al. 2011; Chun et al. 2016) and chemical absorption (Motonari et al. 2012) in NPPs. Activated carbon adsorption has a good effect on treating radioactive CH3I, but there are some problems, such as the adsorbent deactivation, the difficulty of regeneration, the high waste yield, and secondary pollution. Chemical absorption has a slower reaction rate and poor removal efficiency.

CH3I is a halogenated hydrocarbon that is prone to electrochemical reduction due to its strong electron-withdrawing group (Xuan et al. 2013). Therefore, electrochemical reduction of CH3131I is a promising method, which is a new type of green degradation technology that utilizes electrochemical reduction to dehalogenate organic pollutants. It is worth noting that electrocatalytic hydrogenolysis (ECH)), as an electrochemical reduction method, was widely applied because of its advantages such as simple operation, high selectivity, mild reaction conditions, efficient treatment of halogenated organic pollutants, and good compatibility with the environment (Rondinini et al. 2001). The general scheme of ECH cathodic reduction of organic halides can be presented as follows: 
formula
(1)
 
formula
(2)
 
formula
(3)
 
formula
(4)
 
formula
(5)
 
formula
(6)
where M is metal surface, X is halogen and (H)adsM is the reactive hydrogen atom ([H]) produced on a metal surface. According to this scheme, the major reduction pathways are substitution of hydrogen for halogen (Equation (3)). Meanwhile, there is hydrogen evolution reaction (HER) (Equations (5) and (6)), which generates a large amount of hydrogen gas on the surface of the electrode to weaken the reduction of organic halides. Generally, suitable electrode materials and catalysts are the key to solving this problem. Precious metals like Ag, Pd and Pt etc. (Criddle & McCarty 1991; Isse et al. 2006) exhibit significant electrocatalytic activity in the electrochemical reduction of organic halides. However, considering economic costs, we must develop an economical and very effective catalyst.

In recent years, copper has shown attractive application prospects in the electrochemical reduction of organic halides. Isse et al. (2008) studied the reductive cleavage of a series of organic chlorides on Pt, Ag and Cu electrodes by cyclic voltammetry (CV). Results show that Ag, Cu and Pd electrodes have extraordinary electrocatalytic properties towards the reductive cleavage of carbon–chlorine bonds. Tomilov (2001) studied the electrochemical reduction of butyl iodide with different metal materials in organic solvents. It was found that copper cathode shows a high electrocatalytic activity, and the yield of octane based on butyl iodide was 83–84%. Cheng et al. (1997) studied electrochemical degradation of 4-chlorophenol on different electrode surfaces, including a palladized carbon cloth electrode and a palladized graphite cathode. The reactions on the palladized carbon cloth and graphite depend on the adsorption of the chlorinated organic compound on the carbon surface and the reaction with hydrogen at the palladium/carbon interface. For electrochemical reaction, surface morphological structures of electrodes have a great influence on the reaction process. It is well known that carbon fiber materials such as carbon felt (Pimentel et al. 2008) has a three-dimensional (3-D) network structure, and the real surface area is much larger than the geometric surface area, which can provide an advantageous environment for the adsorption of CH3131I on the electrode surface. In our work, we found that the catalytic performance of Cu/CF electrode is more significant than Cu electrode by comparing the deiodination effect of them on CH3I, we chose carbon felt as the electrode material and copper particles deposition on to carbon felt to electrochemically degrade CH3I (1 mg L−1) in aqueous solution. The success of this study provides an efficient approach to remove the radioactive CH3131I released from NPPs.

EXPERIMENTAL

Reagents

Methyl iodide standard (99.5% purity) was purchased from Saan Chemical Technology (Shanghai, China). Copper(II) sulfate pentahydrate (CuSO4•5H2O, analytical reagent), sulfuric acid (H2SO4, analytical reagent), sodium hydroxide (NaOH, analytical reagent) and boracic acid (H3BO3, analytical reagent) were purchased from Sinopharm Chemical Reagent Co., Ltd. Sodium sulfate (Na2SO4, analytical reagent), sodium citrate tribasic dihydrate (Na3C6H5O7•2H2O, ≥99.0%), potassium iodide (KI, analytical reagent), acetone (analytical reagents) and ethanol (analytical reagents), were purchased from Shanghai Titan Scientific Co., Ltd. All solutions were prepared using deionized water with a resistance of 18.25 MΩ cm obtained from a Millipore-Q system (Millipore, USA).

