Dark Reduction of Hg(II) by Dissolved Organic Matter Derived from Aging Microplastics: Mechanisms and Implications

Dissolved organic matter (DOM) plays a critical role in the environmental cycling and transformation of mercury (Hg), primarily due to its strong reducing and complexing properties toward mercuric Hg(II). Microplastics-derived DOM (MPs-DOM), particularly that released during photoaging, represents an emerging source of DOM in aquatic environments. However, its capacity to mediate Hg(II) transformation remains largely unexplored. This study investigated dark reduction of Hg(II) by DOM released from aging polystyrene, polyvinyl chloride, and polylactic acid under simulated environmental conditions. The results show that, under dark conditions, DOM from photoaged MPs suspensions reduced over 30% of Hg(II) within 10 min, whereas DOM from dark-aged MPs suspensions exhibited negligible Hg(II) reduction activity. Further analyses showed that photoaging enhanced the electron-donating capacity of MPs-DOM by increasing phenol-like compounds, which promoted Hg(II) reduction via electron transfer through phenolic hydroxyl groups. Notably, MPs-DOM released during photoaging outperformed the Suwannee River natural organic matter (SRNOM) in reducing Hg(II), likely due to compositional differences in Hg(II)-complexing functional groups. When mixed together, elevated concentrations of MPs-DOM dominated over SRNOM, favoring Hg(II) reduction as the primary pathway. Given the growing prevalence of MPs-DOM in aquatic ecosystems and the persistence of dark reactions in light-limited environments, this study underscores the significant role of MPs-DOM in promoting dark Hg(II) reduction, highlighting a previously underrecognized pathway affecting Hg cycling.