An effective electroanalytical approach for the monitoring of electroactive dyes and intermediate products formed in electro-Fenton treatment

An effective electroanalytical approach for the monitoring of electroactive dyes and intermediate products formed in electro-Fenton treatment

Journal of Electroanalytical Chemistry 808 (2018) 403–411 Contents lists available at ScienceDirect Journal of Electroanalytical Chemistry journal h...

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Journal of Electroanalytical Chemistry 808 (2018) 403–411

Contents lists available at ScienceDirect

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An effective electroanalytical approach for the monitoring of electroactive dyes and intermediate products formed in electro-Fenton treatment


Bakhta Bouzayania,c, Elvira Bocosa, Sourour Chaâbane Elaoudc, Marta Pazosa, Maria Ángeles Sanromána, Elisa González-Romerob,⁎ a b c

Department of Chemical Engineering, University of Vigo, Campus As Lagoas-Marcosende, 36310 Vigo, Spain Department of Analytical and Food Chemistry, University of Vigo, 36310 Vigo, Spain Laboratory of Physical Chemistry of the Solid State, Department of Chemical, University of Sfax, 3000 Sfax, Tunisia



Keywords: Reactive Black 5 H-Acid Electro-Fenton Linear Sweep Voltammetry Screen-printed carbon electrodes Dye degradation monitoring

Reactive Black 5 (RB5) is an electroactive diazo dye compound derivate of H-acid (coupling reaction), which cannot be effectively degraded by conventional or biological processes. In this work, the feasibility of electroFenton (EF) process to treat a simulated effluent polluted by RB5 dye and the effectiveness of electroanalysis by Linear Sweep Voltammetry (LSV) to monitor RB5, intermediates and by-products during the EF treatment have been demonstrated. Several variables on the electrochemical behaviour of RB5 have been determined under EF experimental conditions. The key factors are scan rate and dye concentration (in the presence and the absence of iron) and their effect on shape, position and height of RB5 peaks have been determined to obtain the best sensitivity and selectivity in the voltammetric analysis. Moreover, the iron appears as a strong electrocatalyst that promotes the electron transfer for the oxidation reaction of RB5 on screen-printed carbon electrode, in part, due to a very stable coordinate complex formed with RB5. Besides that, the degradation profiles have revealed the main stages along the EF process and the voltammetric kinetic data has given selective information about the RB5 degradation and the evolution of the electroactive products generated. The formation of aromatic/cyclic organic intermediates, and evolution of carboxylic acids, as well as the inorganic ions released during the treatment were validated by other techniques and a plausible pathway is proposed based on the obtained results.

1. Introduction Around 80,000 tons of synthetic dyes are annually used by a large variety of industries such as textile, leather tanning, hair colouring, food products, etc. [1]. However, 1–10% of employed dyes are lost during their consumption and production, generating 87 million litres of wastewater per day [2]. The important volumes of consumed water and wastewater generated provide aquatic environments with colour and odour, causing an irreparable damage on these ecosystems. Such enormous scale production and the later discharge of these substances into the water streams cause not only non-aesthetic conditions but also eutrophication, entailing a potential risk for living beings health [3]. Within this context, concerns among population and scientific community have force the environmental authorities to create more severe laws in matter of pollution and concentration limits of some chemical. As an example, attempts in this direction in Europe have included the redaction of the Directive 2002/61/EC, created to establish restrictions on the dyeing market and concentration limits of 70 ppm in the use of ⁎

certain azocolourants [4]. Dyes use to be classified according to their chromophore group, being the azo class (eN]Ne) the most used ones at industrial scale. Reactive Black 5 (RB5) is an electroactive diazo dye derivate from Hacid, which is considered having irritating and toxic properties for living beings. Moreover, apart from azo groups RB5 present sulfonic acid groups to increase water solubility that also increase the recalcitrant character of this compound [5]. Severe damages on the ecosystems and aquatic organisms due to their extremely high persistence and low biodegradability [6]. Lamentably, conventional wastewater treatment plants (WWTP) cannot achieve the complete elimination of these azo dyes. Consequently, numerous efforts have been addressed on research of alternative water technologies able to degrade these pollutants [7–10]. As an example, Electrochemical Advanced Oxidation Processes (EAOPs) are currently considered promising candidates able to achieve a complete and quick mineralization of nonbiodegradable organic matter [11]. These processes comprise the generation of hydroxyl radicals (%

Corresponding author. E-mail addresses: [email protected] (E. Bocos), [email protected] (S.C. Elaoud), [email protected] (M. Pazos), [email protected] (M.Á. Sanromán), [email protected] (E. González-Romero). Received 31 March 2017; Received in revised form 20 May 2017; Accepted 19 June 2017 Available online 20 June 2017 1572-6657/ © 2017 Elsevier B.V. All rights reserved.

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OH), highly reactive species up to quickly attack the saturated aromatic rings of organic compounds until their complete elimination. Among them, EF treatment is the most known and popular EAOP [12]. Based in Fenton's reaction (1), on this process, H2O2 is in situ electrogenerated at a carbonaceous cathode continuously fed with oxygen Eq. (2), while Fe2 + is electro-regenerated from Fe3 + to Fe2 + at the cathode surface Eq. (3) [12].

