EFFECT OF ELECTROLYTE ON ELECTROCHEMICAL OXIDATION

EFFECT OF ELECTROLYTE ON ELECTROCHEMICAL OXIDATION
OF THE CARBOFURAN USING Ti/BDD ANODES
M. R. Baldan1*; W. D. Toledo1; M. R. V. Lanza2; N.G. Ferreira1; N. M. Santos1
1
National Institute for Space Research, São José dos Campos, SP, Brazil
2
University of São Paulo (USP/IQSC), São Carlos, Brazil.
Pesticide removal by the electrochemical oxidation was investigated with commercial Carbofuran with an
electrolyte containing aqueous solutions K2SO4 0.1 mol L-1 or H2SO4 0.1 mol L-1 thermo stated at 20ºC, in a
flow reactor with recirculation system. The degradation reactions were performed at constant applied current
densities in the range 10 to 200 mAcm-2, flow rates of 50 and 300 Lh-1 for a total of 5 hours. The electrochemical
oxidation of carbofuran pesticides has been carried using as anode four Ti/BDD electrodes, produced by Hot
Filament Chemical Vapor Deposition reactor. The morphological and structural characterizations of the
electrodes were evaluated by Scanning Electron Microscopy and by Raman Scattering Spectroscopy techniques.
The electrochemical of Ti/BDD electrodes was performed by Cyclic Voltammetry in presence of the electrolytes.
The effect of pH of the electrolyte in the electrochemical degradation of carbofuran was monitored by UV–Vis
Spectrophotometry, Total Organic Carbon and High Performance Liquid Chromatography. The reducing the
TOC concentration promoted the mineralization of pesticide contributing to the formation of carbofuran
intermediates resulting from the reaction of oxidation. A completely degraded of the pesticide was obtained
under an acid pH and current densities of 200 mAcm2.
Keywords: BDD anodes, carbofuran, wastewater treatment, electrochemical oxidation.
Introduction
The electrochemical oxidation (EO) has been gained great interests as outstanding effective
technologies to remove toxic micro pollutants [1-3]. In the EO process, hydroxyl radicals
(OH) is mainly generated at the anode surface from water oxidation and carbon dioxide in a
non-selected way, whose production rate is determined by the character of the anode material
[4]. On the other hand, the high-efficiency electrode of conductive-diamond (BDD) anodes,
with a promotion of higher mineralization rate of organics, has been widely applied to treat
persistent pollutants [5, 6]. The experiments were designed to study the effect of electrolyte,
and current density on EO process of the carbofuran, in which condition would benefit the
higher production of •OH at BDD anode surface. The aim was to find the optimum values for
operating conditions. A detailed study of the oxidation process on carbofuran by EO was
provided to assess the environmental impact of the treatments.
Experimental part
The EO of carbofuran pesticides 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 laminar and turbulent flow conditions and at different applied current
densities for a total of 5 h with an aqueous electrolyte containing potassium sulphate or
sulphuric acid (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 TOCVCPN analyzer. BDD electrodes were produced on pure titanium substrate 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 BDD electrodes morphology and quality were evaluated
by SEM, micro-Raman scattering spectroscopy and the electrochemical characterization was
performed by cyclic voltammetry.
Results and discussion
The representative spectra obtained by UV-Vis showed an absorption band variation centered
on 276 nm that 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 4.20 min corresponding to pesticide.
The Raman scattering spectra of the 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. SEM images revealed continuous and homogeneous BDD electrodes covering the
entire substrates.
Conclusions
This study shows BDD electrodes are capable of rapidly oxidizing carbofuran to inorganic
reaction products. The process likely involves direct oxidation via electron transfer and
indirect oxidation via reaction with hydroxyl radicals produced from water oxidation. It can
be concluded that the EO treatment with Ti/BDD as anode is highly effective for the rapid
degradation and mineralization of carbofuran and the increasing current density accelerates
the degradation and mineralization processes. The total degradation arised from hydroxylation
on the aromatic moiety and also through the homolytic bond dissociation and hydrolysis of
the carbamate C–O bond.
References
[1] Martinez-Huitle, C.A., Ferro, S., Chem. Society Reviews, 2006, 35, 1324.
[2] Pimentel, M., Oturan, N., Dezotti, M., Oturan, M.A., Appl. Catal. B: Env., 2008, 83, 140.
[3] Oturan, M.A., Oturan, N., Lahitte, C., Trevin, S., J. Electroanal. Chem., 2001, 50, 796.
[4] Brillas, E., Sirés, I., Oturan, M.A., Chem. Reviews, 2009,109, 6570.
[5] Panizza, M., Martinez-Huitle, C.A., Chemosphere, 2013, 90, 1455.
[6] Cañizares, P., García-Gómez, J., Lobato, J., Rodrigo, M.A., Ind. & Eng. Chem.
Research, 2003, 42, 956.
Acknowledgement: The authors gratefully acknowledge the following Brazilian funding
authorities for financial support: FAPESP, CAPES and CNPq.

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