Journal of Environmental Sciences 26 (2014) 708–715
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Mechanism of enhanced removal of quinonic intermediates during electrochemical oxidation of Orange II under ultraviolet irradiation Fazhan Li1 , Guoting Li1,∗, Xiwang Zhang2 1. Institute of Environmental and Municipal Engineering, North China University of Water Resources and Electric Power, Zhengzhou 450011, China. E-mail: [email protected] 2. Department of Chemical Engineering, Monash University, Clayton Vic 3800, Australia
article info
abstract
Article history: Received 02 March 2013 revised 18 November 2013 accepted 25 November 2013
The effect of ultraviolet irradiation on generation of radicals and formation of intermediates was investigated in electrochemical oxidation of the azo-dye Orange II using a TiO2 -modified βPbO2 electrode. It was found that a characteristic absorbance of quinonic compounds at 255 nm, which is responsible for the rate-determining step during aromatics degradation, was formed only in electrocatalytic oxidation. The dye can be oxidized by either HO radicals or direct electron transfer. Quinonic compounds were produced concurrently. The removal of TOC by photo-assisted electrocatalytic oxidation was 1.56 times that of the sum of the other two processes, indicating a significant synergetic effect. In addition, once the ultraviolet irradiation was introduced into the process of electrocatalytic oxidation, the degradation rate of quinonic compounds was enhanced by as much as a factor of two. The more efficient generation of HO radicals resulted from the introduction of ultraviolet irradiation in electrocatalytic oxidation led to the significant synergetic effect as well as the inhibiting effect on the accumulation of quinonic compounds.
Keywords: electrocatalysis photocatalysis β-PbO2 electrode ultraviolet Orange II DOI: 10.1016/S1001-0742(13)60435-0
Introduction Advanced oxidation processes (AOPs), e.g. electrocatalysis, photocatalysis and Fenton, have drawn increasing attention in water treatment over the last few years. Compared with conventional water treatment processes, AOPs have better performance in degradation of toxic pollutants (Rosenfeldt and Linden, 2004; Peller et al., 2003; Huber et al., 2003; Pelegrini et al., 2001). Among various AOPs, photocatalysis and electrocatalysis have been widely studied due to their high removal efficiency for various pollutants. However, the two processes both consume large amounts of energy to generate HO radicals for pollutant degradation, which hampers their practical application (Andreozzi et al., 1999; Comninellis and Chen, 2010; Bouya et al., 2012; Chen, 2004; Hoffman et al., ∗ Corresponding
author. E-mail: [email protected]
1995). Hence, it is necessary to adopt novel reactive systems to utilize energy more efficiently. Recent studies showed that integration of photocatalysis and electrocatalysis could be a good approach to solve this problem. For instance, Pelegrini et al. (2001) found that the degradation rate of a photo-assisted electrolysis process using a 70TiO2 /30RuO2 (DSA) electrode was an order of magnitude greater than the sum of those of electrocatalysis and photocatalysis. The integrated process possesses not only the advantages of electrolysis such as high efficiency and ease of operation, but also enhanced photocatalytic properties of the anode were employed in the integrated process (Comninellis and Chen, 2010). It is well accepted that hydroxyl radicals are generated during photocatalytic oxidation as expressed in the following equations: TiO2 + hν −→ TiO2 (h+ + e− )
(1)
h+ + H2 O (HO− ) −→ HO· + H+
(2)
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Journal of Environmental Sciences 26 (2014) 708–715
Separation of photogenerated electrons and holes was proved to be accelerated by application of an anodic bias on a photoanode with immobilized TiO2 particles (Ward and Bard, 1982; Vinodgopal et al., 1993). The efficiency of photocatalysis was greatly enhanced, which was attributed to the use of the immobilized TiO2 film. Although the mechanism of electrocatalytic oxidation has been intensively studied to date, the process is not yet fully understood. Kirk and co-workers found that aniline was oxidized readily, but further oxidation of the intermediates to carbon dioxide was difficult (Kirk et al., 1985). Electro-oxidation of aniline was mainly attributed to the direct electron transfer from aniline to the anode. Electrochemical conversion and combustion were further accepted as the two main alternatives for the electrochemical treatment of organic pollutants by Comninellis (Comninellis, 1994). The performance of the process was strongly dependent on the electrode materials, which are classified into active and nonactive electrodes in terms of their involvement in the reduction. Nonactive electrodes such as PbO2 and SnO2 generally exhibit better performance due to generation of hydroxyl radicals as expressed in Eq. (3): MO x + H2 O −→ MO x (HO·) + H+ + e−
(3)
Hydroxyl radicals, as the strongest oxidants in aqueous solution, are nonselective oxidizing species and can decompose the organics adsorbed on the anode into CO2 and H2 O, which is called electrocatalytic combustion. However, active electrodes such as IrO2 and RuO2 are able to transform organics to biocompatible organics by the production of higher oxidation state compounds as shown in Eq (4): MO x (HO·) −→ MO x+1 + H+ + e−
(4)
The oxidation ability of MO x+1 is lower than that of hydroxyl radical, resulting in selective oxidation on the anode, which is called electrochemical conversion. However, Polcaro and co-workers claimed that the degradation mechanism was dependent on the experimental conditions for Ti/SnO2 and Ti/PbO2 electrodes (Polcaro et al., 1999). A direct oxidation may be dominant at a high concentration of organics, whereas oxidation by HO radicals may prevail for dilute solutions. This study aims to further understand the oxidation of pollutants in photo-assisted electrocatalytic oxidation and explore its properties. A TiO2 -modified β-PbO2 electrode was used in the processes. Besides the photo-assisted electrocatalytic oxidation, photocatalytic oxidation and electrocatalytic oxidation were also investigated under similar conditions as controls. The research results could expand our knowledge of photo-assisted electrocatalytic oxidation for water treatment.
1 Materials and Methods 1.1 Materials Orange II and TiO2 were purchased from Beijing Chemical Reagents Company. Orange II was selected as a model pollutant and used without further purification. The chemical structure of Orange II is illustrated in Fig. 1. TiO2 used in this study was pure anatase. Other materials were of analytical grade. The chemical 1,4-benzoquinone was purified by sublimation before use. The DSA was provided by Beijing Titanium Industrial and Trade Company and it had the same size as that of the TiO2 -modified β-PbO2 electrode. The electrode employed in this study was a 2.0 g TiO2 -modified β-PbO2 electrode and its electrochemically assisted photocatalytic properties were reported in our previous study (Li et al., 2006). 1.2 Catalytic oxidation of Orange II The experiment setup can be referred to our previous study (Li et al., 2006). Briefly, 125 mL simulated dye wastewater (Orange II 50 mg/L; Na2 SO4 0.01 mol/L) was added into a 150 mL reactor. An 8 W low pressure UV lamp emitting at 254 nm was placed parallel to the electrodes. The distance between lamp and electrodes was 20 mm. A current of 10 mA was used for the processes of electrocatalytic oxidation and photo-assisted electrocatalytic oxidation. The stable voltage was supplied by a potentiostat (DH1715A-3, Beijing Dahua Radio Instrument Factory). The simulated wastewater was stirred by a magnetic stirrer during the reactions. 1.3 Analyses The UV-Vis spectra of the samples were recorded from 200 nm to 600 nm by a U-3010 UV-Vis spectrophotometer (Hitachi Co., Japan). The concentration of the dye was determined by measuring its absorbance at 484 nm. Total organic carbon (TOC) in water was measured by a multi N/C 3000 instrument (AnalytikJenaAG, Germany) after SO3Na
SO3Na
N
N
N
N OH
Azo form Fig. 1
H O
Hydrazone form
Molecular structure of Orange II (C.I. 15510).
