Electrochemical advanced oxidation process using
DiaChem
®electrodes
I. Tröster*, L. Schäfer*, M. Fryda** and T. Matthée**
* Fraunhofer Institut für Schicht- und Oberflächentechnik, Bienroder Weg 54e, 38108 Braunschweig, Germany (E-mail: [email protected];[email protected])
** CONDIAS GmbH, Fraunhofer Str. 1b, 25524 Itzehoe, Germany (E-mail: [email protected];[email protected])
Abstract The electrochemical advanced oxidation process (EAOP) using boron doped diamond (DiaChem®, registered trademark of Condias GmbH) has been studied for wastewater treatment and drinking water disinfection. DiaChem®electrodes consist of preferentially metallic base materials coated with a conductive polycrystalline diamond film by hot-filament chemical vapour deposition. They exhibit high overpotential for water electrolysis as well as high chemical inertness and extended lifetime. In particular the high overpotential for water decomposition opens the widest known electrochemical window, allowing the energy efficient production of hydroxyl radicals directly from aqueous solutions. The hydroxyl radicals on the other hand are effectively used for the oxidation of pollutants. The EAOP using DiaChem® electrodes thus facilitates the direct and, if necessary, complete decomposition of even hazardous or persistent pollutants in different wastewaters. Current efficiencies of more than 90%, also without the use of additives for hydroxyl radical generation, have been demonstrated. Additionally, for drinking water preparation diamond electrodes facilitate disinfection with and without the support of chlorine.
Keywords Diamond electrode; electrochemical advanced oxidation process; hydroxyl radicals; water treatment
Introduction
Effluent and water treatment are essential technologies to protect the environment and reduce danger to ourselves. The main goal is the elimination of toxic pollutants or at least their conversion to harmless or biocompatible species, respectively. Current technologies for water treatment consist of physical, chemical and biological treatment methods for the purification of wastewater (Liu and Litpák, 2000). Additionally, tertiary treatment meth-ods, e.g. advanced oxidation processes (AOP) (Andreozzi et al., 1999, Gogate and Pandit, 2003, and references therein), are used to support primary and secondary treatment meth-ods to achieve the elimination of resistant and less reactive pollutants (Rajeshwar and Ibanez, 1997). The role of electrochemistry in water and effluent treatment is relatively small, since conventional electrode materials achieve only low current efficiencies due to the water electrolysis side reaction (Comninellis, 1994; Simonsson, 1997). However, electrochemical methods have already been used e.g. in electro-Fenton and photoelectro-Fenton processes (Brillas et al., 2000) as well as for electro-coagulation and electrochemical separation (Dimoglo et al., 2003). In these cases electrochemistry is broadening the versa-tile activation methods for hydroxyl radical production from precursors and supporting redox systems in AOP.
With the development of a new electrode material, conductive diamond, pure electro-chemical solutions for effluent-treatment problems become more advantageous. Diamond electrodes exhibit high overpotential for water electrolysis as well as high chemical inert-ness and an extended lifetime. In particular, the high overpotential for water decomposition opens the widest known electrochemical window (Angus et al., 1999), allowing the
Water Science and Technology
Vol 49 No 4 pp 207–212
© IWA
Publishing 2004
electrogeneration of hydroxyl radicals directly from aqueous solutions. The evidence of hydroxyl radicals was provided by electron spin resonance and liquid chromatography measurements, using 5,5-dimethyl-1-pyrroline-N-oxide and salicylic acid for spin trap-ping, respectively (Marselli et al., 2002). Additionally, the nonactive behaviour of the sur-face of diamond electrodes promotes almost exclusively the combustion of organic pollutants with high current efficiencies (Comninellis et al., 1999). Figure 1 presents the electrochemical windows of different electrode materials and the potential of formation for hydroxyl radicals.
The electrochemical production of oxidants at the diamond surface can also be exploited for the disinfection of drinking water and removal of colour and odour to prevent water-borne diseases. Current technologies for water disinfection are chemical treatment with disinfectants or physical treatments, like filtration or radiation. Electrochemical methods for the generation of oxidants for disinfection are still under investigation (Rambaud et al., 2001; Maier et al., 2003; Li et al., 2003). The direct and highly efficient electrochemical in-situ production of the most common chemical chlorine-based disinfection agents at the dia-mond surface may achieve more accurate dosage and simplifies the handling of chemicals. Moreover, diamond electrodes are able to establish a chlorine-free, oxygen-based disinfec-tion, avoiding the drawbacks of chlorine and the generation of harmful by-products (Rajeshwar and Ibanez, 1997). Establishing EAOP with diamond electrodes for water treatment technologies offers the benefits of electrolytic processes: routine operation with automated control and low maintenance, controllable and fast start-up and shut-down, continuous monitoring of the process using sensor signals like current and/or potential parameters.
