Contrasting Nickel Binding Mechanisms in Water-Column and Sediment Organic Matter: The Critical Role of Molecular Size and Chemical Composition

The environmental fate of nickel (Ni) is dictated by its interaction with organic matter (OM), yet the specific roles of OM source and molecular size remain unclear. This study investigated the binding characteristics of Ni with size-fractionated dissolved OM (DOM) from the water column and alkaline-extractable OM (AEOM) from sediments in a tropical wetland. Using ultrafiltration and spectroscopy, we found that sedimentary AEOM was predominantly high-molecular-weight (HMW) and terrestrial compounds, whereas aquatic DOM was dominated by low-molecular-weight (LMW), microbial-derived compounds. Counterintuitively, the highest Ni binding affinity (NiBA) for both DOM and AEOM occurred in the smallest-molecular-weight fraction (<0.3 kDa). Predictive models confirmed this divergence: the model for the more chemically homogeneous AEOM was highly predictive (r = 0.89), while the model for the complex DOM was less robust (r = 0.70). Our findings demonstrate that LMW fractions are hotspots for Ni binding, challenging the common assumption that larger molecules are more reactive. We conclude that biogeochemical processing in sediments creates an OM pool that is chemically distinct and more predictable than that in the overlying water. This distinction is critical for accurately assessing Ni mobility and ecological risk in aquatic systems.