Print version ISSN 0327-0793
Lat. Am. appl. res. vol.41 no.2 Bahía Blanca Apr. 2011
Application of electrochemical oxidation as alternative for removing methyl green dye from aqueous solutions
D.A. Carvalho, J.H. Bezerra Rocha, N.S. Fernandes, D.R. Da Silva and C.A. Martínez-Huitle
Universidade Federal do Rio Grande do Norte, Centro de Ciências Exatas e da Terra.
Departamento de Química, Lagoa Nova CEP 59078-970 - Natal, RN, Brasil
Tel/Fax.: +55 (84) 3211-9224. email@example.com
Universidade Federal do Rio Grande do Norte, Núcleo de Estudos em Petróleo e Gás Natural - NEPGN, Lagoa Nova - CEP 59.072-370 Brazil.. firstname.lastname@example.org
Abstract - In this work, the treatment of synthetic wastewaters containing Methyl Green (MG) by anodic oxidation using Ti/PbO2 anodes was investigated. Galvanostatic electrolyses of MG synthetic wastewaters have led to the complete decolourization removal at different current density values (10, 20 and 40 mA cm-2), temperatures (25, 40 and 60°C) and agitation rate (100, 200 and 300 rpm). According to the experimental results obtained in this work, the electrochemical oxidation process is suitable for removing TOC and decolourising wastewaters containing MG dye, due to the electrocatalytic properties of Ti/PbO2 anode. In general, the energy requirements for removing color during galvanostatic electrolyses of MG synthetic solutions depends mainly on the applied current density; it passes from 0.29 kWh at 10 mA cm-2 to 1.64 kWh at 40 mA cm-2 per volume of treated effluent removed (m-3). The results are described and discussed in the light of the existing literature.
Keywords - Anodic Oxidation; Water Treatment; Decolourization; Lead Oxide Electrode.
Nowadays, electrochemical treatment is one of the methods used for removal of organic and inorganic impurities from fresh, drinking and waste waters. Consequently, many researches are attempting to use electrochemical methods as an effective method for detoxification of aquatic systems containing bio-refractory pollutants (Panizza and Cerisola, 2009; Martinez-Huitle and Panizza, 2010; Martinez-Huitle and Ferro, 2006; Martinez-Huitle et al., 2005; Comninellis and Pulgarin, 1993; Comninellis, 1994; Rajeshwar et al., 1994; Rodgers et al., 1999; Galla et al., 2000). Indeed, several works have shown that the electrochemical oxidation represents a feasible alternative in the wastewater treatment (Panizza and Cerisola, 2009; Martinez-Huitle and Panizza, 2010; Martinez-Huitle and Ferro, 2006; Martinez-Huitle et al., 2005; Galla et al., 2000; Nelson, 2002), being possible the application of this technology as a direct or indirect oxidation process (Panizza and Cerisola, 2009; Martinez-Huitle and Panizza, 2010; Martinez-Huitle and Ferro, 2006). Whereas the electron transfer taking place between electrodes and decomposable species is the key factor in direct electrochemical oxidation, the use of mediators electrochemically formed as oxidizing species to destroy the organic compounds is the distinctive aspect in indirect electrochemical oxidation. For these reasons, both direct and indirect electrochemical oxidation can be considered as good alternative for remediation of aquatic systems with pollution problems (Panizza and Cerisola, 2009; Martinez-Huitle and Panizza, 2010; Martinez-Huitle and Ferro, 2006).
