Dominance of adsorption over release: Microplastics' impact on sediment dissolved organic matter dynamics

Microplastics (MPs) are pervasive contaminants in aquatic ecosystems, accumulating in sediments and influencing ecological and biogeochemical processes. While MPs are known to both adsorb and release sediment dissolved organic matter (SOM), the relative dominance of these processes remains unclear. This study investigates the impact of MPs on the quantity and chemical composition of SOM through a 30-day dark incubation experiment using polystyrene (PS) MPs of three different sizes (10, 100, and 1000 mu m). Our results demonstrate that adsorption dominates over release, fundamentally altering SOM dynamics. PS significantly reduced chromophoric and fluorescent SOM, with smaller MPs (10 mu m) causing the greatest reductions-24.2 % and 15.1 %, respectively. Spectroscopic analyses revealed decreases in aromaticity, molecular weight, and humification degree of SOM in the presence of PS, indicating selective adsorption of highly aromatic and humified SOM fractions. Parallel factor analysis identified four humic-like and one protein-like component, with larger MPs (1000 mu m) uniquely increasing the protein-like fluorescence intensity after 30 days. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy further confirmed the chemical modifications of MPs surfaces due to SOM adsorption. These findings underscore the critical role of MPs as adsorbents rather than sources of SOM, with smaller MPs exhibiting stronger adsorption effects. However, this study was conducted under controlled conditions with a single MP type. Future research should explore diverse MP compositions and environmental factors to better assess the ecological implications of MPs on SOM dynamics.