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

Organic acids released from plant roots can form complexes with cadmium (Cd) in the soil solution and influence metal bioavailability not only due to the nature and concentration of the complexes but also due to their lability. The lability of a complex influences its ability to buffer changes in the concentration of free ions (Cd2+); it depends on the association (ka, m3 mol−1 s−1) and dissociation (kd, s−1) rate constants. A resin exchange method was used to estimate kd and kaexp (m3 mol−1 s−1), which is the conditional estimate of ka 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 influence Cd uptake by plants. For all three organic acids, the kaexp and kd estimates were around 2.5 10−3 m3 mol−1 s−1 and 1.3 × 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.