- Email: [email protected]
Sb-SnO2 electrodes
Journal of Hazardous Materials 239–240 (2012) 225–232
Contents lists available at SciVerse ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Comparison of various organic compounds destruction on rare earths doped Ti/Sb-SnO2 electrodes Yu-Hong Cui a,b , Yu-Jie Feng b,∗ , Junfeng Liu b , Nanqi Ren b School of Environmental Science and Engineering, Huazhong University of Science and Technology, No. 1037 Luoyu Road, Hongshan District, Wuhan 430074, PR China State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, No. 73 Huanghe Road, Nangang District, Harbin 150090, PR China
Different REs doping has distinct effect on the Ti/Sb-SnO2 electrode performance. Gd or Eu improves the performance of Ti/Sb-SnO2 on aromatic ring cleavage. Catechol is more refractory to be degraded than benzoquinone and hydroquinone. The molecular structure of organic compound influences its degradation rate.
a r t i c l e
i n f o
Article history: Received 26 February 2012 Received in revised form 27 August 2012 Accepted 28 August 2012 Available online 4 September 2012 Keywords: Electrode Organic pollutant Electrocatalysis Rare earth Wastewater treatment
g r a p h i c a l
a b s t r a c t
Benzoquinone Hydroquinone Catechol Maleic acid Fumaric acid Succinic acid Malonic acid Oxalic acid Acetic acid
40 -1
h i g h l i g h t s
30
-3
b
kTOC (×10 min )
a
20
10
0
0
10
20
30 -3
40
-1
korg (×10 min )
a b s t r a c t Ti/Sb-SnO2 and three kinds of rare earths (REs), namely Ce, Gd, and Eu doped Ti/Sb-SnO2 electrodes were prepared and tested for their capacity on electrocatalytic degradation of three kinds of basal aromatic compounds (benzoquinone, hydroquinone and catechol) and six kinds of aliphatic acids (maleic acid, fumaric acid, succinic acid, malonic acid, oxalic acid and acetic acid). The elimination of selected organics as well as their TOC removal with different doped Ti/Sb-SnO2 electrodes was described by first-order kinetics. Compared with Ti/Sb-SnO2 , the Gd and Eu doped electrodes show better performance on the degradation of most of the selected organics, while Ce doped electrode shows either closely or lower efficiency on the degradation of these selected organics. Besides electrode material, the molecular structure of organic compound has obvious effect on its degradation in the electrocatalytic process. Catechol is more resistant to the electrophilic attack by hydroxyl radicals than benzoquinone and hydroquinone. The compound with more complicate molecular structure or longer carbon chain is more difficult to be mineralized. The aliphatic acid with higher oxygen content or more double bonds is more readily to be oxidized in the electrocatalytic process. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Along with the industrial development, various wastewaters containing organic pollutants are produced. Some of the organics presented in industrial wastewaters are refractory and can not be eliminated by traditional treatment, and then the
∗ Corresponding author. Tel.: +86 451 86283068; fax: +86 451 87162150. E-mail address: [email protected] (Y.-J. Feng). 0304-3894/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2012.08.068
possible release of organic contaminants into the aquatic environment from wastewater effluents has gained much attention recently [1–5]. On the other hand, the overuse of pesticides, pharmaceuticals and personal care products in modern society also brings many kinds of organic pollutants to aquatic environment [2,6,7]. No matter what source of the organic pollutants, their presence in water may affect aquatic organisms and can lead a potential risk on human health through drinking water or consumption of food irrigated by polluted water. Therefore, it is necessary to develop new and effective treatment processes toward organic pollutions in water.
226
Y.-H. Cui et al. / Journal of Hazardous Materials 239–240 (2012) 225–232
Advanced oxidation processes (AOPs) have been developed to deal with organic pollutants in water through in situ generation of strong oxygen-based oxidizers, among which the formation of hydroxyl radical (• OH) is especially desired, since it is the strongest oxidizer and can react non-selectively with various types of organics [2,5,8]. Electrocatalytic oxidation is one of the AOPs and has drawn considerable research attention due to its simplicity and robustness in structure and operation [9–11]. The effectiveness of electrocatalytic oxidation depends largely on the activities of electrode materials toward the target pollutants [5,9,10,12,13]. Therefore, the discovering of more efficient electrodes with good capacity on organics destruction receives an increasing interest, including boron-doped diamond (BDD) electrodes and various doped metal oxide-film electrodes such as PbO2 and SnO2 [1,5,9,10,12]. It has been reported that the surface modification as well as the chemical composition significantly influences the electrode performance for either diamond electrode or metal oxide-film electrode [12,14–16]. With proper doping of selected dopants such as bismuth, fluoride and rare earths (REs), the electrocatalytic activity or stability of PbO2 electrodes can be considerably enhanced [14,17,18]. Modification of SnO2 electrode by dopants to enhance the electrochemical properties has been studied and the most promising electrode is found to be the antimony doped Ti-based SnO2 anodes (Ti/Sb-SnO2 ) [19]. Recent studies indicate that the co-doped SnO2 -Sb-X electrodes (X represents Pt, Ni, Co, etc.) have better performance or some special properties in an electrocatalytic process [20–22]. The previous investigations by our group have found that proper doping the Sb-SnO2 anode with REs metal ions can improve the electrocatalytic oxidation rate of phenol [23]; further analysis suggested that the better performance of REs doped Sb-SnO2 anodes is aroused from the increased production of active oxygen species (e.g. • OH) and the reduced oxygen vacancies in crystal lattice [12]. Besides anode catalyst materials, the target organic pollutant is also very important in determining overall treatment efficiency in an electrocatalytic oxidation process. Although organic substrates are primarily degraded by the in situ generated hydroxyl radicals, the mechanism of different organics degradation may be distinctively different. For example, the electrocatalytic oxidation of di- and tri-nitrophenols on Bi-doped PbO2 electrode is largely influenced by the molecular structures of the nitrophenols [24]. Our previous research also found that the electrocatalytic destruction of BPA and its intermediates may follow different mechanisms due to their different adsorption and activation characters [5]. Surface modification by dopants to develop highly efficient anodes for water treatment has been made in several studies [12,14,17,22,23] and the effects of organics molecule on its degradation using a certain anode were discussed by a few researchers [24]. Yet the performance of various organic compounds destruction on a certain anode material with different dopants has not been systematically investigated. Besides, since some special groups (i.e. NO2 , SO3 H, Cl) or the intermediate active species (i.e. Cl• , HOCl/ClO− ) formed from electrolysis of some parent compounds may cause interference in a comparison study, the present study selected nine kinds of fundamental organics including three kinds of basal aromatic compounds (benzoquinone, hydroquinone and catechol) and six kinds of aliphatic acids (maleic acid, fumaric acid, succinic acid, malonic acid, oxalic acid and acetic acid) which are often observed as intermediate products during the oxidation of many refractory organic compounds [5,10,16,24] as model compounds to verify the performance of three kinds of REs (Ce, Gd and Eu) doped Ti/Sb-SnO2 electrodes and the control electrode (Ti/SbSnO2 ) in electrocatalytic oxidation process. The kinetics and the efficiency of the selected organics degradation on different REs
doped Ti/Sb-SnO2 electrodes were investigated and the degradation pathway as well as mechanism was also discussed. 2. Experimental 2.1. Electrode preparation Ti/Sb-SnO2 and REs doped Ti/Sb-SnO2 (Ti/Re-Sb-SnO2 ) electrodes were prepared by thermal deposition of the coating layers as previously described [12,23], including pretreatment of titanium plates, preparation of inner layer and outer layer coatings. Ti plates with a dimension of 2 cm × 3 cm were used as the base metal for all oxide-coated electrodes. The plate was polished using 40-grit and 320-grit sand papers, degreased in hot 40% NaOH at 80 ◦ C for 2 h, and then etched in 10% boiling oxalic acid for 2 h followed by thorough washing with distilled water, and dried. The electrodeposition of antimony-tin inner layer was applied first followed by thermal oxidation. For electro-deposition, the pretreated Ti plate was placed as the cathode in 100 mL of alcohol solution containing 17.5 g SnCl4 ·5H2 O, 0.73 g Sb2 O3 , and 2 mL of concentrated (37%) HCl. The deposition was carried out at constant current of 0.12 A for 25 min with two counter electrodes made of Ti/RuO2 . After electrodeposition, the electrodes were heated in an oven at 400 ◦ C for 2 h. For the second coating step to get the outer layer, the uniformly coated plate was hot annealed after being dip coated in a solution consisting of 30 g SnCl4 ·5H2 O, 0.8 g Sb2 O3 , 2.5 mL of concentrated (37%) HCl and proper rare earth nitrates (with a molar ratio of Re/Sn of 2:100) in 50 mL n-butanol. The electrode was then dried in an infrared oven. After five cycles of both dipping and drying, the Ti plates were heated in a muffle oven (450 ◦ C for 20 min) for coating pyrolysis. This whole process (dipping, drying and pyrolysis) was repeated 5 times and finally, the electrodes were heated at 650 ◦ C for 3 h. 2.2. Organics electrolysis The electrolysis cells were a series of 100 mL glass beakers. For each cell, a 6 cm2 (2 cm × 3 cm) anode, prepared as described above, and a stainless steel cathode having the same area was placed in the beaker at a spacing of 8 mm between the electrodes. A DC potentiostat with a voltage range of 0–30 V was used as the power supply for organic degradation studies. For an electrolysis test, 100 mL of an aqueous solution with a pre-determined concentration of 200 mg L−1 (for aromatic compounds) or 100 mg L−1 (for aliphatic acids) was placed in the cell. 0.25 M Na2 SO4 was used as supporting electrolyte in the electrolysis cell which was reported to be better for the surface-specific reaction on the working anode [25]. Electrolysis was performed under galvanostatic control at 0.12 A. During organics destruction, the voltage evolution of each electrolytic cell was recorded as an index of system stability. No remarkable difference was found between different electrodes, and the voltages kept almost constant in the electrocatalytic processes. At the end of each run, based on a pre-determined time of electrolysis, the solution in the cell was analyzed for the concentration of original compound, total organic carbon (TOC) and intermediate products. 2.3. Analysis methods TOC was analyzed using a TOC Analyzer (Shimadu 5000A). A high performance liquid chromatograph (HPLC) consisting of a 2695 separation module and 2996 photodiode array detector (Waters) was used for concentration measurement of the organic compound and its intermediate products during electrolysis, including aromatic compounds and organic acids. A Waters Symmetry C18 column (5 m × 50 mm × 3.91 mm) was used as the
Y.-H. Cui et al. / Journal of Hazardous Materials 239–240 (2012) 225–232
227
Table 1 Current efficiency and energy consumption for electrocatalytic degradation of various organics with different Ti/Re-Sb-SnO2 electrodes. Organic compounds
Electrodes Ti/Sb-SnO2
Benzoquinone MCE (%)a Ep (J(mg L−1 TOC)−1 )b Hydroquinone MCE (%)a Ep (J(mg L−1 TOC)−1 )b Catechol MCE (%)a Ep (J(mg L−1 TOC)−1 )b Maleic acid MCE (%)a Ep (J(mg L−1 TOC)−1 )b Fumaric acid MCE (%)a Ep (J(mg L−1 TOC)−1 )b Succinic acid MCE (%)a Ep (J(mg L−1 TOC)−1 )b Malonic acid MCE (%)a Ep (J(mg L−1 TOC)−1 )b Oxalic acid MCE (%)a Ep (J(mg L−1 TOC)−1 )b Acetic acid MCE (%)a Ep (J(mg L−1 TOC)−1 )b a b
Ti/Ce-Sb-SnO2
Ti/Gd-Sb-SnO2
Ti/Eu-Sb-SnO2
29.5 29.1
25.7 33.4
32.6 26.3
39.4 21.8
22.5 40.5
20.5 44.5
34.5 26.4
43.8 20.8
18.2 50.1
14.2 64.2
27.8 32.8
52.1 17.5
5.91 67.6
3.45 115.8
8.23 48.4
11.29 35.4
5.62 71.0
6.44 62.0
11.38 35.0
12.43 32.0
5.85 78.2
5.59 81.9
10.74 42.6
9.01 50.8
6.32 47.0
4.98 59.6
7.09 41.8
7.89 37.6
4.84 17.7
5.12 16.7
6.24 13.7
5.70 15.0
10.11 50.9
6.45 79.8
11.30 45.5
8.76 58.8
Average mineralization current efficiency calculated when 60% of TOC was removed. Average energy consumption per unit of TOC removal calculated when 60% of TOC was removed.
