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Enhancement of oxygen diffusion process on a rotating disk electrode for the electro-Fenton degradation of tetracycline
Electrochimica Acta 182 (2015) 73–80
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Enhancement of oxygen diffusion process on a rotating disk electrode for the electro-Fenton degradation of tetracycline Yan Zhang, Ming-Ming Gao* , Xin-Hua Wang, Shu-Guang Wang, Rui-Ting Liu Shandong Provincial Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Shandong University, Jinan 250100, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 30 April 2015 Received in revised form 24 August 2015 Accepted 26 August 2015 Available online 14 September 2015
An electro-Fenton process was developed for wastewater treatment in which hydrogen peroxide was generated in situ with a rotating graphite disk electrode as cathode. The maximum H2O2 generation rate for the RDE reached 0.90 mg/L/h/cm2 under the rotation speed of 400 rpm at pH 3, and 0.8 V vs SCE. The performance of this electro-Fenton reactor was assessed by tetracycline degradation in an aqueous solution. Experimental results showed the rotation of disk cathode resulted in the efficient production of H2O2 without oxygen aeration, and excellent ability for degrading organic pollutants compared to the electro-Fenton system with fixed cathode. Tetracycline of 50 mg/L was degraded completely within 2 h with the addition of ferrous ion (1.0 mM). The chronoamperometry analysis was employed to investigate the oxygen diffusion on the rotating cathode. The results demonstrated that the diffusion coefficients of dissolved oxygen is 19.45 105 cm2/s, which is greater than that reported in the literature. Further calculation indicated that oxygen is able to diffuse through the film on the rotating cathode within the contact time in each circle. This study proves that enhancement of oxygen diffusion on RDE is benefit for H2O2 generation, thus provides a promising method for organic pollutants degradation by the combination of RDE with electro-Fenton reactor and offers a new insight on the oxygen transform process in this new system. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: electro-Fenton rotating disk electrode tetracycline oxygen transport mineralization
1. Introduction Advanced oxidation processes (AOPs) are regarded as the most attractive methods for the treatment of wastewater containing toxic and non-biodegradable pollutants [1–4]. Among all the processes, Fenton reagents (Fe2+ and H2O2) are particularly powerful for degrading recalcitrant organic pollutants due to the formation of hydroxyl radicals (OH), which can non-selectively react with organic compounds leading to their mineralization to CO2, water and inorganic ions [5]. However, it is expensive and dangerous for the production, transportation and storage of H2O2 [3–8]. In recent decades, electro-Fenton (EF) process has attracted great interests because of the in-situ electro-chemical production of H2O2 and the regeneration of Fe2+ at the cathode (via the reaction 1–3) [6–10]. This can solve the problems caused by the traditional Fenton processes, such as the storage and shipment of H2O2 and the generation of iron sludge [11–13]. O2 þ 2Hþ þ 2e ! H2 O2
* Corresponding author. Tel.: +86 531 88362802; fax: +86 531 88364513. E-mail address: [email protected] (M.-M. Gao). http://dx.doi.org/10.1016/j.electacta.2015.08.134 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
ð1Þ
H2 O2 þ Fe2þ ! Fe3þ þ OH þ OH
ð2Þ
Fe3þ þ e ! Fe2þ
ð3Þ
Although EF process has been proposed as a promising environmental remediation technology, it encounters some drawbacks for further application. To our knowledge, the efficiency of H2O2 production is highly dependent on the diffusion of oxygen of the gaseous phase into the liquid phase [14–16]. Traditionally, the sparging of pure oxygen or air into the solution is usually considered to improve the insufficient of oxygen [7,17,18]. However, the reduction of oxygen in the cathode is controlled by the solubilisation of the molecular oxygen into the solution and then diffusion from the bulk to the electrode surface [19]. Due to the low solubility of oxygen, most oxygen bubbled into the solution cannot reach the electrode surface, resulting in the low oxygen utilization efficiency. As an example, the oxygen utilization efficiency, based on the fraction of oxygen that ended up in the H2O2 from the total amount of oxygen supplied through sparing, was less than 0.1% [20]. Furthermore, the high energy
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requirements of stirred tank and airlift would be a major waste in production process. Another way to increase the surface concentration of oxygen and reduce mass transfer limitations is the application of gas diffusion cathodes. As known, the gas diffusion cathode has excellent performance in the production of H2O2, but their size is commonly small, adding up to high cost and instability for long term operation [21]. Therefore, it is essential to develop a cost-effective EF system with much improved oxygen delivery efficiency. Inspired by the rotating biological contactor and the rotating cathode microbial fuel cells [22–25], the use of rotating disk electrode (RDE) has been introduced in this EF process. In this system, the disk electrode was partially immersed in the solution and its upper part is contained in gas phase. So when a rotational movement is imposed to the disk, a liquid film is brought upwards over the surface of the disk, thus providing a contact of the liquid film with the oxygen. After moving downwards, the liquid film will be taken up again by the liquid in the reactor [26]. What’s more, the contact between phases is generated by maintaining the liquid phase as thin film thus minimizing the mass transfer resistances from gas phase to the liquid phase [26]. A previous study [20] found that the RDE process is feasible in the application. The dissolved oxygen (DO) in this reactor does not impact on the generation rate of H2O2 because adequate amount of oxygen could be supplied during the air exposure cycle. In this study, EF system was fabricated with graphite, a typical carbon material [27–29], as rotating disk cathode to give a common information for carbon-based RDE. The effects of pH, applied potential and rotating speed on the generation of hydrogen peroxide were studied. Then, the performance of this reactor was evaluated for the degradation of tetracycline (TC), compared with cathode-fixed EF process with gas–liquid stirred. Finally, the oxygen transform process was investigated to give an insight on the effects of rotating disk cathode on the generation of H2O2 and electro-Fenton process. 2. Experimental 2.1. EF system assembly A schematic diagram of the RDE reactor used in this study is shown in Fig. 1. The electrochemical generation of hydrogen peroxide and the subsequent removal of TC concentrate were performed in the undivided cell of 250 mL capacity. The cell was made of 0.2 cm thick acrylic material, 10.0 cm high, and was 7.0 cm long and 5.0 cm wide. The electrodes in this study were prepared by spectroscopically pure graphite (SPG) (99.9% porosity,
1.85 g/cm2 bulk density, 13 mV m electric resistivity, Shanghai Yifeng Co., Ltd.) without any modification. The prepared cathode of graphite disk (Ø 80 mm, thickness of 8 mm) was selected as the working electrode, a graphite column (Ø 20 mm) as counter electrode and a saturated calomel electrode (SCE) as reference electrode. The disk cathode were mounted on a horizontal copper shaft (1.0 cm diameter) that was attached to a variable speed motor device by a plastic connector. The cathode disk was 40% submerged in the liquid and the distance between the cathode and the anode was 2.0 cm. 2.2. Operation conditions The H2O2 electro-generation experiments were performed in the homemade RDE reactor in 0.05 M Na2SO4 solution at room temperature. The solution pH was adjusted to 3 using H2SO4 and NaOH and determined with a Sartorius PB-10 pH-meter. The electrical signal for the electrochemical experiments was controlled and recorded by a CHI760D electrochemical workstation (Shanghai Chenhua, China). The disk cathode was rotated at a controlled speed (100, 200, 300, 400 and 500 rpm) driven by a motor. At the 60th min, 2 mL samples were taken for analyzing the concentration of the H2O2. The degradation of tetracycline hydrochloride (Analytical grade, 96% purity, Aladdin Industrial Corporation, Shanghai) by RDE process was carried out in the same apparatus at several initial concentrations (50, 100, 200 and 300 mg/L). After the solution pH was adjusted to 3, FeSO4 was introduced to provide 1.0 mM Fe2+ in the solution as the catalyst and the rotation speed was kept 400 rpm. The TC samples were taken every 20 min and then filtered by 0.45 mm membrane for analysis, and the electric charges were recorded. A comparative reaction for TC degradation was carried out in a conventional gas–liquid stirred tank reactor. A fixed graphite sheet (80 mm 62 mm, 50.2 cm2) was used as cathode, which has the same surface area as the RDE. Other operation parameters (such as pH, applied potential and Fe2+ concentration) were the same as the RDE reactor. The concentration of oxygen in the solution was maintained by continuously bubbling compressed air at 300 ml/min recommended as the optimum amount sparging rate in the EF process [30,31]. 2.3. Analytical method The concentration of H2O2 during such experiments was monitored by UV–Vis spectrophotometer (UV759, Shanghai instrument analysis Co., LTD.) using the potassium titanium (IV)
Fig. 1. Experimental setup of RDE reactor used for tetracycline degradation. The cell was 10 cm high, 7 cm long and 5 cm wide. The distance between the graphite disk cathode (Ø 80 mm, thickness of 8 mm) and the anode (Ø 20 mm) was 2 cm.
