Mechanisms of biochar enhanced Cu₂O photocatalysts in the visible-light photodegradation of sulfamethoxazole

Cu₂O nanoparticles are decorated with biochars derived from spent coffee grounds (denoted as Cu₂O/SCG) and applied as visible-light-active photocatalysts in the sulfamethoxazole (SMX) degradation. The physicochemical properties of Cu₂O/SCG are identified by various spectral analysis, electrochemical and photochemical techniques. As a result, the Cu₂O/SCG exhibits the higher removal efficiency of SMX than the pristine Cu₂O under visible light irradiation. We can observe that Cu₂O could be incorporated onto the SCG biochars with rich oxygen vacancies/adsorbed hydroxyl groups. In addition, the Cu₂O/SCG has the lower charge transfer resistance, faster interfacial electron transfer kinetics, decreased recombination of charge carriers and superior absorbance of visible light. The construction of band diagrams for Cu₂O/SCG and pristine Cu₂O via UV–vis spectra and Mott−Schottky plots suggest that the band energy shifts and higher carrier density of Cu₂O/SCG may be responsible for the photocatalytic activity enhancements. From the radical scavenger experiments and electron paramagnetic resonance spectra, the aforementioned energy shifts could decrease the energy requirement of transferring photoinduced electrons to the potential for the formation of active superoxide radicals (·O₂⁻) via one and two-electron reduction routes in the photocatalytic reaction. A proposed degradation pathway shows that ·O₂⁻ and h⁺ are two main active species which can efficiently degrade SMX into reaction intermediates by oxidation, hydroxylation, and ring opening. This research demonstrates the alternative replacement of conventional carbon materials for the preparation of biochar-assisted Cu₂O photocatalysts which are applied in the environmental decontamination by using solar energy.