S-Scheme Carbon-Doped C3N5/NiAl-LDH Heterojunction for Visible-Light-Driven Ciprofloxacin Oxidation and Cr(VI) Reduction

The coexistence of antibiotics and heavy metals in water requires materials that can oxidize and reduce two chemically distinct pollutants at the same time. Here, we show that carbon doping and heterointerface engineering act cooperatively in a carbon-doped C3N5/NiAl-LDH (CHL) heterojunction to meet this challenge under visible light. Carbon incorporation narrows the bandgap of C3N5 from 2.21 to 1.92 eV, but more importantly raises its Fermi level relative to NiAl-LDH, so that electron transfer across the intimate interface builds an internal electric field that drives an S-scheme, rather than a type-II, pathway, preserving the strongly reducing electrons of C-C3N5 and the strongly oxidizing holes of NiAl-LDH. The optimized 40 wt% composite (CHL-2) degrades 80.8% of ciprofloxacin (CIP) and reduces 78.3% of Cr(VI) within 150 min, with apparent rate constants 2.04 and 2.67 times those of pristine C3N5. ·O2− and h+ dominate CIP oxidation, whereas ·O2− and e− govern Cr(VI) reduction; in the binary system, CIP and Cr(VI) act as complementary electron donor and acceptor, raising both efficiencies to about 83%. CHL-2 retains more than 70% of its activity over five cycles and remains effective in river water, lake water and domestic wastewater. To our knowledge, this is the first S-scheme C-C3N5/NiAl-LDH system applied to the simultaneous oxidation of CIP and reduction of Cr(VI), and it shows that carbon doping and interfacial electronic coupling reinforce rather than merely accompany each other, although practical deployment still requires further validation of scalability and long-term performance.