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Enhancing the electrochemical oxidation of acid-yellow 36 azo dye using boron-doped diamond electrodes by addition of ferrous ion
Journal of Hazardous Materials 167 (2009) 1226–1230
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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Short communication
Enhancing the electrochemical oxidation of acid-yellow 36 azo dye using boron-doped diamond electrodes by addition of ferrous ion M. Villanueva-Rodríguez a , A. Hernández-Ramírez a , J.M. Peralta-Hernández a,∗ , Erick R. Bandala b , Marco A. Quiroz-Alfaro b a b
Universidad Autónoma de Nuevo León, Facultad de Ciencias Químicas, Av. Universidad s/n, Cd. Universitaria, San Nicolás de los Garza, NL. 66400, Mexico Universidad de Las Américas - Puebla, Escuela de Ingeniería y Ciencias, Sta. Catarina Martir - Cholula, Puebla 72820, Mexico
a r t i c l e
i n f o
Article history: Received 27 August 2008 Received in revised form 7 November 2008 Accepted 31 December 2008 Available online 14 January 2009 Keywords: Electrochemical oxidation BDD electrode Ferrous sulphate Ferrate ion
a b s t r a c t This work shows preliminary results on the electrochemical oxidation process (EOP) using boron-doped diamond (BDD) electrode for acidic yellow 36 oxidation, a common azo dye used in textile industry. The study is centred in the synergetic effect of ferrous ions and hydroxyl free radicals for improving discoloration of azo dye. The assays were carried out in a typical glass cell under potentiostatic conditions. On experimental conditions, the EOP was able to partially remove the dye from the reaction mixture. The reaction rate increased significantly by addition of Fe2+ (1 mM as ferrous sulphate) to the system and by (assumed) generation of ferrate ion [Fe(VI)] over BDD electrode. Ferrate is considered as a highly oxidizing reagent capable of removing the colorant from the reaction mixture, in synergistic action with the hydroxyl radicals produced on the BDD surface. Further increases in the Fe2+ concentration lead to depletion of the reaction rate probably due to the hydroxyl radical scavenging effect of Fe2+ excess in the system. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Textile industry is rated as one of the most polluting sector among the different human activities due to their high discharge volume and effluent composition [1]. Operations involved in this industrial activity are very water demanding and the effluents generated contain high concentrations of dyes, surfactants, suspended solids and organic matter [2]. The release of textile wastewater to the natural water courses is the main dispersion path of a wide variety of dyestuff in the environment. Dyes contained in textile effluents retain their color and structural integrity under diverse weather conditions due to their design to persist under oxidizing and reducing conditions, washing and light exposure. They also show a high resistance to microbial degradation on wastewater treatment systems. These characteristics show synthetic dyes as very refractory chemicals able to generate toxicity to aquatic organisms and, in some cases, humans [2]. Among the synthetic dyes and pigments, the azo group is one of the most important. It has been estimated that azo-dyes constitute about 70% of the world dye production and 50,000 metric tons of these chemicals are released to the environment every year [3].
∗ Corresponding author. Tel.: +52 8183294000x6288. E-mail address: [email protected] (J.M. Peralta-Hernández). URL: http://www.uanl.mx (J.M. Peralta-Hernández). 0304-3894/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2008.12.137
A wide variety of physical, chemical and biological water treatment technologies have been tried in the past for the removal of such refractory chemicals from water [4,5]. Among them, emerging methodologies, such as advanced oxidation processes (AOPs) have been identified as highly efficient processes for the removal of dyes in water. For instance, ozonation, H2 O2 /O3 , H2 O2 /UV, Fenton and Fenton-like reactions, TiO2 /UV and electrochemical oxidation, have been tested for the treatment of textile industry effluents [1,2,6–9]. Among AOPs, electrochemical oxidation using BDD electrodes have been identified as a very attractive technology for electrolytic and electroanalytic applications because of its wide electrochemical potential window, low capacitance and electrochemical stability [10]. The application of diamond films in anodic oxidation has been reported for the oxidation of phenols and chlorophenols, cyanide, surfactants, ammonia, alcohols and several other pollutants [10–13] and for deactivation of pathogenic microorganisms in water by the electro-generation of chlorine-based disinfection species and reactive oxygen species (ROS) [14–17]. The great potential of BDD electrodes for the removal of water pollutants has been attributed to the generation of hydroxyl radicals (• OH), widely reported as the most oxidizing specie after fluorine (E = 2.8 V). Nevertheless, it is well known that other oxidizing species, such as ROS, peroxodisulfate or peroxodicarbonate can also be formed as the result of the oxidation of sulfate, carbonate and phosphate in the BDD surface, causing the addition of species containing those cations an enhancement of the electro-
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electrode; platinum wire (Pt) was the counter electrode, and Ag/AgCl was used as the reference electrode. The perchloric acid medium 0.1 M, was chosen in accordance with the literature revised [27], since no ferric or ferrous complexes have been determined in the presence of ClO4 − ion. 2.2. Dye solution preparation Fig. 1. Chemical structure of acid-yellow 36 azo dye.
