Short-term photoaging of conventional and biodegradable mulch film microplastics: divergent surface transformation and dissolved organic matter release

Photoaging alters microplastic (MP) surface properties and dissolved organic matter (DOM) release, yet, early-stage responses of agricultural mulch film-derived MPs remain insufficiently understood. Here, short-term (192 h) photoaging of conventional polyethylene (PE) and biodegradable poly(butylene adipate-co-terephthalate) (PBAT) MPs was investigated in ultrapure water under high-pressure mercury lamp irradiation in a photochemical reactor. Time-dependent changes in surface morphology, hydrophobicity, carbonyl index (CI), hydroxyl radical (·OH) formation, dissolved organic carbon (DOC), and fluorescent DOM characteristics were examined. PE and PBAT showed distinct early-stage photoaging dynamics. PBAT developed surface cracks within 30 h and became hydrophilic after 192 h, whereas PE showed obvious cracking only after 90 h and remained predominantly hydrophobic. The CI increased for PE but decreased for PBAT, indicating different surface carbonyl accumulation and loss patterns. OH signals were detected on both MPs, with stronger signals on PBAT; however, EPR results indicate radical formation rather than demonstrating the exclusive causal role of ·OH in degradation. PBAT released more DOC than PE, with DOM mainly characterized by humic- and tryptophan-like components, while PE-derived DOM was dominated by protein- and tryptophan-like substances. These findings provide an integrated solid-phase and leachate perspective on the early-stage environmental transformation of mulch-derived MPs.