Review and study on the valorisation of cigarette butts

Cigarette butts are one of the most polluting solid wastes. It is estimated that each year the average cigarette production is about 5.7 trillion, and every day a colossal amount of cigarette butts is thrown into nature. Most cigarettes have a filter made of cellulose acetate, which due to modifications of the cellulose increases its filtering efficiency but limits its potential for biodegradation. When cigarette butts are discarded into the environment, significant amounts of toxic chemicals are released and potentially accumulate in waterways and roads, thus posing a serious threat to local organisms and aquatic species, but also causing considerable and significant damage to the environment as a whole. To solve this huge problem, we need to find an intelligent way to collect and store this waste and, above all, develop a sustainable way to recycle it. The scientific and technical literature on this subject is generally very modest. The way explored in the laboratory in this work was the production of activated carbons from cigarette butts, through the pyrolysis of the cellulose acetate fibres that make up the filter of the butts. Pyrolysis tests were carried out at four different temperatures (400, 600, 700 and 800°C) with a residence time from 5 to 120 minutes, using protocols found in the bibliography. Pre- and post-treatments were then carried out to achieve better performance: activation by impregnation with NaOH and KOH, washing with ethanol, microwave treatment and extraction with supercritical CO2. The results showed that it is indeed possible to produce activated carbon from cigarette butts. After thermal treatment, the cellulose acetate filter becomes a black carbon-like material through a morphological transformation. The development of a hierarchical pore structure during the carbonisation process is most likely a consequence of the acetate modification. The activated carbon produced, in general, has a large specific surface area with a porosity ranging from micrometre to nanometre: it was seen that the microporous surface has between 60% and 70% of the total area. The largest specific area is obtained with ramping from room temperature to 800°C, with an area of 720 m2/g, of which 67% is micropores. Other carbons made from cigarette butts’ filters also performed well, with peaks in specific surface area in samples produced at 800°C: these carbons have specific surface area between 500 and 650 m2/g. On the other hand, the carbons produced with pre-treatment on the butts had much worse results (for example with impregnation in NaOH and heat treatment at 800°C for 20 minutes we obtained a result of 28 m2/g); for the NaOH and KOH pre-treatment this decrease was caused by the fact that sodium and potassium crystals are deposited on the surface during the impregnation phase. These crystals, during the heating process, melt and prevent the heat from getting deep into the fibres, which fails to destroy them completely and form a hierarchical carbonaceous structure. Furthermore, the post-treated samples have average specific areas of 400 m2/g. However, not all active carbons produced with post-treatment have been analysed and it is very likely that carbons impregnated in NaOH and KOH and then thermally treated at 800°C have improved results (this is demonstrated in several literature articles). Further work and analysis will have to be done to expand on these results as this was only a preliminary study.