Y. Zhang et al. / Electrochimica Acta 182 (2015) 73–80
oxalate method (l = 402 nm) [32].The current efficiency (CE) for hydrogen peroxide production is defined as follows: CE ¼
ð4Þ
Ao At 100% Ao
ð5Þ
COD analysis was performed according to standard method (APHA, 1995) [33]. Samples were taken at regular time intervals and immediately filtered through 0.45 mm membrane filter. The mixture of 2.0 mL of filtered sample, 4.0 mL of sulfuric acid reagent (containg 10 g/L Ag2SO4) and 1.5 mL of digestion solution (containing 20.432 g/L K2Cr2O7 and 33.3 g/L HgSO4) was heated at 150 C for 2 h. The samples were then cooled and analyzed on a spectrophotometer at 600 nm. The oxygen transform on the RDE were investigated using chronoamperometry (CA) measurements. The diffusion coefficients of dissolved O2 in liquid membrane (D) are calculated from the slopes of linear parts of the plots of Inet against t 1=2 , based on Cottrell equation of semi-infinite diffusion: 1
Inet ¼
no FAD2 C 1
3. Results and discussion 3.1. Hydrogen peroxide production performance
nFC H2 O2 V 100% Rt 0 Idt
Where n is the number of electron transfer of oxygen reduction to H2O2 (n = 2), F is the faradic constant (96,486 C/mol), CH2O2 is the concentration of H2O2 (mol/L), V the bulk volume (L), I the applied current (A), and t is time (s). TC concentration was analyzed on an UV–Vis spectrophotometer equipment at l = 355 nm, and its removal efficiency (h) was calculated by the following formula:
h¼
75
ð6Þ
1
t 2 p2
Where no is the number of electron-transfer (no = 2), and C is the bulk concentration of dissolved oxygen where the disk emerges from the liquid. For a rotating disk gas-liquid contactor, a relationship has been derived between the amount of liquid entrained by the rotation of disk and its rotational velocity [34]: 1
d ¼ KV c 2
ð7Þ
Where d is a mean film thickness on the disk, K is a function of the 1
acceleration of gravity and the kinematic viscosity and V c 2 is the linear velocity. For water at 20 C, K is 1.20 104 m1/2 s1/2 [35].
The effects of the pH on H2O2 production and current efficiency at RDE system were shown in Fig. 2a. Results indicated that the highest EF activity was attained under pH 3.0, where the H2O2 production reached near 37.1 mg/L in 1 h. It shown an agreement between the experimental data and the theory that observed previously, which indicated pH 3.0 was optimal for the Fenton process [36–41]. From Eq. (1), it seems that a low pH was beneficial to the higher electro-generation of H2O2 since its synthesis was in acidic medium. However, a high proton concentration may promote H2 evolution and reduce the current efficiency. What’s more, the efficiency of EF process dropped rapidly especially when pH 4. This is due to H2O2 could be rapidly decomposed to oxygen and water at neutral and higher pH [42]. The cathodic potential determines the formation and current efficiency of H2O2, and thus affects the waste water treatment performance [43]. The effects of cathodic potential on H2O2 production were shown in Fig. 2b. It was indicated that the accumulation of H2O2 increased initially with the increasing cathodic potentials, and the highest H2O2 accumulation was achieved at 0.8 V vs SCE. When the potential exceeded the optimum, the H2O2 accumulation declined sharply. And the current efficiency of H2O2 production at the applied potential of 0.6,0.7,0.8, 0.9 and 1.0 V vs SCE was 15.1%, 16.6%, 17.1%, 9.3% and 8.9%, respectively. The increased applied potential can accelerate the electron transfer between the reactive solutions and promote the oxygen reduction reaction. And when the applied potential was further increased (> 0.8 V vs SCE), the side reactions such as hydrogen evolution on the cathode might strengthen, which resulted in a decreased H2O2 yield and current efficiency. In addition, the further investigation also showed that increased hydrogen evolution at higher potential could destroy the surface structure of graphite cathode. To study the effects of rotation speed on the H2O2 generation, experiments were conducted by varying the speed in the range of 100–500 rpm during 60 min electrolysis. As shown in Fig. 2c, the H2O2 concentration increased from 15.6 to 45.3 mg/L when the rotating speed increased from 100 to 400 rpm. However, no additional improvement in H2O2 concentration was observed when the rotation speed increased further to 500 rpm. In addition, the CE at rotation rate of 100–500 rpm were 10.5%, 17.3%, 17.5%, 17.4% and 9.1%, respectively. This observation indicated that a suitable cathode rotation speed was favorable for electro-
Fig. 2. Effects of pH values (a), cathodic potential (b) and rotation speed (c) on hydrogen peroxide generation (histogram) and current efficiency (solid square) in 0.05 M Na2SO4 solution in 1 h with different conditions: (a) cathodic potential of 0.8 V, the rotation speed of 300 rpm, and pH of 2.0, 3.0, 4.0, 5.0 and 6.0; (b) cathodic potential of 0.6, 0.7, 0.8, 0.9 and 1.0 V, pH of 3.0, and the rotation speed of 300 rpm; (c) pH of 3.0,cathodic potential 0.8 V, and rotation speed is 100, 200, 300, 400 and 500 rpm.
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generation of H2O2, which might be explained by the following factor. The cathode rotation increased the contact area between oxygen and electrode, thus, improved the efficiency of oxygen mass transfer and the generation rate of H2O2. However, when rotation speed was too high, the resistance of electrolyte solution increased with the excessive bubbles in the system, resulting in a drop in the yields of H2O2 [43]. In addition, we observed that the high rotation speed (500 rpm) would cause splashing of the solution, which could be an additional reason for the decrease of H2O2 accumulation at high rotation speed. Based on the results above, H2O2 accumulation arrived at its maximal value of 0.90 mg/L/h/cm2 when the conditions were concluded as 0.8 V vs SCE of the cathodic potential, 400 rpm of rotation speed and 3.0 of pH. Compared with previous reports, it suggested that the generation rate of H2O2 in present work was higher than those cases using graphite in conventional systems involving O2 sparging. For example, several research groups obtained H2O2 generation rates of 0.15 and 0.18 mg/L/h/ cm2 at O2 flow rates of 114 and 6.0 L/h, respectively [7,42]. This observation indicated that the current approach was more efficient, owing to the constant rotation of the disk. The liquid film structure formed on the disk surface may be more conducive to the diffusion of oxygen from gas-phase to the film.
