Sulfate radical-based photo-Fenton reaction derived by CuBi₂O₄ and its composites with Α-Bi₂O₃ under visible light irradiation: Catalyst fabrication, performance and reaction mechanism

Sulfate radical-based photo-Fenton (SR-photo-Fenton) reaction, assisted by visible light irradiation, was achieved by CuBi₂O₄ and its composites with α-Bi₂O₃ for refractory chemical degradation in aqueous solution. Herein, this catalyst was fabricated by a sol-gel method and the fabrication conditions, including calcination temperature and molar ratio of Cu/Bi, were optimized according to the crystal phase composition, catalytic activity and toxic copper ion leaching. The optimal calcination temperature was 500 °C and molar ratio of Bi to Cu was 2.0. The catalyst containing CuBi₂O₄ and α-Bi₂O₃ showed a higher density of surface -OH which might be the key surface active site than pure CuBi₂O₄. The influence of initial solution pH, PMS concentration, catalyst dosage and catalyst reuse on rhodamine B (RhB) degradation was investigated. Importantly, calcination at 500 °C reverted the catalytic activity of catalyst. Results of electron paramagnetic resonance, competitive radical experiments and surface chemical property characterization demonstrated that the reaction mechanism of this novel SR-photo-Fenton reaction is a combination of interface and solution reactions. In the interface reaction, the transfer of photogenerated electron/hole pairs drives the decomposition of PMS to produce SO₄[rad]⁻ and [rad]OH. Furthermore, the cycling of Cu(I)/Cu(II) facilitated effective PMS activation to generate free radical that was responsible for the degradation of RhB. The second order reaction rate constant between RhB and SO₄[rad]⁻ was determined to be 0.595-6.436 × 10¹⁰ M⁻¹ S⁻¹ based on the chemical reaction kinetics of radical, which was a first and important report for SO₄[rad]⁻ chemistry.