H2 O2 + Fe2 + → OH• + OH− + Fe3 +


O2(g) + 2H+ + 2e− → H2 O2


Fe3 + + e− → Fe2 +


received. During electrochemical assays, an initial concentration of 100 mg L− 1 (50% of purity) was used in all cases. Na2SO4 (10 mM) purchased to Sigma Aldrich was used as electrolyte. Iron (II) sulfate heptahydrate (FeSO4·xH2O, used as catalyst source, 99%) was provided by Sigma-Aldrich and used as iron source. H2SO4, or NaOH were used to adjust the pH and purchased to Prolabo and Sigma Aldrich, respectively. Chromatographic mobile phases were prepared with acetonitrile (analytical grade, Sigma Aldrich) for HPLC analysis and the following inorganic salts: Na2CO3 and NaHCO3 (both from Fluka) for IC analysis. All solutions were prepared in Millipore filtered water and volumetric lab equipment. The following commercial products were used in the intermediate studies by voltammetric techniques: 4-Aminophenol (4AP), acetaminophen, sulfanilic acid, H-acid (4-Amino-5-hydroxynaphtalene-2,7-disulfonate), 1-Hydroxynaphtalene-4-sulfonate (1N4S), 4-Hydroxybenzene sulfonate (4PS), 1,2-Naphtoquinone-4-sulfonate (12NQ4S), 4,5-Dihydroxynaphtalene-2,7-disulfonate (45N27S) from ACROS Organic and quinone (Q), hydroquinone (HQ), catechol, pyrogallol, 2-phenilendiamine (PDA), 2-Hydroxynaphtalene-3,6-disulfonate (2N36S) from Sigma-Aldrich.

Hydroxyl radicals attack the organic compounds and the by-products generated may be carcinogenic substances in nature, being crucial the development and evaluation of alternative analytical approaches able to bring an accurate identification of these intermediates [13]. Usually, the evolution of pollutants and oxidation intermediates formed in EAOPs is evaluated by analytical techniques such as HPLC, HPLC-MS, GC–MS, UV–Vis Spectrophotometer, etc. However, some of them (HPLC-MS or GC–MS) have complex interpretation, being necessaries long time of analysis (HPLC, HPLC-MS, GC–MS) and high volumes (GC–MS) of sample. Nonetheless, electroanalytical methodologies may be an interesting tool for monitoring both kind of inorganic and organic pollutants in water, which applicability on this field has been practically disregarded [14,15]. Recently, we have afforded in previous works the challenge of developing alternative electroanalytical approaches with high versatility and efficiency for monitoring electroactive pollutants and the intermediates generated in EF treatments [16–18]. These studies describe the use of traditional electroanalytical techniques such as Linear Sweep Voltammetry (LSV) or Differential Pulse Voltammetry (DPV) in combination with “screen-printed carbon electrodes” (SPCE), which indeed can be considered as very accurate and sensitive devices, being suited to detect contaminants even at trace levels (ppt) [19], a fundamental task to understand degradation mechanisms. As a matter of fact, a study published by Bounab et al. [17] has demonstrated the potentialities of this methodology identifying electroactive compounds formed during the EF treatment of phenolic derivatives. More recently, Bocos et al. [18] reported in an interesting work the novelty of this system as a powerful tool able to partially elucidate the degradation mechanism of the ionic liquid 1-Butyl-3-methylimidazolium triflate during EF treatment. Remarkably, this new device allowed the authors to describe the strange behaviour found by Ionic Chromatographic (IC) analysis. As the molecule 1-Butyl-3-methylimidazolium triflate has 3 atoms of fluoride, it was expected to found a determined amount in the media as mineralization of organics occurs, however no sign of fluoride in solution was found. Eventually, by using the described electroanalytical system, authors could identify a strong interaction occurring between the anion triflate released in the treatment and iron used as catalyst. Thus, by coupling LSV with SPCE, the authors were able to quickly evaluate the evolution of the treatment causing low disruption of the system, since very little volumes of sample (50 μL) are necessaries. The core of this work deals with the application of LSV technique on the monitoring of the RB5 dye degradation and the electroactive intermediates formed during EF process. Likewise, the formation of aromatic/cyclic organic intermediates and evolution of carboxylic acids, as well as the inorganic ions released during the treatment were validated by other conventional techniques.