A series of experiments were carried out to test the influence of initial pollutant concentration on this process. As shown in Fig. 4a, when the initial concentration was 50 mg/L, TC was almost completely removed in 120 min. However, the TC removal percentages decreased to 95.8%, 89.5% and 80.1% with the initial concentration of 100, 200 and 300 mg/L, respectively. The mineralization of TC was monitored by measuring the COD of the solution and the results were presented in Fig. 4c. The mineralization decreases from 70.4% to 48.9% as the increasing initial TC concentration from 100 to 300 mg/L. The Fig. 4b presents the excellent correlation decay considering a pseudo-first-order
3.2. TC removal by electro-Fenton Organics degradation by EF process not only depends on H2O2, but also is controlled by the concentration of Fe2+ as catalyst [44]. Fig. 3 shows the effects of the concentration of externally added ferrous ions on TC degradation. This degradation processes were found to obey pseudo-first-order kinetics. In the absence of Fe2+, a slow degradation rate was observed and only 55.4% of TC decay was attained after 60 min of electrolysis. An obvious increase of the degradation rate was observed by adding Fe2+ into solution. For example, the TC removal efficiency were 92.9%, 96.1% in 0.5 and 1.0 mM Fe2+, respectively. However, no evident improvement (75.8%) was observed when the concentration increased to 2.0 mM. The negative effect of the higher Fe2+ concentration can be explained by increasing rate of the parasitic reaction occurring between the hydroxyl radicals and the excess of Fe2+ [6,38,44,45]. Fe2þ þ OH ! Fe3þ þ OH
ð8Þ
Fig. 3. Influences of the concentration of Fe2+ (0.0, 0.5, 1.0 and 2.0 mM) on the degradation of tetracycline. Conditions: pH 3, cathodic potential 0.8 V, Na2SO4 0.05 M, rotation speed is 400 rpm and TC = 50 mg/L. The inset presents the corresponding kinetics analysis assuming a pseudo first-order reaction.
Fig. 4. Influences of the initial tetracycline on degradation (a) and COD removal (c), and (b) the corresponding tetracycline degradation kinetics at the same conditions: pH = 3.0, [Fe2+] =1.0 mM, Na2SO4 0.05 M, cathodic potential 0.8 V and rotation speed 400 rpm.
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77
Table 1 the apparent rate constants of TC degradation and COD removal with varying initial TC concentration. Concentration of TC (mg/L)
50 100 200 300
RDE system
FDE system
Kapp (min1)
R2
COD removal (%)
Kapp (min1)
R2
COD removal (%)
0.0450 0.0265 0.0180 0.0133
0.9816 0.9983 0.9895 0.9938
78.7 70.4 52.6 48.9
0.0129 0.0110 0.0082 0.0068
0.9880 0.9934 0.9970 0.9910
69.5 55.8 46.5 40.8
reaction rate. The values of the apparent rate constants obtained from the kinetic analyses are reported in Table 1. In order to clarify the changes in the molecule of TC, samples obtained during the electrolysis were analyzed by UV–Vis techniques (Fig. 5). The spectrum of TC before the treatment present two peaks at 275 nm and 355 nm, which correspond to the absorption peaks of benzene ring structure and the p-p* transition. The intensity of absorption at 355 nm decreased during the electrochemical treatment, which indicated the destruction of the naphthalene rings. The adsorption peak at 275 nm diminished and decreased greatly under 60 min of electrolysis. The fixed disk electrode (FDE) process for TC degradation was investigated to identify the influence of RDE on the process. Fig. 6a illustrates the comparative degradation behavior for TC ranging from 50 to 300 mg/L. During this process, about 80.5% of TC removal was achieved within 120 min when TC initial concentration was 50 mg/L. From Fig. 6b, it can be seen that TC degradation followed the first-order reaction kinetics with the different rate constants and detailed values are summarized in Table 1. The effect of the initial concentration on the removal of COD is given in Fig. 6c. For initial TC concentrations of 100, 200, and 300 mg/L, the COD removal efficiencies were 55.8%, 46.5% and 40.8%, respectively. The degradation of TC in RDE and FDE system are compared in Table 1. It was observed that with an increase in the initial TC concentration, the rate constant in the RDE system decreased. It was 0.0450 min1 at 50 mg/L, 0.0265 min1 at 100 mg/L, 0.0180 min1 at 200 mg/L and 0.0133 min1 at 300 mg/L. A similar phenomenon was also observed in the FDE system. When the TC concentration increased from 50 mg/L to 300 mg/L, the rate
Fig. 5. UV–Vis spectral changes with electrolysis time for 100 mg/L tetracycline solution. Experimental conditions: pH 3.0, cathodic potential 0.8 V, [Fe2 + ] = 1.0 mM, rotation speed is 400 rpm, Na2SO4 0.05 M (Samples were diluted for 2 folds before monitoring).
Fig. 6. The fixed-cathode EF on the degradation of tetracycline (a) and the removal of COD (c) at the same conditions: pH = 3.0, [Fe2+] = 1.0 mM, Na2SO4 0.05 M, cathodic potential 0.8 V, oxygen flow rate: 0.3 L/min. (b) represents the kinetic analysis of tetracycline degradation.
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constant decreased apparently from 0.0129 to 0.0068 min1. This was reasonable because a higher initial TC would lead to a higher concentration of intermediates, which could strengthen the competitive reactions and decrease the degradation rate. It also should be noted that the degradation rate constant of the RDE process within 120 min was nearly 3 times that of the FDE system at the same initial TC concentration. These results show that RDE process has obvious advantages in TC degradation, which may be explained as follow: the rotation of cathode is not only advantageous for the oxygen diffusion into the surface of electrode and H2O2 generation, but also plays the role of the stirrer, being conductive to the spread of TC. 3.3. Oxygen transfer process on RDE system As discussed above, the rotation of disk cathode resulted in the efficient production of H2O2 without oxygen aeration, offering a potentially effective EF method for degrading organic pollutants. Some researchers considered the liquid film formed on the disk to be mainly responsible for the oxygen transfer [19]. The liquid film formed on the disk during the air-exposure cycle would renew its surface due to the forces, such as centrifugal. The air–liquid interface and renewed surface of the liquid film are responsible for the oxygen transfer into the liquid. In order to clarify the mechanism of the oxygen transport, the diffusion process of oxygen on this electrode is analyzed as follow. During the continuous rotating, an aqueous film will be formed on the rotating disk cathode surface. In order to clarify the mechanism of the oxygen transport on RDE system, the average film thickness was calculated in the first. We used Eq. (7) suggested by Zeevalkink et al. to compute liquid film thickness (d). The calculated d is 1.31 104 m. Then the chronoamperometry was used to determine the oxygen diffusion in the aqueous film on the RDE surface. Fig. 7 presents the chronoamperometric behaviors of RDE in phosphate buffer solution with pH 3, 300 rpm, and the initial and final electrode potentials at 0 and 0.65 V vs SCE, respectively. Oxygen reduction reaction does not occur when the initial potential is 0. However, almost 0.1 A current was detected in the system when the potential was 0 V, which could be explain by the followings. The disk electrode acted as a capacitor in the case and the current was the discharge current. As shown in the inset in Fig. 7, the plot of Inet against t2 showed a straight in a short time, while it deviated from initial linearity later. The initial straight line for Cottrell plot 1
reflects the O2 reduction was controlled by the O2 diffusion process inside the film. While, the slight deviation (0.321) from initial linearity (0.458) implied that there was no substantial changes in the process of mass transfer. Thus, the slight deviation in slope may be attribute to the instability of the rotating shaft. The diffusion coefficients of oxygen in liquid membrane (D) are calculated from the slopes of linear parts of the plots of Inet against t2 , based on Cottrell equation of semi-infinite diffusion Eq. (6) [46]. The calculated D is 19.45 105 cm2/s as collected in Table 2. It is assumed that the liquid film has a constant thickness related to the disk radius while it moves with the disk cathode, and that the velocity profile is flat across the film thickness. The oxygen balance is developed for an element of volume from the liquid film as follow. The mass transfer coefficient, K m is calculated as 1.48 m/s using Eq. (9). 1
Km ¼
D
ð9Þ
d
Then, the time of oxygen through the aqueous film (T) and the contact time of the rotating disk with air (TR) can be calculated with (11) Eq. (10) and, respectively. T¼
d
ð10Þ
Km
TR ¼
60 2n
ð11Þ
The calculated results are T = 8.8 105 s and TR = 1.67 103 s, which means that the time of this process that oxygen diffuse to the electrode surface from the gas phase is far less than the contact time of the rotating cathode with air. Thus, it can be proposed that the oxygen is able to diffuse through the aqueous film on the graphite within the contact time in each rotating circle. Theoretically, the oxygen diffusion time from air to the surface of the electrode (T) will be equal to the time of disk exposed to the air (TR) when the speed is 1698 rpm. Obviously, the oxygen was kept in a state of saturated under the condition of this experiment. In addition, we monitored the dissolved oxygen changes in the RDE system and found the dissolved oxygen of the solution was saturated, which agreed with the theoretical calculation. These data for O2 diffusion on the RDE system are summarized in Table 2. The detailed values of oxygen transfer parameters in FDE system are also summarized in Table 2. In the RDE system, the oxygen mass transfer process involved multi-phase interfaces. Therefore, it was much more complicated compared with the common gas diffusion process. The calculated diffusion coefficient here was only one apparent diffusion coefficient. It must be noticed that the value of D in present work was nearly 10 times greater than the diffusion coefficient of oxygen in FDE system reported in previous report. This observation indicated that a suitable cathode rotation speed was favorable for the transfer of oxygen, which might be explained by the following two factors. Firstly, a region with high concentration of DO was formed around the cathode because of the rotating aeration. Secondly, a film was formed on the upper part of RDE during continuous rotation, thus oxygen could diffuse directly through the gas–liquid interface existing between the disk and the gas-phase [26]. It was further shown that
Table 2 Parameters for O2 diffusion on RDE and FDE
Fig. 7. Chronoamperograms response on the rotating graphite disk cathode with an initial potential of 0 V and a final potential of 0.65 V. The insets show the plots of the net electrolysis current Inet vs t1/2 at corresponding electrodes.
