Influence of organic matter on the bioavailability and toxicity of nickel to the amphipod hyalella azteca

Understanding Ni speciation in solution and the partitioning of Ni between solution and solid sediment is important in determining Ni bioavailability and toxicity to Hyalella azteca in sediments. Water-only Ni toxicity tests were conducted to evaluate the effects of dissolved organic matter on Ni speciation and bioavailability. Test substances chemically and spectroscopically characterized for use in these tests included Little Bear Lake sediment, peat moss, and Suwannee River humic and fulvic acids. Nickel speciation, bioavailability and toxicity in the presence of dissolved organic matter was assessed via three methods: ion exchange measurements of the free Ni2+ ion, mathematical modeling using the Windermere Humic Aqueous Model (WHAM VI), and toxicity testing with H. azteca. It was found that the main bioavailable Ni species at the pHs tested (pH 8.10 - 8.33) was the free Ni2+ ion. This research also demonstrated that Ni may be significantly complexed, or largely free and labile, depending on the Ni:dissolved organic carbon ratio. Overall, the Ni:dissolved organic carbon ratio plays a greater role than either dissolved organic carbon source or fraction in determining Ni speciation and Ni bioavailability and toxicity to aquatic organisms. Natural sediment was titrated with Ni under anaerobic conditions to evaluate the partitioning of Ni between solution and solid phase as pH varied (pHs 6, 7, 8). There was a noticeable increase in sediment Ni complexation with increasing pH. To evaluate the influence of organic matter on Ni bioavailability and toxicity in sediments, 10-d toxicity tests (using H. azteca) were conducted with Ni spiked over a range of concentrations in both formulated and field-collected sediments. The total organic carbon content of sediment had a significant influence on Ni bioavailability to H. azteca. Formulated sediments with different amounts of organic carbon displayed a clear decrease in toxicity with increasing organic carbon content at the same total Ni concentration. Results from both the formulated and natural sediment tests further indicated that toxicity was strongly correlated with pore-water Ni concentration, and that toxicity estimates based on pore-water Ni exposures were comparable to separate toxicity estimates for Ni in water-only tests. While excess acid-volatile sulfide in sediment appeared to predict the absence of acute Ni toxicity, it did not predict the absence of Ni bioaccumulation. This was potentially due to the presence of multiple pore-water Ni species (i.e., Ni2+, NiHS+) which were bioavailable in the sediment micro-environment of H. azteca