Pretreating of CF substrate

CF obtained from Jinglong Special Carbon Technology Co., Ltd (Beijing, China) with 15 mm × 20 mm × 2 mm dimensions was used as substrate. In the pretreatment procedure, CF was etched in nitric acid solution (1.0 M, 80 °C) stirring for 50 min, sonicated in an ultrasonic bath for 30 min to remove some dirt or oily substances, rinsed with deionized water, dried at 110 °C in an oven and kept in a desiccator before used.

Cu loading over CF substrate

Pulse electrodeposition experiments were carried out in potentiostatic mode with three-electrode system. CF substrate was served as working electrode (WE), a large area Pt grid as counter electrode (CE), and saturated calomel electrode (SCE) as reference electrode (RE). All potentials reported in the electrochemical measurements are quoted versus SCE. The plating solution contained 20 mM KNO3, CuSO4•5H2O (1–10 mM), and Na3C6H5O7•2H2O (1–6 mM). The pH (2–6) was adjusted with 0.5 M H2SO4 solution. The reaction temperature (25–55 °C) was controlled by the thermostatic magnetic stirrer (HCJ-4D, Langyue Instrument Manufacturing, Changzhou, China). The deposition potential (−1.0 V to −3.0 V) was controlled by a potentiostat equipped with a signal detector (ZF-9, Zhengfang Electronic Appliance, Shanghai, China). Pulse potential signal was applied between the electrodes, with on-pulse time of 20 s and off-pulse time of 200 s in one cycle. The full electrodeposition time (t) was 100–400 s without counting the off-pulse time.

Characterization of electrodes

X-ray diffraction (XRD) measurements were carried out with a ItalStructures powder diffractometer (Italy), using a focused and monochromatized Cu Kα source for Pt and PtRu samples supported on KJB (40 kV/100 mA).

The scanning electron microscope (SEM) micrographs were obtained with a Jeol 6301F (15 kV) microscope.

Brunauer–Emmett–Teller (BET) measurements were performed by N2 adsorption–desorption on a Micromeritics FlowSorb II 2300 apparatus using a 30% mixture of nitrogen in helium flowing gas, after 30 min outgassing at 250 °C.

CV was detected by an electrochemical workstation (CHI760e, CH Instruments, Shanghai, China). The CV measurements were conducted in a 150 mL glass cell with a Pt grid as counter electrode and a saturated calomel (SCE) reference electrode. The WE was the Cu-nano/CF electrode. The electrolytic solution was 100 mL solution with or without 100 mg L−1 CH3I +0.2 M Na2SO4.

Electrolysis of CH3I with constant current

The electrolysis experiment was performed in a two-compartment cell (Figure 1) separated by a cation exchange membrane (CMI-7000, 13 Ω cm2, DuPont, USA), in which a CF electrodeposited electrode (15 mm × 20 mm) was used as cathode and graphite sheet (15 mm × 20 mm, ≥99.9%, Jusheng Graphite, Foshan, China) as anode. Sodium sulfate solution (0.5 M, pH = 4.5) was used as supporting electrolyte and added to the cathodic chamber (150 mL) and anodic chamber (150 mL), respectively. The cell was sealed immediately after adding the CH3I reserve solution (200 mg L−1, 0.754 mL) to the cathodic chamber. A MAISHENG MS302D power supply (Guangzhou Xinchuang Instrument, Guangzhou, China) was used in the electrolysis experiment. All electrolysis experiments were carried out at constant current (2.5 mA cm−2) and under stirring, and the reaction temperature was controlled by the thermostatic magnetic stirrer. The concentrations of iodide ion, CH3I and intermediates were monitored during the electrolysis.

Figure 1

Two-compartment cell: (1) sampling port, (2) gas collection, (3) cathode, (4) anode, (5) cation exchange membrane, (6) magnetic stirring bar, (7) power supply.

Figure 1

Two-compartment cell: (1) sampling port, (2) gas collection, (3) cathode, (4) anode, (5) cation exchange membrane, (6) magnetic stirring bar, (7) power supply.