2.2. Voltammetric analysis Cyclic (CV) for electrode surface activation measurements and Linear Sweep Voltammetry (LSV) for electroanalysis of RB5 degradation studies were obtained with a potentiostat/galvanostat AUTOLAB PGSTAT30 connected to the SPCE through a connector DropSens DSC, where the electrochemical cell with a conventional three-electrode system is placed. The system was controlled by the General Purpose Electrochemical Experiments Software 4.9.05 (GPES 4.9.05). The cell configuration consists in a working electrode: carbon (DRP-C110) with a 4 mm diameter from DropSens Company (Spain), a pseudo-reference electrode of silver, and a carbon counter electrode. The volume dispense on the cell was 50 μL, enough amount to cover the three electrodes system. The voltammetric measurements were carried out under room temperature. The LSV was the selected technique for monitoring the evolution of parent compound, RB5, and the electro-active compounds obtained from the EF experiments. Except when stated, the LSV voltammetric curves were recorded in a window potential range from − 0.6 V to 1.2 V at a scan rate of 100 mV s− 1 and a step potential of 5 mV. A “cleaning/activation” step of the working SPCE was required between successive runs and previously to any measurement in order to remove any impurity from the surface and to increase the active area on this working electrode [17–19]. For that, 5 cycles by CV in the potential range from − 1.2 to + 1.0 V at 100 mV s− 1 in 0.05 M H2SO4 solution were applied. 2.3. EF experiments All experiments were carried out in a 0.25 L cylindrical glass reactor with 0.20 L of RB5 solution at initial pH of 3. The solution was continuously agitated with a magnetic stirring to avoid concentration gradients. Iron (0.25 mM) was added into the bulk as catalyst and H2O2 was in situ electrochemically generated through the continuous aeration (1 L min− 1) on the cathode surface. The electric parameters were recorded with a multimeter (Fluke 175). A constant current density of 0.3 A was applied by two electrodes connected to a direct power supply (HP model 3662). Carbon felt (Carbon Lorraine, France) and boron doped diamond (BDD) (4–5 μm diamond film thickness and doping level around 2500 ppm) supplied by DIACHEM® (Germany) were selected as cathode and anode, respectively. Carbon felt was placed on the inner wall of the cell, covering the total internal perimeter (6 × 12 cm) while BDD (3 × 3 cm) was centred in the reactor. All the experiments were carried out at ambient temperature. Na2SO4 at concentration 0.01 M was added as electrolyte in order to increase the conductivity of the solution.

2. Materials and methods 2.1. Chemicals Reactive Black 5 (RB5) was purchased to Sigma Aldrich and used as 404

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Samples were taken periodically from the cell and filtered (0.45 μm). The liquid then was analysed for pH, pollutant concentration and total organic carbon.

80 °C in an oven. After 1 h, the samples were analysed by GC–MS using a 6850 Agilent GC equipped with a 5955C VLMSD and a HP-5-MS column. Hydrogen was the carrier gas at a flow rate of 1.2 mL min− 1. For the GC separation, the GC injection port temperature was set at 280 °C. The program temperature started at 50 °C (held during 5 min). Subsequently, the temperature ramp was set at 5 °C min− 1 to 280 °C and maximum temperature was maintained for 5 min. The MS detector was operated in EI mode (70 eV).

2.4. Analytical methods 2.4.1. RB5 concentration The decolourisation degree was evaluated by monitoring the absorption wavelength (λ) by measuring samples with a UV–Vis Spectrophotometer (Unicam Helios β, Thermo Electron Corp.). The assays were done at less twice with an experimental error, calculated as standard deviation, below of 3%. Dye decolourisation was expressed in terms of percentage and associated with a decrease in absorbance Eq. (4).

D = [(Ai − At ) Ai ] × 100

2.4.6. Kinetic method Kinetic data were obtained by employing LSV technique. Observed rate constants, kobs, were calculated by fitting the peak current, ip, to the linear least squares method for zero-order reaction and both exponential first-order reaction and integrated first-order Eq.(6) fittings,


Ln [ (ipt − ip∞) (ip0 − ip∞)] = −kobs t

where D is decolourisation (%); Ai and At are the absorbance value at the maximum wavelength of the dye at the initial and time, respectively.

where ipt is the peak current of the analyte that is monitored at any time, t; ip0 is its initial peak current and ip∞ is its peak current at infinite time (up to 5 half-lives).

2.4.2. Total organic carbon (TOC) analysis The mineralization of RB5 was monitored from TOC decay, determined using a Lange cuvette test (LCK 380) in a Hanch Lange DR 2800. TOC tests were prepared in tubes and digested in LT200 Hach Lange digester. Thus, samples were introduced in the Lange cuvette. Following the instructions of the test, the carbon forms carbon dioxide, which diffuses through a membrane into an indicator solution. The change of the colour of the indicator solution was then evaluated photometrically. From these results, the TOC percentages abatements were calculated from the following equation Eq. (5):

TOC reduction (%) = [ΔTOCt TOC 0] × 100


3. Results and discussion 3.1. EF treatment Initially, the EF experiments were conducted in an electrochemical cell described above. A constant current density of 0.3 A was applied by carbon felt cathode and BDD anode immersed in RB5 solution (100 mg L− 1, 50% of purity) containing Na2SO4 (10 mM) and FeSO4 (0.25 mM) at pH 3. As can be seen in Fig. 1, the visible spectrum of RB5 shows a maximum absorption (λ = 597 nm) in the range of visible light, which is in accordance with the blue colour of RB5 solutions. In addition, the visible spectra of RB5 before and after treatment are in agreement with the results obtained by Damodar and You [20] who reported that this dye exhibits three characteristic absorbance peaks at 595, 310 and 254 nm, which were progressively changed during the treatment. The peak at 595 nm in the visible region is characteristic of the chromophore containing a long conjugated π-system, linking the two azo double bond groups of RB5, which was used to measure the colour removal. The other characteristic absorption peaks at 310 and 254 nm could correspond to naphthalene and benzene ring structure, respectively, which correspond to π–π* electron transitions [21]. It is clear that the visible peak at 595 nm decreased continuously until its disappearance after about 1 h of treatment, leading to complete colour removal. At this time, only 56.3% of the initial TOC was removed.



