Estimation of the association and dissociation rate constants of Cd complexes with various aminopolycarboxylic acids by an exchange method
Résumé
Plant uptake of Cd depends not only on the concentration of Cd(2+) in the soil solution but also on Cd complexes, the contribution of the latter depending on their association (k(a)) and dissociation (k(d)) rate constants. We used a previously designed exchange method to estimate k(a) and k(d) 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 k(a) and k(d) 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.