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

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.