A comparison of electrodeposited Ti/β-PbO2 and Ti-Pt/β-PbO2 anodes in the electrochemical degradation of the direct yellow 86 dye

ARTIGO

A comparison of electrodeposited Ti/β-PbO₂ and Ti-Pt/β-PbO₂ anodes in the electrochemical degradation of the direct yellow 86 dye^#

José M. Aquino; Kallyni Irikura; Romeu C. Rocha-Filho^*; Nerilso Bocchi; Sonia R. Biaggio

Departamento de Química, Universidade Federal de São Carlos, CP 676, 13560-970 São Carlos - SP, Brasil

ABSTRACT

The electrochemical performance of electrodeposited Ti/β-PbO₂ and Ti-Pt/β-PbO₂ anodes was galvanostatically evaluated (batch mode, 50 mA cm^-2) to degrade the Direct Yellow 86 dye (100 or 200 mg L^-1 in 0.1 mol L^-1 Na₂SO₄ + 1.5 g L^-1 NaCl), investigating the effect of pH and temperature. Similar results were obtained for both electrodes and the best conditions for removal of color and chemical oxygen demand are pH 7 and 40 °C, when 90% decolorization is attained by passing a charge of only ~0.13 A h L^-1 and total mineralization is achieved with expenditure of ~5 kW h m^-3.

Keywords: PbO₂ anode; electrochemical degradation of wastewater; indirect oxidation.

INTRODUCTION

Different technologies for the treatment of textile wastewater have been extensively studied due to its harmful impact on the environment and also to increasingly rigid environmental regulations.^1,2 Textile wastewater is characterized mainly by its intense color and high organic load, due to non-reacted dyestuff. Synthetic dye molecules are by design chemically stable and thus can remain in the environment for long periods,³ eventually causing severe aquatic⁴ and health problems.⁵ Hence the need to use and implement methods to remove these pollutants from textile wastewater before they are discharged.

Biological, physicochemical, and chemical methods are commonly used technologies for textile wastewater treatment, despite innumerable limitations considering pollutant abatement.^1,2,6,7 Electrochemical methods have been extensively used for wastewater decontamination since they offer an environmentally friendly way of removing toxic inorganic or organic species via redox reactions.^8-17 These methods may be a good option for textile wastewater treatment, since they are easy to implement, versatile, and can lead to high organic and color removals;⁶ their main drawbacks regard the use of electrical energy and the poor current efficiencies attained at low pollutant concentrations.^15,16 However, the use of adequate anode materials, coupled with optimized operation conditions, tend to minimize these problems.

The electrochemical oxidation of pollutants can occur in two ways: directly, when the organic pollutant is oxidized by electron transfer directly to the electrode, or indirectly, when this electron transfer is mediated by some electroactive species (an oxidant generated at the anode).¹⁶ The most common electrogenerated oxidants in indirect oxidation processes are: the hydroxyl radical (•OH), from the oxidation of water;^16,17 Cl₂, HOCl, and OCl^- (frequently referred to as active chlorine), from the oxidation of chloride ions;^16,18 and S₂O₈^2-, from the oxidation of sulfate ions.^16,19

According to the nature of the interaction between the hydroxyl radical and anode materials, Kapalka et al.²⁰ classified them from high (boron-doped diamond - BDD) to low (dimensionally stable anodes - DSA) oxidation power anodes; in this classification, lead dioxide (PbO₂) is a medium oxidation power anode. The electrochemical oxidation mediated by active chlorine is commonly used with medium to low oxidation power anodes in order to increase the pollutant oxidation rate and the process current efficiency.²¹ Indeed, it has been shown that oxidation rates comparable to those on BDD anodes are obtained when using PbO₂ or DSA (medium or low oxidation power) anodes in the presence of chloride ions.^6,14,18 When using active chlorine, it might be essential to have the solution pH under control in order to improve the pollutant removal efficiency, since pH dictates the predominant chlorine species in the electrolyzed solution.²² Thus, different pH conditions were reported as the best ones for color and organic removals.^6,18,23 The possible formation of undesirable chloro-organic derivatives is a known drawback of electrooxidation processes in chloride-containing solutions, although very few papers have actually reported these compounds.²⁴ Undesirable inorganic compounds, such as ClO₄^- ions, which are mainly formed in electrooxidations using BDD anodes, also represent serious health problems.²⁵

