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A Feasibility Study of Hydrogen Peroxide Electrogeneration in Seawater for Environmental Remediation

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A Feasibility Study of Hydrogen Peroxide Electrogeneration in Seawater for Environmental Remediation A publication of CHEMICAL ENGINEERING TRANSACTIONS The Italian Association VOL. 28, 2012 of Chemical Engineering Online at: www.aidic.it/cet Guest Editor: Carlo Merli Copyright © 2012, AIDIC Servizi S.r.l., ISBN 978-88-95608-19-8; ISSN 1974-9791 A Feasibility Study of Hydrogen Peroxide Electrogeneration in Seawater for Environmental Remediation Elisabetta Petrucci*, Daniele Montanaro, Luca Di Palma Department of Chemical Engineering Materials & Environment, Sapienza University of Rome, Via Eudossiana, 18 00184, Rome, Italy [email protected] The electrogeneration of hydrogen peroxide in seawater by oxygen reduction on a gas diffusion cathode was investigated considering the feasibility of this process in an undivided reactor. The possible co-presence of different oxidants and the possibility of their synergistic action was evaluated. The loss of hydrogen peroxide due to direct anodic oxidation and to reaction with anodically electrogenerated species was evidenced. To optimize the production efficiencies of both active chlorine and hydrogen peroxide, the influence of electrode materials and current density was studied. An investigation of the possible analytical interferences was also provided. 1. Introduction The treatment of effluents from remediation process of contaminated sediments requires the development and application of green technologies aimed at minimizing waste production and mitigating the negative impact with the adoption of environmentally-friendly chemicals. Hydrogen peroxide, unlike other oxidants, does not pose the risk of gaseous release or chemical residues (Drogui et al, 2001). Moreover, the in situ production, either by catalysed reaction (Biasi et al., 2011) or electrogeneration, avoids the hazard associated to storage, transportation and handling of highly concentrated solutions. In particular the electrogeneration, by means of oxygen cathodic reduction on carbon cathodes, involves the formation of different radical species such as superoxide •- • radicals O2 and hydroperoxide radicals HOO (Alvarez and Pletcher, 1998; Qiang and Huang, 2002). Increasing efficiency and economic save have been obtained by adopting, as the cathode, a gas diffusion electrode either catalysed (Guillet et al, 2006) or uncatalysed (Alcaide et al., 2002). The better performance of a gas diffusion electrode (Da Pozzo et al., 2005) can be attributed to the increased reactive area but also to the porosity that makes gas transport and reagent diffusion easier thus allowing to overcome the difficulty related to the solubility of oxygen into the catholite and its transport from the bulk to the cathode surface. The cathodic electrogeneration of hydrogen peroxide in aqueous solution has been so far extensively studied mostly considering as main applications the disinfection of drinking water and the electro- Fenton’s reaction for the treatment of water and wastewater containing recalcitrant and toxic compounds. However, scarce investigations are still available about electrosynthesis of hydrogen peroxide in seawater. A previous paper (Da Pozzo et al., 2008) indicates that in a two-compartment cell notable concentrations of hydrogen peroxide are obtained with a constant yield in a wide range of charge. However, due to the high salinity of the medium, serious fouling phenomena affect the 91 membranes used for separation thus resulting in increasing values of cell resistance. Besides in the presence of a separation, high investment and operating cost are needed. The electrogeneration of hydrogen peroxide in undivided reactor is expected to lead to lower concentrations. This is due to numerous reactions such as: anodic oxidation with oxygen evolution, cathodic reduction to water, chemical decomposition in the bulk of solution and chemical reaction with species anodically generated - • • as well as reactions with species cathodically generated (O2 , OH, O2H ) as suggested by Agladze et al. (2007). However, in the absence of a separation, the simultaneous anodic electrogeneration of different oxidising species, in particular active chlorine (which is a mixture of chlorine, hypoclorous acid and hypochlorite ions), chlorates and bromates, certainly occurs. It is thus worth assessing whether the synergistic action of different oxidants can compensate for the lower concentration of peroxide. This study was aimed at verifying the feasibility of cathodic electrogeneration of hydrogen peroxide in an undivided electrolyser. The coexistence of different oxidising species and the possibility of their synergistic action were evaluated analyzing the analytical interference in the determination of their concentrations. 