separation column for the aromatic compounds. The mobile phase was a mixture of methanol and water (50:50, v/v), and the flow rate was set as 0.5 mL min−1 . For the separation of organic acids, an Alltech Atlantis dC18 column (5 m × 150 mm × 3.9 mm) was used, with 20 mM NaH2 PO4 solution (pH 2.7) as the mobile phase at a flow rate of 0.5 mL min−1 . Before each analysis, the sample was filtered through a 0.45-m membrane filter (Millex), and the HPLC injection volume was 10 L.
The mineralization current efficiency (MCE) for electrocatalytic degradation of organics is estimated from the expression (1) and the energy consumption per unit of TOC removal Ep is obtained as expression (2) [26]:
2.4. Kinetic analysis
Ep =
Two kinetic models are usually adopted to describe electrocatalytic degradation of organic pollutants. One is first-order kinetics which is used when the electrode reaction is sufficiently rapid and the whole electrolysis is under mass transfer control; the other is zero-order kinetics which is suitable for the electrode reaction in which the electron transfer process is hindered on the electrode surface and the whole electrolysis is under current control. According to our previous study on SnO2 -based electrode kinetics [26], the first-order kinetics was adopted in the present kinetic analysis.
where (TOC)theor is the theoretically calculated TOC abatement considering that the applied electrical charge is only consumed in the corresponding mineralization reaction, (TOC)exp is the experimental TOC removal with a given charge consumption, Q is the charge consumption at a give electrolysis time and U is the cell voltage reading from the DC power supply.
Table 2 Observed organic intermediates in HPLC analysis. Initial compounds
Observed intermediates
Benzoquinone
Hydroquinone, 2-ketoglutaric acid, maleic acid, fumaric acid, succinic acid, oxalic acid, formic acid Benzoquinone, 2-ketoglutaric acid, maleic acid, fumaric acid, oxalic acid 2-Ketoglutaric acid, maleic acid, fumaric acid, formic acid Fumaric acid, malonic acid, oxalic acid, formic acid Oxalic acid, formic acid Malonic acid, formic acid Acetic acid — Formic acid
Hydroquinone Catechol Maleic acid Fumaric acid succinic acid Malonic acid Oxalic acid Acetic acid
2.5. Efficiency calculation
MCE =
(TOC)exp (TOC)theor
× 100
QU (TOC)exp
(1) (2)
3. Results and discussion 3.1. Kinetic investigation of aromatic compounds degradation on Ti/Re-Sb-SnO2 electrodes 3.1.1. Effect of REs doping on the electrode performance Electrocatalytic degradation of benzoquinone, hydroquinone and catechol with different Ti/Re-Sb-SnO2 electrodes was evaluated and the observed rates of their elimination and mineralization (expressed as TOC removal) were fitted with first-order rate constants (Fig. 1 and Figs. S1 and S2 (Supplementary material)). The reaction rate constants korg (for the compound elimination) and kTOC (for the compound mineralization) are shown in Fig. 2. It seems that doping with Gd or Eu can slightly improve the capacity of Ti/Sb-SnO2 electrode on these aromatics elimination as the rate constant korg are 6.3–32.3% higher than that of the control (Ti/SbSnO2 ), while doping with Ce depresses the activity Ti/Sb-SnO2
Y.-H. Cui et al. / Journal of Hazardous Materials 239–240 (2012) 225–232
(a)
5
Ti/Ce-Sb-SnO2
4
Ti/Gd-Sb-SnO2 Ti/Eu-Sb-SnO2
3
50
-1
Ti/Sb-SnO2
40
-3
6
30
korg (×10 min )
-ln([Hydroquinone]/[Hydroquinone]0)
228
2
(a)
20 10
1 0
0
0
30
60
90
120
(b)
3
Ti/Gd-Sb-SnO2 Ti/Eu-Sb-SnO2
2
1
0
10
-3
Ti/Ce-Sb-SnO2
5
0
0
60
120
180
(b) Benzoquinone Hydroquinone Catechol
-1
Ti/Sb-SnO2
kTOC (×10 min )
4
Ti/Sb-SnO2 Ti/Ce-Sb-SnO2 Ti/Gd-Sb-SnO2 Ti/Eu-Sb-SnO2
15
Time (min)
-ln([TOC]/[TOC]0)
Benzoquinone Hydroquinone Catechol
240
300
Time (min) Fig. 1. Kinetics of (a) hydroquinone elimination and (b) TOC removal in electrocatalytic degradation of hydroquinone with different Ti/Re-Sb-SnO2 electrodes. Initial hydroquinone = 200 mg L−1 , volume of solution = 100 mL, I = 0.12 A. The solid lines represent the fitted curves and the dots represent the experimental data.
electrode on these aromatics elimination especially for benzoquinone as its korg is 30% lower than that of the control (Fig. 1(a), Figs. S1(a) and S2(a) (Supplementary material)). For these aromatics mineralization, the effect of REs doping on Ti/Sb-SnO2 electrode performance seems more evident (Fig. 1(b), Figs. S1(b) and S2(b) (Supplementary material)). The Ti/Gd-Sb-SnO2 and Ti/Eu-Sb-SnO2 electrodes show 20–35% (for benzoquinone), 80–92% (for hydroquinone), and 49–115% (for catechol) higher TOC removal rates (kTOC ) compared with the control, respectively; while the Ti/Ce-Sb-SnO2 electrode shows a poor performance as its kTOC is 7.8% (for benzoquinone), 12% (for hydroquinone) and 24% (for catechol) lower than that of the control, respectively. 3.1.2. Effectiveness of electrocatalytic degradation on different aromatic compounds To compare the destruction effect of electrocatalytic process on different organics, the reaction rate constants of the three selected aromatic compounds are summarized and shown in Fig. 2. It seems that catechol is more refractory to be destroyed in the electrocatalytic process as its korg and kTOC are 33–50% and 17–57% lower than that of other two compounds, respectively. Furthermore, the value of korg for catechol elimination on each electrode is significantly lower than other two compounds (Fig. 2(a)), while this phenomenon is not observed as obviously as for that of kTOC (Fig. 2(b)). So, the first step of catechol transformation might be more resistant to the electrocatalytic oxidation process compared with other two aromatic compounds. It is generally accepted that
Ti/Sb-SnO2 Ti/Ce-Sb-SnO2 Ti/Gd-Sb-SnO2 Ti/Eu-Sb-SnO2
Fig. 2. Reaction rate constants of (a) organics elimination and (b) TOC removal for different aromatic compounds in electrocatalytic degradation process with different Ti/Re-Sb-SnO2 electrodes. Initial aromatic compounds = 200 mg L−1 , volume of solution = 100 mL, I = 0.12 A.
electrocatalytic oxidation of aromatics starts with the attack of hydroxyl radicals to form hydroxylated intermediates and then the ring cleavage happens which leads to the formation of aliphatic species, mainly carboxylic acids [5,10]. However, the effect of this transformation process might be dependent on the structure of organic compounds [24]. From the results shown in Fig. 2, the molecular structure of catechol should be more resistant to the electrophilic attack by hydroxyl radicals among the three selected aromatic compounds, which is caused either by the steric hinderance effect or by the electron effect.
3.2. Kinetic investigation of aliphatic acids degradation on Ti/Re-Sb-SnO2 electrodes 3.2.1. Effect of REs doping on the electrode performance The degradation of six kinds of selected aliphatic acids (maleic acid, fumaric acid, succinic acid, malonic acid, oxalic acid and acetic acid) with different Ti/Re-Sb-SnO2 electrodes was investigated and the results are fitted with first-order rate constants (Fig. 3 and Figs. S3–S7 (Supplementary material)). The reaction rate constants korg for these acids elimination and kTOC for their mineralization are shown in Fig. 4. Compared with the control electrode, the Ti/Gd-SbSnO2 and Ti/Eu-Sb-SnO2 electrodes show better performance on the degradation of most of the investigated acids, as their korg and kTOC are 15–90% and 14–123% higher than that of the control (as indicated in Fig. 4). However, the Ti/Ce-Sb-SnO2 electrode shows either closely (for oxalic acid, fumaric acid and succinic acid) or lower (for maleic acid, malonic acid and acetic acid) effectiveness compared with the control electrode.
