New approaches on heterogeneous electro-Fenton treatment of winery wastewater

Accepted Manuscript Title: New approaches on heterogeneous electro-Fenton treatment of winery wastewater Author: Olalla Iglesias Jessica Meijide Elvira Bocos ´ M.Angeles Sanrom´an Marta Pazos PII: DOI: Reference:

S0013-4686(15)00954-8 http://dx.doi.org/doi:10.1016/j.electacta.2015.04.062 EA 24815

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

8-1-2015 8-4-2015 10-4-2015

Please cite this article as: Olalla Iglesias, Jessica Meijide, Elvira Bocos, ´ M.Angeles Sanrom´an, Marta Pazos, New approaches on heterogeneous electro-Fenton treatment of winery wastewater, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2015.04.062 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.

New approaches on heterogeneous electro-Fenton treatment of winery wastewater Olalla Iglesias, Jessica Meijide, ElviraBocos, M. ÁngelesSanromán, Marta Pazos* [email protected] Department of Chemical Engineering, University of Vigo, Isaac Newton Building, Campus As Lagoas, Marcosende 36310, Vigo, Spain *Corresponding author. Tel: +34 986 818723; fax: +34 986 812380.

ABSTRACT A new approach on heterogeneous electro-Fenton of winery wastewater was investigated. The high organic load and the dependence on the wine industry period makes necessary to find an appropriate treatment able to efficiently reduce the environmental risk of this waste. For this purpose, the electro-Fenton process was improved by searching different catalyst supports (manganese alginate gel beads (MnAB); iron alginate gel beads (Fe-AB) and iron loaded activated carbon (Fe-AC)) and evaluating the applied potential difference effect. Fe-AC attained a fast decolorization and high degradation results at 15 V. To further understand the main degradation mechanisms on this new catalyst, the contribution of the HO● generated in solution and on the anode surface was analyzed. In addition, the positive effect of nickel foam on an extra input of H2O2 production through the generation ●O2 - and its effect on thetreatment were proved. Keywords:Nickel foam, activated carbon, heterogeneous electro-Fenton, reaction mechanism, winery wastewater.

1. Introduction The disposal of wastes from industrial and domestic sources is becoming a serious problem throughout the world. Winemaking is one of the industries whose wastes are classified as hazardous to the environment. A huge number of liters of winery wastewater (WW), that are characterized by a low pH value, high organic content, presence of color and unpleasant odors are originated every-year,which constitutes both environmental and aesthetic problems[1,2]. The industrial production of wine follows different steps such as: vintage, racking, or bottling; that produce different volumes of wastewater with different characteristicsand seasonal variability[3]. Color on wines is due to the presence of phenols, in the case of red wines it is mainly due to the flavonoid group [4].Soluble sugars (fructose and glucose), organic acids (tartaric, lactic and acetic), alcohols (glycerol and ethanol) and high-molecular-weight compounds, such as polyphenols,

tannins

and

lignin

are

the

main

organic

compounds

on

WW[5].Furthermore, the addition of sulfur dioxide (SO2) is widely used for its antioxidant and preservative properties, which hinders the efficiency of traditional degradation treatments [6]. Due to the complexity of WW,principally the high chemical oxygen demand (COD)values that vary from 500 to 45000 mg/L [3],theirtreatment increases the cost of wine production. Thus, the identification of effective and low cost degradation processes has increased the attention of researches worldwide [7,8]. Advanced oxidation processes (AOPs) are degradation treatments that involve the generation of highly reactive radical species that are known for their ability to mineralize a wide range of organic compounds. Among them, the electro-Fenton treatment has attracted particular attention in the research community[9-11]. In this process the degradation of organic matter is produced by direct oxidation and transfer of

one electronand through indirect electrochemical oxidation from hydroxyl radicals (HO●), a nonspecific and highly oxidative radical that is responsible of high degradation values [12-14]. The generation of HO●from the electro-Fenton process corresponds with Eq. 1 (electrochemical generation on the electrode surface), Eq. 2(activated carbon catalyst ofHO●)and Eq. 3(transition metal catalyst of H2O2) from Table 1.The electroFenton process has the advantage of the in situ production of H2O2(Eq. 4), avoiding potential risks arisen from transportation, storage and handling; besides, catalysts, usually iron or other transition metals, are continuously recycled due to the redox reactions (Eq. 6) [15]. Theironcatalyst activity can be substituted by other transition metals (such as Mn,Cd, Co, Cr, Cu,Mn, Ni, and Zn)[16].Fernández de Dios et al.[17]demonstrated the feasibility of Mn alginate gel beads as catalysts for HO●production from H2O2 and their effective behaviour in continuous and batch processes for the treatment of recalcitrant organic pollutants. The immobilisation of catalyst facilitates its reuse[17-19] and the development of continuous processes avoiding its continuous addition on the inflow and its loose on the outflow [18]. Several studies have focussed on metalsorption or entrapment in different matrixes for their use in Fenton, electro-Fenton, photo-Fenton or similar processes. The metal can be immobilised in organic structures such as hydrogels [12,13,17,20] that have a good performance, though these structuresare fragile for long treatment times. On the other hand, inorganic structures for metal immobilisationhavealso attained successful results[19,21,22]. Nonetheless, few studies deal with supports that efficiently capture the metal catalyst and that provide catalytic properties themselves.Activated carbon (AC) is characterised by its great absorption capacity[23], therefore it can be used as Fe

