Electrochemical degradation of the reactive red 141 dye on a β-PbO2 anode assessed by the response surface methodology

The electrochemical degradation of the Reactive Red 141 dye using a filter-press reactor with a β-PbO2 anode was investigated through the application of the response surface methodology. The charge required for 90% decolorization (Q90) and the chemical oxygen demand removal percentage after 30 min electrolysis (COD30) were used to model the system. The investigated independent variables were the current density, pH, NaCl concentration, and temperature. Low values of Q90 (0.2-0.3 A h L-1) were obtained at acidic conditions (pH 1-3) and high concentrations of NaCl (1.0-2.0 g L-1), when Cl2 and HOCl are the predominant oxidant species. The best values of COD30 were obtained at high current densities and acidic to neutral conditions (pH 5-7); however, the consequent energy consumption makes the process not economically feasible under these conditions. For strongly acidic solutions, specific energy consumptions associated to Q90 as low as 0.79 kW h m-3 were attained.


Introduction
The contamination of water is one of the greatest current challenges, as it is becoming a scarce natural resource.The textile industry, in particular, stands in a delicate position due to the large volumes of water used and wastewater produced during dyeing process steps. 1 Among the dyes used by this industry, the azo (-N=N-) ones are the most produced and consumed. 2,3It should be noticed that a considerable amount of synthetic dyes are also discharged in the environment during production.The textile effluents are characterized by intense colors originated by un-reacted dyestuff, high organic loads (due to the presence of auxiliary chemicals), and oscillations in pH and temperature.Those first two characteristics make necessary the treatment of these effluents before their discharge into the environment.On the other hand, increasingly rigid environmental regulations also contribute to a search for better and more efficient treatment methods. 4he azo dyes, as highlighted by Zidane et al., 5 are a dye class that is not easily treated by conventional wastewater treatment methods.
As summarized by Martinez-Huitle and Brillas, 3 biological, physico-chemical, and chemical methods, among others, are commonly used for the treatment of industrial wastewaters.All these methods have advantages and disadvantages, although no current technology has universal application. 3,6,7The biological treatment is commonly the most efficient and economic for wastewater chemical oxygen demand (COD) abatement, but is inefficient and time consuming for most textile effluents since the respective dye molecules are recalcitrant to microbiological degradation; 1,6,7 this recalcitrance makes the method quite inefficient for color removal.Physicochemical methods for wastewater treatment, such as adsorption, coagulation, or filtration, are efficient for decolorization, but their application is expensive and leads to a high amount of sludge. 3Chemical methods as well as advanced oxidation processes are also powerful techniques for dye wastewater decolorization, but are associated to high costs as well as operational problems. 3onsidering the above, electrochemical methods might be a good option due to their versatility, easiness of implementation and pollutant high removal rates, despite electrical energy consumption.In such methods, electrode materials play an important role on the degradation efficiency, mechanism and in the resulting products.Depending on the electrode material used, the anodic oxidation of organic pollutants can be distinguished as occurring by selective oxidation (with "active" anodes) or by combustion (with "non-active" anodes). 3,8Recently these types of anodes have been referred to as low and high oxidation power anodes, respectively. 9Low oxidation power anodes are those that interact strongly with hydroxyl radicals (•OH), resulting in a low chemical reactivity for organics oxidation (low current efficiency for organics oxidation).On the contrary, high oxidation power anodes present a weak interaction with •OH radicals, resulting in a high reactivity for organics oxidation (high current efficiency for organics oxidation). 9Organics oxidation mediated by other electrogenerated oxidants, like Cl 2 , ClO -and HClO, is possible in the presence of chloride ions. 3Concerning these electrogenerated oxidants, several works have been published on their reaction schemes, 3,10,11 speciation diagrams, 10 and parameters affecting their production [11][12][13][14] (e.g.anode material, temperature and pH), as well as their role on the dyestuff mineralization. 13,15,16owever, the possible formation of undesirable chloroorganic derivatives, as detected in the electrochemical treatment of a tannery wastewater, 17 might be the main disadvantage of these electrogenerated oxidants.
][20][21][22][23][24][25] According to the criteria discussed by Kapalka et al., 9 PbO 2 anodes could be classified as medium oxidation power anodes.In some cases, poor performance and problems like scaling and dissolution regarding Ti/PbO 2 electrodes were reported, 11,26 although these problems seem to be related to the substrate preparation.[20][21][22][23][24][25] Classical and conventional methods of studying the influence of one variable at a time require a considerable number of experiments and are time consuming.Moreover, not all combinations of variables or all their levels are taken into account.Thus, statistical methodologies, such as the Response Surface Methodology (RSM), 27 enable studying and modeling a system with respect to its variables more efficiently, in order to find and optimize the best experimental conditions.
Therefore, this work aimed to find the best experimental conditions, using RSM, for the electrochemical degradation of the Reactive-Red 141 (RR 141) azo dye on a Ti-Pt/β-PbO 2 anode.

