Toward the Development of a Novel Electro-Fenton System for

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Toward the Development of a Novel Electro-Fenton System for Eliminating Toxic Organic Substances from Water. Part 1. In Situ Generation of Hydrogen Peroxide Konstantinos V. Plakas, Anastasios J. Karabelas,* Stella D. Sklari, and Vassilis T. Zaspalis Chemical Process and Energy Resources Institute, Centre for Research and Technology - Hellas, P.O. Box 60361, sixth km Charilaou-Thermi Road, Thermi, Thessaloniki, GR 57001, Greece ABSTRACT: The use of a novel electrochemical oxidation system is investigated for in situ generation of hydrogen peroxide, which constitutes the major reactant for hydroxyl radical (OH•) production via the Fenton reaction. The novel electro-Fenton (EF) “filter” is comprised of a stack of carbon anodic and cathodic electrode pairs, for operation in continuous mode, with potential applications in elimination of toxic organic substances (e.g., pesticides, pharmaceuticals) from drinking and similar water sources. Experiments are performed to assess the performance of three types of electrodes (made of woven carbon fibers, loose carbon fibers, and powdered carbon) in the synthesis of hydrogen peroxide by supplying to the system a low voltage direct current. The efficiency of H2O2 electro-generation as a function of various process parameters (i.e., electrode potential, solution pH, ionic strength) is studied for the most promising carbon material (i.e., loose carbon fibers). The results indicate that the optimal cathodic potential for H2O2 generation is 1.3 V vs Ag/AgCl reference electrode at pH 3, with initial mean dissolved oxygen concentration 8.5 mg L−1. Under these conditions, the average current density and average current efficiency are 5.2 A·m−2 and 70%, respectively. Reduced electrolyte (Na2SO4) concentration significantly affects the H2O2 electrogeneration rate, whereas increased solution pH leaves the current efficiency unaffected. Research is ongoing regarding optimization of the EF “filter” and the effective impregnation (in the porous cathodic electrodes) of iron nanoparticles, which mediate the continuous degradation of organic substances.

1. INTRODUCTION The increasing need for effective methods to improve water quality, both for drinking purposes and for treatment of effluents from municipal and industrial facilities, provides strong incentives to develop new technologies and improve the performance of existing ones.1 Therefore, R&D on various aspects of water and wastewater treatment has expanded tremendously in recent years. Particular attention is currently focused on the development of simple, efficient, and costeffective processes for the total destruction of the so-called “emerging” organic pollutants. Typical examples of these chemicals are residues of pesticides, dyes, pharmaceuticals, and personal care products (PPCPs), which are frequently detected in rivers, lakes, coastal seawater, and even drinking water sources, due to their continuous release through inadequately treated municipal and industrial effluents as well as agricultural runoff.2 The occurrence and fate of these compounds are of great concern due to potentially adverse health effects associated with them even at very small concentrations (pg/L to ng/L).2,3 In the past three decades, extensive research has been devoted to advanced oxidation processes (AOPs) as promising alternative methods to efficiently remove recalcitrant, toxic, and nonbiodegradable organic micropollutants from water.4,5 These processes are capable of degrading organic pollutants through the formation of hydroxyl radicals (•OH). The latter are reactive electrophiles (electron preferring) that react rapidly and nonselectively6 with nearly all electron-rich organic compounds. They have an oxidation potential (E°) of 2.80 V vs SHE and exhibit faster rates of oxidation reactions compared © 2013 American Chemical Society