support; furthermore its behaviour asheterogeneous catalyston the generation of HO● from dissolved oxygen inwaterwas already proved (Eq. 2)[24]. In addition to this, the selection of proper electrodes that increase the effectiveness of this process is a limiting factor that should be considered. Several authors demonstrated that Boron-Doped Diamond (BDD) electrode, improves the efficiencywhen used as anode [25-27].Cathode material should optimise the generation of H2O2 among other reduction reactions[28]. Foam materials have higher reaction surface, thus the use ofnickel foam as cathode can improve the production of HO●; furthermore the presence of nickel produces an additional H2O2 generation from the superoxide radical (●O2 -) (Eq. 5)[29]. The main objective of this work is to develop a highly efficient heterogeneous electroFenton process by optimizing the generation ofHO●for the degradation of a highly organic loaded effluent such as WW. 2. Materials and methods 2.1. Winery wastewater A simulated WW was generated by dilution (1:4) of commercial red wine. This WWhas an initial chemical oxygen demand (COD) of 52.8 g/L; a total organic carbon (TOC) of13.75 g/L, a pH of 3.1 and a maximum absorbance at 513 nm. 2.2. Catalysts supports 2.2.1. Fe activated carbon For the iron load on activated carbon (Fe-AC), adsorption assays were carried out by mixing a constant volume (0.15 L) of iron aqueous solution at 0.017 M of Fe3+(Fe2(SO4)3; Sigma-Aldrich, Spain) with 3 g of Activated Carbon (AC) (Granulated nº2 QPPanreac Spain) in 0.25 L Erlenmeyer flasks. The flasks were agitated in an incubator (Thermo Forma 420) at 150 rpm and 20ºC for 2hours. In order to analystiron

adsorption, samples were taken from the supernatant and centrifuged (Sigma 3K-18) for 10 minutes at 7000 rpm to remove solid. Atomic Absorption Spectrometry (Agilent 240FS) was used to measure the iron that remained unsorbed in the supernatant liquid. All the adsorption studies were repeated three times and the reported valuesare the average of measurements. Scanning Electron Microscopy (SEM) of Fe-AC was performed on a JEOL JSM-6700F equipped with an Energy Dispersive Microanalysis (EDS) Oxford Inca Energy 300 using an accelerating applied potential difference of 20 kV(Electron Microscopy Service, C.A.C.T.I., University of Vigo). 2.2.2. Fe and Mn alginate gel beads A solution of sodium alginate 2.0% (w/v), purchased from Prolabo(Barcelona, Spain), was dropped through a syringe into the hardening solutions composed of 0.05 M Fe3+ (Fe2SO3; Sigma-Aldrich, Spain) to create the spherical alginate beads loaded with Fe (Fe-AB) and a solution composed of 0.2 M Mn2+ (MnCl2·4H2O; Sigma-Aldrich, Spain) to create the spherical alginate beads loaded with Mn (Mn-AB). These formed particles were cured at 4ºC for 2 hours in the gelling solution then they were filtered off and washed repeatedly with distilled water. Finally, they were stored at 4ºC in distilled water for the electrochemical studies. 2.3. Electrochemical reactor set up The heterogeneous electro-Fenton of WW(EF-WW) was carried out in a cylindrical glass reactor with a working volume of 0.15 L. The electric field was applied by a 1.6 mm thick nickel foam cathode (Goodfellow Cambridge Ltd, United Kingdom) and a BDD anode (DIACHEM®, Germany). The electrodes (surface11 cm2) were placed opposite to each otherat 1 cm above the bottom of the cell and with an electrode gap of 6 cm. A constant potential drop was

applied with a power supply (HP model 3662). Current intensity was monitored along the process with a multimeter (Fluke 175). Continuous saturation of air at atmospheric pressure was ensured by bubbling 1 L/min of compressed air near the cathode. Reaction mixture contains the selected catalyst in 0.15 L of WW with electrolyte Na2SO4 (0.01M). In these experiments the pH was not modified. The mass of catalyst supports was 8.7 g of Fe-AB, 14.25 g of Mn-AB and 3 g of Fe-AC with different iron uptakes. 2.4. Samples preparation In all experiments, samples were taken periodically from the reactor. They were centrifuged (Sigma 3K-18) for 10 minutes at 7000 rpm to remove solidto the following chemical analysis. 2.5. Winery wastewater analysis The initial and residual color of WW were monitored spectrophotometrically(Unicam Helios β, Thermo Electron Corp.) along the process in the visible region between 350nm and 750 nm. The absorption spectrum showed a single peak with a strong absorption in 513nm. WW decolorization (D) was expressed in terms of percentages. =

(

)∗

(Eq. 10)

where D is decolorization (in %); Ai and At are the areas under the WWcurves at the initial and through time, respectively. Color intensity (CI) was determined as the sum of absorbance at 420, 520 and 620 nm. It is a simple measure of how dark thewine is using a summation of absorbance measurementsin the violet, green and red areas of the visible spectrum [30]. Chemical oxygen demand (COD) was measured with the dichromate method (UNE 77004:2002).

