BORON-DOPED DIAMOND ELECTRODES FOR ...
BORON-DOPED DIAMOND ELECTRODES FOR ELECTROCHEMICAL DEGRADATION OF CARBOFURAN N. M. Santos1*; W. D. Toledo1; M. R. V. Lanza2; N.G. Ferreira1; M. R. Baldan1 1 National Institute for Space Research, São José dos Campos, SP, Brazil 2 University of São Paulo (USP/IQSC), São Carlos, Brazil. Boron-doped diamond electrodes were produced on titanium substrate (Ti/BDD) by Hot Filament Chemical Vapor Deposition (HFCVD) reactor for electrochemical degradation of carbofuran pesticide. The morphological and structural characterizations of the electrodes were evaluated by scanning electron microscopy and by Raman scattering Spectroscopy techniques. The electrochemical characterization of electrodes was performed by cyclic voltammetry, and it was observed large potential windows. The electrochemical degradation of the carbofuran were performed at different current densities, flow rates of 50 and 300 Lh-1 for a total of 2 hours with an electrolyte aqueous solutions K2SO40.1M at 20ºC and Carbofuran concentration of 400 mgL -1 in a flow reactor with recirculation. The efficiency of the pesticide degradation process was monitored by UV–Vis spectrophotometry, total organic carbon and high performance liquid chromatography techniques. The obtained results show that Ti/BDD anodes are promising for electrochemical treatment in the wastewater. The reducing the TOC concentration promoted a breakdown of the carbofuran molecule rather than the mineralization of organic matter. This fact may have contributed to the formation of carbofuran intermediates resulting from the reaction of degradation of the pesticide.
Keywords: Ti/BDD electrode, pesticides, anodic oxidation, environmental analysis Introduction Ti/BDD electrodes have been extensively studied for their ability to oxidize a wide range of water contaminants. Contaminants can be oxidized by a combination of direct electron transfer and reaction with hydroxyl radicals (OH*) produced from water oxidation [1-3]. Ti/BDD electrodes are considered as one of the most ideal electrodes for electro analysis and electrolysis [4, 5]. To date, conductive diamond has several superior characteristics compared with other carbon materials and metals, including very low background current density and wide potential window in aqueous solution [6]. The advantage of the diamond electrode is the fact that it allows the detection of redox reactions in certain potentials that would be outside the range of work potential of conventional electrodes, as platinum (~ 2.0 V), glassy carbon (~ 2.5 V), and graphite (~ 2.0 V). The Ti/BDD produced and characterized in this work was very efficient as an electrode for electrochemical degradation of carbofuran pesticide. Experimental part Boron doping diamond electrodes were produced on pure titanium substrate (Ti/BDD) by Hot Filament Chemical Vapor Deposition reactor (HFCVD) using CH4/H2 gas mixtures. Boron was obtained from H2 forced to pass through a bubbler containing trimetylborate BO3(CH3)3 dissolved in methanol (CH3OH). The Ti/BDD electrodes morphology and quality were evaluated by SEM, Raman scattering spectroscopy and the electrochemical characterization was performed by cyclic voltammetry. The electrochemical oxidation of carbofuran has been carried in an electrochemical reactor comprising two parallel polypropylene plates fitted with four Ti/BDD anodes and four 316 L stainless steel cathodes. The reactor was connected to a recirculation system through which electrolytes could be supplied at flow rates of 50 L h−1 or 300 L h−1 (turbulent flow; Re 1900). Degradation reactions were performed at different applied
current densities (jappl) in the range 10 to 200 mA cm−2 for a total of 2 h with an aqueous electrolyte containing potassium sulphate (0.1 mol L−1) and 400 mg L−1 of commercial carbofuran thermo stated at 20 °C. The UV–Vis spectra were collected samples, the concentration of pesticide in the electrolyte was monitored by high performance liquid chromatography (HPLC) and the variation in total organic carbon (TOC) in the samples of electrolyte was measured using a Shimadzu TOC-VCPN analyzer. Results and discussion SEM images revealed continuous and homogeneous BDD films covering the entire substrates. The Raman scattering spectra of the Ti/BDD electrodes presented a sharp peak at around 1332 cm−1 corresponding to the first-order phonon line in diamond, a emergent band at 1220 cm−1 may be attributed to disorder in the diamond structure caused by the incorporation of boron and the band at around 500 cm−1 is associated with the vibration of boron pairs in the diamond lattice. The representative spectra obtained by UV-Vis show that an absorption band variation centered on 276 nm could be detected after 10 min of electrolysis. Increases in the intensity of this band were more pronounced in the spectral set obtained when electrolysis was performed at a density current of 200 mA cm−2. These results indicate that the electrolyte was modified in a time-dependent manner during electrolysis, possibly as a result of the degradation of the components of commercial carbofuran with formation of intermediaries’ products. HPLC chromatograms of the electrolyte containing pesticide showed peaks at 2.16 min corresponding to the supporting electrolyte and peak at 4.2 min corresponding to pesticide. The promising results reported here indicate that parameters relating to the surface structure of the electrode and the operating conditions of the electrochemical reactor must be systematically controlled in order to achieve the best configuration for the degradation of carbofuran. Conclusions It can be concluded that the electrochemical oxidation processes with Ti/BDD as anode could be efficiently applied to remove pesticide and the increasing current density accelerates the degradation and mineralization processes, but with a loss in mineralization current efficiency due to the side reaction and energy loss on the persistent byproducts produced. The aromatic intermediates were oxidized at the early stage by addition of OH on the benzenic ring (hydroxylation) or by H atom abstraction from side chain. Then, the oxidative cleavage of polyhydroxylated aromatic derivatives conducts to the formation of short chain, causing the decrease of solution toxicity. References [1] Chaplin, B.P., Scharader, G. and Farrell, J., 2010. Env. Sci. and Technol. 44, 4263. [2] Kapalka, A., Foti, G., Comninellis, C., 2008. J. of the Electrochem. Soc. 155, E27. [3] Carter, K.E. and Farrell, J., 2008. Env. Sci. and Technol 42, 6111. [4] Gandini, D. et al 2000. J. Appl. Electrochem. 30, 1345. [5] Suffredini, H.B., Machado, S. A. S, Avaca, L. A., Braz, J., 2004. Chem. Soc. 15, 16. [6] De Clements, R., and Mswain, G., 1994. J. Electrochem. Soc. 144, 3382. Acknowledgement: The authors gratefully acknowledge the following Brazilian funding authorities for financial support: FAPESP, CAPES and CNPq.
