Insight into the self-driven O2 activation and interfacial electron transfer for micropollutant degradation by boron-doped biochar: Role of boron moieties and persistent free radicals

Advance oxidation process (AOP) is a promising way for micropollutants treatment, but the consumption of chemicals and energies limits its large-scale application. To address this issue, a novel boron-doped biochar (BBC) system was developed for the efficient removal of micropollutants, requiring no external energy or oxidant. Herein we demonstrated that the synthesized BBC could efficiently activate the dissolved oxygen (O2) in the water or oxidize micropollutant directly. The complete sulfamethoxazole (SMX) could be removed within 30 min by the BBC synthesized at 600 °C (BBC-600). The experiments on material characterization and removal performance with quenchers revealed that the introduction of boron into biochar increased the active sites for O2 activation, including persistent free radicals (PFRs) and boron moieties (BC3, BCO2, and BC2O) on the biochar. The generated reactive oxidation species (ROS), mainly O2−, were responsible for the SMX degradation in this BBC system. Different performance of aged experiments in air and N2 conditions indicated that PFRs could also directly oxidize SMX. Density functional theory (DFT) was used to further illustrate the inner mechanism of enhanced electron transfer via boron moieties for O2 activation and SMX degradation. Electrons of the adsorbed micropollutant could be transferred to boron moieties, and then trapped by O2 in water to produce O2− and 1O2 for SMX degradation. This study provided a new insight into the structure–activity relationship of B-doped biochar for the direct and indirect degradation of micropollutant, and opened a new avenue for micropollutant degradation without external energy and oxidant.