Use of an Exchange Method to Estimate the Association and Dissociation Rate Constants of Cadmium Complexes Formed with Low-Molecular-Weight Organic Acids Commonly Exuded by Plant Roots

摘要

Organic acids released from plant roots can form complexes with cadmium (Cd) in the soil solution and infl uence metal bioavailability not only due to the nature and concentration of the complexes but also due to their lability. Th e lability of a complex infl uences its ability to buff er changes in the concentration of free ions (Cd2+); it depends on the association (k(a), m(3) mol(-1) s(-1)) and dissociation (k(d), s(-1)) rate constants. A resin exchange method was used to estimate k(d) and k(a)(exp) (m(3) mol(-1) s(-1)), which is the conditional estimate of k(a) depending on the calcium (Ca) concentration in solution. The constants were estimated for oxalate, citrate, and malate, three low-molecular-weight organic acids commonly exuded by plant roots and expected to strongly infl uence Cd uptake by plants. For all three organic acids, the k(a)(exp) and k(d) estimates were around 2.5 10(-3) m(3) mol(-1) s(-1) and 1.3 x 10(-4) s(-1), respectively. Based on the literature, these values indicate that the Cd-low-molecular-weight organic acids complexes formed between Cd and low-molecular-weight organic acids may be less labile than complexes formed with soil soluble organic matter but more labile than those formed with aminopolycarboxylic chelates.