2. Experimental 2.1 Materials Reagents were supplied by Carlo Erba Company and Sigma Aldrich and used in their commercially available form without further purification. To ensure reproducibility, all the electrolyses were conducted on solutions of artificial seawater prepared according to Kester et al. (1967). In this recipe: 47.852 g NaCl, 8.016 g Na2SO4, 1.354 g KCl, 0.392 g NaHCO3, 0.196 g KBr, 0.052 g H3BO3, 0.006 g NaF, 10.16 g MgCl2, 2.2954 g CaCl2, 0.0284 g SrCl2, were dissolved in two litres of distilled water in the absence of organic constituents. The initial pH was 7.4 - 7.5 while the conductivity was 40 mS. 2.2 Electrochemical apparatus and procedure Electrolyses were performed under galvanostatic conditions using a potentiostat AMEL 2051. The electrolyser was an undivided glass cell having a volume of 100 mL. The reactor was thermostated and stirred with a magnetic bar. The cathode was an uncatalysed gas diffusion electrode (GDE). This electrode, described in detail in previous work (Petrucci et al, 2009), had a geometric area of about 5 cm2 and was fed with air (130 mL/min). It was coated on both sides by hydrophobic Shawinigan acetylene black carbon (SAB) provided by Industrie De Nora (Milan, Italy). The anodes tested were a commercially available Pt wire (supplied by Amel mod. 805/SPG/12J), a home made Pt disk with a total surface area of 5 cm2, and a Boron-Doped Diamond electrode (supplied by Adamant Technologies) with a total surface area of 5 cm2. The electrodes were placed vertically at a distance of about 1 cm. Unless differently specified tests were conducted at a current density of 400 A m-2, at ambient temperature at least in triplicate. Samples were withdrawn at defined time intervals during the electrolysis for immediate analysis of active chlorine, hydrogen peroxide and anions content. 2.3 Analyses pH was measured using a Crison GLP 421 pH meter, conductivity using a HD9213-R1Delta Ohm meter. Hydrogen peroxide concentrations were determined reflectometrically by means of Merck analytical tests based on a peroxidase catalyzed reaction. The active chlorine concentration was determined using the DPD (N,N-diethyl-p-phenylenediamine) colorimetric method (EN ISO 7393/2, 2000). At circumneutral pH, the colourless N,N-diethyl-p-phenylenediamine is oxidised by active chlorine into a coloured product with an absorbance peak at 510 nm measured by means of a PG Instruments T80+ UV/vis Spectrophotometer and quartz cells of 1 cm path length. Anions were determined using a Dionex 120 ionic chromatograph equipped with an IONPAC AS12A anionic column in isocratic mode and conductometric detector. Fast analyses of perchlorate were carried out using the Ion Chromatograph DX120 equipped with a RFC 30 eluent generator and a IONPAC AS16 anionic column. The conditions adopted were: a gradient from 35 mM at 0 min to 38 − mM at 8 min, a flow rate of 1.5 mL min 1 and an injection volume of 25 L. 92 3. Results To verify the simultaneous electrogeneration of the oxidants anodically and cathodically produced a preliminary series of tests was conducted to quantify the production of hydrogen peroxide and active chlorine, one at a time. In particular, to evaluate the amount of hydrogen peroxide electrogenerated at the cathode, electrolyses were performed in the absence of chloride, on solutions of Na2SO4 0.1M with an initial pH of 6.5. Unless otherwise specified, all the runs were carried out at at room temperature (20 °C) applying a current density of 400 A m-2 using a GDE as cathode with a platinum wire as anode. As expected, in the absence of chloride, as shown in Figure 1, there is no production of active chlorine while hydrogen peroxide presents increasing concentration up to 710 mg L-1. The hydrogen peroxide production rate tends to decrease with time while current efficiency, defined as the ratio of the amount of hydrogen peroxide actually produced to the amount theoretically expected from Faraday's law (Da Pozzo et. al, 2008), drops to a value less than 40 % after 1 h of electrolysis. Although hydrogen peroxide is known to be an interfering species in the determination of active chlorine with DPD method, these data exclude the possibility since in the presence of H2O2 the amount of active chlorine was always found undetectable. Tests here not reported, verified that the analytical error, due to the presence of hydrogen peroxide in the spectrophotometric determination of active chlorine, becomes -1 appreciable for H2O2 concentrations higher than 1000 mg L . To evaluate the concentrations of active chlorine anodically electrogenerated, galvanostatic electrolyses of seawater solutions were conducted at j = 400 A m-2. To avoid the simultaneous production of hydrogen peroxide, a platinum disk was used as cathode. Figure 2 shows that a notable amount of active chlorine is produced during the electrolysis. Moreover, contrary
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