Moreover, to establish the efficacy of degradation other specific energetic parameters are useful. In this study the amount of COD destroyed was evaluated as expresses Eq. 11.

EC 

I·V ·t (COD)VS

(Eq. 11)

whereEC is energy consumption in kWh/kgCOD , I is the average applied current (A), V is the cell applied potential difference (V), t is the treatment time (h), Vs is the solution volume (L) and ΔCODis the decay in COD (g/L). 2.6. Scavenging studies To determine the influence of the several reactions, explained on Table 1, on the electro-Fenton mechanism,scavenging studies were carried out. For that, tert-butyl alcohol (TBA) 5% (v/v) purchased by Sigma-Aldrich (Spain), was used to scavenge essentially all HO● and benzoquinone (BQ) 0.02 mM(Sigma-Aldich, Spain) was employed to scavenge ●O2 -[31]. Degradation of WW in the working conditions was monitored when scavengers were added in the solution. 2.7. Superoxide radicals analysis Superoxide radicals were analysed with the purpose of verifyingtheir production on the new electro-Fenton configuration and therefore, giving insights on its influence on the degradation results obtained. For that, nitrobluetetrazolium (NBT) purchased by SigmaAldrich (Spain), was used to determine the amount that can be specifically reduced by ●

O2- to form an insoluble purple formazan in the aqueous solution [32]. Therefore the

electro-Fenton treatment of NBT (0.05 mM) in the working conditions was used to determine the generation of ●O2 - through the decrease in the absorption at the characteristic maximum wavelength (259 nm).

2.8. Hydroxyl radicals analysis Hydroxyl radicals are responsible of the high degradation results obtained under electroFenton treatment, for that reason they were quantitatively analyzed under different conditions to evaluate the influence of each reaction of Table 1. Tai et al.[33] postulated that one mole of formaldehyde is generated from 2.17 mol of HO●reacting with dimethyl sulfoxide (DMSO). Therefore the generation HO●was obtained from its quantitative relationship with formaldehyde [33,34]. Formaldehyde concentration was determined through derivatization with 2,4dinitrophenylhydrazine (DNPH) to generate the corresponding hydrazone (DNPH0)by HPLC (Agilent 1260 Infinity) equipment, with a poroshell 120 EC-18 column (4.6 x 50 mm i.d., 2.7 µm) as analytical column. The isocratic eluent was a mixture of acetonitrile and water (40:60, v/v), pumped at a rate of 1mL/min for 5 minutes. Detection was carried out with a diode array detector that was set at 255 nm. Injection value was set in 20 µL. Formaldehyde calibration was carried out between 1 µM and 160 µM. Therefore, a solution of DMSO (250 mM) was used on the electro-Fenton treatment under the working conditions and samples (0.5 mL) were taken periodically to add NDPH (6 mM) (0.1 mL) and an acetate buffer solution set on pH 5 (0.5 mL) due to the pH effect on derivatization[33,35]. After a reaction time of 20 minutes [33] samples were filtered through a 0.45 µm Teflon filter. 3. Results and Discussion 3.1 Comparing different catalyst supports The new heterogeneous electro-Fenton with Fe-AC at an iron concentration of 0.72 mM(3 g of Fe-AC with an iron content of 2 mg/g) for the decolorization and degradation of WW was carried out.The process showed a fast decolorization, around

80% after 180 minutes of treatment (Fig. 1) which noticeablyincreases the decolorization obtained using the system with unloaded activated carbon.After 24 hours of treatment, WW wastotally decolorized using Fe-AC (0.72 mM) and CIand COD were reduced 92% 82%, respectively. These results demonstrate the best performance of this treatment in comparison to the control assays and assert that during this heterogeneous treatment there was decolorization, degradation and reduction of organic load (Fig. 2). Activated carbon is widely known for its adsorption capacity and its application for drinking water technologies [36]. The high internal surface area of activated carbon provides high adsorption capacity; however different target compounds have various affinities for the surface phase versus the water phase, making adsorption dependent on the properties of the target compound and the background matrix [36].Devesa-Rey et al. [37] studied the entrapped conditions of activated carbon in calcium alginate beads for the clarification of WW with highly successful results. In the present study, the effect of WW adsorption without electric field was studied in order to evaluate this effect on the decolorization rates attained on the heterogeneous electro-Fenton process. Thus, the catalyst support Fe-AC was kept in agitation at 25 ºC for 24 hours with WW in the ratio used on the heterogeneous electro-Fenton experiments.These tests determined that the colorwas only reduced 23% due to adsorption,which reveals that the heterogeneous electro-Fenton is the main responsible of WW decolorization. To evaluate the effect of iron concentration on AC, the process was repeated with an iron concentration of3.94 mM (3 g of Fe-AC with an iron content of 11 mg/g)to be compared with AC as control assay.The results shown in Fig.1 indicate that the increase on iron concentration did not improve the efficacy of this treatment. These experiments

