An exchange method to investigate the kinetics of Cd complexation in soil solutions

Knowledge of the dynamic speciation of metals in soil solution is necessary for soil-plant transfer models. The lability of a metal in a given context partly depends on the kinetics of interconversion between the free ion (M) and one complex (ML), governed by the association (c(k)a) and dissociation (c(k)d) rate constants of the reaction of formation of ML If only one single complex is assumed to be present in solution, a single kinetic experiment will enable estimation of ckd and cka', that is, the product of cka by the concentration of free ligand in solution. The method consists of the measurements over time of the radioactive metal remaining in solution after the labeling of a Ca resin--solution system in balanceforthe nonlabeled metal initially present. The method is described for Cd and was applied on four soil solution extracts. The order of magnitude of the estimated c(k)a'- and c(k)d-values was about 10(-3) s(-1). The advantages and drawbacks of the method are discussed.