Analysis methods

The concentration of CH3I was analyzed using an Agilent 7890B Gas Chromatograph (GC) equipped with a micro-electron capture detector (μ-ECD). We extracted the CH3I in solution and gas phase with n-hexane firstly, and then detected their concentration with the GC, separately. A DB-VRX capillary column (30.0 m × 250 μm × 1.4 μm, J&W Scientific, Folsom, CA, USA) was used under following conditions: high purity nitrogen was used as the carrier gas at a flow rate of 1.0 mL min−1; the inlet and detector temperatures were 220 and 280 °C; and the oven was held at 100 °C and kept for 6 min. Under these conditions, the retention time of CH3I was 3.6 min. The electrolysis product (CH4) in the gas phase was analyzed on an HP 6890 system GC, equipped with a flame ionization detector (FID). The gas sample was withdrawn with a gas syringe. An HP Plot Q capillary column (30.0 m × 0.537 mm × 0.40 mm, Agilent Technologies, USA) was used under the following conditions: the flow rate of carrier gas was at 4 mL min−1; the inlet and detector temperatures were 200 and 250 °C; and the oven was held at 80 °C for 2 min, then increased to 240 °C at a rate of 25 °C min−1 and held for 2 min. The concentration of iodide ion was detected by a UV-visible spectrophotometer (L5, Inesa analytical instrument, Shanghai, China) at wavelength of 226 nm. Iodide ion standard curve was obtained using standard solutions of 0.001 mg L−1–1.5 mg L−1 KI in the supporting electrolyte (0.5 M Na2SO4, pH = 4.5) used in the electrolysis. Values of experimental results represent the mean ± SD carried out independent experiments performed in triplicate. Degradation efficiency of CH3I was calculated according to the Equation (7): 
formula
(7)
where R is the degradation efficiency of CH3I (mg L−1), C0 is the initial concentration (mg L−1), Ct is the residual concentration of CH3I in solution (mg L−1), and Cg is the concentration of CH3I in Gas-phase (mg L−1). According to the experimental results we have done, we found the concentration of Cg/C0 at 20 °C (experimental temperature in electrolysis) was very small (∼0.9%), the degradation efficiency can be simplified as: 
formula

RESULTS AND DISCUSSION

XRD pattern of CF electrode

Figure 2 shows the XRD pattern of a CF electrodeposited electrode (Figure 2(a)) and a CF sample (insets of Figure 2(b)). We can see that the Cu-nano/CF electrode presents characteristic peaks on Cu (1, 1, 1) and Cu (2, 0, 0) crystal planes (Standard PDF number: 4-836) by comparing with XRD of CF samples, and the 2θ angles are 43.297° and 50.433°, respectively. It indicates that the metal copper has been successfully loaded on the CF electrode by the pulse electrodeposition method.

Figure 2

XRD patterns of carbon felt: CF electrodeposited electrode (a); CF sample (b).

Figure 2

XRD patterns of carbon felt: CF electrodeposited electrode (a); CF sample (b).

SEM micrographs of electrodes

Figure 3 shows the SEM micrographs of CF electrodeposited electrodes under different plating conditions. As it is seen from Figure 3(a), CF sample contains fibers with average 20 μm diameters, and the fiber surface becomes rough and textured after pretreatment. The carbon felt pretreatment is mainly to increase its surface areas and enhance the hydrophilicity (Zhong et al. 2012). Figure 3(g) shows the micrograph of Cu-nano/CF sample. It is clear from Figure 3(g) that a plated layer is formed over the fiber surface. From the results of XRD, we have known that plated layer is Cu particles loaded onto the fiber surface. Figure 3(i) and 3(j) show the morphology of the coating on the outer and inner fiber, respectively. The outer fiber has many more particles than the inner fiber, and the particle size is larger, which leads to the ‘black core’. These SEM micrographs allow us to determine the appropriate plating condition, which we will discuss below.

Figure 3

SEM micrographs of CF electrodeposited material. (a): CF sample; (b): [Cu2+] = 5 mM, [C6H5O73−] = 6 mM, [KNO3] = 20 mM, pH = 3.0, T = 25 °C, t = 400 s, φ = −1.0 V; (c): [Cu2+] = 5 mM, [C6H5O73−] = 6 mM, [KNO3] = 20 mM, pH = 3.0, T = 25 °C, t = 400 s, φ = −3.0 V; (d): [Cu2+] = 5 mM, [C6H5O73−] = 6 mM, [KNO3] = 20 mM, pH = 3.0, T = 25 °C, t = 100 s, φ = −2.5 V; (e): [Cu2+] = 1 mM, [C6H5O73−] = 4 mM, [KNO3] = 20 mM, pH = 3.0, T = 25 °C, t = 300 s, φ = −2.5 V; (f): [Cu2+] = 1 mM, [C6H5O73−] = 4 mM, [KNO3] = 20 mM, pH = 5.0, T = 45 °C, t = 400 s, φ = −2.5 V; (g): [Cu2+] = 1 mM, [C6H5O73−] = 4 mM, [KNO3] = 20 mM, pH = 5.0, T = 45 °C, t = 300 s, φ = −2.5 V; (h): overall morphology of (g); (i): [Cu2+] = 5 mM, [C6H5O73−] = 6 mM, [KNO3] = 20 mM, pH = 3.0, T = 45 °C, t = 300 s, φ = −2.5 V (outer fiber); (j): [Cu2+] = 5 mM, [C6H5O73−] = 6 mM, [KNO3] = 20 mM, pH = 3.0, T = 45 °C, t = 300 s, φ = −2.5 V (inner fiber).

