Nanoengineering of ultrathin N-CQDs/Bi2WO6 S-scheme heterojunction for enhanced photodegradation of antibiotics as emerging contaminants: Mechanism insight and toxicity assessment

Constructing low-cost and wide visible light response S-scheme heterojunctions is crucial for their photocatalytic efficiency and practical applications. Herein, a novel N-CQDs/UBWO composite was designed by combining nitrogen-doped carbon quantum dots (N-CQDs) obtained from bio-waste lignin with Bi2WO6 ultrathin nanosheets (UBWO) using in-situ hydrothermal approach. The work function analyses, electron paramagnetic resonance (EPR) and in-situ X-ray photoelectron spectroscopy (XPS) evidenced an S-scheme charge transfer mechanism between N-CQD and UBWO during photocatalytic reactions, which endows the composite system a high photocatalytic redox and charge space separation capabilities. Besides, the up-conversion properties of NCQDs render N-CQDs/UBWO composites an enhanced visible light response. Therefore, the optimized 3 wt%NCQD/UBWO S-scheme heterojunction exhibited favorable tetracycline degradation performance, with a degradation efficiency of 85.0 % within 40 min of reaction time, and first-order rate constant (k) of 2.6 and 20.3 times greater than that of UBWO and N-CQDs, respectively. Furthermore, referring to Fukui function calculations and liquid chromatography-mass spectrometry (LC-MS), degraded products and three degradation routes for tetracycline were proposed. The results of the toxicity estimation software tool (T.E.S.T) and mung bean cultivation demonstrated that the intermediate products of tetracycline degradation are of low toxicity. This study provides insights into designing superior S-scheme heterojunctions using CQDs derived from waste biomass for green and efficient removal of antibiotics from wastewater.