Efficient activation of peroxydisulfate by Cu-biochar hybrid materials: Source and transfer direction of interface electrons

Electron transfer between active sites on metal–carbon hybrid materials (MHMs) and oxidants facilitates the generation of reactive species for degrading toxic organic pollutants, yet the activation mechanism remains unclear. We developed a Cu-biochar composite (Cu-BC) through pyrolysis of Cu-rich hyperaccumulators. Cu-BC demonstrated exceptional peroxydisulfate (PDS) activation capability, achieving 92 % tetracycline (TC, 10 mg·L⁻¹) removal within 2 h under visible light irradiation. In contrast to the blank BC/PDS/Vis system, which generated only reactive oxygen species (ROS) of ·O₂^– and ¹O₂ for TC degradation at a reaction rate constant of k_obs = 0.0075 min⁻¹, the Cu-BC/PDS/Vis system produced additional ·SO₄⁻ and ·OH, leading to a threefold increase in k_obs to 0.0204 min⁻¹. Scavenging experiments quantified the contributions of ·SO₄⁻ and ·OH to TC degradation at 25.5 % and 2.3 %, respectively. The galvanic oxidation process, masking experiments and density functional theory calculations reveal importance of electron transfer processes. In addition to the π electrons from carbon substrates (<17.9 %), electrons from carbonyl groups (C=O) (18.2 %) are more effectively transferred to adjacent ≡Cu^Ⅱ on the Cu-BC surface. This neglected interfacial electron transfer (IET) enables the Cu^I/Cu^II/Cu^III cycle to activate PDS to ·SO₄⁻, which allows the TC degradation percentage to remain above 77.1 % after 5 cycles. The activity can be restored by regeneration of C=O via calcination (200 ℃) for the IET process. This study clarifies the IET mechanism between carbon substrates and metal sites on the surface of MHMs and is of great significance for developing efficient MHMs. In addition, the system has good environmental applicability and low toxicity.