Advanced oxidation treatment of malachite green dye using a low cost carbon-felt air-diffusion cathode

Accepted Manuscript Title: Advanced oxidation treatment of malachite green dye using a low cost carbon-felt air-diffusion cathode Author: Jennifer A. Ba˜nuelos Orlando Garc´ıa-Rodr´ıguez Abdellatif El-Ghenymy Francisco J. Rodr´ıguez-Valadez Luis A. God´ınez Enric Brillas PII: DOI: Reference:

S2213-3437(16)30091-4 http://dx.doi.org/doi:10.1016/j.jece.2016.03.012 JECE 1014

To appear in: Received date: Revised date: Accepted date:

30-12-2015 2-3-2016 7-3-2016

Please cite this article as: Jennifer A.Ba˜nuelos, Orlando Garc´ia-Rodr´iguez, Abdellatif El-Ghenymy, Francisco J.Rodr´iguez-Valadez, Luis A.God´inez, Enric Brillas, Advanced oxidation treatment of malachite green dye using a low cost carbon-felt air-diffusion cathode, Journal of Environmental Chemical Engineering http://dx.doi.org/10.1016/j.jece.2016.03.012 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Advanced Oxidation Treatment of Malachite Green Dye Using a Low Cost Carbon-Felt Air-Diffusion Cathode

Jennifer A. Bañuelosa, Orlando García-Rodríguezb, Abdellatif El-Ghenymyc, Francisco J. Rodríguez-Valadezb, Luis A. Godínezb,* Enric Brillasc,** a

Centro de Innovación Aplicada en Tecnologías Competitivas, Departamento de

Investigación y Posgrado, Omega 201, Fraccionamiento Industrial Delta. P.O. Box 37545, León, Guanajuato, México. b

Centro de Investigación y Desarrollo Tecnológico en Electroquímica. Parque

Tecnológico Qro Sanfandila, P.O. Box 76703, Pedro Escobedo, Querétaro, México. c

Laboratori d'Electroquímica dels Materials i del Medi Ambient, Departament de Química

Física, Facultat de Química, Universitat de Barcelona, Martí i Franquès 1-11, 08028 Barcelona, Spain.

Corresponding author. *E-mail: [email protected] (Luis A. Godínez) www.cideteq.mx Tel +52(442)2116006; Fax +52(442)2116007 **E-mail: [email protected] (E. Brillas) 1

Abstract A low cost carbon-felt (CF) material has been successfully employed as an air-diffusion cathode for the generation of H2O2 from O2 reduction in 0.05 M Na2SO4 at pH 3. Using a BDD (Boron Doped Diamond)/CF air-diffusion electrode (3 cm2), a maximum of 94 mg L1

of H2O2 were accumulated for the best current density value identified (21.7 mA cm-2).

These results were technically and economically better than other performances observed for similar cathode materials in O2-saturated solutions. An electrochemical reactor utilizing these electrodes was employed to degrade Malachite Green (MG) dye in the same electrolyte containing 0.5 mM of Fe2+ using the electro-Fenton (EF) and photoelectroFenton (PEF) processes. The main oxidant was HO● formed at the BDD surface from water oxidation and in the bulk from Fenton’s reaction between H2O2 and added Fe2+. As expected, the PEF process was more powerful than the EF due to the enhancement of HO● production by photolysis of Fe(III)-hydroxy species and of Fe(III) complexes of the oxalate of the dye, as well as of intermediates. The effect of MG concentration on decolorization and mineralization rates, mineralization current efficiency and energy consumption for both processes were assessed.. The CF air-diffusion cathode lost progressively performance upon consecutive PEF treatment of a 150 mg L-1 MG owed to the gradual deposition of Fe species, as well as an enrichment of O, Na and S species on its surface. The deposition of Fe species was reversible since these were dissolved in the medium in consecutive trials without the need of adding new amounts of Fe2+. Keywords: Malachite Green; Carbon felt; Water treatment; Fenton process; Photo-ElectroFnton process; Anodic oxidation.

2

1. Introduction Malachite green (MG) is a cationic triphenylmethane dye also called Basic Green 4. It is extensively used as biocide in the aquaculture industry because of its effectiveness against important protozoal and fungal organisms and relatively low cost [1, 2]. Furthermore, it is also commonly used for the dyeing of cotton, silk, paper, leather and also in manufacturing of paints and printing inks [3]. Unfortunately, MG is toxic to human cells and might cause liver tumor formation and pose a serious risk to aquatic life [4]. Despite this, due to its availability and low cost, it is still widely used. Several works have reported MG degradation by different conventional processes, including biological treatments [5, 6], sunlight irradiation [7] and sonochemical degradation [8], among others. Over the last 20 years, advanced oxidation processes (AOPs) have emerged as a promising option for degradation of dyes [9, 10]. AOPs have as the common feature, the generation and use of hydroxyl radical (HO●) as a strong oxidant of organic pollutants. The destruction of MG by AOPs using TiO2 photocatalysis [11], Fenton [12], photo-Fenton [12, 13] and zero-valent iron [14], has been described. Among the AOPs, the Fenton reagent, a mixture of ferrous ion and hydrogen peroxide, is one of the most commonly used methods due to its high efficiency, simplicity in operation and wide range of treatable substances [15]. In this method, HO● is produced in the solution from the classical Fenton’s reaction (1) with an optimum working pH of 2.8: H2O2 + Fe2+  Fe3+ + OH + OH

