2-Chlorophenol is an environmental pollutant presenting in water, soil, etc. It is widely used in industries and in our everyday life as a fungicide, wood preservative, and intermediate in the production of chlorinated pesticides or adhesives [1]. Owing to estrogenic, mutagenic and carcinogenic effects, 2-chlorophenol presents a threat to both the aquatic ecosystem and human health. Accordingly, the decomposition of 2-chlorophenol in wastewater has been studied by many methods such as photodegradation [2, 3], chemical oxidation [4, 5, 6, 7], and electrochemical degradation [8, 9].
Among the above reported methods, electrochemical degradation is an attractive option for the decomposition of 2-chlorophenol because of its high oxidation efficiency, fast reaction rate, amenability to automation, high energy efficiency, cost effectiveness, and environmentally friendly attributes [10, 11]. The electrochemical degradation efficiency is largely dependent on the properties of the anodes. An ideal anode must not only exhibit high efficiencies toward contaminant degradation, but also be electrochemical stable and inexpensive [12]. Recently, studies have mainly focused on “non-active” anodes such as boron-doped diamond (BDD), SnO2, and PbO2, which exhibit a high oxygen evolution overpotential and can produce abundant hydroxyl radicals (•OH) [13]. Won’s group [8] examined the electrochemical degradation of 2- chlorophenol using a Pt/Ti electrode and a BDD electrode. The BDD electrode displayed a significantly higher efficiency toward the degradation of 2-chlorophenol than the Pt/Ti electrode. Ortiz’s group [9] also reported the effective electrochemical degradation of 2-chlorophenol using a BDD anode. However, despite the high chemical stability and outstanding efficiency of BDD electrode, its large-scale practical application is limited by its high cost and difficulty in selecting a suitable substrate for the thin diamond layer [14, 15].
Alternatively, PbO2 electrodes have been used as anodes in the chlor-alkali industry for over 70 years because of their high electrical conductivity, which is similar to that of metals, good stability, high overpotential for oxygen evolution, low cost, ease of fabrication, and long service life. In recent decades, PbO2 anodes have been applied to oxidize organic pollutants [10, 16, 17, 18, 19]. An SnO2-Sb layer between the Ti substrate and PbO2 has been commonly introduced to enhance the adhesion of the PbO2 layer [20]. Niu’s group has reported several successful studies on the electrochemical mineralization of environment contaminants such as perfluorooctanoic acid, perfluorocarboxylic acid [21, 22], and sulfamethoxazole [13] using PbO2 electrodes.
PbO2 displays two allotropic forms, orthorhombic and tetragonal (α and β). Compared with α-PbO2 that features a compact structure, β-PbO2 is widely used in electrochemical degradation owing to its porous structure that leads to high electrochemical efficiency and large surface area [23, 24]. However, direct electrodeposition of β-PbO2 on a Ti substrate lessens the anode stability and activity because the porous β-PbO2 structure can easily detach from the Ti substrate.
Some methods have been reported to improve the adhesion of β-PbO2 on the substrate. For instance, the electrodeposited β-PbO2 on a platinized Ti (Ti-Pt) substrate was reported to be stable in the electrochemical degradation of real textile wastewater [25, 26]. A fluoride-doped Ti/β-PbO2 anode showed good performance in the electrochemical degradation of the dimethyl phthalate ester [27]. β-PbO2 electrode modified by either TiO2 or Co3O4 effectively electrochemically oxidized acid orange 7 [28] and bisphenol A [12], respectively. Additionally, to improve the activity and stability of β-PbO2 electrode, several layers of SnO2-Sb2O5-RuO2 and α-PbO2 were introduced between β-PbO2 and the substrate. Zheng et al. [10] investigated the electrochemical degradation of 4-chlorophenol on Ti/SnO2-Sb2O5-RuO2/α-PbO2/β-PbO2, and Chen et al. [23] investigated the influence of doped nano-CeO2 on Al/α-PbO2/ β-PbO2 electrode for enhancing electro-catalytic activity.
