End of life packaging is nowadays one of the major environmental problems due to its short usage time, the low biodegradability, and the big volume occupied. In this context, gasification is one of the most promising chemical recycling techniques. Some non-recyclable or non-compostable waste gasification plants are already operating such as Enerkem Alberta Biofuels in Canada or the Sierra’s FastOx Pathfinder in California. In this review, we have examined works about plastic gasification from the last fifteen years with a specific focus on polyolefin (PP, PE), plastics mix, and co-gasification of plastic with biomass. For each of these, the best operating conditions were investigated. A very in-depth section was dedicated to supercritical water gasification (SCWG). The most used reactors in gasification processes are fluidized bed reactors together with air or steam as gasifying agents. Tar removal is commonly performed using olivine, dolomite, or nickel based catalysts. SCWG has numerous advantages including the inhibition of tar and coke formation and can be used to remove microplastics from the marine environment. In co-gasification of plastic material with coal or biomass, synergistic effects are observed between the raw materials, which improve the performance of the process, allowing to obtain higher gas yields and a syngas with a high energy content.

In this study, we investigate the hydrothermal liquefaction (HTL) of PET separated from a densified postconsumer plastic mix, with the aim of recovering its monomer. This second raw material is made up of 90% polyolefin, while the remaining 10% is made up of PET, traces of metals, paper, and glass. After preliminary separation by density in water, two batch experiments were performed on the sunken fraction (composed mainly of PET) in a stainless steel autoclave at 345 °C for 30 and 20 min. Both trials resulted in similar yields of the three phases. In particular, the solid yield is around 76% by weight. After a purification step, this phase was analyzed by UV–Vis, ¹H-NMR, and FTIR spectroscopy and resulted to be constituted by terephthalic acid (TPA), a product of considerable industrial interest. The study proved that the hydrothermal liquefaction process coupled with density separation in water is effective for obtaining TPA from a densified postconsumer plastic mix, which can be used for new PET synthesis.

Enzymatic digestion is a promising alternative in the upconversion of plastic waste compared to traditional chemical recycling methods, because it warrants the use of milder conditions. However, enzymes are hardly able to penetrate the bulk of the plastic material; thus, a pretreatment is necessary to promote the reaction. In this study we investigate hydrothermal liquefaction as a thermal pretreatment of a commercial polyurethane before performing an enzymatic digestion. The feedstock is a rigid polyurethane foam. The structure and chemical composition of the feedstock were analyzed through FTIR analysis and solid-state ¹³C NMR. The polyurethane was then subjected to hydrothermal liquefaction using either ultrapure water or KOH as a basic catalyst. Enzymatic digestion was then performed on the organic fraction obtained from both experiments using a lipase extracted from Candida rugosa. The LC-MS analysis of the digests shows an increase in some signal intensities due to the degradation of oligomeric fragments. This new way of recycling allows the recovery of important chemicals such as quinolines and 4,4'-methylenedianiline. With this study we demonstrate that hydrothermal liquefaction coupled with enzymatic digestion is a suitable alternative for handling polyurethane waste.

Anaerobic digestion (AD) is a well-known biological conversion process to obtain a gaseous biofuel from organic matter: in fact, upgrading biogas to biomethane is a mean to substitute conventional natural gas. It is also known that biochar can improve the biogas production in AD processes. In this work, different biochars have been produced from various feedstocks at different process conditions. Biochars obtained from the carbonization of wheat straw (WS) and poplar (P) were produced in a Thermo Gravimetric Analyser at lab scale, at a temperature of 400 °C and 2 h of retention time at the maximum temperature, with a heating rate of 20 °C min⁻¹. Another biochar from poplar (Pc) was also produced in a pilot plant (CarbOn, RE-CORD) working in oxidative pyrolysis conditions, at a temperature range between 500 and 600 °C. Biochars were oxidized with Oxone® using two different methods (ball-milling and simple aqueous solution mixing) to increase the amount of functional groups on their surface. Oxidized biochars (Ws_Ox and P_Ox) were characterized by FTIR, BET, and CEC, and their impact on biogas production was investigated through a lab scale biochemical methane potential (BMP) test using maize silage as substrate. 0.33 g of biochar was used for each treatment. BMP test shows that all batches containing biochar as additive produced more biogas than control (C). WS_Ox and P_Ox produced respectively a + 7.7% and + 11.3% of biogas than C, obtaining the higher productivities with respect to not oxidized biochars. The addition of P and Pc biochars were similar performances in AD, thus highlighting that no significant differences are due to different biochar production scales and process parameters from the same feedstock. This study highlights how in addition to the various examined parameters (nature of the feedstock, pyrolysis parameters, size of biochar and its concentration in AD), also the presence of specific functional groups on the biochar surface influences the AD performance.

The influence of a nanodispersion of TiO₂ in water (nanoparticle size: 40 nm, polydispersity index: 0.25), brushed on a Paraloid film and subjected to UV–Vis irradiation was evaluated. The TiO₂ nanodispersions showed a tendency to reduce the molecular weight of Paraloid due to its photocatalytic properties. FTIR and GPC analyses and SEM images suggested the degradation of the polymer, while chromatic variations of the films were scarcely detected. This study is very remarkable in the perspective of using this material for the removal of polymeric films used in conservation.