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the solution was filtered through a 0.45 μm filter. OH radical was measured by a colorimetric method (Steiner and Babbs, 1990; Babbs and Gale, 1987).
2 Results and discussion 2.1 Destruction of Orange II by three AOPs Orange II has four absorbance bands, of which two are in the visible region and the other two in the ultraviolet region. The two bands in the visible region at 484 and 430 nm are due to the hydrazone and azo forms of Orange II, respectively. The other two bands at 230 nm and at 310 nm in the ultraviolet region are ascribed to the benzene and naphthalene rings of the dye, respectively (Wu et al., 2000; Stylidi et al., 2003). The four bands weakened simultaneously during both photocatalytic oxidation and photo-assisted electrocatalytic oxidation, which indicates that the structure units corresponding with the four bands were destroyed. Meanwhile, the dye solution changed from orange to colorless. The typical UV-Vis absorbance changes of Orange II during electrocatalytic oxidation are illustrated in Fig. 2. A new absorbance band at 255 nm forms in the ultraviolet region from the very beginning, though the decay in the visible region occurs concurrently. The strongest absorbance intensity at 255 nm was achieved at about 40 min, which then diminished gradually. The absorbance might be caused by quinonic compounds (Huang and Yu, 1988). Meanwhile, the orange color of the wastewater initially disappeared quickly and afterward the solution became bright-yellow. Then the color of bright-yellow became paler at longer reaction times. The TOC removal during 2-hr degradation by photocatalytic oxidation, electrocatalytic oxidation and photo-assisted electrocatalytic oxidation was 9.6%, 20.3% and 47.3%, respectively. The TOC removal achieved by the photo-assisted electrocatalytic oxidation was 1.56 times 0 min 20 min 40 min 60 min 80 min 100 min
2.5 Absorbance at 255 nm
1.8
2.0 1.5
Abs (255 nm)
3.0
1.6 1.4 1.2 0
20
40
60 80 100 120 Time (min)
120 min
1.0 0.5 0.0 200
300
400 Wavelength (nm)
500
600
Fig. 2 UV-Vis absorbance of Orange II during electrocatalytic oxidation at reaction time 20, 40, 60, 80, 100 and 120 min.
that of the sum of the photocatalytic oxidation and electrocatalytic oxidation, indicating a synergetic effect between the two processes. As a kind of special photoanode, the TiO2 -modified β-PbO2 electrode has good photocatalytic activity. On the other hand, the β-PbO2 electrode was widely studied in recent years and has been proven to be one of the most prominent electrocatalytic electrodes with potential practicability (Wu and Zhou, 2001; Panizza and Cerisola, 2004). Hydroxyl radicals can be generated on the anode by oxidation of adsorbed water as expressed in Eq. (5) (Flesazr and Ploszynska, 1985): PbO2 (h+ )+ H2 Oads (HO− ) −→ PbO2 (HO·) + H+
(5)
A simplified model for the treatment of the aromatic pollutants by electrocatalytic oxidation is well accepted, which includes three irreversible and consecutive steps: (1) oxidation of refractory compounds to quinonic compounds, (2) formation of aliphatic acids via ring opening reaction, (3) mineralization of aliphatic acids to CO2 (Polcaro et al., 1999; Wu and Zhou, 2001). The oxidation of quinonic compounds is regarded as the rate-determining step in phenol degradation (Polcaro et al., 1999; Tahar and Savall, 1998). As discussed above, a large amount of quinonic compounds accumulated during the electrocatalytic oxidation. Santos and co-workers examined the toxicity of the intermediates detected in wet oxidation of phenol and found that the EC50 values of phenol, catechol, hydroquinone and 1,4-benzoquinone are 16.7±4.2, 8.32±2.7, 0.041, and 0.1 mg/L, respectively (Santos et al., 2004). This indicates that the toxicity of the intermediates, hydroquinone and 1,4benzoquinone, is 3 and 2 orders of magnitude higher than that of their parent phenol, respectively. Hence, quinonic compounds are the most serious concern during electrochemical treatment. Stadler et al. (2012) highlighted that a comprehensive understanding of the fate of pollutants as well as the mechanisms and kinetics of transformation product formation and disappearance is needed. The introduction of ultraviolet irradiation could enhance the removal of quinonic intermediates and TOC in electrocatalytic oxidation. Hence, photocatalytic oxidation and electrocatalytic oxidation were reciprocal of each other. 2.2 Effect of HO radicals Alcohols are commonly used as a quencher in photocatalytic oxidation to estimate the oxidation mechanism (Sun and Joseph, 1995). For example, iso-propanol (i-PrOH, 0.1 mol/L) was used by Chen and co-workers to prove the existence of a number of HO radicals (Chen et al., 2005). In this study, Fig. 3 shows that the inhibitive effect of i-PrOH in photocatalytic oxidation is significant. The decolorization of Orange II at 40 min decreases from 59.8% in the absence of i-PrOH to 13.6% in its presence. By contrast, the inhibitive effect is very weak in elec-
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2.3 Reduction of quinonic compounds by commercial DSA and TiO2 modified β-PbO2 electrode
Color removal
0.8
0.6
0.4
0.2
0.0 EO EO(i-PrOH)
PE
PE(i-PrOH)
PO PO(i-PrOH)
Fig. 3 Removal of color by photo-assisted electrocatalytic oxidation (PE), electrocatalytic oxidation (EO) and photocatalytic oxidation (PO) at 40 min in the presence and absence of 0.1 mol/L i-PrOH.