6
(a)
Ti/Sb-SnO2
40
Ti/Ce-Sb-SnO2
-1
5
korg (×10 min )
Ti/Gd-Sb-SnO2
4
Ti/Eu-Sb-SnO2
30
3 2
Maleic acid Fumaric acid Succinic acid Malonic acid Oxalic acid Acetic acid
(a)
20
10
1 0
0
0
60
120
180
240
(b)
30
-3
Ti/Ce-Sb-SnO2 3
-1
Ti/Sb-SnO2
kTOC (×10 min )
40
4
Ti/Sb-SnO2 Ti/Ce-Sb-SnO2 Ti/Gd-Sb-SnO2 Ti/Eu-Sb-SnO2
300
Time (min)
-ln([TOC]/[TOC]0)
229
-3
-ln([Succinic acid]/[Succinic acid]0)
Y.-H. Cui et al. / Journal of Hazardous Materials 239–240 (2012) 225–232
Ti/Gd-Sb-SnO2 Ti/Eu-Sb-SnO2
2
Maleic acid Fumaric acid Succinic acid Malonic acid Oxalic acid Acetic acid
(b)
20
10
1
0
0
0
60
120
180
240
300
360
Time (min) Fig. 3. Kinetics of (a) succinic acid elimination and (b) TOC removal in electrocatalytic degradation of succinic acid with different Ti/Re-Sb-SnO2 electrodes. Initial succinic acid = 100 mg L−1 , volume of solution = 100 mL, I = 0.12 A. The solid lines represent the fitted curves and the dots represent the experimental data.
Ti/Sb-SnO2 Ti/Ce-Sb-SnO2 Ti/Gd-Sb-SnO2 Ti/Eu-Sb-SnO2
Fig. 4. Reaction rate constants of (a) organics elimination and (b) TOC removal for different aliphatic acids in electrocatalytic degradation process with different Ti/ReSb-SnO2 electrodes. Initial aliphatic acid = 100 mg L−1 , volume of solution = 100 mL, I = 0.12 A.
easily attacked by electrophiles (such as • OH) which leads a rapid molecular destruction. 3.3. Efficiency evaluation of the organics degradation on Ti/Re-Sb-SnO2 electrodes
3.2.2. Effectiveness of electrocatalytic degradation on different aliphatic acids As indicated in Fig. 4, for each electrode, the highest korg and kTOC are found in the oxalic acid degradation process among the selected six aliphatic acids, which implies that oxalic acid is more readily to be destroyed in present electrocatalytic process. Further, the value of korg and kTOC for oxalic acid are almost the same on each electrode, indicating that oxalic acid can be directly oxidized to carbon dioxide without the formation of other by-products. It can also be found from Fig. 4 that the korg and kTOC for oxalic acid degradation are 3–4 times and 3–5 times higher than that of acetic acid, respectively, though they are both carboxylic acids with two carbon atoms. The difference between the two compounds is that oxalic acid has more carboxyls than that of acetic acid, and thus has more double bonds and has higher oxygen content. Actually, comparing the compounds with four carbon atoms, namely, maleic acid, fumaric acid and succinic acid, a similar result could be found for most investigated electrodes except for Ti/Ce-Sb-SnO2 , though the difference are not as obviously as that between oxalic acid and acetic acid which might be due to the much higher oxygen content of oxalic acid. So, it seems that the compound with higher oxygen content or more double bonds is more readily to be destroyed in the electrocatalytic process. It could be suggested that the higher oxygen content of an organic compound means that its carbon atoms are in higher valence and do not far from the final oxidation state, and the double bond with electronic structure would be more
The mineralization current efficiency and energy consumption were calculated from Eqs. (1) and (2) based on the experiments. Generally, for the electrocatalytic degradation of each compound on each electrode, the MCE decreases rapidly with the reaction time, while the Ep increases with the reaction time. The loss of MCE and the increase of Ep are mainly due to oxygen generation from water electrolysis which induces additional charge and energy consumption. The average MCE and Ep for the mineralization of organics on different electrodes are summarized and shown in Table 1. It is found that for each organic compound, the trend of MCE variation on different electrodes is in accordance with the kTOC variation, and the trend of Ep variation on different electrodes is in reverse to the kTOC variation, namely, the Ti/Gd-Sb-SnO2 and Ti/EuSb-SnO2 electrodes show better performance on the degradation of most of the investigated organics compared with the control, while the Ti/Ce-Sb-SnO2 electrode shows either closely or lower effectiveness compared with the control. As for each electrode, however, the higher kTOC does not mean the higher MCE on the degradation of different organics, though it still corresponds to the lower Ep . For example, the kTOC of oxalic acid is much higher than those of other compounds, while the MCE of oxalic acid is almost the lowest one among all the investigated organics on each electrode. As aforementioned, oxalic acid has the highest oxygen content and it needs to transfer the least electrons in the electrochemical reaction process. Therefore, at a given constant current in the present study,
230
Y.-H. Cui et al. / Journal of Hazardous Materials 239–240 (2012) 225–232
Benzoquinone Hydroquinone Catechol Maleic acid Fumaric acid Succinic acid Malonic acid Oxalic acid Acetic acid
-1
kTOC (×10 min )
40
-3
30
20
10
0
0
10
20
30 -3
40
-1
korg (×10 min ) Fig. 5. The difference between organics elimination rate constant korg and TOC removal rate constant kTOC for different compounds in electrocatalytic degradation process with different Ti/Re-Sb-SnO2 electrodes. Initial aromatic compounds = 200 mg L−1 , initial aliphatic acid = 100 mg L−1 , volume of solution = 100 mL, I = 0.12 A.
the electron transfer density might be too high to be efficiently used on oxalic acid oxidation and much electric charge is consumed by water electrolysis which causes the lowest MCE. Besides, Table 1 also indicates that the MCE of aromatic compounds are significantly higher than those of aliphatic acids. The higher concentration and the lower oxygen content of the aromatic compounds should be responsible for their higher MCE compared with the aliphatic acids. 3.4. Proposed mechanism of the organics degradation on Ti/Re-Sb-SnO2 electrodes Fig. 5 shows the relationship of korg and kTOC for all the selected organic compounds with the studied electrodes. An interesting observation is that the difference between korg and kTOC for simple molecules (such as oxalic acid and acetic acid) is much smaller than that of complicate molecules, and this phenomenon is more significant if compared with aromatic compounds as they deviate more farther from the diagonal line. Considering that the initial concentration of aromatic compounds (200 mg L−1 ) is two times of the aliphatic acids (100 mg L−1 ), the difference between korg and kTOC for aromatic compounds should be enlarged if their initial concentration is the same as that of aliphatic acids since the rate constant should be approximately proportional increased with lower concentration [26]. Therefore, it is generally found that the difference between korg and kTOC decreases with the reduction of molecular
structure, which should be caused by the less intermediate processes for the smaller molecules. In other words, the mineralization (as represented by kTOC ) for the complicate compounds is more difficult compared with their transformation (as represented by korg ). Nonetheless, Fig. 5 also indicates that if the first transformation step for a certain compound degradation is difficult and then the first stage becomes the control step, the difference between korg and kTOC would be relatively smaller (for example, catechol degradation). The important intermediates in the organics electrolysis were detected by HPLC and the main observed intermediate products are listed in Table 2. Based on the intermediates analysis and the organics degradation efficiency, pathways of the selected organic compounds are summarized in Figs. 6 and 7. As shown in Fig. 6, hydroquinone, catechol and benzoquinone (which is in equilibrium with hydroquinone in an aqueous solution) are first electrophilic attacked by • OH and form hydroxylated intermediates. These hydroxylated intermediates are unstable and the ring cleavage happens which leads to the formation of aliphatic acids. In this process, the • OH generated from water splitting plays a key role and a greater • OH production would lead to a faster aromatics transformation. Therefore, the use of an anode material with a high • OH production is especially desirable for aromatics elimination. From the results shown in Figs. 1 and 2, Figs. S1 and S2 (Supplementary material), it might be deduced that doping with Gd or Eu enhances the capacity of Ti/Sb-SnO2 anode on • OH generation since the three selected aromatics elimination (or namely, ring cleavage because their aromatic ring are readily split without other by-products in the oxidation process) are accelerated. However, the results of electrocatalytic degradation also show that doping with Ce into Ti/Sb-SnO2 anode seems not good at the aromatic ring cleavage. Our previous research has suggested that the enhanced performance of Ti/Gd-Sb-SnO2 on phenol oxidation is attributed to the increased production of active oxygen species (e.g. • OH) and the lower mobility of surface adsorbed active oxygen atoms into SnO2 lattice; while, the redox couple of Ce4+ /Ce3+ on Ti/Ce-Sb-SnO2 electrode surface allows oxygen transfer into the SnO2 lattice, and lowered the electrode’s capacity on phenol degradation [12]. In the present study, the lower efficiency of Ti/CeSb-SnO2 electrode on the degradation of the selected aromatic compounds could be explained by the above conception. Fig. 7 shows the possible transformations of the selected carboxylic acids based on the intermediates analysis. Maleic acid and fumaric acid could be oxidized to oxalic acid in the anodic process; while, in the present undivided cell electrolysis, succinic acid is a possible intermediate product in the maleic acid or fumaric acid
Fig. 6. Proposed reaction pathways for electrocatalytic degradation of selected aromatic compounds.