cathode
D (105cm2/s) d (103 m) Km (105 m/s) DO (mg/L) reference
RDE FDE
19.5 2.0
13.1 74.0
14.8 2.7
8.1 8.3
this work [42]
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the RDE can easily improve the ability of degradation of organic matter by improving the diffusion of dissolved oxygen. 4. Conclusion The conclusions drawn from this study can be summarized as follows: (1) The optimal conditions for H2O2 generation using this RDE were cathodic potential of 0.8 V vs SCE, initial pH 3 and the rotation speed of 400 rpm. Under the optimal conditions, the H2O2 generation rate could reach 0.90 mg/L/h/cm2. (2) Fe2+ concentration strongly influenced the performance of TC degradation by EF process and 1.0 mM was the best catalytic concentration. Under the above optimal conditions, TC (under 100 mg/L) was the almost completely degraded (96–100%) in the RDE processes in relatively short time (120 min). (3) Enhanced mineralization of TC and the removal of COD were obtained during the rotating disk cathode process compared with fixed-cathode EF process, and the possible reasons have been discussed. (4) Continuous rotating could have allowed oxygen to diffuse more easily into the aqueous film on the upper cathode surface, which was beneficial to the oxygen reduction to H2O2. Chronoamperometry analysis demonstrated that the diffusion coefficients of dissolved oxygen on the RDE was 19.45 105 cm2/s, which was greater than that on the FDE. Furthermore, the calculation indicated that the oxygen was able to diffuse through the aqueous film on the graphite within the contact time in each rotating circle. Acknowledgments This work was done under a grant from National Natural Science Foundation of China Grant No.21007033. References [1] I. Sirés, N. Oturan, M.A. Oturan, R.M. Rodríguez, J.A. Garrido, E. Brillas, ElectroFenton degradation of antimicrobials triclosan and triclocarban, Electrochim. Acta 52 (2007) 5493–5503. [2] L. Zhao, Z. Sun, J. Ma, H. Liu, Enhancement Mechanism of Heterogeneous Catalytic Ozonation by Cordierite-Supported Copper for the Degradation of Nitrobenzene in Aqueous Solution, Environ. Sci. Technol. 43 (2009) 2047– 2053. [3] J.Y. Zhemin Shen, Xiaofang Hu, YangMing Lei, XiuLing Ji, Jinpin Jia, WenHua Wang, Dual Electrodes Oxidation of Dye Wastewater with Gas Diffusion Cathode, Environ. Sci. Technol. (2005) 1819–1826. [4] M.A. Oturan, E. Brillas, Electrochemical Advanced Oxidation Processes (EAOPs) for Environmental Applications, Portugaliae Electrochim. Acta: J. Portuguese Electrochem. Soc. 25 (1) (2007) 1–18. [5] M. Luo, S. Yuan, M. Tong, P. Liao, W. Xie, X. Xu, An integrated catalyst of Pd supported on magnetic Fe3O4 nanoparticles: simultaneous production of H2O2 and Fe2+ for efficient electro-Fenton degradation of organic contaminants, Water Res. 48 (2014) 190–199. [6] M. Zhou, Q. Yu, L. Lei, G. Barton, Electro-Fenton method for the removal of methyl red in an efficient electrochemical system, Sep. Purif. Technol. 57 (2007) 380–387. [7] A. Wang, J. Qu, J. Ru, H. Liu, J. Ge, Mineralization of an azo dye Acid Red 14 by electro-Fenton's reagent using an activated carbon fiber cathode, Dyes Pigments 65 (2005) 227–233. [8] B. Balci, N. Oturan, R. Cherrier, M.A. Oturan, Degradation of atrazine in aqueous medium by electrocatalytically generated hydroxyl radicals. A kinetic and mechanistic study, Water Res. 43 (2009) 1924–1934. [9] N. Oturan, J. Wu, H. Zhang, V.K. Sharma, M.A. Oturan, Electrocatalytic destruction of the antibiotic tetracycline in aqueous medium by electrochemical advanced oxidation processes: effect of electrode materials, Appl. Catal. B: Environ. 140-141 (2013) 92–97. [10] M.A. Oturan, N. Oturan, M.C. Edelahi, F.I. Podvorica, K.E. Kacemi, Oxidative degradation of herbicide diuron in aqueous medium by Fenton's reaction based advanced oxidation processes, Chem. Eng. J. 171 (2011) 127–135. [11] L. Zhou, M. Zhou, Z. Hu, Z. Bi, K.G. Serrano, Chemically modified graphite felt as an efficient cathode in electro-Fenton for p-nitrophenol degradation, Electrochim. Acta 140 (2014) 376–383.
