Aging dynamics and subsequent dissolved organic matter evolution of microplastics regulated by UV-salinity conditions

The aging of marine microplastics (MPs) and their release of plastic-derived dissolved organic matter (MP-DOM) have raised growing concern, yet comparative assessments of conventional polyolefins versus biodegradable polyesters under synergistic UV-salinity stress remain limited. Here, we systematically compared the surface alteration and DOM release of polyethylene (PE), polypropylene (PP), and polylactic acid (PLA) across a salinity gradient (0, 15, and 30 PSU; Practical Salinity Unit) under UV-exposed and dark conditions over 60 days. UV irradiation markedly accelerated aging in all three MP types, as evidenced by progressive enrichment of oxygen-containing functional groups. For PE and PP, increased salinity further intensified photo-oxidative degradation, and released DOM exhibited optical shifts from protein-like toward humic-like fluorescence signatures characterized by elevated SUVA₂₅₄ and HIX values. In contrast, PLA degradation was consistent with ester bond hydrolysis, producing 2-6 times greater DOM yields than PE and PP, with released DOM dominated by protein-like fluorescence components. Partial least squares structural equation modeling (PLS-SEM) indicated that UV exposure exerted stronger effects than salinity on the surface oxidation of PE and PP (β = 0.39-0.41 vs. 0.20-0.31), while salinity showed no significant effect on PLA aging. These findings reveal contrasting pathways by which conventional and biodegradable MPs contribute to marine dissolved organic carbon pools, providing a basis for assessing their differential environmental behaviors under variable salinity regimes.