Y.-H. Cui et al. / Journal of Hazardous Materials 239–240 (2012) 225–232
231
Fig. 7. Proposed reaction pathways for electrocatalytic transformation of (a) maleic acid and fumaric acid, (b) succinic acid, (c) malonic acid, (d) oxalic acid and (e) acetic acid. The compounds with a dashed frame represent the possible intermediates and the semi-circle arrow represents the possible transformation between different intermediates.
electrolysis, and then malonic acid, acetic acid and formic acid are produced (Fig. 7(a) and (b)). The malonic acid is degraded to acetic acid and formic acid, and oxalic acid is another possible product (Fig. 7(c)). The oxalic acid is directly mineralized as aforementioned (Fig. 7(d)). The acetic acid, however, might be through a formic acid pathway (Fig. 7(e)), which can explain its much lower destruction rate compared with that of oxalic acid. As shown in Figs. 3 and 4 and Figs. S3–S7 (Supplementary material), for most of the selected acids, Ti/Gd-Sb-SnO2 and Ti/Eu-Sb-SnO2 electrodes show better performance on either their elimination or their mineralization; while, both of the two REs doped electrodes do not show higher effectiveness on acetic acid degradation than that of the control. The Ti/Ce-Sb-SnO2 electrode, however, shows lower effectiveness on the degradation of some acids while it also shows similar effectiveness on the degradation of other acids compared with the control. These results imply that despite the REs doping effect, the molecular structure of organic compound also affect its degradation rate in the electrocatalytic process. Considering that the reactivity of • OH toward small aliphatic molecules is usually much lower than that toward aromatic compounds [27], the felicitously surface property of an electrode that is favorable to the adsorption and activation of a certain organic compound might be important. This suggestion is in accordance with that of our previous research on electrocatalytic degradation of BPA which has found that Ti/Sb-SnO2 shows better performance on the intermediate aliphatic acids oxidation than those of BDD and Pt electrodes [5]. Therefore, considering electrocatalytic oxidation of wastewaters containing mainly the
organics with one type of molecular structure (such as aromatic ring or small aliphatic acid), it would be effective to select a working electrode with proper features (such as • OH generation or adsorption/activation capacity). Further, it might be deduced that in treating wastewaters containing various organics or containing organics with large/complex molecular structure, a multi-stage electrocatalytic oxidation using different working electrodes at different stages may be more efficient compared with one-stage electrocatalytic oxidation using one type of working electrode.
4. Conclusions Nine kinds of organics including three kinds of basal aromatic compounds and six kinds of typical aliphatic acids were selected to investigate the electrocatalytic ability of REs (Ce, Gd, and Eu) doped Ti/Sb-SnO2 anode materials. Doping with Gd or Eu enhances the capacity of Ti/Sb-SnO2 anode on the three selected aromatics elimination and the rate constants korg are 6.3–32.3% increased compared with the control electrode; while, doping with Ce on Ti/Sb-SnO2 anode seems not good at the aromatic ring cleavage as the korg are 4.2–30% decreased compared with the control. The Ti/Gd-Sb-SnO2 and Ti/Eu-Sb-SnO2 electrodes show better performance on electrocatalytic degradation of most selected acids compared with Ti/Sb-SnO2 and the rate constants are 15–90% (for korg ) and 14–123% (for kTOC ) higher than that of the control; while, the Ti/Ce-Sb-SnO2 electrode shows similar or lower effectiveness on degradation of these acids.
232
Y.-H. Cui et al. / Journal of Hazardous Materials 239–240 (2012) 225–232
The molecular structure of organic compound affects its degradation rate in the electrocatalytic process. Catechol seems more resistant to the electrophilic attack by hydroxyl radicals which makes it more refractory to be destroyed than benzoquinone and hydroquinone. The acid with higher oxygen content or more double bonds is more readily to be destroyed in the electrocatalytic process. The compound with more complicate molecular structure or longer carbon chain would be more difficult to be mineralized due to the more complicate intermediate processes. Acknowledgments This research was supported by the National Nature Science Foundation of China (51008139) and the Open Project of State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (QA201107). The authors also appreciated the National Creative Research Group supported by the National Natural Science Foundation of China (50821002) and the National Natural Science Funds for Distinguished Young Scholars of China (51125033). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.jhazmat.2012.08.068 References [1] C.A. Martinez-Huitle, E. Brillas, Decontamination of wastewaters containing synthetic organic dyes by electrochemical methods: a general review, Appl. Catal. B: Environ. 87 (2009) 105–145. [2] M. Magureanu, D. Piroi, N.B. Mandache, V. David, A. Medvedovici, V.I. Parvulescu, Degradation of pharmaceutical compound pentoxifylline in water by non-thermal plasma treatment, Water Res. 44 (2010) 3445–3453. [3] S.X. Yang, Z.Q. Liu, X.H. Huang, B.P. Zhang, Wet air oxidation of epoxy acrylate monomer industrial wastewater, J. Hazard. Mater. 178 (2010) 786–791. [4] A.Z. Li, X. Zhao, Y.N. Hou, H.J. Liu, L.Y. Wu, J.H. Qu, The electrocatalytic dechlorination of chloroacetic acids at electrodeposited Pd/Fe-modified carbon paper electrode, Appl. Catal. B: Environ. 111–112 (2012) 628–635. [5] Y.H. Cui, X.Y. Li, G.H. Chen, Electrochemical degradation of bisphenol A on different anodes, Water Res. 43 (2009) 1968–1976. [6] R. Broseus, S. Vincent, K. Aboulfadl, A. Daneshvar, S. Sauve, B. Barbeau, M. Prevost, Ozone oxidation of pharmaceuticals, endocrine disruptors and pesticides during drinking water treatment, Water Res. 43 (2009) 4707–4717. [7] C. Sirtori, A. Zapata, I. Oller, W. Gernjak, A. Aguera, S. Malato, Solar photo-Fenton as finishing step for biological treatment of a pharmaceutical wastewater, Environ. Sci. Technol. 43 (2009) 1185–1191. [8] Z.Q. Liu, J. Ma, Y.H. Cui, L. Zhao, B.P. Zhang, Influence of different heat treatments on the surface properties and catalytic performance of carbon nanotube in ozonation, Appl. Catal. B: Environ. 101 (2010) 74–80.
[9] G.H. Chen, Electrochemical technologies in wastewater treatment, Sep. Purif. Technol. 38 (2004) 11–41. [10] X.Y. Li, Y.H. Cui, Y.J. Feng, Z.M. Xie, J.D. Gu, Reaction pathways and mechanisms of the electrochemical degradation of phenol on different electrodes, Water Res. 39 (2005) 1972–1981. [11] A. Kapalka, G. Foti, C. Comninellis, Kinetic modeling of the electrochemical mineralization of organic pollutants for wastewater treatment, J. Appl. Electrochem. 38 (2008) 7–16. [12] Y.H. Cui, Y.J. Feng, Z.Q. Liu, Influence of rare earths doping on the structure and electro-catalytic performance of Ti/Sb-SnO2 electrodes, Electrochim. Acta 54 (2009) 4903–4909. [13] A.Y. Bagastyo, J. Radjenovic, Y. Mu, R.A. Rozendal, D.J. Batstone, K. Rabaey, Electrochemical oxidation of reverse osmosis concentrate on mixed metal oxide (MMO) titanium coated electrodes, Water Res. 45 (2011) 4951–4959. [14] J.T. Kong, S.Y. Shi, L.C. Kong, X.P. Zhu, J.R. Ni, Preparation and characterization of PbO2 electrodes doped with different rare earth oxides, Electrochim. Acta 53 (2007) 2048–2054. [15] E. Vanhove, J. de Sanoit, J.C. Arnault, S. Saada, C. Mer, P. Mailley, P. Bergonzo, M. Nesladek, Stability of H-terminated BDD electrodes: an insight into the influence of the surface preparation, Phys. Status Solidi A 204 (2007) 2931–2939. [16] G.H. Zhao, Y.G. Zhang, Y.Z. Lei, B.Y. Lv, J.X. Gao, Y.A. Zhang, D.M. Li, Fabrication and electrochemical treatment application of a novel lead dioxide anode with superhydrophohic surfaces, high oxygen evolution potential, and oxidation capability, Environ. Sci. Technol. 44 (2010) 1754–1759. [17] Y. Mohd, D. Pletcher, The influence of deposition conditions and dopant ions on the structure activity, and stability of lead dioxide anode coatings, J. Electrochem. Soc. 152 (2005) D97–D102. [18] Y. Mohd, D. Pletcher, The fabrication of lead dioxide layers on a titanium substrate, Electrochim. Acta 52 (2006) 786–793. [19] S. Stucki, R. Kötz, B. Carcer, W. Suter, Electrochemical waste water treatment using high overvoltage anodes. Part II: anode performance and applications, J. Appl. Electrochem. 21 (1991) 99–104. [20] F. Montilla, E. Morallon, J.L. Vazquez, Evaluation of the electrocatalytic activity of antimony-doped tin dioxide anodes toward the oxidation of phenol in aqueous solutions, J. Electrochem. Soc. 152 (2005) B421–B427. [21] Y.H. Cui, Y.H. Wang, B. Wang, H.H. Zhou, K.Y. Chan, X.Y. Li, Electrochemical generation of ozone in a membrane electrode assembly cell with convective flow, J. Electrochem. Soc. 156 (2009) E75–E80. [22] S.Y. Yang, Y.S. Choo, S. Kim, S.K. Lim, J. Lee, H. Park, Boosting the electrocatalytic activities of SnO2 electrodes for remediation of aqueous pollutants by doping with various metals, Appl. Catal. B: Environ. 111–112 (2012) 317–325. [23] Y.J. Feng, Y.H. Cui, B.E. Logan, Z.Q. Liu, Performance of Gd-doped Ti-based Sb-SnO2 anodes for electrochemical destruction of phenol, Chemosphere 70 (2008) 1629–1636. [24] Y. Liu, H.L. Liu, J. Ma, X. Wang, Comparison of degradation mechanism of electrochemical oxidation of di- and tri-nitrophenols on Bi-doped lead dioxide electrode: effect of the molecular structure, Appl. Catal. B: Environ. 91 (2009) 284–299. [25] S. Kim, S.K. Choi, B.Y. Yoon, S.K. Lim, H. Park, Effects of electrolyte on the electrocatalytic activities of RuO2 /Ti and Sb-SnO2 /Ti anodes for water treatment, Appl. Catal. B: Environ. 97 (2010) 135–141. [26] Y.H. Cui, Y.J. Feng, X.Y. Li, Kinetics and efficiency analysis of electrochemical oxidation of phenol: influence of anode materials and operational conditions, Chem. Eng. Technol. 34 (2011) 265–272. [27] G.V. Buxton, C.L. Greenstock, W.P. Helman, W.P. Ross, Critical review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals in aqueous solution, J. Phys. Chem. Ref. Data 17 (1988) 513–886.
