Quantifying the Interactions of Nickel with Marine Dissolved Organic Matter Using Fluorescence

Nickel (Ni) is a versatile metal with an abundance of applications, namely its role in stainless steel, electronics, and batteries, making it a popular choice in industry. Unfortunately, with increasing demand and production comes higher amounts of Ni pollution. Nickel enters ocean waters - either directly or indirectly - and can have profound effects on marine life. Nickel has been established as a toxicant to a variety of aquatic biota, with the divalent cation (Ni2+) thought to be the most bioavailable fraction and thus the most toxic. Having a reliable means of quantifying free Ni ion is pertinent toward establishing appropriate water quality recommendations for aquatic life protection. The objective of this study was to compare two speciation techniques to quantify Ni2+ in natural samples. The methods studied in this work were ion-selective electrode (ISE) and fluorescence quenching (FQ) titrations. Results indicated that a Ni-ISE is more easily applicable in low ionic strength samples since electrode potential changes to added Ni were only seen in freshwater. Fluorescence excitation-emission matrices were scanned to identify fluorophores within the samples, and variable angle synchronous spectra were used to monitor titrations. Binding constants (log K) as well as complexing capacities (LT) were derived using nonlinear regression, and Monte Carlo analysis was used to relate these values to EC50 Ni levels from toxicity tests (conducted by a collaborative group) on the same samples. Results showed that the predicted Ni2+ concentrations at EC50 levels had overlapping 95% confidence intervals for Mytilus edulis. The free Ni2+ concentration did not overlap for Strongylocentrotus purpuratus, though it should be noted that there was only one data point. The Mytilus edulis results also agreed with the artificial seawater (ASW) control, highlighting the validity and usefulness of a Biotic Ligand Model (BLM) for marine Ni