oxidation rate [18–20]. Some authors [21,22] have reported current efficiency in BDD electrodes is limited by mass transport effects, when large molecules (such as dyes) or chemical species at low concentrations are being processed. In such cases, addition of specific electrolytes has been suggested as an alternative to enhance the electro-oxidation reaction rate [22,23]. For example, the presence of chloride ions in the reaction mixture has been reported to increase the oxidation initiated by hydroxyl radicals during different dye treatment, avoiding diffusion-limited conditions on these systems [22,24–26]. So far, this effect has not being observed with other ions tested in these works, such as ferrous sulfate [23] and no further studies have been reported dealing with testing the capability of other ions to enhance electro-oxidation using BDD electrodes. In this study, the electro-oxidation of synthetic textile azo dye wastewater containing acid-yellow 36 as well as the effect of addition of different ferrous sulfate concentrations to enhance the electrochemical treatment when using BDD electrode for dye removal were investigated.
The acidic-yellow 36 (Fig. 1, C.I. 13,080, 70% dye content, chemical formula = C18 H14 N3 NaO3 S, molecular weight = 375.384, and max = 527 nm), was used in this study. An accurately weighed quantity of the dye was dissolved in distilled water to prepare the stock solution (100 mg/L). Experimental solutions of desired concentration (40 mg/L) were obtained by successive dilution. The synthetic textile dye wastewater was prepared in accordance with the literature reports [28], where authors reported levels of dye concentrations in textile wastewater in range 10–200 mg/L. 2.3. Electrochemical cell The electrochemical oxidation processes (EOP) of acidic yellow 36 was carried out in a single undivided cell (V = 40 mL) under potentiostatic mode (2.5 V vs. Ag/AgCl) using a Power Supply source model: Minipa/MPL-1303. BDD was used as the anode (geometric area 0.7854 cm2 ) and platinum wire as the cathode, with an electrode gap of 2 cm. Different iron concentrations in form of ferrous sulfate solution (FeSO4 ·7H2 O) were tested in order to evaluate the increase on dye oxidation rate.
2. Experimental
2.4. Analysis procedure
All the chemicals used in this work, perchloric acid (HClO4 ), and ferrous sulfate (FeSO4 ·7H2 O) were purchased to J.T. Baker as A.C.S reagent grade and used as received without further purification. The azo dye, acid-yellow 36 (AY36), industrial grade, was supplied by Orion Co. (Cuernavaca, Mor). BDD was provided by AdamantTechnologies (Switzerland).
At given time intervals, aliquots were sampled and analyzed by recording variations of the dye absorption band at 527 nm for acidic-yellow 36, in the UV–vis spectra of the dye, using a Lambda 12 spectrophotometer (PerkinElmer Co.).
2.1. Voltammetric studies
3.1. Voltammetric study of BDD electrode
Cyclic voltammetric (CV) studies were performed in a conventional three-electrode cell in 40 mL of acidic solution 0.1 M of perchloric acid (HClO4 ) both without iron ions and in the presence of ferrous sulfate using a computer-controlled potentiostat/galvanostat (Gamry/PCI 4750). BDD was used for the working
Fig. 2 shows the typical cyclic voltammetric curve for BDD anode in 0.1 mM HClO4 at 100 mV/s scan rate. In this figure it is possible to observe the higher overpotential of diamond electrode for oxygen evolution. This leads to wide potential window (approximately 3 V) which can be used for electrochemical reactions in aqueous electrolytes [12]. In this context, it is very well known that when the oxygen evolution takes place on a BDD surface, the hydroxyl radical (• OH) generation from water discharge is possible through the reaction proposed by Comninellis [29]:
3. Results and discussion
BDD + H2 O → BDD(• OH) + H+ + e−
(1)
Based on this mechanism, the ion generation can be observed in Fig. 2 at approximately 2.5 V vs. Ag/AgCl region, where free radicals • OH formation is the predominant reaction. For wastewater treatment applications, these hydroxyl radicals are then consumed by organic compound oxidation (Eq. (2)): BDD(• OH) + R → BDD(mCO2 ) + nH2 O
(2)
3.2. Effect of the ferrous sulfate on discoloration of AY36
Fig. 2. Cyclic voltammogram at boron-doped diamond in 0.1 M HClO4 , scan rate v = 100 mV/s.
