Ecotoxicity Measurements of Degraded Textile Dye by Electrochemical Process Using Boron-Doped Diamond Electrodes

This work presents aspects related to production, characterization and application of boron-doped diamond (BDD) films deposited on Titanium (Ti) substrate at different doping levels. These BDD/Ti electrodes were obtained by Hot Filament Chemical Vapor Deposition (HFCVD) technique. The BDD/Ti electrodes with the dimension of (2,5 x 2,5) cm² were prepared from H₂ (99%) and CH₄ (1%) gaseous mixture kept constant in the main gas line. The doping control were obtained from an additional H₂ gas flux passing through a bubbler containing a solution of B₂O₃ dissolved in CH₃OH with the B/C ratio of 2.000 and 15.000 ppm. The morphological and structural characterization was performed by Scanning Electron Microscopy (SEM), Visible Raman Spectroscopy and X-Ray Diffraction (XRD) techniques. The application of BDD/Ti electrodes for the Brilliant Green dye electrochemical oxidation was investigated by chronopotentiometry technique. Electrochemical measurements were made using a platinum mesh as a counter electrode and Ag/AgCl/KCl_(sat) as the reference electrode. The electrooxidation experiments were carried out at current densities of the 100 mA cm^-2 using 100 mg L^-1 of the Brilliant Green dye and K₂SO₄ 0,1 mol L^-1 as supporting electrolyte, in a total treatment time of 300 min. The degradation process was monitored by UV-spectrophotometry, Total Organic Carbon (TOC), High Efficiency Liquid Chromatografhy (HPLC) analysis, and toxicity against the Gram-negative bioluminescent marine bacterium Vibrio fischeri analysis. Concerning to the BDD morphology, the SEM images confirms that the films grown all over the Ti substrate and did not show cracks or fissures. BDD films depicted grains with a preferential (111) crystallographic orientation. Visible Raman spectroscopy confirmed the good quality of the BDD films by the presence of the narrow band at 1332 cm^-1 characteristic to the diamond signature [1]. This peak decreased in intensity due to boron incorporation in diamond films. The effect of boron doping is reflected in the spectral features. There is the appearance of the two bands located at 500 cm^-1 and at 1220 cm^-1 attributed to B-B vibrations and B-C vibrations, respectively. X-ray diffraction showed diamond diffraction planes, assuring the material crystallinity.  It was identified carbides (TiC) and hydrides (TiH) patterns in the films/substrate interface. The electrooxidation experiments were performed using galvanostat mode by applying 100 mA cm^-2. The cell temperature was kept constant at 25 ºC and the system was kept under stirring. Under these experimental conditions, the results showed that the BDD/Ti electrodes were efficient in the degradation of Brilliant Green dye. The electrode of 15.000 ppm showed better efficiency for solution decolourisation as well as higher efficiency in the mineralization of organic compounds compared to those obtained for lightly doped electrode (2.000 ppm). However, the ecotoxicity tests showed the highest efficiency for the 2.000 ppm electrode. This result can be associated to the high conductivity of the 15.000 ppm electrode that may improve the secondary reactions generated through the supporting electrolyte. In this sense, the high generation of peroxodisulfate ions from these secondary reactions provides this high toxicity in the degraded solutions (Table 1). This behavior was also observed in the ecotoxicity measurements using only the supporting electrolyte.

Acknowledgements: FAPESP (2010/18462-2) and CNPq (Brazil).

[1] May, P. W.; Ludlow, W. J.; Hannaway, M.; Heard, P. J.; Smith, J. A.; Rosser, K. N. Diamond and Related Materials 17, (2008), 105-11.