In this study, a Ti/α-PbO2/β-PbO2 electrode was prepared simply by electrodeposition. The layer of α-PbO2 enhanced the adhesion between the β-PbO2 layer and Ti substrate. The prepared Ti/α-PbO2/β-PbO2 electrode displayed high catalytic activity and long lifetime toward the degradation of 2- chlorophenol. The degradation of complex compounds, i.e., 2,4- dichlorophenol and bisphenol A, was also investigated to further evaluate the efficiency of the Ti/α-PbO2/β-PbO2 electrode.
2-Chlorophenol, 2,4-dichlorophenol, and bisphenol A were obtained from Sigma-Aldrich. All other chemicals were purchased from Shanghai Sinopharm Chemical Reagent Co., Ltd. All chemicals were of analytical grade. The solutions were prepared with twice-distilled water.
A Ti metal net (2.0 cm x 2.0 cm x 0.1 cm) was used as the substrate. Before deposition, the Ti substrate underwent sandblasting, ultrasonic cleaning in acetone for 10 min, ultrasonic cleaning in deionized water for 10 min, immersion in 40% NaOH at 80 °C for 2 h, etching in boiling 15% oxalic acid for 2 h, and rinsing with deionized water [29]. After pretreatment, interlayer α-PbO2 was prepared using the pretreated Ti substrate as an anode and a Cu plate as a cathode in alkaline solution (0.1 mol/L PbO, 3.5 mol/L NaOH) at 40 °C and 10 mA/cm2 for 2 h. Finally, top layer β-PbO2 was electrodeposited on the above prepared substrate in acidic solution at 60 °C using a current density of 20 mA/cm2 for 2 h. The acid solution consisted of 50 g/L Cu(NO3)2, 0.5 g/L NaF, and 150 g/L Pb(NO3)2 [23].
Scanning electron microscopy (SEM; HITACHI-4700, Japan) was used to characterize the morphology of the electrode surface. X-ray diffraction (XRD) patterns of the samples were recorded on a D/max-RB X-ray diffractometer (Rigaku, Japan) using Cu Kα radiation (40 kV, 100 mA).
Polarization curves were obtained on the CHI 660C electrochemical workstation (Shanghai ChenHua Instrument Co., Ltd.) using a conventional three-electrode system. A Ti/α-PbO2/ β-PbO2 electrode with an effective surface area of 1 cm2 was used as the working electrode, a saturated calomel electrode (SCE) was used as the reference electrode, and a platinum electrode was used as the counter electrode. The measurements were conducted at room temperature.
Electrochemical oxidation of 2-chlorophenol was performed in a 150 mL beaker using a Cu plate (8 cm2) as the cathode and Ti/α-PbO2/β-PbO2 (8 cm2) as the anode. The anode and cathode were positioned vertically and parallel to each other with a distance of 1 cm. Na2SO4 was chosen as the supporting electrolyte.
During electrochemical degradation, a portion of the reaction solution was withdrawn from the reactor at certain time intervals to determine the concentration of residual 2- chlorophenol in the test solution using a UV spectrophotometer (TU-1901). The removal rate (h) was calculated according to Eq. (1):
where A0 and A are the absorbance values of the organic pollutant at time zero and t (s), respectively.