In the first case, the model of the oxidation process on the substrate has been investigated on different anodic materials, generally metal oxides such as IrO2, PbO2, SnO2 and SnO2-Sb2O5 (Panizza and Cerisola, 2009; Martinez-Huitle and Ferro, 2006; Martinez-Huitle et al., 2004), but also on boron doped diamond (BDD), one of the more recent electrode material for electrooxidation processes (Panizza and Cerisola, 2009; Martinez-Huitle and Ferro, 2006; Martinez-Huitle et al., 2004, Martinez-Huitle et al., 2008; Quiroz et al., 2006). Lead dioxide, with its high oxygen overpotential, is among the most commonly used anodes for the destruction of organics because the rate of organic oxidation has proved to be higher than on other traditional anodes. For example, as recently reviewed in detail (Panizza and Cerisola, 2009; Martinez-Huitle and Panizza, 2010; Martinez-Huitle and Ferro, 2006; Bonfatti et al., 1999) comparing the oxidation of glucose on different electrode materials such as Ti/PbO2, Ti/Pt, and Ti/Pt-SnO2, showed that the incineration of glucose and its oxidation intermediates (i.e. gluconic and glucaric acid) took place at reasonable rate only at Ti/PbO2 electrodes. The electrochemical oxidation of phenol was thoroughly investigated under different experimental conditions by Tahar and Savall (1999). Phenol and its intermediates (benzoquinone, maleic and fumaric acids) were completely eliminated at a pure Ta/PbO2 anode through the intermediation of hydroxyl radicals adsorbed on the active site of the electrode.
On the other hand, synthetic dyes are extensively used in many fields of up to-date technology, such as in various branches of the textile industry, leather tanning industry, paper production, food technology, agricultural research, light-harvesting arrays, photo-electrochemical cells, and in hair colourings. The chemical classes of dyes employed more frequently on industrial scale are the azo, anthraquinone, sulfur, indigoid, triphenylmethyl (trityl), and phthalocyanine derivatives. Due to large-scale production and extensive application, synthetic dyes can cause considerable environmental pollution and are serious health-risk factors (Forgacs et al., 2004). Traditional wastewater treatment technologies have proven to be markedly ineffective for handling wastewater of synthetic textile dyes because of the chemical stability of these pollutants. A wide range of methods has been developed for the removal of synthetic dyes from waters and wastewaters to decrease their impact on the environment. As an innovative alternative, the electrochemical processes for treating wastewater containing dyes have been proposed (Martínez-Huitle and Brillas, 2009).
In recent years, several scientific groups have investigated the application of electrochemical oxidation alternatives for removing dyes from water using different anodes, as recently reviewed in detail by Martínez-Huitle and Brillas (2009); using Ti-supported PbO2 anodes. Therefore, in the present study is proposed the use of electrochemical technology as alternative to remove synthetic dyes from water in order to eliminate their strong colour and their ecotoxicological consequences on aquatic environment. For this purpose the electrocatalytic activity of Ti/PbO2 anodes was investigated as well as the effect of the applied current density, temperature and agitation rate on the colour and TOC removal efficiency of MG of a synthetic solution containing MG (a basic dye widely used as a stain in bacteriology and as an oxidation-reduction indicator, as an antidote to cyanide and as an antiseptic in veterinary work (Fig. 1)) in order to identify optimal experimental conditions which gives high current efficiency and needs low energy requirements.
Fig. 1. Chemical structure of MG: C16H17N4O2SCl.
Ultrapure water was obtained by Simplicity water purification system. Chemicals were of the highest quality commercially available, and were used without further purification. MG and H2SO4 were purchased from Fluka. The model organic compound solution was prepared dissolving 300 mg L-1 of MG in distilled wastewater containing 0.5 M H2SO4. The linear polarizations were recorded in H2SO4 0.5 M because this electrolyte is commonly used in literature to study the oxygen evolution reaction.
B. Apparatus and procedures
Potentiodynamic measurements (polarization curves and cyclic voltammetry) were carried out at 25°C in a conventional three-electrode cell using model Autolab 302N (Methrom, Switzerland) connected to a PC. Ti/PbO2 has been used as working electrode, an Ag/AgCl electrode as a reference and Pt wire as a counter electrode. The exposed apparent area of the working electrodes was 1.5 cm2.