were continued during 24 hour in order to improve the treatment efficiency. After 24 hours of heterogeneous electro-Fenton with iron concentration of 3.94 mM, (Fig. 2) the decolorizationat a maximum wavelength attained was similar to control assay however CI and COD reduction were lower than experiments using Fe-ACwith the 0.72 mM of iron. According toZhang et al.[38] the high iron content of Fe-AC induced a negative effect on the degradation of organic matter, because an excess on Fe2+ on the media consumes HO●as expressed in Eq. 8 and Fe3+ consumes the hydrogen peroxide (Eq. 10). Then, Fe-AB and Mn-AB were selected to compare the behaviour of different catalyst supports on the decolorization and degradation of WW with nickel foam as cathode material and BDD as anodeat a constant potential drop of 15 V. Fig.3 shows the decolorization curves after a treatment time of 180 minuteswhere Fe-AB and Mn-AB showed slightlylower decolourization than the studied catalyst Fe-AC (0.72 mM). These results are in accordance with the obtained byFernández de Dios et al.[17], Irmak et al. [39] and Iglesias et al.[40]. Fig.4 shows the comparison of the results obtained after 24 hours of treatment using the different catalysts. From these results it can be clearly seen that the catalyst Fe-AC (0.72 mM) obtained the best performance in terms of CI, COD and decolorization. In addition, Mn-AB and Fe-ABwere deteriorated after a sustained treatment under a high potential drop, especially Mn-AB whose structure is more fragile. Iglesias et al.[18] demonstrated the high efficiency of Fe-AB on the continuous treatment of dyes Lissamine Green B and Reactive Black 5, however they already detected the need of increasing their physical resistance. Therefore applying a applied potential difference of 15Vand the continuous magnetic stirring for a long treatment time leads to a break of alginate beads that was not detected working with lower applied potential differences [12,17].

In the case of Fe-AC, the results indicated a fasterdecolorization(Fig.3) and high COD reduction after 24 hours (Fig.4). Furthermore, Fe-AC physical appearance was no modified and the microscopic structure was almost unaffected as is shown inSEM images (Fig.5). 3.2 Effect of applied potential difference The heterogeneous electro-Fenton with Fe-AC was initially evaluated at 15 V to obtain a significant degradation of an extremely high organic loaded wastewater (COD 52.8 g/L). Economical costs of these treatments due to the requirement of high energy consumptions are often considered a limiting factor for their application[41,42]. However, the high volume of WW produced in vintage period requiresa fast and highly efficient process able to treat effluents in continuous mode with short residence times. Constructed wetlands are potential wastewater treatment systems for small to medium sized wineries [7] that have proved high COD removal rates [3,43]; nonetheless they require long residence times and large surfaces[43]. Therefore the use of faster treatment systems, as electro-Fenton, could be a solution for the treatment WW at periods of high effluent production with high organic loads such as vintage. One of the key parameters with high influence in the efficiency and costs of the electroFenton treatment is the applied potential difference. For this reason, the effect of applied potential difference on decolorization and reduction of COD was studied for the heterogeneous electro-Fenton using Fe-AC. Fig. 6 shows the faster the decolorization at 15 V compared with 10 V and 5V.Even after 24 hours (Fig. 7),10 V and 5 V treatments were not able of reaching the values reached on decolorization and CI reduction working with an electric field of 15 V. Furthermore, COD reduction after a day of treatment was 82%, while that for 10 V was limited to 79% and for 5 V reduction obtained was only 71% (Fig. 7).

According toMartínez and Bahena[44]the low applied potential differences reduce the electrolytic reactions, thus the reagents of electro-Fenton are in low concentration to react with pollutant molecules reducing the degradation rate. However, the energy consumptions per amount of destroyed COD showed in Fig.7, indicatethat it is possible to obtain a good degradation degree with low energy cost (0.5kWh/kgCOD) when the applied potential difference is 5V. 3.3Reaction mechanisms The heterogeneous electro-Fenton treatment with Fe-AC is a complex system where several reactions (Table 1) are responsible of HO●generation, and consequently, of decolorizationand degradation of organic compound as a result of HO● reactive action (Eq. 8). Eq. 1-3 are responsible of HO● production. Eq. 1 corresponds to anodic generation of HO● on the electrode surface, Eq. 2 shows the catalytic action of AC on the production of HO●and 3 correspond to catalytic reaction of H2O2with metals to produce HO●. The H2O2in situ generated on the system, is produced from oxygen reduction in presence of water (Eq. 4) and from ●O2 - previously generated from nickel oxidation (Eq. 5). In order to estimate the qualitative effect of different reactions in the overall heterogeneous EF-WW with Fe-AC, several assays were carried out. Firstly, the different sources of HO●(Eq.1-3) were studied. Anodic oxidation (AO) (Eq. 1) involves the oxidation of organics in the contaminated solution by direct charge transfer at the anode of the electrolytic cell, although they are much more rapidly removed with physisorbed formed from water oxidation to O2 operating at high current [45,46].The presence of TBA in the solution scavenges essentially all HO● produced on solution by Eq. 2-3[29,31]. Fig.8 shows the decrease on decolorization of WW when

