Competitive ligand exchange/adsorptive cathodic stripping voltammetry (CLE/AdCSV) for kinetic studies of nickel speciation in aqueous environmental samples containing heterogeneous, macromolecular, organic complexants Original Research Article

Competitive ligand exchange/adsorptive cathodic stripping voltammetry (CLE/AdCSV) using rotating disk electrode voltammetry/square wave voltammetry (RDEV/SWV) has been developed and applied to the kinetic speciation of nickel in aqueous environmental samples containing heterogeneous, macromolecular, organic complexants. Dissociation rate coefficients were obtained for nickel complexes in nitrilotriacetic acid (NTA) model solutions, model solutions of a well-characterized fulvic acid (Armadale fulvic acid) and Rideau river surface water (RRSW). The results demonstrated that the dissociation of nickel complexes formed by organic complexants was slow in comparison with the rate of formation of the Ni(DMG)2 complex. Two kinetically distinguishable components were observed in the NTA model solutions: the first with dissociation rate coefficient >10−3 s−1, and the second with a dissociation rate coefficient ≈10−5 s−1. Nickel complexes of the Armadale FA in the model solution yielded two kinetically distinguishable nickel complexes, including an inert component that did not dissociate within the time scale of the experiment. The proportion of the inert component increased as the [FA] to [Ni] mole ratio was increased. The nickel complexes in the RRSW sample yielded one kinetically distinguishable component: an extremely slowly-dissociating component with a first-order dissociation rate coefficient ≈10−6 s−1. The results are in agreement with the earlier results obtained by our laboratory on the lability/inertness of the Ni–FA complexes formed with the Armadale FA and studied by the competing ligand exchange method (CLEM) using Chelex 100 as the competing ligand and inductively-coupled plasma – mass spectrometry (ICP-MS) and graphite furnace atomic absorption spectrometry (GFAAS) for measurement of the kinetics of metal complex dissociation.