PbO₂ film anodes are interesting materials for the electrochemical degradation of pollutants due to their medium to high oxidation power, easiness of preparation, and low cost.^12-14,23 The PbO₂ film electrochemical performance and stabilization are influenced by the presence of doping species,^12,13,26 electrodeposition conditions,^26,27 and substrate preparation. The main concern when using PbO₂ anodes is the possible release of toxic Pb²⁺ ions and problems related to the adherence of the oxide film to the substrate that reduce the lifetime of the anodes. Titanium substrates covered with platinum (Ti-Pt) are well known to lead to good stability of the PbO₂ films,¹² with low levels of Pb²⁺ ions released into solution;²⁸ however the extensive use of platinum is not economically feasible. Thus, many PbO₂ anodes prepared directly on the Ti substrate have been reported, with previous substrate pretreatment or modification: sandblasting followed by etching in concentrated hydrochloric acid;²⁹ polishing with 320-grit paper strips, etching in boiling 20% hydrochloric acid for 1 h followed by deposition of a SnO₂-Sb coating;³⁰ and etching in boiling 1.6 mol L^-1 oxalic acid for 2 h.²⁷ The presence of fluorine ions³¹ or surfactants³² in the electrodeposition bath can contribute to a more stable oxide film due to the diminishment of the amount of oxygen evolution, which could cause cracks in the oxide surface leading to scaling. Furthermore, these species can improve the PbO₂ film adherence²¹ and its crystallinity.³¹

Studies on the degradation of the diazo dye Direct Yellow 86 - DY 86 (CAS number 50925-42-3 and C.I. number 29325 - Figure 1) are quite rare. Shen and Wang³³ reported on its decomposition by a UV/H₂O₂ process in continuous annular photoreactors. Hsueh et al.³⁴ reported on its decolorization by Aeromonas hydrophila, comparatively to other dyes in order to study the effect of chemical structure. On the other hand, Kuo et al.³⁵ reported studies on the possible removal of the DY 86 dye from solution using carbon nanotubes. As far as we are aware, no study on the degradation of this dye by electrochemical methods has been reported.

In our laboratory, the electrochemical degradation of textile dyes has been investigated previously using doped and undoped β-PbO₂ anodes, in the absence or presence of chloride ions, but these electrodes were always electrodeposited onto Ti-Pt substrates.^12,14,23 Thus, the aim of the present paper is to produce and characterize undoped Ti/β-PbO₂ and Ti-Pt/β-PbO₂ anodes, as well as comparatively evaluate their performances in the electrochemical degradation of the DY 86 dye in the presence of chloride ions. For such, the evolution of the color and chemical oxygen demand (COD) of solutions of this dye is followed with time of electrolysis.

EXPERIMENTAL

Chemicals

All chemicals, including Pb(NO₃)₂ (a.r., Acros), sodium lauryl sulfate, SLS (99%, Fisher Scientific), H₂PtCl₆ (99.9%, Aldrich), HCl (36.5%, Mallinckrodt), HNO₃ (69-70%, Mallinckrodt), NaCl (a.r., JT Baker), Na₂SO₄ (a.r., Qhemis), and DY 86 (Quimanil), were used as received. Doubly deionized water (Millipore Milli-Q system, ρ > 18.2 Mω cm) was used for the preparation of all solutions.