This study reports the electrochemical oxidation of 40 mg/L of AY36, over the BDD electrode in HClO4 solution (EOP). Fig. 3 shows that, under the tested conditions, the EOP was only able to achieve about 41% of AY36 degradation after 180 min of reaction time. As it is widely proposed, the oxidation of AY36 in the process is mainly
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Fig. 3. Effect of FeSO4 concentration over decoloration of 40 mg/L of acid-yellow 36 under anodic potential of 2.5 V. (a) EOP, (b) 12 mM FeSO4 , (c) 8 mM FeSO4 , (d) 6 mM FeSO4 , (e) 1 mM FeSO4 .
promoted by hydroxyl radicals (• OH) formed over the BDD electrode at 2.5 V vs. Ag/AgCl, in accordance with Eqs. (1) and (2) [10,11]. However, it is worthy to note that the EOP does not efficiently achieve total mineralization of azo dye, and therefore the reaction (2) is incomplete. This poor efficiency of the surface is attributed to the formation of secondary products of low-molecular weight, such as carboxylic acids, coming from the AY36 oxidation. In our case, this behavior is in accordance with results reported by some authors, who reported that in the potential region before oxygen evolution over BDD electrode, there is no electrocatalytic activity for the oxidation of carboxylic acids, and the oxidation of aromatic compounds results in the deactivation of BDD surface [11,30,31]. In order to enhance the efficiency of the EOP, different ferrous sulfate concentrations were added to the system under the same experimental conditions. It was found that the addition of 1 mM FeSO4 to the reaction mixture achieved about 94% of dye degradation in the same reaction time (180 min) used for EOP. Further increases in FeSO4 concentration did not show increases in the final dye degradation as it is also shown in Fig. 3; less than 58% dye degradation was reached using 12 mM of FeSO4 , while 69% degradation was accomplished with 8 and 6 mM of FeSO4 , respectively. Based on these results, it is interesting to note that keeping iron salt at high concentration levels slows down the degradation of AY36. Since the formation of secondary products is possible for the AY36 oxidation, the initial rate at 15 min (IR15 , mmol/min) was determined as a comparative parameter for all assays. The main findings follow: the lowest initial rate value was found to be for EOP (IR15 = 1.0 × 10−3 mmol/min), followed by the experiment carried out using the highest FeSO4 concentration ([FeSO4 ] = 12 mM) with a IR15 value of 2.0 × 10−3 mmol/min. As the FeSO4 concentration decreased, IR15 values increased until the highest value (5.0 × 10−3 mmol/min) found for the lowest FeSO4 concentration tested (1 mM), five times higher than EOP. The behavior showed by the addition of FeSO4 in the EOP could be rationalized by the formation of ferrate ion (FeO4 −2 ) which is a powerful oxidant in aqueous media. Under acidic conditions, the red/ox potential of Fe(VI) ion is higher than many other oxidants used for wastewater treatment. Spontaneous reduction of Fe(VI) in aqueous solutions is schemed in Eq. (3) [32]: 2FeO4 2− +5H2 O → 2Fe3+ + (3/2)O2 + 10HO−
(3)
Apart from molecular oxygen, as observed from reaction (3), generation of Fe(III), considered as innocuous in the environment,
Fig. 4. Cyclic voltammograms at boron-doped diamond surface in (a) 0.1 M HClO4 for ferrate ion generation tested the following conditions, (a) without FeSO4 , (b) 12 mM FeSO4 , (c) 8 mM FeSO4 , (d) 6 mM FeSO4 , (e) 1 mM FeSO4 . Scan rate 100 mV/s.