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Bi, T. Jin, M. Zhou, Electrogeneration of hydrogen peroxide for electro-Fenton system by oxygen reduction using chemically modified graphite felt cathode, Sep. Purif. Technol. 111 (2013) 131–136. [18] H. Wang, J. Wang, Electrochemical degradation of 4-chlorophenol using a novel Pd/C gas-diffusion electrode, Appl. Catal. B: Environ. 77 (2007) 58–65. [19] J. Lozar, B. Bachelot, G. Falgayrac, A. Savall, Diffusivity and solubility measurement of oxygen in water–acetic acid–sodium acetate solutions on a rotating ring disc electrode, Electrochim. Acta. 43 (1998) 3293–3296. [20] F. Yu, M. Zhou, L. Zhou, R. Peng, A novel electro-fenton process with H2O2 generation in a rotating disk reactor for organic pollutant degradation, Environ. Sci. Technol. Lett. 1 (2014) 320–324. [21] J.A. Bañuelos, A. El-Ghenymy, F.J. Rodríguez, J. Manríquez, E. Bustos, A. Rodríguez, E. Brillas, L.A. Godínez, Study of an air diffusion activated carbon packed electrode for an electro-fenton wastewater treatment, Electrochim. Acta 140 (2014) 412–418. [22] Z. He, H. Shao, L.T. Angenent, Increased power production from a sediment microbial fuel cell with a rotating cathode, Biosens. Bioelectron. 22 (2007) 3252–3255. [23] G. Zhang, H. Zhang, C. Zhang, G. Zhang, F. Yang, G. Yuan, F. Gao, Simultaneous nitrogen and carbon removal in a single chamber microbial fuel cell with a rotating biocathode, Process Biochem. 48 (2013) 893–900. [24] R.R. Sayess, P.E. Saikaly, M. El-Fadel, D. Li, L. Semerjian, Reactor performance in terms of COD and nitrogen removal and bacterial community structure of a three-stage rotating bioelectrochemical contactor, Water Res. 47 (2013) 881– 894. [25] D. Suor, J. Ma, Z. Wang, Y. Li, J. Tang, Z. Wu, Enhanced power production from waste activated sludge in rotating-cathode microbial fuel cells: The effects of aquatic worm predation, Chem. Eng. J. 248 (2014) 415–421. [26] M. Zanfir, X. Sun, A. Gavriilidis, Investigation of a rotating disc reactor for acetone stripping and asymmetric transfer hydrogenation: modelling and experiments, Chem. Eng. Sci. 62 (2007) 741–755. [27] I.R. Bautitz, R.F.P. Nogueira, Degradation of tetracycline by photo-Fenton process—Solar irradiation and matrix effects, J. Photochem. Photobiol. A: Chem. 187 (2007) 33–39. [28] S. Kim, P. Eichhorn, J.N. Jensen, A.S. Weber, D.S. Aga, Removal of antibiotics in wastewater: effect of hydraulic and solid retention times on the fate of tetracycline in the activated sludge process, Environ. Sci. Technol. 39 (2005) 5816–5823. [29] M. Zhou, Q. Tan, Q. Wang, Y. Jiao, N. Oturan, M.A. Oturan, Degradation of organics in reverse osmosis concentrate by electro-Fenton process, J. Hazard. Mater. 215–216 (2012) 287–293. [30] K. Cruz-González, O. Torres-López, A. García-León, J.L. Guzmán-Mar, L.H. Reyes, A. Hernández-Ramírez, J.M. Peralta-Hernández, Determination of optimum operating parameters for acid yellow 36 decolorization by electroFenton process using BDD cathode, Chem. Eng. J. 160 (2010) 199–206. [31] C.-T. Wang, W.-L. Chou, M.-H. Chung, Y.-M. Kuo, COD removal from real dyeing wastewater by electro-Fenton technology using an activated carbon fiber cathode, Desalination 253 (2010) 129–134. [32] R.M. Sellers, Spectrophotometric determination of hydrogen peroxide using potassium titanium (IV) oxalate, Analyst 105 (1980) 950–954. [33] A. Ozcan, Y. Sahin, M.A. Oturan, Complete removal of the insecticide azinphosmethyl from water by the electro-Fenton method–a kinetic and mechanistic study, Water Res. 47 (2013) 1470–1479. [34] J.A. Zhevalkink, P. Kelderman, C. Boelhouwer, Liquid film thickness in a rotating disc gas-liquid contactor, Water Res. 12 (1978) 577–581. [35] H. Wendt, G. Kreysa, Electrochemical engineering: science and technology in chemical and other industries, Springer, 1999. [36] M. 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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Enhancement of oxygen diffusion process on a rotating disk electrode for the electro-Fenton degradation of tetracycline Yan Zhang, Ming-Ming Gao* , Xin-Hua Wang, Shu-Guang Wang, Rui-Ting Liu Shandong Provincial Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Shandong University, Jinan 250100, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 30 April 2015 Received in revised form 24 August 2015 Accepted 26 August 2015 Available online 14 September 2015
An electro-Fenton process was developed for wastewater treatment in which hydrogen peroxide was generated in situ with a rotating graphite disk electrode as cathode. The maximum H2O2 generation rate for the RDE reached 0.90 mg/L/h/cm2 under the rotation speed of 400 rpm at pH 3, and 0.8 V vs SCE. The performance of this electro-Fenton reactor was assessed by tetracycline degradation in an aqueous solution. Experimental results showed the rotation of disk cathode resulted in the efficient production of H2O2 without oxygen aeration, and excellent ability for degrading organic pollutants compared to the electro-Fenton system with fixed cathode. Tetracycline of 50 mg/L was degraded completely within 2 h with the addition of ferrous ion (1.0 mM). The chronoamperometry analysis was employed to investigate the oxygen diffusion on the rotating cathode. The results demonstrated that the diffusion coefficients of dissolved oxygen is 19.45 105 cm2/s, which is greater than that reported in the literature. Further calculation indicated that oxygen is able to diffuse through the film on the rotating cathode within the contact time in each circle. This study proves that enhancement of oxygen diffusion on RDE is benefit for H2O2 generation, thus provides a promising method for organic pollutants degradation by the combination of RDE with electro-Fenton reactor and offers a new insight on the oxygen transform process in this new system. ã 2015 Elsevier Ltd. All rights reserved.
Keywords: electro-Fenton rotating disk electrode tetracycline oxygen transport mineralization
1. Introduction Advanced oxidation processes (AOPs) are regarded as the most attractive methods for the treatment of wastewater containing toxic and non-biodegradable pollutants [1–4]. Among all the processes, Fenton reagents (Fe2+ and H2O2) are particularly powerful for degrading recalcitrant organic pollutants due to the formation of hydroxyl radicals (OH), which can non-selectively react with organic compounds leading to their mineralization to CO2, water and inorganic ions [5]. However, it is expensive and dangerous for the production, transportation and storage of H2O2 [3–8]. In recent decades, electro-Fenton (EF) process has attracted great interests because of the in-situ electro-chemical production of H2O2 and the regeneration of Fe2+ at the cathode (via the reaction 1–3) [6–10]. This can solve the problems caused by the traditional Fenton processes, such as the storage and shipment of H2O2 and the generation of iron sludge [11–13]. O2 þ 2Hþ þ 2e ! H2 O2
* Corresponding author. Tel.: +86 531 88362802; fax: +86 531 88364513. E-mail address: [email protected] (M.-M. Gao). http://dx.doi.org/10.1016/j.electacta.2015.08.134 0013-4686/ ã 2015 Elsevier Ltd. All rights reserved.
ð1Þ
H2 O2 þ Fe2þ ! Fe3þ þ OH þ OH
ð2Þ
Fe3þ þ e ! Fe2þ
ð3Þ
Although EF process has been proposed as a promising environmental remediation technology, it encounters some drawbacks for further application. To our knowledge, the efficiency of H2O2 production is highly dependent on the diffusion of oxygen of the gaseous phase into the liquid phase [14–16]. Traditionally, the sparging of pure oxygen or air into the solution is usually considered to improve the insufficient of oxygen [7,17,18]. However, the reduction of oxygen in the cathode is controlled by the solubilisation of the molecular oxygen into the solution and then diffusion from the bulk to the electrode surface [19]. Due to the low solubility of oxygen, most oxygen bubbled into the solution cannot reach the electrode surface, resulting in the low oxygen utilization efficiency. As an example, the oxygen utilization efficiency, based on the fraction of oxygen that ended up in the H2O2 from the total amount of oxygen supplied through sparing, was less than 0.1% [20]. Furthermore, the high energy
