Rapid and efficient removal of dissolved nickel by oxic bedded sediment

Nickel (Ni) bioavailability in sediment is modified by naturally occurring ligands that can complex Ni and lessen its toxicity. Current sediment quality guidelines for Ni consider reduced sulfur and organic matter as important ligands, but more recently metal oxide minerals have been shown to alter Ni bioavailability. We assessed Ni partitioning to oxidized ligands in natural sediments by incubating five geochemically distinct low-sulfide sediments with dissolved Ni (0.5, 2, or 5 mg L-1) under different pH conditions (pH 5, 7, or 9) for 28 days. Nickel concentrations in overlying water declined rapidly in most treatments and frequently reached equilibrium within a few days, except in acidic and low Ni loading scenarios. At equilibrium, the sediment sorbed 48%-100% of the added Ni, and most test conditions removed more than 70% of the added Ni. The amount of Ni removed was strongly correlated to particle size and properties of metal oxide minerals, specifically the affinity of hydrous ferric oxides (HFOs) for Ni (i.e., NiHFO:FeHFO, KNi-HFO). Interestingly, bulk sediment characteristics were either uncorrelated or negatively correlated with the amount and rate of Ni removal. Our results show that dissolved Ni is rapidly and substantially removed from the overlying water by oxic bedded sediments in non-acidic conditions, with removal extent driven by particle size and the affinity of HFOs for Ni, and removal rate determined by overlying water pH in these sediments.