1.4 1.2 Relative amount of OH radicals
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1.0 0.8 0.6 0.4
Electrocatalytic oxidation Photocatalytic oxidation Photo-assisted electrocatalytic oxidation
0.2 0.0 0
10
20
30 40 Time (min)
50
60
Fig. 4 Relative amount of OH radicals produced in the processes of electrocatalytic oxidation, photocatalytic oxidation and photo-assisted electrocatalytic oxidation.
trocatalytic oxidation and photo-assisted electrocatalytic oxidation, which is extremely different from photocatalytic oxidation. Therefore, it can be concluded that HO radical is the dominated oxidant in photocatalytic oxidation and decolorization in the other two processes should be mainly attributed to the other electrochemical processes such as direct electron transfer in the presence of i-PrOH. Figure 4 shows the amount of OH radicals generated within 60 min of reaction in the three processes. The yield of OH radicals was greatly increased once the ultraviolet irradiation was introduced into the electrocatalytic oxidation system. Even compared with photocatalytic oxidation alone, the yield of OH radicals was still insignificant in electrocatalytic oxidation. On the contrary, decolorization of the dye in electrocatalytic oxidation is faster than in photocatalytic oxidation, indicating that different oxidation paths may be involved.
Although DSA is usually used in the chlor-alkali industry, it also brings significant improvements in water electrolysis, selective synthesis and destructive oxidation (Panizza et al., 2003). DSA and the non-active TiO2 -modified βPbO2 electrode were both tested under the same conditions to compare their performance in terms of formation of quinonic compounds in electrocatalytic oxidation (Fig. 5a) and photo-assisted electrocatalytic oxidation (Fig. 5b). It can be seen that that the quinonic compounds were formed and destroyed at a higher rate by the TiO2 -modified βPbO2 electrode than by the commercial DSA in both the processes. The TOC removal was only 18.1% within 2hr degradation by photo-assisted electrocatalytic oxidation using the commercial DSA electrode, whereas as much as 47.3% was achieved by the TiO2 -modified β-PbO2 electrode. Furthermore, the absorbance at 255 nm reached its peak at ca. 40 min in electrocatalytic oxidation whilst at 20 min in photo-assisted electrocatalytic oxidation using the TiO2 -modified β-PbO2 electrode. Meanwhile, the concentration of quinonic compounds was at a lower level in the process of photo-assisted electro-catalytic oxidation than in electrocatalytic oxidation regardless of the electrode. This means that the ultraviolet light in photo-assisted electro-catalytic oxidation plays a key role in the enhanced elimination of the quinonic compounds, and the non-active TiO2 -modified β-PbO2 electrode outperforms the DSA. 2.4 Enhancement of electrocatalytic oxidation by ultraviolet light The mechanism of electro-catalytic oxidation of organics is complicated in essence. A series of oxidants such as HO radical, H2 O2 and O3 are generated at the anode. The oxidation of the organics is thought to be mainly attributed to the hydroxyl radical and the basic electrochemical step should be involved as well because it is indispensable for the oxidation. Electrochemical catalysis is different from pure chemical catalysis, and loss of electrons from substrates could occur on the anode under desired electrical potential if permitted by thermodynamics. The electrode potential is proportional to the Gibbs free energy, which is described in the following equation: ΔG = −Z × E × F
(6)
where, E represents the electromotive force of the reversible cell; ΔG is free energy change of the redox process occurring in the system; Z is a positive integer equal to the number of elementary charges involved in the redox process; and F is the Faraday constant. As long as the electrode potential is equal to or higher than that of the standard electrode potential for a redox pair, the electrocatalytic reaction would occur theoretically. Electrode potential is an important parameter in elec-
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DSA
PbO2
2.0 1.2
b
1.8 Absorbance at 255 nm
Absorbance at 255 nm
a
1.6
1.4
1.0
0.8
0.6
1.2 0.4 0
20
40
60 80 Time (min)
100
120
0
20
40
60 80 Time (min)
100
120
Fig. 5 Disappearance of the absorbance at 255 nm during electrocatalytic oxidation (a) and photo-assisted electrocatalytic oxidation (b) using DSA and TiO2 -modified β-PbO2 electrode.
trochemical reactions. Electrochemical oxidation above OEP (oxygen evolution potential) is always employed for organics decontamination due to kinetic rather than thermodynamic limitations (Comninellis, 1994). However, the direct electron transfer from substrates to anode can occur under any positive electrical potential applied, whether higher or lower than the OEP. Typically, electrode fouling originates from direct electron transfer in the potential region of water stability due to the production of quinonic intermediates. By contrast, the effect of the active oxidants such as HO radicals and MO x+1 dominates in the potential region of water decomposition, which avoids the electrode fouling to some extent. The total mechanism of electrocatalytic oxidation is illustrated in Fig. 6 (Qu and Liu, 2007). Quinonic compounds such as benzoquinone are considered to be largely generated due to the limited hydroxyl
Oxidation by .OH/MOX+1
E/V (vs.NHE)
Direct electron transfer OEP
Oxidation Direct electron transfer
0 Reduction zone
Reduction
Fig. 6 General mechanism for the electrocatalytic oxidation of organics. OEP is the oxygen evolution potential for the electrocatalytic electrode (Qu and Liu, 2007).
