The environmental and economic impacts of implementing a circular economy in plastic waste supply chains are not well understood. The proposed systems analysis framework assesses environmental, social, and economic impacts of plastic waste supply chains in a circular economy. The first objective of this article is to identify data sets, models, and knowledge gaps associated with waste plastic supply chain processes, mainly in the U.S. Our literature review indicated that the best data sets exist for virgin plastic resin production, mechanical recycling, landfilling, and incineration, with the materials recovery facility being intermediate, and with chemical recycling the lowest. The second objective of this perspective is to develop an illustrative application of the framework by conducting a preliminary systems analysis of PET bottles with closed-loop recycling. The preliminary systems analysis of polyethylene terephthalate (PET) bottles utilized a linear programming optimization method. Our optimization model indicated that both chemical and mechanical recycling processes are needed to achieve a true circular economy of PET bottles with the least greenhouse gas emissions, specifically reductions of 24% when compared with the linear economy. Good quality and standardized life cycle assessment and techno-economic analysis studies are needed to better understand the environmental, economic, and social impacts of advanced sorting and chemical recycling technologies.

Plastics are useful and beneficial materials that contribute to an improved quality of life, yet they generate significant solid wastes and emissions and consume significant energy resources. Systems analysis is incomplete on current linear production systems of plastics supply chains and their associated processes. Our study combines material flow and life cycle assessment data sets of polyethylene terephthalate (PET) and the main polyolefin polymers in the United States, comprising over 70% of plastics flows. This study estimates the total greenhouse gas (GHG) emissions and energy consumption of these supply chains, including transportation and end-of-life processes, lacking in prior studies. We calculate annual GHG emissions and energy consumption of these plastic supply chains to be 101 MMT CO2-eq and 3248 PJ in 2019, respectively. The GHG emissions of these supply chains represented 1.5% of the total U.S. emissions and 5% of the total U.S. industry-related GHG emissions. The total energy consumption of these supply chains represented 3.1% of the total U.S. energy consumption in 2019. Transportation of PET and polyolefin plastic materials contributes 5% and 2% to the total supply chain GHG emissions and energy consumption, respectively. This baseline study provides a benchmark and enables a comparison to future circular production systems for plastics in the United States.

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Enzyme accessibility plays an important role in controlling the rate of conversion of cellulose to glucose in biomass. The goal of this study is to measure the cellulose accessibility due to the effect of dilute acid pretreatment (DAP) and enzymatic hydrolysis (EH) time of Populus biomass. The NMR cryoporometry technique was applied to measure pore size distribution and pore volume over a wide range of pore sizes (range from 0 to 10 000 Å) for both pretreated and enzymatically hydrolyzed biomass substrates. The results showed a clear shift in the pore size ranges (from smaller to larger) as the DAP (acid concentration 0.5% wt and temperature 160 °C) and EH time increased. In most cases, the pore volume increased with the time of treatment, which clearly suggests internal pore expansion. A linear correlation is observed between the initial rate of enzymatic hydrolysis of pretreated poplar and absolute pore volume greater than the critical pore size (51 Å) and shows the effect of residual enzyme activity on the surface of wood chips.

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