Contents lists available at SciVerse ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Comparison of various organic compounds destruction on rare earths doped Ti/Sb-SnO2 electrodes Yu-Hong Cui a,b , Yu-Jie Feng b,∗ , Junfeng Liu b , Nanqi Ren b School of Environmental Science and Engineering, Huazhong University of Science and Technology, No. 1037 Luoyu Road, Hongshan District, Wuhan 430074, PR China State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, No. 73 Huanghe Road, Nangang District, Harbin 150090, PR China
Different REs doping has distinct effect on the Ti/Sb-SnO2 electrode performance. Gd or Eu improves the performance of Ti/Sb-SnO2 on aromatic ring cleavage. Catechol is more refractory to be degraded than benzoquinone and hydroquinone. The molecular structure of organic compound influences its degradation rate.
a r t i c l e
i n f o
Article history: Received 26 February 2012 Received in revised form 27 August 2012 Accepted 28 August 2012 Available online 4 September 2012 Keywords: Electrode Organic pollutant Electrocatalysis Rare earth Wastewater treatment
g r a p h i c a l
a b s t r a c t
Benzoquinone Hydroquinone Catechol Maleic acid Fumaric acid Succinic acid Malonic acid Oxalic acid Acetic acid
40 -1
h i g h l i g h t s
30
-3
b
kTOC (×10 min )
a
20
10
0
0
10
20
30 -3
40
-1
korg (×10 min )
a b s t r a c t Ti/Sb-SnO2 and three kinds of rare earths (REs), namely Ce, Gd, and Eu doped Ti/Sb-SnO2 electrodes were prepared and tested for their capacity on electrocatalytic degradation of three kinds of basal aromatic compounds (benzoquinone, hydroquinone and catechol) and six kinds of aliphatic acids (maleic acid, fumaric acid, succinic acid, malonic acid, oxalic acid and acetic acid). The elimination of selected organics as well as their TOC removal with different doped Ti/Sb-SnO2 electrodes was described by first-order kinetics. Compared with Ti/Sb-SnO2 , the Gd and Eu doped electrodes show better performance on the degradation of most of the selected organics, while Ce doped electrode shows either closely or lower efficiency on the degradation of these selected organics. Besides electrode material, the molecular structure of organic compound has obvious effect on its degradation in the electrocatalytic process. Catechol is more resistant to the electrophilic attack by hydroxyl radicals than benzoquinone and hydroquinone. The compound with more complicate molecular structure or longer carbon chain is more difficult to be mineralized. The aliphatic acid with higher oxygen content or more double bonds is more readily to be oxidized in the electrocatalytic process. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Along with the industrial development, various wastewaters containing organic pollutants are produced. Some of the organics presented in industrial wastewaters are refractory and can not be eliminated by traditional treatment, and then the
∗ Corresponding author. Tel.: +86 451 86283068; fax: +86 451 87162150. E-mail address: [email protected] (Y.-J. Feng). 0304-3894/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2012.08.068
possible release of organic contaminants into the aquatic environment from wastewater effluents has gained much attention recently [1–5]. On the other hand, the overuse of pesticides, pharmaceuticals and personal care products in modern society also brings many kinds of organic pollutants to aquatic environment [2,6,7]. No matter what source of the organic pollutants, their presence in water may affect aquatic organisms and can lead a potential risk on human health through drinking water or consumption of food irrigated by polluted water. Therefore, it is necessary to develop new and effective treatment processes toward organic pollutions in water.
226
Y.-H. Cui et al. / Journal of Hazardous Materials 239–240 (2012) 225–232
Advanced oxidation processes (AOPs) have been developed to deal with organic pollutants in water through in situ generation of strong oxygen-based oxidizers, among which the formation of hydroxyl radical (• OH) is especially desired, since it is the strongest oxidizer and can react non-selectively with various types of organics [2,5,8]. Electrocatalytic oxidation is one of the AOPs and has drawn considerable research attention due to its simplicity and robustness in structure and operation [9–11]. The effectiveness of electrocatalytic oxidation depends largely on the activities of electrode materials toward the target pollutants [5,9,10,12,13]. Therefore, the discovering of more efficient electrodes with good capacity on organics destruction receives an increasing interest, including boron-doped diamond (BDD) electrodes and various doped metal oxide-film electrodes such as PbO2 and SnO2 [1,5,9,10,12]. It has been reported that the surface modification as well as the chemical composition significantly influences the electrode performance for either diamond electrode or metal oxide-film electrode [12,14–16]. With proper doping of selected dopants such as bismuth, fluoride and rare earths (REs), the electrocatalytic activity or stability of PbO2 electrodes can be considerably enhanced [14,17,18]. Modification of SnO2 electrode by dopants to enhance the electrochemical properties has been studied and the most promising electrode is found to be the antimony doped Ti-based SnO2 anodes (Ti/Sb-SnO2 ) [19]. Recent studies indicate that the co-doped SnO2 -Sb-X electrodes (X represents Pt, Ni, Co, etc.) have better performance or some special properties in an electrocatalytic process [20–22]. The previous investigations by our group have found that proper doping the Sb-SnO2 anode with REs metal ions can improve the electrocatalytic oxidation rate of phenol [23]; further analysis suggested that the better performance of REs doped Sb-SnO2 anodes is aroused from the increased production of active oxygen species (e.g. • OH) and the reduced oxygen vacancies in crystal lattice [12]. Besides anode catalyst materials, the target organic pollutant is also very important in determining overall treatment efficiency in an electrocatalytic oxidation process. Although organic substrates are primarily degraded by the in situ generated hydroxyl radicals, the mechanism of different organics degradation may be distinctively different. For example, the electrocatalytic oxidation of di- and tri-nitrophenols on Bi-doped PbO2 electrode is largely influenced by the molecular structures of the nitrophenols [24]. Our previous research also found that the electrocatalytic destruction of BPA and its intermediates may follow different mechanisms due to their different adsorption and activation characters [5]. Surface modification by dopants to develop highly efficient anodes for water treatment has been made in several studies [12,14,17,22,23] and the effects of organics molecule on its degradation using a certain anode were discussed by a few researchers [24]. Yet the performance of various organic compounds destruction on a certain anode material with different dopants has not been systematically investigated. Besides, since some special groups (i.e. NO2 , SO3 H, Cl) or the intermediate active species (i.e. Cl• , HOCl/ClO− ) formed from electrolysis of some parent compounds may cause interference in a comparison study, the present study selected nine kinds of fundamental organics including three kinds of basal aromatic compounds (benzoquinone, hydroquinone and catechol) and six kinds of aliphatic acids (maleic acid, fumaric acid, succinic acid, malonic acid, oxalic acid and acetic acid) which are often observed as intermediate products during the oxidation of many refractory organic compounds [5,10,16,24] as model compounds to verify the performance of three kinds of REs (Ce, Gd and Eu) doped Ti/Sb-SnO2 electrodes and the control electrode (Ti/SbSnO2 ) in electrocatalytic oxidation process. The kinetics and the efficiency of the selected organics degradation on different REs
doped Ti/Sb-SnO2 electrodes were investigated and the degradation pathway as well as mechanism was also discussed. 2. Experimental 2.1. Electrode preparation Ti/Sb-SnO2 and REs doped Ti/Sb-SnO2 (Ti/Re-Sb-SnO2 ) electrodes were prepared by thermal deposition of the coating layers as previously described [12,23], including pretreatment of titanium plates, preparation of inner layer and outer layer coatings. Ti plates with a dimension of 2 cm × 3 cm were used as the base metal for all oxide-coated electrodes. The plate was polished using 40-grit and 320-grit sand papers, degreased in hot 40% NaOH at 80 ◦ C for 2 h, and then etched in 10% boiling oxalic acid for 2 h followed by thorough washing with distilled water, and dried. The electrodeposition of antimony-tin inner layer was applied first followed by thermal oxidation. For electro-deposition, the pretreated Ti plate was placed as the cathode in 100 mL of alcohol solution containing 17.5 g SnCl4 ·5H2 O, 0.73 g Sb2 O3 , and 2 mL of concentrated (37%) HCl. The deposition was carried out at constant current of 0.12 A for 25 min with two counter electrodes made of Ti/RuO2 . After electrodeposition, the electrodes were heated in an oven at 400 ◦ C for 2 h. For the second coating step to get the outer layer, the uniformly coated plate was hot annealed after being dip coated in a solution consisting of 30 g SnCl4 ·5H2 O, 0.8 g Sb2 O3 , 2.5 mL of concentrated (37%) HCl and proper rare earth nitrates (with a molar ratio of Re/Sn of 2:100) in 50 mL n-butanol. The electrode was then dried in an infrared oven. After five cycles of both dipping and drying, the Ti plates were heated in a muffle oven (450 ◦ C for 20 min) for coating pyrolysis. This whole process (dipping, drying and pyrolysis) was repeated 5 times and finally, the electrodes were heated at 650 ◦ C for 3 h. 2.2. Organics electrolysis The electrolysis cells were a series of 100 mL glass beakers. For each cell, a 6 cm2 (2 cm × 3 cm) anode, prepared as described above, and a stainless steel cathode having the same area was placed in the beaker at a spacing of 8 mm between the electrodes. A DC potentiostat with a voltage range of 0–30 V was used as the power supply for organic degradation studies. For an electrolysis test, 100 mL of an aqueous solution with a pre-determined concentration of 200 mg L−1 (for aromatic compounds) or 100 mg L−1 (for aliphatic acids) was placed in the cell. 0.25 M Na2 SO4 was used as supporting electrolyte in the electrolysis cell which was reported to be better for the surface-specific reaction on the working anode [25]. Electrolysis was performed under galvanostatic control at 0.12 A. During organics destruction, the voltage evolution of each electrolytic cell was recorded as an index of system stability. No remarkable difference was found between different electrodes, and the voltages kept almost constant in the electrocatalytic processes. At the end of each run, based on a pre-determined time of electrolysis, the solution in the cell was analyzed for the concentration of original compound, total organic carbon (TOC) and intermediate products. 2.3. Analysis methods TOC was analyzed using a TOC Analyzer (Shimadu 5000A). A high performance liquid chromatograph (HPLC) consisting of a 2695 separation module and 2996 photodiode array detector (Waters) was used for concentration measurement of the organic compound and its intermediate products during electrolysis, including aromatic compounds and organic acids. A Waters Symmetry C18 column (5 m × 50 mm × 3.91 mm) was used as the
Y.-H. Cui et al. / Journal of Hazardous Materials 239–240 (2012) 225–232
227
Table 1 Current efficiency and energy consumption for electrocatalytic degradation of various organics with different Ti/Re-Sb-SnO2 electrodes. Organic compounds
Electrodes Ti/Sb-SnO2
Benzoquinone MCE (%)a Ep (J(mg L−1 TOC)−1 )b Hydroquinone MCE (%)a Ep (J(mg L−1 TOC)−1 )b Catechol MCE (%)a Ep (J(mg L−1 TOC)−1 )b Maleic acid MCE (%)a Ep (J(mg L−1 TOC)−1 )b Fumaric acid MCE (%)a Ep (J(mg L−1 TOC)−1 )b Succinic acid MCE (%)a Ep (J(mg L−1 TOC)−1 )b Malonic acid MCE (%)a Ep (J(mg L−1 TOC)−1 )b Oxalic acid MCE (%)a Ep (J(mg L−1 TOC)−1 )b Acetic acid MCE (%)a Ep (J(mg L−1 TOC)−1 )b a b
Ti/Ce-Sb-SnO2
Ti/Gd-Sb-SnO2
Ti/Eu-Sb-SnO2
29.5 29.1
25.7 33.4
32.6 26.3
39.4 21.8
22.5 40.5
20.5 44.5
34.5 26.4
43.8 20.8
18.2 50.1
14.2 64.2
27.8 32.8
52.1 17.5
5.91 67.6
3.45 115.8
8.23 48.4
11.29 35.4
5.62 71.0
6.44 62.0
11.38 35.0
12.43 32.0
5.85 78.2
5.59 81.9
10.74 42.6
9.01 50.8
6.32 47.0
4.98 59.6
7.09 41.8
7.89 37.6
4.84 17.7
5.12 16.7
6.24 13.7
5.70 15.0
10.11 50.9
6.45 79.8
11.30 45.5
8.76 58.8
Average mineralization current efficiency calculated when 60% of TOC was removed. Average energy consumption per unit of TOC removal calculated when 60% of TOC was removed.
