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Adsorption behaviour of accelerated UV aged PET and PP microplastics towards Pb(II) under varying pH, temperature, and salinity conditions
Summary
UV aging causes PET and PP microplastics to adsorb significantly more lead (Pb) from water, with the extent varying by pH, temperature, and salinity. This means weathered microplastics in the environment may carry greater toxic metal loads than virgin plastic, amplifying their hazard to ecosystems and human health.
Microplastics (MPs) undergo environmental weathering, particularly through UV radiation, which significantly modifies their surface properties and enhances their potential to adsorb contaminants such as heavy metals (HMs). This study investigates the adsorption behaviour of lead (Pb(II)) onto virgin and aged polyethylene Terephthalate (PET) and polypropylene (PP) MPs up to 30 days of Ultraviolet (UV) aging. Environmental parameters such as pH (3-9), salinity (0.5-3 %), and temperature (5-45 °C) were analysed. UV aging process altered the morphological and physicochemical characteristics of the MPs as confirmed by field emission scanning electron microscopy (FESEM) and contact angle measurements, which indicated increased surface roughness, cracking, and enhanced hydrophilicity. Surface chemistry changes were further analysed using Fourier-transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD), revealing the development of new functional groups, such as carbonyl (CO) and hydroxyl groups (OH). An increase in crystallinity suggested the formation of new active adsorption sites. Batch adsorption experiments were conducted under varying aging durations (0, 15, and 30 days) at different contact durations (0 to 48 h). Results confirmed that UV aging significantly enhanced Pb(II) adsorption capacity from 0.0643 to 0.1432 mg/g for PET and from 0.0661 to 0.1277 mg/g for PP, with adsorption equilibrium reached after 24 h. Adsorption was strongly altered by environmental factors, with higher pH (9) and moderate temperature (15-25 °C) favouring the metal uptake on MPs. Kinetic data best fits the pseudo-second-order model (R > 0.95), indicating chemisorption as the dominant mechanism, while isotherms confirmed a closer fit to the Langmuir model, suggesting homogenous monolayer adsorption. This study identifies the effect of UV aging on MPs and its capacity to adsorb metals under varying environmental conditions thereby providing new insights into their underlying mechanisms.
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