Figure 3

SEM micrographs of CF electrodeposited material. (a): CF sample; (b): [Cu2+] = 5 mM, [C6H5O73−] = 6 mM, [KNO3] = 20 mM, pH = 3.0, T = 25 °C, t = 400 s, φ = −1.0 V; (c): [Cu2+] = 5 mM, [C6H5O73−] = 6 mM, [KNO3] = 20 mM, pH = 3.0, T = 25 °C, t = 400 s, φ = −3.0 V; (d): [Cu2+] = 5 mM, [C6H5O73−] = 6 mM, [KNO3] = 20 mM, pH = 3.0, T = 25 °C, t = 100 s, φ = −2.5 V; (e): [Cu2+] = 1 mM, [C6H5O73−] = 4 mM, [KNO3] = 20 mM, pH = 3.0, T = 25 °C, t = 300 s, φ = −2.5 V; (f): [Cu2+] = 1 mM, [C6H5O73−] = 4 mM, [KNO3] = 20 mM, pH = 5.0, T = 45 °C, t = 400 s, φ = −2.5 V; (g): [Cu2+] = 1 mM, [C6H5O73−] = 4 mM, [KNO3] = 20 mM, pH = 5.0, T = 45 °C, t = 300 s, φ = −2.5 V; (h): overall morphology of (g); (i): [Cu2+] = 5 mM, [C6H5O73−] = 6 mM, [KNO3] = 20 mM, pH = 3.0, T = 45 °C, t = 300 s, φ = −2.5 V (outer fiber); (j): [Cu2+] = 5 mM, [C6H5O73−] = 6 mM, [KNO3] = 20 mM, pH = 3.0, T = 45 °C, t = 300 s, φ = −2.5 V (inner fiber).

Effects of plating conditions on electrolysis of methyl iodide

We know that surface morphological structures of electrodes have a great influence on electrochemical reaction. It can be seen from the SEM micrographs that the morphology of the plating under different plating conditions varies considerably. We used the Cu/CF electrodes preformed under different electrodeposition conditions as a cathode to carry out constant current electrolysis of CH3I, respectively. And we evaluated the catalytic performance of the electrode by the degradation efficiency of CH3I with current density 2.5 mA cm−2, electrolysis time 120 min, pH = 4.5, supporting electrolyte 0.5 M Na2SO4.

Deposition potential (φ)

Deposition potential has a close relationship with the overpotential, which directly affects the growth of copper crystals on the carbon fiber in electrodeposition (Yang et al. 2013). We set the deposition potential between −1.0 and −3.0 V (), which not only satisfies the reduction of Cu2+, but also provides a higher current density. In this study, we chose −1.0, −1.5, −2.5 and −3.0 V as the deposition potential, respectively. Figure 4(a) shows the effect of deposition potential on the catalytic performance of the electrodeposited electrode. When the deposition potential was negatively shifted from −1.0 V to −1.5 V, the degradation efficiency was significantly increased. It can be seen from Figure 3(b) that almost no copper particle deposited on the surface of the carbon fiber at the deposition potential of −1.0 V. It is possible that there was almost no copper particle on carbon fiber, which causes the electrodeposited electrode to have no catalytic effect. When the deposition potential was −3.0 V, the degradation efficiency was lower than that at −2.5 V, which indicates that the deposition potential negative shift accelerated the copper particles to deposit on the carbon fiber, but if the deposition potential is too negative, the morphology of the copper particles would be changed. As shown in Figure 3(c), when the deposition potential was −3.0 V, the copper particles were unevenly distributed on the carbon fibers. According to the degradation efficiency of CH3I and the SEM micrographs, we chose −2.5 V as the optimal deposition potential.

