Nickel Binding Affinity with Size-Fractioned Sediment Dissolved and Particulate Organic Matter and Correlation with Optical Indicators

In rivers, the distribution and reactivity of heavy metals (HMs) are affected by their binding affinity with sediment dissolved organic matter (DOM) and particulate organic matter (POM). The HM-OM binding affinity affected by the interaction between DOM and POM is not well studied. This study investigated the Ni binding affinity to size-fractioned overlaying water DOM and alkaline extracted sediment POM solution (AEOM). The DOM/AEOM filtrates (<0.45 μm) were sequentially separated into five nominal molecular weight (MW) solutions. The AEOM optical indicators had lower autochthonous, higher terrestrial sources, and lower aromaticity than the DOM. The Ni mass (72.3 ± 6.4%) was primarily distributed in the low molecular weight DOM (<1 kDa), whereas the Ni (93.5 ± 0.4%) and organic carbon (OC) mass (85.3 ± 1.0%) were predominantly distributed in the high molecular weight AEOM. The Ni and DOM binding affinity, ([Ni]/[DOC])DOM ratio ranging from 0.76 to 27.32 μmol/g-C, was significantly higher than the ([Ni]/[DOC])AEOM ratios, which ranged from 0.64 to 2.64 μmol/g-C. The ([Ni]/[DOC])AEOM ratio correlated significantly with the selected optical indicators (r = 0.87–0.92, p < 0.001), but the ([Ni]/[DOC])DOM ratio correlated weakly with the optical indicators (r = 0.13–0.40, p > 0.05). In the present study, the Ni binding affinity with size-fractioned DOM/AEOM agrees with the hypothesis of the DOM and POM exchange conceptual model in sediment. The POM underwent a hydrolysis/oxidation process; hence, AEOM had a high molecular weight and stable chemical composition and structure. The Ni mainly attached to the high molecular weight AEOM and the ([Ni]/[DOC])AEOM ratios had a strong correlation with the AEOM optical indicators. In contrast, DOM had a high ([Ni]/[DOC])DOM ratio in low molecular weight DOM.