SEM was used to characterize the morphology and surface structure of the etched Ti substrate, Ti/α-PbO2, Ti/β-PbO2, and Ti/α-PbO2/β-PbO2 (Fig. 1). As observed in Fig. 1(a), the etching process in oxalic acid solution generated a Ti surface with a rough texture, which is expected to increase the surface area of the Ti substrate and strengthen the interaction between the deposition layers and Ti substrate [29]. As expected, interlayer α-PbO2 (Fig. 1(b) and (c)) consisted of uniformly shaped particles in close proximity, i.e., rod-like and fiber-like particles that were smaller than those of the β-PbO2 layer. Such a structure is expected to facilitate the deposition of β-PbO2 layer in the subsequent electrode fabrication step. As observed in Fig. 1(d), Ti/β-PbO2 comprised typical pyramid-shaped crystals, as consistent with the findings of reported studies [27, 28, 30]. In contrast, the Ti/α-PbO2/β-PbO2 electrode (Fig. 1(e)) featured a completely different structure from that of Ti/β-PbO2. Specifically, the Ti/α-PbO2/β-PbO2 electrode featured a cauliflower-type structure comprising clusters of PbO2 crystals. Furthermore, some pores were visible between the PbO2 crystals. Such a cauliflower structure is expected to lead to high surface areas for enhancing the adsorption and degradation of pollutants, as demonstrated in these literature studies that reported the synthesis of comparable β-PbO2 cauliflower-like structures using other methods [31, 32]. Accordingly, Ti/α-PbO2/β-PbO2 is expected to exhibit high activity toward the electrochemical degradation of 2-chlorophenol.
Fig. 1(f) shows the XRD patterns of the intermediate layer α-PbO2 and top layer β-PbO2 that agree with those of standard references (PDF 01-075-2415, 01-073-0851), thereby indicating the formation of α-PbO2 and β-PbO2 crystals in the prepared samples. The α-PbO2 grains were small as deduced from the relatively weak and wide α-PbO2 characteristic diffraction peaks. The small α-PbO2 grains are expected to promote the crystallization of β-PbO2 as indicated by the strong and sharp β-PbO2 characteristic diffraction peaks.
Polarization curves are typically measured to determine the oxygen evolution overpotential of anode electrodes. More specifically, the onset of oxygen evolution is indicated by a sudden current increase in the linear polarization curve. Anodic oxygen evolution causes a power loss in organic wastewater treatment because oxygen evolution is a side reaction that reduces the current efficiency of organic oxidation [33]. Thus, the use of anodes with a high oxygen evolution overpotential favors the electrochemical oxidation of organics. The mechanism is as follows [34]:
where PbO2[] represents the electroactive sites of the electrode and RO represents the organic oxidation products. The oxidation of organics is influenced by the amount of adsorbed •OH. Oxygen evolution is an undesirable concomitant reaction.
Furthermore, ozone generates easily when PbO2 electrodes with high overpotentials are employed, as reported [35]. Ozone is an ideal strong oxidizer for the degradation of organic pollutants with zero toxic residues generation. Ozone can either directly react with organic pollutants or react with H2O to generate •OH radicals as follows:
Fig. 2 shows the linear polarization curves obtained in 0.1 mol/L H2SO4 using Ti/β-PbO2 and Ti/α-PbO2/β-PbO2 electrodes. As observed, the oxygen evolution potential of the Ti/α-PbO2/β-PbO2 electrode was 3.1 V, which is much higher than that of Ti/β-PbO2 (1.6 V) and some reported electrodes such as β-PbO2 (1.8 V) [30], Ti/β-PbO2 doped with F− (2.0 V) [27], and TiO2-NTs/SnO2-Sb/PbO2 (2.0 V) [34]. As observed in Fig. 1, Ti/α-PbO2/β-PbO2 featured a cauliflower-like structure with few pores, whereas Ti/β-PbO2 consisted of typical compact pyramid-shaped crystals. Thus, Ti/α-PbO2/β-PbO2 has a larger electroactive surface than Ti/β-PbO2. Accordingly, the generation of •OH radicals on Ti/α-PbO2/β-PbO2 is easier and higher than that on Ti/β-PbO2, and the oxygen evolution potential is considerably higher on the Ti/α-PbO2/β-PbO2 electrode.