Bulk oxidations were performed in a one-compartment electrochemical cell, the reaction compartment having a capacity of 3 L. Ti/PbO2 was used as the anodes (50 cm2 geometrical area), and a Ti mesh was used as the cathode electrode. The distance between both electrodes was 10 cm. Ti-supported PbO2 anode was supplied by De Nora Electrodes Department (Milan, Italy). The anolyte consisted of 300 ppm MG in 0.5 M H2SO4 solution. A concentration of MG, no higher than 300 ppm, was chosen in order to cover the concentration level a little more than those permitted by Brazilian government regulations. The temperature of the electrolyte was fixed at 25°C In these conditions; the estimated mass transfer coefficient in the cell, determined using the ferri/ferrocyanide couple, was 2×10-5 m s-1. Experiments were performed at 25 °C and the stirring rate was kept almost constant by using a magnetic stirrer for studying the role of applied current density (10, 20 and 40 mA cm-2); furthermore, the effects of the temperature and agitation rate were investigated, carrying out experiments in the range from 25 to 60 °C, and 100, 200 and 300 rpm, respectively, under a current density (jappl) of 10 mA cm-2. A DC Regulate Power Supply MPL 3305 was used during electrolysis experiments. During the runs, samples of anolyte were withdrawn and analyzed for the colour and TOC removal of MG.
Experimentally, decontamination of dyes wastewaters is monitored from the abatement of their total organic carbon (TOC), while their decolorization efficiency or percentage of colour removal is determined by the expression:
where ABS0M and ABStM are the average absorbances before electrolysis and after an electrolysis time t, respectively, at the maximum visible wavelength (l max) of the wastewater (Martínez-Huitle and Brillas, 2009). Colour removal was monitored by measuring absorbance decrease, using a Shimadzu Model UV 1800 spectrophotometer. TOC removal measurements were carried out using a Shimadzu model TOC analyser. The energy consumption per volume of treated effluent was estimated and expressed in kW h m-3. The average cell voltage, during the electrolysis, is taken for calculating the energy consumption, as follows (Martínez-Huitle and Brillas, 2009):
where t is the time of electrolysis (h); V and A are the average cell voltage and the electrolysis current, respectively; and Vs is the sample volume (m3).
III. RESULTS AND DISCUSSION
Preliminary experiments have been carried out by polarization curves and cyclic voltammetry, to obtain information on the electroactivity of MG at PbO2 anodes (electrooxidation mechanism), prior to anodic oxygen evolution. Figure 2 shows linear polarization curves of PbO2 anode obtained in 0.5 M H2SO4 (in absence or presence of MG in solution), with a scan rate of 50 mV s-1, which started form the open circuit potential (OCP). The curve (a) is very different respect to other ones and it shows that oxygen evolution potential increases from 1.85 V versus Ag/AgCl for PbO2. This means that Ti/PbO2 has high oxygen evolution overpotential and consequently is poor electrocatalysts for the oxygen evolution reaction (Martínez-Huitle et al., 2010). These outcome is confirmed when two additions of concentration of MG was incorporated in aqueous solution, curves b and c. The electrocatalytic activity showed by using PbO2 anode to oxidize the model organic compound (curves b and c) was higher than in absence of MG (around 1.7 V), confirming that the PbO2 material is a good electrocatalysts for electrochemical oxidation process (Martínez-Huitle et al., 2010; Tahar and Savall, 1999). In addition, polarization curves obtained in presence of MG in solution demonstrated that a weak adsorption mechanism could be attained; the peak observed at 1.75 V corresponds to the oxidation of MG. However, at this electrode material, the generation water decomposition intermediates, mainly hydroxyl radicals, is principally favoured than direct electron transfer from the substrate; therefore, the participation of ·OH radicals in the electrooxidation process is evidently achieved.
Fig. 2. Polarization curves of PbO2 anode in supporting electrolyte solution (curve a) and in presence of 20 and 30 ppm of MG (curves b and c). Inset: Cyclic voltammograms for PbO2 anode in presence and absence of model organic compound in solution. Ag/AgCl (3.0 M) as reference electrode and a platinum wire as counter electrode. Temperature: 25°C.
