Cigarette butts (CBs) are among the most abundant forms of anthropogenic litter worldwide and constitute a persistent source of environmental contamination due to their slow degradation and the release of toxic substances into terrestrial and aquatic ecosystems. This review critically evaluates recent advances in the valorisation of CB waste into value-added functional materials. Particular attention is given to conversion strategies including chemical activation, electrospinning, hydrothermal processing, and nanocomposite fabrication. These approaches enable the production of activated carbons, cellulose-derived nanomaterials, and metal-doped composites with promising applications in water purification, energy storage, vector control, and pollutant remediation. The underlying mechanisms, performance, scalability, and sustainability of these technologies are discussed. Finally, current challenges, research gaps, and future opportunities of large-scale implementation are highlighted. Collectively, the findings demonstrate that CBs can be re-envisioned not only as a persistent environmental pollutant but also as valuable secondary resources that support circular economy and sustainable material development strategies.

Malachite green (MG) requires selective separation from complex wastewater matrices containing coexisting metal ions. In this study, we developed a process-engineered polymer inclusion membrane (PIM) incorporating a crosslinked bio-based carrier, polyeugenol–bisphenol A diglycidyl ether (Poly-BADGE 4:1), and evaluated its applicability for MG removal from aqueous solutions. We systematically investigated the effects of source-phase pH, receiving phase HNO₃ concentration, membrane thickness, carrier concentration and transport time. With optimal conditions (pH 7.0, 0.75 M HNO₃, 0.33 mm membrane thickness, 0.07 M Poly-BADGE 4:1 carrier concentration, and 21 h), the PIM exhibited an MG removal efficiency of 82.24%. In simulated mixed wastewater containing Pb(II) and Cu(II), MG removal decreased to 65.13% due to competitive interactions, while preferential removal of MG over metal ions was maintained. The membrane maintained its chemical and morphological integrity during the transport experiments, as evidenced by Fourier transform infrared (FTIR) spectroscopy and scanning electron microscope (SEM) characterization. These results demonstrate the potential of Poly-BADGE–based PIMs as a selective polishing unit for dye-contaminated, metal-rich wastewater in advanced treatment schemes.