separation column for the aromatic compounds. The mobile phase was a mixture of methanol and water (50:50, v/v), and the flow rate was set as 0.5 mL min−1 . For the separation of organic acids, an Alltech Atlantis dC18 column (5 m × 150 mm × 3.9 mm) was used, with 20 mM NaH2 PO4 solution (pH 2.7) as the mobile phase at a flow rate of 0.5 mL min−1 . Before each analysis, the sample was filtered through a 0.45-m membrane filter (Millex), and the HPLC injection volume was 10 L.
The mineralization current efficiency (MCE) for electrocatalytic degradation of organics is estimated from the expression (1) and the energy consumption per unit of TOC removal Ep is obtained as expression (2) [26]:
2.4. Kinetic analysis
Ep =
Two kinetic models are usually adopted to describe electrocatalytic degradation of organic pollutants. One is first-order kinetics which is used when the electrode reaction is sufficiently rapid and the whole electrolysis is under mass transfer control; the other is zero-order kinetics which is suitable for the electrode reaction in which the electron transfer process is hindered on the electrode surface and the whole electrolysis is under current control. According to our previous study on SnO2 -based electrode kinetics [26], the first-order kinetics was adopted in the present kinetic analysis.
where (TOC)theor is the theoretically calculated TOC abatement considering that the applied electrical charge is only consumed in the corresponding mineralization reaction, (TOC)exp is the experimental TOC removal with a given charge consumption, Q is the charge consumption at a give electrolysis time and U is the cell voltage reading from the DC power supply.
Table 2 Observed organic intermediates in HPLC analysis. Initial compounds
Observed intermediates
Benzoquinone
Hydroquinone, 2-ketoglutaric acid, maleic acid, fumaric acid, succinic acid, oxalic acid, formic acid Benzoquinone, 2-ketoglutaric acid, maleic acid, fumaric acid, oxalic acid 2-Ketoglutaric acid, maleic acid, fumaric acid, formic acid Fumaric acid, malonic acid, oxalic acid, formic acid Oxalic acid, formic acid Malonic acid, formic acid Acetic acid — Formic acid
Hydroquinone Catechol Maleic acid Fumaric acid succinic acid Malonic acid Oxalic acid Acetic acid
2.5. Efficiency calculation
MCE =
(TOC)exp (TOC)theor
× 100
QU (TOC)exp
(1) (2)
3. Results and discussion 3.1. Kinetic investigation of aromatic compounds degradation on Ti/Re-Sb-SnO2 electrodes 3.1.1. Effect of REs doping on the electrode performance Electrocatalytic degradation of benzoquinone, hydroquinone and catechol with different Ti/Re-Sb-SnO2 electrodes was evaluated and the observed rates of their elimination and mineralization (expressed as TOC removal) were fitted with first-order rate constants (Fig. 1 and Figs. S1 and S2 (Supplementary material)). The reaction rate constants korg (for the compound elimination) and kTOC (for the compound mineralization) are shown in Fig. 2. It seems that doping with Gd or Eu can slightly improve the capacity of Ti/Sb-SnO2 electrode on these aromatics elimination as the rate constant korg are 6.3–32.3% higher than that of the control (Ti/SbSnO2 ), while doping with Ce depresses the activity Ti/Sb-SnO2
Y.-H. Cui et al. / Journal of Hazardous Materials 239–240 (2012) 225–232
(a)
5
Ti/Ce-Sb-SnO2
4
Ti/Gd-Sb-SnO2 Ti/Eu-Sb-SnO2
3
50
-1
Ti/Sb-SnO2
40
-3
6
30
korg (×10 min )
-ln([Hydroquinone]/[Hydroquinone]0)
228
2
(a)
20 10
1 0
0
0
30
60
90
120
(b)
3
Ti/Gd-Sb-SnO2 Ti/Eu-Sb-SnO2
2
1
0
10
-3
Ti/Ce-Sb-SnO2
5
0
0
60
120
180
(b) Benzoquinone Hydroquinone Catechol
-1
Ti/Sb-SnO2
kTOC (×10 min )
4
Ti/Sb-SnO2 Ti/Ce-Sb-SnO2 Ti/Gd-Sb-SnO2 Ti/Eu-Sb-SnO2
15
Time (min)
-ln([TOC]/[TOC]0)
Benzoquinone Hydroquinone Catechol
240
300
Time (min) Fig. 1. Kinetics of (a) hydroquinone elimination and (b) TOC removal in electrocatalytic degradation of hydroquinone with different Ti/Re-Sb-SnO2 electrodes. Initial hydroquinone = 200 mg L−1 , volume of solution = 100 mL, I = 0.12 A. The solid lines represent the fitted curves and the dots represent the experimental data.
electrode on these aromatics elimination especially for benzoquinone as its korg is 30% lower than that of the control (Fig. 1(a), Figs. S1(a) and S2(a) (Supplementary material)). For these aromatics mineralization, the effect of REs doping on Ti/Sb-SnO2 electrode performance seems more evident (Fig. 1(b), Figs. S1(b) and S2(b) (Supplementary material)). The Ti/Gd-Sb-SnO2 and Ti/Eu-Sb-SnO2 electrodes show 20–35% (for benzoquinone), 80–92% (for hydroquinone), and 49–115% (for catechol) higher TOC removal rates (kTOC ) compared with the control, respectively; while the Ti/Ce-Sb-SnO2 electrode shows a poor performance as its kTOC is 7.8% (for benzoquinone), 12% (for hydroquinone) and 24% (for catechol) lower than that of the control, respectively. 3.1.2. Effectiveness of electrocatalytic degradation on different aromatic compounds To compare the destruction effect of electrocatalytic process on different organics, the reaction rate constants of the three selected aromatic compounds are summarized and shown in Fig. 2. It seems that catechol is more refractory to be destroyed in the electrocatalytic process as its korg and kTOC are 33–50% and 17–57% lower than that of other two compounds, respectively. Furthermore, the value of korg for catechol elimination on each electrode is significantly lower than other two compounds (Fig. 2(a)), while this phenomenon is not observed as obviously as for that of kTOC (Fig. 2(b)). So, the first step of catechol transformation might be more resistant to the electrocatalytic oxidation process compared with other two aromatic compounds. It is generally accepted that
Ti/Sb-SnO2 Ti/Ce-Sb-SnO2 Ti/Gd-Sb-SnO2 Ti/Eu-Sb-SnO2
Fig. 2. Reaction rate constants of (a) organics elimination and (b) TOC removal for different aromatic compounds in electrocatalytic degradation process with different Ti/Re-Sb-SnO2 electrodes. Initial aromatic compounds = 200 mg L−1 , volume of solution = 100 mL, I = 0.12 A.
electrocatalytic oxidation of aromatics starts with the attack of hydroxyl radicals to form hydroxylated intermediates and then the ring cleavage happens which leads to the formation of aliphatic species, mainly carboxylic acids [5,10]. However, the effect of this transformation process might be dependent on the structure of organic compounds [24]. From the results shown in Fig. 2, the molecular structure of catechol should be more resistant to the electrophilic attack by hydroxyl radicals among the three selected aromatic compounds, which is caused either by the steric hinderance effect or by the electron effect.
3.2. Kinetic investigation of aliphatic acids degradation on Ti/Re-Sb-SnO2 electrodes 3.2.1. Effect of REs doping on the electrode performance The degradation of six kinds of selected aliphatic acids (maleic acid, fumaric acid, succinic acid, malonic acid, oxalic acid and acetic acid) with different Ti/Re-Sb-SnO2 electrodes was investigated and the results are fitted with first-order rate constants (Fig. 3 and Figs. S3–S7 (Supplementary material)). The reaction rate constants korg for these acids elimination and kTOC for their mineralization are shown in Fig. 4. Compared with the control electrode, the Ti/Gd-SbSnO2 and Ti/Eu-Sb-SnO2 electrodes show better performance on the degradation of most of the investigated acids, as their korg and kTOC are 15–90% and 14–123% higher than that of the control (as indicated in Fig. 4). However, the Ti/Ce-Sb-SnO2 electrode shows either closely (for oxalic acid, fumaric acid and succinic acid) or lower (for maleic acid, malonic acid and acetic acid) effectiveness compared with the control electrode.
