Life Cycle Assessment of Plastic Waste, its Treatment, and Application of the Upcycled Product: A Comprehensive Circular Approach

Plastic waste management is a growing global problem. Single-use flexible packaging plastic waste (FPPW) is one of the most challenging types of plastic waste to recycle due to its mixed composition (including bags, containers and films made of LDPE, HDPE, PP, PET and other materials), multi-material characteristic (multiple thin layered plastics adhered together for a single packaged product), and associated impurities (inks, adhesives and residual impurities from the packaged product). This type of plastic waste is often discarded as non-recyclable from the mechanical recycling streams. However, the use of plastics is unavoidable as the benefits outweigh other viable alternative materials in various applications. In this research, three holistic and comprehensive assessments have been conducted, including the (i) life cycle of plastic production and application, (ii) end-of-life pyrolysis treatment and upcycling to carbon nanotubes (CNTs), and (iii) application of CNTs in electrochemical sensing and end-of-life disposal of the CNTs, in order to address the FPPW management.

The specific case of the life cycle assessment (LCA) of grocery bags indicated that the environmental footprint of plastics is lower than the other prevailing alternatives, including cotton and paper, from the perspective of a metropolitan city with end-of-life incineration treatment. However, incineration of the plastic waste abnegates the circularity of material flow. Hence, a feasible and versatile integrated pyrolysis technology was developed for the FPPW treatment. The pyrolysis of plastics was evaluated using incineration ashes as a reforming catalyst in order to valorize the application of incineration ashes and to alleviate the environmental footprint associated with the utilization of synthetic catalysts. The incineration ashes demonstrated potential to be applied as a reforming catalyst in the pyrolysis of plastics to produce oil and non-condensable gas. However, further advancements in the form of pre-treatments are essential to generate performance comparable to commonly used zeolite catalysts. The LCA of the integrated pyrolysis process along with the upcycling of non-condensable gases from pyrolysis of FPPW to CNTs concluded superior environmental benefits when compared to the conventional pyrolysis of FPPW without CNTs synthesis. Notably, the integration of CNTs synthesis with the conventional pyrolysis process benefitted with diminishing the environmental footprint in terms of climate change, human toxicity, fossil depletion, ionizing radiation, and terrestrial ecotoxicity potentials. The inflexion point for the CNTs yield was identified as >2 wt.% to generate a positive effect on the environment.

Pyrolysis treatment of different waste fractions without significantly compromising the product quality highlighted the versatility of the integrated pyrolysis process. Furthermore, the synthesis of novel waste-derived CNTs (WCNTs) provides an additional revenue stream for the pyrolysis plants, enhancing their economic feasibility.

Subsequently, the synthesized WCNTs were tested in electrochemical sensing using screen-printed electrodes (SPEs) due to its enormous growth potential in diverse future applications. The electrochemical performance of WCNTs was comparable to the commercial CNTs in the detection ofheavy metals, therefore, corroborating WCNTs as a viable alternative in SPEs application. The LCA determined that the WCNTs demonstrated considerable environmental advantages in comparison with the predominantly used noble metals, including gold and platinum, as electrode material. Therefore, substitution of the noble metals by WCNTs is recommended.

In conclusion, the integrated LCA approach provided context of the accrued benefits of high-value CNTs derived from plastic waste. Furthermore, the integrated LCA approach provides a measure to enhance the circularity of the material flow by identifying suitable alternatives, accentuating recycling and upcycling technologies, and determining hotspots for improvement, thereby, facilitating the environmental sustainability. The method helps to improve the recycling rates and alleviate the existing unsustainable consumption patterns. Importantly, similar LCA studies are unique and crucial to advance towards a truly circular economy and achieve the sustainable development goals.