Microplastics as a dynamic source of dissolved organic carbon: Desorption kinetics, thermodynamic drivers, and...

Research on microplastics (MPs) has focused on adsorption, while largely ignoring desorption as a route for releasing sequestered organics into aquatic systems. This study examined the desorption of organic carbon from virgin and preloaded microplastics using model oxygenated organic chemicals (ethanol, acetonitrile, and acetone) and a temperature range of 25-55 °C, together with a separate open-vessel boiling experiment at 100 °C, to evaluate their influence on dissolved organic carbon (DOC) release. The results showed that dissolved organic carbon concentrations increased rapidly during the initial hour. Increasing the temperature from 25 to 55 °C increased dissolved organic carbon release by approximately 1.7-fold compared with the 25 °C treatment. Preloaded microplastics desorption exhibited a fast initial release (0-1 h), with the highest apparent DOC release percentage of 98.64% observed for acetone-conditioned microplastics. Kinetic modeling indicated that DOC desorption was well described by pseudo-first-order and Elovich models (R² > 0.95). Equilibrium data were fitted to the Langmuir and Freundlich isotherms, indicating apparent finite-capacity behavior and heterogeneous surface interactions. In the separate open-vessel boiling experiment at 100 °C, bulk DOC decreased by approximately 70%; however, this decrease coincided with substantial evaporative water loss and therefore cannot be unequivocally attributed to degradation or removal of MP-derived carbon. The characterization of microplastics using scanning electron microscopy, Fourier-transform infrared spectroscopy, and Brunauer-Emmett-Teller analysis indicated treatment-associated changes in MP surface morphology, spectral features, and surface area. Overall, microplastics may act as secondary sources of DOC under controlled chemical-conditioning and thermal-stress conditions. Thermal exposure (boiling at 100 °C) substantially reduces measurable DOC and MP-associated carbon, suggesting a potential mitigation route for MP-derived organic carbon in water.