is also reported as Fe(VI) by-product [33,34]. The red/ox potential for (3) is low at pH 9–10 (0.7 V) and it increases to 2.2 V at lower pH values (2–5). Ferrate(VI) oxidant is considered environmentally friendly and capable of treating a wide range of contaminants [35], including unconventional and emerging microorganisms without side reactions or by-products generation [36]; it has been evaluated for water and wastewater treatment [37], oxidation of cyanide [38], phenols [39], hydrogen sulfide [40], thiourea [41,42], organic matter [43], surfactants [44], with satisfactory results. In a previous work, Lee et al. [45] reported ferrate formation in acidic aqueous solution over BDD electrode using 6 mM FeSO4 and 0.1 M HClO4 , similar conditions to those tested in this work. Presence of ferrate ion in the reaction mixture could act in similar manner as reported for chloride ion containing systems [22], where additional substrate oxidation is carried out by electrogenerated HClO, completing the oxidation initiated by hydroxyl radicals. In this case, our hypothesis is electrogenerated ferrate could synergistically oxidize the AY36 azo complementing the reaction initiated by poorly generated hydroxyl radicals under mild current conditions in the BDD.
3.3. Electrochemical species analysis To support the ferrate ion formation theory during the organic compound degradation, several trials were carried out using cyclic voltammetric measurements over the BDD electrode (Fig. 4), sweeping potential at 100 mV/s for every ferrous sulfate concentrations tested in Section 3.2. Curve (a) is trial without ferrous sulfate, showing no redox signal. Curve (b) is trial with 12 mM Ferrous sulfate, showing no redox signal like the previous trial, and a moderate oxygen evolution starting at 2 V. However, when adding 8 mM of ferrous sulfate we were able to observe (curve c) three distinctive peaks, two anodic (AI and AII) and one cathodic (CI), detected by cyclic voltammetric measurements. The AI and CI signals are associated with the Fe3+ /Fe2+ redox pair, whereas the AII signal is attributed to ferrate ion formation (Eq. (4)), occurring at approximately 2.27 V potential and current response of 2.5 A. This result is in accordance to the work of Lee et al. for similar studies [46]. As ferrous sulfate concentration decreased in the solution from 6 mM to 1 mM (curves d and e, respectively) the greatest increase on the voltamperometric response of the CI signal was observed, since at the lowest ferrous sulfate concentration (curve e) we obtained a higher current increase (almost double than curve c), close to
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Fig. 5. Proposed mechanism for the electrochemical oxidation of organic compound with simultaneous ferrate ion formation in the system.
8 × 10−3 A, and a slight shift of the potential towards 2.5 V. Fe3+ + 4H2 O → FeO4 2− + 8H+ + 3e−
Acknowledgements (4)
It is important to mention that no cathodic signal was appreciated for the reduction of [Fe(VI)] in our study. The test was performed both in presence and absence of nitrogen atmosphere in order to probe that oxygen is not involved during the course of the reaction and this parameter did not affected the signals. It is probable that, under the conditions tested for this assays, the • OH concentration available from water discharge over BDD surface is limited by the increase on the Fe2+ ions concentration. This fact is not surprising, since it is well known that when Fe2+ is present in excess, this specie act as strong hydroxyl radical scavenger, in accordance with Eq. (5) [1,3,46,47], leaving the dye oxidation to occur only by the ferrate ion electrogenerated. Fe2+ + • HO → Fe3+ + OH−
(5)
Thus, Fig. 5 shows the proposed mechanism for the electrochemical oxidation of organic compounds with simultaneous ferrate ion formation in the system: (a) water discharge to hydroxyl radicals, (b) oxidation of organic compound by means of • OH, (c) formation of ferrate ion, (d) oxidation of organic compounds by ferrate ion, (e) electrochemical regeneration of Fe3+ to Fe2+ and (f) scavenger action of Fe2+ excess over hydroxyl radicals in accordance with Eq. (5). Based on results presented in this study, the condition at which the synergetic effect can occur (steps b and d) is at low concentrations of ferrous ion, so steps (e) and (f) can be avoided. 4. Conclusions Degradation of acid-yellow 36 (AY36) was achieved under EOP conditions using BDD electrodes. It was observed that, under determined experimental conditions (constant potential), efficiency of the EOP decreased considerably due to a suspected loss in the electrode efficiency to produce hydroxyl radicals. The generation of ferrate ion in the reaction mixture by the BDD electrode was demonstrated by voltammetric experiments which lead us to propose the synergistic performance of ferrate ion and hydroxyl radicals in the oxidation process. The addition of low quantities of Fe2+ , as ferrous sulfate, leads to an increase of the color removal reaction. Higher values of ferrous sulfate concentration did not show the expected increase of reaction rate, presumably due to the scavenging • OH effect caused by the excess of Fe2+ in the solution. Currently, more experimental assays are being carried out to support all these findings.
