Nickel Speciation and Complexation Kinetics in Freshwater by Ligand Exchange and DPCSV

Abstract

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, k_obsd, was about 3 × 10^-5 (s^-1) for Ni(DMG)₂ complex formation with an excess of DMG (μM) over Ni (nM) at pH 7.1−7.7. The second-order exchange kinetic constants, k_exch, were between 1.2 × 10² and 5.7 × 10³ s^-1 M^-1 for ligand exchange of NiEDTA with DMG and between 5 × 10² and 7 × 10³ 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.