TBAis added.Liu and Zhang[29] observed a similar tendency when using this scavenger in a 3D electro-Fenton system with nickel foam as particle electrodes for the removal of rhodamine B.In both cases, there is a clear depression on degradation when HO● is removed from the aqueous system, demonstrating its crucial role on the electro-Fenton treatment of WW. However, there is still decolorization due to Eq. 1 and 2 in addition to other mediated oxidation processes that contribute to oxidation process in bulk [47]. In order to determine the generation of HO●by direct action of AO, HO●concentration was assayedwith DMSO trapping and liquid chromatography method [33-35]. The concentration of formaldehyde produced from HO● was measured as previously explained in experimental section; therefore this relationship allows an indirect quantification of HO●in solution reacting with DMSO, (Eq. 1-3). To evaluate the HO● produced from AO the electrochemical treatment at 5 V with air supplywas carried out for the HO● determination (Fig. 9). These data show that the lack of Fe-AC(iron and AC catalytic action) decreases more than the 30 % the HO● generation after 120 minutes. On the other hand, to evaluate the effect of ●O2-in the production of H2O2 through Eq.5, BQwas added to the mediumunder the operational conditions. This scavenger reactswith ●

O2-present in the solution to form a semiquinone radical anion (BQ-●) [29,31], avoiding

the generation of H2O2 through the second reaction of Eq.5. Fig. 9 demonstrates that when all the reactions that produce HO● are acting on the system (Eq. 1-5) there is the highest production of these radicals (680 µM). Nonetheless, when ●O2 - is not present, due to the effect of BQ, H2O2 is not generated throughEq.5 and for this reason, there is a clear decrease on HO●. In these conditions the hydroxyl radical concentration provided is around20% of the total production after 120 minutes.These factsseem to be in relationship with the reduction in the decolorization level obtainedwhen the heterogeneous electro-Fenton process with Fe-AC and BQ was carried out (Fig.

8).These results indicate that even if Fe-AC is not present (Fig. 9), nickel foam seems to have some catalytic effect and this effect does not allow to measure HO●form AO because Eq. 3 and 4 (when Metal is nickel) are probably generating HO●. To further confirm the generation of ●O2- by O2reaction with nickel, the method described byXu et al.[32] was used. In this method is based on the reaction of●O2- with the NBTpresent in solution. Fig. 10 shows the effect of aeration to the ●O2-generation. It is clear that reaction (Eq. 5(1)) takes place even at very low O2concentrations; after eliminating O2with N2gas the oxygenation from the atmosphere allows the generation of ●

O2-. Liu and Zhang[29] obtained even higher ●O2-generation, that was able to remove

all NBT after 10 minutes of treatment; they also ruled out the possible degradation of NBT with Ni2+, founding it negligible. Based on the afore mentioned results, it can be stablished that the reaction mechanisms for the electro-Fenton process using Fe-AC is a complex system and the main reactions involve in the process are represented in the Fig. 11. 4. Conclusions Wine making industry produces wastewater in a limited period of time with chemical and physical characteristics that vary widely according to vintage and racking periods. From this work, it is possible to found that the heterogeneous electro-Fenton with FeAC is particularly effective in decolorizing WW and in decreasing its COD. It was demonstrated that an iron concentration of 0.72 mMwas able to produce the highest degradation rates (100% of decolorization, 92% of CI reduction and 82% of COD reduction) because it contains the required iron to catalyst the H2O2decomposition to hydroxyl radicals. The comparison of this new catalyst with Mn-AB and Fe-AB evidenced the better physical resistance of Fe-AC for long treatment times and high potential drops compared to alginate beads. The elevated organic load of WW proved to

require 15 V to reduce their environmental riskefficiently. However, from the economical point of view, the use of applied potential differenceof 5 V permits to obtain an adequate degradation level with a lower energy cost. On the other hand, the analysis of the qualitative effect of different reactions taking place on the new heterogeneous electro-Fenton with Fe-AC of WWhasproved the important role of AO and the increase ofHO● production in solution by the catalytic action of AC and the metals involved on the treatment (Fe and Ni) over H2O2.As a final remark, all these results advocate the application of the new heterogeneous electroFenton with Fe-AC to allow an effective treatment of highly organic loaded WW.

Acknowledgements This work has been funded by the Xunta de Galicia and by ERDF Funds (Project EM2012/083 andGRC 2013/003). The authors are grateful to the Spanish Ministry of Economy and Competitiveness for financial support of the researcher Marta Pazos under a Ramón y Cajal program.

References [1] J. Ramond, P.J. Welz, M.I. Tuffin, S.G. Burton, D.A. Cowan, Assessment of temporal and spatial evolution of bacterial communities in a biological sand filter mesocosm treating winery wastewater, J. Appl. Microbiol. 115 (2013) 91-101. [2] R. Mosteo, P. Ormad, E. Mozas, J. Sarasa, J.L. Ovelleiro, Factorial experimental design of winery wastewaters treatment by heterogeneous photo-Fenton process, Water Res. 40 (2006) 1561-1568. [3] L. Serrano, D. de la Varga, I. Ruiz, M. Soto, Winery wastewater treatment in a hybrid constructed wetland, Ecol. Eng. 37 (2011) 744-753. [4] M. Fanzone, A. Peña-Neira, M. Gil, V. Jofré, M. Assof, F. Zamora, Impact of phenolic and polysaccharidic composition on commercial value of Argentinean Malbec and Cabernet Sauvignon wines, Food Res. Int. 45 (2012) 402-414. [5] Jeanette Chapman, Winery Wastewater Handbook: Production, Impacts and Managemen, Winetitles Publishers, Adelaide, South Australia, 2001. [6] R.J. Elias, A.L. Waterhouse, Controlling the fenton reaction in wine, J. Agric. Food Chem. 58 (2010) 1699-1707. [7] K.P.M. Mosse, A.F. Patti, E.W. Christen, T.R. Cavagnaro, Review: Winery wastewater quality and treatment options in Australia, Austr. J. Grape Wine Res. 17 (2011) 111-122. [8] I. Oller, S. Malato, J.A. Sánchez-Pérez, Combination of Advanced Oxidation Processes and biological treatments for wastewater decontamination-A review, Sci. Total Environ. 409 (2011) 4141-4166.

