Upcycling of PET-based plastic bottles into carbon dots for photocatalytic degradation of reactive black five using response surface methodology

Abstract

Water contamination and plastic pollution are the most prominent environmental issues, demanding state-of-the-art, sustainable remediations. This study describes the upcycling of waste polyethylene terephthalate (PET) bottles into carbon dots (CDs) and their application for the degradation of Reactive Black 5 (RB5) dye in water. PET was converted into CDs via a multi-step method and characterized by advanced analytical techniques in order to evaluate the photophysical, structural, and chemical properties. UV Vis spectroscopy provided evidence of normal CD absorption, and the optical band gap calculated was approximately 3.14 eV (Eg 395 nm). The photoluminescence of CDs was strong, with pHsensitive fluorescence, indicating the contribution of surface states to the emission. FTIR analysis confirmed the presence of numerous surface functionalities, whereas XRD implied the presence of largely disordered carbon domains. DLS established nanoscale dispersion in aqueous media, and AFM provided nanoscale morphology and height characteristics in line with CD particles deposited on a substrate. Response Surface Methodology (RSM) with BBD model was utilized to simulate and optimize the degradation of RB5 based on the analysis of the combined and interactive influence of main operation factors. The synthesized CDs exhibit a high removal rate of RB5 under light illumination (79.6-91.2%), whereas dark removal was relatively low (10.7-18.4%), which indicates that the major contribution came through light-driven processes, with the net photocatalytic degradation being high (68.9-72.8%). The RSM study established a region of optimal operation (pH = 7.4, CD dose = 0.50g/L, time = 75 minutes, and RB5 = 26mg/L). The response in the experiment was very similar to the predicted one, and this confirms the accuracy of the statistical optimization. PET-derived CDs offer an inexpensive, green, and efficient medium for dye-contaminated wastewater purification.