radicals relative to phenol (Iniesta et al., 2001). Nevertheless, almost same amount of quinonic compounds is still observed to have accumulated during electro-catalytic oxidation in the presence of i-PrOH as that in the absence of i-PrOH. The contribution of hydroxyl radicals seems to be insignificant in decolorization of Orange II once electrocatalytic oxidation is involved. This is basically due to the direct electron transfer from Orange II molecules to the anode, which occurs in parallel with electrophilic addition by HO radicals. It is worth noting that this phenomenon does not occur in the process of photocatalytic oxidation using the same electrode. The organics are mainly degraded by MO x+1 oxidation and direct electron transfer above OEP at an active electrode such as DSA due to the generation of higher oxidation state oxide MO x+1 resulting from HO radicals. The higher oxidation state MO x+1 has a lower oxidizability, and consequently the removal of quinonic compounds is slow on the DSA electrode. By contrast, HO radicals are formed on nonactive electrodes such as the β-PbO2 electrode, and the fast removal of quinonic compounds was observed. Although the degradation is seemingly dependent on the experimental conditions for nonactive Ti/SnO2 and Ti/PbO2 electrodes (Polcaro et al., 1999), both the oxidation by HO radicals and by direct electron transfer occur during the degradation of the organics concurrently. A synergetic effect is observed when ultraviolet irradiation is introduced into the electrocatalytic system using a DSA electrode. Pelegrini ascribed the synergy to the generation of an increased number of active sites using a DSA type electrode, in which the physisorbed hydroxyl radicals are generated via Eq. (4). Once the illumination by ultraviolet light is applied on the TiO2 -modified βPbO2 electrode, more HO radicals are generated according to Eqs. (1) and (2) besides Eq. (5). Moreover, separation of photogenerated electrons and holes can be accelerated by application of an anodic bias to the TiO2 film and
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thus the lifetime of active holes and electrons is greatly extended (Vinodgopal et al., 1993; Ward and Bard, 1982). The efficiency of photocatalysis is greatly enhanced as well during photo-assisted electrocatalytic oxidation system. Besides, there are H2 O2 , O2 and O3 generated on the anode during electrolytic discharge of water according to (Michaud et al., 2003):
degree of coverage of the electrode by OH radicals in the steady state. Taking into account the kinetic equations and the stoichiometry of the reaction, the following set of differential equations can be obtained:
2HO· −→ H2 O2
(7)
H2 O2 −→O2 + 2H+ + 2e−
(8)
HO· →O· + H+ + e−
(9)
d[B] (18) = K1 [A] − K2 [B] dt Integrating Eqs. (17)–(18) for the following initial conditions:
O· + O2 −→ O3
(10)
Under ultraviolet irradiation, the generated hydrogen peroxide and ozone reacts rapidly with UV light to regenerate HO radicals according to Eqs. (11)–(13), which may inhibit oxygen evolution and have fully utilized HO radicals (Zhang et al., 2003; Ruppert and Bauer, 1994). H2 O2 + hν −→ 2HO·
(11)
O3 + hν (λ < 310 nm)−→ O· + O2
(12)
O· + H2 O −→ 2HO·
(13)
Overall, the generation of HO radicals originated from the introduction of ultraviolet light is more efficient in enhancing the removal of quinonic compounds in photoassisted electrocatalytic oxidation. 2.5 Reaction kinetics According to the simplified model by Polcaro and Wu (Polcaro et al., 1999; Wu and Zhou, 2001), the overall oxidation processes can be roughly represented as follows: K1
K2
A−→B−→C
(14)
A represents Orange II molecules; B is the quinonic compounds and C is the organic acids. K1 (min−1 ) and K2 (min−1 ) are the apparent rate constants. The reaction steps are based on the following assumptions: there is no HO radical accumulation in the solution due to its strong oxidation ability, and HO radicals therefore reach a steady state concentration. All the reactions can be simplified to pseudo-first-order kinetics as follows: ri = Ki [Ri ]
(15)
where, [Ri ] is the concentration of reactant Ri such as A and B. The apparent rate constant Ki is associated with the true rate constant ki by: Ae θOH Ki = ki Vsol
−
d[A] = K1 [A] dt
At time t = 0, [A] = a; [B] = 0
(19)
The generalized model is given by: [A] = ae−K1 t [B] =
(20)
K1 a (e−K1 t − e−K2 t ) K2 − K1
(21)
When [B] reaches its maximal value, d[B] dt = 0, tm = lnK2 −lnK1 . K2 −K1 The tm of electrocatalytic oxidation and photo-assisted electrocatalytic oxidation can be roughly accepted as 40 and 20 min from the UV-Vis analysis, respectively. The removal index of the quinonic compounds is defined as: λ = K2 /K1
(22)
The calculated results are presented in Table 1. It can be seen that the degradation rate of the quinonic compounds (K2 ) during electrocatalytic oxidation is 0.0196 min−1 while that during photo-assisted electrocatalytic oxidation is as much as 0.0652 min−1 . The degradation rate of the quinonic compounds increases by as much as a factor of two when ultraviolet light is imported into the electrocatalytic oxidation process. The removal index of the quinonic compounds λ during photo-assisted electrocatalytic oxidation is almost three times that of the electrocatalytic oxidation process alone. Hence, ultraviolet light is capable of assisting the electrocatalytic oxidation process to decolorize and degrade dye solution, especially for the removal of the quinonic compounds.
Table 1 Apparent rate constants (K1 ) and the calculated results (K2 ) and the removal index of quinonic compounds (λ) during electrocatalytic oxidation and photo-assisted electrocatalytic oxidation of Orange II.
(16)
where,Vsol (m3 ) is the volume of the electrolyte, Ae (m2 ) is the effective area of the electrode and θOH represents the
(17)
K1 K2 λ
Electrocatalytic oxidation
Photo-assisted electrocatalytic oxidation
0.0314 (R2 = 0.93) 0.0196 0.624
0.0374 (R2 = 0.97) 0.0652 1.75
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3 Conclusions The production of quinonic intermediates is an indispensible step during aromatics treatment by advanced oxidation processes. A significant synergetic effect occurred in electrocatalytic oxidation assisted by ultraviolet irradiation. Particularly, the absorbance band of quinonic compounds at 255 nm was formed during electrocatalytic oxidation. It was proved that HO radicals were generated more efficiently in electrocatalytic oxidation assisted by ultraviolet irradiation, which is the so-called photo-assisted electrocatalytic oxidation. As such, the degradation rate of quinonic compounds in electrocatalytic oxidation was enhanced by as much as a factor of two by the introduction of ultraviolet irradiation. The initial decolorization in electrocatalytic oxidation and photo-assisted electrocatalytic oxidation could be mainly attributed to the direct electron transfer from Orange II molecules to the anode. The dye can be oxidized by HO radicals as well as by direct electron transfer during electrocatalytic oxidation. Acknowledgments This work was supported by the starting fund for talents of North China University of Water Resources and Electric Power, and partially by the National Science Foundation of China (No. 51378205). references Andreozzi, R., Caprio, V., Insola, A., Marotta, R., 1999. Advanced oxidation processes (AOP) for water purification and recovery. Catal. Today 53(1), 51–59. Babbs, C. F., Gale, M. J., 1987. Colorimetric assay for methanesulfinic acid in biological samples. Anal. Biochem. 163(1), 67–76. Bouya, H., Errami, M., Salghi, R., Bazzi, L., Zarrouk, A., Al-Deyab, S. S. et al., 2012. Electrochemical degradation of cypermethrin pesticide on a SnO2 Anode. Int. J. Electrochem. Sci. 7(4), 3453–3465. Chen, G. H., 2004. Electrochemical technologies in wastewater treatment. Sep. Purif. Technol. 38(1), 11–41. Chen, Y. X., Yang, S. Y., Wang, K., Lou, L. P., 2005. Role of primary active species and TiO2 surface characteristic in UV-illuminated photodegradation of Acid Orange 7. J. Photochem. Photobiol. A: Chem. 172(1), 47–54. Comninellis, C., 1994. Electrocatalysis in the electrochemical conversion/combustion of organic pollutants for waste water treatment. Electrochim. Acta 39(11-12), 1857–1862. Comninellis, C., Chen, G. H., 2010. Electrochemistry for the Environment. Springer Science Business Media, New York. Flesazr, B., Ploszynska, J., 1985. An attempt to define benzene and phenol electrochemical oxidation mechanism. Electrochim. Acta 30(1), 31–42. Hoffman, M. R., Martin, S. T., Choi, W. Y., Bahnemann, D. W., 1995. Environmental applications of semiconductor photocatalysis. Chemi. Rev. 95(1), 69–96. Huang, L., Yu, D. Q., 1988. Application of UV-Vis Spectra in Organic
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Mechanism of enhanced removal of quinonic intermediates during electrochemical oxidation of Orange II under ultraviolet irradiation Fazhan Li1 , Guoting Li1,∗, Xiwang Zhang2 1. Institute of Environmental and Municipal Engineering, North China University of Water Resources and Electric Power, Zhengzhou 450011, China. E-mail: [email protected] 2. Department of Chemical Engineering, Monash University, Clayton Vic 3800, Australia
article info
abstract
Article history: Received 02 March 2013 revised 18 November 2013 accepted 25 November 2013
The effect of ultraviolet irradiation on generation of radicals and formation of intermediates was investigated in electrochemical oxidation of the azo-dye Orange II using a TiO2 -modified βPbO2 electrode. It was found that a characteristic absorbance of quinonic compounds at 255 nm, which is responsible for the rate-determining step during aromatics degradation, was formed only in electrocatalytic oxidation. The dye can be oxidized by either HO radicals or direct electron transfer. Quinonic compounds were produced concurrently. The removal of TOC by photo-assisted electrocatalytic oxidation was 1.56 times that of the sum of the other two processes, indicating a significant synergetic effect. In addition, once the ultraviolet irradiation was introduced into the process of electrocatalytic oxidation, the degradation rate of quinonic compounds was enhanced by as much as a factor of two. The more efficient generation of HO radicals resulted from the introduction of ultraviolet irradiation in electrocatalytic oxidation led to the significant synergetic effect as well as the inhibiting effect on the accumulation of quinonic compounds.