Methods
Production and properties of DiaChem®electrodes
DiaChem®electrodes consist of preferentially metallic base materials which are coated with a conductive polycrystalline diamond film. Fraunhofer IST, Germany, in cooperation with Condias GmbH, Germany, and CSEM, Switzerland have developed and improved the technology for the deposition of boron doped conductive diamond coatings for diamond electrodes. DiaChem®electrodes with reproducible electrochemical and physical proper-ties are fabricated on base materials of up to 0.5 m ×1 m in size. The fabrication of the
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Potential / V vs. SHE
Figure 1 Electrochemical working potential windows of different electrode materials (MOX: Ti/IrO2). The width of the bars represents the potential region for water stability in electrochemical applications
electrodes comprises cleaning of substrate materials (various metals, doped silicon or graphite), seeding with diamond powder and deposition of boron doped polycrystalline diamond film by hot-filament activated chemical vapour deposition (HFCVD) (Fryda et al., 1999). Typical temperatures for thermal activation at the filaments are 2,200–2,600°C. Substrate temperatures range between 700–925°C. Total pressures of 10–50 mbar are applied at gas phase compositions of 0.5–2.5% methane and 10–200 ppm diborane in a hydrogen support gas. Operating HFCVD processes under these typical deposition param-eters yield growth rates of 0.2–1.2 µm/h and electrical resistances of the deposited diamond films between 10–3and 5 ×10–5Ωm, depending on the boron concentration in the films.
Characterization of the diamond films comprises several methods. Representative DiaChem®samples of each deposition run are inspected by scanning electron microscope (SEM) for diamond morphology and crystal size (Joel JSM 6300 F). Raman spectra are used for the detection of non-diamond carbon modification (Instruments S. A. Explorer at 514.5 nm) and secondary ion mass spectrometry determines the distribution of dopant and impurities in the diamond film (Cameca ims 5f SIMS with Cs+ion cluster method) (Fryda et al., 1999). Figure 2 presents a selection of DiaChem®electrodes and a magnification of the polycrystalline diamond coating.
Cyclic voltammograms in sulfuric acid and sodium hydroxide solution are used as a standard electrochemical characterization (EG&G Parc three electrode flat cell, Zahner electric IM5d potentiostat, Pt counter electrode, Ag/AgCl reference electrode). In diluted sulfuric acid metal-based diamond electrodes exhibit an electrochemical potential of up to 2.8 V for oxygen generation and about –1.3 V for hydrogen generation (Fryda et al., 2001). Diamond electrodes show no fouling of the diamond surface. During severe operating con-ditions (e.g. current densities up to 40,000 A/m2in diluted sulfuric acid), niobium-based electrodes showed no degradation in their electrochemical performance for several months or ten thousands of Ah, respectively.
Water treatment
The EAOP-treatment of different types of model and real wastewaters was carried out in different electrochemical cells especially developed for the application of diamond elec-trodes. Typical operating conditions are a circular flow of the wastewater and anodic cur-rent densities of 300–500 A/m2. The decomposition of pollutants was monitored by the
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reduction of the chemical oxygen demand (COD) for organic pollutants (COD cell test; Merck Spectroquant®, photometrical detection).
