Construction of Fe2+/Fe3+ cycle system at dual-defective carbon nitride interfaces for photogenerated electron utilization

• Fe 2+ /Fe 3+ cycle system of FeO x -BC at dual-defective carbon nitride is constructed. • Dual-defects can narrow band structure and inhibit charge carrier recombination. • KCN plays a vital role in Fe 2+ /Fe 3+ cycle system in the FeO x -BC/KCN. • FeO x -BC/KCN presents an excellent photocatalytic degradation of OFLO. To improve charge carrier transfer in graphite carbon nitride (CN)-based composites, a cycle system at KCl-assisted carbon nitride (KCN) interfaces via growth of FeO x nanowalls inside biochar (FeO x -BC/KCN) was constructed, which achieved an excellent performance for ofloxacin (OFLO) degradation (100%, 30 min) and more than 30% of defluorination rate under visible-light irradiation. The KCN presented dual-defective structures, nitrogen vacancy and cyano group, which could narrow its band gap and modulate the electron structure. XPS and Mössbauer spectra proved that an electron channel between KCN and FeO x -BC was formed, where highly conductive BC accelerated the photogenerated electron transfer from KCN to FeO x nanowalls and these electrons participated in a Fe 2+ /Fe 3+ reversible cycle system to realize effective utilization. Moreover, the degradation mechanisms and pathways of OFLO over the FeO x -BC/KCN were investigated. This study provides an alternative strategy to improve charge carrier transfer in CN-based photocatalysts through construction of a facile cycle system.