(1)

A variant of this method is the photo-Fenton process, where the working solution is also treated with UV light to accelerate the degradation process taking advantage of: (i) the photoreduction of Fe(OH)2+, the pre-eminent species at pH 3, to regenerate Fe2+ with more 3

HO● production by reaction (2) and (ii) the photodecomposition of generated Fe(III)carboxylate intermediates via reaction (3) [15, 16]. Fe(OH)2+ + hv  Fe2+ + HO●

(2)

Fe(OOCR)2+ + hv  Fe2+ + CO2 + R

(3)

Recently, the degradation of a large variety of dyes has also been studied by different electrochemical AOPs (EAOPs) such as electro-Fenton (EF) [17-20] and photoelectroFenton (PEF) [21-23]. In these technologies, H2O2 is directly supplied to the contaminated solution by the two-electron reduction of injected O2 gas at a carbonaceous cathode [15]: O2(g) + 2 H+ + 2 e  H2O2

(4)

Aside from the use of inexpensive and safer H2O2, EF and PEF also possess another important advantage over Fenton and photo-Fenton treatments. Using the electrochemical approach, Fe2+ can be continuously regenerated from the cathodic reduction of Fe3+, strongly enhancing the rate of Fenton’s reaction (1) and hence the mineralization of organics. As a result, EF and PEF have a great efficiency to remove pollutants from wastewater without adding chemicals, with acceptable energy consumption [16, 24]. When EF and PEF are carried out in an undivided cell, organics are also destroyed by physisorbed M(HO●) radicals formed at the anode (M) surface and HO● produced in the bulk from Fenton’s reaction (1) [25]. It has been found that the best anodes for such EAOPs are non-active boron doped diamond (BDD) thin-film electrodes, because they possess a much greater O2-overpotential than other conventional anodes with a very weak physisorption of BDD(HO●) radical produced by reaction (5) that promote organics degradation [25-29]. 4

BDD + H2O  BDD(HO●) + H+ + e

(5)

The high efficiency of EF and PEF for the mineralization of many organic pollutants, including dyes, has been well-proven using carbon-polytetrafluoroethylene (PTFE) gasdiffusion electrodes (GDEs) fed with O2 gas or air for H2O2 production from reaction (4) [15, 16, 25], which was introduced by Brillas et al. [30]. These kind of electrodes consist of a carbon-PTFE cloth bounded to a current collector in which O2 (pure or from air) is injected into its inner side, crosses the 3D electrode and is reduced in its outer side, in a three zone interface where the carbon, electrolyte and gas are all in contact. However, their size and surface area are commonly small, with high cost and instability for long term operation [31]. To try to overcome these drawbacks, we have checked the use of a low cost carbon-felt (CF) material as an alternative GDE. This paper presents the results obtained in this study, showing that this CF air-diffusion cathode is able to produce sufficient amount of H2O2 and possesses sufficient stability for long EF and PEF treatments. The oxidation ability of these EAOPs was checked in acidic sulfate solutions polluted with MG using a BDD anode at the best current found for H2O2 generation. The effect of dye concentration on the performance of the decolorization and mineralization processes of MG was explored. The cost for both treatments was comparatively analyzed, as well.

2. Experimental 2.1. Chemicals Commercial Malachite Green (83% purity as oxalate salt, C52H54N4O12, C.I. 42000) was supplied by Panreac and used as received. Anhydrous sodium sulfate, used as background electrolyte, and iron (II) sulfate heptahydrate, used for Fenton’s reaction, were 5