where ΔTOCt (mg L ) is the removal of TOC at “t” time of treatment and TOC0 (mg L− 1) is the initial value at the beginning of the treatment. 2.4.3. Carboxylic acids Generated aliphatic carboxylic acids were identified by ion-exclusion HPLC using Agilent 1100 HPLC equipped with a Rezex™ ROAOrganic Acid H+ (8%) column (300 × 7.8 mm; Phenomenex) at 55 °C. A 0.005 N of H2SO4 solution at a flow rate of 0.5 mL min− 1 was used as the mobile phase. The identification of intermediates was made by comparison of retention time and UV spectra with those of pure standards. 2.4.4. Ionic analysis The concentration of nitrate and sulfate released during the treatments was measured by ion chromatography (Dionex ICS-3000 equipped with a conductivity detector) using an anionic exchanger column, Metrosep A Supp 5250/4.0 mm (Metrohm) (C.A.C.T.I. University of Vigo). The volume of injection was 20 μL. A solution of 3.2 mM sodium carbonate and 1.0 mM sodium hydrogen carbonate solution at 0.7 mL min− 1 was used as mobile phase. On the other hand, ammonium concentration was analysed by automated segmented flow analyzer (Bran & Luebbe AA3 Autoanalyzer) according to Berthelot reaction (C.A.C.T.I. University of Vigo). 2.4.5. GC/MS analysis The identification of the main degradation products formed during the EF treatment was carried out by GC/MS analysis. Initially, 200 mL of the aqueous sample were extracted three times with 30 mL of ethyl acetate each time. After extraction, samples were dried with a rotary evaporator and taken up to 100 μL of ethyl acetate. Finally, derivatization was carried out by mixing 100 μL of sample with 100 μL n,o bis (trimethylsilyl) trifluoroacetamide (99%) and keeping the mixture at

Fig. 1. Spectra of samples during EF treatment of RB5 (100 mg L− 1, 50% purity), 0.3 A, pH = 3, Na2SO4 (0.01 M).


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Fig. 2. TOC and carboxylic acids concentration (mg/L) evolution along EF treatment of RB5 (100 mg L− 1, 50% purity), 0.3 A, pH = 3, Na2SO4 (0.01 M). TOC: grey bars; malonic acid: black bars; oxalic acid: white bars/black strips and acetic acid: white bars/dots.

However, the TOC profile (Fig. 2) revealed that it is necessary a treatment time of 4 h to achieve the complete mineralization. Thus, the oxidation reactions provoke the cleavage of the dye chromophore group that reduces the colour in the bulk solution. Then, the rate of colour removal was faster than the degradation of aromatic fragments rate which is in agreement with the evolution of the other characteristic peaks in the UV region. For this reason, the changes in the colour are more rapid than those observed in TOC. The oxidation of these complex molecules and the aromatic by-products generated aliphatic short-chain carboxylic acids such as malonic, oxalic or acetic which have been identified and quantified by ion-exclusion HPLC, which justifies the changes in the TOC values (Fig. 2). In addition, changes on the concentration of several ions during EF treatment such as nitrate, ammonium and sulfate with maximum levels of 1.46, 1.57 and 2.77 mg L− 1, respectively, confirmed the rupture of degradation products in which these ions are released to the bulk solution. Based on the evolution of the pollutants and their by-products, by application of voltammetric technique could be possible the rapid decision-making in order to improve the efficiency of the EF process and its energy cost reduction. Then, the next step of this study is the development of an effective electroanalytical approach that can give continuously information of the pollutants level when the EF is applied. This valuable tool will be a helpful technique to elucidate and better understanding a plausible degradation pathway.

Fig. 3. Effect of scan rate on the peak current of RB5 (100 mg L− 1; 50% of purity), Na2SO4 (0.01 M), pH = 3. (A) LSV voltammograms at different scan rates (10, 50, 100, 200, 400, 500, 700 and 1000 mV s− 1) and (B) linear correlation between de peak current and the scan rate for both peaks of RB5.

Moreover, for the same concentration of the analytes, the peak current of RB5 is almost the half that the 2N36S, in good agreement with the specifications of the commercial purity of RB5 from the supplier. As expected, higher peak currents and peak widths were observed as the scan rate was increased. The dependence of peak current for both anodic peaks of RB5 shown a good linear correlation with the scan rate (linear fittings are shown in Fig. 3B), confirming that the reaction between the molecule of RB5 and the interface of SPCE followed an adsorption-controlled process [19,23]. The log ip versus log scan rate plots give us the same information about the process. In our case, the logarithmic plots also display (not shown) a linear correlation with slope values of (0.71 ± 0.02) and (0.88 ± 0.01) with both R2 0.998 for peak 1 and 2 of RB5, respectively, which are higher values to 0.5 that is expected for the pure diffusion-controlled system, but they are not 1.0 that is expected for the species fully confined to the electrode surface. At the same time, the width of peaks was also increased by the effect of the scan rate increasing, thus a scan rate of 100 mV s− 1 was selected to get narrow peaks to obtain better selectivity.