Keywords: electrocatalysis photocatalysis β-PbO2 electrode ultraviolet Orange II DOI: 10.1016/S1001-0742(13)60435-0
Introduction Advanced oxidation processes (AOPs), e.g. electrocatalysis, photocatalysis and Fenton, have drawn increasing attention in water treatment over the last few years. Compared with conventional water treatment processes, AOPs have better performance in degradation of toxic pollutants (Rosenfeldt and Linden, 2004; Peller et al., 2003; Huber et al., 2003; Pelegrini et al., 2001). Among various AOPs, photocatalysis and electrocatalysis have been widely studied due to their high removal efficiency for various pollutants. However, the two processes both consume large amounts of energy to generate HO radicals for pollutant degradation, which hampers their practical application (Andreozzi et al., 1999; Comninellis and Chen, 2010; Bouya et al., 2012; Chen, 2004; Hoffman et al., ∗ Corresponding
author. E-mail: [email protected]
1995). Hence, it is necessary to adopt novel reactive systems to utilize energy more efficiently. Recent studies showed that integration of photocatalysis and electrocatalysis could be a good approach to solve this problem. For instance, Pelegrini et al. (2001) found that the degradation rate of a photo-assisted electrolysis process using a 70TiO2 /30RuO2 (DSA) electrode was an order of magnitude greater than the sum of those of electrocatalysis and photocatalysis. The integrated process possesses not only the advantages of electrolysis such as high efficiency and ease of operation, but also enhanced photocatalytic properties of the anode were employed in the integrated process (Comninellis and Chen, 2010). It is well accepted that hydroxyl radicals are generated during photocatalytic oxidation as expressed in the following equations: TiO2 + hν −→ TiO2 (h+ + e− )
(1)
h+ + H2 O (HO− ) −→ HO· + H+
(2)
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Separation of photogenerated electrons and holes was proved to be accelerated by application of an anodic bias on a photoanode with immobilized TiO2 particles (Ward and Bard, 1982; Vinodgopal et al., 1993). The efficiency of photocatalysis was greatly enhanced, which was attributed to the use of the immobilized TiO2 film. Although the mechanism of electrocatalytic oxidation has been intensively studied to date, the process is not yet fully understood. Kirk and co-workers found that aniline was oxidized readily, but further oxidation of the intermediates to carbon dioxide was difficult (Kirk et al., 1985). Electro-oxidation of aniline was mainly attributed to the direct electron transfer from aniline to the anode. Electrochemical conversion and combustion were further accepted as the two main alternatives for the electrochemical treatment of organic pollutants by Comninellis (Comninellis, 1994). The performance of the process was strongly dependent on the electrode materials, which are classified into active and nonactive electrodes in terms of their involvement in the reduction. Nonactive electrodes such as PbO2 and SnO2 generally exhibit better performance due to generation of hydroxyl radicals as expressed in Eq. (3): MO x + H2 O −→ MO x (HO·) + H+ + e−
(3)
Hydroxyl radicals, as the strongest oxidants in aqueous solution, are nonselective oxidizing species and can decompose the organics adsorbed on the anode into CO2 and H2 O, which is called electrocatalytic combustion. However, active electrodes such as IrO2 and RuO2 are able to transform organics to biocompatible organics by the production of higher oxidation state compounds as shown in Eq (4): MO x (HO·) −→ MO x+1 + H+ + e−
(4)
The oxidation ability of MO x+1 is lower than that of hydroxyl radical, resulting in selective oxidation on the anode, which is called electrochemical conversion. However, Polcaro and co-workers claimed that the degradation mechanism was dependent on the experimental conditions for Ti/SnO2 and Ti/PbO2 electrodes (Polcaro et al., 1999). A direct oxidation may be dominant at a high concentration of organics, whereas oxidation by HO radicals may prevail for dilute solutions. This study aims to further understand the oxidation of pollutants in photo-assisted electrocatalytic oxidation and explore its properties. A TiO2 -modified β-PbO2 electrode was used in the processes. Besides the photo-assisted electrocatalytic oxidation, photocatalytic oxidation and electrocatalytic oxidation were also investigated under similar conditions as controls. The research results could expand our knowledge of photo-assisted electrocatalytic oxidation for water treatment.
1 Materials and Methods 1.1 Materials Orange II and TiO2 were purchased from Beijing Chemical Reagents Company. Orange II was selected as a model pollutant and used without further purification. The chemical structure of Orange II is illustrated in Fig. 1. TiO2 used in this study was pure anatase. Other materials were of analytical grade. The chemical 1,4-benzoquinone was purified by sublimation before use. The DSA was provided by Beijing Titanium Industrial and Trade Company and it had the same size as that of the TiO2 -modified β-PbO2 electrode. The electrode employed in this study was a 2.0 g TiO2 -modified β-PbO2 electrode and its electrochemically assisted photocatalytic properties were reported in our previous study (Li et al., 2006). 1.2 Catalytic oxidation of Orange II The experiment setup can be referred to our previous study (Li et al., 2006). Briefly, 125 mL simulated dye wastewater (Orange II 50 mg/L; Na2 SO4 0.01 mol/L) was added into a 150 mL reactor. An 8 W low pressure UV lamp emitting at 254 nm was placed parallel to the electrodes. The distance between lamp and electrodes was 20 mm. A current of 10 mA was used for the processes of electrocatalytic oxidation and photo-assisted electrocatalytic oxidation. The stable voltage was supplied by a potentiostat (DH1715A-3, Beijing Dahua Radio Instrument Factory). The simulated wastewater was stirred by a magnetic stirrer during the reactions. 1.3 Analyses The UV-Vis spectra of the samples were recorded from 200 nm to 600 nm by a U-3010 UV-Vis spectrophotometer (Hitachi Co., Japan). The concentration of the dye was determined by measuring its absorbance at 484 nm. Total organic carbon (TOC) in water was measured by a multi N/C 3000 instrument (AnalytikJenaAG, Germany) after SO3Na
SO3Na
N
N
N
N OH
Azo form Fig. 1
H O
Hydrazone form
Molecular structure of Orange II (C.I. 15510).