Voltammograms were obtained at 10 mg L-1 of concentration of MG, in acidic media (H2SO4 0.5 M) and at room temperature (25°C). In all cases, the cyclic voltammograms (CV) curves were recorded below the decomposition potential of water and/or supporting electrolyte. Inset in the Fig. 2 shows the CV for background electrolyte and for solution containing 10 mg L-1 of MG at PbO2 anode. CV curves obtained with in H2SO4 0.5 M (Inset in the Fig. 2, black curve), presented the typical behavior of Pb oxide layer. A very different behavior was observed, when, as shown in the inset of Fig. 2 (red curves (line and dashed lines)), a significant current shift could be recorded when MG was added to the solution, compared with the curves recorded in 0.5 M H2SO4, at the same j value. This indicates that the pathway of MG oxidation involves water decomposition intermediates, mainly hydroxyl radicals, which are only available in conditions of oxygen evolution rather than direct electron transfer from the substrate (Panizza and Cerisola, 2009). In fact, on a PbO2 anode, which has a higher overpotential for oxygen evolution (Fig. 2, curve a); this secondary reaction is not favored over organic mineralization (Fig. 2, curves b and c). In addition, as the number of cycles increased, the voltammogram decrease moderately round about the same (Inset in the Fig. 2, scan 1-4), meaning that the electrogeneration of hy droxyl radicals inhibited the electrode fouling which instead occurred on the other anode materials, such as Ti/SnO2 anodes (Martinez-Huitle et al., 2004).
The visible spectra of MG reported in the Fig. 3, show a maximum absorption peak in the range of visible light which are in accordance with the green color of MG solutions. Thus, the measurement of the color removal was obtained using a UV/vis spectrophotometer at 426 nm. Fig. 3 shows the decrease with time of the absorbance band at 426 nm during galvanostatic electrolysis of MG synthetic wastewaters containing 300 mg L-1 by applying different values of current density, 10, 20 and 40 mA cm-2. As can be observed, the absorbance was satisfactorily reduced during the treatment. The intensity of the visible band decreases continuously until its disappearance after about 30-35 min of electrolysis leading to complete solution decolourisation.
Fig. 3. UV absorption spectra of MG in 0.5 M H2SO4 solutions as a function of time, during electrochemical oxidation process at (a) 10 (b) 20 and (c) 40 mA cm-2.
Figures 4 to 5 present the influence of the applied current density on the decay of colour (Fig. 4) and TOC removal (Fig. 5) during electrochemical oxidation of synthetic waste of MG containing 300 mg L-1. As can be observed, the complete removal of colour was achieved, independently of the applied current density; it is due to that the experiments were carried out in a zone controlled by mass transfer and under these conditions the efficiency of the process is very similar (Martinez-Huitle et al., 2004). Although the elimination rate rather increase when the applied current density values were decreased, as can be observed in the Fig. 4. It indicates that the complete dye elimination was attained by direct anodic oxidation (adsorption dye molecules to electrode surface) and indirect oxidation by means of its reaction with electrogenerated OH· radicals (Panizza and Cerisola, 2009). TOC removal values from 43 to 70% were achieved, depending on applied current density (10, 20 and 40 mA cm-2), as can be observed in Figure 5. TOC values showed that by products formed on the final processes avoid the complete elimination of organic matter due to the lower capacity to remove these by-products at the Ti/PbO2 anodes, increasing the time process (Martinez-Huitle et al., 2004).
Fig. 4. Electrochemical oxidation of MG at Ti/PbO2 electrode at different applied current densities (relative absorvance vs time (min). [MG]0 = 300 ppm in 0.5 M H2SO4 and 25°C). Inset: Colour removal efficiency vs time (min) under the same experimental conditions.
Fig. 5. Electrochemical oxidation of MG at Ti/PbO2 electrode at different temperatures (relative absorvance vs time (min). [MG]0 = 300 ppm in 0.5 M H2SO4 and 10 mA cm-2).