3.2. Electrochemical study Initially, a preliminary electrochemical study was performed in order to identify the characteristics peaks related to RB5, optimize scan rate in LSV and develop the calibration curve of the dye in the presence and the absence of Fe(II). 3.2.1. Effect of scan rate The effect of scan rate on peak current of RB5 in the absence of Fe (II) was evaluated by LSV. Thus, scanning rates from 10 mV s− 1 to 1000 mV s− 1 were applied. As shown in Fig. 3A, in all cases two peaks at positive potentials were observed. The first peak at 0.560 V, more sensitive than peak 2, was assigned to eOH (1) group oxidation. To be sure of this affirmation, the LS voltammogram of 2-hydroxynaphtalene3,6-sulfonate (2N36S) at the same conditions was registered (Supplementary information, Fig. S1B). At a scan rate of 100 mV s− 1, both analytes showed an oxidation peak at very closer potential, being 0.552 V for RB5 and 0.663 V for 2N36S. The latter ones is a hydroxynaphtol with similar structure to RB5 (Supplementary information, Fig. S1A and C), but the higher electron cloud density of 2N36S than RB5 (with eNH2 and eN]Negroups as substituents in the naphthalene skeleton) that leaves to higher oxidation potential for 2N36S [22].

3.2.2. Effect of RB5 concentration in the presence of Fe(II) The calibration curves of RB5 with and without Fe(II) and the effect of the Fe(II) over the peak current of RB5 was studied by LSV (Fig. 4). As expected, good linear correlation was found with dye concentration in both situations. However, as shown in Fig. 4B, when RB5 was added to the Fe(II) solution and was mixed in the media, a new peak (peak 0) at lower potentials (c.a. 0.265 V) was observed. The peak current of this new peak was increased with the increase of RB5 concentration up to 10 mg L− 1 and it was kept constant in presence of higher RB5 concentrations. This behaviour indicates the formation of a coordination complex between Fe2 + and RB5, mainly with the eNH2 and eOH groups, but also with the involvement of two eN]Ne (azo) groups 406

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(peak 0 in Fig. 4B) was shifted to much lower potential (about 300 mV) than RB5 alone (peak 1 in Fig. 4A), probably due to the presence of the two azo groups in RB5 (not present in H-acid molecule) that they take active part in the formation of the coordination complex, being the iron the shuttle for the electron transfer of RB5 oxidation reaction on screenprinted activated carbon electrode and an effective electrocatalysis was observed [16,19,22]. As it can be seen in Fig. 4C, despite either in the presence or the absence of Fe(II), the anodic current of peak 1 of RB5 showed a good linear correlation with concentration in the range from 1 to 100 mg L− 1 and the slope was higher in presence of iron, indicating ones again that the electrochemical redox processes is catalysed by Fe. The Limit of Detection, LOD, of RB5 (calculated as the ratio between three times the standard deviation of background current and the sensitivity) in the presence of Fe was 0.77 mg L− 1, lower in 100 times to regulatory limit [4]. The equations for these linear fittings were: ip1(μA) = (0.159 ± 0.035) + (0.022 ± 0.001) CRB5 (mg L− 1) (R2 = 0.997) at 0.557 V, in the absence of Fe, and ip1(μA) = (0.284 ± 0.007) + (0.027 ± 0.001) CRB5 (mg L− 1) (R2 = 0.998) at 0.605 V, in the presence of Fe2 + 0.2 mM. The peaks observed at 0.064 V (indicated as OGS, Oxygen Groups on Surface) in Fig. 4A and B were attributed to the oxygen groups generated on the surface after the electrochemical activation treatment of working electrode in acidic media, which produce the oxidation of hydroxyl majorities on the carbon surface to cetone/quinone ones, when the potential scan of + 1.0 V was reached in the voltammetric scan. 3.3. EF treatment of RB5 and monitorization by LSV After initial characterisation of the system, LSV was applied to monitor RB5 and by-products formed during EF. Thus, LS voltammograms at different treatment times were registered. Fig. 5 shows the voltammograms of the evolution of RB5 (100 mg L− 1, 50% purity) and intermediate by-products formed during EF treatment in the presence of 0.2 mM of Fe2 +, Na2SO4 (0.01 M) at pH = 3 and 0.3 A of applied current (Fig. 5A) and the degradation profiles of RB5 and intermediates formed along EF treatment (Fig. 5B). As it can be observed in Fig. 5A, at zero time (red line), the three characteristics peaks for RB5 in presence of iron were observed (indicated as peak zero, dark red number; peak 1 and 2, dark blue numbers in this Figure). After 3 min of EF treatment (lime line), both peaks of RB5 at 0.605 V and 1.016 V decrease, while the peak of RB5-Fe is divided in two that were assigned, one to lower potential for RB5-Fe complex that decrease very fast and the other one (shifted to higher potential to the peak potential of peak 0 at t = 0) to the formation of intermediates (peak 4, dark green number). Finally, at this time of treatment, a new peak next to −0.2 V (peak 3, light brown number) and a shoulder close to 0.800 V (peak 5, dark violet number) were also observed. The decolourisation of solution was observed after 7 min of treatment, where the peaks for RB5-Fe complex, RB5 (peak 2) and the shoulder (peak 5) disappeared at all. This is consequence of the rupture of azo groups, leaving in solution H-acid and sulfanilic acid derivatives. Once the azo groups were broken by action of %OH and the H-acid was generated in the EF process, suggesting that the complex with iron was also formed. The peak potential shift to 0.4 V and the peak 4 can explain the formation of this complex (Supplementary information, Fig. S2). On the other hand, the sulfanilic acids derivatives suffer a very fast attack by hydroxyl radicals to produce 4-hydroxybenzene sulfonic acid (4PS) (peak 5) and, simultaneously, its transformation to hydroquinone/quinone (peak 3), acetaminophen and small contribution of sulfanilic acid (peak 4), being the reason that the peak current increase in the profile evolution for each of these three peaks (Fig. 5B). Note that the anodic peak close to 0.06 V (OGS) with changes on its shape and height with time was also observed. This peak was also consequence of

