Water, Air, and Soil Pollution

, Volume 202, Issue 1–4, pp 131–140 | Cite as

A Study of Diffusive Gradients in Thin Films for the Chemical Speciation of Zn(II), Cd(II), Pb(II), and Cu(II): The Role of Kinetics

  • Ibraheem Gaabass
  • John D. Murimboh
  • Nouri M. Hassan
Article

Abstract

The lability and mobility of Zn(II)–, Cd(II)–, Pb(II)–, and Cu(II)–humic acid complexes were studied using diffusive gradients in thin films (DGT). A unique feature of this research was (1) the use of DGTs with diffusive layer thicknesses ranging from 0.4 to 2.0 mm to study lability and mobility of Zn(II)–, Cd(II)–, Pb(II)–, and Cu(II)–humic acid complexes, combined with (2) the application of a competing ligand exchange (CLE) method using Chelex 100, the same chelating resin that is used in DGT, to study the kinetic speciation. The CLE experiments were run immediately after the completion of the DGT experiments, thereby allowing effects of the competing ligand to be separated from the effects introduced by the use of the polyacrylamide gel that is used in DGT. The results indicate that Zn(II) and Cd(II) tend to form more labile and more mobile complexes with humic acid than Pb(II) or Cu(II). The dissociation rate constants of Zn(II), Cd(II), and Pb(II) were found to increase with the ionic potential of the metal, suggesting that the binding between some trace metals and humic acid has a significant covalent component. Furthermore, the results suggest that the Eigen mechanism may not be strictly obeyed for metals such as Cu(II) which have high rate constants of water exchange, k w. Consequently, the markedly slow kinetics of Cu(II)-HA species suggests that the usual equilibrium assumption may not be valid in freshwaters.

Keywords

Diffusive gradients in thin films Competing ligand exchange Chelex 100 Trace metals Chemical speciation Humic acid 

Notes

Acknowledgments

J.M. thanks the Natural Sciences and Engineering Research Council of Canada (NSERC) for a research grant. The authors also thank Dr. Stephen Duffy (Mount Allison University) for providing the humic acid and Gary Fisher and Dr. Colin Cameron (Defence Research and Development Canada Halifax) for providing ICP-MS time.

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Copyright information

© Springer Science+Business Media B.V. 2009

Authors and Affiliations

  • Ibraheem Gaabass
    • 1
  • John D. Murimboh
    • 1
  • Nouri M. Hassan
    • 2
  1. 1.Department of ChemistryAcadia UniversityWolfvilleCanada
  2. 2.Worsfold Water Quality CentreTrent UniversityPeterboroughCanada

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