Thermal stress suppresses methanogenesis while Fe(III) alters carbon-related responses during microplastic aging in landfill leachate

Landfill leachate differs from many anaerobic aquatic environments because it combines darkness, oxygen limitation, high dissolved organic matter, reactive iron, plastic contamination, high ionic strength, and localized heat stress. These conditions may precondition microplastics (MPs), but whether MP aging in leachate promotes anaerobic greenhouse-gas production remains unclear. Here, polyethylene (PE), polystyrene (PS), and polylactic acid (PLA) MPs were incubated for 90 d in field-collected landfill leachate at 30 °C or 70 °C, with or without Fe(III) amendment. Temperature dominated the measured carbon-related responses: compared with 30 °C, 70 °C reduced headspace CO₂C accumulation from 2.0 to 3.8 to 0.1-1.1 mg C and eliminated detectable headspace CH₄. Under thermal stress, Fe(III) addition was associated with greater DOC depletion, lower filter-passing Fe concentrations, and increased filter-retained solid-phase C. MP aging was polymer dependent: PE mainly served as a scaffold for Fe-DOM coating, PS showed Fe-sensitive carbonyl-region changes and localized deposits, and PLA exhibited hydrolysis-associated disruption. Heat stress reduced microbial diversity and strongly diminished methanogenesis-associated archaea, whereas putative polymer-associated ASVs remained embedded within Fe-regulated and polymer-specific environments. These findings highlight polymer-specific surface alteration and concurrent changes in carbon fractions under thermal stress and Fe(III) amendment, with temperature exerting the dominant constraint on methanogenesis in the tested leachate system.