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requirements of stirred tank and airlift would be a major waste in production process. Another way to increase the surface concentration of oxygen and reduce mass transfer limitations is the application of gas diffusion cathodes. As known, the gas diffusion cathode has excellent performance in the production of H2O2, but their size is commonly small, adding up to high cost and instability for long term operation [21]. Therefore, it is essential to develop a cost-effective EF system with much improved oxygen delivery efficiency. Inspired by the rotating biological contactor and the rotating cathode microbial fuel cells [22–25], the use of rotating disk electrode (RDE) has been introduced in this EF process. In this system, the disk electrode was partially immersed in the solution and its upper part is contained in gas phase. So when a rotational movement is imposed to the disk, a liquid film is brought upwards over the surface of the disk, thus providing a contact of the liquid film with the oxygen. After moving downwards, the liquid film will be taken up again by the liquid in the reactor [26]. What’s more, the contact between phases is generated by maintaining the liquid phase as thin film thus minimizing the mass transfer resistances from gas phase to the liquid phase [26]. A previous study [20] found that the RDE process is feasible in the application. The dissolved oxygen (DO) in this reactor does not impact on the generation rate of H2O2 because adequate amount of oxygen could be supplied during the air exposure cycle. In this study, EF system was fabricated with graphite, a typical carbon material [27–29], as rotating disk cathode to give a common information for carbon-based RDE. The effects of pH, applied potential and rotating speed on the generation of hydrogen peroxide were studied. Then, the performance of this reactor was evaluated for the degradation of tetracycline (TC), compared with cathode-fixed EF process with gas–liquid stirred. Finally, the oxygen transform process was investigated to give an insight on the effects of rotating disk cathode on the generation of H2O2 and electro-Fenton process. 2. Experimental 2.1. EF system assembly A schematic diagram of the RDE reactor used in this study is shown in Fig. 1. The electrochemical generation of hydrogen peroxide and the subsequent removal of TC concentrate were performed in the undivided cell of 250 mL capacity. The cell was made of 0.2 cm thick acrylic material, 10.0 cm high, and was 7.0 cm long and 5.0 cm wide. The electrodes in this study were prepared by spectroscopically pure graphite (SPG) (99.9% porosity,
1.85 g/cm2 bulk density, 13 mV m electric resistivity, Shanghai Yifeng Co., Ltd.) without any modification. The prepared cathode of graphite disk (Ø 80 mm, thickness of 8 mm) was selected as the working electrode, a graphite column (Ø 20 mm) as counter electrode and a saturated calomel electrode (SCE) as reference electrode. The disk cathode were mounted on a horizontal copper shaft (1.0 cm diameter) that was attached to a variable speed motor device by a plastic connector. The cathode disk was 40% submerged in the liquid and the distance between the cathode and the anode was 2.0 cm. 2.2. Operation conditions The H2O2 electro-generation experiments were performed in the homemade RDE reactor in 0.05 M Na2SO4 solution at room temperature. The solution pH was adjusted to 3 using H2SO4 and NaOH and determined with a Sartorius PB-10 pH-meter. The electrical signal for the electrochemical experiments was controlled and recorded by a CHI760D electrochemical workstation (Shanghai Chenhua, China). The disk cathode was rotated at a controlled speed (100, 200, 300, 400 and 500 rpm) driven by a motor. At the 60th min, 2 mL samples were taken for analyzing the concentration of the H2O2. The degradation of tetracycline hydrochloride (Analytical grade, 96% purity, Aladdin Industrial Corporation, Shanghai) by RDE process was carried out in the same apparatus at several initial concentrations (50, 100, 200 and 300 mg/L). After the solution pH was adjusted to 3, FeSO4 was introduced to provide 1.0 mM Fe2+ in the solution as the catalyst and the rotation speed was kept 400 rpm. The TC samples were taken every 20 min and then filtered by 0.45 mm membrane for analysis, and the electric charges were recorded. A comparative reaction for TC degradation was carried out in a conventional gas–liquid stirred tank reactor. A fixed graphite sheet (80 mm 62 mm, 50.2 cm2) was used as cathode, which has the same surface area as the RDE. Other operation parameters (such as pH, applied potential and Fe2+ concentration) were the same as the RDE reactor. The concentration of oxygen in the solution was maintained by continuously bubbling compressed air at 300 ml/min recommended as the optimum amount sparging rate in the EF process [30,31]. 2.3. Analytical method The concentration of H2O2 during such experiments was monitored by UV–Vis spectrophotometer (UV759, Shanghai instrument analysis Co., LTD.) using the potassium titanium (IV)
Fig. 1. Experimental setup of RDE reactor used for tetracycline degradation. The cell was 10 cm high, 7 cm long and 5 cm wide. The distance between the graphite disk cathode (Ø 80 mm, thickness of 8 mm) and the anode (Ø 20 mm) was 2 cm.
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oxalate method (l = 402 nm) [32].The current efficiency (CE) for hydrogen peroxide production is defined as follows: CE ¼
ð4Þ
Ao At 100% Ao
ð5Þ
COD analysis was performed according to standard method (APHA, 1995) [33]. Samples were taken at regular time intervals and immediately filtered through 0.45 mm membrane filter. The mixture of 2.0 mL of filtered sample, 4.0 mL of sulfuric acid reagent (containg 10 g/L Ag2SO4) and 1.5 mL of digestion solution (containing 20.432 g/L K2Cr2O7 and 33.3 g/L HgSO4) was heated at 150 C for 2 h. The samples were then cooled and analyzed on a spectrophotometer at 600 nm. The oxygen transform on the RDE were investigated using chronoamperometry (CA) measurements. The diffusion coefficients of dissolved O2 in liquid membrane (D) are calculated from the slopes of linear parts of the plots of Inet against t 1=2 , based on Cottrell equation of semi-infinite diffusion: 1
Inet ¼
no FAD2 C 1
3. Results and discussion 3.1. Hydrogen peroxide production performance
nFC H2 O2 V 100% Rt 0 Idt
Where n is the number of electron transfer of oxygen reduction to H2O2 (n = 2), F is the faradic constant (96,486 C/mol), CH2O2 is the concentration of H2O2 (mol/L), V the bulk volume (L), I the applied current (A), and t is time (s). TC concentration was analyzed on an UV–Vis spectrophotometer equipment at l = 355 nm, and its removal efficiency (h) was calculated by the following formula:
h¼
75
ð6Þ
1
t 2 p2
Where no is the number of electron-transfer (no = 2), and C is the bulk concentration of dissolved oxygen where the disk emerges from the liquid. For a rotating disk gas-liquid contactor, a relationship has been derived between the amount of liquid entrained by the rotation of disk and its rotational velocity [34]: 1
d ¼ KV c 2
ð7Þ
Where d is a mean film thickness on the disk, K is a function of the 1
acceleration of gravity and the kinematic viscosity and V c 2 is the linear velocity. For water at 20 C, K is 1.20 104 m1/2 s1/2 [35].
The effects of the pH on H2O2 production and current efficiency at RDE system were shown in Fig. 2a. Results indicated that the highest EF activity was attained under pH 3.0, where the H2O2 production reached near 37.1 mg/L in 1 h. It shown an agreement between the experimental data and the theory that observed previously, which indicated pH 3.0 was optimal for the Fenton process [36–41]. From Eq. (1), it seems that a low pH was beneficial to the higher electro-generation of H2O2 since its synthesis was in acidic medium. However, a high proton concentration may promote H2 evolution and reduce the current efficiency. What’s more, the efficiency of EF process dropped rapidly especially when pH 4. This is due to H2O2 could be rapidly decomposed to oxygen and water at neutral and higher pH [42]. The cathodic potential determines the formation and current efficiency of H2O2, and thus affects the waste water treatment performance [43]. The effects of cathodic potential on H2O2 production were shown in Fig. 2b. It was indicated that the accumulation of H2O2 increased initially with the increasing cathodic potentials, and the highest H2O2 accumulation was achieved at 0.8 V vs SCE. When the potential exceeded the optimum, the H2O2 accumulation declined sharply. And the current efficiency of H2O2 production at the applied potential of 0.6,0.7,0.8, 0.9 and 1.0 V vs SCE was 15.1%, 16.6%, 17.1%, 9.3% and 8.9%, respectively. The increased applied potential can accelerate the electron transfer between the reactive solutions and promote the oxygen reduction reaction. And when the applied potential was further increased (> 0.8 V vs SCE), the side reactions such as hydrogen evolution on the cathode might strengthen, which resulted in a decreased H2O2 yield and current efficiency. In addition, the further investigation also showed that increased hydrogen evolution at higher potential could destroy the surface structure of graphite cathode. To study the effects of rotation speed on the H2O2 generation, experiments were conducted by varying the speed in the range of 100–500 rpm during 60 min electrolysis. As shown in Fig. 2c, the H2O2 concentration increased from 15.6 to 45.3 mg/L when the rotating speed increased from 100 to 400 rpm. However, no additional improvement in H2O2 concentration was observed when the rotation speed increased further to 500 rpm. In addition, the CE at rotation rate of 100–500 rpm were 10.5%, 17.3%, 17.5%, 17.4% and 9.1%, respectively. This observation indicated that a suitable cathode rotation speed was favorable for electro-
Fig. 2. Effects of pH values (a), cathodic potential (b) and rotation speed (c) on hydrogen peroxide generation (histogram) and current efficiency (solid square) in 0.05 M Na2SO4 solution in 1 h with different conditions: (a) cathodic potential of 0.8 V, the rotation speed of 300 rpm, and pH of 2.0, 3.0, 4.0, 5.0 and 6.0; (b) cathodic potential of 0.6, 0.7, 0.8, 0.9 and 1.0 V, pH of 3.0, and the rotation speed of 300 rpm; (c) pH of 3.0,cathodic potential 0.8 V, and rotation speed is 100, 200, 300, 400 and 500 rpm.
