Surface-functionalized cigarette butts as efficient adsorbents for Cr(VI) removal and mechanistic insights

Discarded cigarette butts represent one of the most abundant plastic wastes worldwide; nevertheless, their reuse as functional adsorbents remains underexplored. Here, we report a promising way to reutilize discarded cigarette butts as an adsorbent for Cr(VI) removal using surface functionalization with 5% (v/v) 3-aminopropyltriethoxysilane and 2.5% (v/v) glutaraldehyde. The functionalized butts completely removed 200 mg L⁻¹ of Cr(VI) within 120 h at pH 2.0 and 60 °C, with a maximum capacity of 33.90 mg g⁻¹. The adsorption process fitted a pseudo-second-order model and a Freundlich isotherm well, indicating multilayer chemisorption. Scanning electron microscopy equipped with energy dispersive X-ray spectroscopy (SEM-EDS) images confirmed the morphological changes on the fibers of cigarette butts after surface functionalization and the accumulation of chromium within the fibers after adsorption. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) analyses demonstrated the active participation of ether, carbonyl, hydroxyl, and amine groups in both complexation and the reduction of Cr(VI) to Cr(III). A mechanistic scheme is proposed illustrating the adsorption-complexation-reduction pathway. The adsorbent retained ~ 70% removal efficiency over five regeneration cycles, demonstrating strong reusability. Comparative evaluation with other bio-adsorbents highlights the strength of this simple, waste-derived, chemically functionalized material for Cr(VI) remediation. This study presents a practical and scalable method for upcycling cigarette filter waste into an effective water treatment material.