β-PbO₂ film preparation on Ti and Ti-Pt substrates

Firstly, 3.1 cm × 2.7 cm (each face) Ti plates were sandblasted followed by cleaning with 2-propanol in an ultrasonic bath for 20 min. The procedure involving the Ti substrate platinization is fully described in a previous work.¹²β-PbO₂ films were electrodeposited on the Ti and Ti-Pt substrates in a conventional electrochemical cell, using a calomel reference electrode and two AISI-304 stainless steel plates as counter electrodes. The deposition bath was composed of 0.1 mol L^-1 Pb(NO₃)₂, 1.0 g L^-1 SLS for the Ti substrate or 0.5 g L^-1 SLS for the Ti-Pt substrate, in 0.1 mol L^-1 HNO₃. The electrolysis was carried out at 65 °C for the time necessary to obtain a 50 mg cm^-2 PbO₂ film, assuming 100% faradaic efficiency. The applied current density (j) in the electrodeposition was 5 or 20 mA cm^-2 for the Ti or Ti-Pt substrate, respectively. As soon as the electrodeposition was finished, the electrodes were transferred to and maintained in a vessel containing deionized water initially at 65 °C, until ambient temperature was reached.

Electrodes characterization

X-ray diffraction measurements were carried in order to characterize the phases of the PbO₂ film; the analysis was a 2θ-θ (where θ is the diffraction angle), with a continuous sweeping rate equal to 1° min^-1, ranging from 20° to 80°. The morphology of the β-PbO₂ grains was examined by scanning electron microscopy (SEM).

The electrochemical behavior of the electrodes in a 2 mol L^-1 H₂SO₄ solution was analyzed by cyclic voltammetry. Additionally, an accelerated test of the Ti/β-PbO₂ (3.88 cm²) and Ti-Pt/β-PbO₂ (3.68 cm²) anodes was carried out galvanostatically (150 mA cm^-2) for 12 h at 40 ºC, also in a 2 mol L^-1 H₂SO₄ solution. For such, a conventional electrochemical cell was used with a saturated calomel reference electrode and two AISI-304 stainless steel plates as counter electrodes; the H₂SO₄ solution (100 mL) was continuously stirred. Samples of the solution were collected at the end of the accelerated test, for Pb detection in a flame atomic absorption spectrometer (FAAS); the anodes were also analyzed for mass loss.

Electrochemical degradation of the DY 86 dye

The experiments on the electrochemical degradation of the DY 86 dye were carried out in a stirred cell (600 mL beaker) with turbulent promoters attached to the wall, a PbO₂ anode, a calomel reference electrode, and two AISI-304 stainless steel plates as counter electrodes. The experiments were carried out galvanostatically in the batch mode, using 400 mL of a 0.1 mol L^-1 Na₂SO₄ solution containing 100 or 200 mg L^-1 DY 86; the NaCl concentration was fixed at 1.5 g L^-1. Considering that in previous works on the degradation of dyes²³ we found that optimal conditions for COD removal were attained at a high current density, its value was fixed at 50 mA cm^-2. The investigated variables were pH (3, 7, and 11) and temperature (10, 25 and 40 ºC).

Analyses

The evolution of the color and COD of the DY 86 solutions was followed with time of electrolysis. The dye decolorization was monitored at certain time intervals by sampling of 3 mL of the solution (initial dye concentration of 100 mg L^-1). The solution color was analyzed from 200 to 800 nm in a UV-vis spectrophotometer (HP 8452 diode array detector). The charge per unit volume of the electrolyzed solution required for 90% decolorization (Q⁹⁰) was used to establish the best operation conditions for color removal. Then, using these best operation conditions, the COD removal experiments were carried out by electrolyzing a 200 mg L^-1 DY 86 solution. The COD measurements were done with commercial reagents (Hach) using 2 mL samples of the electrolyzed solution, which were further oxidized by digestion at 150 ºC for 2 h. Then, the absorbance of each sample was read in a Hach DR/890 model spectrophotometer.

RESULTS AND DISCUSSION

Electrodes characterization

Figure 2 shows the X-ray diffraction spectra for the Ti/β-PbO₂ and Ti-Pt/β-PbO₂ anodes. By comparing the obtained phase angles corresponding to the different diffraction lines with those indexed according to the JCPDS 41-1492 card, it was possible to conclude that both PbO₂ films are present as the tetragonal β-phase; no peaks for the α-phase were detected in the spectra. Furthermore, the SEM images (Figure 1S, supplementary materials) revealed the typical pyramidal grains of the PbO₂β-phase. The oxide film of the Ti/β-PbO₂ anode (Figure 1Sa) is made up of larger grains because its electrodeposition rate was slower (applied current density of 5 mA cm^-2), a procedure that should also have led to a reduced number of defects in the film. These characteristics are most probably the reason why the oxide film of the Ti/β-PbO₂ anode presents some peaks of higher intensity in the diffraction spectrum (Figure 2).

