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Dual-phase plastic derivatives: Phase-specific photochemical behaviors and impacts on aquatic carbon
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Lab tests examined how two plastic breakdown products, from polypropylene and polystyrene, behave as tiny particles versus dissolved material in water exposed to light. The dissolved fractions mainly triggered light-driven reactive compounds, while the particles added surfaces that boosted certain reactive byproducts and slowed, but did not stop, how fast carbon changed form over time.
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.
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