6
(a)
Ti/Sb-SnO2
40
Ti/Ce-Sb-SnO2
-1
5
korg (×10 min )
Ti/Gd-Sb-SnO2
4
Ti/Eu-Sb-SnO2
30
3 2
Maleic acid Fumaric acid Succinic acid Malonic acid Oxalic acid Acetic acid
(a)
20
10
1 0
0
0
60
120
180
240
(b)
30
-3
Ti/Ce-Sb-SnO2 3
-1
Ti/Sb-SnO2
kTOC (×10 min )
40
4
Ti/Sb-SnO2 Ti/Ce-Sb-SnO2 Ti/Gd-Sb-SnO2 Ti/Eu-Sb-SnO2
300
Time (min)
-ln([TOC]/[TOC]0)
229
-3
-ln([Succinic acid]/[Succinic acid]0)
Y.-H. Cui et al. / Journal of Hazardous Materials 239–240 (2012) 225–232
Ti/Gd-Sb-SnO2 Ti/Eu-Sb-SnO2
2
Maleic acid Fumaric acid Succinic acid Malonic acid Oxalic acid Acetic acid
(b)
20
10
1
0
0
0
60
120
180
240
300
360
Time (min) Fig. 3. Kinetics of (a) succinic acid elimination and (b) TOC removal in electrocatalytic degradation of succinic acid with different Ti/Re-Sb-SnO2 electrodes. Initial succinic acid = 100 mg L−1 , volume of solution = 100 mL, I = 0.12 A. The solid lines represent the fitted curves and the dots represent the experimental data.
Ti/Sb-SnO2 Ti/Ce-Sb-SnO2 Ti/Gd-Sb-SnO2 Ti/Eu-Sb-SnO2
Fig. 4. Reaction rate constants of (a) organics elimination and (b) TOC removal for different aliphatic acids in electrocatalytic degradation process with different Ti/ReSb-SnO2 electrodes. Initial aliphatic acid = 100 mg L−1 , volume of solution = 100 mL, I = 0.12 A.
easily attacked by electrophiles (such as • OH) which leads a rapid molecular destruction. 3.3. Efficiency evaluation of the organics degradation on Ti/Re-Sb-SnO2 electrodes
3.2.2. Effectiveness of electrocatalytic degradation on different aliphatic acids As indicated in Fig. 4, for each electrode, the highest korg and kTOC are found in the oxalic acid degradation process among the selected six aliphatic acids, which implies that oxalic acid is more readily to be destroyed in present electrocatalytic process. Further, the value of korg and kTOC for oxalic acid are almost the same on each electrode, indicating that oxalic acid can be directly oxidized to carbon dioxide without the formation of other by-products. It can also be found from Fig. 4 that the korg and kTOC for oxalic acid degradation are 3–4 times and 3–5 times higher than that of acetic acid, respectively, though they are both carboxylic acids with two carbon atoms. The difference between the two compounds is that oxalic acid has more carboxyls than that of acetic acid, and thus has more double bonds and has higher oxygen content. Actually, comparing the compounds with four carbon atoms, namely, maleic acid, fumaric acid and succinic acid, a similar result could be found for most investigated electrodes except for Ti/Ce-Sb-SnO2 , though the difference are not as obviously as that between oxalic acid and acetic acid which might be due to the much higher oxygen content of oxalic acid. So, it seems that the compound with higher oxygen content or more double bonds is more readily to be destroyed in the electrocatalytic process. It could be suggested that the higher oxygen content of an organic compound means that its carbon atoms are in higher valence and do not far from the final oxidation state, and the double bond with electronic structure would be more
The mineralization current efficiency and energy consumption were calculated from Eqs. (1) and (2) based on the experiments. Generally, for the electrocatalytic degradation of each compound on each electrode, the MCE decreases rapidly with the reaction time, while the Ep increases with the reaction time. The loss of MCE and the increase of Ep are mainly due to oxygen generation from water electrolysis which induces additional charge and energy consumption. The average MCE and Ep for the mineralization of organics on different electrodes are summarized and shown in Table 1. It is found that for each organic compound, the trend of MCE variation on different electrodes is in accordance with the kTOC variation, and the trend of Ep variation on different electrodes is in reverse to the kTOC variation, namely, the Ti/Gd-Sb-SnO2 and Ti/EuSb-SnO2 electrodes show better performance on the degradation of most of the investigated organics compared with the control, while the Ti/Ce-Sb-SnO2 electrode shows either closely or lower effectiveness compared with the control. As for each electrode, however, the higher kTOC does not mean the higher MCE on the degradation of different organics, though it still corresponds to the lower Ep . For example, the kTOC of oxalic acid is much higher than those of other compounds, while the MCE of oxalic acid is almost the lowest one among all the investigated organics on each electrode. As aforementioned, oxalic acid has the highest oxygen content and it needs to transfer the least electrons in the electrochemical reaction process. Therefore, at a given constant current in the present study,
230
Y.-H. Cui et al. / Journal of Hazardous Materials 239–240 (2012) 225–232
Benzoquinone Hydroquinone Catechol Maleic acid Fumaric acid Succinic acid Malonic acid Oxalic acid Acetic acid
-1
kTOC (×10 min )
40
-3
30
20
10
0
0
10
20
30 -3
40
-1
korg (×10 min ) Fig. 5. The difference between organics elimination rate constant korg and TOC removal rate constant kTOC for different compounds in electrocatalytic degradation process with different Ti/Re-Sb-SnO2 electrodes. Initial aromatic compounds = 200 mg L−1 , initial aliphatic acid = 100 mg L−1 , volume of solution = 100 mL, I = 0.12 A.
the electron transfer density might be too high to be efficiently used on oxalic acid oxidation and much electric charge is consumed by water electrolysis which causes the lowest MCE. Besides, Table 1 also indicates that the MCE of aromatic compounds are significantly higher than those of aliphatic acids. The higher concentration and the lower oxygen content of the aromatic compounds should be responsible for their higher MCE compared with the aliphatic acids. 3.4. Proposed mechanism of the organics degradation on Ti/Re-Sb-SnO2 electrodes Fig. 5 shows the relationship of korg and kTOC for all the selected organic compounds with the studied electrodes. An interesting observation is that the difference between korg and kTOC for simple molecules (such as oxalic acid and acetic acid) is much smaller than that of complicate molecules, and this phenomenon is more significant if compared with aromatic compounds as they deviate more farther from the diagonal line. Considering that the initial concentration of aromatic compounds (200 mg L−1 ) is two times of the aliphatic acids (100 mg L−1 ), the difference between korg and kTOC for aromatic compounds should be enlarged if their initial concentration is the same as that of aliphatic acids since the rate constant should be approximately proportional increased with lower concentration [26]. Therefore, it is generally found that the difference between korg and kTOC decreases with the reduction of molecular
structure, which should be caused by the less intermediate processes for the smaller molecules. In other words, the mineralization (as represented by kTOC ) for the complicate compounds is more difficult compared with their transformation (as represented by korg ). Nonetheless, Fig. 5 also indicates that if the first transformation step for a certain compound degradation is difficult and then the first stage becomes the control step, the difference between korg and kTOC would be relatively smaller (for example, catechol degradation). The important intermediates in the organics electrolysis were detected by HPLC and the main observed intermediate products are listed in Table 2. Based on the intermediates analysis and the organics degradation efficiency, pathways of the selected organic compounds are summarized in Figs. 6 and 7. As shown in Fig. 6, hydroquinone, catechol and benzoquinone (which is in equilibrium with hydroquinone in an aqueous solution) are first electrophilic attacked by • OH and form hydroxylated intermediates. These hydroxylated intermediates are unstable and the ring cleavage happens which leads to the formation of aliphatic acids. In this process, the • OH generated from water splitting plays a key role and a greater • OH production would lead to a faster aromatics transformation. Therefore, the use of an anode material with a high • OH production is especially desirable for aromatics elimination. From the results shown in Figs. 1 and 2, Figs. S1 and S2 (Supplementary material), it might be deduced that doping with Gd or Eu enhances the capacity of Ti/Sb-SnO2 anode on • OH generation since the three selected aromatics elimination (or namely, ring cleavage because their aromatic ring are readily split without other by-products in the oxidation process) are accelerated. However, the results of electrocatalytic degradation also show that doping with Ce into Ti/Sb-SnO2 anode seems not good at the aromatic ring cleavage. Our previous research has suggested that the enhanced performance of Ti/Gd-Sb-SnO2 on phenol oxidation is attributed to the increased production of active oxygen species (e.g. • OH) and the lower mobility of surface adsorbed active oxygen atoms into SnO2 lattice; while, the redox couple of Ce4+ /Ce3+ on Ti/Ce-Sb-SnO2 electrode surface allows oxygen transfer into the SnO2 lattice, and lowered the electrode’s capacity on phenol degradation [12]. In the present study, the lower efficiency of Ti/CeSb-SnO2 electrode on the degradation of the selected aromatic compounds could be explained by the above conception. Fig. 7 shows the possible transformations of the selected carboxylic acids based on the intermediates analysis. Maleic acid and fumaric acid could be oxidized to oxalic acid in the anodic process; while, in the present undivided cell electrolysis, succinic acid is a possible intermediate product in the maleic acid or fumaric acid
Fig. 6. Proposed reaction pathways for electrocatalytic degradation of selected aromatic compounds.
