Determination of Ni2+ using an equilibrium ion exchange technique: Important chemical factors and applicability to environmental samples

In natural waters, the determination of free metal concentrations is a key parameter for studying bioavailability. Unfortunately, few analytical tools are available for determining Ni speciation at the low concentrations found in natural waters. In this paper, an ion exchange technique (IET) that employs a Dowex resin is evaluated for its applicability to measure [Ni2+] in freshwaters. The presence of major cations (e.g.   Na, Ca and Mg) reduced both the times that were required for equilibration and the partition coefficient to the resin (λ′Ni)(λ′Ni). IET measurements of [Ni2+] in the presence of known ligands (citrate, diglycolate, sulfoxine, oxine and diethyldithiocarbamate) were verified by thermodynamic speciation models (MINEQL+ and VisualMINTEQ). Results indicated that the presence of hydrophobic complexes (e.g. Ni(DDC)20) lead to an overestimation of the Ni2+ fraction. On the other hand, [Ni2+] measurements that were made in the presence of amphiphilic complexes formed with humic substances (standard aquatic humic acid (SRHA) and standard aquatic fulvic acid (SRFA)) were well correlated to free ion concentrations that were calculated using a NICA-DONNAN model. An analytical method is also presented here to reduce the complexity of the calibration (due to the presence of many other cations) for the use of Dowex equilibrium ion exchange technique in natural waters.