The cyclic voltammograms (2^nd cycle) of the two anodes in a 2 mol L^-1 H₂SO₄ solution (Figure 3) show an anodic response starting above 1.5 V, before oxygen evolution, and a cathodic response below 1.4 V. These cathodic and anodic responses correspond to the conversion PbO₂→ PbSO₄ and the reverse conversion PbSO₄→ PbO₂, respectively, as previously discussed by Feng and Johnson,³⁶ for instance. No significant potential or current difference is observed in these voltammograms, except for the fact that the oxygen evolution reaction starts at a somewhat lower potential on the Ti/β-PbO₂ anode.

Electrochemical degradation of the DY 86 dye: decolorization

As it was previously shown,^6,14,18 when Cl^- ions are added to the solution the rate of dye decolorization is increased significantly. Figure 4 shows for the two anodes how the relative absorbance (at 380 nm) of the dye solution (100 mg L^-1 DY 86) changed as a function of the electric charge passed through the electrochemical cell per unit volume of solution, at pH 7 for the three temperatures investigated; similar decay curves were obtained at pH 3 and 11 (Figures 2S and 3S, supplementary materials). From these results one can see that the decolorization is significantly favored as the temperature is increased, independently of which anode was used; the best results were obtained at 40 °C, for pH 3 and 7. Table 1 shows the results obtained for Q⁹⁰, kinetic constants, apparent activation energy, cell potential, and energy consumption per unit volume of the solution associated to Q⁹⁰. The best operation conditions (lowest values of Q⁹⁰ and energy consumption) for the decolorization were attained for pH 7 at 40 ºC (Table 1). At this pH value, for the Ti/β-PbO₂ anode, when the temperature is lowered from 40 to 25 or 10 °C, the value of Q⁹⁰ increases twofold or more than fivefold, respectively. On the other hand, when at 40 °C the pH is decreased from 7 to 3, the value of Q⁹⁰ increases by about 40%; however, if the pH is increased from 7 to 11, the value of Q⁹⁰ increases more than sixfold. Similar but not as marked variations are found for the Ti-Pt/β-PbO₂ anode. Clearly, the best conditions for inactivation of the chromophore of the DY 86 dye by electrolysis with Cl^- ions added to the solution are pH 7 and temperature of 40 °C.

As extensively discussed by Cheng and Kelsall,²² when solutions containing Cl^- ions are electrolyzed the generated chlorine gas is hydrolyzed and disproportionates, forming hypochlorous acid (HClO):

For pH < 3.3, Cl₂ is the predominant species in solution, becoming the only species in solution for pH < 1. For pH > 3, reaction 2 becomes irreversible, and HClO is the predominant species in the pH range 3.4 to 7.5. As the pH increases, HClO deprotonates:

For pH > 7.6, the hypochlorite ion (ClO^-) becomes the predominant species; for pH > 10, ClO^- is the only chlorine species in solution. Hence, the best conditions for decolorization of the DY 86 dye (pH 7) occur when the HClO and ClO^- species are the oxidants present in solution, approximately in the proportion 2:1,²² whereas the worst conditions occur when the ClO^- species is the sole oxidant present in solution (pH 11).

High temperatures favor the mediated oxidation probably due to the increasing power of the electrogenerated oxidants. That behavior is in agreement with the higher values of the apparent second-order kinetic constants for reactions at 40 ºC (Table 1). A second-order model was chosen because it better fits the experimental data (R > 0.95). The apparent activation energies have similar values and do not seem to depend on the solution pH; their high values indicate that the DY 86 dye decolorization is highly temperature sensitive,³⁷ as the data in Table 1 clearly show. Moreover, the results here presented are in agreement with the ones of Szpyrkowicz and Radaelli,³⁷ who also reported a second-order kinetic behavior as well as a temperature dependence for the decolorization of an azo dye, as in the present work.