Figure 4

Degradation effect of Cu-nano/CF electrodes obtained by electrodeposition on CH3I. Electrodeposition conditions: [Cu2+] = 5 mM, [C6H5O73−] = 6 mM, [KNO3] = 20 mM, pH = 3.0, T = 25 °C, t = 400 s (a); [Cu2+] = 5 mM, [C6H5O73−] = 6 mM, [KNO3] = 20 mM, pH = 3.0, T = 25 °C, φ = −2.5 V (b); [C6H5O73−] = 6 mM, [KNO3] = 20 mM, pH = 3.0, T = 25 °C, φ = −2.5 V, t = 300 s (c); [Cu2+] = 1 mM, [KNO3] = 20 mM, pH = 3.0, T = 25 °C, φ = −2.5 V, t = 300 s (d); [Cu2+] = 1 mM, [C6H5O73−] = 4 mM, [KNO3] = 20 mM, pH = 3.0, T = 25 °C, φ = −2.5 V, t = 300 s (e); [Cu2+] = 1 mM, [C6H5O73−] = 4 mM, [KNO3] = 20 mM, T = 45 °C, φ = −2.5 V, t = 300 s (f).

Figure 4

Degradation effect of Cu-nano/CF electrodes obtained by electrodeposition on CH3I. Electrodeposition conditions: [Cu2+] = 5 mM, [C6H5O73−] = 6 mM, [KNO3] = 20 mM, pH = 3.0, T = 25 °C, t = 400 s (a); [Cu2+] = 5 mM, [C6H5O73−] = 6 mM, [KNO3] = 20 mM, pH = 3.0, T = 25 °C, φ = −2.5 V (b); [C6H5O73−] = 6 mM, [KNO3] = 20 mM, pH = 3.0, T = 25 °C, φ = −2.5 V, t = 300 s (c); [Cu2+] = 1 mM, [KNO3] = 20 mM, pH = 3.0, T = 25 °C, φ = −2.5 V, t = 300 s (d); [Cu2+] = 1 mM, [C6H5O73−] = 4 mM, [KNO3] = 20 mM, pH = 3.0, T = 25 °C, φ = −2.5 V, t = 300 s (e); [Cu2+] = 1 mM, [C6H5O73−] = 4 mM, [KNO3] = 20 mM, T = 45 °C, φ = −2.5 V, t = 300 s (f).

Electrodeposition time (t)

Figure 4(b) shows the effect of electrodeposition time (without counting the relaxation times) on the catalytic performance of the electrodeposited electrode. The result shows that the deposition time has a great influence on the catalytic performance of the CF electrodeposited material. When the deposition time was short (t = 100 s), few copper particles deposited on the surface of the carbon fiber, which could not provide enough active sites to CH3I, resulting in poor catalytic performance (Figure 3(d)). However, long deposition time could cause the copper particles clustering on the outer fibers (Figure 3(f)) and unplating of the inner fibers (‘black core’) due to plated fibers having high electrical conductivity compared with the unplated ones. Therefore, the plated fibers act as a protective screen to the inner fibers (Wan et al. 1997). Compared with the blank group (t = 0 s), in which 51.5% of CH3I was degraded in 120 min; when the deposition time was 300 s, the catalytic performance of the copper plating layer was significantly improved, and 83.7% of CH3I was degraded in 120 min.

Concentration of Cu2+

Figure 4(c) shows the effect of concentration of Cu2+ on the catalytic performance of the electrodeposited electrode. As the concentration of Cu2+ increased, the degradation efficiency of the electrode on CH3I decreased. When the concentration of Cu2+ was 1 mM, the electrode showed the remarkable catalytic performance, in which 92.5% of CH3I was degraded in 120 min. The diffusion rate of Cu2+ is related to its concentration; increasing the concentration of Cu2+ can enhance the current density, but reduce the dispersing ability of Cu2+, the crystal nucleus grows faster and the nucleation is slow, which is not conducive to form the nanoparticles (Huang et al. 2015). For example, when the concentration of Cu2+ was too high (e.g. 5 mM), the Cu particles size reaches 2 μm (Figure 3(c)). At low concentration (e.g. 1 mM), Cu particles are sized as about 0.1 μm and evenly loaded on the carbon fibers (Figure 3(g)).

