Thermal conversion of waste cigarette butts: Feedstock heterogeneity, process control, valorization pathways, and...

Waste cigarette butts are abundant and widely dispersed composite solid wastes worldwide. They consist of cellulose acetate filters, residual tobacco, cigarette paper, ash, and carry nicotine, polycyclic aromatic hydrocarbons (PAHs), and heavy metals. During weathering, cellulose acetate filters release microfibers and contribute to microplastic pollution, while toxic components migrate into aquatic and soil environments. Conventional landfilling, incineration, and recycling fail to achieve pollution control and resource recovery. This review focuses on the thermal conversion of waste cigarette butts. It compares feedstock heterogeneity, reaction mechanisms, and the effects of different processes on product formation and toxic-component migration, with the aim of assessing resource-recovery feasibility and environmental safety. During thermal conversion, additives such as triacetin first undergo thermal desorption. Cellulose acetate then undergoes deacetylation and backbone cleavage, producing acetic acid and oxygenated volatiles. Cigarette paper releases phenols and aromatic compounds, while residual tobacco releases nicotine, pyridine, and other nitrogen-containing compounds. Volatiles may condense into pyrolysis oil or undergo cracking, reforming, or gasification into syngas at higher temperatures and under controlled atmospheres or catalytic conditions. The remaining carbon framework forms solid char through condensation, aromatization, and structural rearrangement. Different routes are oriented toward different products, but thermal conversion does not necessarily eliminate toxic components. Complete mass-balance data on their distribution among gas, oil, char, ash, and wastewater remain unavailable. Future studies should strengthen feedstock standardization, validation under realistic and continuous operating conditions, and life-cycle and techno-economic assessments, so that the environmental constraints and engineering requirements of waste cigarette butt thermal conversion can be defined more clearly.