Dual-phase plastic derivatives: Phase-specific photochemical behaviors and impacts on aquatic carbon

Aquatic carbon cycling is increasingly perturbed by exogenous organic carbon from plastic derivatives. These derivatives form a dual-phase system, comprising particulate nano-plastics (NPs, reaching 10⁸ particles/mL) and truly dissolved fractions (accounting for ˃60% of the mass), with the potential to redistribute carbon between fast and slow cycling pools. However, the phase-specific contributions to photochemical processes and carbon flow remain poorly defined. Here, combining ultrafiltration and quantitative carbon tracking, we elucidate their intrinsic phase specificity. Morphologically, PP-NPs exist as rough, irregular fragments, whereas PS-NPs exhibit highly regular, smooth spherical structures. Spectroscopic analyses revealed that PP derivatives exhibit weak, dissolved-phase-dominated absorption/fluorescence activities with strong inter-phase structural heterogeneity. Conversely, PS derivatives maintain robust and consistent dual-phase optical properties driven by cross-phase chemical homology. The photoreactivity and carbon fates of both derivatives are synergistically governed by phase- and polymer-specific mechanisms. The dissolved fraction serves as the core precursor driving primary photoactive species (³DOM*) generation, whereas particulate NPs provide solid-liquid interfaces that amplify the yields of secondary reactive species (¹O₂ and •OH) by 1.54- to 2.24-fold. Notably, PS-NPs, whose surfaces are highly enriched with reactive sites (Δ=0.25 µmolₑ⁻ mg^-1 C), can deeply intervene in short-term carbon flows via dual pathways of photon competition and carbon replenishment. Nevertheless, the presence of particulate NPs does not alter the ultimate upper limit of mineralization; rather, they construct an optical shield that decelerates the mid-term rate of carbon phase transformation. By resolving phase specificity via quantitative carbon tracking, this study provides a mechanistic framework for integrating plastic carbon into aquatic biogeochemical models and evaluating its ecological consequences.