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RESEARCH ARTICLE
Contents Vol 6(4)

Estimation of the association and dissociation rate constants of Cd complexes with various aminopolycarboxylic acids by an exchange method

André Schneider A B , Christophe Nguyen A and Laurence Denaix A
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A Institut National de la Recherche Agronomique (INRA), UMR 1220, Bordeaux-Aquitaine, Domaine de la Grande Ferrade, TCEM, 71 avenue Edouard Bourlaux, BP 81, F-33883 Villenave-d’Ornon cedex, France.

B Corresponding author. Email: schneide@bordeaux.inra.fr

Environmental Chemistry 6(4) 334-340 https://doi.org/10.1071/EN09053
Submitted: 30 April 2009  Accepted: 1 July 2009   Published: 25 August 2009

Environmental context. Phytoremediation is a potential way to remove cadmium from polluted soils. The process of plant uptake of cadmium can be enhanced by the addition of chelating compounds. The ability of roots to effectively take up Cd when bound to these complexes is dependent on the speed at which the Cd is associated or dissociated (bound or unbound) from the complex. An exchange method is used here to estimate these association and dissociation rates for a series of Cd–aminopolycarboxylate complexes (some of which have been tested elsewhere in phytoextraction studies). The results of these studies may make it possible to better model the bioavailability of Cd to plant roots.

Abstract. Plant uptake of Cd depends not only on the concentration of Cd2+ in the soil solution but also on Cd complexes, the contribution of the latter depending on their association (ka) and dissociation (kd) rate constants. We used a previously designed exchange method to estimate ka and kd constants of Cd complexed with chelates of the aminopolycarboxylic acid series, i.e. ethylenediamine-N,N′-diacetic acid (EDDA), nitrilotriacetic acid (NTA), N-(2-hydroxyethyl)ethylene-diamine-N,N′,N′-triacetic acid (HEDTA), ethylenediaminetetraacetic acid (EDTA), ethylene glycol-bis(2-aminoethyl)-N,N,N′,N′-tetraacetic acid (EGTA), and trans-1,2-diaminocyclohexane-N,N,N′,N′-tetraacetic acid (CDTA) for future mechanistic modelling of Cd bioavailability including the lability of complexes. The precision of ka and kd estimates depended on the stability of the complexes. For the chelates with the highest stability, HEDTA, EDTA, EGTA and CDTA, the constants were estimated with a good precision. The knowledge of these constants enables improved modelling of bioavailability of Cd to plant roots by considering the contribution of Cd-complexes.

Additional keywords: bioavailability, cationic resin, chelate, lability, trace elements.


Acknowledgements

The authors thank Dr L. Augusto for interesting discussions on this work, and M. H. Bridet and J. P. Touzel for their help in the consultation of various chemical databases.


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