Sustainable Synthesis of Phytoremediated Eichhornia crassipes-Derived Carbon Quantum Dot Supported on Zinc Oxide (CQD-ZnO): Characterisation and Sonocatalytic Activity

Phytoremediation is considered as a green alternative for remediating metal-contaminated soil and water, yet further efforts are needed to minimise secondary pollution after phytoremediation. This study investigates a cost-effective and sustainable method to synthesise carbon quantum dot supported on zinc oxide (CQD-ZnO) composites using extracted zinc (Zn) from post-phytoremediated plants, plant extracts, and CQDs derived from water hyacinth (Eichhornia crassipes) for the sonocatalytic degradation of malachite green. The CQD-ZnO materials were characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), Brunauer–Emmett–Teller (BET) surface analysis, and ultraviolet–visible (UV–Vis) spectroscopy to confirm their crystalline structure, morphology, functional groups, surface area, and optical properties. The composites exhibited disaggregation of agglomerates, high crystallinity, and increased carbon content due to the addition of CQDs containing phenolic functional groups (e.g., polyphenols, flavonoids) from the plant extract. The highest sonocatalytic degradation efficiency (84.52%) was achieved after 90 min of treating 10 ppm malachite green using 1 g/L of the CQD-ZnO composite at a natural pH, with 300 W ultrasonic power at 25 kHz. This study paves the way for the development of environmentally friendly, high-performance sonocatalysts from post-phytoremediated plants for wastewater treatment applications.