Fig. 4. Effect of concentration on the peak current of RB5 (from 1 mg L− 1 to100 mg L− 1; 50% of purity). Voltammograms in the absence (A) and in the presence (B) of Fe(II) 0.2 mM, and both calibration linear fits graph (C). Conditions: RB5 in 10 mM of Na2SO4 at pH 3. The LSV voltammograms are scanned at 50 mV s− 1 (scan rate). OGS peaks were attributed to Oxygen Groups (hydroquinone/quinone) on the Surface obtained after the activation treatment of working electrode.

present in the molecule [24]. On the other hand, the addition of H-acid (4-Amino-5-hydroxynaphtalene-2,7-disulfonate), a compound with similar structure to RB5 (both eNH2 and eOH groups are presented in the molecule) to a solution that contain Fe2 + gave place to a change of the uncoloured solution to dark red colour and a very sensitive voltammetric peak appeared that suggest the formation of a very stable coordination complex with iron. Fig. S2A in Supplementary information illustrates this coordination complex formed between Fe2 + and H-acid. The peak potential of H-acid-Fe complex was centred at 0.411 V (lower in 83 mV to H-acid in the absence of Fe2 +) and its peak current was at least 21 times higher than obtained for the H-acid alone, indicating the high catalytic effect of Fe2 + [16,19,22]. In conclusion, this behaviour for H-acid, in the absence and the presence of Fe(II), was the same that observed for RB5 under same experimental conditions, but the peak potential for RB5-Fe complex 407

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Fig. 5. (A) Voltammograms of the evolution and (B) degradation profiles of RB5 and intermediate by-products formed along EF treatment in the presence of 0.2 mM of Fe2 +, Na2SO4 (0.01 M) at pH = 3 and 0.3 A of applied current. OGS peaks were attributed to both Oxygen Groups on the Surface obtained after the activation treatment of working electrode and the contributions of hydroquinone/quinone reactions from the intermediates generated along EF treatment. LSV Conditions: voltammograms recorded in the potential window from − 0.6 V to 1.2 V at 100 mV s− 1. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)

the oxidation of hydroquinone to quinone from intermediates generated that appears at the same potential that the anodic reactions of same oxygen functionalities (OGS) from electrode surface after activation and the evolution of this peak along EF treatment was not conclusive. At longer times of treatment (up to 7 min), the peak current of peak 1 (RB5), peak 3 (quinone derivatives generated such as 1,2Naphtoquinone-4-sulfonate (12NQ4S) and quinone, Fig. S3 in Supplementary information) and peak 4 (mainly for the formation of Hacid, 4,5-Dihydroxynaphtalene-2,7-disulfonate (45N27S), 4-hydroxybenzene sulfonic acid (4PS), acetoaminophen and sulfanilic acid intermediates, Figs. S2 and S3 in Supplementary information) were decreased continuously until they disappeared after about 25 min of EF treatment (Fig. 5). In conclusion, the stages of EF treatment at any time can be seen from degradation profiles of RB5 (Fig. 5B), following the evolution of the peaks. That is, the rupture of azo groups (peak 0) and the opening of naphthalene (peak 1) and benzene (peaks 3 and 4) rings can be detected at the time that the peak current of each peak reach zero current value. Voltammetric kinetic data were obtained by monitoring the three peaks of RB5 and the rate constant were calculated by using the methodology described in Section 2.4.6. Fig. 6 illustrates the peak current of the main peaks (0, 1 and 2), ip in μA, versus time plots and ln (ipt − ip∞) versus time plots for RB5 degradation by EF treatment. Fig. 6A shows the evolution of voltammetic peak 0 of RB5-Fe complex with time plot depicting complex loss that follows a typically zero-order

Fig. 6. Peak current, ip (μA), versus time plots and ln(ipt − ip∞) versus time plots for RB5 degradation by EF treatment: peak 0 of RB5-Fe complex (A) and peak 1 (B) and peak 2 (C) of RB5.