Concentration of citrate

Citrate has a strong complexing ability with Cu2+ to form the complex of Cu(II)-citrate. As shown in Figure 4(d), as the concentration of citrate increased, the degradation efficiency of the electrode on CH3I also increased. And when the concentration of citrate was 4 mM, the electrode showed the remarkable catalytic performance. Liu et al. (2014) discussed effects of citrate concentration on electrochemical reduction process of Co(II). The Co(II)-citrate complex became harder to be reduced with the improving of citrate concentration, and the reduction potential of the complex is more negative than that of the hydrogen ion. In this study, excess citrate ions in the solution adsorb on the electrode surface, which hinders the reduction of copper citrate (Chassaing et al. 1986). On the other hand, if the citrate concentration is too low, excess Cu2+ would preferentially be deposited on the external fibers because it is easier to be reduced than Cu(II)-citrate. It can be seen from Figure 3(e) that the coating is loose and porous, and the particles are fine.

Electrodeposition temperature (T)

We studied the effect of electrodeposition temperature on the catalytic performance of the electrodeposited electrode (Figure 4(e)). It can be seen that increasing the temperature can improve the catalytic performance of the electrode, and the catalytic performance was the highest when electrodeposition temperature was 45 °C. However, as the temperature continues to rise, the electrode performance decreases. This is probably because temperature has a great impact on diffusion rate (Akhmadeev 1974). Increasing the temperature accelerated the complex of [Cu2+]-[C6H5O73−] diffusion into the inner fibers, which was conducive to load the copper particles on carbon fiber. But when the temperature is too high (e.g. 55 °C), the quickly deposition rate led to rapid growth of the protruding point, plated fibers have high electrical conductivity compared with the unplated ones, which is not conducive to the uniform deposition of copper particles (Canava & Lincot 2000).

Initial pH

In this study, we adjusted the pH of the plating solution between 2.0 and 6.0 to avoid forming hydroxide precipitation of copper(II) near the cathode. Figure 4(f) shows the effect of initial pH on the catalytic performance of the electrodeposited electrode. Results showed that the degradation efficiency of CH3I was higher at initial pH of 5.0 and 6.0, compared to other pH values, suggesting weakly acidic plating solution is more conducive to form a plating with good catalytic performance. When the Cu-nano/CF electrode prepared under the condition of pH 5 was used as the cathode, 98.1% of CH3I was degraded in 120 min. It is worth noting that the degradation efficiency tended to stabilize in about 90 min, suggesting the nano-copper particles plating on carbon fibers significantly improve the catalytic performance of the electrode. It can be seen from the SEM micrograph (Figure 3(g)) that the nano-copper particles have a regular spherical shape with an average of 100 nm diameters.

Cu(II) is very stable when it exists in the form of a complex (Sigel 1975) and the HER dominated if the plating solution is strongly acidic. Wan et al. (1997) found that HER (2H+2e=H2) generate the gas (H2), which is helpful for the Cu2+ to enter the central fibers of the felt. Our experimental results also validate this viewpoint. Figure 3(h) shows the overall morphology of the Cu-nano/CF electrodeposited electrode at pH 5 of the plating solution; we can see that the inner fiber also has a dense nano-copper particles coating.

Based on the above results, we used the analysis of variance to judge the significance of the differences for different electrodeposition conditions. The analysis results are shown in Table 1.

Table 1

Analysis of variance of different electrodeposition conditions

Factor F-value F crit p-value Difference 
deposition potential (V) 34.7 4.76 0.000347 extremely significant 
deposition time (s) 27.5 4.76 0.000667 extremely significant 
Cu2+ (mM) 31.6 4.76 0.000451 extremely significant 
C6H5O73− (mM) 26.5 4.76 0.000734 extremely significant 
deposition temperature (°C) 6.05 4.76 0.0303 significant 
pH 32.9 4.76 0.000401 extremely significant 
Factor F-value F crit p-value Difference 
deposition potential (V) 34.7 4.76 0.000347 extremely significant 
deposition time (s) 27.5 4.76 0.000667 extremely significant 
Cu2+ (mM) 31.6 4.76 0.000451 extremely significant 
C6H5O73− (mM) 26.5 4.76 0.000734 extremely significant 
deposition temperature (°C) 6.05 4.76 0.0303 significant 
pH 32.9 4.76 0.000401 extremely significant 

As expected, the BET surface area of the Cu-nano/CF obtained by this electrodeposition process (0.737 m2 g−1) was higher than the surface area of the CF sample (0.0753 m2 g−1).