of analytical grade purchased from Fluka. The solution pH was initially regulated to pH 3.0 with analytical grade sulfuric acid supplied by Merck. The solutions were prepared with high-purity water obtained from a Millipore Milli-Q system with resistivity > 18 M cm at 25 ºC. Organic solvents and other chemicals employed were either of HPLC or analytical grade supplied by Merck, Fluka and Avocado. 2.2. Electrolytic system Fig. 1 depicts a sketch of the electrolytic system used for the EF and PEF trials. All the experiments were performed in an open, one-compartment cylindrical glass cell equipped with a double jacket chamber for circulation of external thermostated water to regulate the solution temperature at 25 °C using a Thermo Electron Corporation HAAKE DC 10 thermostat. The 100 mL solution was vigorously stirred with a magnetic bar at 800 rpm to ensure mixing and transport of reactants towards/from the electrodes. The cell contained a 3 cm2 BDD thin film electrode purchased from NeoCoat (La-Chaux-de-Fonds, Switzerland) as the anode and a 3 cm2 CF air-diffusion electrode supplied by Grupo Roe (Mexico) as the cathode, separated about 1 cm. The cathode was mounted as described elsewhere [18] and was fed with an air flow rate of 20 mL min−1 for H2O2 electrogeneration. The trials were made at constant current provided by an Amel 2053 potentiostat-galvanostat and the potential difference between electrodes was directly read on a Demestres 6005 digital multimeter. In PEF experiments, a Philips TL 6-W black light blue tube lamp was placed 5 cm above the solution. The lamp supplied a wavelength range of 320-400 nm centered at max = 360 nm, with an average power density of 5 W m-2, as detected with a Kipp&Zonen CUV 5 UV radiometer. Before using the BDD anode, the electrode was polarized in 100 mL of 0.05 M Na2SO4 solution of pH 3.0 at 300 mA for 240 6

min to clean the impurities of its surface. For the same purpose and before each trial, the CF cathode was washed with 10% H2SO4 solution and ultrasonicated in pure water for 20 min. 2.3. Instruments and analytical procedures The solution pH was measured with a Crison GLP 22 pH-meter. Aliquots withdrawn from the electrolyzed solutions were neutralized to stop the degradation process and filtered with 0.45 µm PTFE filters obtained from Whatman before analysis. Hydrogen peroxide concentration was determined by measuring the light absorption of the titanic-hydrogen peroxide complex at λ = 408 nm using an Unicam 1800 UV-Vis spectrophotometer [32, 33]. The decolorization of the dye solutions was followed from the absorbance decay at the maximum wavelength λmax = 616 nm using the spectrophotometer. Measurements were made with 1:10 diluted samples and the percentage of color removal was calculated from the expression [15, 16]: Color removal (%) =

A0 - At A0

 100

(6)

where A0 and At correspond to the absorbance of the initial solution and after a time t of electrolysis, respectively. The mineralization of MG solutions was monitored using their total organic carbon (TOC) abatement, measured with a Shimadzu VCSN total organic carbon analyzer. Samples of 50 µL were injected into the analyzer and reproducible TOC values with an accuracy of ±1% were obtained. The concentrations of NH4+ and NO3 ions were determined by ion chromatography following procedures reported elsewhere [21]. The surface morphology of the CF electrode after the different treatments studied was analyzed using scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDS). In all cases, the samples were placed in a gold plate and electron 7

acceleration voltage of 15 kV was applied for SEM and EDS measurements using a JEOL 5400LV system.

3. Results and discussion 3.1. H2O2 accumulation in the electrolytic cell To evaluate the performance of the BDD/CF air-diffusion cell for EF and PEF processes, several assays were made. In this way, the concentration of H2O2 accumulated in 100 mL of a 0.050 M Na2SO4 solution of pH 3.0, at 25 ºC by applying a current density of 21.7 mA cm-2 for 240 min was determined. In these experiments, no change in the pH solution was observed. Fig. 2a depicts the accumulation of H2O2 in the solution as a function of electrolysis time when air was either injected to the cathode or directly to the solution. In the latter case, the solution was bubbled with air during 45 min before the pass of current to achieve airsaturation conditions [24]. A gradual accumulation of H2O2 until a steady-state content in the solution can always be achived, which is a common behavior for this kind of electrolytic cells [16, 25]. The steady state is attained when the electrogeneration rate for H2O2 from reaction (4) becomes equal to its destruction rate, pre-eminently by its oxidation at the BDD anode to O2 gas with generation of the weak oxidant hydroperoxyl radical (HO2) via reactions (7) and (8) [25]: H2O2  HO2 + H+ + e

(7)

HO2  O2(g) + H+ + e

(8)

