Waste Valorization of Polyethylene Terephthalate and Periwinkle Shells for Sustainable Adsorptive Removal of Trichloroacetic Acid from Domestic Water

Trichloroacetic acid (TCA), a chlorination-derived disinfection by-product, can persist in treated water and requires effective removal approaches. This study evaluated the performance of polyethylene terephthalate-derived activated carbon (PAC), periwinkle shell powder (PSP), and PAC–PSP composites for TCA removal, with emphasis on process optimisation, characterisation, equilibrium behaviour, reusability, and application to field water. PAC was produced from waste PET using cocoa pod husk-derived alkaline activation and carbonisation, while composites were prepared at PAC:PSP ratios of 70:30 (STC) and 40:60 (FSC). Adsorbents were characterised by Fourier-transform infrared spectroscopy and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy. Batch experiments examined pH, adsorbent dosage, contact time, and initial TCA concentration. PAC achieved 99.33% removal at pH 9, 0.06 g dosage, and 4 min contact time, while maximum removals for STC, PSP, and FSC were 95.52%, 89.81%, and 88.86%, respectively. Under optimised conditions, field water TCA removal ranged from 86.51% to 98.04%, with STC achieving 94.28% removal in raw water and 98.04% in conventionally treated water. All adsorbents retained more than 75% removal efficiency through five regeneration cycles, although performance declined with repeated use. The Freundlich model provided a better empirical fit than the Langmuir model based on comparative coefficients of determination (R2). Overall, PAC, PSP and PAC–PSP composites demonstrated promising potential for TCA removal while supporting waste valorisation and circular water treatment strategies. PAC showed a 16.43% carbon yield and 1.97% ash content, while characterisation indicated distinct carbonaceous and mineral-rich surface features. The findings support practical waste-to-resource applications for sustainable domestic water treatment under the investigated operating conditions.