S-CoAl-LDH/Fe-C3N5 Heterojunction for the Efficient Photocatalytic Reduction of Cr(VI) and Degradation of Tetracycline Complex Pollutants

Heterojunction construction is a key strategy for enhancing the photocatalytic efficiency of semiconductors. In this study, a composite of sulfur-doped CoAl layered double hydroxide and Fe-doped C3N5(S-CoAl-LDH/Fe-C3N5) was developed via a hydrothermal method for the synergistic oxidation-reduction degradation of the organic pollutant tetracycline (TC) and the detoxification of heavy-metal ions (Cr(VI)) in wastewater. After optimization, the CAF-4 heterojunction (the composite with 20 wt% Fe-C3N5 loading) exhibited TC degradation rates 5.51 and 3.97 times higher than those of pristine Fe-C3N5 and S-CoAl-LDH, respectively; under simulated sunlight, the Cr(VI) reduction rates were 11.75 and 4.22 times higher, respectively. The as-prepared catalyst demonstrated good stability across a wide pH range, in the presence of various cations and anions, and in different water matrices. Under coexisting pollutant conditions, the composite still achieved removal efficiencies of 82.1% for Cr(VI) and 64.7% for TC. After five cycling runs, the adsorption-photocatalytic efficiency of the composite for the removal of Cr(VI) and TC composite pollutants remained above 80%. Overall, CAF-4 shows great promise for application in the adsorption-photocatalytic treatment of wastewater containing combined Cr(VI) and TC pollution.