Hydrothermal Transformation of Mixed Plastic–Lignocellulosic Waste: Polymer Degradation Pathways and Environmental Implications

Agricultural plastics are frequently contaminated with lignocellulosic residues, limiting mechanical recyclability and contributing to environmental accumulation. Understanding polymer transformation pathways in such mixed systems is critical for enabling circular polymer management. In this study, mild hydrothermal processing (250–300°C) was applied to a model mixture of low-density polyethylene (LDPE) and wheat straw to investigate polymer–biomass interactions and by-product formation. Comprehensive phase characterization revealed the recovery of two distinct solid fractions: a lignocellulosic-derived hydrochar with reduced O:C ratio, and a polymer-rich LDPE phase that incorporated hydrochar into a homogeneous composite structure, suggesting thermally induced polymer restructuring and interfacial integration. Liquid products were dominated by phenolic compounds whose concentrations increased with temperature, while furanic species decreased, indicating temperature-dependent lignocellulosic depolymerization pathways. Gaseous products included CH₄, CO, CO₂, and propene, with light hydrocarbon formation increasing from ~ 95 ppm at 250°C to ~ 800 ppm at 300°C, reflecting progressive polymer backbone scission. Soil incubation assays demonstrated limited impacts of the aqueous phase on dominant microbial phyla, though shifts at the specie level were observed. These findings provide mechanistic insight into polymer behavior during hydrothermal co-processing and highlight both opportunities and environmental considerations for advancing mixed plastic–biomass recycling strategies.