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the solution was filtered through a 0.45 μm filter. OH radical was measured by a colorimetric method (Steiner and Babbs, 1990; Babbs and Gale, 1987).
2 Results and discussion 2.1 Destruction of Orange II by three AOPs Orange II has four absorbance bands, of which two are in the visible region and the other two in the ultraviolet region. The two bands in the visible region at 484 and 430 nm are due to the hydrazone and azo forms of Orange II, respectively. The other two bands at 230 nm and at 310 nm in the ultraviolet region are ascribed to the benzene and naphthalene rings of the dye, respectively (Wu et al., 2000; Stylidi et al., 2003). The four bands weakened simultaneously during both photocatalytic oxidation and photo-assisted electrocatalytic oxidation, which indicates that the structure units corresponding with the four bands were destroyed. Meanwhile, the dye solution changed from orange to colorless. The typical UV-Vis absorbance changes of Orange II during electrocatalytic oxidation are illustrated in Fig. 2. A new absorbance band at 255 nm forms in the ultraviolet region from the very beginning, though the decay in the visible region occurs concurrently. The strongest absorbance intensity at 255 nm was achieved at about 40 min, which then diminished gradually. The absorbance might be caused by quinonic compounds (Huang and Yu, 1988). Meanwhile, the orange color of the wastewater initially disappeared quickly and afterward the solution became bright-yellow. Then the color of bright-yellow became paler at longer reaction times. The TOC removal during 2-hr degradation by photocatalytic oxidation, electrocatalytic oxidation and photo-assisted electrocatalytic oxidation was 9.6%, 20.3% and 47.3%, respectively. The TOC removal achieved by the photo-assisted electrocatalytic oxidation was 1.56 times 0 min 20 min 40 min 60 min 80 min 100 min
2.5 Absorbance at 255 nm
1.8
2.0 1.5
Abs (255 nm)
3.0
1.6 1.4 1.2 0
20
40
60 80 100 120 Time (min)
120 min
1.0 0.5 0.0 200
300
400 Wavelength (nm)
500
600
Fig. 2 UV-Vis absorbance of Orange II during electrocatalytic oxidation at reaction time 20, 40, 60, 80, 100 and 120 min.
that of the sum of the photocatalytic oxidation and electrocatalytic oxidation, indicating a synergetic effect between the two processes. As a kind of special photoanode, the TiO2 -modified β-PbO2 electrode has good photocatalytic activity. On the other hand, the β-PbO2 electrode was widely studied in recent years and has been proven to be one of the most prominent electrocatalytic electrodes with potential practicability (Wu and Zhou, 2001; Panizza and Cerisola, 2004). Hydroxyl radicals can be generated on the anode by oxidation of adsorbed water as expressed in Eq. (5) (Flesazr and Ploszynska, 1985): PbO2 (h+ )+ H2 Oads (HO− ) −→ PbO2 (HO·) + H+
(5)
A simplified model for the treatment of the aromatic pollutants by electrocatalytic oxidation is well accepted, which includes three irreversible and consecutive steps: (1) oxidation of refractory compounds to quinonic compounds, (2) formation of aliphatic acids via ring opening reaction, (3) mineralization of aliphatic acids to CO2 (Polcaro et al., 1999; Wu and Zhou, 2001). The oxidation of quinonic compounds is regarded as the rate-determining step in phenol degradation (Polcaro et al., 1999; Tahar and Savall, 1998). As discussed above, a large amount of quinonic compounds accumulated during the electrocatalytic oxidation. Santos and co-workers examined the toxicity of the intermediates detected in wet oxidation of phenol and found that the EC50 values of phenol, catechol, hydroquinone and 1,4-benzoquinone are 16.7±4.2, 8.32±2.7, 0.041, and 0.1 mg/L, respectively (Santos et al., 2004). This indicates that the toxicity of the intermediates, hydroquinone and 1,4benzoquinone, is 3 and 2 orders of magnitude higher than that of their parent phenol, respectively. Hence, quinonic compounds are the most serious concern during electrochemical treatment. Stadler et al. (2012) highlighted that a comprehensive understanding of the fate of pollutants as well as the mechanisms and kinetics of transformation product formation and disappearance is needed. The introduction of ultraviolet irradiation could enhance the removal of quinonic intermediates and TOC in electrocatalytic oxidation. Hence, photocatalytic oxidation and electrocatalytic oxidation were reciprocal of each other. 2.2 Effect of HO radicals Alcohols are commonly used as a quencher in photocatalytic oxidation to estimate the oxidation mechanism (Sun and Joseph, 1995). For example, iso-propanol (i-PrOH, 0.1 mol/L) was used by Chen and co-workers to prove the existence of a number of HO radicals (Chen et al., 2005). In this study, Fig. 3 shows that the inhibitive effect of i-PrOH in photocatalytic oxidation is significant. The decolorization of Orange II at 40 min decreases from 59.8% in the absence of i-PrOH to 13.6% in its presence. By contrast, the inhibitive effect is very weak in elec-
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2.3 Reduction of quinonic compounds by commercial DSA and TiO2 modified β-PbO2 electrode
Color removal
0.8
0.6
0.4
0.2
0.0 EO EO(i-PrOH)
PE
PE(i-PrOH)
PO PO(i-PrOH)
Fig. 3 Removal of color by photo-assisted electrocatalytic oxidation (PE), electrocatalytic oxidation (EO) and photocatalytic oxidation (PO) at 40 min in the presence and absence of 0.1 mol/L i-PrOH.
