Nickel speciation and complexation kinetics in freshwater by ligand exchange and DPCSV.
A technique of ligand exchange with DMG (dimethylglyoxime) and DPCSV was applied to determine Ni speciation in lake, river, and groundwater samples. The working conditions related to ligand-exchange equilibrium were optimized, and the ligand-exchange kinetics were examined. The observed pseudo-first-order rate, kobsd, was about 3 x 10(-5) (s-1) for Ni(DMG)2 complex formation with an excess of DMG (microM) over Ni (nM) at pH 7.1-7.7. The second-order exchange kinetic constants, kexch, 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 the 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 river affected by inputs from sewage effluents and agriculture. Free ionic Ni2+ 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 bound by organic ligands with strong affinity (log K 12.1-14.9) and low concentrations (13-100 nM) at pH 7.2-8.2. Because of slow ligand-exchange kinetics, Ni speciation in natural waters may in many cases not reach equilibrium.
Xue
H B
HB
Swiss Federal Institute for Environmental Science and Technology (EAWAG), CH-6047 Kastanienbaum, Switzerland. xue@eawag.ch
Jansen
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S
Prasch
A
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Sigg
L
L
D007700:Kinetics; D008024:Ligands; D008962:Models, Theoretical; D009532:Nickel / Q000737:chemistry*; D010091:Oximes; D012722:Sewage; D014873:Water Pollutants / Q000032:analysis*
D008024:Ligands; D010091:Oximes; D012722:Sewage; D014873:Water Pollutants; C013501:dimethylglyoxime; D009532:Nickel