to conventional oxidants such as H2O2 or KMnO4. Key AOPs include heterogeneous and homogeneous photocatalysis based on near-UV or solar visible irradiation, anodic oxidation (AO), ozonation, Fenton reagent, ultrasound, and wet oxidation (also called “hot AOPs”). Less conventional (in various stages of development) processes include ionizing radiation with electron beams and γ-radiolysis, microwaves, pulsed plasma, UV/periodate, and ferrate reagent.5 Recently, the coupling of AOPs with electrochemistry has led to the EAOPs (electrochemical AOPs) which have experienced significant development, showing great effectiveness for the decontamination of wastewater polluted with toxic and persistent pesticides, organic synthetic dyes, PPCPs, and many other industrial pollutants.7 The most popular technique among them is the electro-Fenton (EF) process which forms the basis for a large variety of related processes.7 Although the classic Fenton reactions (Fe2+/H2O2) have been widely considered for the treatment of polluted waters, they have two main drawbacks for large-scale applications in water and wastewater treatment, i.e., relatively high cost and the hazards associated with the transport and handling of commercial concentrated H2O2 as well as the narrow working Special Issue: Recent Advances in Nanotechnology-based Water Purification Methods Received: Revised: Accepted: Published: 13948

February 25, 2013 April 4, 2013 April 4, 2013 April 4, 2013 dx.doi.org/10.1021/ie400613k | Ind. Eng. Chem. Res. 2013, 52, 13948−13956

Industrial & Engineering Chemistry Research

Article

Figure 1. Schematic representation of (a) the experimental setup and (b) the electro-Fenton “filter”.

materials are desirable cathodic electrodes for the EF system because of their stability, conductivity, high surface area, and chemical resistance. Indicative carbonaceous materials used in previous works are carbon-PTFE (also called gas diffused electrodes-GDEs),9 carbon felt,10 activated carbon fiber (ACF),11 reticulated vitreous carbon (RVC),12 carbon sponge,13 and carbon nanotubes (NT).14 Due to the particular hydrodynamic conditions inside the three-dimensional electrodes (e.g., carbon felt, ACF, NT), both large specific electrode areas and high mass transfer coefficients of dissolved oxygen can be obtained, combined with low cost and easy handling of such materials. Moreover, porous carbon can be an especially effective support for dispersing/embedding iron nanoparticles because it provides high surface area, it is resistant to both acidic and basic conditions, its surface can be functionalized to provide controlled metal loading sites, and its pore structure can be tailored for enhanced adsorption. In view of the above considerations, iron (Fe2+/Fe3+) impregnated 3-D carbon electrodes can be utilized for the continuous generation of hydroxyl radicals and the effective degradation of organic micropollutants. This work deals with research and development of a heterogeneous EF system, in which the Fenton reagent (an aqueous mixture of H2O2 and Fe2+) is generated from electrode reactions. Specifically, a “filter”-type device consisting of a pair of anodic and cathodic electrodes made of carbonaceous materials has been designed and constructed, in which a fast O2 reduction and a significant electrosynthesis of H2O2 take place. The reaction of H2O2 with iron-based nanoparticles impregnated in the cathode produces oxidizing agents (HO•, HO2•) which in turn are consumed in reactions involving the degradation of organic matter and the regeneration of Fe2+.

pH range (2−3) because iron ions precipitate as a hydroxide at higher pH values. The EF process appears to be capable of overcoming these two problems because both H2O2 and iron ions can be produced within the electro-Fenton system. Specifically, EF is based on the continuous supply of H2O2 to the contaminated water through an oxygen-reduction reaction (eq 1). In parallel, iron ions (Fe2+ or Fe3+, Fen+) are either added to the contaminated water (homogeneous EF) or are embedded onto suitable electrode materials (heterogeneous EF), in order to catalyze electrogenerated H2O2 to produce the oxidizing agent •OH via Fenton reactions (eq 2). In the EF system, the regeneration of Fe2+ can occur by a direct cathodic reaction (eq 3), by the oxidation of organic species (eq 4), or by the reaction with H2O2 (eq 5), accelerating the production of •OH from Fenton reaction.8 Compared with classical Fenton methods, the EF system can avoid the addition of expensive H2O2 and maintain an almost constant H2O2 concentration by electrogeneration during the pollutant degradation process. O2(g) + 2H+ + 2e− → H 2O2

(1)

Fe 2 + + H 2O2 → Fe3 + + •OH + OH−

(2)

Fe3 + + e− → Fe 2 +

(3)

Fe3 + + •R → Fe 2 + + R+

(4)

