Repurposing waste plastic into a sustainable adsorbent for removing synthetic dye: experimental, optimization and theoretical modeling

Abstract

Plastic waste, particularly polyethylene terephthalate (PET), poses a significant environmental threat due to its persistence and non-biodegradability. This study presents a sustainable approach to repurposing recycled PET bottles into activated carbon for the efficient removal of Brilliant Green (BG) dye from aqueous solutions. The synthesized adsorbent was characterized by FTIR, BET, XRD, XPS, and FESEM analyses, confirming a high surface area (403 m² g⁻¹) and abundant functional groups that are favorable for adsorption. Under optimized conditions of 5 mg L⁻¹ initial dye concentration, pH 7, 0.1 g adsorbent dose, 298 K, and 120 min contact time, the material achieved a removal efficiency of 99.3%. Adsorption followed pseudo-second-order kinetics and was fitted both Langmuir and Freundlich isotherm models, indicating combined physical and chemical interactions. Thermodynamic parameters revealed a spontaneous and endothermic process, while regeneration tests demonstrated excellent stability over five cycles. Density functional theory (DFT) analysis indicated that preferential adsorption occur through π–π stacking and donor–acceptor interactions. Box–Behnken design (BBD) optimization further validated the model's predictive accuracy. Overall, this work provides both dual environmental benefits by transforming PET waste into a cost-effective, and eco-friendly adsorbent for sustainable dye removal and water purification.