Keywords: Ti/BDD electrode, pesticides, anodic oxidation, environmental analysis Introduction Ti/BDD electrodes have been extensively studied for their ability to oxidize a wide range of water contaminants. Contaminants can be oxidized by a combination of direct electron transfer and reaction with hydroxyl radicals (OH*) produced from water oxidation [1-3]. Ti/BDD electrodes are considered as one of the most ideal electrodes for electro analysis and electrolysis [4, 5]. To date, conductive diamond has several superior characteristics compared with other carbon materials and metals, including very low background current density and wide potential window in aqueous solution [6]. The advantage of the diamond electrode is the fact that it allows the detection of redox reactions in certain potentials that would be outside the range of work potential of conventional electrodes, as platinum (~ 2.0 V), glassy carbon (~ 2.5 V), and graphite (~ 2.0 V). The Ti/BDD produced and characterized in this work was very efficient as an electrode for electrochemical degradation of carbofuran pesticide. Experimental part Boron doping diamond electrodes were produced on pure titanium substrate (Ti/BDD) by Hot Filament Chemical Vapor Deposition reactor (HFCVD) using CH4/H2 gas mixtures. Boron was obtained from H2 forced to pass through a bubbler containing trimetylborate BO3(CH3)3 dissolved in methanol (CH3OH). The Ti/BDD electrodes morphology and quality were evaluated by SEM, Raman scattering spectroscopy and the electrochemical characterization was performed by cyclic voltammetry. The electrochemical oxidation of carbofuran has been carried in an electrochemical reactor comprising two parallel polypropylene plates fitted with four Ti/BDD anodes and four 316 L stainless steel cathodes. The reactor was connected to a recirculation system through which electrolytes could be supplied at flow rates of 50 L h−1 or 300 L h−1 (turbulent flow; Re 1900). Degradation reactions were performed at different applied
current densities (jappl) in the range 10 to 200 mA cm−2 for a total of 2 h with an aqueous electrolyte containing potassium sulphate (0.1 mol L−1) and 400 mg L−1 of commercial carbofuran thermo stated at 20 °C. The UV–Vis spectra were collected samples, the concentration of pesticide in the electrolyte was monitored by high performance liquid chromatography (HPLC) and the variation in total organic carbon (TOC) in the samples of electrolyte was measured using a Shimadzu TOC-VCPN analyzer. Results and discussion SEM images revealed continuous and homogeneous BDD films covering the entire substrates. The Raman scattering spectra of the Ti/BDD electrodes presented a sharp peak at around 1332 cm−1 corresponding to the first-order phonon line in diamond, a emergent band at 1220 cm−1 may be attributed to disorder in the diamond structure caused by the incorporation of boron and the band at around 500 cm−1 is associated with the vibration of boron pairs in the diamond lattice. The representative spectra obtained by UV-Vis show that an absorption band variation centered on 276 nm could be detected after 10 min of electrolysis. Increases in the intensity of this band were more pronounced in the spectral set obtained when electrolysis was performed at a density current of 200 mA cm−2. These results indicate that the electrolyte was modified in a time-dependent manner during electrolysis, possibly as a result of the degradation of the components of commercial carbofuran with formation of intermediaries’ products. HPLC chromatograms of the electrolyte containing pesticide showed peaks at 2.16 min corresponding to the supporting electrolyte and peak at 4.2 min corresponding to pesticide. The promising results reported here indicate that parameters relating to the surface structure of the electrode and the operating conditions of the electrochemical reactor must be systematically controlled in order to achieve the best configuration for the degradation of carbofuran. Conclusions It can be concluded that the electrochemical oxidation processes with Ti/BDD as anode could be efficiently applied to remove pesticide and the increasing current density accelerates the degradation and mineralization processes, but with a loss in mineralization current efficiency due to the side reaction and energy loss on the persistent byproducts produced. The aromatic intermediates were oxidized at the early stage by addition of OH on the benzenic ring (hydroxylation) or by H atom abstraction from side chain. Then, the oxidative cleavage of polyhydroxylated aromatic derivatives conducts to the formation of short chain, causing the decrease of solution toxicity. References [1] Chaplin, B.P., Scharader, G. and Farrell, J., 2010. Env. Sci. and Technol. 44, 4263. [2] Kapalka, A., Foti, G., Comninellis, C., 2008. J. of the Electrochem. Soc. 155, E27. [3] Carter, K.E. and Farrell, J., 2008. Env. Sci. and Technol 42, 6111. [4] Gandini, D. et al 2000. J. Appl. Electrochem. 30, 1345. [5] Suffredini, H.B., Machado, S. A. S, Avaca, L. A., Braz, J., 2004. Chem. Soc. 15, 16. [6] De Clements, R., and Mswain, G., 1994. J. Electrochem. Soc. 144, 3382. Acknowledgement: The authors gratefully acknowledge the following Brazilian funding authorities for financial support: FAPESP, CAPES and CNPq.