Fe3 + + H 2O2 ⇆ [Fe−O2 H]2 + + H+ ⇆ Fe2 + + HO2• (5)

Because of the low oxygen solubility in water and the slow mass transport, the reduction of oxygen to produce hydrogen peroxide with a high yield occurs only on certain cathodic materials, such as mercury, gold, or carbon. Carbon-based 13949

dx.doi.org/10.1021/ie400613k | Ind. Eng. Chem. Res. 2013, 52, 13948−13956


Article

rings are placed within the Teflon plates to prevent leakage. The total cross-sectional area of each electrode is 22.05 cm2. Taking into account the thickness of Teflon plates and spacer, the interelectrode gap is 7.5 mm. The fluid recirculation system comprises a digital flow meter, a digital thermometer, a dissolved oxygen meter (Consort, Belgium), a magnetic-drive pump (IWAKI, Japan), and a 3.0 L stainless steel reservoir. The temperature is regulated at 25 °C by thermostatted water circulated through a stainless steel coil in the reservoir. In these experiments the electrolytic cell is operated in batch recirculation mode at a flow rate of 300 mL/ min. Potentiostatic electrolysis (chrono-amperometry) is controlled through a VersaSTAT3 potentiostat/galvanostat (Princeton Applied Research, AMETEK), connected to a PC for continuous monitoring of cell current (VersaStudio software). An Ag/AgCl electrode (E = 0.207 V vs SHE) from Metrohm is used as reference electrode, which is externally connected to the cathodic electrode by means of a Luggin capillary (thus significantly reducing the IR drop). New carbon electrodes are used for each test. It should be noted that no O2 is externally fed into the system (using either compressed air or pure oxygen gas); i.e., the electrogeneration of H2O2 solely depends on the dissolved O2 and the oxygen generated by water oxidation over the anode. 2.5. Experimental Procedure. The capability of different electrode materials to electrogenerate H2O2 through reaction 1 was studied by electrolyzing 0.05 M Na2SO4 solutions at constant potential in the absence of impregnated with iron particles cathodes; i.e., both anode and cathode were of the same carbon material. Controlled-potential electrolysis was used for the optimization of H2O2 electro-generation rate related to potential and electrode material. During electrolysis of Na2SO4 solution, 3 mL samples of the filtrate (solution from the outlet of the “filter”) were obtained at predetermined time intervals, and H2O2 concentration was measured. For this study, the H2O2 electrogeneration rate was determined as a function of potential within the range 0.6 to 1.5 V. Linear sweep voltammetry (LSV) tests were also performed as an alternative tool for assessing the range of potential in which H2O2 is generated at suitable rates.16 Specifically, LSV voltammograms were recorded in 0.05 M Na2SO4 solution at 25 °C and pH 3. The potential was scanned between 0.2 and 1.5 V at a rate of 0.01 V s−1. Before recording, the potential was scanned repeatedly in the same range, at a scan rate of 0.05 V s−1, to remove residual impurities. All potentials were referred to Ag/AgCl electrode. Current efficiency (CE), defined as the ratio of the electricity consumed by the electrode reaction of interest over the total electricity passing through the circuit, can be calculated7 by eq 6:

This paper focuses on the key issue of H2O2 production rate by the novel “filter”, for different electrode materials, as a function of main system parameters (electrode potential, solution pH, ionic strength).

