High release of isotopically depleted CO2 and CH4 from the photo-degradation of plastic: A pilot laboratory study

This pilot laboratory study reports the direct release of photochemically derived isotopically depleted CO2 and CH4 from low-density polyethylene (LDPE) under ambient atmospheric conditions. We investigated the release of CO2 and CH4 from two different size fractions of LDPE; coarse (∼10 mm × 7 mm) and fine (∼1.5 mm × 1 mm) pieces. Emissions observed at the end of 45 days incubation experiment were 94 nmol/g CH4 and 20 μmol/g CO2 for coarse plastics, and 242 nmol/g CH4 and 42 μmol/g CO2 for fine plastics kept in 120 ml vials under same natural sunlight conditions. Release of CO2 and CH4 from the LDPE kept under dark conditions was statistically not distinguishable from blanks (vials without plastics), which attests the role of photo-degradation in their release. Further, >2 times increase in the emissions of CH4 and CO2 for fine plastics have important implications as plastic size decreases continuously with their degradation in different environmental compartments. Isotopic composition of carbon (δ13C) of CH4 (−676‰ to – 496‰) and CO2 (−132‰ to −140‰) released from the LDPE shows conspicuously depleted signatures, which may have important implications for studies using δ13C to understand biogeochemical processes in different environmental compartments. The present study necessitates the accountability of CO2 and CH4 release from plastics while studying the biogeochemical cycling of carbon in terrestrial and aquatic environments. More studies with different types and sizes of plastics under different environmental conditions are needed to assess the role of plastics in CO2 and CH4 budget.