1.4 1.2 Relative amount of OH radicals
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1.0 0.8 0.6 0.4
Electrocatalytic oxidation Photocatalytic oxidation Photo-assisted electrocatalytic oxidation
0.2 0.0 0
10
20
30 40 Time (min)
50
60
Fig. 4 Relative amount of OH radicals produced in the processes of electrocatalytic oxidation, photocatalytic oxidation and photo-assisted electrocatalytic oxidation.
trocatalytic oxidation and photo-assisted electrocatalytic oxidation, which is extremely different from photocatalytic oxidation. Therefore, it can be concluded that HO radical is the dominated oxidant in photocatalytic oxidation and decolorization in the other two processes should be mainly attributed to the other electrochemical processes such as direct electron transfer in the presence of i-PrOH. Figure 4 shows the amount of OH radicals generated within 60 min of reaction in the three processes. The yield of OH radicals was greatly increased once the ultraviolet irradiation was introduced into the electrocatalytic oxidation system. Even compared with photocatalytic oxidation alone, the yield of OH radicals was still insignificant in electrocatalytic oxidation. On the contrary, decolorization of the dye in electrocatalytic oxidation is faster than in photocatalytic oxidation, indicating that different oxidation paths may be involved.
Although DSA is usually used in the chlor-alkali industry, it also brings significant improvements in water electrolysis, selective synthesis and destructive oxidation (Panizza et al., 2003). DSA and the non-active TiO2 -modified βPbO2 electrode were both tested under the same conditions to compare their performance in terms of formation of quinonic compounds in electrocatalytic oxidation (Fig. 5a) and photo-assisted electrocatalytic oxidation (Fig. 5b). It can be seen that that the quinonic compounds were formed and destroyed at a higher rate by the TiO2 -modified βPbO2 electrode than by the commercial DSA in both the processes. The TOC removal was only 18.1% within 2hr degradation by photo-assisted electrocatalytic oxidation using the commercial DSA electrode, whereas as much as 47.3% was achieved by the TiO2 -modified β-PbO2 electrode. Furthermore, the absorbance at 255 nm reached its peak at ca. 40 min in electrocatalytic oxidation whilst at 20 min in photo-assisted electrocatalytic oxidation using the TiO2 -modified β-PbO2 electrode. Meanwhile, the concentration of quinonic compounds was at a lower level in the process of photo-assisted electro-catalytic oxidation than in electrocatalytic oxidation regardless of the electrode. This means that the ultraviolet light in photo-assisted electro-catalytic oxidation plays a key role in the enhanced elimination of the quinonic compounds, and the non-active TiO2 -modified β-PbO2 electrode outperforms the DSA. 2.4 Enhancement of electrocatalytic oxidation by ultraviolet light The mechanism of electro-catalytic oxidation of organics is complicated in essence. A series of oxidants such as HO radical, H2 O2 and O3 are generated at the anode. The oxidation of the organics is thought to be mainly attributed to the hydroxyl radical and the basic electrochemical step should be involved as well because it is indispensable for the oxidation. Electrochemical catalysis is different from pure chemical catalysis, and loss of electrons from substrates could occur on the anode under desired electrical potential if permitted by thermodynamics. The electrode potential is proportional to the Gibbs free energy, which is described in the following equation: ΔG = −Z × E × F
(6)
where, E represents the electromotive force of the reversible cell; ΔG is free energy change of the redox process occurring in the system; Z is a positive integer equal to the number of elementary charges involved in the redox process; and F is the Faraday constant. As long as the electrode potential is equal to or higher than that of the standard electrode potential for a redox pair, the electrocatalytic reaction would occur theoretically. Electrode potential is an important parameter in elec-
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DSA
PbO2
2.0 1.2
b
1.8 Absorbance at 255 nm
Absorbance at 255 nm
a
1.6
1.4
1.0
0.8
0.6
1.2 0.4 0
20
40
60 80 Time (min)
100
120
0
20
40
60 80 Time (min)
100
120
Fig. 5 Disappearance of the absorbance at 255 nm during electrocatalytic oxidation (a) and photo-assisted electrocatalytic oxidation (b) using DSA and TiO2 -modified β-PbO2 electrode.
trochemical reactions. Electrochemical oxidation above OEP (oxygen evolution potential) is always employed for organics decontamination due to kinetic rather than thermodynamic limitations (Comninellis, 1994). However, the direct electron transfer from substrates to anode can occur under any positive electrical potential applied, whether higher or lower than the OEP. Typically, electrode fouling originates from direct electron transfer in the potential region of water stability due to the production of quinonic intermediates. By contrast, the effect of the active oxidants such as HO radicals and MO x+1 dominates in the potential region of water decomposition, which avoids the electrode fouling to some extent. The total mechanism of electrocatalytic oxidation is illustrated in Fig. 6 (Qu and Liu, 2007). Quinonic compounds such as benzoquinone are considered to be largely generated due to the limited hydroxyl
Oxidation by .OH/MOX+1
E/V (vs.NHE)
Direct electron transfer OEP
Oxidation Direct electron transfer
0 Reduction zone
Reduction
Fig. 6 General mechanism for the electrocatalytic oxidation of organics. OEP is the oxygen evolution potential for the electrocatalytic electrode (Qu and Liu, 2007).
