The dye contamination in wastewaters is mainly due to the textile sector which generating large volumes of toxic wastewater. In this context, dye oxidation has been studied using different chemical and electrochemical processes. Particularly, electrochemical technique depends on the high performance of electrode material. Boron doped diamond has appeared as singular material for electrochemical oxidation in wastewater treatment, mainly due to its very large potential widow, that allows reaching high anodic potential for hydroxyl radical (OH•) generation . In this context, carbon fiber (CF) is a great interest substrate to grow BDD films because it exhibits high surface area allowing high active site density in a homogeneous porosity distribution. On the other hand, titanium dioxide (TiO2) is one of the most studied semiconducting oxide for different electrochemical applications because it is stable in both alkaline and acid media, besides its high electrochemically surface area . Thus, ternary and binary composites were produced and characterized considering the constituent materials of carbon fibers (CF), boron doped diamond (BDD), and titanium dioxide (TiO2) as electrodes for electrochemical dye oxidation.
CF samples were produced from polyacrylonitrile (PAN) precursor at heat treatment temperature of 1000 ºC and were cut in 25 x 25 mm2 to produce large electrode area. BDD films were grown on CF substrate by hot filament chemical vapor deposition (HFCVD) the following growth parameters: 750 °C, 30 Torr, 8h and gas mixture of 2/198 % CH4/H2. Boron source was obtained by an additional hydrogen line passing through a bubbler with B2O3 dissolved in methanol by controlling the B/C ppm ratio. TiO2 depositions on BDD/CF substrate was obtained by anodic hydrolysis of TiCl3 under potentiostatic mode, at a fixed potential of 0.75 V for 60 min in a 5 mmol.L-1 TiCl3 + 0.1 mol.L-1 KCl (pH = 2) aqueous solution. All the electrochemical experiments were performed in a conventional three-electrode glass cell, using a platinum wire as a counter electrode an Ag/AgCl/KCl (sat) as the reference electrode. Both composite materials were characterized by field emission gun-scanning electron microscopy (FEG-SEM) images, Raman spectroscopy, and Fourier transform infrared spectroscopy (FTIR). CF, TiO2/CF, and TiO2/BDD/CF were used as electrodes for electrooxidation of the Brilliant Green dye at different current densities using 100 mg L-1 of the Brilliant Green dye in K2SO4 0.1 mol L-1 solution for 300 min. Their efficiency was analyzed by UV/VIS Spectrophotometry and by Total Organic Carbon techniques considering the influence of the electrode characteristics in the degradation process.
The binary and ternary TiO2/CF and TiO2/BDD/CF composites were firstly analyzed by FEG-SEM images taking into account these two sample considering the TiO2 deposits. The FEG-SEM images for TiO2/CF deposited at 30 min showed a thick TiO2 layer covering the fibers, with CF surface scratches or cracks completely filled by TiO2 film. For ternary composite, after 60 min of deposition the rougher film texture is dominant and homogenous covering all diamond grains. This morphology seems to be suitable to keep the contributions of these constituent material properties for TiO2/BDD/CF composite, mainly related to the BDD importance for electrochemical oxidation processes of organic pollutants. Concerning the Brilhant Green Dye electrochemical oxidation, the results showed that all the electrodes were efficient in the solution discolorization. Also, the organic compound mineralization showed a decrease according to TOC measurements of the electrolyzed samples. Considering the synergism between two good anode materials, BDD and TiO2 for organic electrodegradation, both of them binary and ternary composites presented the good performance. However, the ternary electrode, probably associated to its highest electrode surface area, showed the best performance.
Anodic hydrolysis of TiCl3 under potentiostatic mode was a suitable procedure to produce binary and ternary composites. In addition, both of them showed satisfactory results in electrolysis processes to degrade the Brilhant Green Dye.
This work was supported by FAPESP, Process 2016/13393-9, CAPES, and CNPq Brazilian Agencies.
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