[9] X. Florenza, A.M.S. Solano, F. Centellas, C.A. Martínez-Huitle, E. Brillas, S. Garcia-Segura, Degradation of the azo dye Acid Red 1 by anodic oxidation and indirect electrochemical processes based on Fenton's reaction chemistry. Relationship between decolorization, mineralization and products, Electrochim.Acta 142 (2014) 276-288. [10] S. Garcia-Segura, F. Centellas, C. Arias, J.A. Garrido, R.M. Rodríguez, P.L. Cabot, et al., Comparative decolorization of monoazo, diazo and triazo dyes by electro-Fenton process, Electrochim. Acta 58 (2011) 303-311. [11] A. Uranga-Flores, C. De La Rosa-Júarez, S. Gutierrez-Granados, D.C. De Moura, C.A. Martínez-Huitle, J.M. Peralta Hernández, Electrochemical promotion of strong oxidants to degrade Acid Red 211: Effect of supporting electrolytes, J ElectroanalChem 738 (2015) 84-91. [12] O. Iglesias, J. Gómez, M. Pazos, M.A. Sanromán, Electro-Fenton oxidation of imidacloprid by Fe alginate gel beads, Appl. Catal. B Environ. 144 (2013) 416-424. [13] E. Rosales, O. Iglesias, M. Pazos, M.A. Sanromán, Decolourisation of dyes under electro-Fenton process using Fe alginate gel beads, J. Hazard. Mater. 213-214 (2012) 369-377. [14] M.Á. Fernández De Dios, O. Iglesias, M. Pazos, M.Á. Sanromán, Application of electro-fenton technology to remediation of polluted effluents by self-sustaining process, Sci. World J. 2014 (2014). [15] M. Umar, H.A. Aziz, M.S. Yusoff, Trends in the use of Fenton, electro-Fenton and photo-Fenton for the treatment of landfill leachate, Waste Manage. 30 (2010) 21132121.

[16] M. Strlic, J. Kolar, V. Šelih, D. Kocar, B. Pihlar, A comparative study of several transition metals in fenton-like reaction systems at circum-neutral pH, ActaChim. Slov. 50 (2003) 619-632. [17] M.A. Fernández de Dios, E. Rosales, M. Fernández-Fernández, M. Pazos, M. ÁngelesSanromán, Degradation of organic pollutants by heterogeneous electro-Fenton process using Mn-alginate composite, J. Chem. Technol. Biotechnol. (2014). [18] O. Iglesias, E. Rosales, M. Pazos, M.A. Sanromán, Electro-Fenton decolourisation of dyes in an airlift continuous reactor using iron alginate beads, Environ. Sci. Pollut. Res. 20 (2013) 2252-2261. [19] O. Iglesias, M.A. Fernández de Dios, M. Pazos, M.A. Sanromán, Using ironloaded sepiolite obtained by adsorption as a catalyst in the electro-Fenton oxidation of Reactive Black 5, Environ. Sci. Pollut. Res. 20 (2013) 5983-5993. [20] E. Bocos, M. Pazos, M.A. Sanromán, Electro-Fenton decolourization of dyes in batch mode by the use of catalytic activity of iron loaded hydrogels, J. Chem. Technol. Biotechnol. 89 (2014) 1235-1242. [21] N.K. Daud, B.H. Hameed, Fenton-like oxidation of reactive black 5 solution using iron-Montmorillonite K10 catalyst, J. Hazard. Mater. 176 (2010) 1118-1121. [22] A.V. Russo, L.F. Toriggia, S.E. Jacobo, Natural clinoptilolite-zeolite loaded with iron for aromatic hydrocarbons removal from aqueous solutions, J. Mater. Sci. 49 (2014) 614-620. [23] K.Y. Foo, B.H. Hameed, An overview of landfill leachate treatment via activated carbon adsorption process, J. Hazard. Mater. 171 (2009) 54-60.