Electrode stability is an important property in practical application. The stability of PbO2 electrodes is usually influenced by superficial ruptures and detachment of the PbO2 film owing to the permeation of solution. Under normal conditions (such as a current density of 20 mA/cm2), a PbO2 electrode can remain highly functional for several years. To reduce the testing time, an accelerated life test was used to evaluate the service life of the Ti/β-PbO2 and Ti/α-PbO2/β-PbO2 electrodes in 1.0 mol/L H2SO4 at 60 °C at a current density of 4 A/cm2. The electrolysis time when the potential reached 10 V was regarded as the service life of the electrode [14]. As determined herein, the service lives of the Ti/β-PbO2 and Ti/α-PbO2/β-PbO2 electrodes were 10 and 15 h, respectively. Though the electrode lifetime is strongly dependent on several conditions such as current density, pH, and temperature, the lifetimes of the Ti/β-PbO2 and Ti/α-PbO2/β-PbO2 electrodes are estimated to be 45.6 and 68.4 years, respectively, at a current density of 20 mA/cm2 according to Eq. (6) proposed by Hine et al. [12] as follows:
where A1 is the current density in the accelerated test (4 A/cm2), A is the current density in practical applications (0.02 A/cm2), t1 is the lifetime of the electrode in the accelerated test (h), and t is the electrode lifetime in practical applications (h). The longer lifetime of the Ti/α-PbO2/β-PbO2 electrode (compared with that of the Ti/β-PbO2 electrode) indicates that the α-PbO2 interlayer greatly enhances the electrochemical stability of top layer β-PbO2 because of the slow formation of an insulating TiO2 layer owing to the hindered diffusion of O2− ions in water to the Ti substrate in the presence of the compact α-PbO2 interlayer [34].
The one-factor-at-a-time is a classical methodology adopted for investigating the influence of several factors on a particular parameter. However, such a methodology involves numerous experiments, and some important conclusions on the interaction among factors can be missed. In the present study, an orthogonal test was designed to circumvent the above disadvantages [36]. Four parameters were considered: current density, initial concentration, electrolyte concentration, and reaction temperature. Table 1 lists the experimental conditions, and an orthogonal L9 (34) was considered for the optimization of the experimental conditions. Table 2 shows the orthogonal test results. The K and R values are listed in Table 2. Based on the R values, the influence of the various studied parameters on electrochemical oxidation decreased in the order of current density > reaction temperature > electrolyte concentration > initial concentration. The optimal experimental conditions were chosen according to the results in Table 2 as follows: anode current density 20 mA/cm2, reaction temperature 35 °C, electrolyte concentration 0.1 mol/L, and 2-chlorophenol initial concentration 50 mg/L. For instance, the average removal rates of 2-chlorophenol at 5, 10, and 20 mA/cm2 were 87.84%, 91.89%, and 96.72%, respectively. Hence a current density of 20 mA/cm2 was chosen as the optimal current density.
The variations in the 2-chlorophenol removal rate on the Ti/β-PbO2 and Ti/α-PbO2/β-PbO2 electrodes under the selected experimental conditions are shown in Fig. 3(a). As observed, both the Ti/β-PbO2 and Ti/α-PbO2/β-PbO2 electrodes were effective toward the electrochemical degradation of 2- chlorophenol. After 180 min of reaction, the removal rate of 2-chlorophenol on the respective electrodes Ti/β-PbO2 and Ti/α-PbO2/β-PbO2 were 78.4% and 100%, respectively. The higher removal rate of the Ti/α-PbO2/β-PbO2 electrode when compared with that of the Ti/β-PbO2 electrode at the same degradation time supports the fact that using interlayer α-PbO2 can increase the electrode oxidation activity for 2- chlorophenol.
The calculated voltage of cell U(t) is based on the potential at nil current, the overpotentials of the anode and cathode, and the ohmic drop through the solution of resistance R. The cell voltage can be directly measured during electrolysis reaction. As observed, the cell voltage was mostly stable on both the Ti/β-PbO2 and Ti/α-PbO2/β-PbO2 electrodes (Fig. 3(b)) during an electrochemical degradation period of 300 min, thus indicating that the prepared electrodes are very stable. The average values of the cell voltage were 4.62 and 4.34 V for the Ti/β-PbO2 and Ti/α-PbO2/β-PbO2 electrodes, respectively. This finding indicates the onset of a low energy consumption on both electrodes; energy consumption is proportional to the cell voltage [37]. The cost associated with electrical power usage for 1000 kg of wastewater was calculated as 22 and 20 kWh using the Ti/β-PbO2 and Ti/α-PbO2/β-PbO2 electrodes, respectively.