Results and discussion Wastewater treatment
Diamond electrodes yield a continuous electrogeneration of hydroxyl radicals, allowing the reduction of COD in preferentially highly polluted wastewaters with high removal rates. The EAOP has already been used for the treatment of model and real wastewaters containing different types of pollutants, e.g. inorganic cyanides and aromatic organic com-pounds (Perret et al., 1999), concentrates with persistent biotoxic organic compounds from filtrative treatment of water (Verstraete et al., 2002), complex forming agents, cooling lubricants and others (Tröster et al., 2002). Figure 3 presents the decomposition of phenol in a model wastewater starting from different initial concentrations of organic load. The applied current density at the diamond electrode was 300 A/m2in a monopolar flow cell with flow rates of 25 l/h in a circular flow operation mode. The decrease of organic load exhibits a linear region which is independent of the initial phenol concentration. The cur-rent efficiency for decomposition in this linear region is almost 100%. Nevertheless, in the highly concentrated solution fouling of the electrode emerged, due to formation of organic polymers at the cathodic surface. This problem was solved by applying the polarity reversal mode. At low COD values the linear region of COD decomposition changes to an exponen-tial trend. This different behaviour of COD removal rates has been described by Comninellis et al. (2001), presenting a theoretical model to predict COD evolution and cur-rent efficiency during the EAOP treatment of organic pollutants. Due to this model the applied current density, i.e. hydroxyl radical generation, has to be adjusted to the pollutant concentration to obtain fast and efficient COD removal. However, in the decomposition presented in Figure 3 the diamond electrodes were operated with constant current density. Therefore the COD decays present the two operating regimes of EAOP. In the linear region the COD removal is under current control and the obtained current efficiency is nearly 100%. At lower COD concentrations the removal changes to an exponential trend, repre-senting mass transport control with beginning side reactions (such as oxygen evolution), which lead to the decrease of current efficiency. Actually, mass transport limitations and reaction rates are closely linked to system design and operating parameters. Therefore, it is
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Figure 3 Decomposition of phenol model wastewater with different initial concentrations using DiaChem® electrodes
obvious that EAOP systems have to be adapted to specific customer requirements in order to optimise for desired COD reduction rates at highest current efficiencies.
Water disinfection
The EAOP using diamond electrodes has also been applied for the disinfection of chloride-containing swimming pool water. The diamond-based systems exhibit a continuous chlo-rine productivity and a higher disinfecting performance against bacteria in comparison to directly added hypochlorite (NaOCl). This effect was also based on the production of addi-tional oxidizing species such as e. g. hydrogen peroxide in natural mineralised water. In addition it was found that almost all introduced organic matter (e.g. sun protection oils) was destroyed by electrochemical oxidation applying diamond electrodes (Haenni et al., 2002). Aside from based disinfection, diamond electrodes were tested for chlorine-free disinfection at Gerus GmbH, Germany. The performance of diamond electrodes was compared with common electrode materials like Pt and IrO2in a bacteria and sugar contain-ing solution (Figure 4). Treatment with DiaChem®electrodes achieved not only consider-able reduction in bacteria population but also a simultaneous removal of the COD by combustion of the sugar. Both effects can be put down to the formation of hydroxyl radi-cals. Therefore the EAOP offers the improvement of disinfection methods based on, for instance, oxygen containing species. Additionally, the simultaneous COD removal and dis-infection might be highly interesting for industrial process water treatment, e.g. closed loop technologies.
Conclusions
The EAOP using DiaChem®electrodes has been applied for the purification of wastewater and disinfection of drinking water in laboratory-scale and small pilot-scale systems. Based on the energy efficient continuous formation of hydroxyl radicals directly from water this advanced water treatment method enables the elimination of pollutants in a broad range of concentrations. Nevertheless, the emerging technologies of diamond deposition and EAOP treatment for water purification require further development:
1. investigation of treatment efficiencies and applicability in comparison to established advanced treatment methods for resistant and/or toxic pollutants as well as for highly polluted wastewater,
2. improvement and further development of electrochemical cells adjusted to the opera-tion mode of diamond electrodes and taking into account hydrodynamic condiopera-tions,
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Figure 4 Disinfective performance of diamond electrodes in comparison to common electrode materials: sterilisation of a bacteria (14 ×107CFU =colony forming units of E. coli) and sugar (glucose, COD 9 g O
2/l)
3. up-scaling of electrochemical systems, engineering and construction of EAOP-plants and adjustment to specific customer requirements, and
4. improvement of physical properties and lifetime of diamond electrodes for different applications.
References
Andreozzi, R., Caprio, V., Insola, A. and Marotta, R. (1999). Advanced oxidation processes (AOP) for water purification and recovery. Catal. Today, 53, 51–59.
Angus, J.C., Martin, H.B., Landau, U., Evstefeeva, Y.E., Miller, B. and Vinokur, N. (1999). Conducting diamond electrodes: applications in electrochemistry. New Diamond Front. Carbon Tech., 9(3), 175–187.