[24] S. Karthikeyan, G. Sekaran, In situ generation of a hydroxyl radical by nanoporous activated carbon derived from rice husk for environmental applications: Kinetic and thermodynamic constants, Phys. Chem. Chem. Phys. 16 (2014) 3924-3933. [25] E. Brillas, J.A. Garrido, R.M. Rodríguez, C. Arias, P.L. Cabot, F. Centellas, Wastewaters by electrochemical advanced oxidation processes using a BDD anode and electrogenerated H2O2 with Fe(II) and UVA light as catalysts, PortugaliaeElectrochimicaActa 26 (2008) 15-46. [26] S. Garcia-Segura, E. Brillas, Mineralization of the recalcitrant oxalic and oxamic acids by electrochemical advanced oxidation processes using a boron-doped diamond anode, Water Res. 45 (2011) 2975-2984. [27] T.A. Enache, A. Chiorcea-Paquim, O. Fatibello-Filho, A.M. Oliveira-Brett, Hydroxyl radicals electrochemically generated in situ on a boron-doped diamond electrode, Electrochem. Commun. 11 (2009) 1342-1345. [28] M. Panizza, M.A. Oturan, Degradation of Alizarin Red by electro-Fenton process using a graphite-felt cathode, Electrochim.Acta 56 (2011) 7084-7087. [29] W. Liu, Z. Ai, L. Zhang, Design of a neutral three-dimensional electro-Fenton system with foam nickel as particle electrodes for wastewater treatment, J. Hazard. Mater. 243 (2012) 257-264. [30] M. Dimitrovska, E. Tomovska, M. Bocevska, Characterisation of Vranec, Cabernet Sauvignon and Merlot wines based on their chromatic and anthocyanin profiles, J. Serb. Chem. Soc. 78 (2013) 1309-1322. [31] T. Xu, P.V. Kamat, K.E. O'Shea, Mechanistic evaluation of arsenite oxidation in TiO 2 assisted photocatalysis, J PhysChemA 109 (2005) 9070-9075.

[32] X. Xu, X. Duan, Z. Yi, Z. Zhou, X. Fan, Y. Wang, Photocatalytic production of superoxide ion in the aqueous suspensions of two kinds of ZnO under simulated solar light, Catal. Commun. 12 (2010) 169-172. [33] C. Tai, J. Peng, J. Liu, G. Jiang, H. Zou, Determination of hydroxyl radicals in advanced oxidation processes with dimethyl sulfoxide trapping and liquid chromatography, Anal. Chim.Acta 527 (2004) 73-80. [34] Y. Zhong, X. Liang, Z. He, W. Tan, J. Zhu, P. Yuan, et al., The constraints of transition metal substitutions (Ti, Cr, Mn, Co and Ni) in magnetite on its catalytic activity in heterogeneous Fenton and UV/Fenton reaction: From the perspective of hydroxyl radical generation, Applied Catalysis B: Environmental 150–151 (2014) 612618. [35] M.T. Oliva-Teles, P. Paíga, C.M. Delerue-Matos, M.C.M. Alvim-Ferraz, Determination of free formaldehyde in foundry resins as its 2,4-dinitrophenylhydrazone by liquid chromatography, Anal. Chim.Acta 467 (2002) 97-103. [36] G. Bustos, M.A. Carrizales, E. Cervantes, X. Vecino, A.B. Moldes, Treatment of wastewater from sugarcane using entrapped activated carbon, CYTA J. Food 12 (2014) 189-194. [37] R. Devesa-Rey, G. Bustos, J.M. Cruz, A.B. Moldes, Optimisation of entrapped activated carbon conditions to remove coloured compounds from winery wastewaters, Bioresour. Technol. 102 (2011) 6437-6442. [38] H. Zhang, J.C. Heung, C. Huang, Optimization of Fenton process for the treatment of landfill leachate, J. Hazard. Mater. 125 (2005) 166-174.

[39] S. Irmak, H.I. Yavuz, O. Erbatur, Degradation of 4-chloro-2-methylphenol in aqueous solution by electro-Fenton and photoelectro-Fenton processes, Appl. Catal. B Environ. 63 (2006) 243-248. [40] O. Iglesias, M.A.Fernández de Dios, E. Rosales, M. Pazos, M.A. Sanromán, Optimisation of decolourisation and degradation of Reactive Black 5 dye under electroFenton process using Fe alginate gel beads, Environ. Sci. Pollut. Res. 20 (2013) 21722183. [41] E. Pajootan, M. Arami, M. Rahimdokht, Discoloration of wastewater in a continuous electro-Fenton process using modified graphite electrode with multi-walled carbon nanotubes/surfactant, Sep. Purif. Technol. 130 (2014) 34-44. [42] W.R.P. Barros, P.C. Franco, J.R. Steter, R.S. Rocha, M.R.V. Lanza, Electro-Fenton degradation of the food dye amaranth using a gas diffusion electrode modified with cobalt (II) phthalocyanine, J ElectroanalChem 722-723 (2014) 46-53. [43] H.L. Shepherd, M.E. Grismer, G. Tchobanoglous, Treatment of high-strength winery wastewater using a subsurface-flow constructed wetland, Water Environ. Res. 73 (2001) 394-403. [44] S.S. Martínez, C.L. Bahena, Chlorbromuron urea herbicide removal by electroFenton reaction in aqueous effluents, Water Res. 43 (2009) 33-40. [45] C.A. Martínez-Huitle, E. Brillas, Decontamination of wastewaters containing synthetic organic dyes by electrochemical methods: A general review, Appl. Catal. B Environ. 87 (2009) 105-145. [46] M. Panizza, G. Cerisola, Direct and mediated anodic oxidation of organic pollutants, Chem. Rev. 109 (2009) 6541-6569.

[47] F. Sopaj, M.A. Rodrigo, N. Oturan, F.I. Podvorica, J. Pinson, M.A. Oturan, Influence of the anode materials on the electrochemical oxidation efficiency. Application to oxidative degradation of the pharmaceutical amoxicillin, Chem. Eng. J. 262 (2015) 286-294.