radicals relative to phenol (Iniesta et al., 2001). Nevertheless, almost same amount of quinonic compounds is still observed to have accumulated during electro-catalytic oxidation in the presence of i-PrOH as that in the absence of i-PrOH. The contribution of hydroxyl radicals seems to be insignificant in decolorization of Orange II once electrocatalytic oxidation is involved. This is basically due to the direct electron transfer from Orange II molecules to the anode, which occurs in parallel with electrophilic addition by HO radicals. It is worth noting that this phenomenon does not occur in the process of photocatalytic oxidation using the same electrode. The organics are mainly degraded by MO x+1 oxidation and direct electron transfer above OEP at an active electrode such as DSA due to the generation of higher oxidation state oxide MO x+1 resulting from HO radicals. The higher oxidation state MO x+1 has a lower oxidizability, and consequently the removal of quinonic compounds is slow on the DSA electrode. By contrast, HO radicals are formed on nonactive electrodes such as the β-PbO2 electrode, and the fast removal of quinonic compounds was observed. Although the degradation is seemingly dependent on the experimental conditions for nonactive Ti/SnO2 and Ti/PbO2 electrodes (Polcaro et al., 1999), both the oxidation by HO radicals and by direct electron transfer occur during the degradation of the organics concurrently. A synergetic effect is observed when ultraviolet irradiation is introduced into the electrocatalytic system using a DSA electrode. Pelegrini ascribed the synergy to the generation of an increased number of active sites using a DSA type electrode, in which the physisorbed hydroxyl radicals are generated via Eq. (4). Once the illumination by ultraviolet light is applied on the TiO2 -modified βPbO2 electrode, more HO radicals are generated according to Eqs. (1) and (2) besides Eq. (5). Moreover, separation of photogenerated electrons and holes can be accelerated by application of an anodic bias to the TiO2 film and
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thus the lifetime of active holes and electrons is greatly extended (Vinodgopal et al., 1993; Ward and Bard, 1982). The efficiency of photocatalysis is greatly enhanced as well during photo-assisted electrocatalytic oxidation system. Besides, there are H2 O2 , O2 and O3 generated on the anode during electrolytic discharge of water according to (Michaud et al., 2003):
degree of coverage of the electrode by OH radicals in the steady state. Taking into account the kinetic equations and the stoichiometry of the reaction, the following set of differential equations can be obtained:
2HO· −→ H2 O2
(7)
H2 O2 −→O2 + 2H+ + 2e−
(8)
HO· →O· + H+ + e−
(9)
d[B] (18) = K1 [A] − K2 [B] dt Integrating Eqs. (17)–(18) for the following initial conditions:
O· + O2 −→ O3
(10)
Under ultraviolet irradiation, the generated hydrogen peroxide and ozone reacts rapidly with UV light to regenerate HO radicals according to Eqs. (11)–(13), which may inhibit oxygen evolution and have fully utilized HO radicals (Zhang et al., 2003; Ruppert and Bauer, 1994). H2 O2 + hν −→ 2HO·
(11)
O3 + hν (λ < 310 nm)−→ O· + O2
(12)
O· + H2 O −→ 2HO·
(13)
Overall, the generation of HO radicals originated from the introduction of ultraviolet light is more efficient in enhancing the removal of quinonic compounds in photoassisted electrocatalytic oxidation. 2.5 Reaction kinetics According to the simplified model by Polcaro and Wu (Polcaro et al., 1999; Wu and Zhou, 2001), the overall oxidation processes can be roughly represented as follows: K1
K2
A−→B−→C
(14)
A represents Orange II molecules; B is the quinonic compounds and C is the organic acids. K1 (min−1 ) and K2 (min−1 ) are the apparent rate constants. The reaction steps are based on the following assumptions: there is no HO radical accumulation in the solution due to its strong oxidation ability, and HO radicals therefore reach a steady state concentration. All the reactions can be simplified to pseudo-first-order kinetics as follows: ri = Ki [Ri ]
(15)
where, [Ri ] is the concentration of reactant Ri such as A and B. The apparent rate constant Ki is associated with the true rate constant ki by: Ae θOH Ki = ki Vsol
−
d[A] = K1 [A] dt
At time t = 0, [A] = a; [B] = 0
(19)
The generalized model is given by: [A] = ae−K1 t [B] =
(20)
K1 a (e−K1 t − e−K2 t ) K2 − K1
(21)
When [B] reaches its maximal value, d[B] dt = 0, tm = lnK2 −lnK1 . K2 −K1 The tm of electrocatalytic oxidation and photo-assisted electrocatalytic oxidation can be roughly accepted as 40 and 20 min from the UV-Vis analysis, respectively. The removal index of the quinonic compounds is defined as: λ = K2 /K1
(22)
The calculated results are presented in Table 1. It can be seen that the degradation rate of the quinonic compounds (K2 ) during electrocatalytic oxidation is 0.0196 min−1 while that during photo-assisted electrocatalytic oxidation is as much as 0.0652 min−1 . The degradation rate of the quinonic compounds increases by as much as a factor of two when ultraviolet light is imported into the electrocatalytic oxidation process. The removal index of the quinonic compounds λ during photo-assisted electrocatalytic oxidation is almost three times that of the electrocatalytic oxidation process alone. Hence, ultraviolet light is capable of assisting the electrocatalytic oxidation process to decolorize and degrade dye solution, especially for the removal of the quinonic compounds.
Table 1 Apparent rate constants (K1 ) and the calculated results (K2 ) and the removal index of quinonic compounds (λ) during electrocatalytic oxidation and photo-assisted electrocatalytic oxidation of Orange II.
(16)
where,Vsol (m3 ) is the volume of the electrolyte, Ae (m2 ) is the effective area of the electrode and θOH represents the
(17)
K1 K2 λ
Electrocatalytic oxidation
Photo-assisted electrocatalytic oxidation
0.0314 (R2 = 0.93) 0.0196 0.624
0.0374 (R2 = 0.97) 0.0652 1.75
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3 Conclusions The production of quinonic intermediates is an indispensible step during aromatics treatment by advanced oxidation processes. A significant synergetic effect occurred in electrocatalytic oxidation assisted by ultraviolet irradiation. Particularly, the absorbance band of quinonic compounds at 255 nm was formed during electrocatalytic oxidation. It was proved that HO radicals were generated more efficiently in electrocatalytic oxidation assisted by ultraviolet irradiation, which is the so-called photo-assisted electrocatalytic oxidation. As such, the degradation rate of quinonic compounds in electrocatalytic oxidation was enhanced by as much as a factor of two by the introduction of ultraviolet irradiation. The initial decolorization in electrocatalytic oxidation and photo-assisted electrocatalytic oxidation could be mainly attributed to the direct electron transfer from Orange II molecules to the anode. The dye can be oxidized by HO radicals as well as by direct electron transfer during electrocatalytic oxidation. Acknowledgments This work was supported by the starting fund for talents of North China University of Water Resources and Electric Power, and partially by the National Science Foundation of China (No. 51378205). references Andreozzi, R., Caprio, V., Insola, A., Marotta, R., 1999. Advanced oxidation processes (AOP) for water purification and recovery. Catal. Today 53(1), 51–59. Babbs, C. F., Gale, M. J., 1987. Colorimetric assay for methanesulfinic acid in biological samples. Anal. Biochem. 163(1), 67–76. Bouya, H., Errami, M., Salghi, R., Bazzi, L., Zarrouk, A., Al-Deyab, S. S. et al., 2012. Electrochemical degradation of cypermethrin pesticide on a SnO2 Anode. Int. J. Electrochem. Sci. 7(4), 3453–3465. Chen, G. H., 2004. Electrochemical technologies in wastewater treatment. Sep. Purif. Technol. 38(1), 11–41. Chen, Y. X., Yang, S. Y., Wang, K., Lou, L. P., 2005. Role of primary active species and TiO2 surface characteristic in UV-illuminated photodegradation of Acid Orange 7. J. Photochem. Photobiol. A: Chem. 172(1), 47–54. Comninellis, C., 1994. Electrocatalysis in the electrochemical conversion/combustion of organic pollutants for waste water treatment. Electrochim. Acta 39(11-12), 1857–1862. Comninellis, C., Chen, G. H., 2010. Electrochemistry for the Environment. Springer Science Business Media, New York. Flesazr, B., Ploszynska, J., 1985. An attempt to define benzene and phenol electrochemical oxidation mechanism. Electrochim. Acta 30(1), 31–42. Hoffman, M. R., Martin, S. T., Choi, W. Y., Bahnemann, D. W., 1995. Environmental applications of semiconductor photocatalysis. Chemi. Rev. 95(1), 69–96. Huang, L., Yu, D. Q., 1988. Application of UV-Vis Spectra in Organic
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