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Short communication
Enhancing the electrochemical oxidation of acid-yellow 36 azo dye using boron-doped diamond electrodes by addition of ferrous ion M. Villanueva-Rodríguez a , A. Hernández-Ramírez a , J.M. Peralta-Hernández a,∗ , Erick R. Bandala b , Marco A. Quiroz-Alfaro b a b
Universidad Autónoma de Nuevo León, Facultad de Ciencias Químicas, Av. Universidad s/n, Cd. Universitaria, San Nicolás de los Garza, NL. 66400, Mexico Universidad de Las Américas - Puebla, Escuela de Ingeniería y Ciencias, Sta. Catarina Martir - Cholula, Puebla 72820, Mexico
a r t i c l e
i n f o
Article history: Received 27 August 2008 Received in revised form 7 November 2008 Accepted 31 December 2008 Available online 14 January 2009 Keywords: Electrochemical oxidation BDD electrode Ferrous sulphate Ferrate ion
a b s t r a c t This work shows preliminary results on the electrochemical oxidation process (EOP) using boron-doped diamond (BDD) electrode for acidic yellow 36 oxidation, a common azo dye used in textile industry. The study is centred in the synergetic effect of ferrous ions and hydroxyl free radicals for improving discoloration of azo dye. The assays were carried out in a typical glass cell under potentiostatic conditions. On experimental conditions, the EOP was able to partially remove the dye from the reaction mixture. The reaction rate increased significantly by addition of Fe2+ (1 mM as ferrous sulphate) to the system and by (assumed) generation of ferrate ion [Fe(VI)] over BDD electrode. Ferrate is considered as a highly oxidizing reagent capable of removing the colorant from the reaction mixture, in synergistic action with the hydroxyl radicals produced on the BDD surface. Further increases in the Fe2+ concentration lead to depletion of the reaction rate probably due to the hydroxyl radical scavenging effect of Fe2+ excess in the system. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Textile industry is rated as one of the most polluting sector among the different human activities due to their high discharge volume and effluent composition [1]. Operations involved in this industrial activity are very water demanding and the effluents generated contain high concentrations of dyes, surfactants, suspended solids and organic matter [2]. The release of textile wastewater to the natural water courses is the main dispersion path of a wide variety of dyestuff in the environment. Dyes contained in textile effluents retain their color and structural integrity under diverse weather conditions due to their design to persist under oxidizing and reducing conditions, washing and light exposure. They also show a high resistance to microbial degradation on wastewater treatment systems. These characteristics show synthetic dyes as very refractory chemicals able to generate toxicity to aquatic organisms and, in some cases, humans [2]. Among the synthetic dyes and pigments, the azo group is one of the most important. It has been estimated that azo-dyes constitute about 70% of the world dye production and 50,000 metric tons of these chemicals are released to the environment every year [3].
∗ Corresponding author. Tel.: +52 8183294000x6288. E-mail address: [email protected] (J.M. Peralta-Hernández). URL: http://www.uanl.mx (J.M. Peralta-Hernández). 0304-3894/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2008.12.137
A wide variety of physical, chemical and biological water treatment technologies have been tried in the past for the removal of such refractory chemicals from water [4,5]. Among them, emerging methodologies, such as advanced oxidation processes (AOPs) have been identified as highly efficient processes for the removal of dyes in water. For instance, ozonation, H2 O2 /O3 , H2 O2 /UV, Fenton and Fenton-like reactions, TiO2 /UV and electrochemical oxidation, have been tested for the treatment of textile industry effluents [1,2,6–9]. Among AOPs, electrochemical oxidation using BDD electrodes have been identified as a very attractive technology for electrolytic and electroanalytic applications because of its wide electrochemical potential window, low capacitance and electrochemical stability [10]. The application of diamond films in anodic oxidation has been reported for the oxidation of phenols and chlorophenols, cyanide, surfactants, ammonia, alcohols and several other pollutants [10–13] and for deactivation of pathogenic microorganisms in water by the electro-generation of chlorine-based disinfection species and reactive oxygen species (ROS) [14–17]. The great potential of BDD electrodes for the removal of water pollutants has been attributed to the generation of hydroxyl radicals (• OH), widely reported as the most oxidizing specie after fluorine (E = 2.8 V). Nevertheless, it is well known that other oxidizing species, such as ROS, peroxodisulfate or peroxodicarbonate can also be formed as the result of the oxidation of sulfate, carbonate and phosphate in the BDD surface, causing the addition of species containing those cations an enhancement of the electro-
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electrode; platinum wire (Pt) was the counter electrode, and Ag/AgCl was used as the reference electrode. The perchloric acid medium 0.1 M, was chosen in accordance with the literature revised [27], since no ferric or ferrous complexes have been determined in the presence of ClO4 − ion. 2.2. Dye solution preparation Fig. 1. Chemical structure of acid-yellow 36 azo dye.