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generation of H2O2, which might be explained by the following factor. The cathode rotation increased the contact area between oxygen and electrode, thus, improved the efficiency of oxygen mass transfer and the generation rate of H2O2. However, when rotation speed was too high, the resistance of electrolyte solution increased with the excessive bubbles in the system, resulting in a drop in the yields of H2O2 [43]. In addition, we observed that the high rotation speed (500 rpm) would cause splashing of the solution, which could be an additional reason for the decrease of H2O2 accumulation at high rotation speed. Based on the results above, H2O2 accumulation arrived at its maximal value of 0.90 mg/L/h/cm2 when the conditions were concluded as 0.8 V vs SCE of the cathodic potential, 400 rpm of rotation speed and 3.0 of pH. Compared with previous reports, it suggested that the generation rate of H2O2 in present work was higher than those cases using graphite in conventional systems involving O2 sparging. For example, several research groups obtained H2O2 generation rates of 0.15 and 0.18 mg/L/h/ cm2 at O2 flow rates of 114 and 6.0 L/h, respectively [7,42]. This observation indicated that the current approach was more efficient, owing to the constant rotation of the disk. The liquid film structure formed on the disk surface may be more conducive to the diffusion of oxygen from gas-phase to the film.
A series of experiments were carried out to test the influence of initial pollutant concentration on this process. As shown in Fig. 4a, when the initial concentration was 50 mg/L, TC was almost completely removed in 120 min. However, the TC removal percentages decreased to 95.8%, 89.5% and 80.1% with the initial concentration of 100, 200 and 300 mg/L, respectively. The mineralization of TC was monitored by measuring the COD of the solution and the results were presented in Fig. 4c. The mineralization decreases from 70.4% to 48.9% as the increasing initial TC concentration from 100 to 300 mg/L. The Fig. 4b presents the excellent correlation decay considering a pseudo-first-order
3.2. TC removal by electro-Fenton Organics degradation by EF process not only depends on H2O2, but also is controlled by the concentration of Fe2+ as catalyst [44]. Fig. 3 shows the effects of the concentration of externally added ferrous ions on TC degradation. This degradation processes were found to obey pseudo-first-order kinetics. In the absence of Fe2+, a slow degradation rate was observed and only 55.4% of TC decay was attained after 60 min of electrolysis. An obvious increase of the degradation rate was observed by adding Fe2+ into solution. For example, the TC removal efficiency were 92.9%, 96.1% in 0.5 and 1.0 mM Fe2+, respectively. However, no evident improvement (75.8%) was observed when the concentration increased to 2.0 mM. The negative effect of the higher Fe2+ concentration can be explained by increasing rate of the parasitic reaction occurring between the hydroxyl radicals and the excess of Fe2+ [6,38,44,45]. Fe2þ þ OH ! Fe3þ þ OH
ð8Þ
Fig. 3. Influences of the concentration of Fe2+ (0.0, 0.5, 1.0 and 2.0 mM) on the degradation of tetracycline. Conditions: pH 3, cathodic potential 0.8 V, Na2SO4 0.05 M, rotation speed is 400 rpm and TC = 50 mg/L. The inset presents the corresponding kinetics analysis assuming a pseudo first-order reaction.
Fig. 4. Influences of the initial tetracycline on degradation (a) and COD removal (c), and (b) the corresponding tetracycline degradation kinetics at the same conditions: pH = 3.0, [Fe2+] =1.0 mM, Na2SO4 0.05 M, cathodic potential 0.8 V and rotation speed 400 rpm.
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77
Table 1 the apparent rate constants of TC degradation and COD removal with varying initial TC concentration. Concentration of TC (mg/L)
50 100 200 300
RDE system
FDE system
Kapp (min1)
R2
COD removal (%)
Kapp (min1)
R2
COD removal (%)
0.0450 0.0265 0.0180 0.0133
0.9816 0.9983 0.9895 0.9938
78.7 70.4 52.6 48.9
0.0129 0.0110 0.0082 0.0068
0.9880 0.9934 0.9970 0.9910
69.5 55.8 46.5 40.8
reaction rate. The values of the apparent rate constants obtained from the kinetic analyses are reported in Table 1. In order to clarify the changes in the molecule of TC, samples obtained during the electrolysis were analyzed by UV–Vis techniques (Fig. 5). The spectrum of TC before the treatment present two peaks at 275 nm and 355 nm, which correspond to the absorption peaks of benzene ring structure and the p-p* transition. The intensity of absorption at 355 nm decreased during the electrochemical treatment, which indicated the destruction of the naphthalene rings. The adsorption peak at 275 nm diminished and decreased greatly under 60 min of electrolysis. The fixed disk electrode (FDE) process for TC degradation was investigated to identify the influence of RDE on the process. Fig. 6a illustrates the comparative degradation behavior for TC ranging from 50 to 300 mg/L. During this process, about 80.5% of TC removal was achieved within 120 min when TC initial concentration was 50 mg/L. From Fig. 6b, it can be seen that TC degradation followed the first-order reaction kinetics with the different rate constants and detailed values are summarized in Table 1. The effect of the initial concentration on the removal of COD is given in Fig. 6c. For initial TC concentrations of 100, 200, and 300 mg/L, the COD removal efficiencies were 55.8%, 46.5% and 40.8%, respectively. The degradation of TC in RDE and FDE system are compared in Table 1. It was observed that with an increase in the initial TC concentration, the rate constant in the RDE system decreased. It was 0.0450 min1 at 50 mg/L, 0.0265 min1 at 100 mg/L, 0.0180 min1 at 200 mg/L and 0.0133 min1 at 300 mg/L. A similar phenomenon was also observed in the FDE system. When the TC concentration increased from 50 mg/L to 300 mg/L, the rate
Fig. 5. UV–Vis spectral changes with electrolysis time for 100 mg/L tetracycline solution. Experimental conditions: pH 3.0, cathodic potential 0.8 V, [Fe2 + ] = 1.0 mM, rotation speed is 400 rpm, Na2SO4 0.05 M (Samples were diluted for 2 folds before monitoring).
Fig. 6. The fixed-cathode EF on the degradation of tetracycline (a) and the removal of COD (c) at the same conditions: pH = 3.0, [Fe2+] = 1.0 mM, Na2SO4 0.05 M, cathodic potential 0.8 V, oxygen flow rate: 0.3 L/min. (b) represents the kinetic analysis of tetracycline degradation.