The oxidation curves reveal a faster removal of MG at 40 mA cm-2 and a more gradual removal at 10 and 20 mA cm-2, also accompanied by a good faradaic efficiency for the oxidation of the organic substrate (80%). In fact, this outcome confirms the polarization curves and cyclic voltammetry measurements on a Ti/PbO2 anode, where organic mineralization reaction is favoured than oxygen evolution (Fig. 2). Figure 5 shows the influence of temperature on the colour as function of time by applying 10 mA cm-2 of current density. As can be seen, total colour removal was achieved in all cases. It seems that the temperature has a significant impact on the kinetics of the electrochemical oxidation of MG because the rate of colour removal was considerably increased by increasing the temperature (from 25 to 40 or 60°C). The increase of temperature from 25 to 60 °C decreases the electrolysis-time required for the total colour removal from 30 to 15 min. A change in the temperature has slight influence on the electrooxidation with hydroxyl radicals. But, this can be explained by the increase on the chemical reactions rates, in agreement with results reported by other authors (Martinez-Huitle et al., 2004, Panizza and Cerisola, 2009). In addition, incomplete TOC removal was achieved in all cases (data not showed); this may occurs due to favor the side reaction of oxygen evolution at higher temperatures. Nevertheless, TOC decay values achieved at 40 and 60°C were higher (60 and 82% of TOC removal, respectively) than 25°C (40% of TOC removal).
For this electrocatalytic process, PbO2-Organic interaction is assumed, the carbonyl groups can exhibit a short-range interaction with surface Pb(IV) sites favouring the MG and by-products oxidation as proposed for p-nitrophenol (Quiroz et al., 2005), chloranilic acid (Martinez-Huitle et al., 2004) and glucose (Bonfatti et al., 1999).
In this last case, the formation of intermediate species with simpler molecular frames could originate blocking phenomena on superficial lead sites decreasing, consequently, the efficiency of the short-range MG-Pb(IV) sites interactions. Therefore, it is clear that oxidation process is practically dependent on the applied current density and temperature. From figures reported in Table 1, the energy consumption for the MG removal increases from 0.289 to 1.637 kWh m-3 when jappl increases from 10 to 40 mAcm-2. On basis of these results, best performances were achieved at 40 mA cm-2 of applied current density at this anode material. Table 1 also compares the energy consumption values during anodic oxidation at different temperatures. Since the process efficiency remains almost constant, doubling the temperature allowed to diminish the requested treatment time. In addition, energy consumption values at different agitation rates were also reported, confirming the decrease of charge required when a decrease on the hydrodynamic conditions was attained. The faster MG removal with increasing current can then be ascribed to the concomitant greater production of ·OH from reaction that accelerates the oxidation rate of all organics. The simultaneous increase in specific charge for overall mineralization indicates a slower relative proportion of ·OH produced at the PbO2 surface. The electrochemical method also showed great effectiveness to destroy different MG contents, using PbO2 anode. The MG-time plots obtained for 300 mg L-1 MG solutions at 40 mA cm-2 are depicted in Figures 4 and 6. As can be seen, the time required for total mineralization increases from 30 min to 400 min when colour or TOC are removed, latter depending on the presence of more pollutants. This trend can be related to the transport of more organics to the anode causing their faster reaction with greater amounts of ·OH, thus decreasing the loss of this radical by non-oxidizing reactions (Quiroz et al., 2003).
Fig. 6. Electrochemical oxidation of MG at Ti/PbO2 electrode at different applied current densities (TOC removal efficiency vs time (min). [MG]0 = 300 ppm in 0.5 M H2SO4 and 25°C).
Table I - Energy consumption calculated from Eq. 2,a per volume of treated effluent during anodic oxidation of MG for different applied current densities. [MG]0=300 mg L-1, Electrolyte: 0.5 M H2SO4. Cost eliminationb (Brazilian currency).