8

Fig. 2a shows that in the steady state, H2O2 concentrations of 94 and 30 mg L-1 were found with and without injecting air to the cathode, respectively. This finding indicates that the flow of pumped air through the 3D porous structure of the CF cathode supplies a much larger quantity of O2 gas to the electrolyte-cathode surface giving rise to a larger H2O2 electrogeneration. In contrast, the low solubility of O2 in the solution bulk limits the H2O2 content in solution. 3.1.1. Effect of applied current on H2O2 production The influence of applied current density on H2O2 accumulation using the BDD/CF air-diffusion cell was studied in a range between 11.6 and 33.3 mA cm-2, which are common values for EF and PEF processes. The results obtained for 240 min of electrolysis are presented in Fig. 2b. As can be seen in this figure, a steady concentration of 28 mg L-1 was obtained for 11.6 mA cm-2, which only rose to 31 mg L-1 at 15 mA cm-2, suggesting a low increase in rate of reaction (4) to produce the peroxyde species. In contrast, when the current density grew to 18.3 mA cm-2, a much higher H2O2 concentration of 85 mg L-1 was found, increasing up to 94 mg L-1 at 21.7 mA cm-2. This means that in the 18.3-21.7 mA cm-2 range, reaction (4) was effectively accelerated, thus enhancing H2O2 production. The ulterior rise to 33.3 mA cm-2, however, caused a large inhibition in H2O2 accumulation yielding a final content of 6.3 mg L-1. This suggests that at this high current, the competitive reduction of H+ to H2 gas and/or the further two-electron reduction of H2O2 to OH predominates [15], thereby diminishing the rate of reaction (4). Following these results, subsequent experiments were conducted using a current density of 21.7 mA cm-2. 3.1.2. Comparative cost of the cathodic material for H2O2 production.

9

The cost of the CF used in this work was compared with that of other carbonaceous materials reported in the literature to generate H2O2 in an undivided cell [19, 21, 34, 35]. Table 1 collects the cathode material used, its geometric area in electrolysis, the applied potential or current, the maximum H2O2 content obtained and its cost in US dollars cm-2. Since O2 reduction from reaction (4) relies on the active sites of the cathode and the type of material employed [31, 32], the maximum H2O2 content accumulated per cm2 of electrode material is also given in Table 1. A look at this table shows that CF electrodes are comparatively cheaper (only 0.021 US dollars cm-2) than carbon cloth, graphite cloth and activated graphite felt. These data also shows that the use of CF is advantageous due to its maximum H2O2 production (31.3 mg L-1 cm-2) when used as an air-diffusion cathode, a possibility not tested for the other materials. 3.2. Electro-Fenton degradation of Malachite Green solutions EF experiments were made by electrolyzing 100 mL of a 150 mg L-1 MG solution in 0.05 M Na2SO4 with 0.5 mM Fe2+ at pH 3.0 using a BDD/CF cell with and without injecting air to the cathode at 21.7 mA cm-2 in order to check the effect of H2O2 production on the performance of dye degradation. The Fe2+ concentration of 0.50 mM was chosen because it is reported as optimal for the treatment of many organics by EF [15, 16]. Fig. 3a shows that the solution was gradually decolorized in both cases. While total decolorization was attained after 120 min for the CF air-diffusion cathode, a slightly lower color removal of 97% was observed when air was bubbled through the solution. A clear superiority of the air-diffusion cathode approach can be observed at 30 min of electrolysis when near 85% color removal was already achieved. At this time, MG is attacked by similar amounts of BDD(HO●) formed from reaction (5) in both experiments and in larger

10

extent by the greater amounts of HO● produced in the bulk from Fenton’s reaction (1) when more H2O2 is generated at the air-diffusion cathode. At longer times, the decolorization rate is very slow and quite similar for both experiments, suggesting the formation of more recalcitrant aromatic intermediates that absorb at analogous wavelength to the max of the dye and that are very slowly destroyed by hydroxyl radicals, as reported for other dyes [19]. Fig. 3b shows a very different behavior for the mineralization process. TOC was much more rapidly reduced using the CF air-diffusion cathode, up to 61% in 360 min, a value much higher than the 43% found by injecting air to the solution. From these results, it is possible to conclude that the enhancement of the oxidation power of the EF process in the air-diffusion cathode case is clearly due to the greater production of H2O2 (see Fig. 2a) leading to higher quantities of HO● from Fenton's reaction (1) that destroy more rapidly the mineralizable organic products of MG. The influence of dye concentration in the 50 and 150 mg L-1 range on the performance of the EF process with a BDD/CF air-diffusion cell at the best current density of 21.7 mA cm-2 was explored. During these trials, the initial pH of 3.0 did not vary significantly and final values near 2.8-2.9 were found. Fig. 4a show that greater MG concentration caused a slight drop in the percentage of color removal at the beginning of the process, although all the solutions became completely decolorized after 120 min. For example, after 30 min of electrolysis 92%, 89% and 86% of color removal was attained for 50, 100 and 150 mg L-1 MG, respectively. At longer times, the color decayed very slowly, probably due to slow oxidation of colored aromatic products, as pointed out above. The same tendency can be observed in Fig. 4b for the normalized TOC abatement, which diminished at larger dye content. After 360 min of EF treatment, 66%, 64% and 61% mineralization was obtained for 50, 100 and 150 mg L-1 of dye, respectively. The decay of 11