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constant decreased apparently from 0.0129 to 0.0068 min1. This was reasonable because a higher initial TC would lead to a higher concentration of intermediates, which could strengthen the competitive reactions and decrease the degradation rate. It also should be noted that the degradation rate constant of the RDE process within 120 min was nearly 3 times that of the FDE system at the same initial TC concentration. These results show that RDE process has obvious advantages in TC degradation, which may be explained as follow: the rotation of cathode is not only advantageous for the oxygen diffusion into the surface of electrode and H2O2 generation, but also plays the role of the stirrer, being conductive to the spread of TC. 3.3. Oxygen transfer process on RDE system As discussed above, the rotation of disk cathode resulted in the efficient production of H2O2 without oxygen aeration, offering a potentially effective EF method for degrading organic pollutants. Some researchers considered the liquid film formed on the disk to be mainly responsible for the oxygen transfer [19]. The liquid film formed on the disk during the air-exposure cycle would renew its surface due to the forces, such as centrifugal. The air–liquid interface and renewed surface of the liquid film are responsible for the oxygen transfer into the liquid. In order to clarify the mechanism of the oxygen transport, the diffusion process of oxygen on this electrode is analyzed as follow. During the continuous rotating, an aqueous film will be formed on the rotating disk cathode surface. In order to clarify the mechanism of the oxygen transport on RDE system, the average film thickness was calculated in the first. We used Eq. (7) suggested by Zeevalkink et al. to compute liquid film thickness (d). The calculated d is 1.31 104 m. Then the chronoamperometry was used to determine the oxygen diffusion in the aqueous film on the RDE surface. Fig. 7 presents the chronoamperometric behaviors of RDE in phosphate buffer solution with pH 3, 300 rpm, and the initial and final electrode potentials at 0 and 0.65 V vs SCE, respectively. Oxygen reduction reaction does not occur when the initial potential is 0. However, almost 0.1 A current was detected in the system when the potential was 0 V, which could be explain by the followings. The disk electrode acted as a capacitor in the case and the current was the discharge current. As shown in the inset in Fig. 7, the plot of Inet against t2 showed a straight in a short time, while it deviated from initial linearity later. The initial straight line for Cottrell plot 1
reflects the O2 reduction was controlled by the O2 diffusion process inside the film. While, the slight deviation (0.321) from initial linearity (0.458) implied that there was no substantial changes in the process of mass transfer. Thus, the slight deviation in slope may be attribute to the instability of the rotating shaft. The diffusion coefficients of oxygen in liquid membrane (D) are calculated from the slopes of linear parts of the plots of Inet against t2 , based on Cottrell equation of semi-infinite diffusion Eq. (6) [46]. The calculated D is 19.45 105 cm2/s as collected in Table 2. It is assumed that the liquid film has a constant thickness related to the disk radius while it moves with the disk cathode, and that the velocity profile is flat across the film thickness. The oxygen balance is developed for an element of volume from the liquid film as follow. The mass transfer coefficient, K m is calculated as 1.48 m/s using Eq. (9). 1
Km ¼
D
ð9Þ
d
Then, the time of oxygen through the aqueous film (T) and the contact time of the rotating disk with air (TR) can be calculated with (11) Eq. (10) and, respectively. T¼
d
ð10Þ
Km
TR ¼
60 2n
ð11Þ
The calculated results are T = 8.8 105 s and TR = 1.67 103 s, which means that the time of this process that oxygen diffuse to the electrode surface from the gas phase is far less than the contact time of the rotating cathode with air. Thus, it can be proposed that the oxygen is able to diffuse through the aqueous film on the graphite within the contact time in each rotating circle. Theoretically, the oxygen diffusion time from air to the surface of the electrode (T) will be equal to the time of disk exposed to the air (TR) when the speed is 1698 rpm. Obviously, the oxygen was kept in a state of saturated under the condition of this experiment. In addition, we monitored the dissolved oxygen changes in the RDE system and found the dissolved oxygen of the solution was saturated, which agreed with the theoretical calculation. These data for O2 diffusion on the RDE system are summarized in Table 2. The detailed values of oxygen transfer parameters in FDE system are also summarized in Table 2. In the RDE system, the oxygen mass transfer process involved multi-phase interfaces. Therefore, it was much more complicated compared with the common gas diffusion process. The calculated diffusion coefficient here was only one apparent diffusion coefficient. It must be noticed that the value of D in present work was nearly 10 times greater than the diffusion coefficient of oxygen in FDE system reported in previous report. This observation indicated that a suitable cathode rotation speed was favorable for the transfer of oxygen, which might be explained by the following two factors. Firstly, a region with high concentration of DO was formed around the cathode because of the rotating aeration. Secondly, a film was formed on the upper part of RDE during continuous rotation, thus oxygen could diffuse directly through the gas–liquid interface existing between the disk and the gas-phase [26]. It was further shown that
Table 2 Parameters for O2 diffusion on RDE and FDE
Fig. 7. Chronoamperograms response on the rotating graphite disk cathode with an initial potential of 0 V and a final potential of 0.65 V. The insets show the plots of the net electrolysis current Inet vs t1/2 at corresponding electrodes.
cathode
D (105cm2/s) d (103 m) Km (105 m/s) DO (mg/L) reference
RDE FDE
19.5 2.0
13.1 74.0
14.8 2.7
8.1 8.3
this work [42]
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the RDE can easily improve the ability of degradation of organic matter by improving the diffusion of dissolved oxygen. 4. Conclusion The conclusions drawn from this study can be summarized as follows: (1) The optimal conditions for H2O2 generation using this RDE were cathodic potential of 0.8 V vs SCE, initial pH 3 and the rotation speed of 400 rpm. Under the optimal conditions, the H2O2 generation rate could reach 0.90 mg/L/h/cm2. (2) Fe2+ concentration strongly influenced the performance of TC degradation by EF process and 1.0 mM was the best catalytic concentration. Under the above optimal conditions, TC (under 100 mg/L) was the almost completely degraded (96–100%) in the RDE processes in relatively short time (120 min). (3) Enhanced mineralization of TC and the removal of COD were obtained during the rotating disk cathode process compared with fixed-cathode EF process, and the possible reasons have been discussed. (4) Continuous rotating could have allowed oxygen to diffuse more easily into the aqueous film on the upper cathode surface, which was beneficial to the oxygen reduction to H2O2. Chronoamperometry analysis demonstrated that the diffusion coefficients of dissolved oxygen on the RDE was 19.45 105 cm2/s, which was greater than that on the FDE. Furthermore, the calculation indicated that the oxygen was able to diffuse through the aqueous film on the graphite within the contact time in each rotating circle. Acknowledgments This work was done under a grant from National Natural Science Foundation of China Grant No.21007033. References [1] I. Sirés, N. Oturan, M.A. Oturan, R.M. Rodríguez, J.A. Garrido, E. Brillas, ElectroFenton degradation of antimicrobials triclosan and triclocarban, Electrochim. Acta 52 (2007) 5493–5503. [2] L. Zhao, Z. Sun, J. Ma, H. Liu, Enhancement Mechanism of Heterogeneous Catalytic Ozonation by Cordierite-Supported Copper for the Degradation of Nitrobenzene in Aqueous Solution, Environ. Sci. Technol. 43 (2009) 2047– 2053. [3] J.Y. Zhemin Shen, Xiaofang Hu, YangMing Lei, XiuLing Ji, Jinpin Jia, WenHua Wang, Dual Electrodes Oxidation of Dye Wastewater with Gas Diffusion Cathode, Environ. Sci. Technol. (2005) 1819–1826. [4] M.A. Oturan, E. Brillas, Electrochemical Advanced Oxidation Processes (EAOPs) for Environmental Applications, Portugaliae Electrochim. Acta: J. Portuguese Electrochem. Soc. 25 (1) (2007) 1–18. [5] M. Luo, S. Yuan, M. Tong, P. Liao, W. Xie, X. Xu, An integrated catalyst of Pd supported on magnetic Fe3O4 nanoparticles: simultaneous production of H2O2 and Fe2+ for efficient electro-Fenton degradation of organic contaminants, Water Res. 48 (2014) 190–199. [6] M. Zhou, Q. Yu, L. Lei, G. Barton, Electro-Fenton method for the removal of methyl red in an efficient electrochemical system, Sep. Purif. Technol. 57 (2007) 380–387. [7] A. Wang, J. Qu, J. Ru, H. Liu, J. Ge, Mineralization of an azo dye Acid Red 14 by electro-Fenton's reagent using an activated carbon fiber cathode, Dyes Pigments 65 (2005) 227–233. [8] B. Balci, N. Oturan, R. Cherrier, M.A. Oturan, Degradation of atrazine in aqueous medium by electrocatalytically generated hydroxyl radicals. A kinetic and mechanistic study, Water Res. 43 (2009) 1924–1934. [9] N. Oturan, J. Wu, H. Zhang, V.K. Sharma, M.A. Oturan, Electrocatalytic destruction of the antibiotic tetracycline in aqueous medium by electrochemical advanced oxidation processes: effect of electrode materials, Appl. Catal. B: Environ. 140-141 (2013) 92–97. [10] M.A. Oturan, N. Oturan, M.C. Edelahi, F.I. Podvorica, K.E. Kacemi, Oxidative degradation of herbicide diuron in aqueous medium by Fenton's reaction based advanced oxidation processes, Chem. Eng. J. 171 (2011) 127–135. [11] L. Zhou, M. Zhou, Z. Hu, Z. Bi, K.G. Serrano, Chemically modified graphite felt as an efficient cathode in electro-Fenton for p-nitrophenol degradation, Electrochim. Acta 140 (2014) 376–383.
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