catalysed reaction and yields an observed rate-constant kobs of (67 ± 7)×10− 2 M min− 1 (R2 = 0.99). For RB5 degradation peaks (Fig. 6B and C), the rate constant values, kobs (integrated pseudo-first order reactions, R2 = 0.99), were (15.4 ± 0.9)× 10− 2 min− 1 and (140 ± 16)× 10− 2 min− 1 for the voltammetric peaks 1 and 2 of RB5, respectively. The comparison of the kinetic data of RB5 degradation with those obtained for H-acid degradation (Supplementary information, Fig. S2B), where the H-acid loss is quite slow (kobs = (2.6 ± 0.1)× 10− 2 min− 1, and R2 ≥ 0.991 for about 5 half-lives), demonstrating the important role of azo groups in RB5 molecule that, once broken the bonds, the RB5 structure was much fragile and, thus, the attack to this structure by the %OH radical was much easier and consequently, faster its degradation. Note that the peak current lightly increase during the evolution profile for peak 1 of RB5 between 7 and 12 min of EF treatment time (Figs. 5B and 6B), indicating that the 2N36S was formed at this interval of time, together with other by-products as 1-Hydroxynaphtalene-4-sulfonate (1N4S) and sulfanilic acid (Figs. S1 and S3 in Supplementary information), where the 2N36S was degraded by action of hydroxyl radicals with similar 408

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Fig. 7. Proposed degradation pathway of RB5 under EF treatment. Acronyms meaning of the by-products formed: 4-Aminophenol (4AP), acetaminophen, sulfanilic acid, 1Hydroxynaphtalene-4-sulfonate (1N4S), 4-Hydroxybenzene sulfonate (4PS), 1,2-Naphtoquinone-4-sulfonate (12NQ4S), 4,5-Dihydroxynaphtalene-2,7-disulfonate (45N27S), quinone (Q), hydroquinone (HQ), pyrogallol and 2-Hydroxynaphtalene-3,6-disulfonate (2N36S).

order of rate constant, kobs, with a value of (14.8 ± 0.9)× 10− 2 min− 1 (integrated pseudo-first order reaction R2 = 0.996 for about 4 halflives) (Supplementary information, Fig. S1C).

3.4. Proposed degradation pathway Regarding the intermediates detected during EF treatment by GC/ MS, HPLC and LSV a plausible degradation pathway is proposed in Fig. 7. These analyses revealed the conversion of the molecule of RB5 in shorter compounds considering %OH as the main oxidant of RB5. 409

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The azo groups, responsible of colour, are very susceptible to •OH attack given the presence of an easily breakable π bond [13]. Thus, as expected the initial attack of RB5 azo groups took place, generating Hacid and sulfanilic acid derivatives. As a matter of fact, the rupture of azo groups contained in RB5 dye, triggers the fast decolourisation of solution observed after 7 min of treatment. Likewise, aromatic amines (eN]Ne) were detected by GC/MS, being formed by the partial or complete cleavage of this bond over reduction reactions that take place at the cathode analysis [1]. In the second step, the oxidation of H-acid provokes the loss of an amine group, releasing in solution the intermediate 2-hydroxynaphthalene-3,6-disulfonic acid disodium (2N36S) and NH4+ ions, further oxidized in NO3−. Afterwards, the attack of 2N36S by %OH caused its hydroxylation and transformation in 4,5 dihydroxynaphtalene-2,7-disulfonic acid (45N27S). Simultaneously, sulfanilic derivatives formerly detected are oxidized by %OH, being transformed in sulfanilic acid, which are further oxidized in 4-hydroxybenzenesulfonic acid and 4-aminophenol. Subsequently, de-amination of 4-aminophenol and 4-hydroxybenzene sulfonate generates hydroquinone and acetaminophen, respectively, with release of NO3− and SO42 − in solution. Then, the attack of later sulfanilic and H-acid derivatives generates other intermediate products such as quinone and 1,2naphthoquinone-4-sulfonic acid. Subsequent rupture of benzene rings present in quinone, hydroquinone, sulfanilic acid, 4-aminophenol, pyrogallol and acetaminophen leads to the formation of short carboxylic acids such as oxalic acid, malonic acid and acetic acid. Eventually, the formation of a complex between oxalic acid and Fe3 + slowed down the destruction of organics, remaining constant the TOC concentration in solution after 4 h of treatment, due to recalcitrant nature of this complex.