the percentage of color and TOC removals with increasing dye concentration is therefore consistent with the reaction of a larger amount of organics with similar quantities of produced hydroxyl radicals. 3.3. Degradation of Malachite Green solutions by photoelectro-Fenton The low mineralization reached for the MG solutions using EF is indicative of the formation of intermidiates that are recalcitrant to the action of BDD(HO●) and HO●. As previously mentioned, photo-assisted Fenton processes are known to be effective with these intermidiates and therefore, the effect of UV light irradiation on the degradation process was investigated by treating the above solutions with the PEF process. Fig. 4c shows the color performance of the PEF process and its inspection reveals that the solution color was more rapidly lost at the beginning of PEF with 50 mg L-1 MG than for 150 mg L-1 MG. Nevertheless, for all dye contents total decolorization was found in 80-90 min, a time about 30-40 min shorter than that observed for the EF treatments (see Fig. 4a). The faster loss in color by PEF suggests the production of additional HO● induced from the photolytic reaction (2), the photodecomposition of complexes of Fe(III) with initial oxalate from the dye by reaction (4) [36, 37] and that UV light photolyzes the colored intermediates formed during MG degradation, thus enhancing the decolorization process. The positive effect of UV radiation on TOC abatement is shown in Fig. 4d. TOC decayed faster than in the non-irradiated samples and thus, 78%, 77% and 67% mineralization was finally reached for 50, 100 and 150 mg L-1 MG, respectively, which were roughly 20% greater than those determined under EF conditions (see Fig. 4b). This enhancement can be ascribed to the photodecomposition of final Fe(III)-carboxylate

12

complexes by UV light from reaction (3), which are very slowly destroyed with BDD(HO●) and HO● [38, 39]. Ion chromatograms of the final electrolyzed solutions revealed the pre-eminent accumulation of NH4+ ion, along with a small amount of NO3 ion. For example, for 150 mg L-1 MG, the initial N was mineralized as 83% and 13% in the form of NH4+ and NO3 ions, respectively. 3.4. Comparative TOC kinetics, current efficiency and cost of the EF and PEF processes The exponential TOC decay of Figs. 4b and 4d were analyzed by a pseudo-first-order kinetic equation. Good linear correlations were obtained for the ln (TOCt/TOC0)-t plots and the corresponding apparent rate constants for TOC removal (kTOC), along with their square regression coefficient (R2), are collected in Table 2. These data confirm the superiority of PEF over EF for each dye content, as a result of its greater oxidation power because oxidizing hydroxyl radicals are coupled to the photo-oxidation of UV light. Since the mineralization of MG involves its conversion into carbon dioxide and preeminently NH4+ ion, as stated above, its overall reaction can be written as follows [40]: C52H54N4O12 + 92 H2O  52 CO2 + 4 NH4+ + 222 H+ + 226 e

(9)

Taking this reaction, the mineralization current efficiency (MCE) for the EF and PEF degradations of MG were estimated by Eq. (10) [41, 42]: MCE (%) =

n F Vs (TOC)exp 4.32  107 m I t

 100

(10)

where n is the number of electrons consumed in the mineralization process (226 from reaction (9)), F is the Faraday constant (96487 C mol-1), Vs is the solution volume (L), 13

(TOC)exp is the experimental TOC decay (mg L-1), 4.32  107 is a conversion factor (3600 s h-1 x 12000 mg C mol-1),

is the number of carbon atoms of the molecule (52 in

this case), is the applied current (A) and t is the electrolysis time (h). Figs. 5a and b show the MCE values calculated from Eq. (10) for the trials of Figs. 4b and 4d, respectively. As expected, higher current efficiencies were found for PEF than for EF under comparable conditions. Note that most MCE values at short electrolysis times were > 100%, because a maximum MCE value of 200% can be obtained since hydroxyl radicals are generated from electrode reactions involved in both, the anode and cathode. However, Eq. (10) does not consider the photo-oxidation due to UV light. As can be seen in Figs. 5a and b, the current efficiency always dropped dramatically with electrolysis time. For 150 mg L-1 of dye, for example, the MCE value decayed from 139% at 60 min to 46% at 360 min under PEF treatment. This trend can be ascribed to the progressive loss of organic matter and the generation of more recalcitrant intermediates [19, 43]. However, the current efficiency grew with increasing dye content, despite the opposite tendency found for the decay in TOC. Thus, after 60 min of the 50, 100 and 150 mg L-1 MG solutions, increasing MCE values of 72%, 107% and 134% for EF and 82%, 115% and 139% for PEF, respectively, were determined. This means that the oxidation ability of both EAOPs was enhanced as organic load raised, a behavior that can be ascribed to the reaction of more quantity of organics with greater amounts of hydroxyl radicals because of the deceleration of their non-oxidizing parasitic reactions. These reactions include, for example, the oxidation of BDD(HO●) to O2 via reaction (11) and the removal of HO● with Fe2+ and H2O2 from reactions (12) and (13), respectively [15, 16]: 2 BDD(HO●)  2 BDD + O2 + 2 H+ + 2 e 14