2. EXPERIMENTAL WORK 2.1. Materials. Three different carbon materials were tested as anodic and/or cathodic electrodes in this investigation. Two of them were provided by the University of Alicante, Spain, designated as “CF-1371” (carbon fiber with a specific surface area 1371 m2/g and thickness 1 mm) and “CF-1410” (carbon fiber with a specific surface area 1410 m2/g and thickness ∼2 mm). The third one, “CCB-470″, was made of powdered carbon which was compressed to form discs (diameter 4.2 cm, thickness 3.7 mm) with a specific surface area 470 m2/g; the original powder was obtained from a Coconut Carbon Black cartridge (Pentair CCBC-10). H2O2 electrogeneration was measured in salt solutions of various concentrations, prepared by dissolving analytical grade anhydrous sodium sulfate (SigmaAldrich) in ultrapure water (resistivity >18 MΩ·cm) from a Milli-Q purification system (Millipore, Milford, MA, USA). 2.2. Electrode Characterization. Powder XRD data were collected in a Siemens D-500 diffractometer using Cu Kα radiation. The diffraction patterns were obtained in the 2θ range from 5 to 80°, in steps of 0.04° and 1 s counting time per step. SEM images were recorded using a JEOL JSM6300 microscope operating at 20 kV. The samples were gold sputtered to avoid charging effects on the images. The N2 adsorption−desorption isotherms were measured at 77 K on a Micromeritics TriStar porosimeter. Specific surface areas (SBET) were determined with the Brunauer−Emmett−Teller (BET) method using adsorption data points in the relative pressure P/ P0 range 0.01 to 0.30. The samples were outgassed at 200 °C for 18 h under high vacuum before the measurements. The electrical resistivity is determined by a digital multimeter (Omega) by placing the pointed probes on the material surface at a distance of approximately 1 cm between them. To account for differences of surface resistance at least five readings were taken at different positions across the samples and the average was computed. 2.3. Analysis of Electro-Generated H2O2. Hydrogen peroxide concentration was determined spectrophotometrically (UV-1700 Pharmaspec, Shimadzu) by the iodide method.15 Two calibration curves were prepared for low (0−180 μg L−1; quartz cells of 10 cm) and high (200−3400 μg L−1; quartz cells of 1 cm) H2O2 concentrations (detection limit of 20 μg L−1). All reagents used were of commercially available analytical grade; i.e., potassium iodide (Riedel-de Häen), potassium hydrogen phthalate (KHP) (J.T. Baker), ammonium heptamolybdate-4-hydrate (J.T. Baker), NaOH (Merck), and hydrogen peroxide (30% w/v) (Panreac). Before analysis, samples withdrawn from electrolyzed solutions were filtered through 0.45 μm PTFE Millipore membranes. The pH of the electrolytic solutions was adjusted to the desired values using H3PO4 and NaOH dilute solutions. 2.4. Electro-Fenton “Filter”Experimental Setup. Figure 1a presents the experimental setup used for constantpotential electrolyses in the flow-through electro-Fenton “filter” specially designed for this work. The “filter” is built within a closed cylindrical cell (made of Acetal resin, DuPont-Delrin) of 6.1 cm inner diameter; it comprises pairs of special ring-type Teflon plates and carbon electrodes that are sequentially placed one on the top of the other, as shown in Figure 1b. Rubber O-

CE =

nFC H2O2V 106M H2O2Q

× 100% (6)

where n represents the stoichiometric number of electrons transferred in reaction 1, F is the Faraday constant (96487 C mol−1), CH2O2 the concentration of H2O2 in bulk solution (μg L−1), V the volume of the treated solution (L), MH2O2 the molecular weight of H2O2 (34 g mol−1), and Q the charge consumed during the electrolysis (C). 13950



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3. RESULTS AND DISCUSSION 3.1. Electrode Characterization. Figure 2 presents the XRD patterns of the samples. As shown in this figure, the

Table 1. Specific Surface Area and Electrical Resistance of the Electrode Materials sample

CCB-470 powder

CCB-470 disk

CF-1371

CF-1410

SBET (m2 g−1) resistance (Ω)