oxidation rate [18–20]. Some authors [21,22] have reported current efficiency in BDD electrodes is limited by mass transport effects, when large molecules (such as dyes) or chemical species at low concentrations are being processed. In such cases, addition of specific electrolytes has been suggested as an alternative to enhance the electro-oxidation reaction rate [22,23]. For example, the presence of chloride ions in the reaction mixture has been reported to increase the oxidation initiated by hydroxyl radicals during different dye treatment, avoiding diffusion-limited conditions on these systems [22,24–26]. So far, this effect has not being observed with other ions tested in these works, such as ferrous sulfate [23] and no further studies have been reported dealing with testing the capability of other ions to enhance electro-oxidation using BDD electrodes. In this study, the electro-oxidation of synthetic textile azo dye wastewater containing acid-yellow 36 as well as the effect of addition of different ferrous sulfate concentrations to enhance the electrochemical treatment when using BDD electrode for dye removal were investigated.
The acidic-yellow 36 (Fig. 1, C.I. 13,080, 70% dye content, chemical formula = C18 H14 N3 NaO3 S, molecular weight = 375.384, and max = 527 nm), was used in this study. An accurately weighed quantity of the dye was dissolved in distilled water to prepare the stock solution (100 mg/L). Experimental solutions of desired concentration (40 mg/L) were obtained by successive dilution. The synthetic textile dye wastewater was prepared in accordance with the literature reports [28], where authors reported levels of dye concentrations in textile wastewater in range 10–200 mg/L. 2.3. Electrochemical cell The electrochemical oxidation processes (EOP) of acidic yellow 36 was carried out in a single undivided cell (V = 40 mL) under potentiostatic mode (2.5 V vs. Ag/AgCl) using a Power Supply source model: Minipa/MPL-1303. BDD was used as the anode (geometric area 0.7854 cm2 ) and platinum wire as the cathode, with an electrode gap of 2 cm. Different iron concentrations in form of ferrous sulfate solution (FeSO4 ·7H2 O) were tested in order to evaluate the increase on dye oxidation rate.
2. Experimental
2.4. Analysis procedure
All the chemicals used in this work, perchloric acid (HClO4 ), and ferrous sulfate (FeSO4 ·7H2 O) were purchased to J.T. Baker as A.C.S reagent grade and used as received without further purification. The azo dye, acid-yellow 36 (AY36), industrial grade, was supplied by Orion Co. (Cuernavaca, Mor). BDD was provided by AdamantTechnologies (Switzerland).
At given time intervals, aliquots were sampled and analyzed by recording variations of the dye absorption band at 527 nm for acidic-yellow 36, in the UV–vis spectra of the dye, using a Lambda 12 spectrophotometer (PerkinElmer Co.).
2.1. Voltammetric studies
3.1. Voltammetric study of BDD electrode
Cyclic voltammetric (CV) studies were performed in a conventional three-electrode cell in 40 mL of acidic solution 0.1 M of perchloric acid (HClO4 ) both without iron ions and in the presence of ferrous sulfate using a computer-controlled potentiostat/galvanostat (Gamry/PCI 4750). BDD was used for the working
Fig. 2 shows the typical cyclic voltammetric curve for BDD anode in 0.1 mM HClO4 at 100 mV/s scan rate. In this figure it is possible to observe the higher overpotential of diamond electrode for oxygen evolution. This leads to wide potential window (approximately 3 V) which can be used for electrochemical reactions in aqueous electrolytes [12]. In this context, it is very well known that when the oxygen evolution takes place on a BDD surface, the hydroxyl radical (• OH) generation from water discharge is possible through the reaction proposed by Comninellis [29]:
3. Results and discussion
BDD + H2 O → BDD(• OH) + H+ + e−
(1)
Based on this mechanism, the ion generation can be observed in Fig. 2 at approximately 2.5 V vs. Ag/AgCl region, where free radicals • OH formation is the predominant reaction. For wastewater treatment applications, these hydroxyl radicals are then consumed by organic compound oxidation (Eq. (2)): BDD(• OH) + R → BDD(mCO2 ) + nH2 O
(2)
3.2. Effect of the ferrous sulfate on discoloration of AY36
Fig. 2. Cyclic voltammogram at boron-doped diamond in 0.1 M HClO4 , scan rate v = 100 mV/s.