(11)

Fe2+ + HO●  Fe3+ + OH

(12)

H2O2 + HO●  HO2 + H2O

(13)

For the above EF and PEF trials, the specific energy cost per unit mass of TOC removed (ECTOC) and the specific energy consumption per unit volume (ECV) at the total electrolysis time tM (t) were calculated from Eq. (14) and (15), respectively [43, 44]:

ECTOC (kWh g-1 TOC) = ECV (kWh m-3) =

Ecell I tM

(TOC)exp Vs Ecell I tM

(14) (15)

Vs

where Ecell is the potential difference of the cell (9.0 V) and the rest of parameters have been defined above. The energy consumptions thus obtained are summarized in Table 2, along with the electrical cost estimated from the ECV value assuming an average retail price of electricity for the industrial sector of US$ 0.076 per KWh (Comisión Federal de Electricidad, Mexico). It should be noted that the ECTOC and ECV values for each experiment increased with electrolysis time and consequently, the data given in Table 2, are those at longer times. Inspection of Table 2 also reveals that ECTOC became smaller with increasing dye concentration due to the decay of a greater amount of TOC, being lower for PEF because of its higher oxidation power. For 150 mg L-1 MG, for example, the final ECTOC value was 0.39 KWh g-1 TOC for EF, dropping to 0.32 KWh g-1 TOC for PEF. In contrast, the electrical cost per m3 was the same for both treatments (2.67 US$ m-3), regardless of the organic load utilized, although greater mineralization was obtained by PEF owed to the additional photo-oxidation with UV light. However, the high energy cost of the 6 W UV 15

lamp (see the last column of Table 2) prevents the PEF application in practice. To overcome this, it has been proposed the solar PEF process with sunlight as renewable and inexpensive energy source [41, 44] and even the use of an autonomous solar flow plant without electrical energy consumption [9]. The EF process might appear to be more expensive than ozonation and other advanced oxidation processes. In this regard, there is some evidence that for certain applications, EF is in the same range of cost than the Fenton process [45], which in turn has been reported to be cheaper than ozonation [46]. In addition to cost-competitiveness for EF, it is also important to note that the EF process are generally considered to be more efficient than the Fenton approach, probably due to the production of a greater quantity of free radicals [45, 47, 48]. 3.5. Stability of the CF air-diffusion cathode for Malachite Green degradation To investigate the stability of the CF air-diffusion cathode in the EAOPs tested, a series of ten cycles was made by consecutively electrolyzing a 150 mg L-1 MG solution in 0.05 M Na2SO4 with 0.50 mM Fe2+ under PEF conditions at 21.7 mA cm-2 for 360 min. In these assays, the cathode was not rinsed with 10% H2SO4 to assess the possible effect of the medium. Fig. 6 shows a progressive decay in TOC removal percentage with the increasing number of cycles of treatment. As can be seen, TOC after 360 min of electrolysis was reduced from 67% to 23% for the assays made from the 1st to 10th cycle. This gradual reduction of the oxidation ability of the PEF process can be attributed to the loss of active sites for H2O2 electrogeneration by reaction (4), probably by precipitation of Fe(OH)3 on its surface because of the highly alkaline medium at its vicinity.

16

To confirm the above hypothesis, the surface of the CF cathode after 10 cycles was analyzed by SEM. The image of Fig. 7a with a magnification of 100 xs shows thin crosslinked and disordered strands. The image of Fig. 7b with a larger magnification (5000x) clearly evidences the existence of conglomerates of iron hydroxide covering the surface of CF. This was confirmed from the mapping images presented in Fig. 8a for 100x and Fig. 8b for 1000x. After 10 cycles of PEF treatment, a high amount of iron containing particles is deposited onto the CF surface, thus blocking in large extent the sites of carbon for H2O2 generation. Complementary EDS analysis of the elements detected at the CF surface also corroborated the deposition of iron. Table 3 shows that 15.61 wt% Fe was present at the surface of the cathode material after 10 cycles of PEF treatment, whereas this element was undetected in the raw material. The large presence of Fe can be associated to the high increase of oxygenated groups (like -OH) on the material, as shown in Table 3. From the data of this table, one can conclude that carbon undergoes a large decay from 89.46 to 22.59 wt% also due to the presence of a larger percentage of Na and S coming from the supporting electrolyte. In view of the large influence of the inorganic species of the solution on the performance of the CF air-diffusion cathode, a last series of 11 consecutive trials were carried out by electrolyzing a 150 mg L-1 MG solution in 0.05 M Na2SO4 by EF at 21.7 mA cm-2 for 360 min. Up to the 10th cycle, 0.50 mM Fe2+ was added to each solution, whereas for the 11th cycle, no catalyst was added. Fig. 8c reveals that TOC was more rapidly reduced in the absence of added Fe2+ and thus, about 78% TOC decay was obtained at the end of the 11th cycle, a value even higher than that obtained in the 1st cycle (see Fig. 4b). This phenomenon can be ascribed to the gradual dissolution of Fe species deposited onto 17

