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Polyethylene microplastics and nanoplastics colored with inorganic pigments in aquatic environments: Effects of mechanical aging on physicochemical properties, aggregation kinetics, and metal release
Summary
Researchers studied how mechanical aging affects colored polyethylene microplastics and nanoplastics containing inorganic pigments in aquatic environments. They found that plastics with certain pigments, particularly ultramarine blue, degraded faster and released more metals than transparent or iron oxide-pigmented plastics. The study reveals that pigment type significantly influences how colored plastics aggregate, break down, and release potentially harmful metals into water.
Mechanical aging affects the environmental fate and transport of colored plastics. This study investigated the physicochemical properties, aggregation kinetics, and metal release of polyethylene (PE) microplastics (PEMPs) and nanoplastics (PENPs) aged from masterbatches. Colored PE with inorganic pigments, including White-Ti-PE (titanium dioxide), Red-Fe-PE (iron oxide), Yellow-Cr/Pb-PE (lead chromate), and Blue-Al-PE (ultramarine), was compared with Transparent-PE. Structural degradation and oxidation ranked Blue-Al-PE > White-Ti-PE > Yellow-Cr/Pb-PE > Red-Fe-PE > Transparent-PE, indicating that PE with more pigments and weaker abrasion-resistance experienced greater aging and metal release. Aggregation rates ranked Blue-Al-PENPs (0.74-1.11 nm/s) > Yellow-Cr/Pb-PENPs (0.20-0.55 nm/s) and White-Ti-PENPs (0.28-0.40 nm/s) > Transparent-PENPs (0.04-0.31 nm/s) > Red-Fe-PENPs (0.04-0.19 nm/s). Raising pH initially promoted and later suppressed Blue-Al-PENPs and White-Ti-PENPs aggregation through charge reversal, while inhibiting others by electrostatic repulsion. For pigmented PENPs, aggregation was driven by electrical double-layer compression, pigment metal bridging, and electrostatic attraction in acidic conditions, while pigment metal precipitation promoted aggregation under alkaline conditions. FeO inhibited Red-Fe-PENPs aggregation via hydration repulsion across all conditions. Humic acid exhibited a salinity-dependent dual role: it stabilized PENPs at low salinity through steric hindrance and electrostatic repulsion, but promoted aggregation at high salinity (35 practical salinity units) via ionic bridging in composite water samples. At 20 mg/L PENPs, metal releases from Yellow-Cr/Pb-PENPs (Cr: 5.45-28.55 μg/L; Pb: 0.66-95.56 μg/L) and Blue-Al-PENPs (Al: 10.05-1466.08 μg/L) exceeded aquatic-life criteria. The findings underscore the impact of mechanical aging on fate and risks of PE colored with inorganic pigments.
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