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UV Weathering of Microplastics: Linking Surface Oxidation, Bulk Molecular and Mechanical Degradation, and Fragmentation Metrics Within a QA/QC Framework for Biomonitoring
Summary
Sunlight doesn't just make plastic waste look worn—it chemically breaks it down into smaller and smaller fragments, and this review pulls together existing research to explain how scientists measure that breakdown process. This matters because smaller plastic fragments (down to the nano scale) are more likely to enter our bodies through water, food, or air, so understanding how sunlight speeds up this fragmentation helps researchers better predict and track the tiniest, potentially most harmful microplastics we're exposed to. The paper doesn't report new health risks itself, but it proposes better, more consistent methods for studying plastic breakdown—
Sunlight-driven UV weathering is a major transformation pathway of environmental microplastics, promoting surface oxidation, molecular degradation, embrittlement, and progressive fragmentation toward smaller size fractions. However, comparisons across studies remain difficult because weathering is often described using descriptors that probe different aspects of degradation without being clearly distinguished. Surface-sensitive oxidation metrics, such as carbonyl or oxidation indices (CI/OI), are frequently emphasized, whereas fragmentation and embrittlement are more directly governed by bulk molecular-weight loss, mechanical weakening, and particle-size evolution. This review examines UV weathering of common polymers through a coupled chemico-mechanical perspective relevant to the micro-to-nano transition. We distinguish surface chemical descriptors, bulk molecular and mechanical descriptors, and fragmentation-related metrics, and critically assess the analytical methods used to measure them, including FTIR, Raman spectroscopy, GPC/SEC, thermal methods, mechanical testing, and particle-size analyses. We argue that no single metric is sufficient to describe weathering progression, and that meaningful interpretation requires joint reporting of oxidation state, Mn/Mw changes, mechanical deterioration where available, and particle-size distribution as a function of cumulative or spectrum-weighted UV dose. We further propose a minimal QA/QC reporting framework including UV metadata, temperature, oxygen availability, blanks, replicates, recovery tests, and matrix-specific detection limits. By separating what different methods actually probe and linking them to fragmentation mechanisms, this review provides a more operational basis for interpreting UV-aged microplastics in environmental sampling and biomonitoring.