Recycled PET/Sepiolite Composite Adsorbent for Methylene Blue Removal: Formulation, RSM Optimisation, Reusability, and Adsorption Mechanism.

This study explores the valorisation of post-industrial recycled polyethylene terephthalate (rPET) via incorporation of sepiolite clay to develop a composite adsorbent for methylene blue (MB) dye removal. The composite formulation was evaluated by varying the rPET concentration, TFA:DCM solvent ratio, sepiolite content, and blending time. The selected formulation had an average particle size of 267.10 nm, a specific surface area of 0.67 m2/g, and a pore volume of 0.003925 cm3/g. Adsorption conditions were then optimised using a five-factor, three-level Box-Behnken design, considering contact time, MB concentration, adsorbent dosage, pH, and temperature. The resulting quadratic model was statistically significant (F = 114.18, p < 0.0001), with R2 = 0.9892, adjusted R2 = 0.9805, and predicted R2 = 0.9568; however, the lack-of-fit was also significant (F = 90.90, p < 0.0001), indicating that model predictions should be interpreted within the investigated factor ranges. At the selected conditions of 62.5 min, 25.5 mg/L MB, 2.55 g/50 mL adsorbent, pH 7, and 55 °C, the model predicted an MB removal efficiency of 99.03%. The adsorbent retained at least 93% reusability efficiency after six adsorption-washing-reuse cycles. The kinetic data were described more closely by the pseudo-second-order model, whereas the Freundlich model provided a better representation of the equilibrium data (R2 = 0.9976). Thermodynamic results indicated that adsorption was favourable and exothermic within the investigated temperature range. Overall, the results show that post-industrial PET can be converted into a value-added sepiolite-containing adsorbent for MB removal.