Y.-H. Cui et al. / Journal of Hazardous Materials 239–240 (2012) 225–232
231
Fig. 7. Proposed reaction pathways for electrocatalytic transformation of (a) maleic acid and fumaric acid, (b) succinic acid, (c) malonic acid, (d) oxalic acid and (e) acetic acid. The compounds with a dashed frame represent the possible intermediates and the semi-circle arrow represents the possible transformation between different intermediates.
electrolysis, and then malonic acid, acetic acid and formic acid are produced (Fig. 7(a) and (b)). The malonic acid is degraded to acetic acid and formic acid, and oxalic acid is another possible product (Fig. 7(c)). The oxalic acid is directly mineralized as aforementioned (Fig. 7(d)). The acetic acid, however, might be through a formic acid pathway (Fig. 7(e)), which can explain its much lower destruction rate compared with that of oxalic acid. As shown in Figs. 3 and 4 and Figs. S3–S7 (Supplementary material), for most of the selected acids, Ti/Gd-Sb-SnO2 and Ti/Eu-Sb-SnO2 electrodes show better performance on either their elimination or their mineralization; while, both of the two REs doped electrodes do not show higher effectiveness on acetic acid degradation than that of the control. The Ti/Ce-Sb-SnO2 electrode, however, shows lower effectiveness on the degradation of some acids while it also shows similar effectiveness on the degradation of other acids compared with the control. These results imply that despite the REs doping effect, the molecular structure of organic compound also affect its degradation rate in the electrocatalytic process. Considering that the reactivity of • OH toward small aliphatic molecules is usually much lower than that toward aromatic compounds [27], the felicitously surface property of an electrode that is favorable to the adsorption and activation of a certain organic compound might be important. This suggestion is in accordance with that of our previous research on electrocatalytic degradation of BPA which has found that Ti/Sb-SnO2 shows better performance on the intermediate aliphatic acids oxidation than those of BDD and Pt electrodes [5]. Therefore, considering electrocatalytic oxidation of wastewaters containing mainly the
organics with one type of molecular structure (such as aromatic ring or small aliphatic acid), it would be effective to select a working electrode with proper features (such as • OH generation or adsorption/activation capacity). Further, it might be deduced that in treating wastewaters containing various organics or containing organics with large/complex molecular structure, a multi-stage electrocatalytic oxidation using different working electrodes at different stages may be more efficient compared with one-stage electrocatalytic oxidation using one type of working electrode.
4. Conclusions Nine kinds of organics including three kinds of basal aromatic compounds and six kinds of typical aliphatic acids were selected to investigate the electrocatalytic ability of REs (Ce, Gd, and Eu) doped Ti/Sb-SnO2 anode materials. Doping with Gd or Eu enhances the capacity of Ti/Sb-SnO2 anode on the three selected aromatics elimination and the rate constants korg are 6.3–32.3% increased compared with the control electrode; while, doping with Ce on Ti/Sb-SnO2 anode seems not good at the aromatic ring cleavage as the korg are 4.2–30% decreased compared with the control. The Ti/Gd-Sb-SnO2 and Ti/Eu-Sb-SnO2 electrodes show better performance on electrocatalytic degradation of most selected acids compared with Ti/Sb-SnO2 and the rate constants are 15–90% (for korg ) and 14–123% (for kTOC ) higher than that of the control; while, the Ti/Ce-Sb-SnO2 electrode shows similar or lower effectiveness on degradation of these acids.
232
Y.-H. Cui et al. / Journal of Hazardous Materials 239–240 (2012) 225–232
The molecular structure of organic compound affects its degradation rate in the electrocatalytic process. Catechol seems more resistant to the electrophilic attack by hydroxyl radicals which makes it more refractory to be destroyed than benzoquinone and hydroquinone. The acid with higher oxygen content or more double bonds is more readily to be destroyed in the electrocatalytic process. The compound with more complicate molecular structure or longer carbon chain would be more difficult to be mineralized due to the more complicate intermediate processes. Acknowledgments This research was supported by the National Nature Science Foundation of China (51008139) and the Open Project of State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (QA201107). The authors also appreciated the National Creative Research Group supported by the National Natural Science Foundation of China (50821002) and the National Natural Science Funds for Distinguished Young Scholars of China (51125033). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.jhazmat.2012.08.068 References [1] C.A. Martinez-Huitle, E. Brillas, Decontamination of wastewaters containing synthetic organic dyes by electrochemical methods: a general review, Appl. Catal. B: Environ. 87 (2009) 105–145. [2] M. Magureanu, D. Piroi, N.B. Mandache, V. David, A. Medvedovici, V.I. Parvulescu, Degradation of pharmaceutical compound pentoxifylline in water by non-thermal plasma treatment, Water Res. 44 (2010) 3445–3453. [3] S.X. Yang, Z.Q. Liu, X.H. Huang, B.P. Zhang, Wet air oxidation of epoxy acrylate monomer industrial wastewater, J. Hazard. Mater. 178 (2010) 786–791. [4] A.Z. Li, X. Zhao, Y.N. Hou, H.J. Liu, L.Y. Wu, J.H. Qu, The electrocatalytic dechlorination of chloroacetic acids at electrodeposited Pd/Fe-modified carbon paper electrode, Appl. Catal. B: Environ. 111–112 (2012) 628–635. [5] Y.H. Cui, X.Y. Li, G.H. Chen, Electrochemical degradation of bisphenol A on different anodes, Water Res. 43 (2009) 1968–1976. [6] R. Broseus, S. Vincent, K. Aboulfadl, A. Daneshvar, S. Sauve, B. Barbeau, M. Prevost, Ozone oxidation of pharmaceuticals, endocrine disruptors and pesticides during drinking water treatment, Water Res. 43 (2009) 4707–4717. [7] C. Sirtori, A. Zapata, I. Oller, W. Gernjak, A. Aguera, S. Malato, Solar photo-Fenton as finishing step for biological treatment of a pharmaceutical wastewater, Environ. Sci. Technol. 43 (2009) 1185–1191. [8] Z.Q. Liu, J. Ma, Y.H. Cui, L. Zhao, B.P. Zhang, Influence of different heat treatments on the surface properties and catalytic performance of carbon nanotube in ozonation, Appl. Catal. B: Environ. 101 (2010) 74–80.
[9] G.H. Chen, Electrochemical technologies in wastewater treatment, Sep. Purif. Technol. 38 (2004) 11–41. [10] X.Y. Li, Y.H. Cui, Y.J. Feng, Z.M. Xie, J.D. Gu, Reaction pathways and mechanisms of the electrochemical degradation of phenol on different electrodes, Water Res. 39 (2005) 1972–1981. [11] A. Kapalka, G. Foti, C. Comninellis, Kinetic modeling of the electrochemical mineralization of organic pollutants for wastewater treatment, J. Appl. Electrochem. 38 (2008) 7–16. [12] Y.H. Cui, Y.J. Feng, Z.Q. Liu, Influence of rare earths doping on the structure and electro-catalytic performance of Ti/Sb-SnO2 electrodes, Electrochim. Acta 54 (2009) 4903–4909. [13] A.Y. Bagastyo, J. Radjenovic, Y. Mu, R.A. Rozendal, D.J. Batstone, K. Rabaey, Electrochemical oxidation of reverse osmosis concentrate on mixed metal oxide (MMO) titanium coated electrodes, Water Res. 45 (2011) 4951–4959. [14] J.T. Kong, S.Y. Shi, L.C. Kong, X.P. Zhu, J.R. Ni, Preparation and characterization of PbO2 electrodes doped with different rare earth oxides, Electrochim. Acta 53 (2007) 2048–2054. [15] E. Vanhove, J. de Sanoit, J.C. Arnault, S. Saada, C. Mer, P. Mailley, P. Bergonzo, M. Nesladek, Stability of H-terminated BDD electrodes: an insight into the influence of the surface preparation, Phys. Status Solidi A 204 (2007) 2931–2939. [16] G.H. Zhao, Y.G. Zhang, Y.Z. Lei, B.Y. Lv, J.X. Gao, Y.A. Zhang, D.M. Li, Fabrication and electrochemical treatment application of a novel lead dioxide anode with superhydrophohic surfaces, high oxygen evolution potential, and oxidation capability, Environ. Sci. Technol. 44 (2010) 1754–1759. [17] Y. Mohd, D. Pletcher, The influence of deposition conditions and dopant ions on the structure activity, and stability of lead dioxide anode coatings, J. Electrochem. Soc. 152 (2005) D97–D102. [18] Y. Mohd, D. Pletcher, The fabrication of lead dioxide layers on a titanium substrate, Electrochim. Acta 52 (2006) 786–793. [19] S. Stucki, R. Kötz, B. Carcer, W. Suter, Electrochemical waste water treatment using high overvoltage anodes. Part II: anode performance and applications, J. Appl. Electrochem. 21 (1991) 99–104. [20] F. Montilla, E. Morallon, J.L. Vazquez, Evaluation of the electrocatalytic activity of antimony-doped tin dioxide anodes toward the oxidation of phenol in aqueous solutions, J. Electrochem. Soc. 152 (2005) B421–B427. [21] Y.H. Cui, Y.H. Wang, B. Wang, H.H. Zhou, K.Y. Chan, X.Y. Li, Electrochemical generation of ozone in a membrane electrode assembly cell with convective flow, J. Electrochem. Soc. 156 (2009) E75–E80. [22] S.Y. Yang, Y.S. Choo, S. Kim, S.K. Lim, J. Lee, H. Park, Boosting the electrocatalytic activities of SnO2 electrodes for remediation of aqueous pollutants by doping with various metals, Appl. Catal. B: Environ. 111–112 (2012) 317–325. [23] Y.J. Feng, Y.H. Cui, B.E. Logan, Z.Q. Liu, Performance of Gd-doped Ti-based Sb-SnO2 anodes for electrochemical destruction of phenol, Chemosphere 70 (2008) 1629–1636. [24] Y. Liu, H.L. Liu, J. Ma, X. Wang, Comparison of degradation mechanism of electrochemical oxidation of di- and tri-nitrophenols on Bi-doped lead dioxide electrode: effect of the molecular structure, Appl. Catal. B: Environ. 91 (2009) 284–299. [25] S. Kim, S.K. Choi, B.Y. Yoon, S.K. Lim, H. Park, Effects of electrolyte on the electrocatalytic activities of RuO2 /Ti and Sb-SnO2 /Ti anodes for water treatment, Appl. Catal. B: Environ. 97 (2010) 135–141. [26] Y.H. Cui, Y.J. Feng, X.Y. Li, Kinetics and efficiency analysis of electrochemical oxidation of phenol: influence of anode materials and operational conditions, Chem. Eng. Technol. 34 (2011) 265–272. [27] G.V. Buxton, C.L. Greenstock, W.P. Helman, W.P. Ross, Critical review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals in aqueous solution, J. Phys. Chem. Ref. Data 17 (1988) 513–886.