Reservoirs frequently require dredging to remove accumulated sediment. This sediment is currently unused, but represents a possible source of humic acids (HAs) that have potential as iron-binding supplements in fertilizers. In order to evaluate the utility of these materials, we investigated the structural features and Fe(II)-binding constants or capacities of some HAs obtained from reservoir sediments. The histidine content in the HAs was correlated to the Freundlich binding constant for an Fe(II)-HA complex, indicating that indole moieties in histidine function as strong binding sites for Fe(II). The binding capacities (2.2-3.2 mmol g(-1)) indicated that the HAs have the capacity to be loaded with 12-18% Fe(II), comparable to the iron content of previously reported Fe fertilizers that employ HAs from other sources (2-17%). The Fe(II)-binding capacities were positively correlated with the content of phenolic hydroxyl groups. In addition, the positive correlation between Fe(II)-binding capacity and vanillyl moieties in the HAs indicates that 3,4-dihydroxybenzene and/or benzoic acid derivatives also serve as Fe(II) binding sites.