Thermochemical and chemo-biological molecular recycling of plastic waste and plastic-biomass waste mixtures: an updated review

Massive amounts of plastic and biomass waste are mismanaged worldwide, causing detrimental consequences to human health and the environment. In fact, the disposal of residues through landfills without further processing and burning for household heating and cooking are common practices. Thermochemical processing, such as pyrolysis, chemical depolymerization and bioprocessing, has proven feasible for recovering valuable building block molecules from plastic residues. The main goal of pyrolysis is to obtain aliphatic hydrocarbons useful as fuel, while chemical processing generates constitutive molecules of plastic (i.e., monomers and polyols) that can be repolymerized and reintroduced in the market. Alternatively, the bioprocessing of plastic waste requires prior chemical depolymerization in order to unleash the building blocks. Chemo-enzymatic treatment of waste plastic-biomass mixtures is an open challenge due to the diverse composition of their residues, along with the presence of additives and contaminants. The few reports found in the literature regarding the bioprocessing of plastic residues with lignocellulosic biomass and paper indicate that chemical pretreatment cannot be avoided and that some substances present in the residues can act as fermentation inhibitors that affect waste bioprocessing.