310

470 220

1371 130

1410 520

comprising both materials. However, there is some difference in the isotherm types of these materials (Figure 3), which is attributed to their different structure shown in the SEM pictures of Figure 4c,d; indeed, the CF-1371 woven (braidtype) structure (Figure 4c) is much denser than that of CF1410 loose fibers. The effect of these electrode material structural characteristics will be subsequently discussed in the context of electrolysis and H2O2 production performance. As expected, all carbonaceous materials used exhibit a rather low electrical resistance (Table 1). Among the three electrode materials, CF-1371 displays the lowest electrical resistance followed by CCB-470 and CF-1410. 3.2. Linear Sweep Voltammetry (LSV) for O2 Reduction at Different Cathode Materials. LSV curves reflect the transient current response with respect to applied cathodic potential (Ec) and offer a first insight into the electron and/or mass transfer controlled reactions in an electrochemical system. Figure 5 shows the results of the LSV tests performed with the three types of electrode materials used. As shown in Figure 5, the current increases rather sharply with increasing Ec, with no distinct “plateau”, for all three carbonaceous materials. An exception is observed in the case of CF-1410 electrodes, since a rather small increase of current is recorded in the region between 0.6 and 0.8 V/Ag/AgCl. In general, a limiting current “plateau” represents the cathodic potential region for the efficient electro-generation of H2O2. In this region, reaction 1 is controlled by mass transfer of the dissolved oxygen through the cathode−solution diffusion layer, rather than by the electron transfer between dissolved oxygen and cathode.16 The absence of such a “plateau”, especially in the case of the carbon CCB-470 disc-electrodes, is probably related to ohmic losses within the porous electrodes. According to Badellino et al.,17 these ohmic losses may change a mass transfer limited regime to a mixed controlled process, where electron plays a role. In this case, LSV current responses may not allow a clear identification of mass transfer electrochemical reactions. Since a selection must be made regarding the optimum working potential for the three different carbonaceous materials, numerous experiments have been performed by measuring the H2O2 electrogeneration rate at controlled cathodic-potential electrolysis of Na2SO4 solutions. These results are presented in the next subsection. 3.3. Hydrogen Peroxide Electrogeneration. Figure 6 shows the time-dependent changes of H2O2 concentration at various applied cathodic potentials in the case of anode and cathode electrodes of CF-1371 or CF-1410 materials. All experiments were conducted using 0.05 M Na2SO4 acidic solutions (pH 3) at a constant temperature of 25 °C and an initial dissolved O2 concentration of ∼8.5 mg L−1. In the case of CCB-470 electrodes, the H2O2 production was only measured at 0.5 V/Ag/AgCl. Specifically, the application of higher potentials led to increased current densities (121.5 A m−2 and 180 A m−2 at 1.0 and 1.3 V/Ag/AgCl, respectively) which in turn resulted in the formation of gases in the cell (evidenced by the bubbles appearing at the outlet of the Luggin capillary);

Figure 2. XRD patterns of CCB-470 powder (a), CCB-470 disk (b), CF-1371 (c), and CF-1410 (d).

samples are amorphous and their patterns exhibit two broad relatively intense reflections near 22° and 42° which can be attributed to the sample holder. The surface area of the samples was determined from nitrogen isotherm analysis. As shown in Figure 3 the samples are microporous with the exception of the

Figure 3. Nitrogen adsorption−desorption isotherms of the samples: CCB-470 powder (square), CCB-470 disk (circle), CF-1371 (up triangle), and CF-1410 (down triangle). Adsorption: full symbols; desorption: open symbols.

CF-1371 material which contained also a fraction of mesopores and displayed a type-IV isotherm, while hysteresis was observed indicating the occurrence of capillary condensation in the pores. Using these data, the specific surface area (SBET) of the materials used in this study was calculated (Table 1). Figure 4 includes scanning electron micrographs of the electrode materials tested; the small specific surface of powder and disk material (Figure 4a,b) is indicated in these pictures. It is interesting that the specific surface areas of materials CF1371 (woven carbon fibers) and CF-1410 (loose fibers) are very close, despite their different macroscopic appearance and structure; this may be due to the same primary fiber diameters 13951



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Figure 4. Scanning electron micrographs of (a) CCB-470 powder, (b) CCB-470 disk, (c) CF-1371, and (d) CF-1410 materials.

Figure 5. Linear sweep voltammograms for O2 reduction on (a) CF1371 or CF-1410 cathodes and (b) CCB-470 cathode. Experimental conditions: sweeping rate 0.01 V s−1, pH 3, temperature 25 °C, 0.05 M Na2SO4 solution.

