Differentiating natural organic matter roles in freshwater floc and bed sediment lead dynamics

Natural organic matter (NOM) is shown to be an important determinant of Pb dynamics across nine highly variable freshwater (wetland, stream, lake) ecosystems of Ontario, Canada. Soluble organic–Pb complexes emerged as the dominant Pb aqueous species (68–99%) in NOM-rich ecosystems; while distinct surficial sediment and suspended floc Pb sequestration and solid-solution partitioning patterns were observed across sites, driven by differential roles of living versus refractory NOM respectively within these two compartments. Field results suggest organo-Pb complexes dominate bed sediment Pb solid-solution partitioning, as sites with the highest pH values showed the greatest Pb mobilization, despite site-specific solid-phase Pb sequestration patterns. Contrasting bed sediment results, amorphous Fe oxyhydroxides (FeOOH) collected by floc microbes and extracellular polymeric substances, were the best predictor of floc Pb solid-solution partitioning, more consistent with cationic Pb behavior, as well as the key floc Pb sequestration phase (43–82%) across systems.