Recently we reported on the electrochemical degradation of the dyes Reactive Red 141 and Acid Blue 62 (100 mg L^-1 in 0.1 mol L^-1 Na₂SO₄) on Ti-Pt/β-PbO₂ anodes.²³ For the Reactive Red 141 dye, the best conditions for decolorization occur at very low pH values, at temperatures greater than 25 °C, with at least ~1.4 g L^-1 NaCl added to the solution. In the case of the Acid Blue 62 dye, these conditions are pH 4, at temperatures equal or lower than 25 °C, with 1.0 - 2.0 g L^-1 NaCl added to the solution. Based on these results and the ones here reported, clearly the necessary NaCl concentration is about 1.5 g L^-1 NaCl, but the pH and temperature vary with the dye being degraded.

Electrochemical degradation of the DY 86 dye: COD removal

Previously,¹⁴ when the performance of a Ti-Pt/β-PbO₂ anode was compared to that of a boron-doped diamond (BDD) anode in the degradation of the Reactive Orange 16 dye (85 mg L^-1 in 0.1 mol L^-1 Na₂SO₄), we showed that when Cl^- ions are added to the solution the extent of dye degradation using the Ti-Pt/β-PbO₂ anode was increased significantly, but not enough to lead to total mineralization (only 80% COD removal, compared to 100% for the BDD anode at a lower value of the applied electric charge).

Figure 5 shows for the two anodes how the COD removal percentage of the dye solution (200 mg L^-1 DY 86) changed as a function of the electric charge passed through the electrochemical cell per unit volume of solution, for three different values of pH, at 40 °C; this temperature was chosen because it led to better decolorization results. By analyzing Figure 5 one can conclude that both anodes are capable of mineralizing the DY 86 dye, independently of the solution pH; however, clearly the degradation is faster in neutral (pH 7) or basic (pH 11) solution, when lower values of the applied electric charge are necessary to attain total mineralization. Although the OCl^- species has a lower oxidation potential than the Cl₂ or HClO ones, its concentration might be higher because it is an ionic species (the other two are gaseous); this might explain why high COD removal rates are also attained for pH 11 solutions. Nevertheless, looking at the degradation process from the point of view of removing both color and COD, clearly the best operation conditions are pH 7 and 40 °C. Table 2 shows the values of the kinetic constants of the fitting of an apparent first-order model (R > 0.91), cell potential, and energy consumption per unit volume of the electrolyzed solution to attain complete mineralization. As expected from the results presented before, the lowest values of energy consumption were also obtained for the neutral and basic solutions.

As mentioned above, the electrochemical degradation of the dye Acid Blue 62 (100 mg L^-1 in 0.1 mol L^-1 Na₂SO₄ + 1.5 g L^-1 NaCl) using a Ti-Pt/β-PbO₂ anode was previously studied in our laboratory.²³ For this dye, the best conditions for COD removal are pH 11, at 45 °C. Based on these results and the ones here reported, clearly better degradation results are obtained at a higher temperature.

Accelerated tests

Tables 1 and 2 show that the electrolyses carried out with the Ti/β-PbO₂ anode needed a somewhat higher value of cell potential than when the Ti-Pt/β-PbO₂ anode was used; this suggests that some passivation of the Ti substrate occurred during the PbO₂ film electrodeposition to obtain the Ti/β-PbO₂ anode.