b-PbO 2 film preparation on a Ti-Pt substrate
Firstly, both sides of Ti plates (3.1 cm × 2.7 cm, 99.9% Aldrich) were sandblasted using 60-70 µm glass microspheres, followed by cleaning for 20 min in an ultrasonic bath containing 2-propanol.Then, platinum films were electrodeposited (250 mA cm -2 for 10 min) on these Ti plates, using a conventional one-compartment cell with two Pt foil counter electrodes.The electrodeposition bath consisted of 20 g L -1 H 2 PtCl 6 in 8.2 mol L -1 HCl at 65 ºC. 28he β-PbO 2 films were electrodeposited on the platinized Ti substrates in a conventional cell, using a calomel reference electrode and two AISI-304 stainless steel plates as counter electrodes.The deposition bath consisted of 0.1 mol L -1 Pb(NO 3 ) 2 , 0.5 g L -1 sodium lauryl sulfate (SLS), in 0.1 mol L -1 HNO 3 at 65 °C. 25The electrolysis (20 mA cm -2 ) was carried out for the time necessary to obtain a 50 mg cm -2 PbO 2 film, assuming 100% faradaic efficiency.As soon as the electrodeposition was finished, the electrode was transferred to and maintained in a vessel containing deionized water initially at 65 °C, until ambient temperature was reached.

Electrochemical degradation of RR 141
The electrochemical experiments were carried out in a one-compartment filter-press reactor composed of the Ti-Pt/ β-PbO 2 electrode (exposed area: 3.1 cm × 1.9 cm, each face) and two nickel plates as anode and cathodes, respectively.The experimental setup and the electrochemical reactor are schematically shown in Figures 1 and 2, respectively.In order to investigate and find the best experimental conditions for the RR 141 electrochemical degradation, RSM by a central composite design (CCD) was used.The investigated variables were: current density, pH, NaCl concentration, and temperature.Five different levels (-2, -1, 0, 1, 2) were studied for each variable.Table 1 shows the range and the levels of these variables.Three replications were carried out at the design center in order to evaluate the pure error and, consequently, the lack of fit.All the experiments were carried out randomly.
The RR 141 concentration in a 0.1 mol L -1 Na 2 SO 4 solution was fixed at 100 mg L -1 .Other fixed parameters were the dye solution volume (0.4 L) and the flow rate (360 L h -1 ).

Analyses
The electrochemical performance was analyzed through solution color and COD removals.The decolorization was monitored in situ (at 544 nm) by pumping the dye solution from the electrochemical system reservoir to an UV-Vis spectrophotometer (Ultrospec 2100 pro, from Amersham Pharmacia Biotech), and then back to the reservoir.The charge per unit volume of the electrolyzed solution required for 90% decolorization (Q 90 ) was used to model the electrochemical system performance.
The COD measurements were carried out after 30 min of electrolysis (COD 30 ) using a 2.5-mL sample of the electrolyzed dye solution.This sample was oxidized by digestion at 150 °C for 2 h in a H 2 SO 4 solution with K 2 Cr 2 O 7 , Ag 2 SO 4 , and HgSO 4 .The sample absorbance was read at 620 nm in a Hach DR/890 model spectrophotometer.The COD 30 values were calculated using a previously calibrated curve and the modeling was conducted through the percentage of COD removal.
The quadratic equation used to model the RSM responses was Y = β 0 + Σβ i X i + Σβ ii X i 2 + Σβ ij X i X j , where β 0,i,ii,ij are model coefficients and X i,j the independent variables.Detailed description and discussion of this equation can be found in Montgomery's book. 27