Figure 6. Generation of H2O2 as a function of electrolysis time at various applied cathodic potentials for (a) CF-1371 electrodes and (b) CF-1410 electrodes. Experimental conditions: Solution of 0.05 M Na2SO4, pH 3, recirculation liquid flow 300 mL min−1, temperature 25 °C.

these bubbles negatively affected the connection between the reference electrode and the cathode, thus forcing termination of the experiment (due to overload of potentiostat). According to eq 7, a high cathodic potential enhances the evolution of hydrogen gas through the consumption of protons at the cathode, thus reducing the system capability to produce H2O2.

2H+ + 2e− → H 2

slight decrease of the oxygen concentration (from 8.5 mg L−1 to 6.5 mg L−1) observed after the initiation of electrolysis is followed by a stabilization at longer electrolysis times. The stabilization of H2O2 concentration may be also the result of electro-generation and consumption reactions taking place at similar rates within the system. The consumption of H2O2 may be attributed to a number of parasitic reactions, such as the electrochemical reduction of H2O2 at the cathodic electrode, as reported for all the electro-chemical cells described in the literature, regardless of cell configuration:7

(7)

Results with the woven carbon fiber material CF-1371 (Figure 6a) show a linear increase of H2O2 concentration during the first 45 min of the electrolysis, subsequently exhibiting a tendency for stabilization. This trend seems to be in full agreement with the dissolved oxygen concentrations measured in the bulk solution (data not shown here) since a 13952


Industrial & Engineering Chemistry Research H 2O2 + 2H+ + 2e− → 2H 2O

Article

(8)

As shown in Figure 6a, the hydrogen peroxide concentration does not stabilize for larger than 1.3 V potentials (e.g., 1.5 V); instead, it tends to decrease with time, reaching a minimum value. This implies that higher cathodic potentials favor the formation of water through the reduction of the accumulated H2O2 at the cathode−solution interface [reaction 8] or the reduction of O2 directly to H2O, in a four-electron exchange reaction:16,17 O2(g) + 4H+ + 4e− → 2H 2O

(9)

The concentration curves of hydrogen peroxide in the case of (loose fiber) CF-1410 electrodes (Figure 6b) follow patterns similar to those of CF-1371 electrodes. The main differences observed are the slow initiation of H2O2 electrogeneration and the significantly higher concentrations of H2O2 in the bulk under steady-state conditions. As in the case of carbon CF-1371 electrodes, the utilization of the loose-fiber CF-1410 material as anode and cathode displays an optimum hydrogen peroxide generation at a potential value of ∼1.3 V/Ag/AgCl. At greater cathodic potentials (e.g., 1.5 V) the H2O2 concentration increases steeply at the beginning of the electrolysis, showing a tendency for stabilization at rather lower concentration values. The superiority of the carbon CF-1410 electrodes is also highlighted in Figures 7a−c, where the time-dependent changes of H2O2 concentration, current efficiency, and current density (at the optimum cathodic potentials) are presented for the three carbon materials tested. Figure 7a shows H2O2 accumulated in the recycled solution with concentration (at steady-state conditions) 10.7 mg L−1, 3.2 mg L−1, and 1.3 mg L−1 in the cases of loose carbon fibers (CF1410, felt), carbon disks (CCB-470) and woven carbon fiber (CF-1371) electrodes, respectively. The improved performance of the “filter” with CF-1410 electrodes is also highlighted by the lower total surface area required to achieve a high hydrogen peroxide concentration rate, in comparison to CCB-470 electrodes. Specifically, the total surface area of the different pairs of electrodes varied in the range 900−1000 m2 for the CF1371 and CF-1410 electrodes to 3500−3700 m2 in the case of CCB-470 electrodes; these estimates were obtained by accounting for the specific surface area of each material and the weight of the different specimens used. In parallel, a remarkable increase of current efficiency is observed with the electrolysis time when the CF-1410 electrodes are used in the “filter”, with an average steady-state value at 70%. On the contrary, current efficiency decreases with electrolysis time for the other two materials, with values

Toward the Development of a Novel Electro-Fenton System for

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