After the accelerated tests of the anodes (150 mA cm^-2, for 12 h in a 2 mol L^-1 H₂SO₄ solution at 40 ºC), a mass loss of 0.9% was detected for the Ti/β-PbO₂ anode, whereas no significant mass loss was found for the Ti-Pt/β-PbO₂ anode. However, when the used H₂SO₄ solutions were tested, Pb²⁺ contents of 3.5 and 1.9 ppm were found for the Ti/β-PbO₂ and Ti-Pt/β-PbO₂ anodes, respectively. Finally, it should be mentioned that after the 12 h test, scaling of the oxide film was detected on the Ti/β-PbO₂ anode; nevertheless, no sign at all of film detachment was detected after the electrochemical degradation experiments. From these results one can conclude that the β-PbO₂ film on the Ti-Pt/β-PbO₂ anode is stabler than the one on the Ti/β-PbO₂ anode; hence, the electrodeposition of the oxide directly on the Ti substrate needs to be further investigated so as to further increase the stability of the oxide film and minimize the passivation of the substrate.

CONCLUSIONS

The electrodeposition of the β-PbO₂ phase on a titanium substrate was successfully accomplished, with no alpha phase present; however, the cell potentials attained when this Ti/β-PbO₂ anode was used (somewhat higher than those attained when a Ti-Pt/β-PbO₂ anode was used) for the electrolysis of the DY 86 dye indicate that the Ti substrate was partly passivated during the electrodeposition of the β-PbO₂ film. Additionally, the results of the accelerated tests indicate that the Ti/β-PbO₂ anode is not as stable as the Ti-Pt/β-PbO₂ anode; hence, further investigations need to be carried out on the electrodeposition of the β-PbO₂ directly on Ti in order to further improve the stability of the Ti/β-PbO₂ anode.

Despite the above highlighted differences in the stability of the two anodes, their performances in the color or COD removal of the DY 86 dye solution did not present any significant difference. The best conditions for decolorization of the DY 86 dye (100 mg L^-1 in 0.1 mol L^-1 Na₂SO₄, 400 mL) in the presence of Cl^- ions (1.5 g L^-1) using any of the anodes are pH 7 and 40 °C; under these conditions, 90% decolorization is attained passing an electric charge per unit volume of the electrolyzed solution of only about 0.13 A h L^-1. On the other hand, using any of the anodes at 40 °C, the mineralization of the dye (200 mg L^-1 in 0.1 mol L^-1 Na₂SO₄ + 1.5 g L^-1 NaCl, 400 mL) is faster if the solution is neutral (pH 7) or basic (pH 11), when lower values of applied charge are necessary to attain total mineralization; under these conditions, total mineralization is attained with an energy consumption per unit volume of the electrolyzed solution of around 5 kW h m^-3. Consequently, from the point of view of removing both color and COD, clearly the best operation conditions are pH 7 and 40 °C, when the HClO and ClO^- species are the oxidants present in solution, approximately in the proportion 2:1.

Finally, the very low values of electric charge needed to attain 90% decolorization and the low values of energy consumption to attain complete mineralization, per unit volume of dye solution, indicate the feasibility of applying the electrochemical process to degrade the DY 86 dye using β-PbO₂ anodes in the presence of chloride ions added to the solution.

SUPPLEMENTARY MATERIAL

Three figures are available as supplementary material: SEM micrographs of the β-PbO₂ films (Figure 1S); relative absorbance as a function of the applied electric charge for the decolorization of a DY 86 solution at pH 3 (Figure 2S) and at pH 11 (Figure 3S), at different temperatures, using Ti/β-PbO₂ and Ti-Pt/β-PbO₂ anodes. These figures can be downloaded as a PDF file from http://quimicanova.sbq.org.br.

ACKNOWLEDGMENTS

The authors gratefully acknowledge financial support and scholarships from the Brazilian funding agencies CNPq, CAPES, and FAPESP. Quimanil (Brazil) is acknowledged for supplying the dye sample. Profs. L. A. M. Ruotolo (DEQ-UFSCar), E. R. Pereira Filho, E. R. Leite, and A. A. Batista (DQ-UFSCar) are also gratefully acknowledged for access to different apparatuses.

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Recebido em 1/7/10; aceito em 6/9/10; publicado na web em 20/10/10

^#

It is with great pleasure that we dedicate this article in honor of "brother Hans", to whom SBQ owes much.

Publication Dates

• Publication in this collection
  16 Dec 2010
• Date of issue
  2010

History

• Received
  01 July 2010
• Accepted
  06 Sept 2010