One-step synthesis of carbon quantum dot-carbon nanotube composites on waste eggshell-derived catalysts for enhanced adsorption of methylene blue

Abstract Pristine CNTs always suffer from a slow adsorption rate or low adsorption capacity. Development of CNT-based adsorbents is highly desirable to improve their adsorption performance. In this study, novel carbon quantum dot-carbon nanotube composite (CQD-CNT) adsorbents and defective CNTs were synthesized for the first time by one-step chemical vapor deposition using catalysts derived from waste eggshells. The coverage of CQDs on CNTs endowed CQD-CNT with a larger surface area (324.4 m3/g), more abundant pore structures and surface functional groups compared to defective CNTs. The adsorption of methylene blue (MB) on CQD-CNT was systematically studied, and CQD-CNT demonstrated good adsorption capacity and extremely fast adsorption rate for MB at a wide pH range (2–12) and different temperatures (293–313 K). A high removal efficiency of 99.5% was achieved in 5 min, and the maximum adsorption capacity was 299.4 mg/g, which was 1.3 and 4.6 times higher than that of defective CNTs and pristine CNTs, respectively. The adsorption results were closely fitted by Langmuir isotherm model and pseudo-first-order kinetic model. The adsorption of MB on CQD-CNT involved multiple adsorption mechanisms, including electrostatic interactions, hydrogen bonding, π-π stacking interaction and pore filling. In addition, CQD-CNT showed good reusability and stability, retaining a removal efficiency of 95.1% after 5 cycles. Finally, high decolorization rates (98.6–100%) were obtained using CQD-CNT in the treatment of simulated textile wastewater. These results demonstrate that CQD-CNT benefitted from one-step green synthesis and the effective utilization of waste eggshells has potential applications in wastewater treatment.