Figure caption Fig.1. Decolorization of WW using: Fe-ACat a final iron concentration of 3.94 mM (white circles); Fe-AC at a final iron concentration of 0.72 mM (black triangles) and control assay with unloaded AC (black circles); at 15 V, an air flow of 1 L/min, pH 3.2. Fig.2. Percentages of COD reduction (black bars) and CI reduction (light grey bars) and color reduction (dark grey bars) of WW with different iron loads and AC: Fe-ACat final iron concentration of 3.94 mM and 0.72 mM; and control assay wwithout iron (AC) after 24 hours of WW treatment at 15 V, an air flow of 1 L/min and pH 3.2. Fig. 3.Decolorization of WW with different iron supports: Fe-AB (black circles); MnAB (white circles); Fe-AC (black triangles) at 15 V, an air flow of 1 L/min, pH 3.2. Fig.4. Percentages of COD reduction (black bars) and CI reduction (light grey bars) and decolorization (dark grey bars) of WW with different iron supports: Fe-AB, Mn-AB, Fe-AC after 24 hours of electro-Fenton treatment of WW at 15 V, an air flow of 1 L/min and pH 3.2. Fig.5.SEM images of: (a) initial Fe-AC and (b) Fe-AC after 24 hours of electro-Fenton treatment of WW at 15 V.

Fig.6. Decolorization of WW with Fe-AC at different constant drops: 15 V (black triangles); 10 V (black squares); 5 V (black circles), with an air flow of 1 L/min and pH 3.2. Fig.7. Percentages of COD reduction (black bars) and CI reduction (light grey bars), decolorization (dark grey bars) and EC (white bars) of WW with Fe-AC at different constant potential drops: 15 V, 10 V and 5 V after 24hours of electro-Fenton treatment of WW at an air flow of 1 L/min and pH 3.2. Fig.8. Decolorization of WW with heterogeneous electro-Fenton with Fe-AC in presence of scavengers: control without scavenger (black triangles); BQ scavenger (white circles) and TBA scavenger (black circles) at 15 V, an air flow of 1 L/min and pH 3.2. Fig.9.HO● concentration in the heterogeneous electro-Fenton with Fe-AC (black circles); the heterogeneous electro-Fenton with Fe-AC and BQ (white circles); the electrochemical treatment (black squares) and the electrochemical treatment with BQ (white squares), 5V and an air supply of 1 L/minat pH 3.2. Fig.10. Removal of NBT in the heterogeneous electro-Fenton with Fe-AC with 1 L/min of air supply and magnetic stirring (black circles); magnetic stirring (white triangles) and after nitrogen aeration without agitation nor air supply (black triangles) at 5 V and pH 3.2. Fig. 11. Proposed reaction mechanisms in heterogeneous electro-Fenton reaction using Nickel foam as cathode and BDD as anode.

Table 1.- Main reactions taking place in the heterogeneous electro-Fenton configuration developed in this work.

Eq.

Description

Reaction

1

Anionic oxidation

EM1 + H2O → EM(HO●) + H+ + e-

Result

AC(ecb-) + O2→AC(O2●) 2

AC catalyst of HO●

AC(hvb+) + H2O →AC(2HO●) + H+

HO●generation

AC(O2●) + H+ →AC(2HO●) Metal2 catalyst of

H2O2 + Metal(n)+→HO● + OH- +

HO●

Metal(n+1)+

Oxygen reduction

O2(g) + 2 H+ + 2 e- → H2O2

3

4

1) Superoxide Ni + 2O2 → Ni2+ + 2 ●O2generation

H2O2 generation

5 2) Superoxide ●

O2- + e- + 2H+ → H2O2

reduction 6

Metal reduction

Metaln+ + e- →Metal(n+1)+

7

HO● reactive action

R + HO●→ CO2+ H2O

Catalyst recycing Degradation of organic compound

Metal(n+1)++ HO●→ Metaln++ OH-

8

HO●consumption

Reactions of Metal H2O2+ Metaln+→Metal(n+1)++ 9

in excess HO2●+ H2O

1

EM is electrode surface

2

Metal is a transition metal (Fe, Mn or Ni).

H2O2consumption

Fig. 1

100

Decolorization (%)

80

60

40

20

0 0

20

40

60

80

100

120

140

Time (minutes)

Fig. 2

100

Reduction (%)

80

60

40

20

0 3.94 mM Fe

0.72 mM Fe

control

160

180

Fig. 3

100

Decolorization (%)

80

60

40

20

0 0

20

40

60

80

100

120

140

Time (minutes)

Fig. 4

100

Reduction (%)

80

60

40

20

0 Mn-AB

Fe-AB

Fe-AC

160

180

Fig. 5

a

b

Fig. 6

100

Decolorization (%)

80

60

40

20

0 0

20

40

60

80

100

Time (minutes)

120

140

160

180

100

10

80

8

60

6

40

4

20

2

0

0 15 V

10 V

5V

Fig. 8

100

Decolorization (%)

80

60

40

20

0 0

20

40

60

80

100

Time (minutes)

120

140

160

180

EC(kWh/kg COD)

Reduction (%)

Fig. 7

Fig. 9

Hydroxyl radical ()

600

400

200

0 0

20

40

60

80

100

120

Time (minutes)

Fig. 10

100

Removal(%)

80

60

40

20

0 0

20

40

60

80

100

Time (minutes)

120

140

160

180

Fig. 11