Results and Discussion
The CCD experimental matrix as well as the observed and predicted responses for COD 30 removal (%) and Q 90 (A h L -1 ) are shown in Table 2.The quadratic equations that describe the COD 30 removal and Q 90 behaviors are given below.In these equations, non-significant coefficients were excluded based on the results for each model analysis of variance (ANOVA) and the student t test (at 95% confidence level).
According to the F-test, the Q 90 modeling presented a considerable lack of fit (F 10,2 (95%) = 170.0).However, the NaCl concentration, X 3 (g L -1 ) 0 0.58 1.17  observed and the predicted Q 90 values presented a good correlation as shown in Figure 3a.On the other hand, the COD 30 removal modeling was satisfactorily adjustable (F 10,2 (95%) = 1.1), with a significant correlation between the observed and predicted percentage values (Figure 3b). Figure 4 shows some of the response surfaces for Q 90 as a function of the independent variables: NaCl concentration and pH (Figure 4a), temperature and pH (Figure 4b), and current density and pH (Figure 4c).Among these variables, pH is the most important due to different chloro-species generation.The current density and the NaCl concentration determine the rate and amount of the generated chlorospecies, respectively.The temperature determines mainly the oxidants diffusion rate, as well as the chloro-species generation and stabilization.
Figure 4a (data obtained at 35 °C and using 75 mA cm -2 ) shows that the best operational conditions (low Q 90 values) occur for very low pH values and high NaCl concentrations.According to the literature, 3,10,16,22 the predominant active chlorine species in these conditions are Cl 2 and HClO: As these species have a higher oxidation potential than the OCl -species, which are mainly produced in neutral to basic conditions (pH ≥ 7), the mediated oxidation of the chromophore double bonds is rapidly accomplished.Figure 4b (data obtained using 75 mA cm -2 and 1.17 g L -1 NaCl) shows that Q 90 has a region of minimum at 35 °C and very low pH values.This is consistent with the temperature associated to the results presented in Figure 4a.The increase in Q 90 at temperatures lower than 35 °C is probably due to a decrease in the reaction rate.On the other hand, the increase in Q 90 at temperatures higher than 35 °C is possibly due to a lower Cl 2 solubility, as well as increased O 2 evolution reaction or waste chemical reactions: 3,12,13,20 2HOCl(aq) + OCl -(aq That is why Q 90 increases faster at high temperatures than at low temperatures, as the pH increases. Figure 4c (data obtained at 35 °C and using 1.17 g L -1 NaCl) also shows the best operation conditions at very low   pH values, at which, clearly Q 90 has a minimum value region at about 70 mA cm -2 .These conditions are related to Cl 2 and HOCl generation.The high values of Q 90 at low and high current densities (two maxima) are possibly related to the amount of generated oxidants and to their waste chemical reactions, respectively.
The response surfaces for COD 30 removal are shown in Figure 5.In order to compare Q 90 and COD 30 removal, the response surfaces of the latter are represented as a function of the same independent variables as in Figure 4. Figure 5a (data obtained at 35 °C and using 75 mA cm -2 ) shows that the best pH conditions for COD 30 removal are different from those for Q 90 .The optimal pH region (5 to 7) corresponds to slightly acidic to neutral solutions.Additionally, high NaCl concentrations seem to be unnecessary in the electroxidation of solutions containing the RR 141 dye.Different pH conditions for COD and color removals are not new in the literature. 3,12,15,16,26These results indicate that the dye molecule is slowly mineralized by the Cl 2 and HClO species, mainly due to their low solubility in the solution (reaction 6).On the other hand, neutral to basic conditions (pH ≥ 7) favor the production of high amounts of the OCl -species: As this oxidant concentration increase, a greater COD removal is more likely to be accomplished despite its lower oxidation potential.
Figure 5b (data obtained using 75 mA cm -2 and 1.17 g L -1 NaCl) shows that the COD 30 removal is nearly independent of the pH at the lowest (< 20 °C) and highest (55 °C) temperatures.The highest COD 30 removals were attained at the lowest temperature because at the highest temperature this removal tends to deteriorate due to the oxidant waste reactions (equations 3 to 6).Hence, the maximum rates of degradation are achieved at around 35 °C and pH 5. It should be recalled that low Q 90 values were also optimized at conditions around 35 °C.
The COD 30 removal response surface as a function of pH and current density is shown in Figure 5c (data obtained at 35 °C and using 1.17 g L -1 NaCl).The value of COD 30 removal increases sharply with the current density due to the high rates of oxidants electrogeneration.In these conditions the low pH values lead to the best COD 30 removal performances owing to the higher concentrations of the Cl 2 and HClO species, which have higher oxidation potentials than the OCl -species.
The lowest values of specific energy consumption attained for Q 90 were 0.78 kW h m -3 , for strongly acidic solutions (experiment 22: 75 mA cm -2 , 1.17 g L -1 NaCl, and 35 °C), and about 1.6 kW h m -3 , independently of the temperature (experiments 2 and 11: pH 3, 50 mA cm -2 and 1.75 g L -1 NaCl).These values are much lower than the ones reported by Rajkumar and Kim, 16 who, using a dimensionally stable anode (36 mA cm -2 , 100 ppm RR 141, initial pH 6.2-6.5, 1.5 g L -1 NaCl and 25 °C), reported a value of 3.453 kW h m -3 to attain at least 95% decolorization.
Finally, as it was mentioned before, the possible formation of undesirable chloro-organic derivatives might be a disadvantage of oxidants electrogenerated from chloride ions.This possibility will be investigated in a future work on the electroxidation of the RR 141 dye.

