In this study, mineralization of organic pollutants in polyaniline (PANI) wastewater was achieved by a combination of photoelectro-Fenton (PEF) and heterogeneous photocatalysis with black TiO2 nanotubes. Organics were decomposed by the •OH radicals and SO4–• radicals formed on the black TiO2 nanotube photocatalyst and at the IrO2/Ti anode during water oxidation and from the Fenton’s reaction between Fe2+ and generated H2O2. Poor chemical oxygen demand (COD) removal was obtained utilizing visible light/TiO2, anodic oxidation with generated H2O2 (AO-H2O2), visible light/black TiO2/AO-H2O2, and electro-Fenton. The mineralization was strongly increased by the PEF/black TiO2 process because of the photolysis of products under visible light irradiation; it was improved by coupled heterogeneous photocatalysis due to the additional •OH radicals at the black TiO2 nanotube surface. The results demonstrated that about 96.4% COD and 85% total organic carbon of PANI wastewater were removed after 360 min by the PEF/black TiO2 process under the optimal conditions. The black TiO2 nanotubes exhibited good stability as the mineralization ratio in 360 min which was still above 80% after 10 cycles’ degradation, showing that the black TiO2 nanotubes are powerful and promising photocatalysts for improving the removal efficiency of PANI wastewater.
Photoelectro-Fenton (PEF) process can continuously promote the occurrence of Fenton reaction and the generation of active species, which is an advanced oxidation technology for pollutant degradation. However, the lack of bifunctional catalysts restricts the development of PEF technology. In this study, the electronic rearrangement MOF-525 modified by metalloporphyrin (named MOF-525-Fe/Zr) was prepared, to load on the carbon felt as a novel cathode catalyst, which is used in PEF process. A series of characterization and photoelectric chemical properties tests combined with DFT calculation showed that the modification of MOF-525 could not only have the large specific surface area and multistage pore structure but also co-stimulate the metal-to-ligand charge transfer (MLCT) and ligand-to-cluster charge transfer (LCCT) by photoelectric synergy. These charge transitions provide periodic electron donor-acceptor conduction paths in MOF-525-Fe/Zr, which can improve the active species formation and transfer efficiency. Owing to their favorable pore and electronic structure as well as stability, MOF-525-Fe/Zr shows great promise for the application in the catalytic process of PEF. Sulfamethoxazole (SMX) degradation was enhanced by MOF-525-Fe/Zr with the TOC removal rate above 75% both in river water and tap water. Finally, the reasonable pathway of PEF catalytic degradation of SMX was proposed by HPLC-MS analysis. In conclusion, this study provides a new idea for reconstructing the electronic structure of MOFs catalyst and broadening the practical application of PEF technology.Keywords:photoelectro-Fenton;MOF-525;metal-to-ligand charge transfer;ligand-to-cluster charge transfer;sulfamethoxazole
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