This study reports the electrochemical oxidation of 40 mg/L of AY36, over the BDD electrode in HClO4 solution (EOP). Fig. 3 shows that, under the tested conditions, the EOP was only able to achieve about 41% of AY36 degradation after 180 min of reaction time. As it is widely proposed, the oxidation of AY36 in the process is mainly
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Fig. 3. Effect of FeSO4 concentration over decoloration of 40 mg/L of acid-yellow 36 under anodic potential of 2.5 V. (a) EOP, (b) 12 mM FeSO4 , (c) 8 mM FeSO4 , (d) 6 mM FeSO4 , (e) 1 mM FeSO4 .
promoted by hydroxyl radicals (• OH) formed over the BDD electrode at 2.5 V vs. Ag/AgCl, in accordance with Eqs. (1) and (2) [10,11]. However, it is worthy to note that the EOP does not efficiently achieve total mineralization of azo dye, and therefore the reaction (2) is incomplete. This poor efficiency of the surface is attributed to the formation of secondary products of low-molecular weight, such as carboxylic acids, coming from the AY36 oxidation. In our case, this behavior is in accordance with results reported by some authors, who reported that in the potential region before oxygen evolution over BDD electrode, there is no electrocatalytic activity for the oxidation of carboxylic acids, and the oxidation of aromatic compounds results in the deactivation of BDD surface [11,30,31]. In order to enhance the efficiency of the EOP, different ferrous sulfate concentrations were added to the system under the same experimental conditions. It was found that the addition of 1 mM FeSO4 to the reaction mixture achieved about 94% of dye degradation in the same reaction time (180 min) used for EOP. Further increases in FeSO4 concentration did not show increases in the final dye degradation as it is also shown in Fig. 3; less than 58% dye degradation was reached using 12 mM of FeSO4 , while 69% degradation was accomplished with 8 and 6 mM of FeSO4 , respectively. Based on these results, it is interesting to note that keeping iron salt at high concentration levels slows down the degradation of AY36. Since the formation of secondary products is possible for the AY36 oxidation, the initial rate at 15 min (IR15 , mmol/min) was determined as a comparative parameter for all assays. The main findings follow: the lowest initial rate value was found to be for EOP (IR15 = 1.0 × 10−3 mmol/min), followed by the experiment carried out using the highest FeSO4 concentration ([FeSO4 ] = 12 mM) with a IR15 value of 2.0 × 10−3 mmol/min. As the FeSO4 concentration decreased, IR15 values increased until the highest value (5.0 × 10−3 mmol/min) found for the lowest FeSO4 concentration tested (1 mM), five times higher than EOP. The behavior showed by the addition of FeSO4 in the EOP could be rationalized by the formation of ferrate ion (FeO4 −2 ) which is a powerful oxidant in aqueous media. Under acidic conditions, the red/ox potential of Fe(VI) ion is higher than many other oxidants used for wastewater treatment. Spontaneous reduction of Fe(VI) in aqueous solutions is schemed in Eq. (3) [32]: 2FeO4 2− +5H2 O → 2Fe3+ + (3/2)O2 + 10HO−
(3)
Apart from molecular oxygen, as observed from reaction (3), generation of Fe(III), considered as innocuous in the environment,
Fig. 4. Cyclic voltammograms at boron-doped diamond surface in (a) 0.1 M HClO4 for ferrate ion generation tested the following conditions, (a) without FeSO4 , (b) 12 mM FeSO4 , (c) 8 mM FeSO4 , (d) 6 mM FeSO4 , (e) 1 mM FeSO4 . Scan rate 100 mV/s.