electroanalysis by LSV on activated SPCE to monitor RB5, intermediates and by-products during the EF treatment have been demonstrated. Conflict of interest Authors declare no conflict of interest. Acknowledgements This research has been funded by the Spanish Ministry of Economy and Competitiveness (MINECO), Xunta de Galicia and ERDF Funds (Projects CTM2014 52471-R, CTQ2015-71650-REDT, CTQ2015-71955REDT and GRC 2013/003). The authors would like to thank to University of Sfax for the grant of B. Bouzayani and MINECO for financial support of E. Bocos under FPI program. The authors are grateful to J. Pérez-Juste (Colloid Group/UVIGO) for his assistance with kinetic data results. Appendix B. Supplementary data Structures of RB5 and intermediates; LSV voltammograms at EF conditions of RB5, 2-Hydroxynaphtalene-3,6-Sulfonate (2N36S) and Hacid; Peak current, ip (μA), versus time plots and ln(ipt − ip∞) versus time plots of 2N36S and H-acids under their EF treatment. Supplementary data associated with this article can be found in the online version, at doi: 035. References [1] A.J. Méndez-Martínez, M.M. Dávila-Jiménez, O. Ornelas-Dávila, M.P. ElizaldeGonzález, U. Arroyo-Abad, I. Sirés, E. Brillas, Electrochemical reduction and oxidation pathways for Reactive Black 5 dye using nickel electrodes in divided and undivided cells, Electrochim. Acta 59 (2012) 140–149. [2] P. Aravind, H. Selvaraj, S. Ferro, M. Sundaram, An integrated (electro- and biooxidation) approach for remediation of industrial wastewater containing azo-dyes: understanding the degradation mechanism and toxicity assessment, J. Hazard. Mater. 316 (2016) 203–215. [3] C.A. Martínez-Huitle, E. Brillas, Decontamination of wastewaters containing synthetic organic dyes by electrochemical methods: a general review, Appl. Catal. B Environ. 87 (3–4) (2009) 105–145. [4] Directive 2002/61/EC, DIRECTIVE 2002/61/EC OF THE EUROPEAN PARLIAMENT AND OF THE COUNCIL of 19 July 2002 Amending for the Nineteenth Time Council Directive 76/769/EEC Relating to Restrictions on the Marketing and Use of Certain Dangerous Substances and Preparations (azocolourants), European Union, 2002. [5] M.A. Kertesz, A.M. Cook, T. Leisinger, Microbial metabolism of sulfur- and phosphorus-containing xenobiotics, FEMS Microbiol. Rev. 15 (1994) 195–215. [6] E. Bocos, M. Pazos, M.A. Sanromán, Electro-Fenton decolourisation of dyes in batch mode by the use of catalytic activity of iron loaded hydrogels, J. Chem. Technol. Biotechnol. 89 (8) (2014) 1235–1242. [7] E. Forgacs, T. Cserháti, G. Oros, Removal of synthetic dyes from wastewaters: a review, Environ. Int. 30 (2004) 953–971. [8] A.B. dos Santos, F.J. Cervantes, J.B. van Lier, Review paper on current technologies for decolourisation of textile wastewaters: perspectives for anaerobic biotechnology, Bioresour. Technol. 98 (2007) 2369–2385. [9] T. Robinson, G. McMullan, R. Marchant, P. Nigam, Remediation of dyes in textile effluent: a critical review on current treatment technologies with a proposed alternative, Bioresour. Technol. 77 (2001) 247–255. [10] S. Song, L. Xu, Z. He, J. Chen, X. Xiao, B. Yan, Mechanism of the photocatalytic degradation of C.I. reactive black 5 at pH 12.0 using SrTiO3/CeO2 as the catalyst, Environ. Sci. Technol. 41 (2007) 5846–5853. [11] C. Martínez-Huitle, M. Rodrigo, I. Sirés, O. Scialdone, Single and coupled electrochemical processes and reactors for the abatement of organic water pollutants: a critical review, Chem. Rev. 115 (2015) 13362–13407. [12] F.C. Moreira, R.A.R. Boaventura, E. Brillas, V.J.P. Vilar, Electrochemical advanced oxidation processes: a review on their application to synthetic and real wastewaters, Appl. Catal. B Environ. 202 (2017) 217–261. [13] S. Hisaindee, M.A. Meetani, M.A. Rauf, Application of LC-MS to the analysis of advanced oxidation process (AOP) degradation of dye products and reaction mechanism, Trends Anal. Chem. 49 (2013) 31–44. [14] V. Pifferi, F. Spadavecchia, G. Cappelletti, E.A. Paoli, C.L. Bianchi, L. Falciola, Electrodeposited nano-titania films for photocatalytic Cr(VI) reduction, Catal. Today 209 (2013) 8–12. [15] V. Pifferi, G. Cappelletti, C. Di Bari, D. Meroni, F. Spadavecchia, L. Falciola, Multiwalled carbon nanotubes (MWCNTs) modified electrodes: effect of purification and functionalization on the electroanalytical performances, Electrochim. Acta 146

4. Conclusions In this work, the application of electroanalysis by LSV as powerful tool to elucidate the degradation mechanism of the pollutants seems to be demonstrated and the trends found are summarizing in the following bullet points:

• The electrochemical process of RB5 is adsorption–controlled, then the SPCE surface should be cleaned and activated before use The • RB5 as ligand and Fe(II) complex is formed and Fe(II) catalyze the oxidation reaction of RB5 on SPCEs • The EF removal of RB5 is reached at 30 min and LSV technique is a •

• •

good tool for monitoring the EF treatment of electroactive compounds (reactives and intermediates) The EF monitorization by LSV shows three main steps: 1. Rapid dye decolourisation (peak 0 evolution of zero-order catalysed reaction) 2. Naphtalene rings breakdown at 20 min (peak 1 evolution of pseudo-first-order reaction) 3. Benzene rings breakdown at 25–30 min (peaks 3 and 4 evolution of intermediates) The EF monitorization by HPLC shows that oxalic acid degradation is delayed by the formation of Fe-complex with oxalate with stoichiometry 1:3 The ionic analysis shows the formation of NH4+, NO3− and SO42 − and the TOC shows that the mineralization is reached up to 4 h

According to these statements, it is possible to conclude that the EF treatment, under the conditions used in this study, of the electroactive dye was completely eliminated and the TOC concentration remaining at the last stage of the process is clearly due to the recalcitrant nature of the complex formed between the Fe3 + and the by-product oxalate detected by HPLC. Therefore, the feasibility of EF process to treat a simulated effluent polluted by the diazo dye RB5 and the effectiveness of 410

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