the CF cathode, in a sufficient amount to efficiently generate HO● from Fenton’s reaction (1). Further studies are then needed to understand the reversible behavior of the iron species deposited at the CF surface for improving the degradation process of organics by EF and PEF.

4. Conclusions It has been demonstrated the feasibility of using a CF as air-diffusion cathode for the generation of H2O2 from O2 reduction and its application to the degradation of aqueous sulfate solutions of MG dye by EF and PEF with 0.50 mM Fe2+ using an undivided cell with a BDD anode. This cathode material shows high specific 3D surface area, good chemical and mechanical stability and an excellent low cost. It has been found that 94 mg L-1 of H2O2 can be accumulated in 100 mL of a 0.05 M Na2SO4 solution of pH 3.0 operating with the BDD/CF air-diffusion cell at the best current density of 21.7 mA cm-2. The MG solutions were decolorized and mineralized more quickly for PEF than for EF because the production of HO● from Fenton’s reaction was enhanced by the additional photolysis of Fe(III)-hydroxy species and Fe(III) complexes with oxalate of the dye. In both EAOPs, the normalized TOC abatement decreased with increasing the MG content from 50 to 150 mg L-1, but the current efficiency increased by the concomitant deceleration of nonoxidizing waste reactions of hydroxyl radicals. Stability tests of the electrolytic system showed a gradual loss in PEF performance due to the loss of availability of the CF airdiffusion cathode to generate H2O2. SEM and EDS analysis confirmed the deposition of Fe species, as well as a high increase in the proportion of O, Na and S on the CF material after 10 cycles of treatments. The performance of this cathode was largely improved by consecutively treating a MG solution without adding Fe2+, thus posing the need of further 18

work to clarify the reversibility of the deposition of iron species at the CF surface for improving the degradation of organics by EF and PEF.

Acknowledgments The authors are grateful to MINECO (Spain) under project CTQ2013-48897-C2-1-R, cofinanced with FEDER funds and to the Mexican Council for Science and Technology (CONACyT), Mexico, for financial support of this work.

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25

Figure and Table Captions Fig. 1.- Sketch of the experimental set-up for the degradation of 100 mL of a Malachite Green solution by electro-Fenton (EF) or photoelectro-Fenton (PEF) process with a BDD/CF-air diffusion arrengement. Fig. 2. (a) Time-course of the concentration of accumulated H2O2 in a BDD/CF cell with () and without () injecting air to the cathode and applied current of 21.7 mA cm-2 and (b) influence of applied current on H2O2 accumulation with electrolysis time in a BDD/CF air-diffusion cell and current density of () 11.6, () 15 mA, () 18.3 mA, () 21.7 and (◄) 33.3 mA cm-2. General Conditions: Volume 100 mL, Na2SO4 concentration 0.050 M, pH 3.0; temperature 25 °C. Fig. 3. (a) Percentage of color removal and (b) normalized TOC decay vs. electrolysis time for the EF process of a 150 mg L-1 Malachite Green air-saturated solution using a BDD/CF cell with () and without () injecting air to the cathode. Volume 100 mL, Na2SO4 concentration 0.050 M, Fe2+ concentration 0.50 mM, pH 3.0, temperature 25 °C and applied current density 21.7 mA cm-2. Fig. 4. Effect of initial dye concentration on the change of (a) percentage of color removal and (b) normalized TOC with electrolysis time for Malachite Green solutions treated by EF and (c) percentage of color removal and (d) normalized TOC with electrolysis time for PEF degradation of Malachite Green solutions with a BDD/CF air-diffusion cell. Volume 100 mL, Na2SO4 concentration 0.05 M, Fe2+ concentration 0.5 mM, pH 3.0; temperature 25 °C, applied current density 21.7 mA cm-2 and dye concentration of (▲) 50 mg L-1, () 100 mg L-1 and () 150 mg L-1. Fig. 5. Mineralization current efficiency vs. electrolysis time for the (a) EF and (b) PEF process performances of Fig. 5b and 6b, respectively. MG concentration: (▲) 50 mg L-1, () 100 mg L-1 and () 150 mg L-1. Fig. 6. Percentage of TOC removal at 360 min of 10 consecutive cycles of a MG solution treatment by PEF using the same BDD/CF air-diffusion cell. Volume 100 mL, dye concentration 150 mg L-1, Na2SO4 concentration 0.050 M, Fe2+ concentration 0.5 mM, pH 3.0, temperature 25 °C and applied current density 21.7 mA cm-2. Fig. 7. SEM images at (a) 100x and (b) 5000x of the CF electrode after ten cycles of PEF treatment of a 150 mg L-1 MG solution with 0.5 mM Fe2+ at 21.7 mA cm-2 for 360 min. Fig. 8. Mapping images of carbon and iron at (a) 100x and (b) 1000x of the CF electrode under the conditions of Fig. 9, and (c) influence of iron concentration on normalized TOC decay vs. electrolysis time for consecutive cycles of EF treatment of MG solutions: () 6th Cycle with consecutive addition of 0.50 mM Fe2+ from the 1st cycle and () 11th cycle without Fe2+ addition.