Brillas, E., Calpe, J.C. and Casado, J. (2000). Mineralization of 2,4-D by advanced electrochemical oxidation processes. Wat. Res., 34(8), 2253–2262.
Comninellis, Ch. (1994). Electrocatalysis in the electrochemical conversion/combustion of organic pollutants for wastewater treatment. Electrochim. Acta, 39(11), 1857–1862.
Comninellis, Ch., Fóti, G., Gandini, D., Perret, A. and Haenni, W. (1999). Oxidation of organics by intermediates of water discharge on IrO2and synthetic diamond anodes. Electrochem. Solid-State Lett., 2(5), 228–230.
Comninellis, Ch., Panizza, M., Michaud, P.A. and Cerisola, G. (2001). Electrochemical treatment of wastewaters containing organic pollutants on boron-doped diamond electrodes: Prediction of specific energy consumption and required electrode are. Electrochem. Comm., 3, 336–339.
Dimoglo, A., Inan, H., Simsek, H. and Karpuczcu, M. (2003). Olive oil mill wastewater treatment by means of electro-coagulation. Sep. Purif. Techn.(in press).
Fryda, M., Schäfer, L. and Tröster, I. (2001). Doped diamond – A new material for industrial
electrochemistry. In: Recent Research Developments in Electrochemistry, S.G. Pandalai (ed.), vol 4, Transworld Research Network, Trivandrum, 85–97.
Fryda, M., Herrmann, D., Schäfer, L., Klages, C.-P., Perret, A., Haenni, W., Comninellis, Ch. and Gandini, D. (1999). Properties of diamond electrodes for wastewater treatment. New Diamond Front. Carbon
Technol., 9, 229–240.
Gogate, P.R. and Pandit, A.B. (2003). A review of imperative technologies for wastewater treatment I: oxidation technologies at ambient conditions; A review of imperative technologies for wastewater treatment II: hybrid methods. Adv. Environ. Res., (in press).
Haenni, W., Gobet, J., Perret, A., Pupunat, L., Rychen, P., Comninellis, Ch. and Correa, B. (2002). Loop-controlled production of chlorine for disinfection of pool water using boron-doped diamond electrodes.
New Diamond Front. Carbon Tech., 12(2), 83–88.
Li, Z.M., Diao, H.F., Gu, J.D., Shi, H.C. and Xie, Z.M. (2003). Electron microscopic investigation of the bacterial action of electrochemical disinfection in comparison with chlorination, ozonation and Fenton reaction. Process Biochem.(in press).
Liu, D.H.F. and Lipták, B.G. (2000). Wastewater Treatment, Lewis Publishers, Boca Raton, FL. Mier, R.M., Drees, K.P. and Abbaszadegan, M. (2003). Comparative electrochemical inactivation of
bacteria and bacteriophage. Wat. Res., 37(10), 2291–2300.
Marselli, B., Garcia-Gomez, J., Michaud, P.-A., Rodrigo, M.A. and Comninellis, Ch. (2002).
Electrogeneration of Hydroxyl Radicals on Boron-Doped Diamond Electrodes. J. Electrochem. Soc., 150(3), D79–D83.
Perret, A., Haenni, W., Skinner, N., Tang, X.-M., Gandini, D., Comninellis, Ch., Correa, B. and Fóti, G. (1999). Electrochemical behavior of synthetic diamond thin film electrodes. Diamond Rel. Mat., 8, 820–823.
Rajeshwar, K. and Ibanez, J. (1997). Environmental Electrochemistry, Fundamentals and Applications in
Pollution Abatement, Academic Press, San Diego, pp. 36–49.
Rambaud, A., Drogui, P., Elmaleh, S., Rumeau, M. and Bernard, C. (2001). Oxidising and disinfecting by hydrogen peroxide produced in a two-electrode cell. Wat. Res., 35(13), 3235–3241.
Simonsson, D. (1997). Electrochemistry for a cleaner environment. Chem. Soc. Rev., 26, 181–189. Tröster, I., Schäfer, L. and Fryda, M. (2002). Recent developments in production and application of
DiaChem®electrodes for wastewater treatment. New Diamond Front. Carbon Tech., 12(2), 89–97.
Verstraete, W., Van Hege, K. and Verhaege, M. (2002). Indirect electrochemical oxidation of reverse osmosis membrane concentrates at boron-doped diamond electrodes. Electrochem. Comm., 4, 296–300.
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et al.
