Partitioning and availability of uranium and nickel in contaminated riparian sediments

The effects of iron oxides and organic matter on the partitioning and chemical lability of U and Ni were examined for contaminated riparian sediments from the U.S. Department of Energy's Savannah River Site. In sequential extractions of four sediments that ranged from 12.7 to 82.2 g kg(-1) in organic carbon, U was found almost exclusively in moderately labile fractions (93% in acid-soluble + organically bound). Nickel was distributed across all operationally defined fractions, including substantial amounts in the very labile fractions (4-15% in water-soluble + exchangeable), noncrystalline and crystalline iron oxides (38-49%), and in the nonlabile residual fraction (25-34%). Aqueous U concentrations in 1:1 sediment-water extracts were highly correlated to dissolved organic carbon (DOC) (R2 = 0.96; p < 0.0001) and ranged from 29 to 410 microg L(-1). Aqueous concentrations of Ni exceeded U by two to three orders of magnitude (124-2227 microg L(-1)) but were not correlated with DOC (R2 = 0.04; p = 0.53). Partitioning and solubility trends suggest that Ni availability is controlled primarily by iron-oxide phases, whereas U availability is dominated by naturally occurring organic carbon. Discrete mineral phases were also identified as nonlabile reservoirs of anthropogenic metals. In spite of comparably high sediment concentrations, Ni appears to be significantly more available than U in riparian sediments and therefore warrants greater consideration in terms of environmental consequences (i.e., transport, biological uptake, and toxicity).