Visible Light-Induced Photoelectrocalalytic Degradation of 4-Nitrophenol on BiVO4/Carbon Nanotube Electrode

Abstract

The 4-Nitrophenol (1-Hydroxy-4-nitrobenzene, 4-NP) has been appointed as hazardous waste and priority toxic pollutants by U. S. Environmental Protection Agency (EPA). The 4-NP is present in wastewater of several industries and it presents a carcinogenic characteristic. Therefore, efficiently and low cost process for wastewater treatment for 4-NP is necessary. On the other hand, great interests in photocatalysis have been focused on degradation of organic pollutants under visible light irradiation. Thus, the aim of this work was to develop a photoanode based in bismuth vanadate (BiVO4) and multi-walled carbon nanotubes (MWNT) for 4-NP photoelectrochemical degradation under visible-light. A three-electrode conventional electrochemical cell was used. The Ag/AgCl/Cl– sat, platinum and BiVO4/MWCNT/FTO were used as the reference, auxiliary and working electrode, respectively. BiVO4/FTO and BiVO4/MWCNT/FTO electrodes were prepared using polyethylene glycol (PEG 300) as precursor solution. The BiVO4 photoanodes were characterized by X-ray diffraction, diffuse reflectance and cyclic voltammetry. The photocatalytic activity of the electrodes was evaluated using a 150 W metallic vapor lamp as the light source. The degradation experiments were carried in a cell with 20 mL of 0.5 mol L– 1 Na2SO4 and 10 mg L– 1 4-NP under magnetic stirring. The current density of 5 mA cm– 2 was applied for 1h. The concentration of 4-NP was monitored by UV-vis spectrophotometry. The presence of characteristics diffractions peaks are in agreement with crystal structure of the type monoclinic scheelite. The bandgap energy estimated by diffuse reflectance experiments for BiVO4/FTO and BiVO4/MWCNT/FTO were 2.43 eV and 2.42 eV, respectively. These results also confirm the presence of monoclinic structure of BiVO4. Cyclic voltammetry experiments showed both anodic and cathodic photocurrent. The higher photocurrent was observed to BiVO4/MWCNT/FTO electrode, indicating that the carbon nanotube improve the change transfer in the photoanode and decreasing of electron-hole recombination process. UV-vis spectra of 4-NP before degradation experiments exhibited two characteristic absorption peaks at 319 nm (band I) and 227 nm (band II). Electrochemical degradation of 4-NP at BiVO4/MWCNT/FTO were observed decrease of 32.5 % (band I) and 34.6 % (band II) due to formation of hydroxyl radical that attack the 4-NP. Moreover, under photoelectrochemical conditions at BiVO4/MWCNT/FTO the UV-vis spectrum did not show the band II and reduction of 67.6 % of band I. This result is indicative that generation of electron-hole pair improved the formations of hydroxyl radical and also participates of mechanism the degradation. When the photoelectrochemical degradation was conducted using the BiVO4/FTO electrode, only reduction of 42.7 % at band I was observed. So, it is possible to conclude that occurred a synergic effect between BiVO4 and MWCNT which provides more efficiently decomposition of 4-NP. Therefore, photoelectrochemical technology allied with BiVO4/MWCNT and visible-light is suitable for removal of organic pollutants. Acknowledgments: FAPESP (2014/06704-2; 2013/07296-2), INCTMN 2008/57872-1 CNPq 573636/2008-7 and CAPES.