Conclusions
Application of RSM allowed investigating a high number of variables that affect the electrochemical degradation of the RR 141 dye, leading to knowledge of their effects and interactions.The results obtained for the decolorization of the dye solution and its COD removal clearly showed that different experimental conditions are needed in each case.In the presence of chloride ions, the pH was the most significant variable affecting these conditions.This fact is strongly related to the nature and amount of oxidant species generated: i) Cl 2 and HOCl at acidic conditions (pH 3) for decolorization and ii) OCl -at neutral or basic conditions (pH ≥ 7) for COD removal.Finally, RSM applied to the RR 141 electrochemical degradation allowed optimizing the experimental conditions to attain low values of the specific energy consumption, especially when compared to the values reported in the literature.

Figure 3 .
Figure 3. Observed and predicted plot for: (a) charge required for 90% decolorization and (b) COD removal after a 30 min electrolysis.

Figure 4 .
Figure 4. Response surfaces for the charge required for 90% decolorization, as a function of: (a) pH and NaCl concentration (at 35 °C and using 75 mA cm -2 ), (b) pH and temperature (using 75 mA cm -2 and 1.17 g L -1 NaCl) and (c) pH and current density (at 35 °C and using 1.17 g L -1 NaCl).

Figure 5 .
Figure 5.Response surfaces for the COD removal percentage after a 30 min electrolysis, as a function of: (a) pH and NaCl concentration (at 35 °C and using 75 mA cm -2 ), (b) pH and temperature effects (using 75 mA cm -2 and 1.17 g L -1 NaCl) and (c) pH and current density effects (at 35 °C and using 1.17 g L -1 NaCl).

Table 1 .
Range and codification of the independent variables (X i ) used in the experimental design

Table 2 .
Central composite design matrix for the eletrooxidation of the Reactive Red 141 dye and respective predicted and observed charges (90% decolorization) and COD removal * Specific energy consumption related to the charge used for 90% decolorization.