Nickel speciation and complexation kinetics in freshwater by ligand exchange and DPCSV

A technique of ligand exchange with DMG (dimethylglyoxime) and DPCSV was ap plied to determine Ni speciation in lake, river, and groundwater samples. T he working conditions related to ligand-exchange equilibrium were optimized , and the ligand-exchange kinetics were examined. The observed pseudo-first -order rate, k(obsd), was about 3 x 10(-5) (s(-1)) for Ni(DMG)(2) complex f ormation with an excess of DMG (muM) over Ni (nM) at pH 7.1-7.7. The second -order exchange kinetic constants, k(exch), were between 1.2 x 10(2) and 5. 7 x 10(3) s(-1) M-1 for ligand exchange of NiEDTA with DMG and between 5 x 10(2) and 7 x 10(3) s(-1) M-1 for exchange of natural ligands with DMG in t he freshwater samples under similar conditions. Ni ligand exchange between natural ligands and DMG occurred over days with half-lifes of 5-95 h. Total dissolved Ni concentrations in samples from various freshwater systems in Switzerland ranged from 4 nM in an oligotrophic lake to 30 nM in a small ri ver affected by inputs from sewage effluents and agriculture. Free ionic Ni 2+ concentrations were determined in the range of 10(-13)-10(-15) M (pNi = 12.2 - 14.7), indicating that more than 99.9% of dissolved Ni was hound by organic ligands with strong affinity (log K 12.1 - 14.9) and low concentrat ions (13 - 100 nM) at pH 7.2 - 8.2. Because of slow ligand-exchange kinetic s, Ni speciation in natural waters may in many cases not reach equilibrium.