Comninellis (1994) explained the different behavior of electrodes in electrochemical oxidation considering two limiting cases: the so-called "active" and "non-active" anodes. Typical examples are Pt, IrO2 and RuO2 for the former and PbO2, SnO2 and BDD for the latter. The proposed model assumes that the initial reaction in both kind of anodes (generically denoted as M) corresponds to the oxidation of water molecules leading to the formation of physisorbed hydroxyl radical (M(·OH)). Both the electrochemical and chemical reactivity of heterogeneous M(·OH) are dependent on the nature of the electrode material. The surface of active anodes interacts strongly with ·OH and then, a so-called higher oxide or superoxide (MO) may be formed. This may occur when higher oxidation states are available for a metal oxide anode, above the standard potential for oxygen evolution (E° = 1.23 V vs. SHE). The redox couple (MO/M) formed acts as a mediator in the oxidation of organics, which competes with the side reaction of oxygen evolution via chemical decomposition of the higher oxide species.
In contrast, the surface of a "non-active" anode interacts so weakly with ·OH that allows the direct reaction of organics. Therefore, our results confirm that not only the chemical structure influences the time process; but also the electrode material used strongly affects the decolorization efficiency or TOC removal. This behavior has been reported by other authors, as recently reviewed and described in detail by Martínez-Huitle and Brillas (2009).
The aim of the present work was to evaluate the applicability and the efficiency of the electrochemical oxidation process. The results obtained confirm that it is possible to apply this method for the elimination of MG from aqueous solutions. Removal levels and efficiency percentages were investigated as a function of applied current density, obtaining TOC removal efficiencies from 43 to 70% and 90% of colour removal, depending on applied current density. In the electro-oxidation process a strong influence of jappl values on MG oxidation was observed, during elimination of TOC. By changing the jappl values, it was possible to carry out the oxidation obtaining good current and removal efficiency on Ti/PbO2 anode. Colour decay as a function of time during bulk electrolyses of MG synthetic wastewaters was significantly affected by temperature and agitation a rate, meaning that oxidation rate that was increased.
In the case of effect on temperature, TOC decay efficiencies achieved at 40 and 60°C were higher (60 and 82% of TOC removal, respectively) than 25°C (40% of TOC removal); while colour removal was more than 90% in all cases. Finally, agitation rate effect only increases slightly the efficiency of the colour removal; but, it decreases the energy consumption favoring the application of this process for treating industrial effluents.
As a final remark on possible use of PbO2-based electrodes, we have to bear in mind that PbO2 is a good anodic material for the elimination of organic pollutants, but its application in the wastewater treatment may be limited by the risk of lead contamination, due to its dissolution under specific anodic polarization and solution composition. In this connection, however, it may be also worth mentioning that the wear resistance of lead dioxide is significantly conditioned by the preparation method. For instance, Ti/PbO2 electrodes prepared according to the procedure outlined in Shimamune and Nakajima (1996) are much more stable, compared with simpler preparations). The energy consumption makes inadequate Ti/PbO2 anodic oxidation for complete elimination of wastewaters polluted with dyes (TOC removal) but it can be a feasible process for decolorizing wastewaters containing dyes as a pre-treatment process. Finally, taking into consideration an electrical energy cost of about R$ 0.3 (Brazilian price, taxes excluded) per kW h (Agência Nacional de Energia Elétrica, Brazil), the process expenditure was estimated and reported in Table 1 in order to show the viability of this process as a green alternative.
D.A.C. gratefully acknowledges the CNPQ and PROPESQ/UFRN for his fellowship (Iniciação Científica - PIBIC). J. H. B. R. gratefully acknowledges the CAPES for Master fellowship. The authors thank the financial support provided by PETROBRAS and they also thank Industrie De Nora S.p.A. (Milan, Italy) for providing the Ti/PbO2 electrodes.
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Received: March 4, 2010.
Accepted: May 27, 2010.
Recommended by Subject Editor Ricardo Gómez.