Transforming henna: from natural dye to photosensitizer for efficient photocatalytic degradation of 4-nitrophenol using a green-synthesized ZnO@henna nanocomposite

This research develops a green synthesis process for the ZnO@henna nanocomposite and examines its performance in degrading the 4-nitrophenol (4-NP) pollutant in water under visible light illumination. The method of synthesizing ZnO nanoparticles (ZnONPs) started with the use of kaffir lime extract, followed by conjugating henna extract, which contains the natural photosensitizer lawsone. The analysis of the synthesized ZnO@henna nanocomposite included FTIR, XRD, FESEM, EDS, TEM, UV-vis DRS, zeta potential, PL, and BET surface area to validate its formation and show property improvements. The band gap energy of ZnO decreased to 2.80 eV during UV-vis DRS analysis, thus extending the optical absorption into visible light wavelengths. Under blue LED light illumination, the ZnO@henna nanocomposite achieved 93% degradation of 4-NP within 120 minutes. The optimized photocatalytic degradation process occurred under a catalyst dosage of 25 mg combined with a pH value of 10 and an initial 4-NP concentration at 50 ppm. After four successive reaction cycles, the catalyst managed to maintain 85% efficiency in 4-NP mineralization. Laboratory investigations using scavenger experiments along with mechanistic studies proved that hydroxyl radicals (˙OH) and superoxide radicals (O₂˙⁻) were the leading contributors to the degradation system. ZnO@henna shows promise as an economical and environmentally friendly photocatalyst for carrying out environmental remediation operations.