Electrocatalytic reduction pathways of Cu-nano/CF electrode for CH3I

Figure 5(a) shows the representative CV curves obtained using a Cu-nano/CF electrode and a CF sample electrode as cathode, respectively, in a 0.5 M Na2SO4 solution at pH 5 containing 100 mg L−1 CH3I. The Cu-nano/CF electrode appeared a reduction peak at −1.122 V, which could be attributed to the electron transfer from electrode to CH3I. However, the CF electrode did not appear a reduction peak of CH3I in the scanning range. The results show that the CF sample electrode has no catalytic effect on the degradation of CH3I, while the Cu-nano/CF electrode has significant catalytic performance. Figure 5(b) shows the relationship between the peak current (Ip) and the scan rate (V = 0.01–0.05 V/s) of the Cu-nano/CF electrode in a Na2SO4 solution containing 100 mg L−1 CH3I, which could explain the process of electrochemical reduction of CH3I by Cu-nano/carbon felt electrode further. When the scan rate (V) was increased from 0.01 V/s to 0.05 V/s, the peak potentials shifted negatively, and the reduction peak intensity increased. This means that electrochemical reduction of CH3I is an irreversible redox reaction. The cathodic peak current has a linear relationship to the V1/2 (insets of Figure 5(b)), the equation can be simplified to: Ip=kV1/2, where k (the fitting line slopes of Ip vs. V1/2, k = 0.3155) is related to the mass transfer rate. Thus, the electrochemical reduction of CH3I reaction is a diffusion-controlled process (Song et al. 2017).

Figure 5

CV curves of the Cu-nano/CF electrode and the CF sample electrode in 0.5 M Na2SO4 solution containing 100 mg L−1 CH3I (a); CV curves of the Cu-nano/CF electrode with different scan rate in 0.5 M Na2SO4 solution containing 100 mg L−1 (b); mass balance analysis (c), (electrolytic conditions: initial concentration of CH3I was 1 mg L−1, current density was 2.5 mA cm−2, pH was 5.5).

Figure 5

CV curves of the Cu-nano/CF electrode and the CF sample electrode in 0.5 M Na2SO4 solution containing 100 mg L−1 CH3I (a); CV curves of the Cu-nano/CF electrode with different scan rate in 0.5 M Na2SO4 solution containing 100 mg L−1 (b); mass balance analysis (c), (electrolytic conditions: initial concentration of CH3I was 1 mg L−1, current density was 2.5 mA cm−2, pH was 5.5).

In order to study and understand the electrode electrochemical reduction of CH3I and the mechanism of dehalogenation, we performed mass balance analysis with quantitative determination of CH3I, CH4, and I (Figure 5(c)). The concentration of CH3I gradually decreased with the electrolysis time, while the concentrations of CH4 and I gradually increased. They eventually decreased to 0.037 mg L−1, 0.107 mg L−1 and 0.861 mg L−1 after electrolysis of 90 min, respectively. However, the total amount of C and I remained almost the constant throughout the electrolysis reaction. This indicates that CH4, and I are the main products of electrochemical dehalogenation of CH3I.

CONCLUSION

Preparation of Cu-nano/CF electrode by pulse electrodeposition method has practical application value. We successfully loaded nano-copper particles over the carbon felt substrate by this method. Optimum electrodeposition conditions were obtained by SEM characterization and electrolysis of CH3I: 1 mM CuSO4•5H2O + 4 mM Na3C6H5O7•2H2O + 20 mM KNO3, pH = 5.0, T = 45 °C, φ = −2.5 V, t = 300 s. The Cu-nano/CF electrode with average 100 nm diameters presents high surface areas (0.737 m2 g−1) and high catalytic activity. High current density and low concentration of Cu2+ can solve the problem of black core and particle agglomeration in continuous deposition.

Voltammetric investigation shows that the Cu-nano/CF electrodeposited electrode has significant electrochemical reduction catalytic performance for CH3I, and the electrochemical reduction of CH3I reaction is a diffusion-controlled process. CH4, and I are the main products and the total amount of C and I remained almost the constant throughout the electrolysis reaction.

ACKNOWLEDGEMENTS

The authors sincerely appreciate the help of the analysts from Center of Analysis and Test, Laboratory for Resource and Environmental Education, and School of Chemical Engineering in East China University of Science and Technology.

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