26

Figure 1

Figure 2

100

a

90

[Accumulated H2O2] / mg L

-1

80 70 60 50 40 30 20 10 0 0

30

60

90

120

150

180

210

240

180

210

240

Time (min)

100

b

90

[Accumulated H2O2] / mg L

-1

80 70 60 50 40 30 20 10 0 0

30

60

90

120

150

Time (min)

Figure 3

1.0

b

a

100

0.8

TOCt / TOC0

Color Removal (%)

80

60

40

0.6

0.4

0.2

20

0.0

0 0

20

40

60

Time (min)

80

100

120

0

60

120

180

Time (min)

240

300

360

Figure 4

1.0

a

100

b

0.9 0.8 0.7

TOCt/TOC0

Color Removal (%)

80

60

40

0.6 0.5 0.4 0.3 0.2

20

0.1 0.0

0 0

20

40

60

80

100

120

0

Time (min)

120

180

240

300

360

240

300

360

Time (min)

1.0

c

100

60

d

0.9 0.8 0.7

TOCt/TOC0

Color Removal (%)

80

60

40

0.6 0.5 0.4 0.3 0.2

20

0.1 0.0

0 0

20

40

60

Time (min)

80

100

0

60

120

180

Time (min)

Figure 5

140

a

120

% MCE

100 80 60 40

. 20

140

b

120

% MCE

100 80 60 40 20 60

120

180

240

Time (min)

300

360

Figure 6

Figure 7

Figure 8

c 1.0 0.9 0.8

TOCt/TOC0

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0

60

120

180

Time (min)

240

300

360

Table 1. H2O2 production and cost for different cathode materials. Table 2. Comparative electrochemical characteristics for the EF and PEF treatments of Malachite Green solutions in a BDD/CF air-diffusion cell at 21.7 mA cm-2 for 360 min. Table 3. Weight percentage of the elements present in the CF surface of the raw material and after ten cycles of PEF treatment of a 150 mg L-1 Malachite Green solution with 0.50 mM Fe2+ at 21.7 mA cm-2 for 360 min, determined by EDS analysis.

27

Table 1

Geometric area (cm2)

Applied potential or current

H2O2 production (mg L-1)

H2O2 production (mg L-1 cm-2)

Material Carbon felt

3

21.7

Material costa

Supplier

Ref.

94

31.3

0.021

Roe Group

This work

mA cm-2 Carbon cloth

22.06

-700 mV vs. Hg/HgSO4

53

2.4

0.066

Carbone Lorraine

[20]

Graphite cloth

164

300 mA cm-2

50

0.31

0.097

Carbone Lorraine

[17]

Carbon felt

350

6.25 A m-2

3.4

9.7  10-3

0.052

SGL Carbon

[34]

Activated graphite felt

4

-700 mV vs. SCE

79.2

19.8

0.073

Hunan Jihua Carbon High-Tech

[35]

a

In US dollars cm-2 at exchange rate of 13/07/2015

28

Table 2

Method

[MG] -1

EF

PEF

a

R2

kTOC -3

-1

(mg L )

(10 min )

50

6.6

100

ECTOC -1

ECV

Electrical -3

a

UV energy cost a

(kWh g TOC)

(kWh m )

cost

0.997

1.21

35.1

2.67

7.1

0.931

0.53

35.1

2.67

150

9.3

0.976

0.39

35.1

2.67

-

50

9.4

0.955

0.9

35.1

2.67

27.4

100

5.1

0.999

0.46

35.1

2.67

27.4

150

11.9

0.981

0.32

35.1

2.67

27.4

In US dollars m-3

29

Table 3

Weight (%) Sample C

O

Na

S

Fe

CF before treatment

89.46

8.3

1.34

0.84

---

CF after 10 cycles

22.59

40.60

14.70

8.01

15.61

30