More complex compounds, 2,4-dichlorophenol and bisphenol A, were selected to further investigate the performance of the Ti/α-PbO2/β-PbO2 electrode toward electrochemical degradation under the optimum conditions established above. Fig. 4 shows the removal rates of 2,4-dichlorophenol and bisphenol A as a function of degradation time using the Ti/α-PbO2/β-PbO2 anode. As observed, the Ti/α-PbO2/β-PbO2 anode also displayed high activity toward the electrochemical degradation of these two complex organic pollutants. As observed from Fig. 3(a) and Fig. 4, the removal rates of 2-chlorophenol and 2,4-dichlorophenol could reach 100% in 180 and 240 min of electrolysis, respectively, whereas the removal rate of bisphenol A could reach 78% after 240 min of electrolysis.
Determining the electrochemical degradation mechanism is of importance for the control of pollutants. Many research studies have focused on the electrochemical oxidation mechanism of chlorophenol. However, the mechanism is not very clear because detection of the transient intermediate is very difficult. To date, the electrochemical degradation mechanism of chlorophenol has been based on the presence of some state intermediates, as identified by high-performance liquid chromatography. Based on resonance Raman spectroscopy, high-performance liquid chromatography, UV-visible spectroscopy, and theoretical calculations, Cong et al. [38] reported that some of the chlorophenols were oxidized to benzoquinone, while others directly degraded into organic acids. However, oxidation to benzoquinone proceeded primarily at the early stages of the degradation process. Other possible intermediates were catechol, phenol, resorcin, hydroquinone, fumaric acid, maleic acid, and oxalic acid. The UV-visible spectrum of 2-chlorophenol shows two absorbance peaks at 217 and 274 nm, which are due to the phenol structure. As observed in Fig. 5, the absorbance peaks changed slightly within 30 min of reaction, which may be due to the influence of intermediates featuring benzene rings such as benzoquinone, catechol, resorcin, phenol, and hydroquinone. The absorbance peaks decreased after 60 min of degradation owing to the degradation of 2-chlorophenol into organic acids such as fumaric acid, maleic acid, and oxalic acid.
Fig. 6 shows the results of the kinetic analysis conducted under optimal conditions over a short electrolysis period of 150 min. As observed, a good linear relationship between ln(C0/C) and degradation time was obtained. A regression coefficient of 0.9829 and a pseudo rate constant (k) of 0.013 min−1 were obtained when the degradation data of 2-chlorophenol were fitted to a pseudo-first order kinetics model.
A Ti/α-PbO2/β-PbO2 electrode with cauliflower-like structure was prepared by electrodeposition. The prepared electrode featured high oxygen evolution overpotential and high electrochemical stability. The efficiency of the Ti/α-PbO2/ β-PbO2 electrode was assessed toward the electrochemical degradation of 2-chlorophenol. Under the optimal conditions established herein, the removal rate of 2-chlorophenol attained 100% at 180 min of electrolysis. Furthermore, the removal rates of more complex compounds 2,4-dichlorophenol and bisphenol A were 100% and 78% at 240 min of electrolysis, respectively. Kinetics analysis revealed that the degradation of 2-chlorophenol followed a pseudo-first order reaction assessed over a short electrolysis time of 150 min, with a regression coefficient of 0.9829 and a pseudo rate constant of 0.013 min−1.
2015, Vol. 36 Issue (7): 975-981
PDF (767 KB)
2015, Vol. 36 Issue (7): 975-981
PDF (767 KB)