title: Measurement of metals using DGT: impact of ionic strength and kinetics of dissociation of complexes in the resin domain.
abstract: As the measurement of metals by DGT (diffusion gradients in thin films) in low salinity media has been controversial, a thorough study of the impact of ionic strength (I) is timely. DGT accumulations of Cd, Co, and Ni in the presence of NTA at pH 7.5 with I in the range from 10(-4) to 0.5 M were obtained. An observed decrease in the metal accumulation as the ionic strength of the system decreased is partially explained by the electrostatic repulsion between the negatively charged resin domain and the dominant negatively charged complex species M-NTA. This electrostatic effect reduces the complex penetration into the resin domain, especially for nonlabile complexes, which do not fully dissociate in the gel domain. Analytical expressions, based on the Donnan model, were able to quantify these electrostatic effects. Additionally, the data indicate that the kinetic dissociation constant of M-NTA complexes in the resin layer is higher than Eigen predictions, suggesting a ligand-assisted dissociation mechanism. As the ionic strength decreases, the rate of reaction in the resin layer decreases due to the repulsion between the negatively charged resin sites and the complex species. This decrease contributes to the decrease in metal accumulation. These novel, previously unconsidered, effects of ionic strength and the ligand-assisted dissociation mechanism in the resin domain will affect DGT measurements made in freshwaters and soils.
authors.affiliation: Departament de Química, Universitat de Lleida and AGROTECNIO , Rovira Roure 191, 25198, Lleida, Spain.
authors.forename: Jaume
authors.initials: J
authors.lastname: Puy
authors.forename: Josep
authors.initials: J
authors.lastname: Galceran
authors.forename: Sara
authors.initials: S
authors.lastname: Cruz-González
authors.forename: Calin A
authors.initials: CA
authors.lastname: David
authors.forename: Ramiro
authors.initials: R
authors.lastname: Uribe
authors.forename: Chun
authors.initials: C
authors.lastname: Lin
authors.forename: Hao
authors.initials: H
authors.lastname: Zhang
authors.forename: William
authors.initials: W
authors.lastname: Davison
mesh_terms: D004058:Diffusion; D007700:Kinetics; D008670:Metals / Q000737:chemistry*; D009994:Osmolar Concentration; D055672:Static Electricity
chemical_list: D008670:Metals