is also reported as Fe(VI) by-product [33,34]. The red/ox potential for (3) is low at pH 9–10 (0.7 V) and it increases to 2.2 V at lower pH values (2–5). Ferrate(VI) oxidant is considered environmentally friendly and capable of treating a wide range of contaminants [35], including unconventional and emerging microorganisms without side reactions or by-products generation [36]; it has been evaluated for water and wastewater treatment [37], oxidation of cyanide [38], phenols [39], hydrogen sulfide [40], thiourea [41,42], organic matter [43], surfactants [44], with satisfactory results. In a previous work, Lee et al. [45] reported ferrate formation in acidic aqueous solution over BDD electrode using 6 mM FeSO4 and 0.1 M HClO4 , similar conditions to those tested in this work. Presence of ferrate ion in the reaction mixture could act in similar manner as reported for chloride ion containing systems [22], where additional substrate oxidation is carried out by electrogenerated HClO, completing the oxidation initiated by hydroxyl radicals. In this case, our hypothesis is electrogenerated ferrate could synergistically oxidize the AY36 azo complementing the reaction initiated by poorly generated hydroxyl radicals under mild current conditions in the BDD.
3.3. Electrochemical species analysis To support the ferrate ion formation theory during the organic compound degradation, several trials were carried out using cyclic voltammetric measurements over the BDD electrode (Fig. 4), sweeping potential at 100 mV/s for every ferrous sulfate concentrations tested in Section 3.2. Curve (a) is trial without ferrous sulfate, showing no redox signal. Curve (b) is trial with 12 mM Ferrous sulfate, showing no redox signal like the previous trial, and a moderate oxygen evolution starting at 2 V. However, when adding 8 mM of ferrous sulfate we were able to observe (curve c) three distinctive peaks, two anodic (AI and AII) and one cathodic (CI), detected by cyclic voltammetric measurements. The AI and CI signals are associated with the Fe3+ /Fe2+ redox pair, whereas the AII signal is attributed to ferrate ion formation (Eq. (4)), occurring at approximately 2.27 V potential and current response of 2.5 A. This result is in accordance to the work of Lee et al. for similar studies [46]. As ferrous sulfate concentration decreased in the solution from 6 mM to 1 mM (curves d and e, respectively) the greatest increase on the voltamperometric response of the CI signal was observed, since at the lowest ferrous sulfate concentration (curve e) we obtained a higher current increase (almost double than curve c), close to
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Fig. 5. Proposed mechanism for the electrochemical oxidation of organic compound with simultaneous ferrate ion formation in the system.
8 × 10−3 A, and a slight shift of the potential towards 2.5 V. Fe3+ + 4H2 O → FeO4 2− + 8H+ + 3e−
Acknowledgements (4)
It is important to mention that no cathodic signal was appreciated for the reduction of [Fe(VI)] in our study. The test was performed both in presence and absence of nitrogen atmosphere in order to probe that oxygen is not involved during the course of the reaction and this parameter did not affected the signals. It is probable that, under the conditions tested for this assays, the • OH concentration available from water discharge over BDD surface is limited by the increase on the Fe2+ ions concentration. This fact is not surprising, since it is well known that when Fe2+ is present in excess, this specie act as strong hydroxyl radical scavenger, in accordance with Eq. (5) [1,3,46,47], leaving the dye oxidation to occur only by the ferrate ion electrogenerated. Fe2+ + • HO → Fe3+ + OH−
(5)
Thus, Fig. 5 shows the proposed mechanism for the electrochemical oxidation of organic compounds with simultaneous ferrate ion formation in the system: (a) water discharge to hydroxyl radicals, (b) oxidation of organic compound by means of • OH, (c) formation of ferrate ion, (d) oxidation of organic compounds by ferrate ion, (e) electrochemical regeneration of Fe3+ to Fe2+ and (f) scavenger action of Fe2+ excess over hydroxyl radicals in accordance with Eq. (5). Based on results presented in this study, the condition at which the synergetic effect can occur (steps b and d) is at low concentrations of ferrous ion, so steps (e) and (f) can be avoided. 4. Conclusions Degradation of acid-yellow 36 (AY36) was achieved under EOP conditions using BDD electrodes. It was observed that, under determined experimental conditions (constant potential), efficiency of the EOP decreased considerably due to a suspected loss in the electrode efficiency to produce hydroxyl radicals. The generation of ferrate ion in the reaction mixture by the BDD electrode was demonstrated by voltammetric experiments which lead us to propose the synergistic performance of ferrate ion and hydroxyl radicals in the oxidation process. The addition of low quantities of Fe2+ , as ferrous sulfate, leads to an increase of the color removal reaction. Higher values of ferrous sulfate concentration did not show the expected increase of reaction rate, presumably due to the scavenging • OH effect caused by the excess of Fe2+ in the solution. Currently, more experimental assays are being carried out to support all these findings.
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