Sedimentary DOM composition governs the environmental mobility of palladium in tropical island estuaries

As an emerging contaminant, the environmental fate of palladium (Pd) in estuaries depends critically on dissolved organic matter (DOM), yet its binding mechanisms in tropical systems remain unclear. We characterized sedimentary DOM and its complexation with Pd across Hainan Island estuaries. Multiple techniques were employed: ultraviolet-visible spectroscopy (SUA₂₅₄, A₂₅₃/A₂₀₃), excitation-emission matrix fluorescence with parallel factor analysis (EEM-PARAFAC), and fluorescence quenching titration. Sedimentary DOM exhibited low molecular weight and low humification, with enrichment of tryptophan-like proteins and microbial metabolites. A distinct land-sea gradient showed declining terrestrial inputs and increasing marine microbial sources offshore. Humic-like and fulvic-like components occurred only in southeastern estuaries. Crucially, DOM composition controlled Pd complexation strength and migration risk. In eastern and southern estuaries, macromolecular components provided strong complexation (>50%), effectively sequestering Pd. This yielded lower migration risks (36-48%). In contrast, western estuaries were dominated by low-molecular-weight fractions, which showed weak complexation (<20%) and enhanced Pd mobility, resulting in a 61% migration risk. Unlike highly humified temperate estuaries such as the Yangtze River Estuary and Chesapeake Bay-where HIX values frequently exceed 10 and DOM is dominated by terrestrial humic substances-the weaker complexation capacity of tropical DOM leads to higher Pd mobility. This highlights the unique environmental vulnerability of tropical island estuaries to emerging metal contaminants. Our findings establish that spatial heterogeneity in DOM composition critically controls Pd migration risk, providing a theoretical framework for predicting the environmental behavior of platinum-group elements (PGEs) in analogous tropical coastal environments globally.